Zeitschriftenaufsätze und Buchbeiträge

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Koch, Juliane; Liborius, Lisa; Kleinschmidt, Peter; Prost, Werner; Weimann, Nils; Hannappel, Thomas
Impact of the tip-to-semiconductor contact in the electrical characterization of nanowires. - In: ACS omega, ISSN 2470-1343, Bd. 9 (2024), 5, S. 5788-5797

Well-defined semiconductor heterostructures are a basic requirement for the development of high-performance optoelectronic devices. In order to achieve the desired properties, a thorough study of the electrical behavior with a suitable spatial resolution is essential. For this, various sophisticated tip-based methods can be employed, such as conductive atomic force microscopy or multitip scanning tunneling microscopy (MT-STM). We demonstrate that in any tip-based measurement method, the tip-to-semiconductor contact is decisive for reliable and precise measurements and in interpreting the properties of the sample. For that, we used our ultrahigh-vacuum-based MT-STM coupled in vacuo to a reactor for the preparation of nanowires (NWs) with metal organic vapor phase epitaxy, and operated our MT-STM as a four-point nanoprober on III-V semiconductor NW heterostructures. We investigated a variety of upright, free-standing NWs with axial as well as coaxial heterostructures on the growth substrates. Our investigation reveals charging currents at the interface between the measuring tip and the semiconductor via native insulating oxide layers, which act as a metal-insulator-semiconductor capacitor with charging and discharging conditions in the operating voltage range. We analyze in detail the observed I-V characteristics and propose a strategy to achieve an optimized tip-to-semiconductor junction, which includes the influence of the native oxide layer on the overall electrical measurements. Our advanced experimental procedure enables a direct relation between the tip-to-NW junction and the electronic properties of as-grown (co)axial NWs providing precise guidance for all future tip-based investigations.

Liu, Fengli; Yan, Yong; Chen, Ge; Wang, Dong
Recent advances in ambient electrochemical methane conversion to oxygenates using metal oxide electrocatalysts. - In: Green chemistry, ISSN 1463-9270, Bd. 26 (2024), 2, S. 655-677

To reach a decarbonized future, the conversion of greenhouse gases into green fuels and valuable chemicals is of crucial importance. Methane emissions are the second most significant contributor to global warming. Recent advances in electrocatalytic partial oxidation of methane to high-value fuels at ambient temperatures promise to sidestep the requirement of high temperature in conventional thermal catalysis and provide a revolutionary, sustainable, and decentralized alternative to flaring. Electrocatalysts that can selectively produce valuable compounds from methane under mild conditions are essential for commercialization. This review covers current developments in the electrochemical partial oxidation of methane to oxygenates, with an emphasis on metal oxide electrocatalysts. The regularly deployed strategies, including doping and interface engineering, are systematically reviewed in detail. In addition, the design of the electrolytic cell, the electrolyte, time, potential, and temperature are examined thoroughly and discussed.

Li, Feitao; Tan, Xinu; Flock, Dominik; Oliva Ramírez, Manuel; Wang, Dong; Qiu, Risheng; Schaaf, Peter
Structure-dependent oxidation behavior of Au-Cu nanoparticles. - In: Journal of alloys and compounds, ISSN 1873-4669, Bd. 976 (2024), 173179, S. 1-8

Thermal oxidation is an easily controlled method to change the physical and chemical properties of nanoparticles, thus optimizing and expanding their applications. Unfortunately, less attention has been paid to the role of the crystal structure whose atomic arrangements can be critical for oxidation. Au-Cu nanoparticles showing a fast order-disorder transformation are oxidized at two temperatures of ordered (L10) and disordered (A1) phase regions. The oxidation rates between the two phases are compared by the Arrhenius equation, and a lower oxidation rate is determined in the L10 lattice than in the A1 lattice based on the time required for the complete oxidation. One possible reason is attributed to the longer diffusion length in the L10 lattice compared to the A1 lattice due to the anisotropic diffusion path of the former while isotropic diffusion of the latter, resulting in longer oxidation time and then slower oxidation for the ordered sample. The crystalline phase of Au-Cu nanoparticles can be straightforwardly tuned and the resulting atomic disposition is a powerful tool to control oxidation evolution.

Grad, Marius; Honig, Hauke; Diemar, Andreas; Flock, Dominik; Spieß, Lothar
Complex material analysis of a TiC coating produced by hot pressing with optical light microscopy, EDS, XRD, GDOES and EBSD. - In: Surface and coatings technology, ISSN 1879-3347, Bd. 476 (2024), 130265, S. 1-11

The present study investigates the interface between carbon steel and titanium samples annealed at different temperatures (ϑ1 = Image 1 and ϑ2 = Image 2). In both cases, an observable layer forms at the interface, with its thickness increasing from tϑ1 = 2.75 ± Image 3 at Image 1 to tϑ2 = 8.86 ± Image 4 at Image 2. The layer's composition and thickness evolve with temperature. Analysis reveals approximately 40 at.-% carbon concentration in the exterior region, indicating likely titanium carbide creation. X-ray diffraction identifies titanium carbide peaks, while microscopy and elemental mapping confirm compositional gradients at the interface. Electron Backscatter Diffraction (EBSD) shows a gradient in grain size near the TiC surface, reflecting TiC nucleation rates. XRD data detect both titanium carbide and titanium phases, with TiC becoming more prominent at Image 2. Rietveld analysis further confirms TiC formation. Notably, distinct diffraction patterns on the contact and rear sides suggest a Ti(C, O, N) presence. Depth profiles exhibit varying surface and depth carbon concentrations, attributed to temperature effects. The study successfully demonstrates TiC coating fabrication through hot pressing, wherein Ti(C, O, N) coatings arise from titanium's affinity for reacting with oxygen and nitrogen. This research contributes to the understanding of phase transformations and interfacial properties in titanium-carbon steel systems.

Lampouras, Ioannis; Holz, Mathias; Strehle, Steffen; Körner, Julia
Precisely controlled batch-fabrication of highly sensitive co-resonant cantilever sensors from silicon-nitride. - In: Journal of micromechanics and microengineering, ISSN 1361-6439, Bd. 34 (2024), 1, 015005, S. 1-14

Dynamic-mode cantilever sensors are based on the principle of a one-side clamped beam being excited to oscillate at or close to its resonance frequency. An external interaction on the cantilever alters its oscillatory state, and this change can be detected and used for quantification of the external influence (e.g. a force or mass load). A very promising approach to significantly improve sensitivity without modifying the established laser-based oscillation transduction is the co-resonant coupling of a micro- and a nanocantilever. Thereby, each resonator is optimized for a specific purpose, i.e. the microcantilever for reliable oscillation detection and the nanocantilever for highest sensitivity through low rigidity and mass. To achieve the co-resonant state, the eigenfrequencies of micro- and nanocantilever need to be adjusted so that they differ by less than approximately 20%. This can either be realized by mass deposition or trimming of the nanocantilever, or by choice of dimensions. While the former is a manual and error-prone process, the latter would enable reproducible batch fabrication of coupled systems with predefined eigenfrequency matching states and therefore sensor properties. However, the approach is very challenging as it requires a precisely controlled fabrication process. Here, for the first time, such a process for batch fabrication of inherently geometrically eigenfrequency matched co-resonant cantilever structures is presented and characterized. It is based on conventional microfabrication techniques and the structures are made from 1 µm thick low-stress silicon nitride. They comprise the microcantilever and high aspect ratio nanocantilever (width 2 µm, thickness about 100 nm, lengths up to 80 µm) which are successfully realized with only minimal bending. An average yield of % of intact complete sensor structures per wafer is achieved. Desired geometric dimensions can be realized within ±1% variation for length and width of the microcantilever and nanocantilever length, ±10% and ±20% for the nanocantilever width and thickness, respectively, resulting in an average variation of its eigenfrequency by 11%. Furthermore, the dynamic oscillation properties are verified by vibration experiments in a scanning electron microscope. The developed process allows for the first time the batch fabrication of co-resonantly coupled systems with predefined properties and controlled matching states. This is an important step and crucial foundation for a broader applicability of the co-resonant approach for sensitivity enhancement of dynamic-mode cantilever sensors.

Omar, Nurul Amanina Binti; Köster, Frank; Hahn, Frank; Bund, Andreas
Hardness and tribological behaviour of annealed electroless nickel phosphorus composite layers with incorporated boron particles. - In: Surface and coatings technology, ISSN 1879-3347, Bd. 476 (2024), 130261, S. 1-17

In this study, a possible alternative to hard chromium coatings is investigated. Amorphous boron particles have been incorporated in electroless nickel‑phosphorus (NiP) deposits, yielding a dispersion coating. The distribution of the particles is homogenous and the maximum mass fraction of particles embedded in the coating is 6.2 ± 0.2 wt%. Measurements of the zeta potential and particle size of amorphous boron particles in a diluted electrolyte showed that the particles withstood agglomeration until 120 days. Primary and secondary hardness maxima are observed after thermal annealing at 400 ˚C and 860 ˚C due to the formation of nickel phosphide, nickel boride and nickel boride phosphide phases. X-ray diffractometry shows an increase in nickel and nickel phosphide crystal size at 400 ˚C before levelling off at 600 ˚C. The annealing duration should be kept between 30 and 60 min for optimal hardness. The wear resistance increases when the coating is annealed at 400 ˚C. DSC measurements on nickel phosphorus incorporated with boron particles, Ni-P-B, bulk material with P-content (9.6 ± 0,6 wt%) and B-content (4.5 ± 0.8 wt%) showed that the solidus line lies at 926 ˚C, which is why a maximum annealing temperature of 860 ˚C was chosen to avoid melting of the material. The relative texture and phase coefficients, RTC and RPC, showed that the nickel phase is preferred in the NiP system at 400 ˚C and 600 ˚C while the Ni3P phase is preferred in the Ni-P-B system at the same annealing temperature. REM and EDX area analyses are used to show the areal distribution of nickel, phosphorus, and boron before and after the annealing process along the thickness of the coating. A diffusion layer between substrate and coating that contains iron nickel boride and iron nickel phosphide lamellar structure is observed.

Reuter, Christoph; Ecke, Gernot; Strehle, Steffen
Exploring the surface oxidation and environmental instability of 2H-/1T’-MoTe2 using field emission based scanning probe lithography. - In: Advanced materials, ISSN 1521-4095, Bd. 36 (2024), 4, 2310887, S. 1-14

An unconventional approach for the resistless nanopatterning 2H- and 1T’-MoTe2 by means of scanning probe lithography is presented. A Fowler-Nordheim tunneling current of low energetic electrons (E = 30-60 eV) emitted from the tip of an atomic force microscopy (AFM) cantilever is utilized to induce a nanoscale oxidation on a MoTe2 nanosheet surface under ambient conditions. Due to the water solubility of the generated oxide, a direct pattern transfer into the MoTe2 surface can be achieved by a simple immersion of the sample in deionized water. The tip-grown oxide was characterized using Auger electron and Raman spectroscopy, revealing it consists of amorphous MoO3/MoOx as well as TeO2/TeOx. With the presented technology in combination with subsequent AFM imaging it was possible to demonstrate a strong anisotropic sensitivity of 1T’-/(Td)-MoTe2 to aqueous environments. We finally used the discussed approach to structure a nanoribbon field effect transistor out of a few-layer 2H-MoTe2 nanosheet. This article is protected by copyright. All rights reserved

Luo, Wenjun; Xuan, Xinmiao; Shen, Jinfeng; Cheng, Pengfei; Wang, Dong; Schaaf, Peter; Zhang, Zhang; Liu, Junming
High performance photothermal carbon nanotubes/nanostructured hydrogel for solar electricity production and solar water sterilization. - In: Applied surface science, Bd. 643 (2024), 158680

Solar energy is a promising renewable energy source with the potential to contribute to sustainable development. Efficient photothermal conversion is critical for solar energy acquisition and conversion. Here, carbon nanotubes (CNTs) were gelatinized to obtain the nanostructured CNT/hydrogel, and then highly light-absorbing CNT/n-hydrogels with surface texture were obtained by replicating the micrometer structure from the black silicon (b-Si) surface onto CNT/hydrogels by using a PDMS mold. Through the synergistic effect of both surface texture and nanostructures, it demonstrates high efficiency of solar electricity production and solar sterilization. A small thermoelectric (TE) module with an area of 4 × 4 cm2 is integrated with CNT/n-hydrogel absorber for the investigation of photo-thermoelectric conversion. The output power of the CNT/n-hydrogel TE device is 1.42 W•m−2 under 1 sun. And by connecting four devices in series, it has successfully demonstrated for charging mobile phones under two different solar illuminations. This work provides a cost-effective and easy fabrication method for opening up the hydrogel as a photothermal absorber, which is low-cost, reproducible, high-efficiency solar water sterilization and high photothermal conversion efficiency.

Supreeti, Shraddha; Fischer, Michael; Fritz, Mathias; Müller, Jens
High-resolution patterning on LTCC by transfer of photolithography-based metallic microstructures. - In: International journal of applied ceramic technology, ISSN 1744-7402, Bd. 21 (2024), 2, S. 1180-1190

The growing applications and constant miniaturization of electronic devices and of low-temperature co-fired ceramics (LTCC) in various fields, such as aviation, telecommunications, automotive, satellite communications, and military, have led to an increase in the demand for LTCC. Such prospects arise due to the continuous scaling down of components and high-density interconnection in electronics packaging. This paper reports a technique for the transfer of high-resolution microstructures from silicon substrates to LTCC. In this method, gold and copper patterns were formed by photolithography, electrodeposition, and residual layer stripping on silicon substrate. Lithography provides the opportunity to create and transfer complex patterns for use in several different applications and electroplating enables the use of pure metal for excellent electrical properties. The developed structures were transferred onto a top layer of LTCC tape using hot embossing. Then, the subsequent layers were stacked, laminated, and sintered. A resolution of 1.5 μm after free sintering and 4.5 μm after pressure-assisted sintering was achieved. This distinctive method can be useful for several applications requiring high-resolution and superior electrical properties.

Wang, Honglei; Bo, Yifan; Klingenhof, Malte Philipp Helmuth; Peng, Jiali; Wang, Dong; Wu, Bing; Pezoldt, Jörg; Cheng, Pengfei; Knauer, Andrea; Hua, Weibo; Wang, Hongguang; Aken, Peter Antonie van; Sofer, Zdeněk; Strasser, Peter; Guldi, Dirk; Schaaf, Peter
A universal design strategy based on NiPS3 nanosheets towards efficient photothermal conversion and solar desalination. - In: Advanced functional materials, ISSN 1616-3028, Bd. n/a (2023), n/a, 2310942, S. 1-11

2D nanomaterials are proposed as promising photothermal materials for interfacial photothermal water evaporation. However, low evaporation efficiency, the use of hazardous hydrofluoric solution, and poor stability severely limit their practical applications. Here, a mixed solvent exfoliation surface deposition (MSESD) strategy for the preparation of NiPS3 nanosheets and NiPS3/polyvinyl alcohol (PVA) converter is successfully developed. The converter is obtained by drop-casting the NiPS3/PVA nanosheets onto a sponge. The PVA is mainly deposited on the edge of NiPS3 nanosheets, which not only improves the stability of NiPS3 nanosheets, but also adheres to the sponge to prepare a 3D photothermal converter, which shows an evaporation rate of 1.48 kg m−2 h−1 and the average photothermal conversion efficiency (PTCE) of 93.5% under a light intensity of 1 kW m−2. The photothermal conversion mechanism reveals that the energy of absorbed photons in NiPS3 nanosheets can be effectively converted into heat through non-radiative photon transitions as well as multiple optical interactions. To the best of the knowledge, this is the first report on the application of 2D metal-phosphorus-chalcogen (MPChx) for solar desalination, which provides new insights and guidance for the development of high-performance 2D photothermal materials.