Dr. Nicolas Wöhrl

Thin Film Technology
University of Duisburg-Essen

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  • Defect spectroscopy on the dielectric material aluminum oxide
    Oing, D. and Geller, M. and Stahl, L. and Kerski, J. and Lorke, A. and Wöhrl, N.
    Scientific Reports 10 (2020)
    A method for defect characterization is presented that allows to measure the activation energy, capture cross-section, and defect density in dielectric materials. This is exemplarily performed on aluminum oxide thin films deposited on hydrogen-terminated diamond. During the measurement, samples were illuminated using a 405 nm laser, charging the defects while simultaneously measuring the surface conductivity of the diamond at different temperatures. By applying the standard boxcar evaluation known from deep-level transient spectroscopy, we found five different defect levels in Al 2O 3. One can be identified as substitutional silicon in aluminum oxide, while the others are most likely connected to either aluminum interstitials or carbon and nitrogen impurities. © 2020, The Author(s).
    view abstract10.1038/s41598-020-69240-3
  • One-step synthesis of carbon-supported electrocatalysts
    Tigges, S. and Wöhrl, N. and Radev, I. and Hagemann, U. and Heidelmann, M. and Nguyen, T.B. and Gorelkov, S. and Schulz, S. and Lorke, A.
    Beilstein Journal of Nanotechnology 11 (2020)
    Cost-efficiency, durability, and reliability of catalysts, as well as their operational lifetime, are the main challenges in chemical energy conversion. Here, we present a novel, one-step approach for the synthesis of Pt/C hybrid material by plasma-enhanced chemical vapor deposition (PE-CVD). The platinum loading, degree of oxidation, and the very narrow particle size distribution are precisely adjusted in the Pt/C hybrid material due to the simultaneous deposition of platinum and carbon during the process. The as-synthesized Pt/C hybrid materials are promising electrocatalysts for use in fuel cell applications as they show significantly improved electrochemical long-term stability compared to the industrial standard HiSPEC 4000. The PE-CVD process is furthermore expected to be extendable to the general deposition of metal-containing carbon materials from other commercially available metal acetylacetonate precursors. © 2020 Tigges et al.; licensee Beilstein-Institut.
    view abstract10.3762/BJNANO.11.126
  • Synthesis of graphene-related carbon nanoparticles from a liquid isopropanol precursor by a one-step atmospheric plasma process
    Bodnar, W. and Schiorlin, M. and Frank, A. and Schulz, T. and Wöhrl, N. and Miron, C. and Scheu, C. and Kolb, J.F. and Kruth, A.
    Applied Surface Science 514 (2020)
    This study presents a cost-efficient single-step-method to synthesize nanographite from isopropanol by bipolar pulsed electric discharges. The influence of pulse width within the nanosecond range, repetition frequency within the kilohertz range and processing time on the product was systematically investigated by Raman spectroscopy, high-resolution transmission electron microscopy and gas chromatography - mass spectrometry. It was found that long pulses in the microsecond range promote the creation of amorphous and oxidic carbon structures. Although, hydrocarbon cracking and subsequent graphitization do occur, these process conditions are not suitable to drive intermediate reduction processes. In contrast, applying short pulses in the nanosecond regime ensures fast reduction processes and formation of graphene-related nanostructures. The number of observed nanographite layers lies in the range of 3–13 with an average interlayer spacing of 3.4(0.3) Å and an average distance between defects of 11.5(6.0) nm meaning that the produced nanographite is in the area of small defect density. Furthermore, no significant influence of process times on the product properties over a period up to 15 min was observed, indicating good process homogeneity. © 2020 Elsevier B.V.
    view abstract10.1016/j.apsusc.2020.145926
  • The effect of metal-oxide incorporation on the morphology of carbon nanostructures
    Tigges, S. and Wöhrl, N. and Hagemann, U. and Ney, M. and Lorke, A.
    Journal of Physics D: Applied Physics 53 (2020)
    Metal-organic, single-source, low-temperature, morphology-controlled growth of carbon nanostructures is achieved, using an inductively coupled plasma-enhanced chemical vapor deposition system. Three distinctive morphologies, namely nanoflakes, nanowalls (CNWs) and nanorods (and intermediates between these morphologies), can be reproducibly deposited, depending on the process parameters. The synthesized structures can be described as hybrid materials consisting of metal oxide incorporated in a carbon matrix material. Since the incorporation of metal oxide into the carbon structure significantly influences their growth, the synthesis cannot be described solely with the existing models for the growth of CNWs. Optical emission spectroscopy is used to measure the relative number density of suspected growth and etching species in the plasma, while physical and chemical surface analysis techniques (scanning electron microscopy, Raman spectroscopy, scanning Auger microscopy and x-ray photoelectron spectroscopy) were employed to characterize the properties of the different nanostructures. Therefore, by using methods for both plasma and surface characterization, the growth process can be understood. The precursor dissociation in the plasma can be directly linked to the deposited morphology, as the incorporation of Al2O3 into the nanostructures is found to be a major cause for the transition between morphologies, by changing the dominant type of defect within the carbon structure. © 2020 The Author(s). Published by IOP Publishing Ltd.
    view abstract10.1088/1361-6463/ab6946
  • CVD growth of ultrapure diamond, generation of NV centers by ion implantation, and their spectroscopic characterization for quantum technological applications
    Chakraborty, T. and Lehmann, F. and Zhang, J. and Borgsdorf, S. and Wöhrl, N. and Remfort, R. and Buck, V. and Köhler, U. and Suter, D.
    Physical Review Materials 3 (2019)
    Applications of nitrogen-vacancy (NV) centers in diamond in quantum technology have attracted considerable attention in recent years. Deterministic generation of ensembles of NV centers can advance the research on quantum sensing, many-body quantum systems, multipartite entanglement, and so on. Here we report the complete process of controlled generation of NV centers in diamond as well as their characterization: growing diamond films through chemical vapor deposition (CVD), ion implantation, and spectroscopic characterization of the defect centers using a confocal microscope. A microwave-assisted CVD setup is presented which we constructed for the preparation of single-crystalline homoepitaxial diamond films. The films were prepared with minimized nitrogen concentration, which is confirmed through photoluminescence measurements. We demonstrate an in situ ultrahigh vacuum (UHV) implantation and heating process for creation of NV centers using a novel experimental setup. For the first time hot implantation has been shown which prevents surface charging effects. We do not observe graphitization due to UHV heating. By optimizing the implantation parameters it has been possible to implant NV centers in a precise way. We present large area mapping of the samples to determine the distribution of the centers and describe the characterization of the centers by spectroscopic techniques. Reducing the decoherence caused by environmental noise is of primary importance for many applications in quantum technology. We demonstrate improvement on coherence time T2 of the NV spins by suppression of their interaction with the surrounding spin bath using robust dynamical decoupling sequences. © 2019 American Physical Society.
    view abstract10.1103/PhysRevMaterials.3.065205
  • Gradients of microstructure, stresses and mechanical properties in a multi-layered diamond thin film revealed by correlative cross-sectional nano-analytics
    Gruber, D.P. and Todt, J. and Wöhrl, N. and Zalesak, J. and Tkadletz, M. and Kubec, A. and Niese, S. and Burghammer, M. and Rosenthal, M. and Sternschulte, H. and Pfeifenberger, M.J. and Sartory, B. and Keckes, J.
    Carbon 144 (2019)
    Thin diamond films deposited by chemical vapour deposition (CVD) usually feature cross-sectional gradients of microstructure, residual stress and mechanical properties, which decisively influence their functional properties. This work introduces a novel correlative cross-sectional nano-analytics approach, which is applied to a multi-layered CVD diamond film grown using microwave plasma-enhanced CVD and consisting of a ∼8 μm thick nanocrystalline (NCD) base and a ∼14.5 μm thick polycrystalline (PCD) top diamond sublayers. Complementary cross-sectional 30 nm beam synchrotron X-ray diffraction, depth-resolved micro-cantilever and hardness testing and electron microscopy analyses reveal correlations between microstructure, residual stress and mechanical properties. The NCD sublayer exhibits a 1.5 μm thick isotropic nucleation region with the highest stresses of ∼1.3 GPa and defect-rich nanocrystallites. With increasing sublayer thickness, a 110 fibre texture evolves gradually, accompanied by an increase in crystallite size and a decrease in stress. At the NCD/PCD sublayer interface, texture, stresses and crystallite size change abruptly and the PCD sublayer exhibits the presence of Zone T competitive grain growth microstructure. NCD and PCD sublayers differ in fracture stresses of ∼14 and ∼31 GPa, respectively, as well as in elastic moduli and hardness, which are correlated with their particular microstructures. In summary, the introduced nano-analytics approach provides complex correlations between microstructure, stresses, functional properties and deposition conditions. © 2018 Elsevier Ltd
    view abstract10.1016/j.carbon.2018.12.093
  • Patterning of diamond with 10 nm resolution by electron-beam-induced etching
    Dergianlis, V. and Geller, M. and Oing, D. and Wöhrl, N. and Lorke, A.
    Nanotechnology 30 (2019)
    We report on mask-less, high resolution etching of diamond surfaces, featuring sizes down to 10 nm. We use a scanning electron microscope (SEM) together with water vapor, which was injected by a needle directly onto the sample surface. Using this versatile and low-damage technique, trenches with different depths were etched. Cross sections of each trench were obtained by focused ion beam milling and used to calculate the achieved aspect ratios. The developed technique opens up the possibility of mask- and resist-less patterning of diamond for nano-optical and electronic applications. © 2019 IOP Publishing Ltd.
    view abstract10.1088/1361-6528/ab25fe
  • Tunable carrier density and high mobility of two-dimensional hole gases on diamond: The role of oxygen adsorption and surface roughness
    Oing, D. and Geller, M. and Lorke, A. and Wöhrl, N.
    Diamond and Related Materials 97 (2019)
    The transport properties of two-dimensional hole gases (2DHGs) on chemical-vapor-deposition (CVD)-grown diamond are investigated. A hydrogen plasma treatment and exposure to ambient atmosphere are used to establish and tailor the properties of the 2DHG. The transport parameters of the 2DHGs (namely carrier density and mobility) are characterized by temperature-dependent Hall measurements. The importance of the surface oxygen adsorption, determined by X-ray photoelectron spectroscopy (XPS), on the carrier density and mobility is shown. Hall measurements reveal that for oxygen concentrations below 2.2% (relative XPS signal) the carrier density is increasing from 1.4 ∙ 1010 cm−2 to 1.5 ∙ 1013 cm−2 with increasing oxygen adsorption. For oxygen concentrations above 2.2%, the charge carrier density decreases again. The carrier density remains constant over a temperature range between 4.2 K and 325 K. At room temperature, the mobility increases with decreasing carrier concentration. The opposite behavior is observed for 4.2 K. By decreasing the surface roughness to 8.2 nm, we were able to increase the mobility to above 250 cm2/V s at room temperature for a carrier density of 1.2 ∙ 1013 cm−2. This is among the highest values reported for 2DHGs on diamond. © 2019 Elsevier B.V.
    view abstract10.1016/j.diamond.2019.107450
  • Kelvin probe force microscopy studies of the charge effects upon adsorption of carbon nanotubes and C60 fullerenes on hydrogen-terminated diamond
    Kölsch, S. and Fritz, F. and Fenner, M.A. and Kurch, S. and Wöhrl, N. and Mayne, A.J. and Dujardin, G. and Meyer, C.
    Journal of Applied Physics 123 (2018)
    Hydrogen-terminated diamond is known for its unusually high surface conductivity that is ascribed to its negative electron affinity. In the presence of acceptor molecules, electrons are expected to transfer from the surface to the acceptor, resulting in p-type surface conductivity. Here, we present Kelvin probe force microscopy (KPFM) measurements on carbon nanotubes and C60 adsorbed onto a hydrogen-terminated diamond(001) surface. A clear reduction in the Kelvin signal is observed at the position of the carbon nanotubes and C60 molecules as compared with the bare, air-exposed surface. This result can be explained by the high positive electron affinity of carbon nanotubes and C60, resulting in electron transfer from the surface to the adsorbates. When an oxygen-terminated diamond(001) is used instead, no reduction in the Kelvin signal is obtained. While the presence of a charged adsorbate or a difference in work function could induce a change in the KPFM signal, a charge transfer effect of the hydrogen-terminated diamond surface, by the adsorption of the carbon nanotubes and the C60 fullerenes, is consistent with previous theoretical studies. © 2018 Author(s).
    view abstract10.1063/1.5019486
  • Synthesis of carbon nanowalls from a single-source metal-organic precursor
    Giese, A. and Schipporeit, S. and Buck, V. and Wöhrl, N.
    Beilstein Journal of Nanotechnology 9 (2018)
    In this work, the deposition of carbon nanowalls (CNWs) by inductively coupled plasma enhanced chemical vapor deposition (ICPPECVD) is investigated. The CNWs are electrically conducting and show a large specific surface area, which is a key characteristic to make them interesting for sensors, catalytic applications or energy-storage systems. It was recently discovered that CNW films can be deposited by the use of the single-source metal-organic precursor aluminium acetylacetonate. This precursor is relatively unknown in combination with the ICP-PECVD deposition method in literature and, thus, based on our previous publication is further investigated in this work to better understand the influence of the various deposition parameters on the growth. Silicon, stainless steel, nickel and copper are used as substrate materials. The CNWs deposited are characterized by scanning electron microscopy (SEM), Raman spectroscopy and Auger electron spectroscopy (AES). The combination of bias voltage, the temperature of the substrate and the substrate material had a strong influence on the morphology of the graphitic carbon nanowall structures. With regard to these results, a first growth model for the deposition of CNWs by ICP-PECVD and aluminium acetylacetonate is proposed. This model explains the formation of four different morphologies (nanorods as well as thorny, straight and curled CNWs) by taking the surface diffusion into account. The surface diffusion depends on the particle energies and the substrate material and thus explains the influence of these parameters. © 2018 Giese et al.
    view abstract10.3762/bjnano.9.181
  • Nanocrystalline diamond films as a protective coating for implantable bio- devices
    Sharma, R. and Sharma, N. and Parmar, R. and Chatterjee, V. and Kumar, A. and Wöhrl, N. and Buck, V. and Barhai, P.K.
    Advanced Materials Letters 7 (2016)
    Nanocrystalline diamond (NCD) films are deposited on silicon substrates using Microwave Plasma Enhanced Chemical Vapor Deposition technique with the variation of microwave power from 800 W to 1800 W at 200 mbar for 5 hrs. Methane is used as a precursor along with argon and hydrogen as carriers for deposition. Deposited films are characterized by using Raman, FTIR, optical contact angle, AFM and SEM. The biocompatibility study has been carried out by cell viability assay, haemolysis test and simulated body fluid (SBF) adsorption assay. The Lymphocytes and Fibroblast cell lines are cultured on the NCD coated samples and cell viability has been determined by MTT assay. The surface morphology of the samples has been studied by using AFM, before and after interaction with SBF. It has been observed that NCD coated substrates are biocompatible, haemocompatible and also promote the growth of the cells, while the uncoated substrates cause cell death. © 2016 VBRI Press.
    view abstract10.5185/amlett.2016.6050
  • Simultaneous synthesis of nanodiamonds and graphene via plasma enhanced chemical vapor deposition (MW PE-CVD) on copper
    Gottlieb, S. and Wöhrl, N. and Schulz, S. and Buck, V.
    SpringerPlus 5 (2016)
    The simultaneous growth of both nanodiamonds and graphene on copper samples is described for the first time. A PE-CVD process is used to synthesize graphene layers and nanodiamond clusters from a hydrogen/methane gas mixture as it is typically done successfully in thermal CVD processes for graphene synthesis. However, the standard thermal CVD process is not without problems since the deposition of graphene is affected by the evaporation of a notable amount of copper caused by the slow temperature increase typical for thermal CVD resulting in a long process time. In sharp contrast, the synthesis of graphene by PE-CVD can circumvent this problem by substantially shortening the process time at holding out the prospect of a lower substrate temperature. The reduced thermal load and the possibility to industrially scale-up the PE-CVD process makes it a very attractive alternative to the thermal CVD process with respect to the graphene production in the future. Nanodiamonds are synthesized in PE-CVD reactors for a long time because these processes offer a high degree of control over the film’s nanostructure and simultaneously providing a significant high deposition rate. To model the co-deposition process, the three relevant macroscopic parameters (pressure, gas mixture and microwave power) are correlated with three relevant process properties (plasma ball size, substrate temperature and C2/Hα-ratio) and the influence on the quality of the deposited carbon allotropes is investigated. For the evaluation of the graphene as well as the nanodiamond quality, Raman spectroscopy used whereas the plasma properties are measured by optical methods. It is found that the diamond nucleation can be influenced by the C2/Hα-ratio in the plasma, while the graphene quality remains mostly unchanged by this parameter. Moreover it is derived from the experimental data that the direct plasma contact with the copper surface is beneficial for the nucleation of the diamond while the growth and quality of the graphene benefits from a larger distance to the plasma. Therefore, this work presents a basis for a method to tailor the deposition of graphene–diamond hybrid films using a MW PE-CVD process or to suppress the diamond deposition entirely if desired. © 2016, The Author(s).
    view abstract10.1186/s40064-016-2201-x
  • Chemical vapor deposition of Si/SiC nano-multilayer thin films
    Weber, A. and Remfort, R. and Wöhrl, N. and Assenmacher, W. and Schulz, S.
    Thin Solid Films 593 (2015)
    Stoichiometric SiC films were deposited with the commercially available single source precursor Et3SiH by classical thermal chemical vapor deposition (CVD) as well as plasma-enhanced CVD at low temperatures in the absence of any other reactive gases. Temperature-variable deposition studies revealed that polycrystalline films containing different SiC polytypes with a Si to carbon ratio of close to 1:1 are formed at 1000°C in thermal CVD process and below 100°C in the plasma-enhanced CVD process. The plasma enhanced CVD process enables the reduction of residual stress in the deposited films and offers the deposition on temperature sensitive substrates in the future. In both deposition processes the film thickness can be controlled by variation of the process parameters such as the substrate temperature and the deposition time. The resulting material films were characterized with respect to their chemical composition and their crystallinity using scanning electron microscope, energy dispersive X-ray spectroscopy (XRD), atomic force microscopy, X-ray diffraction, grazing incidence X-ray diffraction, secondary ion mass spectrometry and Raman spectroscopy. Finally, Si/SiC multilayers of up to 10 individual layers of equal thickness (about 450 nm) were deposited at 1000°C using Et3SiH and SiH4. The resulting multilayers features amorphous SiC films alternating with Si films, which feature larger crystals up to 300 nm size as measured by transmission electron microscopy as well as by XRD. XRD features three distinct peaks for Si(111), Si(220) and Si(311). © 2015 Published by Elsevier B.V.
    view abstract10.1016/j.tsf.2015.08.042
  • Plasma-enhanced chemical vapor deposition of graphene on copper substrates
    Wöhrl, N. and Ochedowski, O. and Gottlieb, S. and Shibasaki, K. and Schulz, S.
    AIP Advances 4 (2014)
    A plasma enhanced vapor deposition process is used to synthesize graphene from a hydrogen/methane gas mixture on copper samples. The graphene samples were transferred onto SiO2 substrates and characterized by Raman spectroscopic mapping and atomic force microscope topographical mapping. Analysis of the Raman bands shows that the deposited graphene is clearly SLG and that the sheets are deposited on large areas of several mm2. The defect density in the graphene sheets is calculated using Raman measurements and the influence of the process pressure on the defect density is measured. Furthermore the origin of these defects is discussed with respect to the process parameters and hence the plasma environment. © 2014 Author(s).
    view abstract10.1063/1.4873157
  • Synthesis and characterization of graphene films by hot filament chemical vapor deposition
    Stojanović, D. and Wöhrl, N. and Buck, V.
    Physica Scripta (2012)
    In this study, we tried to deposit graphene films on copper substrates using hot filament chemical vapor deposition. The quality and the electronic and structural properties of deposited carbon films were investigated by Raman spectroscopy. Three samples of carbon films on copper substrates were produced and designated as A, B and C according to the preparation conditions. Experiments were conducted at temperatures of 800 and 1000°C, a gas pressure of 500torr and exposure times of 1, 5 and 30min. The intensity of the disorder-induced D peak is observed relative to that of the G peak. The shape of the 2D peak gives us information about the number of layers. The lineshape of the 2D peak of sample C indicates few-layer graphene, whereas Raman spectra of samples A and B indicate hydrogenated few-layer graphene. © 2012 The Royal Swedish Academy of Sciences.
    view abstract10.1088/0031-8949/2012/T149/014068
  • Effect of microwave power and C2 emission intensity on structural and surface properties of nanocrystalline diamond films
    Sharma, R. and Wöhrl, N. and Vrućinić, M. and Timpner, M. and Buck, V. and Barhai, P.K.
    Thin Solid Films 519 (2011)
    Nanocrystalline diamond (NCD) films are synthesized using microwave plasma enhanced chemical vapour deposition technique at 2 × 104 Pa and 600 °C with microwave power of 600-1600 W. Deposition is carried out on n-type (100) silicon wafer with Ar/H2/CH4 gas mixtures. The film properties are analyzed using micro Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), scanning electron microscopy and atomic force microscopy. Raman spectra show two predominant peaks centered around 1335 cm-1 and 1560 cm- 1 and two humps around 1160 cm- 1 and 1450 cm- 1, respectively. FTIR spectra show C:H stretching modes around 3000 cm - 1. XRD patterns show a peak at 44° (2θ). In situ diagnostic of plasma is carried out using Optical Emission Spectroscopy. It has been observed that C2 dimer plays an important role in the nucleation of diamond crystals during NCD film deposition and the emission intensity of C2 can be adjusted by varying the microwave power. It has also been observed that the structural properties like growth rate, surface morphology and grain size of the growing film are dependent on the C2 intensity during deposition. © 2011 Elsevier B.V. All rights reseved.
    view abstract10.1016/j.tsf.2011.05.006
  • Process control of CVD deposition of nanocrystalline diamond films by plasma diagnostics
    Wöhrl, N. and Buck, V.
    Zeitschrift fur Physikalische Chemie 225 (2011)
    Ultrananocrystalline Diamond (UNCD) Films were deposited by MW-CVD from an Ar/H2/CH4 plasma. The plasma properties were measured in situ by optical emission spectroscopy and mass spectroscopy. The intensity of the C2 emission line was systematically varied as a key plasma parameter and a correlation with resulting film properties was found. © by Oldenbourg Wissenschaftsverlag, München.
    view abstract10.1524/zpch.2011.0184
  • Ultrananocrystalline diamond films
    Wöhrl, N. and Buck, V.
    Vakuum in Forschung und Praxis 23 (2011)
    Innovative materials are the driving force for technological progress and with it for the international competitiveness of the industry. Recently a new area of work was opened here: nanotechnology. Hence, a matter of particular interest are self-organized nanostructured materials. Ultra-Nanocrystalline Diamond (UNCD) is a product of this area. The key to structuring those UNCD films is to control the ratio of the hard diamond crystals and the matrix surrounding the nanosized diamond crystals. Essential film properties can be tailored by adjusting this ratio. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/vipr.201100442
  • atomic force microscopy

  • chemical vapor deposition

  • diamonds

  • graphene

  • thin films

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