Prof. Dr. Gabi Schierning

Applied Quantum Materials
University of Duisburg-Essen


  • Density-Dependence of Surface Transport in Tellurium-Enriched Nanograined Bulk Bi2Te3
    Izadi, S. and Bhattacharya, A. and Salloum, S. and Han, J.W. and Schnatmann, L. and Wolff, U. and Perez, N. and Bendt, G. and Ennen, I. and Hütten, A. and Nielsch, K. and Schulz, S. and Mittendorff, M. and Schierning, G.
    Small 19 (2023)
    view abstract10.1002/smll.202204850
  • The role of electrons during the martensitic phase transformation in NiTi-based shape memory alloys
    Kunzmann, A. and Frenzel, J. and Wolff, U. and Han, J.W. and Giebeler, L. and Piorunek, D. and Mittendorff, M. and Scheiter, J. and Reith, H. and Perez, N. and Nielsch, K. and Eggeler, G. and Schierning, G.
    Materials Today Physics 24 (2022)
    view abstract10.1016/j.mtphys.2022.100671
  • Influence of Nanoparticle Processing on the Thermoelectric Properties of (BixSb1−X)2Te3 Ternary Alloys
    Salloum, S. and Bendt, G. and Heidelmann, M. and Loza, K. and Bayesteh, S. and Sepideh Izadi, M. and Kawulok, P. and He, R. and Schlörb, H. and Perez, N. and Reith, H. and Nielsch, K. and Schierning, G. and Schulz, S.
    ChemistryOpen (2021)
    The synthesis of phase-pure ternary solutions of tetradymite-type materials (BixSb1−x)2Te3 (x=0.25; 0.50; 0.75) in an ionic liquid approach has been carried out. The nanoparticles are characterized by means of energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), and transmission electron microscopy. In addition, the role of different processing approaches on the thermoelectric properties - Seebeck coefficient as well as electrical and thermal conductivity - is demonstrated. © 2021 The Authors. Published by The Chemical Society of Japan & Wiley-VCH GmbH
    view abstract10.1002/open.202000257
  • Interface-Dominated Topological Transport in Nanograined Bulk Bi2Te3
    Izadi, S. and Han, J.W. and Salloum, S. and Wolff, U. and Schnatmann, L. and Asaithambi, A. and Matschy, S. and Schlörb, H. and Reith, H. and Perez, N. and Nielsch, K. and Schulz, S. and Mittendorff, M. and Schierning, G.
    Small 17 (2021)
    3D topological insulators (TI) host surface carriers with extremely high mobility. However, their transport properties are typically dominated by bulk carriers that outnumber the surface carriers by orders of magnitude. A strategy is herein presented to overcome the problem of bulk carrier domination by using 3D TI nanoparticles, which are compacted by hot pressing to macroscopic nanograined bulk samples. Bi2Te3 nanoparticles well known for their excellent thermoelectric and 3D TI properties serve as the model system. As key enabler for this approach, a specific synthesis is applied that creates nanoparticles with a low level of impurities and surface contamination. The compacted nanograined bulk contains a high number of interfaces and grain boundaries. Here it is shown that these samples exhibit metallic-like electrical transport properties and a distinct weak antilocalization. A downward trend in the electrical resistivity at temperatures below 5 K is attributed to an increase in the coherence length by applying the Hikami–Larkin–Nagaoka model. THz time-domain spectroscopy reveals a dominance of the surface transport at low frequencies with a mobility of above 103 cm2 V−1 s−1 even at room temperature. These findings clearly demonstrate that nanograined bulk Bi2Te3 features surface carrier properties that are of importance for technical applications. © 2021 The Authors. Small published by Wiley-VCH GmbH
    view abstract10.1002/smll.202103281
  • Ionic Liquid-Based Low-Temperature Synthesis of Phase-Pure Tetradymite-Type Materials and Their Thermoelectric Properties
    Loor, M. and Salloum, S. and Kawulok, P. and Izadi, S. and Bendt, G. and Guschlbauer, J. and Sundermeyer, J. and Perez, N. and Nielsch, K. and Schierning, G. and Schulz, S.
    Inorganic Chemistry 59 (2020)
    Phase-pure crystalline Bi2Se3 and Bi2Te3 nanoparticles are formed in reactions of [C4C1Im]3[Bi3I12] (C4C1Im = 1-butyl-3-methylimidazolium) with [C4C1Pyr][ESiMe3] (E = Se or Te; C4C1Pyr = 1-butyl-1-methylpyrrolidinium) in the ionic liquid (IL) [C4C1Im]I. The resulting crystalline tetradymite-type nanoparticles exhibit stoichiometric Bi:E (E = Se or Te) molar ratios (2:3). Because all synthetic steps were performed under strict inert gas conditions, the surfaces of the Bi2Se3 and Bi2Te3 nanoparticles are free of metal oxide species. As proven by infrared and X-ray photoelectron spectroscopy analyses, the nanoparticle surfaces reveal only minor organic contamination from solvent residues ([C4C1Im]I). The nanomaterials show high Seebeck coefficients of -124 μV K-1 (Bi2Se3) and -155 μV K-1 (Bi2Te3) and feature high electrical conductivities (328 and 946 S cm-1, respectively) at the highest tested temperature (240 °C). The corresponding thermal conductivities (0.8 and 2.3 W m-1 K-1, respectively, at 30 °C) are comparable to those of single crystals and recently reported ab initio calculations, which is in remarkable contrast to typical findings of nanograined bulk materials obtained from compacted nanoparticles. These findings emphasize the low level of impurities, surface contamination, and, in general, defects produced by the synthetic approach reported here. The figure of merit in the in-plane direction of the compacted pellets reached peak values 0.45 for Bi2Se3 and 0.4 for Bi2Te3. © 2020 American Chemical Society.
    view abstract10.1021/acs.inorgchem.9b03060
  • 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
  • Structural and thermoelectrical characterization of epitaxial Sb2Te3 high quality thin films grown by thermal evaporation
    Bendt, G. and Kaiser, K. and Heckel, A. and Rieger, F. and Oing, D. and Lorke, A. and Rodriguez, N.P. and Schierning, G. and Jooss, C. and Schulz, S.
    Semiconductor Science and Technology 33 (2018)
    Thermal evaporation of Sb2Te3 powder was systematically studied under various pressure and temperature conditions. Low pressure experiments (5 •10-6 mbar) conducted inside a horizontal tube reactor at a temperature range of 500 °C-600 °C generated rough polycrystalline films on Si(100) substrates. Based on these experiments, the chemical composition of the resulting films were determined by the furnace temperature. Enhancing the reactor pressure to 20 mbar shifted the growth zone towards higher temperature ranges and yielded highly c-oriented Sb2Te3 films on Si(100) and Al2O3(0001) substrates. Additional experiments were conducted inside a special reactor containing two independent heaters to study the effects of the evaporator and substrate temperatures independently. In contrast to the samples generated in the previous reactor, a two-zone heating reactor allowed the growth of epitaxial Sb2Te3 films with a very smooth surface topology on Al2O3(0001) substrates, as shown by SEM, EDX, XPS, and HRTEM. The electrical in-plane conductivity of the Sb2Te3 films decreased with increasing temperature, ultimately reaching 3950 S •cm-1 at 300 K. The films showed a p-type carrier concentration of 4.3 •10-19 cm-3 at 300 K and a very high carrier mobility of 558 cm2 •V-1 •s-1. The Seebeck coefficient increased monotonically from 94 μV •K-1 at 270 K to 127 μV •K-1 at 420 K. © 2018 IOP Publishing Ltd.
    view abstract10.1088/1361-6641/aad7a3
  • Improving the zT value of thermoelectrics by nanostructuring: tuning the nanoparticle morphology of Sb2Te3 by using ionic liquids
    Schaumann, J. and Loor, M. and Ünal, D. and Mudring, A. and Heimann, S. and Hagemann, U. and Schulz, S. and Maculewicz, F. and Schierning, G.
    Dalton Transactions 46 (2017)
    A systematic study on the microwave-assisted thermolysis of the single source precursor (Et2Sb)2Te (1) in different asymmetric 1-alkyl-3-methylimidazolium- and symmetric 1,3-dialkylimidazolium-based ionic liquids (ILs) reveals the distinctive role of both the anion and the cation in tuning the morphology and microstructure of the resulting Sb2Te3 nanoparticles as evidenced by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS). A comparison of the electrical and thermal conductivities as well as the Seebeck coefficient of the Sb2Te3 nanoparticles obtained from different ILs reveals the strong influence of the specific IL, from which C4mimI was identified as the best solvent, on the thermoelectric properties of as-prepared nanosized Sb2Te3. This work provides design guidelines for ILs, which allow the synthesis of nanostructured thermoelectrics with improved performances. © The Royal Society of Chemistry.
    view abstract10.1039/c6dt04323b
  • 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
  • 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
  • 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
  • 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
  • Record figure of merit values of highly stoichiometric Sb2Te3 porous bulk synthesized from tailor-made molecular precursors in ionic liquids
    Heimann, S. and Schulz, S. and Schaumann, J. and Mudring, A. and Stötzel, J. and MacUlewicz, F. and Schierning, G.
    Journal of Materials Chemistry C 3 (2015)
    We report on the synthesis of Sb2Te3 nanoparticles with record-high figure of merit values of up to 1.5. The central thermoelectric parameters, electrical conductivity, thermal conductivity and Seebeck coefficient, were independently optimized. The critical influence of porosity for the fabrication of highly efficient thermoelectric materials is firstly demonstrated, giving a strong guidance for the optimization of other thermoelectric materials. © The Royal Society of Chemistry 2015.
    view abstract10.1039/c5tc01248a
  • 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
  • Thermoelectric properties of pulsed current sintered nanocrystalline Al-doped ZnO by chemical vapour synthesis
    Gautam, D. and Engenhorst, M. and Schilling, C. and Schierning, G. and Schmechel, R. and Winterer, M.
    Journal of Materials Chemistry A 3 (2015)
    ZnO is a promising n-type oxide thermoelectric material, which is stable in air at elevated temperatures. In the present study, we report the bottom-up approach to create Al-doped ZnO nanocomposites from nanopowders, which are prepared by chemical vapour synthesis. With our synthesis route, we are able to create highly doped Al-containing ZnO nanocomposites that exhibit bulk-like electrical conductivity. Moreover, the impact of the microstructure of the nanocomposites on their thermal conductivity is enormous, with a value of 1.0 W m-1 K-1 for 1% Al-ZnO at room temperature, which is one of the lowest values reported, to date, on ZnO nanocomposites. The optimization of the Al-doping and microstructure with respect to the transport properties of bulk Al-ZnO nanocomposites leads to a zT value of about 0.24 at 950 K, underlining the potential of our technique. This journal is © The Royal Society of Chemistry 2015.
    view abstract10.1039/c4ta04355c
  • Thermoelectric transport properties of boron-doped nanocrystalline diamond foils
    Engenhorst, M. and Fecher, J. and Notthoff, C. and Schierning, G. and Schmechel, R. and Rosiwal, S.M.
    Carbon 81 (2015)
    Natural diamond is known for its outstanding thermal conductivity and electrical insulation. However, synthetic production allows for doping and tailoring microstructural and transport properties. Despite some motivation in the literature and the ongoing search for abundant and non-toxic thermoelectric materials, the first experimental study on a set of eight substrate-free boron-doped nanocrystalline diamond foils is presented herein. All transport coefficients were determined in the same direction within the same foils over a broad temperature range up to 900 °C. It is found that nanostructuring reduces the thermal conductivity by two orders of magnitude, but the mobility decreases significantly to around 1 cm2 V-1 s-1, too. Although degenerate transport can be concluded from the temperature dependence of the Seebeck coefficient, charge carriers notably scatter at grain boundaries where sp2-carbon modifications and amorphous boron-rich phases form during synthesis. A detailed analysis of doping efficiency yields an acceptor fraction of only 8-18 at%, meaning that during synthesis excess boron thermodynamically prefers electrically inactive sites. Decent power factors above 10-4Wm-1 K-2 at 900 °C are found despite the low mobility, and a Jonker-type analysis grants a deeper insight into this issue. Together with the high thermal conductivity, the thermoelectric figure of merit zT does not exceed 0.01 at 900 °C. © 2014 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2014.10.002
  • 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
  • Fabrication of high-temperature-stable thermoelectric generator modules based on nanocrystalline silicon
    Kessler, V. and Dehnen, M. and Chavez, R. and Engenhorst, M. and Stoetzel, J. and Petermann, N. and Hesse, K. and Huelser, T. and Spree, M. and Stiewe, C. and Ziolkowski, P. and Schierning, G. and Schmechel, R.
    Journal of Electronic Materials 43 (2014)
    High-temperature-stable thermoelectric generator modules (TGMs) based on nanocrystalline silicon have been fabricated, characterized by the Harman technique, and measured in a generator test facility at the German Aerospace Center. Starting with highly doped p- and n-type silicon nanoparticles from a scalable gas-phase process, nanocrystalline bulk silicon was obtained using a current-activated sintering technique. Electrochemical plating methods were employed to metalize the nanocrystalline silicon. The specific electrical contact resistance ρ c of the semiconductor-metal interface was characterized by a transfer length method. Values as low as ρ c < 1 × 10-6 Ω cm2 were measured. The device figure of merit of a TGM with 64 legs was approximately ZT = 0.13 at 600°C as measured by the Harman technique. Using a generator test facility, the maximum electrical power output of a TGM with 100 legs was measured to be roughly 1 W at hot-side temperature of 600°C and cold-side temperature of 300°C. © 2014 TMS.
    view abstract10.1007/s11664-014-3093-6
  • 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
  • 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
  • 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
  • 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
  • A thermoelectric generator concept using a p-n junction: Experimental proof of principle
    Becker, A. and Chavez, R. and Petermann, N. and Schierning, G. and Schmechel, R.
    Journal of Electronic Materials 42 (2013)
    Conventional thermoelectric generators (TEGs) use single p- and n-doped legs for thermoelectric energy harvesting. We explore a concept using thermoelectric p-n junctions made from densified silicon nanoparticles. The nanoparticle powder was synthesized in a microwave plasma reactor using silane, diborane and phosphine as precursors. To achieve a bulk sample with a p-n junction, a layer of boron-doped nanoparticle powder was stacked on a layer of phosphorus-doped powder and compacted by current-activated pressure- assisted densification. To use the p-n structure as a TEG, a temperature gradient was applied along the p-n junction. It is expected that this temperature gradient leads to electron-hole pair generation and separation in the junction and diffusion of the charge carriers. A reference method was used to characterize the open-circuit voltage of the p-n junction TEG. © 2013 TMS.
    view abstract10.1007/s11664-012-2399-5
  • 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
  • Electrical contact resistance of electroless nickel to nanocrystalline silicon and the fabrication of a thermoelectric generator
    Kessler, V. and Dehnen, M. and Chavez, R. and Engenhorst, M. and Stoetzel, J. and Petermann, N. and Hesse, K. and Huelser, T. and Spree, M. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1553 (2013)
    We present the fabrication of a high-temperature stable thermoelectric generator based on nanocrystalline silicon. Highly doped silicon nanoparticles were sintered by a current activated sintering technique to get nanocrystalline bulk silicon. The metalization of silicon was realized by (electro-)chemical plating and the specific electrical contact resistance ρc of the semiconductor-metal interface was measured by a transfer length method. Values as low as \rho -C < 1 \cdot 10^{ - 6} \,\Omega cm-2 were measured. The metalized nanocrystalline silicon legs were sintered to metalized ceramic substrates using a silver-based sinter paste. The device figure of merit of the thermoelectric generator was determined by a Harman measurement with a maximum ZT of approximately 0.13 at 600 °C. Copyright © Materials Research Society 2013.
    view abstract10.1557/opl.2013.863
  • Impact of rapid thermal annealing on thermoelectric properties of bulk nanostructured zinc oxide
    Engenhorst, M. and Gautam, D. and Schilling, C. and Winterer, M. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1543 (2013)
    In search for non-toxic thermoelectric materials that are stable in air at elevated temperatures, zinc oxide has been shown to be one of only few efficient n-type oxidic materials. Our bottom-up approach starts with very small (< 10 nm) Al-doped ZnO nanoparticles prepared from organometallic precursors by chemical vapor synthesis using nominal doping concentrations of 2 at% and 8 at%. In order to obtain bulk nanostructured solids, the powders were compacted in a current-activated pressure-assisted densification process. Rapid thermal annealing was studied systematically as a means of further dopant activation. The thermoelectric properties are evaluated with regard to charge carrier concentration and mobility. A Jonker-type analysis reveals the potential of our approach to achieve high power factors. In the present study, power factors larger than 4× 10-4 Wm-1K-2 were measured at temperatures higher than 600 °C. © 2013 Materials Research Society.
    view abstract10.1557/opl.2013.932
  • Note: High resolution alternating current/direct current Harman technique
    Chavez, R. and Becker, A. and Bartel, M. and Kessler, V. and Schierning, G. and Schmechel, R.
    Review of Scientific Instruments 84 (2013)
    This note describes the construction and engineering of a high precision Harman set-up for metrology of the thermoelectric figure of merit (ZT) of modules and materials based on steady state AC and DC measurements. The Harman technique presented in this article has a resolution of milli-ZT and it does not employ lock-in amplifiers or AC bridges; rather, the technique is developed to avoid typical complications experienced in AC Harman systems. By one-time reference measurements the best operation point for the system is chosen, minimizing the effects of capacitive loads due to AC signals. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4825118
  • Simulation of current-activated pressure-assisted densification
    Angst, S. and Schierning, G. and Wolf, D.E.
    AIP Conference Proceedings 1542 (2013)
    Cohe sive particles usually form very porous agglomerates. They support loads up to a consolidation pressure, which increases with decreasing particle size. Compaction of nano-powders can therefore be very costly and time consuming. If the particles are electrically conducting, which is the case e.g. for novel nano-structured thermoelectric materials, the technique of current-activated pressure-assisted densification (CAPAD) turns out to have many advantages. Electrical power deposited locally as Joule heat lowers the consolidation pressure such that higher densities without much coarsening are obtained. We present a new model combining particle dynamics, calculated by molecular dynamic methods, with a network model including thermoelectric properties. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4812001
  • 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
  • Influence of annealing atmospheres and synthetic air treatment on solution processed zinc oxide thin film transistors
    Busch, C. and Schierning, G. and Theissmann, R. and Schmechel, R.
    Journal of Applied Physics 112 (2012)
    Thin film transistors (TFTs) based on active layers of zinc oxide prepared from a solution process were fabricated under different annealing conditions. The influence of the annealing gas as well as the influence of a subsequent exposure to synthetic air to the device properties is considered. Annealing under N 2 or H 2 atmosphere leads to a strong negative threshold voltage shift. With respect to known defect states in ZnO, two different donor states are suggested to be responsible for the negative threshold voltage. A subsequent synthetic air treatment causes in general a positive threshold voltage shift. However, transistors annealed under H 2 degrade very fast under synthetic air in contrast to transistors annealed under N 2. In order to obtain more information about the density of states (DOS) distribution, a transistor model for thin film transistors in the hopping transport regime (Vissenberg model) was utilized. For positive threshold voltages, the DOS distribution is independent from the gas treatment and the threshold voltage within the experimental accuracy. This indicates a shift of the Fermi-level within an exponentially decaying DOS. The change in the charge carrier density is either due to shallow donors or due to a charge transfer with acceptors at the surface. In contrast, for negative threshold voltages, the DOS distribution parameter rises, indicating a flatter DOS distribution. We suggest that the difference is due to the change from accumulation mode to the depletion mode of the device. © 2012 American Institute of Physics.
    view abstract10.1063/1.4742976
  • 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
  • Synthesis of hexagonal Sb 2Te 3 nanoplates by thermal decomposition of the single-source precursor (Et 2Sb) 2Te.
    Schulz, S. and Heimann, S. and Friedrich, J. and Engenhorst, M. and Schierning, G. and Assenmacher, W.
    Chemistry of Materials 24 (2012)
    The size-selective synthesis of hexagonal Sb 2Te 3 nanoplates by thermal decomposition of the single-source precursor bis(diethylstibino)telluride (Et 2Sb) 2Te is described for the first time. The role of the thermolysis temperature and the concentration of the capping agent (PVP*) on the growth of the nanoplates was investigated. The thermal properties of (Et 2Sb) 2Te were investigated by differential scanning calorimetry (DSC), and the resulting Sb 2Te 3 nanoplates were characterized by XRD, SEM, TEM, EDX, and SAED. Moreover, electrical conductivity, Seebeck coefficient and thermal conductivity of the nanoplates were determined, clearly proving the enhanced thermoelectric properties of nanosized antimony telluride. © 2012 American Chemical Society.
    view abstract10.1021/cm301259u
  • 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
  • Thermoreflectance imaging of percolation effects and dynamic resistance in indium tin oxide nanoparticle layers
    Chavez, R. and Angst, S. and Maize, K. and Gondorf, A. and Schierning, G. and Wolf, D.E. and Lorke, A. and Shakouri, A.
    Journal of Applied Physics 112 (2012)
    Thin films of indium tin oxide nanoparticles are studied using charge-coupled device thoermoreflectance. High resolution sub-micron thermal images confirm that percolation in current conduction induces strongly inhomogeneous heat loads on the thin film. We experimentally show that the inhomogeneous current densities induce thousands of micro-hotspots that can be 20 hotter than the average Joule heating in the thin film layer and show comparable behavior in a resistor network. In addition to the percolation induced micro-hotspots, we report major hotspots, with non-Joule behavior, whose temperature response is greater than I 2. We demonstrate that a temperature dependent resistor can account for an effective exponent larger than 2. Finally, it is shown that while ambient molecules modify the thin film conductivity by at least 20, current conduction and percolation effects remain largely unchanged, but such chemical reactions can be nonetheless detected with thermoreflectance. © 2012 American Institute of Physics.
    view abstract10.1063/1.4757960
  • Thin-film transistors with a channel composed of semiconducting metal oxide nanoparticles deposited from the gas phase
    Busch, C. and Schierning, G. and Theissmann, R. and Nedic, A. and Kruis, F.E. and Schmechel, R.
    Journal of Nanoparticle Research 14 (2012)
    The fabrication of semiconducting functional layers using low-temperature processes is of high interest for flexible printable electronics applications. Here, the one-step deposition of semiconducting nanoparticles from the gas phase for an active layer within a thin-film transistor is described. Layers of semiconducting nanoparticles with a particle size between 10 and 25 nm were prepared by the use of a simple aerosol deposition system, excluding potentially unwanted technological procedures like substrate heating or the use of solvents. The nanoparticles were deposited directly onto standard thin-film transistor test devices, using thermally grown silicon oxide as gate dielectric. Proof-of-principle experiments were done deploying two different wide-band gap semiconducting oxides, tin oxide, SnO x, and indium oxide, In 2O 3. The tin oxide spots prepared from the gas phase were too conducting to be used as channel material in thin-film transistors, most probably due to a high concentration of oxygen defects. Using indium oxide nanoparticles, thin-film transistor devices with significant field effect were obtained. Even though the electron mobility of the investigated devices was only in the range of 10 -6 cm 2V -1s -1, the operability of this method for the fabrication of transistors was demonstrated. With respect to the possibilities to control the particle size and layer morphology in situ during deposition, improvements are expected. © 2012 Springer Science+Business Media B.V.
    view abstract10.1007/s11051-012-0888-3
  • 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
  • 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
  • High performance low temperature solution-processed zinc oxide thin film transistor
    Theissmann, R. and Bubel, S. and Sanlialp, M. and Busch, C. and Schierning, G. and Schmechel, R.
    Thin Solid Films 519 (2011)
    Amorphous zinc oxide thin films have been processed out of an aqueous solution applying a one step synthesis procedure. For this, zinc oxide containing crystalline water (ZnO· × H2O) is dissolved in aqueous ammonia (NH3), making use of the higher solubility of ZnO· × H2O compared with the commonly used zinc oxide. Characteristically, as-produced layers have a thickness of below 10 nm. The films have been probed in standard thin film transistor devices, using silicon dioxide as dielectric layer. Keeping the maximum process temperature at 125 °C, a device mobility of 0.25 cm2V- 1s- 1 at an on/off ratio of 106 was demonstrated. At an annealing temperature of 300 °C, the performance could be optimized up to a mobility of 0.8 cm2V- 1s- 1. © 2011 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.tsf.2011.02.073
  • Influence of the annealing atmosphere on solution based zinc oxide thin film transistors
    Busch, C. and Theissmann, R. and Bubel, S. and Schierning, G. and Schmechel, R.
    Materials Research Society Symposium Proceedings 1359 (2011)
    Zinc oxide layers with a thickness of less than 10 nanometers have been synthesized from an aqueous solution for the application as active layer in thin film transistors. They have been conditioned by applying different oxidizing and reducing atmospheres during an annealing process at a temperature of 125°C. It is shown that the charge carrier mobility and threshold voltage is strongly influenced by the annealing atmosphere. Samples annealed in 10% forming gas (H 2 in N 2 - reducing atmosphere) show the highest field-effect-mobility of 0.6 cm 2V -1s -1, but no saturation of the drain current, due to a high free carrier concentration. Samples treated under oxygen (strongest oxidizing atmosphere) show significantly lower mobilities. Subsequently, the samples have been exposed to synthetic air, with varying exposure times. Samples which have been annealed under hydrogen atmospheres show a pronounced decay of the drain current if exposed to synthetic air, whereas all samples conditioned under hydrogen-free atmospheres are significantly more stable under synthetic air. This enhanced sensitivity against oxygen after hydrogen treatment is attributed to residual hydrogen content in the sample that supports the formation of OH-groups which act as electron acceptors. © 2011 Materials Research Society.
    view abstract10.1557/opl.2011.754
  • 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

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