Atomic surface structure of MOVPE-prepared GaP(111)B. - In: Applied surface science, Bd. 534 (2020), 147346
Controlling the surface formation of the group-V face of (111)-oriented III-V semiconductors is crucial for subsequent successful growth of III-V nanowires for electronic and optoelectronic applications. With a view to preparing GaP/Si(111) virtual substrates, we investigate the atomic structure of the MOVPE (metalorganic vapor phase epitaxy)-prepared GaP(111)B surface (phosphorus face). We find that upon high-temperature annealing in the H2-based MOVPE process ambience, the surface is phosphorus-depleted, as evidenced by X-ray photoemission spectroscopy (XPS). However, a combination of density functional theory calculations and scanning tunneling microscopy (STM) suggests the formation of a partially H-terminated phosphorus surface, where the STM contrast is due to electrons tunneling from non-terminated dangling bonds of the phosphorus face. Atomic force microscopy (AFM) reveals that a high proportion of the surface is covered by islands, which are confirmed as Ga-rich by Auger electron spectroscopy (AES). We conclude that the STM images of the samples after high-temperature annealing only reflect the flat regions of the partially H-terminated phosphorus face, whereas an increasing coverage with Ga-rich islands, as detected by AFM and AES, forms upon annealing and underlies the higher proportion of Ga in the XPS measurements.
https://doi.org/10.1016/j.apsusc.2020.147346
Investigation of the reaction kinetics of chromium(III) ions with carboxylic acids in aqueous solutions and the associated effects on chromium deposition. - In: Journal of the Electrochemical Society, ISSN 1945-7111, Bd. 167 (2020), 16, 162509, insges. 8 S.
Electroplated chromium is an important industrial coating. Both in the decorative and functional field, research is being carried out to develop trivalent chromium electrolytes. In this work, the interaction between CrIII ions and carboxylic acids (malonic acid, malic acid, oxalic acid and glycolic acid) is investigated. The use of carboxylic acids to improve the current efficiency in trivalent chromium plating baths has been known for a long time. But the thermodynamics and kinetics of the associated complexing reactions are not well understood. It is shown that the complexes form at different rates depending on the acid and its concentration and have a varying influence on chromium deposition. For the development of a technical bath, care must be taken to ensure that the molar ratios between chromium and carboxylic acids can be controlled. This is possible with the HPLC method presented here.
https://doi.org/10.1149/1945-7111/abd1f4
Electrogravimetry and structural properties of thin silicon layers deposited in sulfolane and ionic liquid electrolytes. - In: ACS applied materials & interfaces, ISSN 1944-8252, Bd. 12 (2020), 51, S. 57526-57538
Potentiostatic deposition of silicon is performed in sulfolane (SL) and ionic liquid (IL) electrolytes. Electrochemical quartz crystal microbalance with damping monitoring (EQCM-D) is used as main analytical tool for the characterization of the reduction process. The apparent molar mass (Mapp) is applied for in situ estimation of the layer contamination. By means of this approach, appropriate electrolyte composition and substrate type are selected to optimize the structural properties of the layers. The application of SL electrolyte results in silicon deposition with higher efficiency compared to the IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMP][TFSI]. This has been associated with the instability of the IL in the presence of silicon tetrachloride and the enhanced incorporation of IL decomposition products into the growing silicon deposit. X-ray photoelectron spectroscopy (XPS) analysis supports the results about the layer composition, as suggested from the microgravimetric experiments. Attention has been given to the impact of practically relevant substrates (i.e., Cu, Ni, and vitreous carbon) on the reduction process. An effective deposition can be carried out on the metal electrodes in both electrolytes due to accelerated reaction kinetics for these types of substrates. However, on vitreous carbon (VC), a successful reduction of SiCl4 can only be accomplished in the IL, while the electroreduction process in SL is dominated by the decomposition of the electrolyte. For short deposition times, the scanning electron microscopy (SEM) images display rough morphologies in the nanometer range, which evolve further to structures with increased length scale of the surface roughness. The development of a rough interface during deposition, resulting in QCM damping at advanced stages of the process, is interpreted by a model accounting for the resistive force caused by the interaction of the liquid with a nonuniform layer interface. By using this approach, the individual contributions of the surface roughness and viscoelastic effects to the measured damping values are estimated.
https://doi.org/10.1021/acsami.0c14694
Metastable atomic layer deposition: 3D self-assembly toward ultradark materials. - In: ACS nano, ISSN 1936-086X, Bd. 14 (2020), 11, S. 15023-15031
Black body materials are promising candidates to meet future energy demands, as they are able to harvest energy from the total bandwidth of solar radiation. Here, we report on high-absorption near-blackbody-like structures (>98% for a wide solar spectrum range from 220 to 2500 nm) consisting of a silica scaffold and Ag nanoparticles with a layer thickness below 10 m, fabricated using metastable atomic layer deposition (MS-ALD). Several effects contribute collectively and in a synergistic manner to the ultrahigh absorption, including the pronounced heterogeneity of the nanoparticles in size and shape, particle plasmon hybridization, and the trapping of omnidirectionally scattered light in the 3D hierarchical hybrid structures. We propose that, in the future, MS-ALD needs to be considered as a simple and promising method to fabricate blackbody materials with excellent broadband absorption.
https://doi.org/10.1021/acsnano.0c04974
Ni3N-coated Ni nanorod arrays for hydrogen and oxygen evolution in electrochemical water splitting. - In: ACS applied nano materials, ISSN 2574-0970, Bd. 3 (2020), 11, S. 10986-10995
Im Titel ist "3" tiefgestellt
Designing high efficiency non-noble metal catalysts for electrochemical hydrogen and oxygen evolution reactions (HER and OER) is a challenging and promising task. In the present work, a large-scale self-supported black nickel sheet composed of nickel nanorod arrays (B-Ni) was successfully fabricated via electroplating with a nanostructured Si template assisted strategy, and Ni3N coating layer was coated on the structured surface followed by a nitrogen plasma treatment with different treatment durations. The black color is due to the immensely enhanced light absorption through the surface nanostructures with high aspect ratio. The samples show promising catalytic performance when used as electrochemical catalysts for both HER and OER in alkaline electrolyte. Benefiting from the nanostructure and the Ni3N coating, B-Ni_N2_10 min exhibits low onset potentials of 1.560 and 0.165 V at a current density of 10 mA/cm2 for OER and HER, respectively. The calculated electrochemical surface area (ECSA) for B-Ni is 10 times higher than the pristine Ni sheet (P-Ni), while after nitrogen plasma treatment, the ECSA for B-Ni_N2_10 min is 21 times higher than that of the P-Ni. In addition, all the catalysts show good electrochemical stabilities. All the improvements for the electrochemical performances suggest that nanostructured nickel nitride could be a promising inexpensive catalyst system for both OER and HER processes.
https://doi.org/10.1021/acsanm.0c02222
Physics-based modeling and parameter identification for lithium ion batteries under high current discharge conditions. - In: Journal of the Electrochemical Society, ISSN 1945-7111, Volume 167 (2020), number 14, 140549
Extreme scenarios of high discharge current must be understood for better battery management system design. Physics-based modeling can give a better insight into the battery response but can be challenging due to the large number of parameters. In this work, an electrochemical pseudo-2D model is developed and used in the parameter identification and validated under high current discharge conditions. Commercial 18650 cells with maximum rated current of 20 A (13.3 C) are characterized with discharge rates up to 40 C under controlled thermal conditions. The proposed three-step parameter identification procedure starts with the open circuit voltage being used to estimate the equilibrium potentials. In a second step, kinetic parameters are identified under high current aided by a parameter sensitivity analysis and parameter optimization with an evolutionary algorithm. The third step is the verification by comparing simulation results with measurements resulting in root main square error under 89 mV for currents until 26.6 C. Limits of the model are explored in the 33.3 C case, where a parameter re-fit shows that polarization effects change for very high current.
https://doi.org/10.1149/1945-7111/abc726
Localized and programmable chemical vapor deposition using an electrically charged and guided molecular flux. - In: ACS nano, ISSN 1936-086X, Bd. 14 (2020), 10, S. 12885-12894
Chemical vapor deposition is a widely used material deposition technique. It commonly provides a uniform material flux to the substrate to cause uniform thin film growth. However, the ability to precisely adjust the local deposition rate would be highly preferable. This communication reports on a chemical vapor deposition method performed in a localized and programmable fashion by introducing an electrically charged and guided molecular flux. This allows for local adjustments of the deposition rate and three-dimensional shape by controlling the electric fields. Specifically, the precursor molecules are charged and then guided by arrays of electrodynamic funnels, which are created by a patterned dielectric layer, to predetermined deposition locations with a minimal spot size of 250 nm. Furthermore, nearest neighbor coupling is reported as a shaping method to cause the deposition of three-dimensional nanostructures. Additionally, the integration of individually addressable domain electrodes offers programmable charge dissipation to achieve an ON/OFF control. The described method is applicable to a wide variety of materials and precursors. Here, the localized and programmable deposition of three-dimensional copper oxide, chromium oxide, zinc oxide, and carbon nanowires is demonstrated.
https://doi.org/10.1021/acsnano.0c03726
Metal-insulator transition in crystalline V2O3 thin films probed at atomic-scale using emission Mössbauer spectroscopy. - In: Thin solid films, ISSN 1879-2731, Bd. 714 (2020), 138389
Im Titel sind "2" und "3" tiefgestellt
Microscopic understanding the metal-to-insulator transition (MIT) in strongly correlated materials is critical to the design and control of modern "beyond silicon" Mott nanodevices. In this work, the local MIT behaviors in single crystalline V2O3 thin films were probed on an atomic scale by online 57Fe emission Mössbauer spectroscopy (eMS) following dilute (<10^-3 at.%) implantation of 57Mn+ (T1/2 = 90 s). Both the epitaxial and the textured V2O3 thin films grown by direct current magnetron sputtering were studied. Three structural components were resolved and identified in the eMS spectra with parameters characteristic of Fe in the 2+ valence state, which are attributable to Fe in either lattice damage or structural defects and Fe in the intrinsic crystal structure of V2O3, respectively. The results prove that the oxygen vacancies are common in the V2O3 thin films. With co-existence of both the non-stoichiometry and epitaxial strain in the thin films, the epitaxial strain plays a dominant role in controlling the global MIT properties of the film. The atomic scale structural transition captured by the eMS affirms the early-stage dynamics of the MIT of V2O3 thin film reported previously. These results approve the feasibility to tune the electronic transport of the V2O3 thin films for the next-generation Mott nanodevices by the epitaxial strain via the sample growth parameters.
https://doi.org/10.1016/j.tsf.2020.138389
Investigation of epitaxial graphene via Raman spectroscopy: origins of phonon mode asymmetries and line width deviations. - In: Carbon, ISSN 1873-3891, Bd. 170 (2020), S. 666-676
In this work a comprehensive study is presented for the analysis of epitaxial graphene layers using Raman spectroscopy. A wide range of graphene types is covered, from defective/polycrystalline single layer graphene to multilayer graphene with low defect density. On this basis the influence of strain type, Fermi level and number of layers on the Raman spectrum of graphene is investigated. A detailed view on the 2D/G dispersion and the respective slopes of uniaxially and biaxially strained graphene is given and its implications on the asymmetry of the G peak analyzed. A linear dependency of the phonon mode asymmetry on uniaxial strain is presented in addition to the known Fermi level dependence. Additional impacts on the asymmetry are found to be arising from the defect density and transfer doping of adsorbates. The discovered transfer doping mechanism is contrary to pure phonon excitation through excitons and exhibits increasing asymmetry with increasing Fermi level. A new characteristic correlation between the 2D mode line width and the inverse I(D)/I(G) ratio is introduced that allows the determination of the strain type and layer number and explains the difference between Raman line widths of monolayer graphene on different substrates.
https://doi.org/10.1016/j.carbon.2020.07.016
Application of hydrosilane-free atmospheric pressure chemical vapor deposition of SiOx films in the manufacture of crystalline silicon solar cells. - In: Thin solid films, ISSN 1879-2731, Bd. 713 (2020), 138338
In this work we present SiOx films deposited in cost-effective laboratory scale three-dimensional printed atmospheric pressure chemical vapor deposition setup. As SiOx films are deposited at room temperature without complex vacuum systems, they can be a good candidate for the use in commercial c-Si solar cell production lines. The quality of the deposited films was investigated as to their integrity, conformity with various surfaces, and post-treatment resilience such as stability against etchants and annealing. Several applications of the SiOx film prepared with the atmospheric pressure chemical vapor deposition (APCVD) were discussed. In one application, the APCVD SiOx was utilized to effectively promote single-side texturing of Float Zone and Czochralski Si wafers by coating only one side with SiOx and subsequently annealing prior to texturing in an alkaline aqueous solution. Another application was to exploit the APCVD SiOx as a plating mask for silicon heterojunction solar cells. Two processing options prior to the oxide-film deposition were investigated: i) application of an Ag seed-layer, which promotes subsequent electroplating, and ii) printing of an organic grid, which, after stripping, creates openings in the SiOx that facilitate electroplating of the solar cell's electrode on the underlying transparent conducting oxide. In a different application, the APCVD SiOx films acted as protection against parasitic plating on the front side of passivated emitter and rear solar cells. The deposited films were characterized by ellipsometry, hemispherical reflectance measurements, scanning electron microscopy, energy dispersive X-ray spectroscopy and optical microscopy.
https://doi.org/10.1016/j.tsf.2020.138338