Dr. Hartmut Wiggers

Institute for Combustion and Gas Dynamics – Reactive Fluids
University of Duisburg-Essen

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  • Atmospheric-pressure particle mass spectrometer for investigating particle growth in spray flames
    Suleiman, S. and Nanjaiah, M. and Skenderovic, I. and Rosenberger, T. and Kunze, F. and Wlokas, I. and Kruis, F.E. and Wiggers, H. and Schulz, C.
    Journal of Aerosol Science 158 (2021)
    In this work, we introduce a new particle mass spectrometer (AP-PMS) that is able to detect particle-size distributions at ambient pressure using a three-stage pumping design. This device is demonstrated for direct sampling from the particle formation in spray-flame synthesis of iron oxide nanoparticles. Aerosol sampling is performed by a probe with integrated dilution that has been characterized and configured by computational fluid dynamics simulations and the chamber-skimmer system has been investigated by schlieren imaging. The system was validated by detailed characterization of a standardized sooting flame and by iron oxide nanoparticles generated in the SpraySyn burner from iron nitrate dissolved in a mixture of ethanol and 2-ethylhexanoic acid. The PMS results are compared to additional inline measurements with SMPS and ELPI + as well as with TEM measurements of thermophoretically sampled materials from the same location in the spray flame. © 2021 The Authors
    view abstract10.1016/j.jaerosci.2021.105827
  • Direct gas phase synthesis of amorphous Si/C nanoparticles as anode material for lithium ion battery
    Orthner, H. and Wiggers, H. and Loewenich, M. and Kilian, S. and Bade, S. and Lyubina, J.
    Journal of Alloys and Compounds 870 (2021)
    High specific capacity of silicon is very attractive for its application as anode material in lithium ion batteries. However, the implementation of silicon is challenging due to its large volume expansion on lithiation leading to pulverization and buildup of a solid-electrolyte interphase. Nanostructuring and design of silicon alloys are a promising strategy to circumvent these challenges. Here we demonstrate an industrially scalable gas phase synthesis method using thermal decomposition of silane and ethylene to produce novel amorphous silicon/carbon-based particles with enhanced electrochemical performance. Fundamental principles and kinetics considerations for the design of high-performance silicon/carbon-based materials are discussed. © 2021 Elsevier B.V.
    view abstract10.1016/j.jallcom.2021.159315
  • Gas-Phase Synthesis of Silicon-Rich Silicon Nitride Nanoparticles for High Performance Lithium–Ion Batteries
    Kilian, S.O. and Wiggers, H.
    Particle and Particle Systems Characterization 38 (2021)
    The practical application of silicon-based anodes is severely hindered by continuous capacity fade during cycling. A very promising way to stabilize silicon in lithium–ion battery (LIB) anodes is the utilization of nanostructured silicon-rich silicon nitride (SiNx), a conversion-type anode material. Here, SiNx with structure sizes in the sub-micrometer range have been synthesized in a hot-wall reactor by pyrolysis of monosilane and ammonia. This work focusses on understanding process parameter–particle property correlations. Further, a model for the growth of SiNx nanoparticles in this hot–wall–reactor design is proposed. This synthesis concept is of specific interest regarding simplicity, flexibility, and scalability: A way utilizing any mixtures of precursor gases to build multi-functional nanoparticles that can be directly used for LIBs instead of focusing on modification of nanostructures after they have been formed. Lab-scale production rates as high as 30 g h−1 can be easily achieved and further scaled. SiN0.7 nanoparticles provide a first cycle coulombic efficiency of 54%, a specific discharge capacity of 1367 mAh g−1, and a capacity retention over 80% after 300 cycles at 0.5 C (j = 0.68 mA cm−2). These results imply that silicon-rich silicon nitrides are promising candidates for high-performance LIBs with very high durability. © 2021 The Authors. Particle & Particle Systems Characterization published by Wiley-VCH GmbH
    view abstract10.1002/ppsc.202100007
  • Investigation of the combustion of iron pentacarbonyl and the formation of key intermediates in iron oxide synthesis flames
    Karakaya, Y. and Kluge, S. and Wiggers, H. and Schulz, C. and Kasper, T.
    Chemical Engineering Science 230 (2021)
    The information of the gas phase kinetics are relevant for the development of detailed reaction mechanisms as well as for process design and control in flame synthesis. In this study, the decomposition of iron pentacarbonyl and the reaction pathways towards iron oxide clusters and particles in laminar H2/O2/Ar low-pressure synthesis flames are investigated. Gas-phase species are analyzed by photoionization and electron ionization mass spectrometry. The extraction of a representative sample from the particle-laden flow of a synthesis flame by an intrusive sampling technique for the analysis is challenging, because iron-intermediate species can condense easily. Cations can be extracted from the flame with a high efficient ion sampling technique that results in high sensitivity. Iron-containing cations provide evidence of the presence of key intermediates, e.g., Fe(OH)2, Fe(OH)3, Fe2O3, and larger Fe-O-clusters which are the dominant intermediates with respect to particle formation and need to be considered in future gas-phase reaction mechanisms. © 2020 Elsevier Ltd
    view abstract10.1016/j.ces.2020.116169
  • Large-scale synthesis of iron oxide/graphene hybrid materials as highly efficient photo-Fenton catalyst for water remediation
    Hammad, M. and Fortugno, P. and Hardt, S. and Kim, C. and Salamon, S. and Schmidt, T.C. and Wende, H. and Schulz, C. and Wiggers, H.
    Environmental Technology and Innovation 21 (2021)
    The Photo-Fenton reaction is an advanced oxidation process to break down organic pollutants in aqueous systems. Moreover, the scalable synthesis and engineering of stable catalysts with a high specific surface area is extremely important for the practical application of the Photo-Fenton process. In the current study, we developed a low-cost method for large-scale production of iron oxide/graphene nanostructures with a controllable graphene loading for the photo-Fenton reaction. Under optimal condition, high efficiencies of degradation (>99%) of methylene blue, rhodamine B, acid orange 7, and phenol at a concentration (60 mg/mL) were reached in 60 min under UV-A irradiation (1.6 mW/cm2) with mineralization of 72, 77, 82, and 48%, respectively. More importantly, the iron oxide/graphene nanocomposites exhibited good stability over a wide range of pH (from 3 to 9) and can be magnetically separated from the solution and repeatedly used with consistent photocatalytic performance. This enhanced removal efficiency of the iron oxide/graphene nanostructure compared to iron oxide nanoparticles is attributed to the accelerated transfer of photo-generated electrons between iron oxide and graphene and its relatively large surface area. The results demonstrate that the iron oxide/graphene system could be potentially utilized for many environmental treatment processes. © 2020 Elsevier B.V.
    view abstract10.1016/j.eti.2020.101239
  • Spray flame synthesis (Sfs) of lithium lanthanum zirconate (llzo) solid electrolyte
    Ali, M.Y. and Orthner, H. and Wiggers, H.
    Materials 14 (2021)
    A spray-flame reaction step followed by a short 1-h sintering step under O2 atmosphere was used to synthesize nanocrystalline cubic Al-doped Li7La3Zr2O12 (LLZO). The as-synthesized nanoparticles from spray-flame synthesis consisted of the crystalline La2Zr2O7 (LZO) pyrochlore phase while Li was present on the nanoparticles’ surface as amorphous carbonate. However, a short annealing step was sufficient to obtain phase pure cubic LLZO. To investigate whether the initial mixing of all cations is mandatory for synthesizing nanoparticulate cubic LLZO, we also synthesized Li free LZO and subsequently added different solid Li precursors before the annealing step. The resulting materials were all tetragonal LLZO (I41 /acd) instead of the intended cubic phase, suggesting that an intimate intermixing of the Li precursor during the spray-flame synthesis is mandatory to form a nanoscale product. Based on these results, we propose a model to describe the spray-flame based synthesis process, considering the precipitation of LZO and the subsequent condensation of lithium carbonate on the particles’ surface. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/ma14133472
  • Spray-Flame Synthesis of LaMnO3+δNanoparticles for Selective CO Oxidation (SELOX)
    Angel, S. and Tapia, J.D. and Gallego, J. and Hagemann, U. and Wiggers, H.
    Energy and Fuels (2021)
    LaMnO3+δ nanoperovskites were prepared via the continuous and scalable spray-flame synthesis (SFS) technique from metal nitrate-based solutions by using either ethanol (EtOH) as solvent or a mixture of ethanol (50 vol %) and 2-ethylhexanoic acid (50 vol %) (EtOH/2-EHA). Solutions based on pure EtOH generated a mixture of several phases and a broad and multimodal particle size distribution, which is attributed to a combination of gas-to-particle and droplet-to particle formation of particles. The product contained a bimodal distribution of the orthorhombic (Pnma II) LaMnO3 perovskite-like phase and additional, unwanted phases such as La2O3 and sub-20 nm Mn-rich amorphous/poorly crystalline particles. The incorporation of 2-EHA led to high surface area (>100 m2 g-1), small, and crystalline LaMnO3+δ nanoparticles with sizes ranging between 4 and 15 nm in the presence of few sub-200 nm particles (<10 wt %). This sample is mainly composed of the orthorhombic Mn4+ rich (Pnma I) LaMnO3+δ phase, and it counts with a very high specific surface area that makes it highly promising for catalytic applications. FTIR and UV-VIS spectroscopy of the precursor solutions revealed the oxidation of the Mn2+ precursor in advance of the particle formation process along with the esterification of the solvent mixture. It is assumed that the observed liquid-phase oxidation supports the formation of Mn4+-rich perovskites. According to O2-TPD and H2-TPR measurements, the EtOH/2-EHA sample presented a much higher formation of adsorbed active oxygen species and higher reducibility than the EtOH-made material, leading to a superior performance for both the catalytic oxidation of CO and the selective oxidation (SELOX) of CO. © 2021 American Chemical Society.
    view abstract10.1021/acs.energyfuels.0c03659
  • Spray-flame synthesis of LaMO3(M = Mn, Fe, Co) perovskite nanomaterials: Effect of spray droplet size and esterification on particle size distribution
    Angel, S. and Schneider, F. and Apazeller, S. and Kaziur-Cegla, W. and Schmidt, T.C. and Schulz, C. and Wiggers, H.
    Proceedings of the Combustion Institute 38 (2021)
    Perovskite nanomaterials such as LaMnO3, LaFeO3, and LaCoO3were synthesized in a spray flame from metal nitrates dissolved in combustible liquids. The addition of low-boiling solvents such as 2-ethylhexanoic acid (2-EHA) to the ethanol-based solutions supports the formation of phase-pure particles with unimodal particle-size distribution in the 10-nm range attributed to enhanced evaporation through micro-explosions. Nevertheless, in many cases, a second particle mode with sizes of a few hundred nanometers is formed. In this paper, we investigate two possible reasons for the appearance of large particles. Firstly, we analyze the effect of the oxygen dispersion gas flow applied in the two-fluid nozzle on the droplet size distributions of burning sprays using phase Doppler anemometry. We identified that an increase of the dispersion gas flow significantly decreases the number concentration of large droplets (&gt;30 μm), which causes a significant increase of the BET surface area of as-synthesized LaMnO3and LaCoO3with increasing dispersion gas flow from 60 m2/g (5 slm dispersion gas) to 100 m2/g (8 slm). Secondly, the esterification in the mixture of solvents towards ethyl-2-ethylhexanoate, which is associated with the release of water as a byproduct, was analyzed by GC/MS. The ester concentration in the iron-containing solution was found to be up to nine times higher than in cobalt or manganese precursor solutions. Simultaneously, the produced LaFeO3materials show lower BET surface areas and the increasing dispersion gas flow has a minor effect on this material than on the cobalt and manganese perovskite cases. We attribute this to the fact that water formed during esterification forces the hydrolysis of iron nitrate and the formation of large particles within the droplets. © 2021 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.proci.2020.07.116
  • Assembly, Stability, and Electrical Properties of Sparse Crystalline Silicon Nanoparticle Networks Applied to Solution-Processed Field-Effect Transistors
    Chryssikos, D. and Wiesinger, M. and Bienek, O. and Wiggers, H. and Stutzmann, M. and Cattani-Scholz, A. and Pereira, R.N.
    ACS Applied Electronic Materials 2 (2020)
    Thin films of crystalline silicon nanoparticles (Si NPs) processed from liquid dispersions of NPs (NP inks) using printing-type deposition methods are currently being intensively investigated for the development of electronic and optoelectronic nanotechnologies. Various (opto)electronic applications have already been demonstrated based on these materials, but so far, devices exhibit modest performance because of relatively low electrical conductivity and charge carrier mobility. In this work, we aim at unveiling the major factors affecting the long-range transport of charges in Si NP thin films. For this, we monitor the electrical properties of thin-film field effect transistors (FETs) as the active channel of the devices is gradually filled with Si NPs. To produce these FET devices featuring stable, sparse Si NP networks within the active channel, we developed a fabrication protocol based on NP deposition by device substrate immersion in a NP ink, made of Si NPs and chlorobenzene, followed by annealing and ultrasonication. We found that both the electrical conductivity and the charge carrier mobility of the FETs increase extremely rapidly as the device channel coverage with NPs increases. Thus, the NP network corresponds effectively to an inhomogeneous blend of conducting and insulating Si NPs, with the most efficient charge percolation paths involving only a fraction of the NPs. We discuss the factors that may lead to this behavior, in view of developing Si NP thin films with competitive charge transport characteristics. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acsaelm.9b00786
  • Effect of Spray Parameters in a Spray Flame Reactor During FexOy Nanoparticles Synthesis
    Carvajal, L. and Buitrago-Sierra, R. and Santamaría, A. and Angel, S. and Wiggers, H. and Gallego, J.
    Journal of Thermal Spray Technology 29 (2020)
    Abstract: Synthesis and characterization of FexOy nanoparticles were carried out in order to study reaction parameters influence in a spray flame reactor. FexOy powders were prepared with three different precursors aiming to understand how the reactor conditions, dispersion gas flow, and precursor solution flow affect morphology, shape, particle size distribution, crystalline phases, and residue content of the obtained materials. Thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy (TEM), x-ray diffraction (XRD), and Raman spectroscopy were employed to characterize the materials. In addition, magnetic behavior of the obtained samples was evaluated. It was found that the evaluated parameters influenced the residue contents obtaining weight changes from 10 to 35%. Particle size distribution centers also showed differences between 17 and 24 nm. By XRD, Raman, and TEM, the presence of hematite (a-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4) was evidenced and explained based on the gas and liquid content in the flame. Additionally, the saturation magnetization was measured for selected samples, obtaining values between 26 and 32 emu g−1. These magnetic measurements were correlated with the crystalline phase composition and particle size distributions. Graphic Abstract: [Figure not available: see fulltext.] © 2020, ASM International.
    view abstract10.1007/s11666-020-00991-1
  • Gas-phase synthesis of iron oxide nanoparticles for improved magnetic hyperthermia performance
    Hammad, M. and Hardt, S. and Mues, B. and Salamon, S. and Landers, J. and Slabu, I. and Wende, H. and Schulz, C. and Wiggers, H.
    Journal of Alloys and Compounds 824 (2020)
    Magnetic nanoparticle-mediated hyperthermia has shown great potential in cancer therapy. However, upscaling of the synthesis of iron oxide nanoparticle with the required narrow size distribution remains challenging. This paper describes the reproducible and scalable synthesis of citric acid-functionalized iron oxide nanoparticles optimized for hyperthermia treatment. Iron oxide nanoparticles were synthesized by a spray flame method, which is eco-friendly and cost-effective. To the best of our knowledge, this is the first study reporting spray-flame synthesis of small iron oxide nanoparticles (approx. 7 nm) with narrow size distribution (polydispersity index ≪ 0.1). The citric acid-coated iron oxide nanoparticles revealed a hydrodynamic size of approx. 37 nm and a high magnetic saturation of 69 Am2/kg at room temperature. The magnetic hyperthermia study showed a significantly enhanced value of the intrinsic loss power (4.8 nHm2/kg), which is 1.5-fold higher than the best commercially available equivalents. The improved heating efficiency and small hydrodynamic size of citric acid-coated iron oxide nanoparticles demonstrate that the system could potentially be used as a nanoplatform for hyperthermia treatment. © 2020 Elsevier B.V.
    view abstract10.1016/j.jallcom.2020.153814
  • Nanoparticle Formation and Behavior in Turbulent Spray Flames Investigated by DNS
    Abdelsamie, A. and Kruis, F.E. and Wiggers, H. and Thévenin, D.
    Flow, Turbulence and Combustion 105 (2020)
    This work is a first direct numerical simulation of a configuration closely related to the SpraySyn burner (Schneider et al. in Rev Sci Instrum 90:085108, 2019). This burner has been recently developed at the University of Duisburg-Essen to investigate experimentally nanoparticle synthesis in spray flames for a variety of materials. The present simulations are performed for ethanol and titanium tetraisopropoxide as a solvent and precursor, respectively, in order to produce titanium dioxide nanoparticles. In the direct numerical simulations, the complete scenario leading to the production of well-defined nanoparticles is taken into account, including evaporation of the liquid mixture (solvent and precursor) injected as a spray, multi-step kinetics for gas-phase combustion, and finally nanoparticle synthesis. The employed models are described in this article. Additionally, the impact of the inlet velocity of the pilot flame on the nanoparticle synthesis is investigated. It has been found that increasing this speed delays spray flame ignition, decreases nanoparticle concentration, but leads to a narrower size distribution at early stage. © 2020, The Author(s).
    view abstract10.1007/s10494-020-00144-y
  • Plasma-assisted gas-phase synthesis and in-line coating of silicon nanoparticles
    Dasgupta, M. and Fortugno, P. and Wiggers, H.
    Plasma Processes and Polymers 17 (2020)
    This study investigates the feasibility of plasma-supported in-line functionalization of silicon nanoparticles (NPs) in an atmospheric pressure gas-phase reactor. The approach utilizes the synthesis of core silicon NPs and their subsequent coating downstream of the particle formation zone. In-line coating is accomplished with a cylindrical coating nozzle to achieve homogenous mixing of coating precursor vapors with in-coming NPs. Multiple siloxanes were tested for their coating suitability and their ability towards coating homogeneity. It was found that tetraethyl orthosilicate is favored for thin layers consisting of almost pure silica while hexamethyldisiloxane and octamethylcyclotetrasiloxane (OMCTS) coatings contained reasonable amounts of hydrocarbons. Moreover, OMCTS showed a pronounced tendency towards homogeneous nucleation, thus leading to the additional formation of silica NPs due to homogeneous nucleation. © 2020 The Authors. Plasma Processes and Polymers published by WILEY-VCH Verlag GmbH & Co. KGaA
    view abstract10.1002/ppap.201900245
  • Selective cyclohexene oxidation with O2, H2O2and: Tert -butyl hydroperoxide over spray-flame synthesized LaCo1- xFexO3nanoparticles
    Büker, J. and Alkan, B. and Fu, Q. and Xia, W. and Schulwitz, J. and Waffel, D. and Falk, T. and Schulz, C. and Wiggers, H. and Muhler, M. and Peng, B.
    Catalysis Science and Technology 10 (2020)
    The elimination of waste and by-product generation and reduced dependence on hazardous chemicals are the key steps towards environmentally sustainable chemical transformations. Heterogeneously catalysed oxidation of cyclohexene with environmentally friendly oxidizing agents such as O2, H2O2 and tert-butyl hydroperoxide (TBHP) has great potential to replace existing processes using stoichiometric oxidants. A series of spray-flame synthesised nanoparticulate LaCo1-xFexO3 catalysts was employed for cyclohexene oxidation, and the comparative results showed that TBHP led to the highest initial activity and allylic selectivity, but O2 resulted in higher conversion for longer reaction times. Furthermore, the influence of Fe substitution was studied, which did not show any beneficial synergistic effects. LaCoO3 was found to be the optimum catalyst for cyclohexene oxidation with O2, following first-order reaction kinetics with an apparent activation energy of 57 kJ mol-1. The catalyst showed good reusability due to its highly stable particle size, morphology and perovskite structure. 7-Oxabicyclo[4.1.0]heptan-2-one was identified to be formed by the oxidation of 2-cyclohexene-1-one with 2-cyclohexene-1-hydroperoxide. © 2020 The Royal Society of Chemistry.
    view abstract10.1039/d0cy00906g
  • Self-assembled nano-silicon/graphite hybrid embedded in a conductive polyaniline matrix for the performance enhancement of industrial applicable lithium-ion battery anodes
    Wiggers, H. and Sehlleier, Y.H. and Kunze, F. and Xiao, L. and Schnurre, S.M. and Schulz, C.
    Solid State Ionics 344 (2020)
    Nano-structured silicon-based composite materials have generated significant excitement for use as anode materials in high-performance Li-ion batteries. For making these materials commercially applicable, a high Coulombic efficiency at the first cycle must be achieved. Additionally, scalable synthesis routes need to be developed to provide access to practically-relevant material quantities. In this work, we propose a strategy for the production of Si/graphite/polyaniline (Si/graphite/PANI) composites that addresses both above mentioned challenges. Si nanoparticles were produced in a pilot-plant-scale microwave-plasma reactor using monosilane (SiH4) as precursor. This process enables the formation of high-purity Si nanoparticles with controllable crystal sizes at a production rate of 45 g/h. Si/graphite hybrids are fabricated through self-assembly by electrostatic attraction. The Si/graphite/PANI nanocomposite is then prepared by in situ polymerization of aniline monomer in the presence of the Si/graphite hybrid. With this approach, ~40 g of Si/graphite/PANI composite per batch can be produced at lab scale. The scalability of the underlying processes enables the use for commercial products. The nanocomposite shows favorable characteristics inherited from its three components: Si nanoparticles provide high capacity, graphite acts as an electrical conductor and gives a high Coulombic efficiency, and the polyaniline coating further enhances the electrical conductivity and protects the entire structure. A very good Coulombic efficiency of 86.2% at the initial cycle is recorded for this nanocomposite material. Galvanostatic charge/discharge tests demonstrate that this material can deliver a discharge capacity of 2000 mAh/g with a very good capacity retention of 76% after 500 cycles at a discharge rate of 0.5C (1.25 A/g). The capacity is 870 mAh/g measured at 5C (12.5 A/g). © 2019 Elsevier B.V.
    view abstract10.1016/j.ssi.2019.115117
  • Silicon Nanoparticle Films Infilled with Al2O3Using Atomic Layer Deposition for Photosensor, Light Emission, and Photovoltaic Applications
    Botas, A.M.P. and Leitão, J.P. and Falcão, B.P. and Wiesinger, M. and Eckmann, F. and Teixeira, J.P. and Wiggers, H. and Stutzmann, M. and Ferreira, R.A.S. and Pereira, R.N.
    ACS Applied Nano Materials 3 (2020)
    Solution-processed thin films of crystalline silicon nanoparticles (Si NPs) have a great potential for a wide variety of electronic and optoelectronic applications. However, such films are inherently unstable due to their huge surface-to-volume ratios and high surface energies, making them prone to degradation associated with spontaneous oxidation in ambient conditions. In this work, we explore the use of atomic layer deposition (ALD) as a means to stabilize and potentially functionalize solution-processed thin films of Si NPs for (opto)electronics, e.g., thin-film transistors, photosensors, light-emitting devices, and photovoltaics. We prepared films of randomly distributed Si NPs with ultrashort surface ligands (Si-H termination) using wet chemistry and spray-coating and then use ALD to infill the films with Al2O3. Through microscopy and optical structural/morphological analysis, we demonstrate the achievability of ALD infilling of films of Si NPs and probe the stability of these films against oxidation. Moreover, we show that the ALD infilling leads to changes in the light emission properties of the Si NP films, including a relative quenching of disorder-related emission features and variations in surface-related dielectric confinement effects. Our studies reveal ALD as a relevant technique toward manufacturing de facto robust, functional nanomaterials based on Si NPs and on nanoscale silicon materials more generally. © 2020 American Chemical Society.
    view abstract10.1021/acsanm.0c00116
  • Spray-flame synthesis of BaTi1-xZrxO3 nanoparticles for energy storage applications
    Tarasov, A. and Shvartsman, V.V. and Shoja, S. and Lewin, D. and Lupascu, D.C. and Wiggers, H.
    Ceramics International 46 (2020)
    BaTi1-xZrxO3 nanoparticles (x = 0, 0.05, 0.1, 0.15, 0.2) were successfully produced by the spray-flame synthesis method. The as-synthesized powders are characterized by small (~10 nm) particle sizes as shown by TEM images. The as-synthesized powders were pre-heated at 800 °C to remove organic residuals from the surface. Pellets were then pressed and sintered at 1100 °C for 3 h. XRD measurements of the sintered materials show that the crystallite size decreases with increasing Zr concentration, which was additionally confirmed by TEM. Dielectric measurements show that the Curie temperature shifts towards lower temperatures with increasing Zr concentration accompanied by a decrease in the dielectric permittivity values which is attributed to a decreasing crystallite/particle size. In addition, a frequency dispersion of the permittivity values is discovered. This is mostly ascribed to Maxwell-Wagner polarization effects typical for nanograined ceramics. © 2020 Elsevier Ltd and Techna Group S.r.l.
    view abstract10.1016/j.ceramint.2020.02.187
  • Spray-flame synthesis of La(Fe, Co)O3 nano-perovskites from metal nitrates
    Angel, S. and Neises, J. and Dreyer, M. and Friedel Ortega, K. and Behrens, M. and Wang, Y. and Arandiyan, H. and Schulz, C. and Wiggers, H.
    AIChE Journal 66 (2020)
    Nano-sized perovskites were synthesized in a spray flame from nitrate precursors dissolved in ethanol and in ethanol/2-ethylhexanoic acid (2-EHA) mixtures. Experiments with ethanol led to a broad particle-size distribution and to the formation of undesired phases such as La2CoO4, La2O3, and Co3O4. The addition of 2-EHA can initiate micro explosions of the burning droplets and has been systematically investigated toward the formation of single-phase, high-surface-area LaCoO3 and LaFeO3 with a narrow size distribution. To investigate the effect of 2-EHA, temperature-dependent changes of the chemical composition of the precursor solutions were analyzed with ATR-FTIR between 23 and 70°C. In all cases, the formation of esters was identified while in the solutions containing iron, additional formation of carboxylates was observed. The synthesized materials were characterized by BET SSA, XRD, SAED and EDX-TEM and their catalytic activity was analyzed, reaching 50% CO conversion at temperatures below 160 and 300°C for LaCoO3 and LaFeO3, respectively. © 2019 The Authors. AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers.
    view abstract10.1002/aic.16748
  • Spray-flame synthesis of LaMO3 (M = Mn, Fe, Co) perovskite nanomaterials: Effect of spray droplet size and esterification on particle size distribution
    Angel, S. and Schneider, F. and Apazeller, S. and Kaziur-Cegla, W. and Schmidt, T.C. and Schulz, C. and Wiggers, H.
    Proceedings of the Combustion Institute (2020)
    Perovskite nanomaterials such as LaMnO3, LaFeO3, and LaCoO3 were synthesized in a spray flame from metal nitrates dissolved in combustible liquids. The addition of low-boiling solvents such as 2-ethylhexanoic acid (2-EHA) to the ethanol-based solutions supports the formation of phase-pure particles with unimodal particle-size distribution in the 10-nm range attributed to enhanced evaporation through micro-explosions. Nevertheless, in many cases, a second particle mode with sizes of a few hundred nanometers is formed. In this paper, we investigate two possible reasons for the appearance of large particles. Firstly, we analyze the effect of the oxygen dispersion gas flow applied in the two-fluid nozzle on the droplet size distributions of burning sprays using phase Doppler anemometry. We identified that an increase of the dispersion gas flow significantly decreases the number concentration of large droplets (&gt;30 μm), which causes a significant increase of the BET surface area of as-synthesized LaMnO3 and LaCoO3 with increasing dispersion gas flow from 60 m2/g (5 slm dispersion gas) to 100 m2/g (8 slm). Secondly, the esterification in the mixture of solvents towards ethyl-2-ethylhexanoate, which is associated with the release of water as a byproduct, was analyzed by GC/MS. The ester concentration in the iron-containing solution was found to be up to nine times higher than in cobalt or manganese precursor solutions. Simultaneously, the produced LaFeO3 materials show lower BET surface areas and the increasing dispersion gas flow has a minor effect on this material than on the cobalt and manganese perovskite cases. We attribute this to the fact that water formed during esterification forces the hydrolysis of iron nitrate and the formation of large particles within the droplets. © 2020 The Combustion Institute.
    view abstract10.1016/j.proci.2020.07.116
  • Spray-Flame-Prepared LaCo1–xFexO3 Perovskite Nanoparticles as Active OER Catalysts: Influence of Fe Content and Low-Temperature Heating
    Alkan, B. and Medina, D. and Landers, J. and Heidelmann, M. and Hagemann, U. and Salamon, S. and Andronescu, C. and Wende, H. and Schulz, C. and Schuhmann, W. and Wiggers, H.
    ChemElectroChem 7 (2020)
    Spray-flame synthesis was used to produce high-surface-area perovskite electrocatalysts with high phase purity, minimum surface contamination, and high electrochemical stability. In this study, as-prepared LaCo1–xFexO3 perovskite nanoparticles (x=0.2, 0.3, and 0.4) were found to contain a high degree of combustion residuals, and mostly consist of both, stoichiometric and oxygen-deficient perovskite phases. Heating them at moderate temperature (250 °C) in oxygen could remove combustion residuals and increases the content of stoichiometric perovskite while preventing particle growth. A higher surface crystallinity was observed with increasing iron content coming along with a rise in oxygen deficient phases. With heat treatment, OER activity and stability of perovskites improved at 30 and 40 at.% Fe while deteriorating at 20 at.% Fe. This study highlights spray-flame synthesis as a promising technique to synthesize highly active nanoscale perovskite catalysts with improved OER activity. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.201902051
  • Tailored SiNx-based Anode Processing for Li-Ion Batteries
    Bapat, S. and Oezcan, F. and Kilian, S.O. and Wiggers, H. and Segets, D.
    ECS Transactions 97 (2020)
    In order to successfully implement promising new battery materials at industrial production rates, it is important to provide suitable recipes for information. Typically, active materials are processed as dispersions in liquid-phase together with further additives aiming at the improvement of battery slurry properties. Interactions between particles and dispersing liquid(s) are decisive for electrode manufacturing and performance. Hence, we combined Hansen parameter approach with analytical centrifugation for dispersing SiNx nanoparticles as promising next generation battery material. Suitable probe liquids were chosen for the identification of beneficial dispersion properties. Electron microscopy was employed for a first qualitative plausibility analysis. Diacetone alcohol showed favorable dispersion properties for SiNx particles while toluene was found to be not suitable. Our approach and findings are an excellent starting point for the systematic characterization and evaluation of new battery materials with regard to processability. © 2020 ECS - The Electrochemical Society.
    view abstract10.1149/09707.0185ecst
  • Comparative study of flame-based SiO 2 nanoparticle synthesis from TMS and HMDSO: SiO-LIF concentration measurement and detailed simulation
    Chrystie, R.S.M. and Janbazi, H. and Dreier, T. and Wiggers, H. and Wlokas, I. and Schulz, C.
    Proceedings of the Combustion Institute 37 (2019)
    Depending on the chemical nature of precursor species, the flame-based synthesis of silicon dioxide nanoparticles in lean hydrogen/oxygen flames proceeds via different chemical routes, which affects the generated particle characteristics. Modeling the flame chemistry and particle formation therefore can provide valuable understanding of the underlying gas-phase and particle-formation pathways. In the present study we compare experimentally obtained temperature and semi-quantified SiO-concentration profiles in low-pressure (3 kPa), lean (? &lt; 0.6), inert-gas diluted H 2 /O 2 /Ar burner-stabilized flat flames doped with 200-4000 ppm hexamethyldisiloxane (HMDSO) or tetramethylsilane (TMS) with results from kinetics modeling. Temperature fields in the flames were determined via multi-line laser-induced fluorescence (LIF) imaging using both added NO and native SiO as target species. Gas-phase silicon monoxide (SiO) was detected via LIF by exciting the rovibrational Q(42) transition in the A 1 Π-X 1 Σ + (1,0) vibronic band system at 230.998 nm that provides a weak temperature dependence when analyzing relative SiO mole fractions. Semi-quantitative SiO mole-fraction profiles as a function of height-above-burner (HAB) were obtained for all flames from the measured SiO-LIF intensities corrected for variations of the temperature-dependent ground-state population and the collisional quenching using measured temperatures and effective fluorescence lifetimes, respectively. The experimental data were compared with results of appropriate chemical kinetics mechanisms from the literature with suitable modifications to best reproduce measured SiO mole-fraction profiles. Modeling initial cluster formation is important in this study to unravel the observed 'double-peak'-structure of the SiO concentration profiles assumed to originate from resublimed SiO from early-formed SiO 2 nanoparticles in the rising temperature gradient during initial particle nucleation, and which may be altered by the availability of oxygen in the precursor species. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.07.024
  • Comparative study of flame-based SiO2 nanoparticle synthesis from TMS and HMDSO: SiO-LIF concentration measurement and detailed simulation
    Chrystie, R.S.M. and Janbazi, H. and Dreier, T. and Wiggers, H. and Wlokas, I. and Schulz, C.
    Proceedings of the Combustion Institute 37 (2019)
    Depending on the chemical nature of precursor species, the flame-based synthesis of silicon dioxide nanoparticles in lean hydrogen/oxygen flames proceeds via different chemical routes, which affects the generated particle characteristics. Modeling the flame chemistry and particle formation therefore can provide valuable understanding of the underlying gas-phase and particle-formation pathways. In the present study we compare experimentally obtained temperature and semi-quantified SiO-concentration profiles in low-pressure (3 kPa), lean (? &lt; 0.6), inert-gas diluted H2/O2/Ar burner-stabilized flat flames doped with 200-4000 ppm hexamethyldisiloxane (HMDSO) or tetramethylsilane (TMS) with results from kinetics modeling. Temperature fields in the flames were determined via multi-line laser-induced fluorescence (LIF) imaging using both added NO and native SiO as target species. Gas-phase silicon monoxide (SiO) was detected via LIF by exciting the rovibrational Q(42) transition in the A1Π-X1Σ+ (1,0) vibronic band system at 230.998 nm that provides a weak temperature dependence when analyzing relative SiO mole fractions. Semi-quantitative SiO mole-fraction profiles as a function of height-above-burner (HAB) were obtained for all flames from the measured SiO-LIF intensities corrected for variations of the temperature-dependent ground-state population and the collisional quenching using measured temperatures and effective fluorescence lifetimes, respectively. The experimental data were compared with results of appropriate chemical kinetics mechanisms from the literature with suitable modifications to best reproduce measured SiO mole-fraction profiles. Modeling initial cluster formation is important in this study to unravel the observed 'double-peak'-structure of the SiO concentration profiles assumed to originate from resublimed SiO from early-formed SiO2 nanoparticles in the rising temperature gradient during initial particle nucleation, and which may be altered by the availability of oxygen in the precursor species. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.07.024
  • Detailed simulation of iron oxide nanoparticle forming flames: Buoyancy and probe effects
    Sellmann, J. and Rahinov, I. and Kluge, S. and Jünger, H. and Fomin, A. and Cheskis, S. and Schulz, C. and Wiggers, H. and Kempf, A. and Wlokas, I.
    Proceedings of the Combustion Institute 37 (2019)
    Nanoparticle formation in flames is strongly influenced by the residence-time-temperature history inside the flame. We study how the temperature history can be intentionally modified by orienting flames either in an upward-firing or downward-firing configuration. We also investigate the influence of unintended residence-time modifications caused by sampling nozzles. These phenomena are investigated by experiments and simulations for the synthesis of iron oxide nanoparticles from premixed iron-pentacarbonyl-doped hydrogen/oxygen flat flames. The experiments apply molecular-beam sampling with a particle mass spectrometer to measure particle sizes and a quartz microbalance to detect the presence of condensed matter. The simulations rely on a finite-rate chemistry approach with species-specific diffusion, particle dynamics are described by a bi-modal population balance model. It is demonstrated that the downward-burning flame forms a detached stagnation point, causing longer residence times at elevated temperature than an upward- or horizontally firing flame, permitting the growth of larger particles. These iron oxide particles are eventually formed in the recombination zone of the flame, but no condensed matter was found in the reaction zone. The experiments also observed the formation of particles in the preheat zone, but their composition and all aspects of their disappearance remain uncertain. Current models do, however, suggest the formation of iron particles and their subsequent evaporation and combustion. © 2018 The Combustion Institute.
    view abstract10.1016/j.proci.2018.06.041
  • Durability study of platinum nanoparticles supported on gas-phase synthesized graphene in oxygen reduction reaction conditions
    Bertin, E. and Münzer, A. and Reichenberger, S. and Streubel, R. and Vinnay, T. and Wiggers, H. and Schulz, C. and Barcikowski, S. and Marzun, G.
    Applied Surface Science 467-468 (2019)
    Ligand-free platinum nanoparticles were prepared by pulsed laser ablation in liquids (PLAL) and employed as a benchmarking catalyst to evaluate the durability of a new gas-phase synthesized graphene support in oxygen reduction conditions. Raman measurements showed that the graphene, as compared to Vulcan, was almost defect free. Transmission electron microscopy and initial electrochemically active surface area measurements confirmed good dispersion of the catalysts on both supports. During durability tests, graphene supported Pt nanoparticles showed much better ECSA retention (75% on graphene as compared to 38% on Vulcan), ultimately retaining a higher ECSA than a commercial sample subjected to the same procedure. © 2018 Elsevier B.V.
    view abstract10.1016/j.apsusc.2018.10.061
  • Microwave plasma-assisted silicon nanoparticles: Cytotoxic, molecular, and numerical responses against cancer cells
    Wahab, R. and Khan, F. and Gupta, A. and Wiggers, H. and Saquib, Q. and Faisal, M. and Ansari, S.M.
    RSC Advances 9 (2019)
    Silicon nanoparticles (SiNPs), which have a special place in material science due to their strong luminescent property and wide applicability in various physicochemical arenas, such as solar cells and LEDs, were synthesised by a microwave plasma-assisted process using an argon-silane mixture. Several characterization tools were applied to check the crystallinity (XRD) and morphological (FESEM, TEM, ∼20 ± 2 nm size) and topographical (AFM, ∼20 nm) details of the NPs. The high-purity SiNPs were applied on myoblast cancer cells to investigate the reactivity of the NPs at different doses (200, 1000 and 2000 ng mL-1) for different incubation periods (24 h, 48 h &amp; 72 h). The MTT assay was utilized to determine the percentage of viable and non-viable cells, while the cell organization was observed via microscopy and CLSM. Additionally, the molecular responses (RT-PCR), such as apoptosis, were analyzed in presence of caspase 3 and 7, and the results showed an upregulation with SiNPs. To validate the obtained data, analytical studies were also performed for the SiNPs via statistical analysis and the most reliable data values were evaluated and acceptable as per the ICH guidelines. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8ra10185j
  • Spray-Flame-Synthesized LaCo1−xFexO3 Perovskite Nanoparticles as Electrocatalysts for Water and Ethanol Oxidation
    Alkan, B. and Cychy, S. and Varhade, S. and Muhler, M. and Schulz, C. and Schuhmann, W. and Wiggers, H. and Andronescu, C.
    ChemElectroChem 6 (2019)
    Coupling electrochemical generation of hydrogen with the concomitant formation of an industrially valuable product at the anode instead of oxygen can balance the high costs usually associated with water electrolysis. We report the synthesis of a variety of nanoparticulate LaCo1−xFexO3 perovskite materials through a specifically optimized spray-flame nanoparticle synthesis method, using different ratios of La, Co, and Fe precursor compounds. Structural characterization of the resulting materials by XRD, TEM, FTIR, and XPS analysis revealed the formation of mainly perovskite-type materials. The electrocatalytic performance of the formed perovskite-type materials towards the oxygen evolution reaction and the ethanol oxidation reaction was investigated by using rotating disk electrode voltammetry. An increased Fe content in the precursor mixture leads to a decrease in the electrocatalytic activity of the nanoparticles. The selectivity towards alcohol oxidation in alkaline media was assessed by using the ethanol oxidation reaction as a model reaction. Operando electrochemistry/ATR-IR spectroscopy results reveal that acetate and acetaldehyde are the final products, depending on the catalyst composition as well as on the applied potential. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201900168
  • SpraySyn-A standardized burner configuration for nanoparticle synthesis in spray flames
    Schneider, F. and Suleiman, S. and Menser, J. and Borukhovich, E. and Wlokas, I. and Kempf, A. and Wiggers, H. and Schulz, C.
    Review of Scientific Instruments 90 (2019)
    In many scientific communities, the definition of standardized experiments has enabled major progress in process understanding. The investigation of the spray-flame synthesis of nanoparticles at a well-defined standard burner by experiment and simulation makes it possible to produce a comprehensive data set with various established and novel measuring methods. In this work, we introduce the design of the SpraySyn burner as a new standard for a free-jet type burner that offers well-defined and simulation-friendly boundary conditions and geometries as well as accessibility for optical diagnostics. A combustible precursor solution is fed through a centrally located capillary and aerosolized with an oxygen dispersion gas flow. The spray flame is stabilized by a premixed flat methane/oxygen pilot flame fed via a porous bronze matrix surrounded by a stabilizing nitrogen coflow emanating through the same porous matrix, providing easy-to-calculate boundary conditions for simulations. This burner design enables the use of a wide choice of solvents, precursors, and precursor combinations. Best-practice operating instructions and parameters are given, and large-eddy simulations are performed demonstrating the suitability of the SpraySyn burner for computational fluid dynamics simulations. For ensuring reproducible operation across labs, we define a consumer-camera-based flame characterization scheme for the quantitative assessment of the flame geometry such as flame length, diameter, tilt angle, and photometric distribution of visible chemiluminescence along the center axis. These parameters can be used for benchmarking the pilot and spray flame by each user of the SpraySyn burner with the reference flames. © 2019 Author(s).
    view abstract10.1063/1.5090232
  • Synthesis of silicon nanoparticles in a pilot-plant-scale microwave plasma reactor: Impact of flow rates and precursor concentration on the nanoparticle size and aggregation
    Kunze, F. and Kuns, S. and Spree, M. and Hülser, T. and Schulz, C. and Wiggers, H. and Schnurre, S.M.
    Powder Technology 342 (2019)
    This work is devoted to scale-up the microwave plasma synthesis of silicon nanoparticles from gaseous precursor monosilane (SiH4), previously investigated in lab-scale processes, to the pilot-plant-scale with production rates up to 200 g/h. The aim is to ensure reproducible, long-term operation of the reactor through gas-dynamic stabilization of the reacting flow and to control particle size and morphology via the gas flow velocity and the precursor concentration. Based on a newly designed nozzle, the lab-scale approach of stabilizing the plasma flow via a tangential sheath gas flow and an axial precursor gas flow was successfully transferred to the pilot-plant scale. At precursor concentrations up to 16 vol% of SiH4 diluted in argon and hydrogen, the as-synthesized particles have similar characteristics compared to those from lab-scale reactors. They are spherical, crystalline, mostly soft-agglomerated, and show a log-normal size distribution with a geometric standard deviation around 1.45 as expected for self-preserving aerosol size-distributions. In contrast to lab-scale experiments, an increase in SiH4 concentration up to 48 vol% does not lead to further growth of isolated primary particles but promotes aggregate formation from smaller primary particles. This is attributed to massive initial nucleation of very small particles due to strong supersaturation and their subsequent strong aggregation while suppressing complete coalescence due to the limited residence time at high temperature. © 2018 Elsevier B.V.
    view abstract10.1016/j.powtec.2018.10.042
  • Towards Mechanistic Understanding of Liquid-Phase Cinnamyl Alcohol Oxidation with tert-Butyl Hydroperoxide over Noble-Metal-Free LaCo1–xFexO3 Perovskites
    Waffel, D. and Alkan, B. and Fu, Q. and Chen, Y.-T. and Schmidt, S. and Schulz, C. and Wiggers, H. and Muhler, M. and Peng, B.
    ChemPlusChem 84 (2019)
    Noble-metal-free perovskite oxides are promising and well-known catalysts for high-temperature gas-phase oxidation reactions, but their application in selective oxidation reactions in the liquid phase has rarely been studied. We report the liquid-phase oxidation of cinnamyl alcohol over spray-flame synthesized LaCo1–xFexO3 perovskite nanoparticles with tert-butyl hydroperoxide (TBHP) as the oxidizing agent under mild reaction conditions. The catalysts were characterized by XRD, BET, EDS and elemental analysis. LaCo0.8Fe0.2O3 showed the best catalytic properties indicating a synergistic effect between cobalt and iron. The catalysts were found to be stable against metal leaching as proven by hot filtration, and the observed slight deactivation is presumably due to segregation as determined by EDS. Kinetic studies revealed an apparent activation energy of 63.6 kJ mol−1. Combining kinetic findings with TBHP decomposition as well as control experiments revealed a complex reaction network. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201900429
  • All gas-phase synthesis of graphene: Characterization and its utilization for silicon-based lithium-ion batteries
    Münzer, A. and Xiao, L. and Sehlleier, Y.H. and Schulz, C. and Wiggers, H.
    Electrochimica Acta 272 (2018)
    We report on a gas-phase synthesis method for the preparation of free-standing few-layer graphene in a microwave plasma reactor using pure ethanol as precursor. This scalable synthesis route produces gas-phase graphene (GPG) with lab-scale production rates up to a few hundred mg/h. The physico-chemical properties of the resulting GPG were characterized by XRD, FTIR-, and Raman spectroscopy, electrical conductivity measurements, XPS, and HRTEM in combination with EELS. The materials’ properties were compared with those of reduced graphene oxide (rGO) made by the established Hummers’ method. The results indicate that the gas-phase synthesis method provides highly-ordered few-layer graphene with extraordinary high purity, very low oxygen content of less than 1 at.%, and high specific conductivity. Both graphene materials were processed in combination with gas-phase synthesized silicon nanoparticles towards silicon-graphene nanocomposites for Li-ion battery anodes. Subsequent electrochemical testing revealed that the gas-phase graphene significantly enhances the long-term stability and Coulomb efficiency of the composite compared to pristine silicon and outperforms the composite fabricated from reduced graphene oxide. © 2018 Elsevier Ltd
    view abstract10.1016/j.electacta.2018.03.137
  • Efficient p-n junction-based thermoelectric generator that can operate at extreme temperature conditions
    Chavez, R. and Angst, S. and Hall, J. and Maculewicz, F. and Stoetzel, J. and Wiggers, H. and Thanh Hung, L. and Van Nong, N. and Pryds, N. and Span, G. and Wolf, D.E. and Schmechel, R. and Schierning, G.
    Journal of Physics D: Applied Physics 51 (2018)
    In many industrial processes, a large proportion of energy is lost in the form of heat. Thermoelectric generators can convert this waste heat into electricity by means of the Seebeck effect. However, the use of thermoelectric generators in practical applications on an industrial scale is limited in part because electrical, thermal, and mechanical bonding contacts between the semiconductor materials and the metal electrodes in current designs are not capable of withstanding thermal-mechanical stress and alloying of the metal-semiconductor interface when exposed to the high temperatures occurring in many real-world applications. Here we demonstrate a concept for thermoelectric generators that can address this issue by replacing the metallization and electrode bonding on the hot side of the device by a p-n junction between the two semiconductor materials, making the device robust against temperature induced failure. In our proof-of-principle demonstration, a p-n junction device made from nanocrystalline silicon is at least comparable in its efficiency and power output to conventional devices of the same material and fabrication process, but with the advantage of sustaining high hot side temperatures and oxidative atmosphere. © 2017 IOP Publishing Ltd.
    view abstract10.1088/1361-6463/aa9b6a
  • Ejector-based sampling from low-pressure aerosol reactors
    Rosenberger, T. and Münzer, A. and Kiesler, D. and Wiggers, H. and Kruis, F.E.
    Journal of Aerosol Science 123 (2018)
    Online measurements of nanoparticles are necessary when rapid information about the particle size and mass distribution is needed. Currently, the application of online measurement techniques with commonly used instruments such as SMPS, CPMA and ELPI+ is not possible at low-pressure conditions. In this work, a commercial vacuum ejector is used as a simple tool to transfer nanoparticles from a low-pressure region to atmospheric pressure. The vacuum ejector is investigated for different process pressures between 120 and 170 mbar to measure size-selected aerosols in the range from 10 to 100 nm. It was found that the sampling with the vacuum ejector does not change the particle size. The gas and particle dilution factors as well as the particle losses are determined, so that quantitative measurements of the aerosol size distribution can be obtained. Additionally, the applicability of the vacuum ejector is tested during particle synthesis in a low-pressure microwave plasma reactor with a combination of online instrumentation. The direct transfer of the aerosol to atmospheric pressure allows real-time measurements. The primary particle size, mass mobility exponent and effective density are calculated exemplary based on parallel online ELPI+, SMPS and CPMA measurements and are compared to offline TEM analysis. © 2018 Elsevier Ltd
    view abstract10.1016/j.jaerosci.2018.06.003
  • Electrostatic Self-Assembly Enabling Integrated Bulk and Interfacial Sodium Storage in 3D Titania-Graphene Hybrid
    Xu, G.-L. and Xiao, L. and Sheng, T. and Liu, J. and Hu, Y.-X. and Ma, T. and Amine, R. and Xie, Y. and Zhang, X. and Liu, Y. and Ren, Y. and Sun, C.-J. and Heald, S.M. and Kovacevic, J. and Sehlleier, Y.H. and Schulz, C. and Mattis, W.L. and Sun, S.-G. and Wiggers, H. and Chen, Z. and Amine, K.
    Nano Letters 18 (2018)
    Room-temperature sodium-ion batteries have attracted increased attention for energy storage due to the natural abundance of sodium. However, it remains a huge challenge to develop versatile electrode materials with favorable properties, which requires smart structure design and good mechanistic understanding. Herein, we reported a general and scalable approach to synthesize three-dimensional (3D) titania-graphene hybrid via electrostatic-interaction-induced self-assembly. Synchrotron X-ray probe, transmission electron microscopy, and computational modeling revealed that the strong interaction between titania and graphene through comparably strong van der Waals forces not only facilitates bulk Na+ intercalation but also enhances the interfacial sodium storage. As a result, the titania-graphene hybrid exhibits exceptional long-term cycle stability up to 5000 cycles, and ultrahigh rate capability up to 20 C for sodium storage. Furthermore, density function theory calculation indicated that the interfacial Li+, K+, Mg2+, and Al3+ storage can be enhanced as well. The proposed general strategy opens up new avenues to create versatile materials for advanced battery systems. © 2017 American Chemical Society.
    view abstract10.1021/acs.nanolett.7b04193
  • Gas-phase synthesis of functional nanomaterials: Challenges to kinetics, diagnostics, and process development
    Schulz, C. and Dreier, T. and Fikri, M. and Wiggers, H.
    Proceedings of the Combustion Institute (2018)
    Gas-phase synthesis of nanoparticles enables production of high-purity materials with well-controlled properties in continuous flow processes. It is an established technology for a couple of (mostly inorganic) commodities with more or less specific materials characteristics. However, increasing process understanding and control provides a chance for scale-up of highly specialized lab-scale technologies used for the manufacturing of unique materials to industrial scale. Nanoparticles with adjustable composition and size distributions are of interest for a wide variety of applications from coatings to electronics to functional materials, e.g., for energy conversion and storage. For the synthesis of materials with desired properties, the reaction conditions must be well controlled. Understanding the decomposition kinetics and mechanisms of vaporized precursor compounds, cluster formation, and the potential interaction with bath gases and flame chemistry is a prerequisite for a targeted synthesis of materials. The scientific challenges concerning the precursor chemistry and particle formation and the experimental and theoretical approaches to overcome them have a large overlap with those in combustion science. Kinetics experiments are carried out in shock-tube reactors with optical and mass spectrometric detection of intermediate and product species, and in flow reactors with laser-based detection of temperature and species concentration as well as molecular-beam sampling techniques. Reaction conditions such as temperature, intermediate species concentration and particle size must be determined . in situ in lab-scale nanoparticle reactors and the definition of standardized experiments that allow to build-up large data bases for model development is important. The scale-up to pilot-plant-scale based on experimentally validated simulations finally helps to prove the viability of new technologies and their application on mass markets such as materials for batteries or catalysis. © 2018.
    view abstract10.1016/j.proci.2018.06.231
  • Intra- and inter-nanocrystal charge transport in nanocrystal films
    Aigner, W. and Bienek, O. and Falcão, B.P. and Ahmed, S.U. and Wiggers, H. and Stutzmann, M. and Pereira, R.N.
    Nanoscale 10 (2018)
    The exploitation of semiconductor nanocrystal (NC) films in novel electronic and optoelectronic applications requires a better understanding of charge transport in these systems. Here, we develop a model of charge transport in NC films, based on a generalization of the concept of transport energy level ET to nanocrystal assemblies, which considers both intra- and inter-NC charge transfer processes. We conclude that the role played by each of these processes can be probed from temperature-dependent measurements of charge carrier density n and mobility μ in the same films. The model also enables the determination of the position of the Fermi energy level EF with respect to ET, an important parameter of charge transport in semiconductor materials, from the temperature dependence of n. Moreover, we provide support to an essentially temperature-independent intra-NC charge carrier mobility, considered in the transport level concept, and consequently the frequently observed temperature dependence of the overall mobility μ in NC films results from a temperature variation of the inter-NC charge transport processes. Importantly, we also conclude that the temperature dependence of conductivity in NC films should result in general from a combination of temperature variations of both n and μ. By applying the model to solution-processed Si NC films, we conclude that transport within each NC is similar to that in amorphous Si (a-Si), with charges hopping along band tail states located below the conduction band edge. For Si NCs, we obtain values of ET - EF of ∼0.25 eV. The overall mobility μ in Si NC films is significantly further reduced with respect to that typically found in a-Si due to the additional transport constraints imposed by inter-NC transfer processes inherent to a nanoparticulate film. Our model accounting for inter- and intra-NC charge transport processes provides a simple and more general description of charge transport that can be broadly applied to films of semiconductor NCs. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8nr00250a
  • Experimental and numerical study of a HMDSO-seeded premixed laminar low-pressure flame for SiO2 nanoparticle synthesis
    Feroughi, O.M. and Deng, L. and Kluge, S. and Dreier, T. and Wiggers, H. and Wlokas, I. and Schulz, C.
    Proceedings of the Combustion Institute 36 (2017)
    Silicon dioxide nanoparticles are generated in a lean hydrogen/oxygen flat flame doped with small amounts of hexamethyldisiloxane (HMDSO) stabilized by a water-cooled sintered bronze matrix. The burner is housed in an optically-accessible low-pressure (3kPa) chamber. Temperature fields were determined via multi-line laser-induced fluorescence (LIF) using added NO as target species. Gas-phase silicon oxide (SiO) was detected via laser-induced fluorescence (LIF) by exciting the weakly temperature-dependent rovibrational Q11(32) transition in the A-X (0,0) vibronic band system at 235.087nm. Semi-quantitative concentration profiles as a function of height-above-burner (HAB) were obtained after exploiting the measured temperature fields and correcting measured LIF intensities for the temperature-dependence of the ground-state population and collisional quenching using measured effective fluorescence lifetimes. Particle sizes were determined as a function of HAB via molecular-beam sampling with subsequent particle mass spectrometry (PMS). The experimental data were used to develop a simple kinetics model of HMDSO combustion and SiO2 particle precursor formation with subsequent nucleation and particle growth in the H2/O2 flame. The model was incorporated in a CFD simulation to account for facility effects that arise from modified flow fields and heat transfer between the flame and the reactor chamber. © 2016 Elsevier Ltd.
    view abstract10.1016/j.proci.2016.07.131
  • Inline coating of silicon nanoparticles in a plasma reactor: Reactor design, simulation and experiment
    Münzer, A. and Sellmann, J. and Fortugno, P. and Kempf, A. and Schulz, C. and Wiggers, H.
    Materials Today: Proceedings 4 (2017)
    Gas-phase synthesis of nanoparticles (NPs) in hot plasmas is a promising approach to produce pure, highly specific, and complex nanomaterials at large production rates. Post-processing of the material by particle coating, embedding, or surface functionalization is often required to adjust the materials' properties with respect to their utilization in functional structures. Due to the high surface-to-volume ratio, the nanoparticles' surface properties strongly influence the processing and thus their applicability. We report on a scalable and continuous gas-phase synthesis process of silicon nanoparticles by a high-temperature single-step plasma process with subsequent inline coating. Our process requires a two-stage supply of process gases: First, silicon nanoparticles (Si-NPs) are formed from the gaseous precursor monosilane (SiH4) after its decomposition in the plasma zone. Secondly, the coating agent ethylene (C2H4) is mixed with the hot, particle-laden gas flow downstream of the plasma zone via a specifically-designed coating nozzle. To facilitate a homogeneous intermixing of C2H4 and the nanoparticle-laden gas stream, fluid dynamics simulations were performed to design and optimize the geometry of the coating nozzle. The process conditions can be varied to tune the decomposition process of gaseous C2H4 in respect to coating the Si-NP surface. As a result, we are able to tune the composition of the nanoparticles. Product characterization by X-ray diffraction, Raman, FTIR and X-ray photoelectron spectroscopy revealed that either SiC, or silicon with a carbon-like or a polyethylene-like shell is produced respectively, with increasing distance of the coating nozzle from the plasma. For all process conditions, spherical, coated particles with a highly-crystalline silicon core were observed as indicated by TEM measurements.
    view abstract10.1016/j.matpr.2017.09.176
  • Light-induced nonthermal population of optical phonons in nanocrystals
    Falcão, B.P. and Leitão, J.P. and Correia, M.R. and Soares, M.R. and Wiggers, H. and Cantarero, A. and Pereira, R.N.
    Physical Review B - Condensed Matter and Materials Physics 95 (2017)
    Raman spectroscopy is widely used to study bulk and nanomaterials, where information is frequently obtained from spectral line positions and intensities. In this study, we monitored the Raman spectrum of ensembles of semiconductor nanocrystals (NCs) as a function of optical excitation intensity (optical excitation experiments). We observe that in NCs the red-shift of the Raman peak position with increasing light power density is much steeper than that recorded for the corresponding bulk material. The increase in optical excitation intensity results also in an increasingly higher temperature of the NCs as obtained with Raman thermometry through the commonly used Stokes/anti-Stokes intensity ratio. More significantly, the obtained dependence of the Raman peak position on temperature in optical excitation experiments is markedly different from that observed when the same NCs are excited only thermally (thermal excitation experiments). This difference is not observed for the control bulk material. The inefficient diffusion of photogenerated charges in nanoparticulate systems, due to their inherently low electrical conductivity, results in a higher steady-state density of photoexcited charges and, consequently, also in a stronger excitation of optical phonons that cannot decay quickly enough into acoustic phonons. This results in a nonthermal population of optical phonons and thus the Raman spectrum deviates from that expected for the temperature of the system. Our study has major consequences to the general application of Raman spectroscopy to nanomaterials. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.95.115439
  • Mass spectrometric analysis of clusters and nanoparticles during the gas-phase synthesis of tungsten oxide
    Kluge, S. and Wiggers, H. and Schulz, C.
    Proceedings of the Combustion Institute 36 (2017)
    The combustion synthesis of nanoscale tungsten-oxide particles from tungsten hexafluoride is investigated in a low-pressure hydrogen/oxygen flat flame. The reactor is equipped with molecular-beam sampling of post-flame gases at variable height above burner (HAB). Main species of the flame, intermediate tungsten species, and tungsten-oxide clusters are studied with time-of-flight mass spectrometry (TOF-MS) as a function of HAB. Various WO x (OH) y are identified within the flame front. With increasing HAB, (WO3) n clusters with increasing cluster size appear in the burnt gases at the expense of the concentration of W1 species. Clusters with n =3-7 arise at 70mm HAB, followed by larger clusters at even larger heights. Clusters up to (WO3)38 were identified. The subsequent formation of nanoparticles is detected with particle mass spectrometry (PMS) and a quartz crystal microbalance (QCM) from 120mm HAB and the increasing particle size and mass flux have been determined. © 2016.
    view abstract10.1016/j.proci.2016.06.165
  • Micrometer-sized nano-structured silicon/carbon composites for lithium-ion battery anodes synthesized based on a three-step Hansen solubility parameter (HSP) concept
    Sehlleier, Y.H. and Dobrowolny, S. and Xiao, L. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Industrial and Engineering Chemistry 52 (2017)
    The processing towards Si/C composites, components and synthesis parameters were selected based on the concept of Hansen solubility parameters (HSP). Si/polymer composites were generated through modified bulk polymerization and subsequent pyrolysis transformed the polymer into the desired porous carbon matrix. Coulombic efficiencies (CE) in excess of 76% after the first cycle and 99.95% after solid electrolyte interphase (SEI) formation have been achieved. A notably high specific delithiation capacity of around 1600 mAh/g with an extremely stable cycling performance even after 400 cycles is obtained. This scalable and economical synthesis approach is readily applicable to the commercial production of anode materials. © 2017 The Korean Society of Industrial and Engineering Chemistry
    view abstract10.1016/j.jiec.2017.04.001
  • Microstructure and thermoelectric properties of Si-WSi2 nanocomposites
    Stoetzel, J. and Schneider, T. and Mueller, M.M. and Kleebe, H.-J. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Acta Materialia 125 (2017)
    Nanocomposites of n-doped Si/WSi2 were prepared and morphologically and thermoelectrically investigated. The composites were densified by spark-plasma-sintering of doped Si nanoparticles with WSi2 nanoinclusions. The nanoparticles were synthesized in a gas-phase process. The microstructure of the bulk nanocomposite shows an inhomogeneous distribution of the WSi2 nanoinclusions in form of WSi2-rich and -depleted regions. This inhomogeneity is not present in the starting material and is assigned to a self-organizing process during sintering. The inhomogeneities are in the micrometer range and may act as scattering centers for long-wavelength phonons. The WSi2 nanoinclusions grow during sintering from originally 3–7 nm up to 30–143 nm depending on the total W content and might act as scattering centers for the medium wavelength range of phonons. Further, the growth of Si grains is suppressed by the WSi2 inclusions, which leads to an enhanced grain boundary density. Adding 1 at% W reduces lattice thermal conductivity by almost 35% within the temperature range from 300 K to 1250 K compared to pure, nanocrystalline silicon (doped). By addition of 6 at% W a reduction of 54% in lattice thermal conductivity is achieved. Although little amounts of W slightly reduce the power factor an enhancement of the thermoelectric figure of merit of 50% at 1250 K compared to a tungsten-free reference was realized. © 2016
    view abstract10.1016/j.actamat.2016.11.069
  • Novel Si-CNT/polyaniline nanocomposites as Lithium-ion battery anodes for improved cycling performance
    Xiao, L. and Sehlleier, Y.H. and Dobrowolny, S. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Materials Today: Proceedings 4 (2017)
    A novel nanocomposite consisting of gas-phased produced Si nanoparticles, carbon nanotubes (CNTs), and polyaniline (PANi) is developed as an anode material (Si-CNT/PANi) for lithium-ion batteries. This nanocomposite integrates the merits from its three components, where Si nanoparticles provide high capacity, CNTs act as an electrically conductive and mechanically flexible network, and PANi coating further enhances the electrical conductivity and protects the silicon structure. An anode made of this nanocomposite shows a high reversible capacity of 2430 mAh/g with good capacity retention over 500 cycles compared to pristine Si. The Si-CNT/PANi nanocomposite also demonstrated a high Coulombic efficiency and improved rate-capabilities. © 2017 Elsevier Ltd.
    view abstract10.1016/j.matpr.2017.09.197
  • Optoelectronic properties and depth profile of charge transport in nanocrystal films
    Aigner, W. and Bienek, O. and Desta, D. and Wiggers, H. and Stutzmann, M. and Pereira, R.N.
    Physical Review B 96 (2017)
    We investigate the charge transport in nanocrystal (NC) films using field effect transistors (FETs) of silicon NCs. By studying films with various thicknesses in the dark and under illumination with photons with different penetration depths (UV and red light), we are able to predictably change the spatial distribution of charge carriers across the films' profile. The experimental data are compared with photoinduced charge carrier generation rates computed using finite-difference time-domain (FDTD) simulations complemented with optical measurements. This enables us to understand the optoelectronic properties of NC films and the depth profile dependence of the charge transport properties. From electrical measurements, we extract the total (bulk) photoinduced charge carrier densities (nphoto) and the photoinduced charge carrier densities in the FETs channel (nphoto∗). We observe that the values of nphoto and their dependence on film thickness are similar for UV and red light illumination, whereas a significant difference is observed for the values of nphoto∗. The dependencies of nphoto and nphoto∗ on film thickness and illumination wavelength are compared with data from FDTD simulations. Combining experimental data and simulation results, we find that charge carriers in the top rough surface of the films cannot contribute to the macroscopic charge transport. Moreover, we conclude that below the top rough surface of NC films, the efficiency of charge transport, including the charge carrier mobility, is homogeneous across the film thickness. Our work shows that the use of NC films as photoactive layers in applications requiring harvesting of strongly absorbed photons such as photodetectors and photovoltaics demands a very rigorous control over the films' roughness. © 2017 American Physical Society.
    view abstract10.1103/PhysRevB.96.035404
  • Parasitic Reactions in Nanosized Silicon Anodes for Lithium-Ion Batteries
    Gao, H. and Xiao, L. and Plümel, I. and Xu, G.-L. and Ren, Y. and Zuo, X. and Liu, Y. and Schulz, C. and Wiggers, H. and Amine, K. and Chen, Z.
    Nano Letters 17 (2017)
    When designing nano-Si electrodes for lithium-ion batteries, the detrimental effect of the c-Li15Si4 phase formed upon full lithiation is often a concern. In this study, Si nanoparticles with controlled particle sizes and morphology were synthesized, and parasitic reactions of the metastable c-Li15Si4 phase with the nonaqueous electrolyte was investigated. The use of smaller Si nanoparticles (∼60 nm) and the addition of fluoroethylene carbonate additive played decisive roles in the parasitic reactions such that the c-Li15Si4 phase could disappear at the end of lithiation. This suppression of c-Li15Si4 improved the cycle life of the nano-Si electrodes but with a little loss of specific capacity. In addition, the characteristic c-Li15Si4 peak in the differential capacity (dQ/dV) plots can be used as an early-stage indicator of cell capacity fade during cycling. Our findings can contribute to the design guidelines of Si electrodes and allow us to quantify another factor to the performance of the Si electrodes. © 2017 American Chemical Society.
    view abstract10.1021/acs.nanolett.6b04551
  • A novel magnetically-separable porous iron-oxide nanocomposite as an adsorbent for methylene blue (MB) dye
    Sehlleier, Y.H. and Hardt, S. and Schulz, C. and Wiggers, H.
    Journal of Environmental Chemical Engineering 4 (2016)
    In this study, efficient and magnetically separable adsorbents for the removal of organic pollutants from water are developed, which are both, environmental friendly and cheap to produce. A new type of porous iron-oxide/polymer nanocomposite was synthesized by a two-step process utilizing surface modification of gas-phase synthesized iron-oxide nanoparticles and a subsequent polymerization process. The potential of iron-oxide/polymer composite adsorbents with a large surface area for the removal of organic components was studied using methylene blue (MB) as a test substance. Adsorption isotherms fitted well with the Langmuir isotherm model and the adsorption capacity of MB on this adsorbent was found to be as high as 298 mg/g which is several times higher than the adsorption capacity of a number of recently reported potential adsorbents. Owing to its magnetic properties, the polluted adsorbent can be easily separated from aqueous solutions. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jece.2016.08.018
  • High-yield and scalable synthesis of a Silicon/Aminosilane-functionalized Carbon NanoTubes/Carbon (Si/A-CNT/C) composite as a high-capacity anode for lithium-ion batteries
    Sehlleier, Y.H. and Dobrowolny, S. and Plümel, I. and Xiao, L. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Applied Electrochemistry 46 (2016)
    In this study, we present a novel anode architecture for high-performance lithium-ion batteries based on a Silicon/3-aminosilane-functionalized CNT/Carbon (Si/A-CNT/C) composite. A high-yield, low-cost approach has been developed to stabilize and support silicon as an active anode material. Silicon (Si) nanoparticles synthesized in a hot-wall reactor and aminosilane-functionalized carbon nanotubes (A-CNT) were dispersed in styrene and divinylbenzene (DVB) and subsequently polymerized forming a porous Si/A-CNT/C composite. Transmission electron microscopy showed that this method enables the interconnection and a uniform encapsulation of Si nanoparticles within a porous carbon matrix especially using aminosilane-functionalized CNT (A-CNT). Electrochemical characterization shows that this material can deliver a delithiation capacity of 2293 mAh g−1 with a capacity retention of more than 90 % after 200 cycles at lithiation and delithiation rate of 0.5 C. We conclude that the porous Si/A-CNT/C composite material can accommodate sufficient space for Si volume expansion and extraction and improve the electronic and ionic conduction. Excellent electrochemical performance during repeated cycling can thus be achieved. © 2015, Springer Science+Business Media Dordrecht.
    view abstract10.1007/s10800-015-0897-x
  • Lattice dynamics and thermoelectric properties of nanocrystalline silicon-germanium alloys
    Claudio, T. and Stein, N. and Petermann, N. and Stroppa, D.G. and Koza, M.M. and Wiggers, H. and Klobes, B. and Schierning, G. and Hermann, R.P.
    Physica Status Solidi (A) Applications and Materials Science 213 (2016)
    The lattice dynamics and thermoelectric properties of sintered phosphorus-doped nanostructured silicon-germanium alloys obtained by gas-phase synthesis were studied. Measurements of the density of phonon states by inelastic neutron scattering were combined with measurements of the elastic constants and the low-temperature heat capacity. A strong influence of nanostructuring and alloying on the lattice dynamics was observed. The thermoelectric transport properties of samples with different doping as well as samples sintered at different temperature were characterized between room temperature and 1000°C. A peak figure of merit zT=0.88 at 900°C is observed and is comparatively insensitive to the aforementioned parameter variations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201532500
  • Novel back-reflector architecture with nanoparticle based buried light-scattering microstructures for improved solar cell performance
    Desta, D. and Ram, S.K. and Rizzoli, R. and Bellettato, M. and Summonte, C. and Jeppesen, B.R. and Jensen, P.B. and Tsao, Y.-C. and Wiggers, H. and Pereira, R.N. and Balling, P. and Larsen, A.N.
    Nanoscale 8 (2016)
    A new back-reflector architecture for light-management in thin-film solar cells is proposed that includes a morphologically smooth top surface with light-scattering microstructures buried within. The microstructures are pyramid shaped, fabricated on a planar reflector using TiO2 nanoparticles and subsequently covered with a layer of Si nanoparticles to obtain a flattened top surface, thus enabling growth of good quality thin-film solar cells. The optical properties of this back-reflector show high broadband haze parameter and wide angular distribution of diffuse light-scattering. The n-i-p amorphous silicon thin-film solar cells grown on such a back-reflector show enhanced light absorption resulting in improved external quantum efficiency. The benefit of the light trapping in those solar cells is evidenced by the gains in short-circuit current density and efficiency up to 15.6% and 19.3% respectively, compared to the reference flat solar cells. This improvement in the current generation in the solar cells grown on the flat-topped (buried pyramid) back-reflector is observed even when the irradiation takes place at large oblique angles of incidence. Finite-difference-time-domain simulation results of optical absorption and ideal short-circuit current density values agree well with the experimental findings. The proposed approach uses a low cost and simple fabrication technique and allows effective light manipulation by utilizing the optical properties of micro-scale structures and nanoscale constituent particles. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c6nr00259e
  • Silicon-based nanocomposites for thermoelectric application
    Schierning, G. and Stoetzel, J. and Chavez, R. and Kessler, V. and Hall, J. and Schmechel, R. and Schneider, T. and Petermann, N. and Wiggers, H. and Angst, S. and Wolf, D.E. and Stoib, B. and Greppmair, A. and Stutzmann, M. and Brandt, M.S.
    Physica Status Solidi (A) Applications and Materials Science 213 (2016)
    Here we present the realization of efficient and sustainable silicon-based thermoelectric materials from nanoparticles. We employ a gas phase synthesis for the nanoparticles which is capable of producing doped silicon (Si) nanoparticles, doped alloy nanoparticles of silicon and germanium (Ge), SixGe1-x, and doped composites of Si nanoparticles with embedded metal silicide precipitation phases. Hence, the so-called "nanoparticle in alloy" approach, theoretically proposed in the literature, forms a guideline for the material development. For bulk samples, a current-activated pressure-assisted densification process of the nanoparticles was optimized in order to obtain the desired microstructure. For thin films, a laser annealing process was developed. Thermoelectric transport properties were characterized on nanocrystalline bulk samples and laser-sintered-thin films. Devices were produced from nanocrystalline bulk silicon in the form of p-n junction thermoelectric generators, and their electrical output data were measured up to hot side temperatures of 750°C. In order to get a deeper insight into thermoelectric properties and structure forming processes, a 3D-Onsager network model was developed. This model was extended further to study the p-n junction thermoelectric generator and understand the fundamental working principle of this novel device architecture. Gas phase synthesis of composite nanoparticles; nanocrystalline bulk with optimized composite microstructure; laser-annealed thin film. The authors fabricated thermoelectric nanomaterials from doped silicon and silicon and germanium alloy nanoparticles, as well as composites of Si nanoparticles with embedded metal silicide nanoparticles. Processing was performed applying a current-activated pressure-assisted densification process for bulk samples and a laser annealing process for thin film samples. Devices were produced in the form of pn junction thermoelectric generators. A 3D-Onsager network model was used to understand the fundamental working principle of this novel device architecture. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201532602
  • Three-Dimensional Percolation and Performance of Nanocrystal Field-Effect Transistors
    Aigner, W. and Wiesinger, M. and Wiggers, H. and Stutzmann, M. and Pereira, R.N.
    Physical Review Applied 5 (2016)
    The understanding of charge transport through films of semiconductor nanocrystals (NCs) is fundamental for most applications envisaged for these materials, e.g., light-emitting diodes, solar cells, and thin-film field-effect transistors (FETs). In this work, we show that three-dimensional film-thickness-dependent percolation effects taking place above the percolation threshold strongly affect the charge transport in NC films and greatly determine the performance of NC devices such as NC FETs. We use thin films of Si NCs with a wide range of thicknesses controllable by spray coating of NC inks to thoroughly investigate the electronic properties and charge transport in thin NC films. We find a steep (superlinear) increase of the electrical conductivity with increasing film thickness, which is not observed in bulk semiconductor thin films with bandlike charge transport. We explain this increase by an exponentially increasing number of charge percolation paths in a system dominated by hopping charge transport. Thin-film NC FETs reveal thickness-independent field-effect mobilities and threshold voltages, whereas on:off current ratios decrease quickly with increasing film thickness. We show that the steep enhancement of electrical conductivity with increasing film thickness provided by three-dimensional percolation effects is, in fact, responsible for the dramatic degradation of NC FET performance observed with increasing film thickness. Our work demonstrates that the performance of NC FETs is much more critically sensitive to film thickness than in conventional FET-based bulk semiconductor materials. © 2016 American Physical Society.
    view abstract10.1103/PhysRevApplied.5.054017
  • Charge storage in β -FeSi2 nanoparticles
    Theis, J. and Bywalez, R. and Küpper, S. and Lorke, A. and Wiggers, H.
    Journal of Applied Physics 117 (2015)
    We report on the observation of a surprisingly high specific capacitance of β-FeSi2 nanoparticle layers. Lateral, interdigitated capacitor structures were fabricated on thermally grown silicon dioxide and covered with β-FeSi2 particles by drop or spin casting. The β-FeSi2-nanoparticles, with crystallite sizes in the range of 10-30nm, were fabricated by gas phase synthesis in a hot wall reactor. Compared to the bare electrodes, the nanoparticle-coated samples exhibit a 3-4 orders of magnitude increased capacitance. Time-resolved current voltage measurements show that for short times (seconds to minutes), the material is capable of storing up to 1 As/g at voltages of around 1V. The devices are robust and exhibit long-term stability under ambient conditions. The specific capacitance is highest for a saturated relative humidity, while for a relative humidity below 40% the capacitance is almost indistinguishable from a nanoparticle-free reference sample. The devices work without the need of a fluid phase, the charge storing material is abundant and cost effective, and the sample design is easy to fabricate. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4906500
  • Direct self-assembly of Fe2O3/reduced graphene oxide nanocomposite for high-performance lithium-ion batteries
    Xiao, L. and Schroeder, M. and Kluge, S. and Balducci, A. and Hagemann, U. and Schulz, C. and Wiggers, H.
    Journal of Materials Chemistry A 3 (2015)
    In this study, Fe2O3/reduced graphene oxide (rGO) nanocomposites were prepared using a direct self-assembly of oppositely charged Fe2O3 nanoparticles (NPs) and graphene oxide (GO) sheets, followed with a lowerature hydrothermal reduction process. The characterization of the nanocomposite shows that Fe2O3 NPs with an average diameter of about 9 nm are uniformly distributed on well-exfoliated rGO layers. The nanocomposites show a high iron oxide mass loading of 63%. The electrical conductivity of the composite was significantly enhanced by about 6 orders of magnitude in comparison to pure Fe2O3 NPs. The characterization of the composite as an anode material for lithium-ion batteries (LIBs) demonstrated a strong positive synergistic effect with respect to its electrochemical performance. Fe2O3/rGO exhibited a capacity of 600 mA h g-1 at a current density of 0.1 A g-1, and even more than 180 mA h g-1 at 10 A g-1 (approx. 17 C), indicating its superior high-rate performance. In addition, it features high efficiency at high rates and very good cyclic stability over a long cycle life of more than 550 cycles. This journal is © The Royal Society of Chemistry.
    view abstract10.1039/c5ta02549d
  • Impact of ambient pressure on titania nanoparticle formation during spray-flame synthesis
    Hardt, S. and Wlokas, I. and Schulz, C. and Wiggers, H.
    Journal of Nanoscience and Nanotechnology 15 (2015)
    Nanocrystalline titania was synthesized via liquid-fed spray-flame synthesis in a hermetically closed system at various pressures. Titanium tetraisopropoxide dissolved in isopropanol was used as precursor. The size, crystal structure, degree of agglomeration, morphology and the band gap of the as-prepared particles were investigated ex situ by nitrogen adsorption, transmission electron microscopy, X-ray diffraction, and UV-VIS absorption spectroscopy. In comparison to synthesis at atmospheric pressure it was found that decreasing pressure has a significant influence on the particle size distribution leading to smaller particles with reduced geometric standard deviation while particle morphology and crystal structure are not affected. Computational fluid dynamics simulations support the experimental findings also indicating a significant decrease in particle size at reduced pressure. Although it is well known that decreasing pressure leads to smaller particle sizes, it is (to our knowledge) the first time that this relation was investigated for spray-flame synthesis. Copyright © 2015 American Scientific Publishers All rights reserved.
    view abstract10.1166/jnn.2015.10607
  • Influence of carbon content, particle size, and partial manganese substitution on the electrochemical performance of LiFexMn1-xPO4/carbon composites
    Hamid, N.A. and Wennig, S. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Ionics 21 (2015)
    LiFePO<inf>4</inf>/C and LiFe<inf>x</inf>Mn<inf>1-x</inf>PO<inf>4</inf>/C (x = 0.7) nanocomposites were successfully synthesized via scalable spray-flame synthesis followed by solid-state reaction. A solution of iron (III) acetylacetonate and tributyl phosphate in toluene was used to produce amorphous, nanosized FePO<inf>4</inf>⋅H<inf>2</inf>O in a spray-flame reactor which was then milled with Li<inf>2</inf>CO<inf>3</inf> and glucose to produce a LiFePO<inf>4</inf>/C composite material in a solid-state reaction. The influence of calcination temperature and carbon content on the properties of the resulting material was investigated using specific surface area measurements (BET), X-ray diffraction (XRD), electron microscopy, and electrochemical characterization. The impact of manganese addition on the electrochemical behavior was analyzed using cyclic voltammetry (CV) and constant-current (CC) measurements. XRD shows that the combination of gas-phase synthesis and subsequent solid-state reaction yields highly pure LiFePO<inf>4</inf>/C. BET measurement revealed that the particle size of LiFePO<inf>4</inf> in the composite depends on the amount of glucose. A discharge capacity of more than 140 mAh/g at C/20 is achieved for LiFePO<inf>4</inf>/C with a carbon content of 6 wt%. This material supports high charge as well as discharge rates delivering more than 60 mAh/g at 16 C and sustains good cycle stability providing 115 mAh/g at 1 C. The energy density of the olivine increases about 10 % by substituting 30 mol% of iron by manganese while preserving the electrochemical performance of pure LiFePO<inf>4</inf>/C. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s11581-015-1366-6
  • Initial reaction steps during flame synthesis of iron-oxide nanoparticles
    Kluge, S. and Deng, L. and Feroughi, O. and Schneider, F. and Poliak, M. and Fomin, A. and Tsionsky, V. and Cheskis, S. and Wlokas, I. and Rahinov, I. and Dreier, T. and Kempf, A. and Wiggers, H. and Schulz, C.
    CrystEngComm 17 (2015)
    Premixed, laminar H2/O2/Ar and CH4/O2/N2 low-pressure flat flames doped with iron pentacarbonyl (Fe(CO)5) were used to investigate the initial steps towards the formation of iron-oxide nanoparticles. The particles were extracted from the flame using a molecular beam sampling probe and the mass flow rate of condensed material was measured by a quartz crystal microbalance (QCM). It was observed that particles are already formed on the cold side of the flame, and vanish quickly once they pass through the flame front. To understand the process and assess the perturbations caused by the sampling probe, spatially resolved laser-based measurements of temperature, Fe and FeO concentration as well as molecular-beam sampling with particle mass spectrometry (PMS) were carried out. Numerical flow simulations of the synthesis flames, the reactor, and the sampling were performed and the simulations confirmed the experimental findings of very early particle formation. The detailed knowledge of the perturbation caused by invasive probing enabled further insight into the iron-oxide nanoparticle formation mechanism. From the results it is concluded that neither Fe atoms nor FeO molecules belong to the growth species of iron-oxide nanoparticles from flame synthesis. © The Royal Society of Chemistry.
    view abstract10.1039/c5ce00456j
  • Laser-Assisted Wet-Chemical Doping of Sintered Si and Ge Nanoparticle Films
    Stoib, B. and Greppmair, A. and Petermann, N. and Wiggers, H. and Stutzmann, M. and Brandt, M.S.
    Advanced Electronic Materials 1 (2015)
    Doped thin films of group-IV semiconductors can be fabricated using the adsorption of dopant species from a liquid source to a precursor nanoparticle film, followed by laser-sintering to incorporate and activate the dopants in the sintered thin film. A detailed study of the doping of germanium films with arsenic reveals diffusion of dopants into the film and their adsorption to the nanoparticle surface as kinetically governing steps, benefiting from the large internal surface area of the nanoparticle film. The resulting charge carrier concentration can be adjusted by the internal surface area via the nano­particle diameter, by controlling the dopant concentration in the liquid, and by the immersion time and temperature. It is shown that the method can be successfully transferred to silicon and silicon–germanium alloy films using group-III and -V elements, which lead to p- and n-type conductivity, respectively. Atomic dopant concentrations above 1020 cm−3 can be realized by laser-sintering, which are electrically active to a high extent and lead to effective conductivities well above 10 S cm–1 in the mesoporous films is investigated here. The method allows flexible printing of devices using inks for the nanoparticles and the dopant and avoids toxic substances for the doping of nanoparticles in the gas phase. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/aelm.201400029
  • Laser-based in situ measurement and simulation of gas-phase temperature and iron atom concentration in a pilot-plant nanoparticle synthesis reactor
    Feroughi, O.M. and Hardt, S. and Wlokas, I. and Hülser, T. and Wiggers, H. and Dreier, T. and Schulz, C.
    Proceedings of the Combustion Institute 35 (2015)
    A scaled-up flame reactor for nanoparticle synthesis was investigated through a combination of in-situ laser-induced fluorescence (LIF) measurements and computational fluid dynamics (CFD) simulations with detailed chemistry. Multi-line NO-LIF was used for imaging gas-temperature and Fe-LIF for measurement of iron atom concentration. Despite the challenging environment of production reactors in an industrial environment, various conditions for stable flames with different gas flows with and without adding Fe(CO)5 as precursor for the synthesis of iron-oxide nanoparticles were investigated. In contrast to previous measurements in laminar lab-scale flames, a second mechanism for forming iron oxide nanoparticles was found via intermediate formation of iron clusters and elemental iron particles in hot, oxygen-free gas streams followed by subsequent oxidation. © 2014 Published by Elsevier Inc. on behalf of The Combustion Institute.
    view abstract10.1016/j.proci.2014.05.039
  • Mesoporous Sulfonated Carbon Materials Prepared by Spray Pyrolysis
    Duyckaerts, N. and Trotuş, I.-T. and Nese, V. and Swertz, A.-C. and Auris, S. and Wiggers, H. and Schüth, F.
    ChemCatChem 7 (2015)
    A one-step approach was developed for the production of mesoporous sulfonated carbon materials by means of an aerosol synthesis. Nebulizing a clear aqueous solution of sucrose and sulfuric acid through a heated oven leads to subsequent dehydration, carbonization and sulfonation of the carbohydrate structure, in less than two seconds residence time. Acid site concentrations ranging from 0.1 to 0.6 mmol g-1 can be obtained. Porosity can easily be introduced via salt templating, and can be adjusted by varying the loading and type of salt used. The highest surface area was obtained with Li<inf>2</inf>SO<inf>4</inf>, giving a BET surface area of 506 m2 g-1 and a mesopore size distribution between 2 and 8 nm. Fructose dehydration and inulin hydrolysis showed that the porous materials synthesized by salt templating are more active than the bulk ones, especially for inulin hydrolysis, for which the initial activity is enhanced by a factor of seven, making these materials competitive with the most active commercial resins. A one-step synthesis for the production of mesoporous sulfonated carbon materials is presented, by which sulfuric acid and an organic precursor are converted, in one step, to an active sulfonated carbon material containing high surface area. Fructose dehydration and inulin hydrolysis reveal the competitiveness with commercial acidic resins. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201500483
  • Microwave plasma synthesis of Si/Ge and Si/WSi2 nanoparticles for thermoelectric applications
    Petermann, N. and Schneider, T. and Stötzel, J. and Stein, N. and Weise, C. and Wlokas, I. and Schierning, G. and Wiggers, H.
    Journal of Physics D: Applied Physics 48 (2015)
    The utilization of microwave-based plasma systems enables a contamination-free synthesis of highly specific nanoparticles in the gas phase. A reactor setup allowing stable, long-term operation was developed with the support of computational fluid dynamics. This paper highlights the prospects of gas-phase plasma synthesis to produce specific materials for bulk thermoelectrics. Taking advantage of specific plasma reactor properties such as Coulomb repulsion in combination with gas temperatures considerably higher than 1000 K, spherical and non-aggregated nanoparticles of multiple compositions are accessible. Different strategies towards various nanostructured composites and alloys are discussed. It is shown that, based on doped silicon/germanium alloys and composites, thermoelectric materials with zT values up to almost unity can be synthesized in one step. First experimental results concerning silicon/tungsten silicide thermoelectrics applying the nanoparticle-in-alloy idea are presented indicating that this concept might work. However, it is found that tungsten silicides show a surprising sinter activity more than 1000 K below their melting temperature. © 2015 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/48/31/314010
  • Resonant photothermal laser processing of hybrid gold/titania nanoparticle films
    Schade, L. and Franzka, S. and Dzialkowski, K. and Hardt, S. and Wiggers, H. and Reichenberger, S. and Wagener, P. and Hartmann, N.
    Applied Surface Science 336 (2015)
    Photothermal processing of thin anatase TiO2 and hybrid Au/anatase TiO2 nanoparticle films on glass supports is investigated using continuous-wave microfocused lasers at λ = 355 nm and λ = 532 nm. UV/Vis spectroscopy, Raman spectroscopy, optical microscopy, atomic force microscopy and scanning electron microscopy are used for characterization. Processing of TiO2 nanoparticle films is feasible at λ = 355 nm only. In contrast, the addition of Au nanoparticles enhances the overall absorbance of the material in the visible range and enables processing at both wavelengths, i.e. at λ = 355 nm and λ = 532 nm. Generally, laser heating induces a transition from anatase to rutile. The modification degree increases with increasing laser power and laser irradiation time. Resonant laser processing of hybrid Au/TiO2-mesoporous films provide promising perspectives in various applications, e.g. in photovoltaics, where embedded nanoparticulate Au could be exploited to enhance light trapping. © 2014 Published by Elsevier B.V.
    view abstract10.1016/j.apsusc.2014.09.118
  • Si-CNT/rGO Nanoheterostructures as High-Performance Lithium-Ion-Battery Anodes
    Xiao, L. and Sehlleier, Y.H. and Dobrowolny, S. and Orthner, H. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    ChemElectroChem 2 (2015)
    A robust and electrochemically stable 3D nanoheterostructure consisting of Si nanoparticles (NPs), carbon nanotubes (CNTs) and reduced graphene oxide (rGO) is developed as an anode material (Si-CNT/rGO) for lithium-ion batteries (LIBs). It integrates the benefits from its three building blocks of Si NPs, CNTs, and rGO; Si NPs offer high capacity, CNTs act as a mechanical, electrically conductive support to connect Si NPs, and highly electrically conductive and flexible rGO provides a robust matrix with enough void space to accommodate the volume changes of Si NPs upon lithiation/delithiation and to simultaneously assure good electric contact. The composite material shows a high reversible capacity of 1665mAhg-1 with good capacity retention of 88.6% over 500 cycles when cycled at 0.5C, that is, a 0.02% capacity decay per cycle. The high-power capability is demonstrated at 10C (16.2Ag-1) where 755mAhg-1 are delivered, thus indicating promising characteristics of this material for high-performance LIBs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201500323
  • Silicon-based nanocomposites for thermoelectric high temperature waste heat recovery
    Schierning, G. and Wiggers, H. and Schmechel, R.
    ECS Transactions 69 (2015)
    Thermoelectric generators can recover waste heat from high temperature heat sources. Given a scalable and affordable technology, this may be part of a future energy mix. For this, suitable thermoelectric converter materials need to be indentified and their efficiency improved. Besides, thermoelectric generators may also be further developed for high temperature applications. We here present results on silicon-based nanocomposites and thermoelectric generators which can meet these criteria. © The Electrochemical Society.
    view abstract10.1149/06909.0003ecst
  • Thermoelectrics from silicon nanoparticles: the influence of native oxide
    Petermann, N. and Stötzel, J. and Stein, N. and Kessler, V. and Wiggers, H. and Theissmann, R. and Schierning, G. and Schmechel, R.
    European Physical Journal B 88 (2015)
    Thermoelectric materials were synthesized by current-assisted sintering of doped silicon nanoparticles produced in a microwave-plasma reactor. Due to their affinity to oxygen, the nanoparticles start to oxidize when handled in air and even a thin surface layer of native silicon oxide leads to a significant increase in the oxide volume ratio. This results in a considerable incorporation of oxygen into the sintered pellets, thus affecting the thermoelectric performance. To investigate the necessity of inert handling of the raw materials, the thermoelectric transport properties of sintered nanocrystalline silicon samples were characterized with respect to their oxygen content. An innovative method allowing a quantitative silicon oxide analysis by means of electron microscopy was applied: the contrast between areas of high and low electrical conductivity was attributed to the silicon matrix and silicon oxide precipitates, respectively. Thermoelectric characterization revealed that both, electron mobility and thermal conductivity decrease with increasing silicon oxide content. A maximum figure of merit with zT = 0.45 at 950 °C was achieved for samples with a silicon oxide mass fraction of 9.5 and 21.4% while the sample with more than 25% of oxygen clearly indicates a negative impact of the oxygen on the electron mobility. © 2015, EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.
    view abstract10.1140/epjb/e2015-50594-7
  • Towards solar cell emitters based on colloidal Si nanocrystals
    Leendertz, C. and Chirvony, V.S. and García-Calzada, R. and Görög, L. and Töfflinger, J.A. and Korte, L. and Agouram, S. and Martínez-Pastor, J. and Petermann, N. and Wiggers, H. and Ulyashin, A.G.
    Physica Status Solidi (A) Applications and Materials Science 212 (2015)
    The application of layers of doped colloidal silicon nanocrystals sandwiched between hydrogenated amorphous silicon layers as emitters in silicon heterojunction solar cells is explored. It is shown that such emitters provide excellent interface passivation and reasonable conductivity. Final solar cells with such nanoparticle emitters reach conversion efficiencies on the same level as solar cells with conventional hetero emitters. Quantum efficiency measurements indicate that the light absorbed in the nanocrystals contributes to the current extracted from the solar cell. The remaining challenges that need to be addressed before the application of such colloidal silicon nanocrystals for the processing of low-cost and potentially printable emitter layers becomes feasible are discussed. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/pssa.201431264
  • High temperature thermoelectric device concept using large area PN junctions
    Chavez, R. and Angst, S. and Hall, J. and Stoetzel, J. and Kessler, V. and Bitzer, L. and Maculewicz, F. and Benson, N. and Wiggers, H. and Wolf, D. and Schierning, G. and Schmechel, R.
    Journal of Electronic Materials 43 (2014)
    A new high temperature thermoelectric device concept using large area nanostructured silicon p-type and n-type (PN) junctions is presented. In contrast to conventional thermoelectric generators, where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we experimentally demonstrate a device concept in which a large area PN junction made from highly doped densified silicon nanoparticles is subject to a temperature gradient parallel to the PN interface. In the proposed device concept, the electrical contacts are made at the cold side eliminating the hot side substrate and difficulties that go along with high temperature electrical contacts. This concept allows temperature gradients greater than 300 K to be experimentally applied with hot side temperatures larger than 800 K. Electronic properties of the PN junctions and power output characterizations are presented. A fundamental working principle is discussed using a particle network model with temperature and electric fields as variables, and which considers electrical conductivity and thermal conductivity according to Fourier's law, as well as Peltier and Seebeck effects. © 2014 TMS.
    view abstract10.1007/s11664-014-3073-x
  • Ignition delay times of shock-heated tetraethoxysilane, hexamethyldisiloxane, and titanium tetraisopropoxide
    Abdali, A. and Fikri, M. and Orthner, H. and Wiggers, H. and Schulz, C.
    Chemical Physics Letters 601 (2014)
    Ignition delay times of tetraethoxysilane (TEOS), hexamethyldisiloxane (HMDSO) and titanium tetraisopropoxide (TTIP) were determined from the onset of chemiluminescence in shock-tube experiments behind reflected shock waves in dry as well as in humid gas mixtures. Additionally, the ignition delay times of TEOS and HMDSO have been investigated in humid air and as a function of water vapor concentration in the initial gas mixture. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.cplett.2014.03.079
  • In situ nanoparticle size measurements of gas-borne silicon nanoparticles by time-resolved laser-induced incandescence
    Sipkens, T.A. and Mansmann, R. and Daun, K.J. and Petermann, N. and Titantah, J.T. and Karttunen, M. and Wiggers, H. and Dreier, T. and Schulz, C.
    Applied Physics B: Lasers and Optics 116 (2014)
    This paper describes the application of time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used mainly for measuring soot primary particles, to size silicon nanoparticles formed within a plasma reactor. Inferring nanoparticle sizes from TiRe-LII data requires knowledge of the heat transfer through which the laser-heated nanoparticles equilibrate with their surroundings. Models of the free molecular conduction and evaporation are derived, including a thermal accommodation coefficient found through molecular dynamics. The model is used to analyze TiRe-LII measurements made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon and hydrogen. Nanoparticle sizes inferred from the TiRe-LII data agree with the results of a Brunauer-Emmett-Teller analysis. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00340-013-5745-2
  • In Situ Particle Size Measurements of Gas-borne Silicon Nanoparticles by Time-resolved Laser-induced Incandescence
    Sipkens, T. A. and Petermann, N. and Daun, K. J. and Titantah, J. and Karttunen, M. and Wiggers, H. and Dreier, T. and Schulz, C.
    Proceedings of the Asme Summer Heat Transfer Conference - 2013, Vol 1 (2014)
    The functionality of silicon nanoparticles is strongly size-dependent, so there is a pressing need for laser diagnostics that can characterize aerosolized silicon nanoparticles. The present work is the first attempt to extend time-resolved laser-induced incandescence (TiRe-LII), a combustion diagnostic used for sizing soot, to size silicon nanoparticles. TiRe-LII measurements are made on silicon nanoparticles synthesized in a low-pressure plasma reactor containing argon. Molecular dynamics (MD) is used to predict the accommodation coefficient between silicon nanoparticles and argon and helium, which is needed to interpret the TiRe-LII data. The MD-derived thermal accommodation coefficients will be validated by comparing them to experimentally-derived values found using transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis.
    view abstract10.1115/HT2013-17246
  • Iron oxide/polymer-based nanocomposite material for hydrogen sulfide adsorption applications
    Blatt, O. and Helmich, M. and Steuten, B. and Hardt, S. and Bathen, D. and Wiggers, H.
    Chemical Engineering and Technology 37 (2014)
    The processing of iron oxide nanoparticles derived from spray flame synthesis for specific adsorption applications is described. After the as-prepared particles proved the ability for H2S removal in pure gas treatment, two different nanoparticle- based composite materials were prepared. While impregnation of activated carbon with the as-prepared nanoparticles showed the expected increase in H2S adsorption capacities, a significant enhancement in desulfurization performance was observed for a novel iron oxide nanoparticle composite material. H2S adsorption was tested in fixed-bed breakthrough curve measurements. The H2S removal efficiency of the novel material under ambient conditions indicates highly promising properties for potential use in industrial and air pollution control applications. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/ceat.201400303
  • Nanocrystalline silicon: Lattice dynamics and enhanced thermoelectric properties
    Claudio, T. and Stein, N. and Stroppa, D.G. and Klobes, B. and Koza, M.M. and Kudejova, P. and Petermann, N. and Wiggers, H. and Schierning, G. and Hermann, R.P.
    Physical Chemistry Chemical Physics 16 (2014)
    Silicon has several advantages when compared to other thermoelectric materials, but until recently it was not used for thermoelectric applications due to its high thermal conductivity, 156 W K-1 m-1 at room temperature. Nanostructuration as means to decrease thermal transport through enhanced phonon scattering has been a subject of many studies. In this work we have evaluated the effects of nanostructuration on the lattice dynamics of bulk nanocrystalline doped silicon. The samples were prepared by gas phase synthesis, followed by current and pressure assisted sintering. The heat capacity, density of phonons states, and elastic constants were measured, which all reveal a significant, ≈25%, reduction in the speed of sound. The samples present a significantly decreased lattice thermal conductivity, ≈25 W K-1 m-1, which, combined with a very high carrier mobility, results in a dimensionless figure of merit with a competitive value that peaks at ZT ≈ 0.57 at 973°C. Due to its easily scalable and extremely low-cost production process, nanocrystalline Si prepared by gas phase synthesis followed by sintering could become the material of choice for high temperature thermoelectric generators. © the Owner Societies 2014.
    view abstract10.1039/c3cp53749h
  • Resonant electronic coupling enabled by small molecules in nanocrystal solids
    Pereira, R.N. and Coutinho, J. and Niesar, S. and Oliveira, T.A. and Aigner, W. and Wiggers, H. and Rayson, M.J. and Briddon, P.R. and Brandt, M.S. and Stutzmann, M.
    Nano Letters 14 (2014)
    The future exploitation of the exceptional properties of nanocrystal (NC) thin films deposited from liquid dispersions of nanoparticles relies upon our ability to produce films with improved electrical properties by simple and inexpensive means. Here, we demonstrate that the electronic conduction of solution-processed NC films can be strongly enhanced without the need of postdeposition treatments, via specific molecules adsorbed at the surfaces of adjacent NCs. This effect is demonstrated for Si NC films doped with the strong molecular oxidizing agent tetrafluoro-tetracyanoquinodimethane (F 4-TCNQ). Density functional calculations were carried out with molecule-doped superlattice solid models. It is shown that, when populated by electrons, hybrid molecule/NC states edge (and may actually resonate with) the conduction-band states of the NC solid. This provides extra electronic connectivity across the NC network as the molecules effectively flatten the electronic potential barriers for electron transfer across the otherwise vacuum-filled network interstitialcies. © 2014 American Chemical Society.
    view abstract10.1021/nl500932q
  • Silicon/Polyaniline nanocomposites as anode material for Lithium ion batteries
    Kummer, M. and Badillo, J.P. and Schmitz, A. and Bremes, H.-G. and Winter, M. and Schulz, C. and Wiggers, H.
    Journal of the Electrochemical Society 161 (2014)
    Due to of its high Li storage capacity, silicon is a promising anode material for lithium ion batteries. Unfortunately, this high specific capacity leads to extreme volume expansion of about 300% during lithiation and delithiation, that may lead to mechanical disintegration of the electrode and poor cycle life. To improve the cycling behavior, we combined nano-silicon (n-Si) active material with an inactive material that acts as a binder and buffering matrix. Stability, flexibility and conductivity are the main requirements for such matrix material. Polyaniline (PANi), a conducting polymer, meets all these requirements. With a theoretical capacity of 643 mAh g -1, the prepared n-Si/PANi sample showed a higher capacity in respect to the commonly used anode material, graphite. The electrochemical performance of the n-Si/PANi composite is stable compared to the performance of nano-silicon without PANi. After 300 cycles the composite still retains more than 60% of its theoretical capacity. © 2013 The Electrochemical Society.
    view abstract10.1149/2.020401jes
  • Spatially resolved determination of thermal conductivity by Raman spectroscopy
    Stoib, B. and Filser, S. and Stötzel, J. and Greppmair, A. and Petermann, N. and Wiggers, H. and Schierning, G. and Stutzmann, M. and Brandt, M.S.
    Semiconductor Science and Technology 29 (2014)
    We review the Raman shift method as a non-destructive optical tool to investigate the thermal conductivity and demonstrate the possibility to map this quantity with a micrometer resolution by studying thin film and bulk materials for thermoelectric applications. In this method, a focused laser beam both thermally excites a sample and undergoes Raman scattering at the excitation spot. The temperature dependence of the phonon energies measured is used as a local thermometer. We discuss that the temperature measured is an effective one and describe how the thermal conductivity is deduced from single temperature measurements to full temperature maps, with the help of analytical or numerical treatments of heat diffusion. We validate the method and its analysis on three- and two-dimensional single crystalline samples before applying it to more complex Si-based materials. A suspended thin mesoporous film of phosphorus-doped lasersintered Si 78 Ge 22 nanoparticles is investigated to extract the in-plane thermal conductivity from the effective temperatures, measured as a function of the distance to the heat sink. Using an iterative multigrid Gauss-Seidel algorithm the experimental data can be modelled yielding a thermal conductivity of 0.1 W/m K after normalizing by the porosity. As a second application we map the surface of a phosphorus-doped three-dimensional bulk-nanocrystalline Si sample which exhibits anisotropic and oxygen-rich precipitates. Thermal conductivities as low as 11 W/m K are found in the regions of the precipitates, signi ficantly lower than the 17 W/m K in the surrounding matrix. The present work serves as a basis to more routinely use the Raman shift method as a versatile tool for thermal conductivity investigations, both for samples with high and low thermal conductivity and in a variety of geometries. © 2014 IOP Publishing Ltd.
    view abstract10.1088/0268-1242/29/12/124005
  • Structural and electronic properties of β - 2 nanoparticles: The role of stacking fault domains
    Imlau, R. and Kovács, A. and Mehmedovic, E. and Xu, P. and Stewart, A.A. and Leidinger, C. and Dunin-Borkowski, R.E. and Bihlmayer, G. and Wiggers, H. and Carius, R. and Kolb, U. and Luysberg, M.
    Physical Review B - Condensed Matter and Materials Physics 89 (2014)
    We use conventional and aberration-corrected transmission electron microscopy (TEM) and ab initio calculations to investigate the structural and electronic properties of β-FeSi2 nanoparticles, which are a promising material for photovoltaic applications due to a band gap of <1 eV and a high absorption coefficient. The nanoparticles have average sizes of ∼20 nm, form aggregates, and are prepared by gas-phase synthesis. Amorphous SiOx shells with thicknesses of ∼1.7 nm around β-FeSi2 cores are identified on individual nanoparticles using electron energy-loss spectroscopy, while stacking fault domains in the nanoparticles are observed using high-resolution TEM, nanobeam electron diffraction, and automated diffraction tomography. Ab initio calculations indicate only minor changes in band structure in the faulted structure when compared to perfect β-FeSi2. The optical properties of imperfect β-FeSi2 nanoparticles are therefore expected to be the same as those of the perfect structure, suggesting that β-FeSi2 nanoparticles are suitable candidates for use in optical absorber layers in thin film solar cells. © 2014 American Physical Society.
    view abstract10.1103/PhysRevB.89.054104
  • Surface functionalization of microwave plasma-synthesized silica nanoparticles for enhancing the stability of dispersions
    Sehlleier, Y.H. and Abdali, A. and Schnurre, S.M. and Wiggers, H. and Schulz, C.
    Journal of Nanoparticle Research 16 (2014)
    Gas phase-synthesized silica nanoparticles were functionalized with three different silane coupling agents (SCAs) including amine, amine/phosphonate and octyltriethoxy functional groups and the stability of dispersions in polar and non-polar dispersing media such as water, ethanol, methanol, chloroform, benzene, and toluene was studied. Fourier transform infrared spectroscopy showed that all three SCAs are chemically attached to the surface of silica nanoparticles. Amine-functionalized particles using steric dispersion stabilization alone showed limited stability. Thus, an additional SCA with sufficiently long hydrocarbon chains and strong positively charged phosphonate groups was introduced in order to achieve electrosteric stabilization. Steric stabilization was successful with hydrophobic octyltriethoxy-functionalized silica nanoparticles in non-polar solvents. The results from dynamic light scattering measurements showed that in dispersions of amine/phosphonate- and octyltriethoxy-functionalized silica particles are dispersed on a primary particle level. Stable dispersions were successfully prepared from initially agglomerated nanoparticles synthesized in a microwave plasma reactor by designing the surface functionalization. © 2014 Springer Science+Business Media.
    view abstract10.1007/s11051-014-2557-1
  • Thermal conductivity of mesoporous films measured by Raman spectroscopy
    Stoib, B. and Filser, S. and Petermann, N. and Wiggers, H. and Stutzmann, M. and Brandt, M.S.
    Applied Physics Letters 104 (2014)
    We measure the in-plane thermal conductance of mesoporous Ge and SiGe thin films using the Raman-shift method and, based on a finite differences simulation accounting for the geometry of the sample, extract the in-plane thermal conductivity. For a suspended thin film of laser-sintered SiGe nanoparticles doped with phosphorus, we find an effective in-plane thermal conductivity of 0.05 W/m K in vacuum for a temperature difference of 400 K and a mean temperature of 500 K. Under similar conditions, the effective in-plane thermal conductivity of a laser-sintered undoped Ge nanoparticle film is 0.5 W/m K. Accounting for a porosity of approximately 50%, the normalized thermal conductivities are 0.1 W/m K and 1 W/m K, respectively. The thermoelectric performance is discussed, considering that the electrical in-plane conductivity is also affected by the mesoporosity. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4873539
  • A new thermoelectric concept using large area PN junctions
    Chavez, R. and Becker, A. and Kessler, V. and Engenhorst, M. and Petermann, N. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1543 (2013)
    A new thermoelectric concept using large area silicon PN junctions is experimentally demonstrated. In contrast to conventional thermoelectric generators where the n-type and p-type semiconductors are connected electrically in series and thermally in parallel, we demonstrate a large area PN junction made from densified silicon nanoparticles that combines thermally induced charge generation and separation in a space charge region with the conventional Seebeck effect by applying a temperature gradient parallel to the PN junction. In the proposed concept, the electrical contacts are made at the cold side eliminating the need for contacts at the hot side allowing temperature gradients greater than 100K to be applied. The investigated PN junction devices are produced by stacking n-type and p-type nanopowder prior to a densification process. The nanoparticulate nature of the densified PN junction lowers thermal conductivity and increases the intraband traps density which we propose is beneficial for transport across the PN junction thus enhancing the thermoelectric properties. A fundamental working principle of the proposed concept is suggested, along with characterization of power output and output voltages per temperature difference that are close to those one would expect from a conventional thermoelectric generator. © 2013 Materials Research Society.
    view abstract10.1557/opl.2013.954
  • Buoyancy induced limits for nanoparticle synthesis experiments in horizontal premixed low-pressure flat-flame reactors
    Weise, C. and Faccinetto, A. and Kluge, S. and Kasper, T. and Wiggers, H. and Schulz, C. and Wlokas, I. and Kempf, A.
    Combustion Theory and Modelling 17 (2013)
    Premixed low-pressure flat-flame reactors can be used to investigate the synthesis of nanoparticles. The present work examines the flow field inside such a reactor during the formation of carbon (soot) and iron oxide (from Fe(CO)5) nanoparticles, and how it affects the measurements of nanoparticle size distribution. The symmetry of the flow and the impact of buoyancy were analysed by three-dimensional simulations and the nanoparticle size distribution was obtained by particle mass spectrometry (PMS) via molecular beam sampling at different distances from the burner. The PMS measurements showed a striking, sudden increase in particle size at a critical distance from the burner, which could be explained by the flow field predicted in the simulations. The simulation results illustrate different fluid mechanical phenomena which have caused this sudden rise in the measured particle growth. Up to the critical distance, buoyancy does not affect the flow, and an (almost) linear growth is observed in the PMS experiments. Downstream of this critical distance, buoyancy deflects the hot gas stream and leads to an asymmetric flow field with strong recirculation. These recirculation zones increase the particle residence time, inducing very large particle sizes as measured by PMS. This deviation from the assumed symmetric, one-dimensional flow field prevents the correct interpretation of the PMS results. To overcome this problem, modifications to the reactor were investigated; their suitability to reduce the flow asymmetry was analysed. Furthermore, 'safe' operating conditions were identified for which accurate measurements are feasible in premixed low-pressure flat-flame reactors that are transferrable to other experiments in this type of reactor. The present work supports experimentalists to find the best setup and operating conditions for their purpose. © 2013 Copyright Taylor and Francis Group, LLC.
    view abstract10.1080/13647830.2013.781224
  • Depassivation kinetics in crystalline silicon nanoparticles
    Pereira, R.N. and Niesar, S. and Wiggers, H. and Brandt, M.S. and Stutzmann, M.S.
    Physical Review B - Condensed Matter and Materials Physics 88 (2013)
    The formation of silicon dangling bond (Si-db) defects in crystalline silicon nanoparticles (Si-NPs) is studied by electron paramagnetic resonance combined with vacuum-annealing experiments. The kinetics of Si-db formation due to H desorption is found to be reliably described by a first-order-rate thermal model with a mean activation energy Ed=2.25 eV and a spread σEd=0.28 eV in the activation energy distribution. These values deviate from those reported in previous studies of other Si-based materials, which is attributed to the presence of different interfacial hydrides Si 4-n-Si-Hn. Hence, the generation Si-db defects in Si-NPs initiates at a much lower temperature than one would expect based on the previously reported kinetics parameters. Unlike the case of planar Si/SiO 2 interfaces, no permanent interface degradation is observed upon annealing of Si-NPs at temperatures &gsim;600°C. This, together with the observation of an interfacial Si-db density similar to that typically incorporated in high quality thermally-grown SiO2 on bulk silicon, indicates the formation of a rather relaxed and thermally stable surface oxide shell during natural oxidation of Si-NPs. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.88.155430
  • Direct gas-phase synthesis of single-phase β-FeSi2 nanoparticles
    Bywalez, R. and Orthner, H. and Mehmedovic, E. and Imlau, R. and Kovacs, A. and Luysberg, M. and Wiggers, H.
    Journal of Nanoparticle Research 15 (2013)
    For the first time, phase-pure β-FeSi2 nanoparticles were successfully produced by gas-phase synthesis. We present a method to fabricate larger quantities of semiconducting β-FeSi2 nanoparticles, with crystallite sizes between 10 and 30 nm, for solar and thermoelectric applications utilizing a hot-wall reactor. A general outline for the production of those particles by thermal decomposition of silane and iron pentacarbonyl is provided based on kinetic data. The synthesized particles are investigated by X-ray diffraction and transmission electron microscopy, providing evidence that the as-prepared materials are indeed β-FeSi2, while revealing morphological characteristics inherent to the nanoparticles created. © 2013 Springer Science+Business Media.
    view abstract10.1007/s11051-013-1878-9
  • Effects of impurities on the lattice dynamics of nanocrystalline silicon for thermoelectric application
    Claudio, T. and Schierning, G. and Theissmann, R. and Wiggers, H. and Schober, H. and Koza, M.M. and Hermann, R.P.
    Journal of Materials Science 48 (2013)
    Doped silicon nanoparticles were exposed to air and sintered to form nanocrystalline silicon. The composition, microstructure, and structural defects were investigated with TEM, XRD, and PDF and the lattice dynamics was evaluated with measurements of the heat capacity, of the elastic constants with resonant ultrasound spectroscopy and of the density of phonon states (DPS) with inelastic neutron scattering. The results were combined and reveal that the samples contain a large amount of silicon dioxide and exhibit properties that deviate from bulk silicon. Both in the reduced DPS and in the heat capacity a Boson peak at low energies, characteristic of amorphous SiO2, is observed. The thermal conductivity is strongly reduced due to nanostructuration and the incorporation of impurities. © 2012 The Author(s).
    view abstract10.1007/s10853-012-6827-y
  • Erratum: Thermoelectric properties of nanocrystalline silicon from a scaled-up synthesis plant (Advanced Engineering Materials (2013) 15 (379) DOI: 10.1002/adem.201200233)
    Kessler, V. and Gautam, D. and Hülser, T. and Spree, M. and Theissmann, R. and Winterer, M. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Advanced Engineering Materials 15 (2013)
    view abstract10.1002/adem.201300182
  • Excimer laser doping using highly doped silicon nanoparticles
    Meseth, M. and Kunert, B.C. and Bitzer, L. and Kunze, F. and Meyer, S. and Kiefer, F. and Dehnen, M. and Orthner, H. and Petermann, N. and Kummer, M. and Wiggers, H. and Harder, N.-P. and Benson, N. and Schmechel, R.
    Physica Status Solidi (A) Applications and Materials Science 210 (2013)
    Laser doping of crystalline Si (c-Si) using highly doped Si nanoparticles (NPs) as the dopant source is investigated. For this purpose Si NPs are deposited onto c-Si substrates from dispersion using a spin coater and subsequently laser annealed by scanning over the sample with a 248 nm line profile excimer laser. Scanning electron microscope (SEM) investigations demonstrate that the laser intensity as well as the oxide concentration in the NP thin film strongly influence the film forming properties of the annealed NPs. Substrate doping is substantiated using electrochemical capacitance voltage (ECV) measurements on realized pn-junctions. In dependence of the laser fluencies ranging from 0.81 to 2.54 J cm-2, the effective doping depth is determined to be in the range of 50 to 250 nm. The rectifying behaviour of the pn- or np-junctions is verified by current voltage measurements. A homogeneous in-plane doping distribution realized by the laser doping process is demonstrated on the μm scale by light beam induced current measurements. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201329012
  • Fabrication of periodic surface topographies via sequential photothermal laser microsintering of silicon nanoparticle films
    Behrenberg, D. and Franzka, S. and Petermann, N. and Wiggers, H. and Hartmann, N.
    Applied Surface Science 278 (2013)
    Photothermal laser microprocessing is exploited in order to induce sintering and compaction of thin silicon nanoparticle (Si NP) films. Ethanolic dispersions of Si NPs with an average diameter of 45 nm are spin-coated on silicon substrates yielding films with a thickness of about 400-500 nm. Scanning electron and atomic force microscopy are used for characterization of the resulting surface morphologies. Sequential processing of the coated layer with a microfocused cw-laser beam at a wavelength of 532 nm generates periodic surface structures. The periodicity of these structures is determined by the distance between adjacent laser-written lines. Despite a 1/e laser spot size of 1.4 μm, fabrication of topographic surface structures with submicrometer periodicities is feasible. In particular, surface topographies with periodicities of 600 nm and a topographic amplitude of 80 nm are fabricated. These results point to a high nonlinearity, which is attributed to the strongly activated, temperature-dependent laser sintering process. These experimental observations are reproduced qualitatively considering a simple photothermal model and an activated sintering process. Prospects of photothermal laser microsintering of nanoparticle films to fabricate biomimetic surface structures are discussed. © 2013 Elsevier B.V.
    view abstract10.1016/j.apsusc.2013.01.205
  • Impact of composition and morphology on the optical properties of Si-NC/P3HT thin films processed from solution
    Rodrigues, F.D. and Cunha, M. and Hilliou, L. and Rino, L. and Correia, M.R. and Busani, T. and Bernardo, G. and Wiggers, H. and Filonovich, S.A. and Pereira, R.N.
    Applied Physics A: Materials Science and Processing 113 (2013)
    Blends of Si nanocrystals (Si-NCs) and organic semiconductors are promising materials for new optical and electronic devices processed from solutions. Here, we study how the optical properties of composite films containing Si- NCs and the organic semiconductor poly(3-hexylthiophene) (P3HT) are influenced by the composition and morphology resulting from different solution-processing parameters and different solvents used dichlorobenzene vs. chloroform). The optical spectra of the hybrid films are described using a simple phenomenological model, with which we can discern the contribution of each material in the films to the optical properties. From this analysis, we obtain quantitativeinformation about the composition and morphology of the hybrid nanostructured films, which otherwise would be obtained from more demanding microscopy and spectroscopy techniques. For the case of the Si-NC/P3HT blend, we find that in films deposited from dichlorobenzene solutions the Si-NCs contribute sizably to light absorption. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s00339-012-7540-z
  • Laser-doping of crystalline silicon substrates using doped silicon nanoparticles
    Meseth, M. and Lamine, K. and Dehnen, M. and Kayser, S. and Brock, W. and Behrenberg, D. and Orthner, H. and Elsukova, A. and Hartmann, N. and Wiggers, H. and Hülser, T. and Nienhaus, H. and Benson, N. and Schmechel, R.
    Thin Solid Films 548 (2013)
    Crystalline Si substrates are doped by laser annealing of solution processed Si. For this experiment, dispersions of highly B-doped Si nanoparticles are deposited onto intrinsic Si and laser processed using an 807.5 nm continuous wave laser. During laser processing the particles as well as a surface-near substrate layer are melted to subsequently crystallize in the same orientation as the substrate. The doping profile is investigated by secondary ion mass spectroscopy revealing a constant B concentration of 2 × 10 18 cm- 3 throughout the entire analyzed depth of 5 μm. Four-point probe measurements demonstrate that the effective conductivity of the doped sample is increased by almost two orders of magnitude. The absolute doping depth is estimated to be in between 8 μm and 100 μm. Further, a pn-diode is created by laser doping an n-type c-Si substrate using the Si NPs. © 2013 Published by Elsevier B.V.
    view abstract10.1016/j.tsf.2013.09.056
  • Low temperature diffusion of Li atoms into Si nanoparticles and surfaces
    Nienhaus, H. and Karacuban, H. and Krix, D. and Becker, F. and Hagemann, U. and Steeger, D. and Bywalez, R. and Schulz, C. and Wiggers, H.
    Journal of Applied Physics 114 (2013)
    The diffusion of Li atoms deposited on hydrogen-passivated Si(001) surfaces, chemically oxidized Si(001) surfaces, Si nanoparticle films, and thick SiO2 layers is investigated with electron-beam induced Auger electron spectroscopy. The nanoparticles exhibit an average diameter of 24 nm. The Li metal film is evaporated at a sample temperature below 120 K. The reappearance of the Si substrate Auger signal as a function of time and temperature can be measured to study the Li diffusion into the bulk material. Values for the diffusion barrier of 0.5 eV for H:Si(001) and 0.3 eV for the ox-Si(001) and Si nanoparticle films are obtained. The diffusion of the Li atoms results in the disruption of the crystalline Si surfaces observed with atomic force microscopy. Contrasting to that, the Si nanoparticle films show less disruption by Li diffusion due to filling of the porous films detected with cross section electron microscopy. Silicon dioxide acts as a diffusion barrier for temperatures up to 300 K. However, the electron beam induces a reaction between Li and SiO2, leading to LiOx and elemental Si floating on the surface. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4813872
  • Morphology control of thin P3HT-Si-NCs composite films for hybrid photovoltaic cells
    Cunha, M. and Bernardo, G. and Hilliou, L. and Wiggers, H. and Pereira, R.N.
    Materials Science Forum 730-732 (2013)
    This work deals with an experimental investigation of the microstructure/morphology of spin-casted composite thin films of poly(3-hexylthiophene) (P3HT) and silicon nanocrystals (Si- NCs), in the weight proportion 1:1, which develop under different deposition conditions. The experimental parameters considered were the following: i) solvent quality; ii) spinning rate; iii) spinning time and iv) solution concentration. The developed morphologies were characterized by means of optical microscopy and X-ray diffraction (XRD) measurements. The present work aims at a) establishing the relationship between processing conditions and resultant morphology and b) defining the most relevant processing parameters that govern and are of significance for the induced morphology. © (2013) Trans Tech Publications, Switzerland.
    view abstract10.4028/www.scientific.net/MSF.730-732.227
  • Morphology, thermoelectric properties and wet-chemical doping of laser-sintered germanium nanoparticles
    Stoib, B. and Langmann, T. and Petermann, N. and Matich, S. and Sachsenhauser, M. and Wiggers, H. and Stutzmann, M. and Brandt, M.S.
    Physica Status Solidi (A) Applications and Materials Science 210 (2013)
    Porous, highly doped semiconductors are potential candidates for thermoelectric energy conversion elements. We report on the fabrication of thin films of Ge via short-pulse laser-sintering of Ge nanoparticles (NPs) in vacuum and study the macroporous morphology of the samples by secondary electron microscopy (SEM) imaging. The temperature dependence of the electrical conductivity and the Seebeck coefficient of undoped Ge is discussed in conjunction with the formation of a defect band near the valence band. We further introduce a versatile method of doping the resulting films with a variety of common dopant elements in group-IV semiconductors by using a liquid containing the dopant atoms. This method is fully compatible with laser-direct writing and suited to fabricate small scale thermoelectric generators. The incorporation of the dopants is verified by X-ray photoelectron spectroscopy (XPS) and their electrical activation is studied by conductivity and thermopower measurements. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssa.201228392
  • Separation of semiconducting and ferromagnetic FeSi2- nanoparticles by magnetic filtering
    Aigner, W. and Niesar, S. and Mehmedovic, E. and Opel, M. and Wagner, F.E. and Wiggers, H. and Stutzmann, M.
    Journal of Applied Physics 114 (2013)
    We have investigated the potential of solution-processed β-phase iron disilicide (FeSi2) nanoparticles as a novel semiconducting material for photovoltaic applications. Combined ultraviolet-visible absorption and photothermal deflection spectroscopy measurements have revealed a direct band gap of 0.85 eV and, therefore, a particularly high absorption in the near infrared. With the help of Fourier-transform infrared and X-ray photoelectron spectroscopy, we have observed that exposure to air primarily leads to the formation of a silicon oxide rather than iron oxide. Mössbauer measurements have confirmed that the nanoparticles possess a phase purity of more than 99%. To diminish the small fraction of metallic iron impurities, which were detected by superconducting quantum interference device magnetometry and which would act as unwanted Auger recombination centers, we present a novel concept to magnetically separate the FeSi2 nanoparticles (NPs). This process leads to a reduction of more than 95% of the iron impurities. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4824293
  • Sintering of thin titanium dioxide nanoparticle films via photothermal processing with ultraviolet continuous-wave lasers
    Schade, L. and Franzka, S. and Hardt, S. and Wiggers, H. and Hartmann, N.
    Applied Surface Science 278 (2013)
    Photothermal laser processing of thin films of anatase titania nanoparticles (TiO2 NPs, diameter: 8-10 nm) with a thickness of about 500 nm is addressed. Laser processing in ambient air is carried out using a microfocused continuous-wave laser setup operating at a wavelength of 355 nm and a 1/e laser spot size of 1.6 μm. In conjunction with scanning electron microscopy, this approach provides a highly reproducible and convenient means in order to modify the local film structure and study the dependence of the resulting film morphology on the laser parameters. Generally, sintering of the nanoparticles is observed. At high laser power densities and/or long irradiation times the average particle/grain size increases reaching values of 200 nm and more. This opens up an opportunity to introduce scattering centers and optimize light trapping within the film, e.g., targeting photovoltaic or photocatalytic applications. © 2012 Elsevier B.V.
    view abstract10.1016/j.apsusc.2012.11.077
  • Solid state NMR structural studies of the lithiation of nano-silicon:: Effects of charging capacities, host-doping, and thermal treatment
    Cattaneo, A.S. and Dupke, S. and Schmitz, A. and Badillo, J.P. and Winter, M. and Wiggers, H. and Eckert, H.
    Solid State Ionics 249-250 (2013)
    The electrochemical lithiation of undoped, P-doped and B-doped nano-silicon particles (100-200 nm diameter) has been studied during the first cycle by ex-situ 6Li and 7Li magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. Samples were charged within pouch cells up to capacities of 4000 mAh/g, at C/40 followed by NMR analysis. The spectra reveal important quantitative information on the local lithium environments during the various stages of the intercalation process. Approximate Li/Si ratios of the lithium silicides present in the nanoparticles can be deduced, based on the initial formation of the SEI layer, which accounts for an irreversible capacity of up to 500 mAh/g. Surface lithium silicide environments with high Li concentrations (corresponding to the composition Li15Si4) are preferentially formed at charging capacities near 1000 mAh/g. At higher charging capacities, irreversible capacity losses are lower and a wide distribution of lithium silicide environments is found, resembling those present in the crystalline phases Li12Si7, Li7Si 3, and Li13Si4. At a charging capacity of and above 2000 mAh/g the large majority of silicon is converted to lithiated silicide particles. Boron-doped nano-Si materials behave generally similar, while phosphorus-doping reveals clear beneficial effects, in particular concerning the initial lithiation stages. Both irreversible capacity losses and surface "over-lithiation" are significantly diminished in these samples. Exposure of lithiated nano-Si samples to elevated temperatures (400-440 K) results in the crystallization of Li7Si3 in all of those nano-Si samples charged with at least 1500 mAh/g. © 2013 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.ssi.2013.07.013
  • Synthesis of small carbon nanoparticles in a microwave plasma flow reactor
    Gurentsov, E. and Priemchenko, K. and Grimm, H. and Orthner, H. and Wiggers, H. and Borchers, C. and Jander, H. and Eremin, A. and Schulz, C.
    Zeitschrift fur Physikalische Chemie 227 (2013)
    Unusually small carbon nanoparticles were synthesized in a microwave plasma flow-reactor by pyrolysis of 0.3-1.2% CH4, C2H 4, and C2H2 with 0.3-3.6% addition of molecular hydrogen in argon. Final particle sizes were analyzed by in-line particle-mass spectrometry (PMS) and by transmission electron microscopy (TEM). TEM measurements of primary particle sizes were found to be in a good agreement with PMS data. The carbon particles formed in the plasma generated by a 2.45 GHz magnetron with an applied power of 180 W and a total pressure of 13 mbar have diameters of 4-6 nm. The type of hydrocarbon precursor and 0.3-3.6% of hydrogen addition did not noticeably influence the final particle sizes. The formation of such small particles is attributed to the low pressure and the comparably low operation power. This method of small carbon nanoparticles synthesis could be useful for the production of carbon black material, where large surface area is important. © by Oldenbourg Wissenschaftsverlag.
    view abstract10.1524/zpch.2013.0369
  • Thermoelectric properties of nanocrystalline silicon from a scaled-up synthesis plant
    Kessler, V. and Gautam, D. and Hülser, T. and Spree, M. and Theissmann, R. and Winterer, M. and Wiggers, H. and Schierning, G. and Schmechel, R.
    Advanced Engineering Materials 15 (2013)
    Silicon based thermoelectrics are promising candidates for high temperature energy scavenging applications. We present the properties of thermoelectrics made from highly boron doped silicon nanoparticles. The particles were produced by a continuous gas phase process in a scaled-up synthesis plant enabling production rates in the kg h-1 regime. The silicon nanoparticles were compacted by direct current assisted sintering to yield nanocrystalline bulk silicon with average crystallite size between 40 and 80 nm and relative densities above 97% of the density of single crystalline silicon. The influence of the sintering temperature on the thermoelectric properties is investigated. It was found that high sintering temperatures are beneficial for an enhancement of the power factor, while the thermal conductivity was only moderately affected. The optimization of the compaction procedure with respect to the transport properties leads to zT values of the p-type nanosilicon of 0.32 at 700 °C, demonstrating the potential of our method. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adem.201200233
  • A sintered nanoparticle p-n junction observed by a Seebeck microscan
    Becker, A. and Schierning, G. and Theissmann, R. and Meseth, M. and Benson, N. and Schmechel, R. and Schwesig, D. and Petermann, N. and Wiggers, H. and Ziolkowski, P.
    Journal of Applied Physics 111 (2012)
    A nanoparticular p-n junction was realized by a field-assisted sintering process, using p-type and n-type doped silicon nanoparticles. A spatially resolved Seebeck microscan showed a broad transition from the positively doped to the negatively doped range. Overshoots on both sides are characteristic for the transition. Despite the tip size being much larger than the mean particle size, information about the dopant distribution between the particles is deduced from modeling the measured data under different assumptions, including the limited spatial resolution of the tip. The best match between measured and modeled data is achieved by the idea of doping compensation, due to the sintering process. Due to a short time at high temperature during the field-assisted sintering process, solid state diffusion is too slow to be solely responsible for the observed compensation of donors and acceptors over a wide range. Therefore, these measurements support a densification mechanism based on (partial) melting and recrystallization. © 2012 American Institute of Physics.
    view abstract10.1063/1.3693609
  • Comparison of micro- and nanoscale Fe +3-containing (Hematite) particles for their toxicological properties in human lung cells in vitro
    Bhattacharya, K. and Hoffmann, E. and Schins, R.F.P. and Boertz, J. and Prantl, E.-M. and Alink, G.M. and Byrne, H.J. and Kuhlbusch, T.A.J. and Rahman, Q. and Wiggers, H. and Schulz, C. and Dopp, E.
    Toxicological Sciences 126 (2012)
    The specific properties of nanoscale particles, large surface-to-mass ratios and highly reactive surfaces, have increased their commercial application in many fields. However, the same properties are also important for the interaction and bioaccumulation of the nonbiodegradable nanoscale particles in a biological system and are a cause for concern. Hematite (α-Fe 2O 3), being a mineral form of Fe(III) oxide, is one of the most used iron oxides besides magnetite. The aim of our study was the characterization and comparison of biophysical reactivity and toxicological effects of α-Fe 2O 3 nano- (d < 100 nm) and microscale (d < 5 μm) particles in human lung cells. Our study demonstrates that the surface reactivity of nanoscale α-Fe 2O 3 differs from that of microscale particles with respect to the state of agglomeration, radical formation potential, and cellular toxicity. The presence of proteins in culture medium and agglomeration were found to affect the catalytic properties of the hematite nano- and microscale particles. Both the nano- and microscale α-Fe 2O 3 particles were actively taken up by human lung cells in vitro, although they were not found in the nuclei and mitochondria. Significant genotoxic effects were only found at very high particle concentrations (> 50 μg/ml). The nanoscale particles were slightly more potent in causing cyto- and genotoxicity as compared with their microscale counterparts. Both types of particles induced intracellular generation of reactive oxygen species. This study underlines that α-Fe 2O 3 nanoscale particles trigger different toxicological reaction pathways than microscale particles. However, the immediate environment of the particles (biomolecules, physiological properties of medium) modulates their toxicity on the basis of agglomeration rather than their actual size. © The Author 2012. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved.
    view abstract10.1093/toxsci/kfs014
  • Electrical transport in semiconductor nanoparticle arrays: Conductivity, sensing and modeling
    Hartner, S. and Schwesig, D. and Plümel, I. and Wolf, D.E. and Lorke, A. and Wiggers, H.
    NanoScience and Technology 79 (2012)
    Electrical properties of nanoparticle ensembles are dominated by interparticle transport processes, mainly due to particle-particle and particle-contact interactions. Thismakes their electrical properties dependent on the network properties such as porosity and particle size and is a main prerequisite for solid- state gas sensors, as the surrounding gas atmosphere influences the depletion layer surrounding each particle. Different kinds of nanoparticle arrays such as pressed pellets, printed layer, and thin films prepared by molecular beam-assisted deposition are characterized with respect to their electrical transport properties. Experimental results are shown for the electrical and sensing properties of several metal oxide nanoparticle ensembles and the influence of porosity is investigated during compaction of nanoparticle powders exposed to an external force. A model describing these properties is developed and it is shown that for a given material only porosity, geometry, and particle size influence the overall electrical properties. The model developed for the description of current transport in particulate matter can also be utilized to describe current-assisted sintering. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_10
  • Exchange-coupled donor dimers in nanocrystal quantum dots
    Pereira, R.N. and Almeida, A.J. and Stegner, A.R. and Brandt, M.S. and Wiggers, H.
    Physical Review Letters 108 (2012)
    Doping of semiconductor nanocrystals (NCs) is expected to enable the control of key NC properties, yet its practical exploitation requires an understanding of exchange interactions when multiple dopants are incorporated in a single NC. Here, we experimentally probe the exchange of donor dimers in NCs via a deviation of their triplet-state magnetic resonance from Curie paramagnetism. We show that the exchange coupling of the closely spaced donors can be well described by effective mass theory, which allows the consideration of statistical effects crucial in NC ensembles. While a dimer induces discrete states in a NC, their energy splitting differs by up to 3 orders of magnitude for randomly placed dimers in a NC ensemble, due to an enormous dependence of the exchange energy on the dimer configuration. © 2012 American Physical Society.
    view abstract10.1103/PhysRevLett.108.126806
  • Functionalization of SiO2 nanoparticles and their superhydrophobic surface coating
    Sehlleier, Y. H. and Abdali, A. and Hulser, T. and Wiggers, H. and Schulz, C.
    Nanoformulation (2012)
    view abstract10.1039/9781849735247-00113
  • High-capacity cathodes for lithium-ion batteries from nanostructured LiFePO 4 synthesized by highly-flexible and scalable flame spray pyrolysis
    Hamid, N.A. and Wennig, S. and Hardt, S. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Power Sources 216 (2012)
    Olivine, LiFePO 4 is a promising cathode material for lithium-ion batteries due to its low cost, environmental acceptability and high stability. Its low electric conductivity prevented it for a long time from being used in large-scale applications. Decreasing its particle size along with carbon coating significantly improves electronic conductivity and lithium diffusion. With respect to the controlled formation of very small particles with large specific surface, gas-phase synthesis opens an economic and flexible route towards high-quality battery materials. Amorphous FePO 4 was synthesized as precursor material for LiFePO 4 by flame spray pyrolysis of a solution of iron acetylacetonate and tributyl phosphate in toluene. The pristine FePO 4 with a specific surface from 126-218 m 2 g -1 was post-processed to LiFePO 4/C composite material via a solid-state reaction using Li 2CO 3 and glucose. The final olivine LiFePO 4/C particles still showed a large specific surface of 24 m 2 g -1 and were characterized using X-ray diffraction (XRD), electron microscopy, X-ray photoelectron spectrocopy (XPS) and elemental analysis. Electrochemical investigations of the final LiFePO 4/C composites show reversible capacities of more than 145 mAh g -1 (about 115 mAh g -1 with respect to the total coating mass). The material supports high drain rates at 16 C while delivering 40 mAh g -1 and causes excellent cycle stability. © 2012 Elsevier B.V.
    view abstract10.1016/j.jpowsour.2012.05.047
  • Laser-sintered thin films of doped SiGe nanoparticles
    Stoib, B. and Langmann, T. and Matich, S. and Antesberger, T. and Stein, N. and Angst, S. and Petermann, N. and Schmechel, R. and Schierning, G. and Wolf, D.E. and Wiggers, H. and Stutzmann, M. and Brandt, M.S.
    Applied Physics Letters 100 (2012)
    We present a study of the morphology and the thermoelectric properties of short-pulse laser-sintered (LS) nanoparticle (NP) thin films, consisting of SiGe alloy NPs or composites of Si and Ge NPs. Laser-sintering of spin-coated NP films in vacuum results in a macroporous percolating network with a typical thickness of 300 nm. The Seebeck coefficient for LS samples is the same as for bulk samples prepared by current-assisted sintering and is typical for degenerate doping. The electrical conductivity of LS films is influenced by two-dimensional percolation effects and rises with increasing temperature, approximately following a power-law. © 2012 American Institute of Physics.
    view abstract10.1063/1.4726041
  • Low-cost post-growth treatments of crystalline silicon nanoparticles improving surface and electronic properties
    Niesar, S. and Pereira, R.N. and Stegner, A.R. and Erhard, N. and Hoeb, M. and Baumer, A. and Wiggers, H. and Brandt, M.S. and Stutzmann, M.
    Advanced Functional Materials 22 (2012)
    Freestanding silicon nanocrystals (Si-ncs) offer unique optical and electronic properties for new photovoltaic, thermoelectric, and other electronic devices. A method to fabricate Si-ncs which is scalable to industrial usage has been developed in recent years. However, barriers to the widespread utilization of these nanocrystals are the presence of charge-trapping defects and an oxide shell formed upon ambient atmosphere exposure hindering the charge transport. Here, we exploit low-cost post-growth treatment routes based on wet-etching in hydrofluoric acid plus surface hydrosilylation or annealing enabling a complete native oxide removal and a reduction of the defect density by up to two orders of magnitude. Moreover, when compared with only H-terminated Si-ncs we report an enhancement of the conductivity by up to a factor of 400 for films of HF etched and annealed Si-ncs, which retain a defect density below that of untreated Si-ncs even after several months of air exposure. Further, we demonstrate that HF etched and hydrosilylated Si-ncs are extremely stable against oxidation and maintain a very low defect density after a long-term storage in air, opening the possibility of device processing in ambient atmosphere. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/adfm.201101811
  • Monodisperse titania microspheres via controlled nanoparticle aggregation
    Schunk, D. and Hardt, S. and Wiggers, H. and Marlow, F.
    Physical Chemistry Chemical Physics 14 (2012)
    This paper describes the fabrication of highly monodisperse TiO 2 nanoparticle aggregates (NPAs) by controlled aggregation of nanoparticles in a water-in-oil emulsion. Equally sized drops containing a titanium dioxide nanoparticle suspension are produced in a T-channel device. This procedure has a high tuning potential. Increasing the velocity of the titania suspension phase leads to an enlargement of the droplets, while raising the velocity of the oil phase reduces the drop diameter. The technique enables the preparation of monodisperse (< 2%) drops between 150 and 400 μm in diameter. Evaporation of the suspension medium leads to nanoparticle aggregates. There is no significant change in dispersity from emulsion drops to NPAs, if the drying is carried out slowly. © 2012 the Owner Societies.
    view abstract10.1039/c2cp40658f
  • Photothermal laser processing of thin silicon nanoparticle films: On the impact of oxide formation on film morphology
    Behrenberg, D. and Franzka, S. and Petermann, N. and Wiggers, H. and Hartmann, N.
    Applied Physics A: Materials Science and Processing 106 (2012)
    Photothermal laser processing of thin films of H-terminated silicon nanoparticles (Si NPs) is investigated. Ethanolic dispersions of Si NPs with an average diameter of 45 nm are spin-coated on silicon substrates yielding films with thicknesses ≤500 nm. Small-area laser processing is carried out using a microfocused scanning cw-laser setup operating at a wavelength of 532 nm and a 1/e laser spot size of 1.4 μm. In conjunction with microscopic techniques, this provides a highly reproducible and convenient approach in order to study the dependence of the resulting film morphology and composition on the experimental parameters. Processing in air results in strongly oxidized granular structures with sizes between 100 and 200 nm. The formation of these structures is dominated by surface oxidation. In particular, changing the processing parameters (i.e., laser power, writing speed, and/or the background air pressure) has little effect on the morphology. Only in vacuum at pressures <1 mbar, oxygen adsorption, and hence oxide formation, is largely suppressed. Under these conditions, irradiation at low laser powers results in mesoporous surface layers, whereas compact silicon films are formed at high laser powers. In agreement with these results, comparative experiments with films of H-terminated and surface-oxidized Si NPs reveal a strong impact of the surface oxide layer on the film morphology. Mechanistic aspects and implications for photothermal processing techniques, e.g., targeting photovoltaic and thermoelectric applications, are discussed. © 2012 Springer-Verlag.
    view abstract10.1007/s00339-012-6779-8
  • Stabilization of mid-sized silicon nanoparticles by functionalization with acrylic acid
    Bywalez, R. and Karacuban, H. and Nienhaus, H. and Schulz, C. and Wiggers, H.
    Nanoscale Research Letters 7 (2012)
    We present an enhanced method to form stable dispersions of medium-sized silicon nanoparticles for solar cell applications by thermally induced grafting of acrylic acid to the nanoparticle surface. In order to confirm their covalent attachment on the silicon nanoparticles and to assess the quality of the functionalization, X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier spectroscopy measurements were carried out. The stability of the dispersion was elucidated by dynamic light scattering and Zeta-potential measurements, showing no sign of degradation for months. © 2012 Bywalez et al.
    view abstract10.1186/1556-276X-7-76
  • Synthesis of tailored nanoparticles in flames: Chemical kinetics, in situ diagnostics, numerical simulation, and process development
    Wiggers, H. and Fikri, M. and Wlokas, I. and Roth, P. and Schulz, C.
    NanoScience and Technology 79 (2012)
    Flame synthesis of nanoparticles provides access to a wide variety of metal oxide nanoparticles. Detailed understanding of the underlying fundamental processes is a prerequisite for the synthesis of specific materials with well-defined properties. Multiple steps from gas-phase chemistry, inception of first particles and particle growth are thus investigated in detail to provide the information required for setting up chemistry and particle dynamics models that allow simulating particle synthesis apparatus. Experiments are carried out in shock wave and flow reactors with in situ optical diagnostics, such as absorption, laser-induced fluorescence, and laser-induced incandescence, with in-line sampling via mass spectrometry as well as with thermophoretic sampling for ex situ microscopic analysis and electronic characterization. Focus is on tuning particle size as well as crystallinity and stoichiometry, with a specific focus on sub-stoichiometric materials with tunable composition. © Springer-Verlag Berlin Heidelberg 2012.
    view abstract10.1007/978-3-642-28546-2_1
  • The effect of Peltier heat during current activated densification
    Becker, A. and Angst, S. and Schmitz, A. and Engenhorst, M. and Stoetzel, J. and Gautam, D. and Wiggers, H. and Wolf, D.E. and Schierning, G. and Schmechel, R.
    Applied Physics Letters 101 (2012)
    It is shown that current-activated pressure-assisted densification (CAPAD) is sensitive to the Peltier effect. Under CAPAD, the Peltier effect leads to a significant redistribution of heat within the sample during the densification. The densification of highly p-doped silicon nanoparticles during CAPAD and the properties of the obtained samples are investigated experimentally and by computer simulation. Both, simulation and experiments, indicate clearly a higher temperature on the cathode side and a decreasing temperature from the center to the outer shell. Furthermore, computer simulations provide additional insights into the temperature profile which explain the anisotropic properties of the measured sample. © 2012 American Institute of Physics.
    view abstract10.1063/1.4731272
  • The realization of a pn-diode using only silicon nanoparticles
    Meseth, M. and Ziolkowski, P. and Schierning, G. and Theissmann, R. and Petermann, N. and Wiggers, H. and Benson, N. and Schmechel, R.
    Scripta Materialia 67 (2012)
    Si nanoparticles (Si-NPs) are a non-toxic and low-cost material resource that can be processed from dispersion for electrical thin film or from powder for bulk application using various sintering techniques. So far research on electronic applications using Si-NPs is limited. Few reports exist on thermoelectric research, or hybrid photovoltaic applications. In the following we demonstrate the realization of the first Si pn-diode using only Si-NPs in combination with field-assisted sintering. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.scriptamat.2012.04.039
  • Thermally induced reactions between lithiated nano-silicon electrode and electrolyte for lithium-ion batteries
    Profatilova, I.A. and Langer, T. and Badillo, J.P. and Schmitz, A. and Orthner, H. and Wiggers, H. and Passerini, S. and Winter, M.
    Journal of the Electrochemical Society 159 (2012)
    The thermal stability of nano-silicon electrodes before and after lithiation was studied by means of differential scanning calorimetry (DSC). It was found that pristine Si electrodes heated in presence of ECDEC 1M LiPF 6 electrolyte show exothermic reactions between sodium carboxymethylcellulose (Na CMC binder) and LiPF 6. The products of thermal decomposition of a lithiated nano-Si electrode with electrolyte at different temperatures were identified using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). SEI layer was found to be responsible for the thermal reactions in the range between 77 and 107°C. Exothermic events between 107 and 140°C were caused by partial decomposition of LiPF 6 salt, which products initiated further transformations of SEI layer compounds and esterification of Na CMC. Interaction between nano-Li xSi and ECDEC 1M LiPF 6 was the reason for the main exothermic peaks at temperatures between 150 and 300°C. Nano-Li xSi heated with ECDEC solvent mixture without LiPF 6 resulted in electrolyte decomposition at much lower temperatures (&gt;105°C). Therefore, the important role of LiPF 6 in the thermal stabilization of nano-Li xSi with electrolyte at temperatures < 140°C was confirmed while LiTFSI salt added to ECDEC was ineffective in the prevention of the main exothermic reaction starting at 105°C. © 2012 The Electrochemical Society.
    view abstract10.1149/2.095205jes
  • Artificially nanostructured n-type SiGe bulk thermoelectrics through plasma enhanced growth of alloy nanoparticles from the gas phase
    Stein, N. and Petermann, N. and Theissmann, R. and Schierning, G. and Schmechel, R. and Wiggers, H.
    Journal of Materials Research 26 (2011)
    SiGe alloys belong to the class of classic high temperature thermoelectric materials. By the means of nanostructuring, the performance of this well-known material can be further enhanced. Additional grain boundaries and point defects added to the alloy structure result in a strong decrease in thermal conductivity because of reduced lattice contribution to the overall thermal conductivity. Hence, the figure of merit can be increased. To obtain a nanostructured bulk material, a nanosized raw material is essential. In this work, a new approach toward nanostructured SiGe alloys is presented where alloyed nanoparticles are synthesized from a homogeneous mixture of the respective precursors in a microwave plasma reactor. As-prepared nanoparticles are compacted to a dense bulk material by a field assisted sintering technique. A figure of merit of zT = 0.5 ± 0.09 at 450 °C and a peak zT of 0.8 ± 0.15 at 1000 °C could be achieved for a nanostructured, 0.8% phosphorus-doped Si 80Ge20 alloy without any further optimization. Copyright © Materials Research Society 2011.
    view abstract10.1557/jmr.2011.117
  • Efficiency enhancement in hybrid P3HT/silicon nanocrystal solar cells
    Niesar, S. and Fabian, W. and Petermann, N. and Herrmann, D. and Riedle, E. and Wiggers, H. and Brandt, M.S. and Stutzmann, M.
    Green 1 (2011)
    Hybrid organic-inorganic solar cells from poly(3-hexylthiophene) (P3HT) and freestanding silicon nanocrystals (Si-ncs) combine the advantages of siliconbased photovoltaics with the cost-efficient solution processing technique. At present, the microwave-plasma synthesis of Si-ncs that allows for a future upscaling to industrial demands is at the expense of the Si-nc surface quality and the number of charge-trapping defects. Here, we present an enhancement of the solar cell performance by identifying the major factors which are limiting the device efficiency. With the help of low-cost post-growth treatments of the Si-ncs and the optimization of various device parameters, P3HT:Si-ncs bulk heterojunction solar cells with an efficiency up to 1.1% are achieved. In particular, etching of the Si-ncs with hydrofluoric acid to remove the surface oxide shells and surface defects has a strong impact on the solar cell performance. An intermediate Si weight ratio of around 60% is found to lead to the highest current densities. For Si-ncs with very small diameters, an additional enhancement of the open circuit voltage was observed. Moreover, we show that the structural order of P3HT has a strong influence on the efficiency, which can be explained by an improved charge carrier separation at the P3HT/Si-ncs interface in combination with an enhanced charge transport in the P3HT phase. Copyright © 2011 De Gruyter.
    view abstract10.1515/GREEN.2011.034
  • Erratum: Artificially nanostructured n-type SiGe bulk thermoelectrics through plasma enhanced growth of alloy nanoparticles from the gas phase (Journal of Materials Research (2011) 26: 15 (1872) DOI: 10.1557/jmr.2011.117)
    Stein, N. and Petermann, N. and Theissmann, R. and Schierning, G. and Schmechel, R. and Wiggers, H.
    Journal of Materials Research 26 (2011)
    view abstract10.1557/jmr.2011.311
  • Freestanding silicon quantum dots: Origin of red and blue luminescence
    Gupta, A. and Wiggers, H.
    Nanotechnology 22 (2011)
    In this paper, we studied the behavior of silicon quantum dots (Si-QDs) after etching and surface oxidation by means of photoluminescence (PL) measurements, Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance spectroscopy (EPR). We observed that etching of red luminescing Si-QDs with HF acid drastically reduces the concentration of defects and significantly enhances their PL intensity together with a small shift in the emission spectrum. Additionally, we observed the emergence of blue luminescence from Si-QDs during the re-oxidation of freshly etched particles. Our results indicate that the red emission is related to the quantum confinement effect, while the blue emission from Si-QDs is related to defect states at the newly formed silicon oxide surface. © 2011 IOP Publishing Ltd Printed in the UK & the USA.
    view abstract10.1088/0957-4484/22/5/055707
  • From nanoparticles to nanocrystalline bulk: Percolation effects in field assisted sintering of silicon nanoparticles
    Schwesig, D. and Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Wiggers, H. and Schmechel, R. and Wolf, D.E.
    Nanotechnology 22 (2011)
    Nanocrystalline bulk materials are desirable for many applications as they combine mechanical strength and specific electronic transport properties. Our bottom-up approach starts with tailored nanoparticles. Compaction and thermal treatment are crucial, but usually the final stage sintering is accompanied by rapid grain growth which spoils nanocrystallinity. For electrically conducting nanoparticles, field activated sintering techniques overcome this problem. Small grain sizes have been maintained in spite of consolidation. Nevertheless, the underlying principles, which are of high practical importance, have not been fully elucidated yet. In this combined experimental and theoretical work, we show how the developing microstructure during sintering correlates with the percolation paths of the current through the powder using highly doped silicon nanoparticles as a model system. It is possible to achieve a nanocrystalline bulk material and a homogeneous microstructure. For this, not only the generation of current paths due to compaction, but also the disintegration due to Joule heating is required. The observed density fluctuations on the micrometer scale are attributed to the heat profile of the simulated powder networks. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/22/13/135601
  • Plasma synthesis of nanostructures for improved thermoelectric properties
    Petermann, N. and Stein, N. and Schierning, G. and Theissmann, R. and Stoib, B. and Brandt, M.S. and Hecht, C. and Schulz, C. and Wiggers, H.
    Journal of Physics D: Applied Physics 44 (2011)
    The utilization of silicon-based materials for thermoelectrics is studied with respect to the synthesis and processing of doped silicon nanoparticles from gas phase plasma synthesis. It is found that plasma synthesis enables the formation of spherical, highly crystalline and soft-agglomerated materials. We discuss the requirements for the formation of dense sintered bodies, while keeping the crystallite size small. Small particles a few tens of nanometres and below that are easily achievable from plasma synthesis, and a weak surface oxidation, both lead to a pronounced sinter activity about 350 K below the temperature usually needed for the successful densification of silicon. The thermoelectric properties of our sintered materials are comparable to the best results found for nanocrystalline silicon prepared by methods other than plasma synthesis. © 2011 IOP Publishing Ltd.
    view abstract10.1088/0022-3727/44/17/174034
  • Role of oxygen on microstructure and thermoelectric properties of silicon nanocomposites
    Schierning, G. and Theissmann, R. and Stein, N. and Petermann, N. and Becker, A. and Engenhorst, M. and Kessler, V. and Geller, M. and Beckel, A. and Wiggers, H. and Schmechel, R.
    Journal of Applied Physics 110 (2011)
    Phosphorus-doped silicon nanopowder from a gas phase process was compacted by DC-current sintering in order to obtain thermoelectrically active, nanocrystalline bulk silicon. A density between 95 and 96 compared to the density of single crystalline silicon was achieved, while preserving the nanocrystalline character with an average crystallite size of best 25 nm. As a native surface oxidation of the nanopowder usually occurs during nanopowder handling, a focus of this work is on the role of oxygen on microstructure and transport properties of the nanocomposite. A characterization with transmission electron microscopy (TEM) showed that the original core/shell structure of the nanoparticles was not found within the sintered nanocomposites. Two different types of oxide precipitates could be identified by energy filtered imaging technique. For a detailed analysis, 3-dimensional tomography with reconstruction was done using a needle-shaped sample prepared by focused ion beam (FIB). The 3-dimensional distribution of silicon dioxide precipitates confirmed that the initial core/shell structure breaks down and precipitates are formed. It is further found that residual pores are exclusively located within oxide precipitates. Thermoelectric characterization was done on silicon nanocomposites sintered between 960 C and 1060 C with varying oxygen content between room temperature and 950 C. The higher sintering temperature led to a better electrical activation of the phosphorus dopant. The oxidic precipitates support densification and seem to be able to reduce the thermal conductivity therefore enhancing thermoelectric properties. A peak figure of merit, zT, of 0.5 at 950 C was measured for a sample sintered at 1060 C with a mean crystallite size of 46 nm. © 2011 American Institute of Physics.
    view abstract10.1063/1.3658021
  • Solution-processed networks of silicon nanocrystals: The role of internanocrystal medium on semiconducting behavior
    Pereira, R.N. and Niesar, S. and You, W.B. and Da Cunha, A.F. and Erhard, N. and Stegner, A.R. and Wiggers, H. and Willinger, M.-G. and Stutzmann, M. and Brandt, M.S.
    Journal of Physical Chemistry C 115 (2011)
    We have produced networks of surface-oxidized and hydrogen-terminated silicon nanocrystals (Si-NCs), both intrinsic and n-type doped, on flexible plastic foil from nanoparticle inks. The charge transport in these networks was comprehensively studied by means of time-dependent conductivity, steady-state current versus voltage characteristics, and steady-state photocurrent measurements as a function of incident light intensity. These measurements were complemented by surface chemistry and structural/morphological analysis from Fourier transform infrared spectroscopy and electron microscopy. Whereas H-terminated Si-NC networks function as semiconductors (both in air and in vacuum), where conductivity enhancement upon impurity doping and photoconductivity were observed, these characteristics are not present in networks of surface-oxidized Si-NCs. For both network types, the observation of a power law behavior for steady-state current versus voltage and a current decaying with time at constant bias indicate that charge transport is controlled by space-charge-limited current (involving trap states) via percolation paths through the networks. We have also monitored the evolution of the networks (photo)conductivity when the internanocrystal separating medium formed by Si-H bonds is progressively replaced by a native oxide upon exposure to air. Although a decrease in the (photo)conductivity is observed, the networks still behave as semiconductors even after a long-term air exposure. From an analysis of all (photo)current data, we deduce that in networks of oxidized Si-NCs inter-NC charge transfer requires the participation of oxide-related electronic states, whereas in H-terminated Si-NC networks direct inter-NC charge transfer plays a major role in the overall long-range conduction process. © 2011 American Chemical Society.
    view abstract10.1021/jp205984m
  • Stable aqueous dispersions of ZnO nanoparticles for ink-jet printed gas sensors
    Khalil, A.S.G. and Hartner, S. and Ali, M. and Wiggers, H. and Winterer, M.
    Journal of Nanoscience and Nanotechnology 11 (2011)
    For the preparation of printed devices based on ZnO nanoparticles (ZnO NPs), stable colloidal dispersions of these materials are highly desirable. ZnO NPs have been synthesized by Chemical Vapor Synthesis. The particles have a spherical shape with a narrow size distribution. Stable aqueous dispersions of the ZnO NPs have been successfully prepared after the addition of a polymeric stabilizer. These stable dispersions have been used to print ZnO NP films on interdigital gold structures on silicon by ink-jet printing. The printing parameters have been optimized for forming layers with high quality. Close-packed ZnO NP thin films with a thickness between 100-250 nm have been prepared. Impedance spectroscopy has been used to study the gas sensing properties of the printed films at different temperatures in air and in hydrogen. The impedance spectra show the semi-circles typical for semiconducting materials. The conductance of the printed films has been measured at room temperature with high accuracy. In hydrogen gas, the conductance is larger as expected and this behavior is reversible. © 2011 American Scientific Publishers.
    view abstract10.1166/jnn.2011.4043
  • Synthesis and ink-jet printing of highly luminescing silicon nanoparticles for printable electronics
    Gupta, A. and Khalil, A.S.G. and Offer, M. and Geller, M. P. and Winterer, M. and Lorke, A. and Wiggers, H.
    Journal of Nanoscience and Nanotechnology 11 (2011)
    The formation of stable colloidal dispersions of silicon nanoparticles (Si-NPs) is essential for the manufacturing of silicon based electronic and optoelectronic devices using cost-effective printing technologies. However, the development of Si-NPs based printable electronics has so far been hampered by the lack of long-term stability, low production rate and poor optical properties of Si- NPs ink. In this paper, we synthesized Si-NPs in a gas phase microwave plasma reactor with very high production rate, which were later treated to form a stable colloidal dispersion. These particles can be readily dispersed in a variety of organic solvents and the dispersion is stable for months. The particles show excellent optical properties (quantum yields of about 15%) and long-term photoluminescence (PL) stability. The stable ink containing functionalized Si-NPs was successfully used to print structures on glass substrates by ink-jet printing. The homogeneity and uniformity of large-area printed film was investigated using photoluminescence (PL) mapping. Copyright © 2011 American Scientific Publishers.
    view abstract10.1166/jnn.2011.4184
  • Synthesis of tailored WO3 and WOx (2.9 \< x \< 3) nanoparticles by adjusting the combustion conditions in a H2/O 2/Ar premixed flame reactor
    Gupta, A. and Ifeacho, P. and Schulz, C. and Wiggers, H.
    Proceedings of the Combustion Institute 33 (2011)
    Flame synthesis of WO3 and WOx (2.9 < x < 3) nanoparticles is carried out by adding a dilute concentration of WF6 as precursor in a low-pressure H2/O2/Ar premixed flame reactor. The reactor is equipped with molecular-beam sampling and particle mass spectroscopy (PMS) to determine particle composition and sizes as a function of height above burner. Varying the H2/O2 ratio allowed us to tune the stoichiometry of the product. With a H2/O2 ratio of 0.67 white colored stoichiometric WO3 is formed, whereas the H2/O2 ratio &gt;0.8 yields blue colored non-stoichiometric WOx (2.9 < x < 3) nanoparticles. The size of nanoparticles can be controlled by varying the residence time in the high-temperature zone of the reactor as observed by molecular-beam sampling with subsequent analysis using PMS. Transmission electron microscopy (TEM) images of as-synthesized nanoparticles show that particles are non-agglomerated and have an almost spherical morphology. The X-ray diffraction (XRD) pattern of the as-synthesized material indicates that the powders exhibit poor crystallinity, however, subsequent thermal annealing of the sample in air changes its structure from amorphous to crystalline phase. It is observed that particles with sub-stoichiometric composition (WOx) show higher conductivity compared to the stoichiometric WO3 sample. © 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
    view abstract10.1016/j.proci.2010.06.162
  • Defect reduction in silicon nanoparticles by low-temperature vacuum annealing
    Niesar, S. and Stegner, A.R. and Pereira, R.N. and Hoeb, M. and Wiggers, H. and Brandt, M.S. and Stutzmann, M.
    Applied Physics Letters 96 (2010)
    Using electron paramagnetic resonance, we find that vacuum annealing at 200 °C leads to a significant reduction in the silicon dangling bond (Si-db) defect density in silicon nanoparticles (Si-NPs). The best improvement of the Si-db density by a factor of 10 is obtained when the vacuum annealing is combined with an etching step in hydrofluoric acid (HF), whereas HF etching alone only removes the Si-dbs at the Si/ SiO2 interface. The reduction in the Si-db defect density is confirmed by photothermal deflection spectroscopy and photoconductivity measurements on thin Si-NPs films. © 2010 American Institute of Physics.
    view abstract10.1063/1.3428359
  • Electroluminescence from silicon nanoparticles fabricated from the gas phase
    Theis, J. and Geller, M. and Lorke, A. and Wiggers, H. and Wieck, A. and Meier, C.
    Nanotechnology 21 (2010)
    Electroluminescence from as-prepared silicon nanoparticles, fabricated by gas phase synthesis, is demonstrated. The particles are embedded between an n-doped GaAs substrate and a semitransparent indium tin oxide top electrode. The total electroluminescence intensity of the Si nanoparticles is more than a factor of three higher than the corresponding signal from the epitaxial III-V semiconductor. This, together with the low threshold voltage for electroluminescence, shows the good optical properties of these untreated particles and the efficient electrical injection into the device. Impact ionization by electrons emitted from the top electrode is identified as the origin of the electrically driven light emission. © 2010 IOP Publishing Ltd.
    view abstract10.1088/0957-4484/21/45/455201
  • Freestanding spherical silicon nanocrystals: A model system for studying confined excitons
    Goller, B. and Polisski, S. and Wiggers, H. and Kovalev, D.
    Applied Physics Letters 97 (2010)
    We report on the light emitting properties of freestanding hydrogen-terminated spherical silicon nanocrystals. The nanocrystals exhibit size-dependent tunable light emission properties. Many light emission properties of this system are found to be very similar to those known for other systems containing silicon nanocrystals. However, we found specific emission properties of this system ascribed to the spherical shape of silicon nanocrystals and their spatial separation. We attributed all observations to the spatial confinement of excitons within the crystalline core of the indirect band gap silicon nanocrystals. © 2010 American Institute of Physics.
    view abstract10.1063/1.3470103
  • Optical and electrical properties of silicon nanoparticles
    Gupta, A. and Hartner, S. and Wiggers, H.
    INEC 2010 - 2010 3rd International Nanoelectronics Conference, Proceedings (2010)
    For the fabrication of optoelectronic devices based on silicon nanoparticles (Si-NPs), it is very important to understand their optical and electrical behavior. In this paper, we present the optical and electrical properties of Si-NPs. We demonstrate that the optical properties of Si-NPs depend on their size as well as their surface chemistry. The size of Si-NPs was finely tuned by etching them in a mixture of hydrofluoric acid (HF) and nitric acid (HNO 3) for different times. The resulting Si-NPs exhibit bright luminescence across the visible spectrum. In order to stabilize the optical emission, the surface of freshly etched Si-NPs was successfully functionalized with organic molecules. As the surface chemistry is also expected to strongly influence the electrical transport between Si-NPs and therefore the electrical properties of Si-NP ensembles, the conductivity of pellets consisting of Si-NPs was measured using impedance spectroscopy. The surface oxide of Si-NPs was removed by etching them with HF acid. The freshly etched Si-NPs showed much higher conductivity compared to as-prepared samples. The surface functionalization of freshly etched Si-NPs slightly decreases their conductivity. However, it was observed that the conductivity was still much higher compared to as-prepared samples. ©2010 IEEE.
    view abstract10.1109/INEC.2010.5424734
  • Silicon nanocrystals dispersed in water: Photosensitizers for molecular oxygen
    Goller, B. and Polisski, S. and Wiggers, H. and Kovalev, D.
    Applied Physics Letters 96 (2010)
    We report on the synthesis of freestanding silicon spheres having sizes in the range of 3-10 nm. As-prepared luminescent silicon nanocrystals have H-passivated surface. Therefore, energy transfer from excitons confined in Si nanocrystals to oxygen molecules is found to be efficient. It is demonstrated that a termination of silicon nanocrystal H-passivated hydrophobic surface by lipids provides their water solubility. We found that this procedure preserves photosensitizing ability of silicon nanocrystals. Therefore, this material system can potentially be employed for a variety of biomedical applications. © 2010 American Institute of Physics.
    view abstract10.1063/1.3432349
  • Stable aqueous dispersions of ZnO nanoparticles for ink-jet printed gas sensors
    Khalil, A.S.G. and Hartner, S. and Ali, M. and Gupta, A. and Wiggers, H. and Winterer, M.
    INEC 2010 - 2010 3rd International Nanoelectronics Conference, Proceedings (2010)
    For the preparation of printable devices based on ZnO nanoparticles (ZnO NP), stable colloidal dispersions of these materials are highly desirable. ZnO NP have been synthesized by Chemical Vapor Synthesis. The particles have a spherical shape with a narrow size distribution. Stable aqueous dispersions of the ZnO NP have been successfully prepared after the addition of a polymeric stabilizer. The prepared dispersions are stable for at least 2 months without observable sedimentation. These stable dispersions are used to prepare ZnO NP films on different substrates by ink-jet printing. The viscosity and the surface tension of the dispersion as well as the printing parameters have been optimized for forming layers with high quality. Dense and low porosity layers of ZnO NP with a thickness between 100-250 nm have been prepared on different substrates. First measurements on ink-jet printed ZnO films are done on self fabricated inter digital capacitors (IDCs) at room temperature. The ZnO films show resistivity at room temperature of 7.76 kΩ.cm. For sensing measurements in hydrogen atmosphere, the sheet resistance decreases rapidly until it reaching metallic behavior. This behavior is reversible. ©2010 IEEE.
    view abstract10.1109/INEC.2010.5424503
  • Stable colloidal dispersions of silicon nanoparticles for the fabrication of films using inkjet printing technology
    Gupta, A. and Khalil, A.S.G. and Winterer, M. and Wiggers, H.
    INEC 2010 - 2010 3rd International Nanoelectronics Conference, Proceedings (2010)
    The formation of stable colloidal dispersions of nanoparticles is essential for the manufacture of electronic and optoelectronic devices using cost-effective printing technologies, In this study, we examined the stability of silicon nanoparticles (Si-NPs) in aqueous medium at different pH. The Si-NPs show high zeta potential values within pH = 6.5 - 8.5. In addition, the Si-NPs do not show any isoelectric point in the pH range studied, It IS observed that the stability of Si-NPs in aqueous medium increases after the addition of ethanol. In order to stabilize Si-NPs in organic solvents, their surface is functionalized with alkyl groups via a thermally induced alkylation process. The functionalized Si-NPs form nice, transparent dispersions in a variety of organic solvents and no sedimentation of functionalized samples was observed over any period of time. Fabricating films of Si-NPs using inkjet printing is currently under investigation. ©2010 IEEE.
    view abstract10.1109/INEC.2010.5425068
  • batteries

  • electrical properties of mesoscopic systems

  • flame synthesis

  • gas-phase synthesis

  • gas-phase synthesis

  • lithium-ion batteries

  • mesoscopic systems

  • nanoparticles

  • sensors

  • spectroscopy

  • synthesis

  • thermoelectrics

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