Winfried Daum

Clausthal University of Technology

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G. Lilienkamp

Clausthal University of Technology

A. Waag

Technische Universität Braunschweig

Benjamin Borkenhagen

Clausthal University of Technology

Björn Braunschweig

University of Münster

Lorenzo Caccamo

Technische Universität Braunschweig

Joan Daniel Prades

Universitat de Barcelona

Thomas Hannappel

Technische Universität Ilmenau

Hutomo Suryo Wasisto

Technische Universität Braunschweig

Nurhalis Majid

National Research and Innovation Agency

Henning Döscher

Fraunhofer Institute for Systems and Innovation Research

Cooperative Institutions

Technische Universität Braunschweig

Clausthal University of Technology

Universitat de Barcelona

Institut de Nanociència i Nanotecnologia de la Universitat de Barcelona

Helmholtz-Zentrum Berlin für Materialien und Energie

Technische Universität Ilmenau

Supply Chain Competence Center (Germany)

University of Freiburg

FC Barcelona

Physikalisch-Technische Bundesanstalt

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High-density array of Au nanowires coupled by plasmon modes

Acta Physica Sinica (2012)

Hongdan Yan P. Lemmens Johannes Ahrens Martin Br ring Sven Burger

Au nanowire arrays with high aspect ratios are prepared in anode aluminum oxide templates by electrochemical deposition. The obtained structures are investigated by scanning Auger microscopy. Surface plasmon resonances of Au nanowire arrays induce a transverse mode (T mode) and a longitudinal mode (L mode) in the optical absorption, which indicates the strong anisotropy of the Au nanowires. The L mode intensity is related to the angle and polarization of the incident light. The L mode position shows a shift with the increase of aspect ratio of the nanowires. The plasmon coupling between Au nanowires is compared with the H-aggregation of organic chromophores. The blue shift of the L mode in the arrays compared with a single nanowire is induced by multi-coupling of the electromagnetic field between neighbouring nanowires. A finite element method is used to simulate the electric field distributions of a single Au nanowire and an array of plasmonically coupled wires.

Localized surface plasmon

Transverse mode

10.7498/aps.61.237105

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Citations (7)

Cu/Au(111) Surfaces and AuCu Intermetallics for Electrocatalytic Reduction of CO₂ in Ionic Liquid Electrolytes

ACS Catalysis (2024)

Björn Ratschmeier Christian Paulsen Klaus Stallberg Gina Roß Winfried Daum

Room-temperature ionic liquids (RTIL) are important alternatives to aqueous electrolytes in electrocatalytic reactions, batteries, and fuel cells. They are known to reduce existing high overpotentials and increase CO2 solubility as well as product selectivity in CO2 reduction reactions (CO2RR). In our work, we have studied the activity for CO2RR of Au(111), Cu(111), and Cu-modified Au(111) electrodes with 1/3, 2/3, and 3/3 Cu monolayers, as well as of AuCu and AuCu3 intermetallics in contact with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][NTf2] electrolytes with 1.5 M H2O. Using offline gas chromatography (GC), we demonstrate the formation of H2 and mainly CO as the only reduction products at Au(111), while exclusively H2 is formed at Cu(111). Synergistic electronic and geometric effects lead to higher levels of CO formation at Cu-modified Au(111) electrodes in comparison to neat Au(111). Operando IR absorption spectroscopy (IRAS) of the bulk electrolyte shows the formation of a 2-imidazolium carboxylic acid intermediate that can lower the overpotential for CO2 reduction and does not require stabilization of a CO2– radical anion as an alternative intermediate at the interface. Systematic variation of the copper content at the catalysts' surfaces enables us to adjust the H2/CO syngas ratio to a maximum of 1.8 for Cu-modified Au(111) electrodes and ∼3.2 for AuCu3 catalysts at electrolysis times of 20 min, demonstrating a large tunability of the syngas ratio with electrode potential. The observed range of H2/CO ratios includes the ideal ratio of 2 for the Fischer–Tropsch process to produce hydrocarbons and the ratio of 3 needed for methanation.

Overpotential

10.1021/acscatal.3c04592

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Citations (11)

Non-Destructive, Large-Scale Imaging of Anti-Phase Disorder in GaP Epilayers on Si(001) Using Low-Energy Electron Microscopy

Meeting abstracts/Meeting abstracts (Electrochemical Society. CD-ROM) (2012)

Benjamin Borkenhagen G. Lilienkamp Winfried Daum Henning Döscher Thomas Hannappel

Abstract not Available.

Low-energy electron microscopy

10.1149/ma2012-01/17/786

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A Highly Selective and Self‐Powered Gas Sensor Via Organic Surface Functionalization of p‐Si/n‐ZnO Diodes

Advanced Materials (2014)

Martin Hoffmann Leonhard Mayrhofer Olga Casals Lorenzo Caccamo Francisco Hernández-Ramírez

Selectivity and low power consumption are major challenges in the development of sophisticated gas sensor devices. A sensor system is presented that unifies selective sensor-gas interactions and energy-harvesting properties, using defined organic-inorganic hybrid materials. Simulations of chemical-binding interactions and the consequent electronic surface modulation give more insight into the complex sensing mechanism of selective gas detection.

Surface Modification

10.1002/adma.201403073

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Citations (111)

Catalytic CO oxidation on Pt under near ambient pressure: A NAP-LEEM study

Ultramicroscopy (2019)

Torsten Franz Bernhard von Boehn Helder Marchetto Benjamin Borkenhagen G. Lilienkamp

Low-energy electron microscopy

Ambient pressure

Photoemission electron microscopy

10.1016/j.ultramic.2019.02.024

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Structure and Origin of Antiphase Domains and Related Defects in Thin GaP Epilayers on As-Modified Si(100)

Crystal Growth & Design (2022)

Benjamin Borkenhagen Agnieszka Paszuk Franz Niklas Knoop Oliver Supplie Manali Nandy

We study the origin and formation of antiphase domains (APDs) and related defects in 7 nm thin, lattice-matched GaP buffer layers deposited by metal–organic chemical vapor deposition (MOCVD) on well-defined, nearly single-domain, double-layer stepped, low-miscut Si(100) substrates obtained by specific treatment with arsenic. Using dark-field imaging modes in low-energy electron microscopy (LEEM), the minority reconstruction domains of Si(100):As and the APDs of the deposited GaP epilayer are identified, quantified, and compared. We show that residual (2 × 1)-reconstructed terraces of the minority domain on the Si substrate cause the formation of APDs and that the fraction of the minority domain of the substrate (≅0.07) entails a comparable fraction of APDs in thin GaP epilayers. The topographies of APDs are revealed by atomic force microscopy (AFM) and by scanning tunneling microscopy (STM). We observe two very different APD-related defects in the GaP epilayer, both pinned to residual monolayer steps of the substrate. GaP growth on minority domain terraces with widths in the range of 40–100 nm gives rise to APDs of comparable lateral dimensions. Minority domain terraces of the substrate with widths <20 nm cause the formation of 7–20 nm wide trenches in the GaP layer with rampart-like mounds along their rims. Using nanoscale Auger electron spectroscopy (AES), we provide evidence that these trenches extend through the GaP layer down to the exposed, uncovered Si substrate. We conclude that nucleation of GaP on small minority domain terraces is largely inhibited as most Ga and P atoms deposited on these terraces diffuse across the domain boundary and side walls of emerging trenches to adjacent majority domain terraces where they form the observed mounds. Nucleation of GaP does take place on minority domain terraces with widths ≥40 nm and leads to the growth of APDs.

10.1021/acs.cgd.2c00697

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Citations (2)

Molecular Structure of Charged α-Al2O3(0001) Surfaces in Aqueous Electrolytes Probed with Sum-Frequency Vibration Spectroscopy

Meeting abstracts/Meeting abstracts (Electrochemical Society. CD-ROM) (2012)

Winfried Daum Armin Rumpel Björn Braunschweig

Abstract not Available.

10.1149/ma2012-01/42/1563

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Laser lassen Molekülschwingungen an Grenzflächen erkennen

Physik in unserer Zeit (1996)

Winfried Daum

Abstract Mit Hilfe der optischen Frequenzmischung unter Verwendung kurzer, intensiver Laserpulse können Moleküle an Grenzflächen spektroskopisch nachgewiesen werden. Diese Methode wird am Forschungszentrum Jülich eingesetzt, um Anlagerungen und Reaktionen von Molekülen an Grenzflächen zwischen Festkörpern und Flüssigkeiten zu untersuchen.

10.1002/piuz.19960270608

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Transferable Substrateless GaN LED Chips Produced by Femtosecond Laser Lift-Off for Flexible Sensor Applications

Nursidik Yulianto Steffen Bornemann Lars Daul Christoph Margenfeld Irene Manglano Clavero

Transferable substrate-less InGaN/GaN light-emitting diode (LED) chips have successfully been fabricated in a laser lift-off (LLO) process employing high power ultrashort laser pulses with a wavelength of 520 nm. The irradiation of the sample was conducted in two sequential steps involving high and low pulse energies from the backside of the sapphire substrate, which led to self-detachment of the GaN stack layer without any additional tape release procedure. To guarantee their optoelectrical function and surface quality, the lifted LED chips were assessed in scanning electron microscopy (SEM) and electroluminescence (EL) measurements. Moreover, surface characterizations were done using atomic force microscopy (AFM) and Auger Electron Spectroscopy (AES).

Auger electron spectroscopy

10.3390/proceedings2130891

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Charge Transfer across the n-Gallium Phosphide(100) Photoanode/Electrolyte Interface during Photoelectrochemical Water Splitting

Meeting abstracts/Meeting abstracts (Electrochemical Society. CD-ROM) (2018)

Waqas Saddique G. Lilienkamp Winfried Daum

A detailed understanding of charge transfer mechanisms at the electrode/electrolyte interface is important for the development of electrodes for efficient photoelectrochemical (PEC) processes. We have studied the charge transfer processes across the n-GaP(100) photoanode/electrolyte interface. In a 0.02 M HCl electrolyte, high photoanodic currents from the n-GaP(100) photoanodes related to photolytic water splitting were measured at low anodic potentials but these photocurrents diminished at cathodic potentials and high anodic potentials. Electrochemical impedance spectroscopy (EIS) was carried out for n-GaP(100) photoanodes at different potentials to analyze the relevant charge transfer processes. Our EIS results suggest that the adsorption of hydroxide on metal-like surface Ga and their subsequent oxidative transformations – formation of these surface species is most favorable at low anodic potentials – is the driving force for high photoanodic currents at low anodic potentials. III-V semiconductors are prone to corrosion during PEC water splitting processes with corrosion-related decrease of efficiency. Our n-GaP(100) photoanodes were surface-conditioned via oxidizing at 0.8 V vs RHE and subsequently hydrogenated to passivate the defects in the oxide film. After this preparation, water splitting was observed at potentials between 0 and 0.3 V. The Nyquist plots derived from our EIS measurements for the n-GaP(100) photoanode consist of three semicircles indicating three distinct charge transfer mechanisms. The plots were analyzed with two different equivalent electrical circuits for the electrode/electrolyte system. These equivalent electrical circuits allowed to evaluate the relevance of different possible charge transfer pathways from the n-GaP(100) photoanode to the electrolyte. For each equivalent electrical circuit, the potential dependence of the resistances and capacitances including constant phase elements were determined from the fit and then compared with the potential variation of the current in the cyclic voltammogram. In a theoretical study Bertoluzzi et al. discussed the PEC implications of charge transfer from an n-doped semiconductor to the electrolyte that takes place via two different pathways 1 : by a direct charge transfer from the valence band of the semiconductor to the electrolyte, and by an indirect charge transfer from the valence band to the electrolyte via surface states (or defect states at the surface). The latter gives rise to a pronounced maximum of the interface- or defect-related capacitance as a function of potential. Our EIS results suggest that at low anodic potentials, the effective charge transfer process occurs from the valence band of the n-GaP(100) photoanode to defect states present in a thin Ga 2 O 3 -like layer at the surface, and subsequently, from these defect states to the electrolyte. This indirect charge transfer pathway is derived from a pronounced maximum of the capacitance as a function of applied potential. Our experimental results also suggest that subsequent charge transfer from these defect states leads to the formation of a molecular hydroxide layer the surface of metal-like Ga regions at the n-GaP(100) photoanode. Metal-like Ga at the surface of the photoanode was identified by XPS. A tafel slop of 63 mV/decade measured at the onset potential of the photoanodic current strongly points to the presence of adsorbed metal hydroxide (GaOH) species and subsequent oxidation products GaO and GaOOH as surface intermediates for the oxygen evolution which are finally catalyzed to molecular oxygen 2 . Our electrochemical impedance spectroscopy results also suggest that at low anodic potentials, overpotentials for the formation of hydroxide absorbates and their subsequent oxidation are lowest whereas at higher anodic potentials these hydroxide absorbates are stable and do not oxidize further to produce oxygen. At cathodic potentials, on the other hand, these surface hydroxides are reduced. As a consequence, large PEC currents associated with oxygen formation are possible only at low anodic potentials. By analyzing and understanding the charge transfer processes and the effect of different electrochemical parameters on these processes, we were able to optimize the PEC conditions which finally led to photolytic water splitting without applied voltage or addition of catalysts. Bertoluzzi, L; Lopez-Varo, P; Tejada, J. A. J; Bisquert, J., J. Mater. Chem. A., 2016, 4 , 2873. Antoine, O; Butel, Y; Durand, R., J. Electroanal. Chem. 2001, 499 , 85.

Passivation

Photoelectrochemical cell

Photoelectrochemistry

Gallium phosphide

Half-cell

10.1149/ma2018-01/39/2290

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