apl. Prof. Dr. Hermann Nienhaus

Surface Physics
University of Duisburg-Essen

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  • Following the motion of a charged conducting sphere by electrostatic induction in a parallel plate capacitor
    Kaponig, M. and Mölleken, A. and Tarasevitch, D. and Utzat, D. and Nienhaus, H. and Möller, R.
    Journal of Electrostatics 103 (2020)
    The charges induced in the plates of a parallel plate capacitor due to a conducting charged moving sphere have been measured up to the mechanical contact. For larger distances the induced charge scales linearly with the distance. However, when the sphere approaches the plate further the charges on the sphere are attracted by the induced charges in the plate and move on the surface of the sphere towards the plate. This leads to a further increase of the induced charge. The experimental results compare well to an approximate formula which will be discussed in detail. © 2019
    view abstract10.1016/j.elstat.2019.103411
  • Temperature dependent electrical characteristics of a junction field effect transistor for cryogenic sub-attoampere charge detection
    Kavangary, A. and Graf, P. and Azazoglu, H. and Flebbe, M. and Huba, K. and Nienhaus, H. and Möller, R.
    AIP Advances 9 (2019)
    Junction field effect transistors (JFET) at cryogenic temperatures can be employed as almost perfect charge detectors with leakage currents of less than 1 aA. The electrical input and output characteristics of the commercial n-channel silicon JFET BF545B is studied as a function of temperature in the range between 30 and 300 K. As long as the charge carrier concentration is constant an increasing drain current is observed for reduced temperatures and low gate voltages. Using a constant mobility model for the device this behaviour can be explained with the higher electron mobility in the source-drain channel. The mobility is found to increase with T -1 at lower temperatures which corresponds to a dopant concentration of 7 . 10 16 /cm 3 . For larger negative gate voltages a source-drain voltage is found at which the drain current is almost temperature independent. As soon as the charge carriers freeze out the input characteristics changes significantly due to the exponential decrease of the carrier concentration. The effective reduction of the gate leakage current is crucial for ultimate sensitive charge detection. Hence, the current is measured through the entire temperature range in an open gate configuration. The leakage current decreases exponentially with lower temperatures across more than six orders of magnitude and reaches values of 10 -20 A below 160 K. It is exclusively due to the generation of electron-hole pairs in the depletion layer since the data are in full agreement with the Shockley-Read-Hall model of the generation current. © 2019 Author(s).
    view abstract10.1063/1.5077039
  • Three dimensional tracing of a charged particle by electrostatic detection
    Genc, E. and Mölleken, A. and Tarasevitch, D. and Utzat, D. and Nienhaus, H. and Möller, R.
    Review of Scientific Instruments 90 (2019)
    Using a configuration of electrodes connected to charge sensitive amplifiers, the position of a charged particle in free space can be determined in all three dimensions. In our experiment, spheres with a diameter of a millimeter and a charge of about 0.1 pC are traced while they are bouncing at a surface. A spatial resolution of about 0.5 mm combined with a temporal resolution better than 10 μs is achieved. Moreover, the transfer of electric charges when touching a surface can be evaluated. © 2019 Author(s).
    view abstract10.1063/1.5093988
  • Measuring displacement currents during fabrication of Mg/Si Schottky diodes due to band-bending evolution
    Hagemann, U. and Huba, K. and Nienhaus, H.
    Journal of Applied Physics 124 (2018)
    The generation of a rectifying metal-semiconductor contact forms a charge depletion layer in the semiconductor surface. The resulting space charge leads to a surface band bending and the formation of a Schottky barrier. The present study introduces an unconventional method to measure and monitor the surface band bending during metal atom deposition by recording the displacement current between the metal and the semiconductor. Magnesium atoms are evaporated at 130 K onto hydrogen-passivated p-Si(001) surfaces. During deposition, the time-dependent reverse current in the diode is detected. A sharp current peak of a few nA can be attributed to the displaced charge when the first monolayers of the Mg film are formed. The currents are proportional to the number of Mg atoms impinging onto the surface. Integrating the observed displacement currents over time yields the total space charge densities at the interface between 8 and 23 nC/cm 2. This is in excellent agreement with the calculated value for a Schottky barrier of 0.5 eV and assuming flatband condition for hydrogen-passivated Si(001) surfaces. © 2018 Author(s).
    view abstract10.1063/1.5055206
  • Scanning probe microscopy and potentiometry using a junction field effect transistor based sensor
    Graf, P. and Flebbe, M. and Hoepken, S. and Utzat, D. and Nienhaus, H. and Möller, R.
    Applied Physics Letters 113 (2018)
    Scanning tunneling microscopy in its conventional form relies on a steady state tunneling current of 10−12-10−6 A. However, for various applications, it is desirable to reduce the current load to a minimum. Here, we present first experiments using a cooled junction field effect transistor in open gate operation, thereby reducing the DC-current to less than 10−19 A. This enables almost ideal measurements of the local electrochemical potential on a surface. Various methods applying dynamic modes can be used to maintain a constant distance between the scanning probe and the sample surface. Here, we use an AC-bias applied to the sample and a lock-in amplifier connected to the preamplifier to evaluate the conductance of the tunneling gap. © 2018 Author(s).
    view abstract10.1063/1.5054349
  • Electrometer with sub-attoampere current load
    Graf, P. and Flebbe, M. and Utzat, D. and Nienhaus, H. and Möller, R.
    Review of Scientific Instruments 88 (2017)
    By cooling a conventional junction field-effect transistor below 150 K, a simple and versatile electrometer with extremely high impedance can be realized. At operating condition, the leakage current to the gate amounts to a few hundredths of an attoampere. The electrometer can be used from DC up to a frequency of 10 kHz. Without reduction of the bandwidth, a sensitivity of a few μV is obtained. Working at low frequencies, currents as low as a few attoamperes can be detected. If the input voltage is out of the operational range, the forward current or the Zener current of the gate junction protects the transistor against destructive charging. © 2017 Author(s).
    view abstract10.1063/1.4998979
  • Surface plasmon coupled chemiluminescence during adsorption of oxygen on magnesium surfaces
    Hagemann, U. and Nienhaus, H.
    Journal of Chemical Physics 143 (2015)
    The dissociative adsorption of oxygen molecules on magnesium surfaces represents a non-adiabatic reaction exhibiting exoelectron emission, chemicurrent generation, and weak chemiluminescence. Using thin film Mg/Ag/p-Si(111) Schottky diodes with 1 nm Mg on a 10-60 nm thick Ag layer as 2π-photodetectors, the chemiluminescence is internally detected with a much larger efficiency than external methods. The chemically induced photoyield shows a maximum for a Ag film thickness of 45 nm. The enhancement is explained by surface plasmon coupled chemiluminescence, i.e., surface plasmon polaritons are effectively excited in the Ag layer by the oxidation reaction and decay radiatively leading to the observed photocurrent. Model calculations of the maximum absorption in attenuated total reflection geometry support the interpretation. The study demonstrates the extreme sensitivity and the practical usage of internal detection schemes for investigating surface chemiluminescence. © 2015 AIP Publishing LLC.
    view abstract10.1063/1.4938996
  • In situ non-DLVO stabilization of surfactant-free, plasmonic gold nanoparticles: Effect of Hofmeister's anions
    Merk, V. and Rehbock, C. and Becker, F. and Hagemann, U. and Nienhaus, H. and Barcikowski, S.
    Langmuir 30 (2014)
    Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water-metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV-vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br-, SCN-, or I-) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F - or SO4 2-) seem to destabilize the gold colloid, whereas Cl- and NO3 - give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount. © 2014 American Chemical Society.
    view abstract10.1021/la404556a
  • Non-adiabatic processes in the charge transfer reaction of O2 molecules with potassium surfaces without dissociation
    Krix, D. and Nienhaus, H.
    Journal of Chemical Physics 141 (2014)
    Thin potassium films grown on Si(001) substrates are used to measure internal chemicurrents and the external emission of exoelectrons simultaneously during adsorption of molecular oxygen on K surfaces at 120 K. The experiments clarify the dynamics of electronic excitations at a simple metal with a narrow valence band. X-ray photoemission reveals that for exposures below 5 L almost exclusively peroxide K2O2 is formed, i.e., no dissociation of the molecule occurs during interaction. Still a significant chemicurrent and a delayed exoelectron emission are detected due to a rapid injection of unoccupied molecular levels below the Fermi level. Since the valence band width of potassium is approximately equal to the potassium work function (2.4 eV) the underlying mechanism of exoemission is an Auger relaxation whereas chemicurrents are detected after resonant charge transfer from the metal valence band into the injected level. The change of the chemicurrent and exoemission efficiencies with oxygen coverage can be deduced from the kinetics of the reaction and the recorded internal and external emission currents traces. It is shown that the non-adiabaticity of the reaction increases with coverage due to a reduction of the electronic density of states at the surface while the work function does not vary significantly. Therefore, the peroxide formation is one of the first reaction systems which exhibits varying non-adiabaticity and efficiencies during the reaction. Non-adiabatic calculations based on model Hamiltonians and density functional theory support the picture of chemicurrent generation and explain the rapid injection of hot hole states by an intramolecular motion, i.e., the expansion of the oxygen molecule on the timescale of a quarter of a vibrational period. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4892805
  • Quantum size effects in chemicurrent measurements during low-temperature oxidation of Mg(0001) epilayers
    Hagemann, U. and Nienhaus, H.
    New Journal of Physics 16 (2014)
    The reactivity of Mg epilayers on Si(111)-7 × 7 towards molecular oxygen is investigated as a function of the metal film thickness in the range between 7 and 45 monolayers. Quantum well and surface states are characterized with ultra-violet photoelectron spectroscopy demonstrating the epitaxial and single-crystalline structure of the Mg films. The oxidation rate is monitored during the reaction by measuring chemicurrents at 110K in the Mg/p-Si(111) Schottky diodes due to the non-adiabatic character of at least one step in the reaction chain. For film thicknesses around 9 and 13 monolayers the chemicurrent transients demonstrate that the reaction rate is strongly enhanced by a factor of more than two. With Mg 2p core level spectroscopy, a similar enhancement can be found for the total oxygen uptake for long exposures indicating that the chemicurrent increase measures solely a quantum size effect on the reactivity and no device-related effects. The enhanced reactivity can be explained by the increased first charge transfer into the affinity level of the approaching molecule when a quantum well state appears at the Fermi level and increases the density of electronic states. A linear relationship between the photoelectron intensity at the Fermi level and the maximum chemicurrent is clearly observed. On the other hand, the surface work function and the Schottky barrier height exhibit almost no correlation with the enhanced reactivity. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
    view abstract10.1088/1367-2630/16/11/113035
  • Internal detection of surface plasmon coupled chemiluminescence during chlorination of potassium thin films
    Becker, F. and Krix, D. and Hagemann, U. and Nienhaus, H.
    Journal of Chemical Physics 138 (2013)
    The interaction of chlorine with potassium surfaces is a prototype reaction with a strong non-adiabatic energy transfer leading to exoemission and chemiluminescence. Thin film K/Ag/p-Si(111) Schottky diodes with 8 nm potassium on a 5-200 nm thick Ag layer are used as 2π-photodetectors for the chemiluminescence during chlorination of the K film at 110 K. The observed photocurrent shows a sharp maximum for small exposures and decreases gradually with the increasing chloride layer. The time dependence can be explained by the reaction kinetics, which is governed initially by second-order adsorption processes followed by an electric field-assisted diffusion. The detector current corresponds to a yield of a few percent of elementary charge per reacting chlorine molecule and is orders of magnitude larger than for external detection. The photoyield can be enhanced by increasing the Ag film thickness. For Ag films of 30 and 50 nm, the yield exhibits a maximum indicating surface plasmon coupled chemiluminescence. Surface plasmon polaritons in the Ag layer are excited by the reaction and decay radiatively into Si leading to the observed currents. A model calculation for the reverse process in attenuated total reflection is applied to explain the observed current yield maxima. © 2013 American Institute of Physics.
    view abstract10.1063/1.4776156
  • Laser-doping of crystalline silicon substrates using doped silicon nanoparticles
    Meseth, M. and Lamine, K. and Dehnen, M. and Kayser, S. and Brock, W. and Behrenberg, D. and Orthner, H. and Elsukova, A. and Hartmann, N. and Wiggers, H. and Hülser, T. and Nienhaus, H. and Benson, N. and Schmechel, R.
    Thin Solid Films 548 (2013)
    Crystalline Si substrates are doped by laser annealing of solution processed Si. For this experiment, dispersions of highly B-doped Si nanoparticles are deposited onto intrinsic Si and laser processed using an 807.5 nm continuous wave laser. During laser processing the particles as well as a surface-near substrate layer are melted to subsequently crystallize in the same orientation as the substrate. The doping profile is investigated by secondary ion mass spectroscopy revealing a constant B concentration of 2 × 10 18 cm- 3 throughout the entire analyzed depth of 5 μm. Four-point probe measurements demonstrate that the effective conductivity of the doped sample is increased by almost two orders of magnitude. The absolute doping depth is estimated to be in between 8 μm and 100 μm. Further, a pn-diode is created by laser doping an n-type c-Si substrate using the Si NPs. © 2013 Published by Elsevier B.V.
    view abstract10.1016/j.tsf.2013.09.056
  • Low temperature diffusion of Li atoms into Si nanoparticles and surfaces
    Nienhaus, H. and Karacuban, H. and Krix, D. and Becker, F. and Hagemann, U. and Steeger, D. and Bywalez, R. and Schulz, C. and Wiggers, H.
    Journal of Applied Physics 114 (2013)
    The diffusion of Li atoms deposited on hydrogen-passivated Si(001) surfaces, chemically oxidized Si(001) surfaces, Si nanoparticle films, and thick SiO2 layers is investigated with electron-beam induced Auger electron spectroscopy. The nanoparticles exhibit an average diameter of 24 nm. The Li metal film is evaporated at a sample temperature below 120 K. The reappearance of the Si substrate Auger signal as a function of time and temperature can be measured to study the Li diffusion into the bulk material. Values for the diffusion barrier of 0.5 eV for H:Si(001) and 0.3 eV for the ox-Si(001) and Si nanoparticle films are obtained. The diffusion of the Li atoms results in the disruption of the crystalline Si surfaces observed with atomic force microscopy. Contrasting to that, the Si nanoparticle films show less disruption by Li diffusion due to filling of the porous films detected with cross section electron microscopy. Silicon dioxide acts as a diffusion barrier for temperatures up to 300 K. However, the electron beam induces a reaction between Li and SiO2, leading to LiOx and elemental Si floating on the surface. © 2013 AIP Publishing LLC.
    view abstract10.1063/1.4813872
  • Low-temperature oxidation of alkali overlayers: Ionic species and reaction kinetics
    Krix, D. and Nienhaus, H.
    Applied Surface Science 270 (2013)
    Clean and oxidized alkali metal films have been studied using X-ray photoelectron spectroscopy (XPS). Thin films, typically 10 nm thick, of lithium, sodium, potassium, rubidium and cesium have been deposited on silicon substrates and oxidized at 120 K. Plasmon losses were found to dress the primary photo emission structures of the metals' core lines which confirms the metallic, bulk like nature of the films. The emission from the O 1s core levels was used to determine the chemical composition and the reaction kinetics during the exposure to molecular oxygen at low pressures. Molecular oxide ions O2- and O22- as well as atomic oxygen ions O2- were detected in varying amounts depending on the alkali metal used. Diffusive transport of material in the film is shown to greatly determine the composition of the oxides. Especially, the growth of potassium superoxide is explained by the diffusion of potassium atoms to the surface and growth at the surface in a Deal-Grove like model.
    view abstract10.1016/j.apsusc.2013.01.008
  • Stabilization of mid-sized silicon nanoparticles by functionalization with acrylic acid
    Bywalez, R. and Karacuban, H. and Nienhaus, H. and Schulz, C. and Wiggers, H.
    Nanoscale Research Letters 7 (2012)
    We present an enhanced method to form stable dispersions of medium-sized silicon nanoparticles for solar cell applications by thermally induced grafting of acrylic acid to the nanoparticle surface. In order to confirm their covalent attachment on the silicon nanoparticles and to assess the quality of the functionalization, X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier spectroscopy measurements were carried out. The stability of the dispersion was elucidated by dynamic light scattering and Zeta-potential measurements, showing no sign of degradation for months. © 2012 Bywalez et al.
    view abstract10.1186/1556-276X-7-76
  • Carbon nanowalls deposited by inductively coupled plasma enhanced chemical vapor deposition using aluminum acetylacetonate as precursor
    Jain, H.G. and Karacuban, H. and Krix, D. and Becker, H.-W. and Nienhaus, H. and Buck, V.
    Carbon 49 (2011)
    Well aligned carbon nanowalls, a few nanometers thick, were fabricated by continuous flow of aluminum acetylacetonate (Al (acac)3) without a catalyst, and independent of substrate material. The nanowalls were grown on Si, and steel substrates using inductively coupled plasma-enhanced chemical vapor deposition. Deposition parameters like flow of argon gas and substrate temperature were correlated with the growth of carbon nanowalls. For a high flow of argon carrier gas, an increased amount of aluminum in the film and a reduced lateral size of the carbon walls were found. The aluminum is present inside the carbon nanowall matrix in the form of well crystallized nanosized Al4C3 precipitates. © 2011 Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.carbon.2011.07.002
  • Stimuli-responsive track-etched membranes via surface-initiated controlled radical polymerization: Influence of grafting density and pore size
    Tomicki, F. and Krix, D. and Nienhaus, H. and Ulbricht, M.
    Journal of Membrane Science 377 (2011)
    The surface-functionalization of poly(ethylene terephthalate) track-etched membranes of different nominal pore sizes (400, 1000 and 3000. nm) with stimuli-responsive poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) via surface-initiated (SI) atom transfer radical polymerization (ATRP) was performed. Variations of grafting density and grafted chain length were achieved by variation of synthesis conditions. It could be clearly demonstrated that mixtures of reaction solutions containing different ratios of acyl bromides, only one bearing the initiator group necessary for the SI ATRP, led to different initiator group densities on the resulting track-etched membrane surface which had been verified by X-ray photoelectron spectroscopy. Moreover the mass increase as function of reaction time strongly correlated with the amount of initiator bound to the membrane surface indicating that the ATRP reaction was not limited by monomer diffusion into the pores. Scanning electron microscopy images and permporometry measurements indicated an even functionalization on the entire membrane surface which was the basis for further investigations. The stimuli-responsive properties of PDMAEMA grafted track-etched membranes were studied by permeability measurements with citrate and glycine buffers as function of pH (2 and 10) and temperature (25 and 60 °C). By that the barrier properties of the membranes could be effectively changed in two steps. The results agree with the expectation that a change in grafting density and chain length has an effect on the stimuli-responsive properties of the membrane. Results for membranes having similar degrees of grafting clearly showed that the reversible swelling of grafted polymeric layers was more pronounced for lower grafting density. © 2011 Elsevier B.V.
    view abstract10.1016/j.memsci.2011.04.028
  • Electronic excitations generated by the deposition of Mg on Mg films
    Hagemann, U. and Krix, D. and Nienhaus, H.
    Physical Review Letters 104 (2010)
    Nonadiabatic processes are observed during growth of Mg atoms from the gas phase on Mg films. Chemicurrents are measured in thin film Mg/p-Si(001) Schottky diodes which are exposed to thermally evaporated Mg atoms. The photonic and chemical contributions to the observed reverse currents in the devices can be distinguished by varying the Mg atom flux and by independent measurements using an empty evaporator as a source of heat radiation. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevLett.104.028301
  • Electronic excitations in magnesium epitaxy: Experiment and theory
    Hagemann, U. and Timmer, M. and Krix, D. and Kratzer, P. and Nienhaus, H.
    Physical Review B - Condensed Matter and Materials Physics 82 (2010)
    The nonadiabatic response of the electronic system during growth of Mg films is investigated both experimentally by measuring chemicurrents in Mg/p-Si (001) Schottky diodes, and theoretically by time-dependent perturbation theory applied to first-principles electronic-structure calculations. Reverse currents are detected in the diodes when they are exposed to thermally evaporated Mg atoms. Dissipation of condensation energy to the electronic system as well as absorption of infrared photons due to heat radiation are the current-generating mechanisms. They can be distinguished by studying the dependence of the currents on the evaporator temperature and on the Mg film thickness. In contrast to the photocurrents, the chemicurrent is proportional to the Mg atom flux as it reproduces the enthalpy of Mg sublimation in an Arrhenius diagram. Independent measurements of photocurrents by use of an empty evaporator as a source of heat radiation provide further evidence for a chemicurrent contribution to the overall signal. The presence of chemicurrents in Mg epitaxy is further supported by simulations of monolayer growth and calculations of the pertinent rates for nonadiabatic electronic transitions in Mg adsorption. The simulations show that the grown surface is atomically rough with many step and kink sites. Adsorption at these sites is sufficiently exothermic to induce energetic electron-hole pairs that give rise to a detectable current across the Schottky barrier of the diode. The calculated spectra of the excited electrons and holes are found to display high-energy tails above 0.4 eV. While the contribution of the electronic channel to the dissipation of condensation energy is very small (less than 1%), the calculated probability for high-energy electronic excitations in Mg epitaxy is compatible with the chemicurrent contribution extracted from the experimental data. © 2010 The American Physical Society.
    view abstract10.1103/PhysRevB.82.155420
  • chemically induced electronic excitations

  • chemiluminescence

  • electronic properties

  • interfaces

  • nanoparticles

  • nanostructures

  • sensors

  • surfaces

  • vibrational properties

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