Prof. Dr. Nils Hartmann
University of Duisburg-Essen
- Segregation-Enhanced Epitaxy of Borophene on Ir(111) by Thermal Decomposition of Borazine
Omambac, K.M. and Petrović, M. and Bampoulis, P. and Brand, C. and Kriegel, M.A. and Dreher, P. and Janoschka, D. and Hagemann, U. and Hartmann, N. and Valerius, P. and Michely, T. and Meyer Zu Heringdorf, F.J. and Horn-von Hoegen, M.
ACS Nano 15 (2021)Like other 2D materials, the boron-based borophene exhibits interesting structural and electronic properties. While borophene is typically prepared by molecular beam epitaxy, we report here on an alternative way of synthesizing large single-phase borophene domains by segregation-enhanced epitaxy. X-ray photoelectron spectroscopy shows that borazine dosing at 1100 °C onto Ir(111) yields a boron-rich surface without traces of nitrogen. At high temperatures, the borazine thermally decomposes, nitrogen desorbs, and boron diffuses into the substrate. Using time-of-flight secondary ion mass spectrometry, we show that during cooldown the subsurface boron segregates back to the surface where it forms borophene. In this case, electron diffraction reveals a (6 × 2) reconstructed borophene χ6-polymorph, and scanning tunneling spectroscopy suggests a Dirac-like behavior. Studying the kinetics of borophene formation in low energy electron microscopy shows that surface steps are bunched during the borophene formation, resulting in elongated and extended borophene domains with exceptional structural order. ©
view abstract 10.1021/acsnano.1c00819
- The Effect of Water on the 2-Propanol Oxidation Activity of Co-Substituted LaFe1−CoxO3 Perovskites
Dreyer, M. and Cruz, D. and Hagemann, U. and Zeller, P. and Heidelmann, M. and Salamon, S. and Landers, J. and Rabe, A. and Ortega, K.F. and Najafishirtari, S. and Wende, H. and Hartmann, N. and Knop-Gericke, A. and Schlögl, R. and Behrens, M.
Chemistry - A European Journal (2021)Perovskites are interesting oxidation catalysts due to their chemical flexibility enabling the tuning of several properties. In this work, we synthesized LaFe1−xCoxO3 catalysts by co-precipitation and thermal decomposition, characterized them thoroughly and studied their 2-propanol oxidation activity under dry and wet conditions to bridge the knowledge gap between gas and liquid phase reactions. Transient tests showed a highly active, unstable low-temperature (LT) reaction channel in conversion profiles and a stable, less-active high-temperature (HT) channel. Cobalt incorporation had a positive effect on the activity. The effect of water was negative on the LT channel, whereas the HT channel activity was boosted for x>0.15. The boost may originate from a slower deactivation rate of the Co3+ sites under wet conditions and a higher amount of hydroxide species on the surface comparing wet to dry feeds. Water addition resulted in a slower deactivation for Co-rich catalysts and higher activity in the HT channel state. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
view abstract 10.1002/chem.202102791
- Experimental evidence for the separation of thermally excited bipolar charge carries within a p-n junction: A new approach to thermoelectric materials and generators
MacUlewicz, F. and Wagner, T. and Arzi, K. and Hartmann, N. and Weimann, N. and Schmechel, R.
Journal of Applied Physics 125 (2019)Common thermoelectric generators are based on the Seebeck effect, which describes the thermal diffusion of majority charge carriers within a temperature gradient in a solid. It is a unipolar transport phenomenon that gets suppressed if bipolar charge carriers occur. Here, we demonstrate by experiments that thermally excited bipolar charge carriers can be separated by the built-in field without external bias within a p-n junction. Such a phenomenon has been predicted theoretically before but was never proven experimentally. In the experiment, a nominal intrinsic silicon wafer (doping concentration less than 1013cm-3) was inserted in a p-i-n structure. It could be shown that electric power can be extracted from the space charge region (i-region), while the conventional thermoelectric contribution from the p-and n-regions is suppressed by short-circuiting. While the measured and simulated overall Seebeck effect of intrinsic silicon predicts a zero crossing of output power with increasing hot-side temperature due to a transition from p-type to n-type transport, the measured and simulated output power of the p-i-n structure increases monotonically with increasing hot-side temperature, indicating clearly the different nature of both mechanisms. © 2019 Author(s).
view abstract 10.1063/1.5081998
- Lead diffusion in CaTiO3: A combined study using Rutherford backscattering and TOF-SIMS for depth profiling to reveal the role of lattice strain in diffusion processes
Beyer, C. and Dohmen, R. and Rogalla, D. and Becker, H.-W. and Marquardt, K. and Vollmer, C. and Hagemann, U. and Hartmann, N. and Chakraborty, S.
American Mineralogist 104 (2019)We present experimental data on the diffusivity of Pb in CaTiO3 perovskite, which is commonly used for dating kimberlites and carbonatites. Experiments were performed on oriented synthetic and natural CaTiO3 single crystals. The Pb-source was either a laser deposited (Ca0.83Pb0.07)Ti1.05O3 thin film or a (Ca0.9Pb0.1)TiO3 powder reservoir. The crystals were annealed in a high-temperature furnace between 736 and 1135 °C and for durations from 2 to 283 h. The diffusion profiles were measured with Rutherford backscattering and time-of-flight secondary ion mass spectrometry in the depth-profiling mode. The concentration profiles measured on the same samples with the two analytical methods are in agreement. The measured concentration profiles show two regions - a steep gradient at the diffusion interface that transitions sharply (at ~50 to 150 nm from the surface) to a low concentration tail that penetrates deeper into the crystal. This diffusion behavior could be modeled best using diffusion coefficients that are a function of the Pb concentration, with a different set of diffusion coefficient for the high and the low concentration region of the profile, respectively. The diffusion coefficients extracted from the thin film and powder source experiments are similar within uncertainties. Pb diffuses slower at concentrations between 8.5 and 0.6 wt% and 1.6 to 2.6 log units faster below ~0.5 wt% Pb. Temperature dependency for each region is discussed in the text, and the Arrhenius relation for the fast diffusion regime that is most relevant for natural samples is DPbfast=2.5×10-13×exp(-158(24)kJ/mol/RT)m2/s. $$begin array D- text Pb- text fast=2.5 times 10- -13times exp Big(-158big(24big)text kJ/mol/RTBig)m-2/s. end array $$ We found a distinct change in the structure of CaTiO3 in the surface region of the single crystal that is coincidental with the change in diffusivity. This initial region is dominated by planar defects. We propose that Pb is trapped in planar defects that have formed due to the high strain introduced into the perovskite structure caused by the mismatch in the ionic radius between Ca2+ and Pb2+. The activation energies obtained here yield closure temperature for Pb in CaTiO3 between 300 and 400 °C for a range of different cooling scenarios if diffusive resetting of Pb in CaTiO3 occurs at all. At typical cooling rates of hours to days for ascending kimberlite, the age of crystal growth is preserved, with closure temperatures similar to the magma temperature. © 2019 Walter de Gruyter GmbH, Berlin/Boston 2019.
view abstract 10.2138/am-2019-6730
- UV-light assisted patterned metallization of textile fabrics
Bahners, T. and Gebert, B. and Prager, A. and Hartmann, N. and Hagemann, U. and Gutmann, J.S.
Applied Surface Science 436 (2018)A UV-assisted process allows full-faced or local deposition of silver domains on textiles made of natural as well as synthetic fibers, which act as nuclei for subsequent galvanic metallization. SEM and XPS analyses indicate that the process generates particulate depositions – particles, aggregates – of elementary silver. Masking the UV irradiation confines silver deposition strictly to the exposed areas thus allowing patterning. Adhesion of the deposited silver is high on the studied natural fiber cotton and polyamide fibers. Adhesion on smooth and chemically inert synthethic fibers such as, e.g., poly(ethylene terephthalate) or para- and meta-aramids could be enhanced by finishing with poly(vinylamine) thus providing complex-forming amino groups. Although the process does not deposit a closed, electrically conducting layer, all studied samples could be metallized by galvanization. The resulting metal coatings exhibit high conductivity and wash stability. Following a patterned silver deposition, the subsequent galvanic metallization produced conductive patterns of identical geometry thus opening an avenue towards printed circuits on textile fabrics. © 2017 Elsevier B.V.
view abstract 10.1016/j.apsusc.2017.12.119
- Adjusting the catalytic properties of cobalt ferrite nanoparticles by pulsed laser fragmentation in water with defined energy dose
Waag, F. and Gökce, B. and Kalapu, C. and Bendt, G. and Salamon, S. and Landers, J. and Hagemann, U. and Heidelmann, M. and Schulz, S. and Wende, H. and Hartmann, N. and Behrens, M. and Barcikowski, S.
Scientific Reports 7 (2017)Highly active, structurally disordered CoFe2O4/CoO electrocatalysts are synthesized by pulsed laser fragmentation in liquid (PLFL) of a commercial CoFe2O4 powder dispersed in water. A partial transformation of the CoFe2O4 educt to CoO is observed and proposed to be a thermal decomposition process induced by the picosecond pulsed laser irradiation. The overpotential in the OER in aqueous alkaline media at 10 mA cm-2 is reduced by 23% compared to the educt down to 0.32 V with a Tafel slope of 71 mV dec-1. Importantly, the catalytic activity is systematically adjustable by the number of PLFL treatment cycles. The occurrence of thermal melting and decomposition during one PLFL cycle is verified by modelling the laser beam energy distribution within the irradiated colloid volume and comparing the by single particles absorbed part to threshold energies. Thermal decomposition leads to a massive reduction in particle size and crystal transformations towards crystalline CoO and amorphous CoFe2O4. Subsequently, thermal melting forms multi-phase spherical and network-like particles. Additionally, Fe-based layered double hydroxides at higher process cycle repetitions emerge as a byproduct. The results show that PLFL is a promising method that allows modification of the structural order in oxides and thus access to catalytically interesting materials. © 2017 The Author(s).
view abstract 10.1038/s41598-017-13333-zf
- Gradual modification of ITO particle's crystal structure and optical properties by pulsed UV laser irradiation in a free liquid jet
Lau, M. and Straube, T. and Aggarwal, A.V. and Hagemann, U. and De Oliveira Viestel, B. and Hartmann, N. and Textor, T. and Lutz, H. and Gutmann, J.S. and Barcikowski, S.
Dalton Transactions 46 (2017)Indium tin oxide (ITO) particle coatings are known for high transparency in the visible, good conductive properties and near-infrared absorption. These properties depend on ITO particle's stoichiometric composition, defects and size. Here we present a method to gradually change ITO particle's optical properties by a simple and controlled laser irradiation process. The defined irradiation process and controlled energy dose input allows one to engineer the absorption and transmission of coatings made from these particles. We investigate the role of the surrounding solvent, influence of laser fluence and the specific energy dose targeting modification of the ITO particle's morphology and chemistry by stepwise laser irradiation in a free liquid jet. TEM, SEM, EDX, XPS, XRD and Raman are used to elucidate the structural, morphological and chemical changes of the laser-induced ITO particles. On the basis of these results the observed modification of the optical properties is tentatively attributed to chemical changes, e.g. laser-induced defects or partial reduction. © 2017 The Royal Society of Chemistry.
view abstract 10.1039/c7dt00010c
- Laser-Induced Functionalization of Organo/Carbon Interfaces for Selective Adsorption of Au Nanoparticles in Microsized Domains
Schade, M. and Franzka, S. and Hartmann, N.
Langmuir 33 (2017)Laser microprocessing of highly oriented pyrolytic graphite (HOPG) in conjunction with chemical functionalization routines is used to fabricate functional microsized domains. Infrared and Auger electron spectroscopy, contact angle measurements, and electron microscopy are used for characterization of laser-fabricated structures. HOPG samples are coated with alkylsiloxane monolayers. Laser-induced bromination of coated HOPG samples in gaseous bromine is carried out using a microfocused laser beam at a wavelength of 514 nm and 1/e2 laser spot diameter of about 2 μm. Subsequent azidation and amination results in functional domains with sizes in the range of 1.2 to 40 μm and more. At low laser powers and irradiation times fully functionalized circular-shaped structures are formed. At high laser powers and irradiation times laser processing results in decomposition of the organic monolayer and substrate in the center of the structures yielding donut-shaped structures. After laser processing and chemical transformation Au nanoparticles are selectively adsorbed onto the functional domains. This provides an opportunity to build up functional nanoparticle microarrays on carbon-based materials, e.g., for applications in sensing and electrocatalysis. © 2017 American Chemical Society.
view abstract 10.1021/acs.langmuir.7b00695
- Laser-induced surface activation of biocomposites for electroless metallization
Rytlewski, P. and Bahners, T. and Polewski, F. and Gebert, B. and Gutmann, J.S. and Hartmann, N. and Hagemann, U. and Moraczewski, K.
Surface and Coatings Technology 311 (2017)In this work biocomposites containing polylactide (PLA), polycaprolactone (PCL), copper(II) oxide and copper acetylacetonate were manufactured by an extrusion process. The extruded composites differed with respect to the PLA/PCL ratio whereas the content of mixed copper(II) oxide and copper acetylacetonate powders was held constant at 20 wt%. The main aims for the addition of PCL was to increase impact strength resistance, improve surface catalytic properties and reduce the temperature of extrusion, thus limiting degradation effects initiated by copper acetylacetonate. The composite samples were irradiated with an ArF excimer laser varying the number of laser pulses and then metalized by electroless plating. Based on optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) measurements, it was found that (i) PCL was dispersed in the form of droplets in all volume of PLA, (ii) the copper compounds were preferably located in the dispersed PCL phase, and (iii) composites with higher PCL content were more effectively metalized. © 2016 Elsevier B.V.
view abstract 10.1016/j.surfcoat.2016.12.048
- Resonant laser processing of nanoparticulate Au/TiO2 films on glass supports: Photothermal modification of a photocatalytic nanomaterial
Schade, L. and Franzka, S. and Thomas, M. and Hagemann, U. and Hartmann, N.
Surface Science 650 (2016)Resonant laser processing at λ = 532 nm is used to modify thin Au/TiO2 nanoparticle films on soda lime glass plates. A microfocused continuous-wave laser is employed for local patterning at distinct laser powers. In conjunction with microscopic techniques this approach allows for reproducible high-throughput screening of laser-induced material modifications. Optical microscopy and microspectroscopy reveal laser darkening, i.e. a significantly increased optical absorbance. Scanning electron microscopy and X-ray photoelectron spectroscopy show laser-induced film growth and roughening along with the integration of SiO2 from the glass supports. Raman spectroscopy displays a phase transition from anatase to rutile. Au evaporation and/or integration only takes place at high laser powers. All these modifications provide promising perspectives in view of photocatalytic applications. Data from complementary laser experiments with unblended pure TiO2 coatings at λ = 532 nm and λ = 355 nm point to a photothermal process, in which the optical energy is selectively deposited in the Au nanoparticles and transformed into heat. As a result, thermally activated modifications take place. General prospects of laser processing in targeted modification of nanomaterials for photocatalysis are emphasized. © 2016 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.susc.2016.01.006
- Surface-enhanced Raman spectroscopy on laser-engineered ruthenium dye-functionalized nanoporous gold
Schade, L. and Franzka, S. and Biener, M. and Biener, J. and Hartmann, N.
Applied Surface Science 374 (2016)Photothermal processing of nanoporous gold with a microfocused continuous-wave laser at λ = 532 nm provides a facile means in order engineer the pore and ligament size of nanoporous gold. In this report we take advantage of this approach in order to investigate the size-dependence of enhancement effects in surface-enhanced Raman spectroscopy (SERS). Surface structures with laterally varying pore sizes from 25 nm to ≥200 nm are characterized using scanning electron microscopy and then functionalized with N719, a commercial ruthenium complex, which is widely used in dye-sensitized solar cells. Raman spectroscopy reveals the characteristic spectral features of N719. Peak intensities strongly depend on the pore size. Highest intensities are observed on the native support, i.e. on nanoporous gold with pore sizes around 25 nm. These results demonstrate the particular perspectives of laser-fabricated nanoporous gold structures in fundamental SERS studies. In particular, it is emphasized that laser-engineered porous gold substrates represent a very well defined platform in order to study size-dependent effects with high reproducibility and precision and resolve conflicting results in previous studies. ©2015 Elsevier B.V. All rights reserved.
view abstract 10.1016/j.apsusc.2015.08.168
- Photothermally induced bromination of carbon/polymer bipolar plate materials for fuel cell applications
Schade, M. and Franzka, S. and Cappuccio, F. and Peinecke, V. and Heinzel, A. and Hartmann, N.
Applied Surface Science 336 (2015)A facile photothermal procedure for direct functionalization of carbon/polymer bipolar plate materials is demonstrated. Through irradiation with a microfocused beam of an Ar+-laser at λ = 514 nm in gaseous bromine and distinct laser powers and pulse lengths local bromination of the carbon/polymer material takes place. At a 1/e spot diameter of 2.1 μm, functionalized surface areas with diameters down to 5 μm are fabricated. In complementary experiments large-area bromination is investigated using an ordinary tungsten lamp. For characterization contact angle goniometry, X-ray photoelectron spectroscopy and electron microscopy in conjunction with labeling techniques are employed. After irradiation bromine groups can easily be substituted by other chemical functionalities, e.g. azide and amine groups. This provides a facile approach in order to fabricate surface patterns and gradient structures with varying wetting characteristics. Mechanistic aspects and prospects of photothermal routines in micropatterning of carbon/polymer materials are discussed. © 2014 Published by Elsevier B.V.
view abstract 10.1016/j.apsusc.2014.09.161
- Poly(N,N-dimethylaminoethyl methacrylate) Brushes: PH-Dependent Switching Kinetics of a Surface-Grafted Thermoresponsive Polyelectrolyte
Thomas, M. and Gajda, M. and Amiri Naini, C. and Franzka, S. and Ulbricht, M. and Hartmann, N.
Langmuir 31 (2015)The temperature-dependent switching behavior of poly(N,N-dimethylaminoethyl methacrylate) brushes in alkaline, neutral, and acidic solutions is examined. A novel microscopic laser temperature-jump technique is employed in order to study characteristic thermodynamic and kinetic parameters. Static laser micromanipulation experiments allow one to determine the temperature-dependent variation of the swelling ratio. The data reveal a strong shift of the volume phase transition of the polymer brushes to higher temperatures when going from pH = 10 to pH = 4. Dynamic laser micromanipulation experiments offer a temporal resolution on a submillisecond time scale and provide a means to determine the intrinsic rate constants. Both the swelling and the deswelling rates strongly decrease in acidic solutions. Complementary experiments using in situ atomic force microscopy show an increased polymer layer thickness at these conditions. The data are discussed on the basis of pH-dependent structural changes of the polymer brushes including protonation of the amine groups and conformational rearrangements. Generally, repulsive electrostatic interactions and steric effects are assumed to hamper and slow down temperature-induced switching in acidic solutions. This imposes significant restrictions for smart polymer surfaces, sensors, and devices requiring fast response times. © 2015 American Chemical Society.
view abstract 10.1021/acs.langmuir.5b03448
- 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 abstract 10.1016/j.apsusc.2014.09.118
- Chemical functionalization of carbon/polymer bipolar plate materials via oxygen plasma activation and subsequent silanization
Schade, M. and Franzka, S. and Schröter, A. and Cappuccio, F. and Gajda, M. and Peinecke, V. and Heinzel, A. and Hartmann, N.
Surface and Coatings Technology 240 (2014)A simple coating routine in order to tune the wettability of carbon/polymer bipolar plate materials is presented. Standard carbon/polypropylene composite materials as used for commercial bipolar plates for polymer-electrolyte-membrane fuel cell application are chemically modified via oxygen plasma activation and subsequent silanization using distinct precursor molecules including perfluorodecyltrichlorosilane and aminopropyltrimethoxysilane. For characterization of the samples contact angle measurements, infrared and Auger electron spectroscopy and scanning electron and atomic force microscopy are employed. Spectroscopic data provides direct evidence for successful functionalization of the substrates. Microscopic data reveals the inherent roughness of the micro-/nanostructured substrate surfaces. Depending on the particular silane precursor, the coating procedure yields hydrophilic and hydrophobic surfaces with static water contact angles ranging from 55° to 160°. The wettability of these substrates remains unchanged upon storage in clean air over a period of one year and more. Prospects of the coating procedure targeting the optimization of the water management in fuel cell applications are discussed. © 2013.
view abstract 10.1016/j.surfcoat.2013.12.037
- Laser Patterning of Silanized Carbon/Polymer Bipolar Plates with Tailored Wettability for Fuel Cell Applications
Schade, M. and Franzka, S. and Schröter, A. and Cappuccio, F. and Peinecke, V. and Heinzel, A. and Hartmann, N.
Laser Surface Modification and Adhesion (2014)Laser processing provides a powerful means to locally remove silane-based coatings and fabricate patterns with laterally varying wettability. Previous work predominantly focused on silane-based organic monolayers on flat substrate materials such as silicon wafers and quartz glass. In this chapter a simple laserassisted routine for patterning of silanized carbon/polymer bipolar plate materials is presented. Standard carbon/polypropylene composite materials as used for commercial bipolar plates for proton-exchange-membrane fuel cell applications can be chemically modifi ed via oxygen plasma activation and subsequent silanization using different precursor molecules including perfluorodecyltrichlorosilane and aminopropyltrimethoxysilane. Subsequently, laser ablation allows one to remove the coating. Additionally, substrate ablation occurs leaving a rough surface behind. For characterization of the samples contact angle measurements, infrared and Auger electron spectroscopy, scanning electron microscopy and profi lometry are employed. Spectroscopic data provide direct evidence for successful functionalization of the substrates. Microscopic and profi lometric data reveal the inherent roughness of the micro-/nanostructured substrate surfaces. Depending on the particular silane precursor, the coating procedure yields hydrophilic and hydrophobic surfaces with static water contact angles ranging from 55° to 160°. Laser ablation yields surfaces with static water contact angles of 135±5°. Hence, surface patterns with strongly varying wettability characteristics can be created. Combining silanization and laser processing provides a means to tailor the surface wettability in the channel structures of bipolar plates. Prospects of this approach for the optimization of the water management in fuel cell applications are discussed. © 2015 by Scrivener Publishing LLC. All rights reserved.
view abstract 10.1002/9781118831670.ch7
- Micro-patterning of self-assembled organic monolayers by using tunable ultrafast laser pulses
Maragkaki, S. and Aumann, A. and Schulz, F. and Schröter, A. and Schöps, B. and Franzka, S. and Hartmann, N. and Ostendorf, A.
Proceedings of SPIE - The International Society for Optical Engineering 8972 (2014)We study the application of tunable ultrafast laser pulses in micropatterning self- assembled organic monolayer (SAMs) employing non collinear optical parametric amplification (NOPA). SAMs are ultrathin organic monolayers, which can be used in a variety of ways to assemble functionalized surface structures. In our study, we investigate the characteristics of SAMs as monomolecular resists during etching of gold. NOPA is a versatile method which provides the generation of ultrafast laser pulses, with a tunable wavelength in the visible and near infrared range. Due to the noncollinear geometry, a broadened spectral range can be amplified. The NOPA delivers wavelengths in the range of 480 nm to 950 nm at laser pulse lengths in the sub- 30 femtosecond range using a prism compressor after the nonlinear conversion. The ultrashort laser technology together with the advantages of the NOPA system guarantee high precision and allows us to determine the optimum conditions of sub-wavelength patterning by studying the effects of the fluence and the wavelength. At the same time, single-pulse processing allows us to selectively remove the ultrathin organic coating, while it ensures short processing time. In our study we used thiol-based SAMs as ultrathin layers on gold-coated glass substrates with a film thickness of 1-2 nm and 40 nm respectively. © 2014 SPIE.
view abstract 10.1117/12.2037716
- Photothermal laser fabrication of micro- and nanostructured chemical templates for directed protein immobilization
Schröter, A. and Franzka, S. and Hartmann, N.
Langmuir 30 (2014)Photothermal patterning of poly(ethylene glycol) terminated organic monolayers on surface-oxidized silicon substrates is carried out using a microfocused beam of a CW laser operated at a wavelength of 532 nm. Trichlorosilane and trimethoxysilane precursors are used for coating. Monolayers from trimethoxysilane precursors show negligible unspecific protein adsorption in the background, i.e., provide platforms of superior protein repellency. Laser patterning results in decomposition of the monolayers and yields chemical templates for directed immobilization of proteins at predefined positions. Characterization is carried out via complementary analytical methods including fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. Appropriate labeling techniques (fluorescent markers and gold clusters) and substrates (native and thermally oxidized silicon substrates) are chosen in order to facilitate identification of protein adsorption and ensure high sensitivity and selectivity. Variation of the laser parameters at a 1/e2 spot diameter of 2.8 μm allows for fabrication of protein binding domains with diameters on the micrometer and nanometer length scale. Minimum domain sizes are about 300 nm. In addition to unspecific protein adsorption on as-patterned monolayers, biotin-streptavidin coupling chemistry is exploited for specific protein binding. This approach represents a novel facile laser-based means for fabrication of protein micro- and nanopatterns. The routine is readily applicable to femtosecond laser processing of glass substrates for the fabrication of transparent templates. (Graph Presented). © 2014 American Chemical Society.
view abstract 10.1021/la503814n
- Photothermal laser microsintering of nanoporous gold
Schade, L. and Franzka, S. and Mathieu, M. and Biener, M.M. and Biener, J. and Hartmann, N.
Langmuir 30 (2014)Photothermal processing of nanoporous gold using a microfocused continuous-wave laser at a wavelength of 532 nm and a 1/e2 spot diameter of 2.9 μm has been studied. In addition, complementary experiments have been carried out via conventional annealing. Scanning electron microscopy has been used for characterization. Local laser irradiation at distinct laser powers and pulse lengths results in coarsening of the porous gold structures. During laser processing the pore size of the native nanoporous gold increases to maximum values in the range of 0.25-3 μm. The affected areas exhibit lateral dimensions in the range of 2-10 μm. Overall two regions are distinguished. An inner region, where large pores and ligaments are formed and an outer region, where the pore size and ligament size gradually change and approach the feature sizes of the native material. A qualitative thermokinetic model allows one to reproduce the experimentally observed dependence of the laser-induced morphologies on the laser parameters. On the basis of this model the underlying processes are attributed to sintering and melting of the gold structures. The presented results demonstrate the prospects of photothermal laser processing in engineering porous gold with spatially varying porosities on micrometer to nanometer length scales. © 2014 American Chemical Society.
view abstract 10.1021/la5011192
- Fabrication of chemical templates via selective laser-induced desorption of hexadecanethiol self-assembled monolayers
Schröter, A. and Mathieu, M. and Franzka, S. and Feydt, J. and Irsen, S. and Hartmann, N.
Applied Surface Science 278 (2013)A nonlinear photothermal laser patterning technique for rapid fabrication of chemical templates is demonstrated. Hexadecanethiol monolayers on Au-coated Si substrates are processed at λ = 532 nm, a 1/e2 spot diameter of d1/e2=2.8Tμm and ambient conditions. Local laser irradiation at high laser powers and short irradiation times in the micro-/millisecond range induces desorption of thiol molecules. The laser-depleted areas are backfilled with mercaptohexadecanoic acid in order to build up chemical templates. Atomic force microscopy, scanning electron microscopy and scanning Auger electron spectroscopy are used for characterization of these templates. In agreement with a selective laser process, the results indicate the formation of flat chemical patterns with well-defined boundaries. Complementary condensation experiments demonstrate the functionality of the patterns as hydrophilic/hydrophobic templates. In particular, upon decreasing the temperature below the dew point, selective formation of water droplets on the backfilled areas is observed. © 2013 Elsevier B.V.
view abstract 10.1016/j.apsusc.2012.12.135
- 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 abstract 10.1016/j.apsusc.2013.01.205
- Femtosecond-laser processing of nitrobiphenylthiol self-assembled monolayers
Schröter, A. and Franzka, S. and Koch, J. and Chichkov, B.N. and Ostendorf, A. and Hartmann, N.
Applied Surface Science 278 (2013)Single-pulse femtosecond laser patterning of nitrobiphenylthiol monolayers on Au-coated Si substrates at λ = 800 nm, τ < 30 fs and ambient conditions has been investigated. After laser processing wet etching experiments are performed. Laser irradiation reduces the chemical resistance of the coating. In particular, the monolayer acts as a positive-tone resist. Burr-free pattern transfer is feasible at laser pulse fluences between 1 and 2.7 J/cm 2. Minimum structure sizes at a 1/e laser spot diameter of about 1 μm are close to 300 nm, i.e. sub-wavelength processing is demonstrated. Noteworthy, however, no indications for negative-tone resist properties of processed monolayers are evident, that is, cross-linking of the biphenyl moieties, if at all, is marginal. Also, complementary labeling experiments provide no evidence for chemical transformation of the nitro end groups into amine functionalities. Perspectives of resonant fs-laser processing in exploiting the particular prospects of nitrobiphenylthiol monolayers as negative-tone resists and chemically patternable platforms are discussed. © 2013 Elsevier B.V.
view abstract 10.1016/j.apsusc.2013.01.170
- Hofmeister effect of sodium halides on the switching energetics of thermoresponsive polymer brushes
Naini, C.A. and Thomas, M. and Franzka, S. and Frost, S. and Ulbricht, M. and Hartmann, N.
Macromolecular Rapid Communications 34 (2013)A laser temperature-jump technique is used to probe the impact of sodium halides on the temperature-dependent switching kinetics and thermodynamics of poly(N-isopropylacrylamide) brushes. An analysis on the basis of a two-state model reveals van't Hoff enthalpy and entropy changes. Sodium halides increase the endothermicity and the entropic gain of the switching process below and above Tc following the Hofmeister series: NaCl > NaBr > NaI. In contrast, enthalpic and entropic changes at Tc remain virtually unaffected. This provides an unprecedented insight into the underlying switching energetics of this classic stimuli-responsive polymer. Because of its model character, these results represent an essential reference on the way to unpuzzle the molecular driving forces of the Hofmeister effect. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/marc.201200681
- 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 abstract 10.1016/j.tsf.2013.09.056
- 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 abstract 10.1016/j.apsusc.2012.11.077
- Fabrication, laser structuring and field emission properties of carbon nanowalls
Neubert, M. and Behrenberg, D. and Hartmann, N. and Buck, V. and Serbun, P. and Navitski, A. and Muller, G.
Technical Digest - 25th International Vacuum Nanoelectronics Conference, IVNC 2012 (2012)We have synthesized carbon nanowalls (CNWs) on Si substrate by inductively/capacitively coupled plasma enhanced chemical vapor deposition (ICP/CCP-PECVD). The shape and density of CNWs are controlled by adjusting the synthesis parameters. Local field emission measurements of Ø 30 μm spots reproducibly yielded stable current up to 1 μA. Integral pulsed measurements results on the unstructured cathodes showed fairly homogeneous emission and current density of at least 3 mA/cm 2 at 5.6 V/μm, limited by the power load on the phosphor screen. In order to get cathodes for gate controlled devices and to improve the emission homogeneity, structuring by laser was performed. © 2012 IEEE.
view abstract 10.1109/IVNC.2012.6316966
- 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 abstract 10.1007/s00339-012-6779-8
- Substrate-mediated effects in photothermal patterning of alkanethiol self-assembled monolayers with microfocused continuous-wave lasers
Schröter, A. and Kalus, M. and Hartmann, N.
Beilstein Journal of Nanotechnology 3 (2012)In recent years, self-assembled monolayers (SAMs) have been demonstrated to provide promising new approaches to nonlinear laser processing. Most notably, because of their ultrathin nature, indirect excitation mechanisms can be exploited in order to fabricate subwavelength structures. In photothermal processing, for example, microfocused lasers are used to locally heat the substrate surface and initiate desorption or decomposition of the coating. Because of the strongly temperature-dependent desorption kinetics, the overall process is highly nonlinear in the applied laser power. For this reason, subwavelength patterning is feasible employing ordinary continuous-wave lasers. The lateral resolution, generally, depends on both the type of the organic monolayer and the nature of the substrate. In previous studies we reported on photothermal patterning of distinct types of SAMs on Si supports. In this contribution, a systematic study on the impact of the substrate is presented. Alkanethiol SAMs on Au-coated glass and silicon substrates were patterned by using a microfocused laser beam at a wavelength of 532 nm. Temperature calculations and thermokinetic simulations were carried out in order to clarify the processes that determine the performance of the patterning technique. Because of the strongly temperature-dependent thermal conductivity of Si, surface-temperature profiles on Au/Si substrates are very narrow ensuring a particularly high lateral resolution. At a 1/e spot diameter of 2 μm, fabrication of subwavelength structures with diameters of 300-400 nm is feasible. Rapid heat dissipation, though, requires high laser powers. In contrast, patterning of SAMs on Au/glass substrates is strongly affected by the largely distinct heat conduction within the Au film and in the glass support. This results in broad surface temperature profiles. Hence, minimum structure sizes are larger when compared with respective values on Au/Si substrates. The required laser powers, though, are more than one order of magnitude lower. Also, the laser power needed for patterning decreases with decreasing Au layer thickness. These results demonstrate the impact of the substrate on the overall patterning process and provide new perspectives in photothermal laser patterning of ultrathin organic coatings. © 2012 Schröter et al.
view abstract 10.3762/bjnano.3.8
- Probing the intrinsic switching kinetics of ultrathin thermoresponsive polymer brushes
Amiri Naini, C. and Franzka, S. and Frost, S. and Ulbricht, M. and Hartmann, N.
Angewandte Chemie - International Edition 50 (2011)Laser-stimulated polymer brushes: The temperature-dependent switching kinetics of surface-grafted thermoresponsive polymer brushes were investigated by a stroboscopic micromanipulation/-characterization technique for real-time parallel measurements (see picture). Intrinsic response times range from the microsecond to the millisecond time scale; these results could lead to fabrication of nanosized polymeric actuators and sensors with unprecedented responsivities. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/anie.201100140
- Micro- and nanopatterning of functional organic monolayers on oxide-free silicon by laser-induced photothermal desorption
Scheres, L. and Klingebiel, B. and Ter Maat, J. and Giesbers, M. and De Jong, H. and Hartmann, N. and Zuilhof, H.
Small 6 (2010)The photothermal laser patterning of functional organic monolayers, prepared on oxide-free hydrogen-terminated silicon, and subsequent backfi lling of the laser-written lines with a second organic monolayer that differs in its terminal functionality, is described. Since the thermal monolayer decomposition process is highly nonlinear in the applied laser power density, subwavelength patterning of the organic monolayers is feasible. After photothermal laser patterning of hexadecenyl monolayers, the lines freed up by the laser are backfi lled with functional acid fl uoride monolayers. Coupling of cysteamine to the acid fl uoride groups and subsequent attachment of Au nanoparticles allows easy characterization of the functional lines by atomic force microscopy (AFM) and scanning electron microscopy (SEM). Depending on the laser power and writing speed, functional lines with widths between 1.1 μm and 250 nm can be created. In addition, trifl uoroethyl-terminated (TFE) monolayers are also patterned. Subsequently, the decomposed lines are backfi lled with a nonfunctional hexadecenyl monolayer, the TFE stripes are converted into thiol stripes, and then finally covered with Au nanoparticles. By reducing the lateral distance between the laser lines, Au-nanoparticle stripes with widths close to 100 nm are obtained. Finally, in view of the great potential of this type of monolayer in the fi eld of biosensing, the ease of fabricating biofunctional patterns is demonstrated by covalent binding of fl uorescently labeled oligo-DNA to acidfl uoride-backfi lled laser lines, which-as shown by fl uorescence microscopy-is accessible for hybridization.Copyright © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
view abstract 10.1002/smll.201000189
- Nonlinear femtosecond laser processing of alkylsiloxane monolayers on surface-oxidized silicon substrates
Franzka, S. and Koch, J. and Chichkov, B.N. and Hartmann, N.
Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 28 (2010)Femtosecond laser patterning of octadecylsiloxane monolayers on surface-oxidized silicon substrates via single-pulse processing at λ=800 nm, τ< 30 fs, and ambient conditions has been investigated. Depending on the laser pulse fluence, local irradiation results in circular spots of distinct size and morphology. At high fluences, a particular rich complexity of distinct surface morphologies is observed including hole, rim, and ripple formation, and a faint boundary area where monolayer decomposition sets in. At low fluences, subwavelength patterning of the organic monolayer is feasible. In particular, at a 1/e laser spot diameter of 1.3 μm, surface spots with a width down to 300 nm are fabricated. Selective processing of the organic monolayer, though, is restricted to a very narrow range of fluences between 1.1 and 1.2 J/ cm 2. A significantly larger parameter range for selective processing is anticipated in the case of functional monolayers that incorporate aromatic groups. Promising perspectives in femtosecond laser processing of organic monolayers are discussed. © 2010 American Vacuum Society.
view abstract 10.1116/1.3281296
- Photothermal micro- and nanopatterning of organic/silicon interfaces
Klingebiel, B. and Scheres, L. and Franzka, S. and Zuilhof, H. and Hartmann, N.
Langmuir 26 (2010)Photothermal laser processing of organic monolayers on oxide-free silicon substrates under ambient conditions is investigated. Organic monolayers on Si(100) and Si(111) substrates are prepared via hydrosilylation of H-terminated silicon samples in neat 1-hexadecene and 1-hexadecyne, respectively. Laser processing at λ = 514 nm and a 1/e2 spot diameter of 2.6 μm results in local decomposition of the monolayers and oxidation of the exposed substrate. In agreement with the high thermal and chemical stability of these monolayers, a thermokinetic analysis of the data from experiments at distinct laser powers and pulse lengths points to a highly activated process. As a result, processing is strongly nonlinear and allows for subwavelength patterning, with line widths between 0.4 and 1.4 μm. Most remarkably, upon fabrication of dense line patterns, narrow organic monolayer stripes with sharp edges and lateral dimensions of 80 nm are formed. This opens up new perspectives in photothermal engineering of organic/silicon interfaces, e.g., for hybrid microelectronic and sensor applications. © 2010 American Chemical Society.
view abstract 10.1021/la903926z
- Photothermally induced bromination and decomposition of alkylsiloxane monolayers on surface-oxidized silicon substrates
Klingebiel, B. and Schröter, A. and Franzka, S. and Hartmann, N.
Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 28 (2010)Photothermal laser processing of alkylsiloxane monolayers in gaseous bromine is investigated. Surface-oxidized silicon samples are coated with octadecylsiloxane monolayers and locally irradiated with a focused beam of an Ar+ -laser at λ=514 nm and a 1/ e2 spot diameter of 3 μm. For characterization, atomic force microscopy, scanning electron microscopy, and optical microscopy in conjunction with labeling techniques and condensation experiments are used. At low laser powers, monolayer bromination in micron-sized areas is observed. Additionally, at high laser powers, decomposition of the monolayer takes place at the center of the brominated areas. Prospects and limitations of this procedure in fabrication of multifunctional templates are discussed. © 2010 American Vacuum Society.
view abstract 10.1116/1.3386590
- Self-assembled organic monolayers as high-resolution resists in rapid nonlinear processing with single femtosecond laser pulses
Mathieu, M. and Franzka, S. and Koch, J. and Chichkov, B.N. and Hartmann, N.
Applied Physics A: Materials Science and Processing 101 (2010)Femtosecond laser patterning of alkanethiol monolayers on gold-coated silicon substrates at λ=800 nm, τ< 30 fs and ambient conditions has been investigated. Single-pulse processing allows one to selectively remove the organic coating. Subsequently, pattern transfer into the gold film via wet etching in ferri-/ferrocyanide solution is achieved. As demonstrated, burr-free patterning can be carried out over an extremely wide range of laser pulse fluences from above 2 J/cm2 down to 0.5 J/cm2. Moreover, at low fluences, sub-wavelength processing down to λ/5 is feasible. In particular, at a 1/e laser spot diameter of about 1 μm, holes with diameters of 160 nm and step edges below 80 nm are fabricated. These results emphasize the prospects of organic monolayers as high-resolution resists in rapid nonlinear femtosecond laser processing. © 2010 Springer-Verlag.
view abstract 10.1007/s00339-010-5899-2
- Sub-wavelength patterning of organic monolayers via nonlinear processing with continuous-wave lasers
Mathieu, M. and Hartmann, N.
New Journal of Physics 12 (2010)In recent years, nonlinear processing with continuous-wave lasers has been demonstrated to be a facile means of rapid nanopatterning of organic monolayers down to the sub-100 nm range. In this study, we report on laser patterning of thiol-based organic monolayers with sub-wavelength resolution. Au-coated silicon substrates are functionalized with 1-hexadecanethiol. Irradiation with a focused beam of an Ar+ laser operating at λ = 514 nm allows one to locally remove the monolayer. Subsequently, the patterns are transferred into the Au film via selective etching in a ferri-/ferrocyanide solution. Despite a 1/e2 spot diameter of about 2.8 μm, structures with lateral dimensions down to 250 nm are fabricated. The underlying nonlinear dependence of the patterning process on laser intensity is traced back to the interplay between the laser-induced transient local temperature rise and the thermally activated desorption of the thiol molecules. A simple thermokinetic analysis of the data allows us to determine the effective kinetic parameters. These results complement our previous work on photothermal laser patterning of ultrathin organic coatings, such as silane-based organic monolayers, organo/silicon interfaces and supported membranes. A general introduction to nonlinear laser processing of organic monolayers is presented. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
view abstract 10.1088/1367-2630/12/12/125017
- Temporal stability of photothermally fabricated micropatterns in supported phospholipid multilayers
Mathieu, M. and Schunk, D. and Franzka, S. and Mayer, C. and Hartmann, N.
Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films 28 (2010)The temporal stability of photothermally fabricated micropatterns in multilayered films of 1,2-dioleoyl-sn-glycero-3-phosphate on surface-oxidized silicon substrates is investigated. Multilayered films are fabricated via spin coating of diluted phospholipid solutions. A focused beam of an Ar+ laser at λ=514 nm and a 1/ e2 spot diameter of 2.5 μm is used to locally remove the coating and fabricate micropatterns. Subsequently, the temporal stability of such patterns at ambient conditions in air and in water is examined using optical microscopy. Generally, these patterns are stable on a time scale of several hours to days, both in air and in water, and hence provide sufficient time for most follow-on experiments. The spin-coated phospholipid films, though, are intrinsically unstable in water. In particular, some hours after immersion in water, dewetting of the film starts to set in. The implications of these results for potential applications are discussed. © 2010 American Vacuum Society.
view abstract 10.1116/1.3271156