Synchrotron studies of top-down grown silicon nanowires

Morphology of the top-down grown silicon nanowires obtained by metal-assisted wet-chemical approach on silicon substrates with different resistance were studied by scanning electron microscopy. Obtained arrays of compact grown Si nanowires were a subject for the high resolution electronic structures studies by X-ray absorption near edge structure technique performed with the usage of high intensity synchrotron radiation of the SRC storage ring of the University of Wisconsin-Madison. The different oxidation rates were found by investigation of silicon atoms local surrounding specificity of the highly developed surface and near surface layer that is not exceeded 70 nm. Flexibility of the wires arrays surface morphology and its composition is demonstrated allowing smoothly form necessary surface oxidation rate and using Si nanowires as a useful matrixes for a wide range of further functionalization

Nanostructured Silicon Matrix for Materials Engineering

Tin-containing layers with different degrees of oxidation are uniformly distributed along the length of silicon nanowires formed by a top-down method by applying metalorganic chemical vapor deposition. The electronic and atomic structure of the obtained layers is investigated by applying nondestructive surface-sensitive X-ray absorption near edge spectroscopy using synchrotron radiation. The results demonstrated, for the first time, a distribution effect of the tin-containing phases in the nanostructured silicon matrix compared to the results obtained for planar structures at the same deposition temperatures. The amount and distribution of tin-containing phases can be effectively varied and controlled by adjusting the geometric parameters (pore diameter and length) of the initial matrix of nanostructured silicon. Due to the occurrence of intense interactions between precursor molecules and decomposition by-products in the nanocapillary, as a consequence of random thermal motion of molecules in the nanocapillary, which leads to additional kinetic energy and formation of reducing agents, resulting in effective reduction of tin-based compounds to a metallic tin state for molecules with the highest penetration depth in the nanostructured silicon matrix. This effect will enable clear control of the phase distributions of functional materials in 3D matrices for a wide range of applications

Spectromicroscopy Studies of Silicon Nanowires Array Covered by Tin Oxide Layers

The composition and atomic and electronic structure of a silicon nanowire (SiNW) array coated with tin oxide are studied at the spectromicroscopic level. SiNWs are covered from top to down with a wide bandgap tin oxide layer using a metal–organic chemical vapor deposition technique. Results obtained via scanning electron microscopy and X-ray diffraction showed that tin-oxide nanocrystals, 20 nm in size, form a continuous and highly developed surface with a complex phase composition responsible for the observed electronic structure transformation. The “one spot” combination, containing a chemically sensitive morphology and spectroscopic data, is examined via photoemission electron microscopy in the X-ray absorption near-edge structure spectroscopy (XANES) mode. The observed spectromicroscopy results showed that the entire SiNW surface is covered with a tin(IV) oxide layer and traces of tin(II) oxide and metallic tin phases. The deviation from stoichiometric SnO2 leads to the formation of the density of states sub-band in the atop tin oxide layer bandgap close to the bottom of the SnO2 conduction band. These observations open up the possibility of the precise surface electronic structures estimation using photo-electron microscopy in XANES mode

XPS investigations of MOCVD tin oxide thin layers on Si nanowires array

Tin oxide thin layers were grown by metal-organic chemical vapor deposition technique on the top-down nanostructured silicon nanowires array obtained by metal-assisted wet-chemical technique from single crystalline silicon wafers. The composition of the formed layers were studied by high-resolution X-ray photoelectron spectroscopy of tin (Sn 3d) and oxygen (O 1 s) atoms core levels. The ion beam etching was applied to study the layers depth composition profiles. The composition studies of grown tin oxide layers is shown that the surface of layers contains tin dioxide, but the deeper part contains intermediate tin dioxide and metallic tin phases

Optical Properties of Silicon Nanowires Fabricated by Environment-Friendly Chemistry

Silicon nanowires (SiNWs) were fabricated by metal-assisted chemical etching (MACE) where hydrofluoric acid (HF), which is typically used in this method, was changed into ammonium fluoride (NH4F). The structure and optical properties of the obtained SiNWs were investigated in details. The length of the SiNW arrays is about 2 μm for 5 min of etching, and the mean diameter of the SiNWs is between 50 and 200 nm. The formed SiNWs demonstrate a strong decrease of the total reflectance near 5-15 % in the spectral region λ < 1 μm in comparison to crystalline silicon (c-Si) substrate. The interband photoluminescence (PL) and Raman scattering intensities increase strongly for SiNWs in comparison with the corresponding values of the c-Si substrate. These effects can be interpreted as an increase of the excitation intensity of SiNWs due to the strong light scattering and the partial light localization in an inhomogeneous optical medium. Along with the interband PL was also detected the PL of SiNWs in the spectral region of 500-1100 nm with a maximum at 750 nm, which can be explained by the radiative recombination of excitons in small Si nanocrystals at nanowire sidewalls in terms of a quantum confinement model. So SiNWs, which are fabricated by environment-friendly chemistry, have a great potential for use in photovoltaic and photonics applications

Spektroskopische Charakterisierung der Wechselwirkungen in Halbleiter-Molekül-Hybriden

Die vorliegende Arbeit beschäftigt sich mit der Entwicklung und Evaluierung von Strategien zur Kopplung von FeFe-H2asen Modellsystemen zu der Oberfläche von CdSe Quantenpunkten. Hierbei wurden Modelle aufgezeigt, mit denen die Art der Wechselwirkungen von FeFe-H2ase Modellsystemen mit CdSe Quantenpunkten auf Basis von Emissionslöschexperimenten beschrieben werden können. Weiterhin wurden Methoden bereitgestellt, die Affinität von FeFe-H2ase Modellsystemen zu der Quantenpunktoberfläche mit verschiedenen Kopplungsgruppen zu evaluieren und zu erhöhen. Hierbei bilden insbesondere Modellsysteme mit Carboxyl- und in situ erzeugten Dithiocarbamat-Gruppen die stabilsten Halbleiter-Molekül-Hybride aus. Der erstmalig nachgewiesene schnelle Elektronentransferprozess von heißen und relaxierten Leitungsbandzuständen der CdSe Quantenpunkte zu einem sich in der Nähe zu der Oberfläche befindlichen FeFe-H2ase Modellsystem ohne Kopp-lungsgruppe zeigt, dass große Potenzial dieser Hybride für die photokatalytische Protonenreduktion. Gleichzeitig wird durch den schnellen Rücktransfer und Rekombination der Elektronen mit in Defektstellen gefangenen Löchern ersichtlich, dass Elektronen- und Lochtransferprozesse aufeinander abgestimmt werden müs-sen. Die vorgestellte Möglichkeit, dünne Schichten aus CdSe Quantenpunkten mit dem ursprünglichen Ligandensystem mit FeFe-H2ase Modellsystemen zu funktionalisieren und in photokatalytischen Experimenten erfolgreich einzusetzen, verdeut-licht das Potenzial solcher Systeme für die heterogene photokatalytische Protonenreduktion. Somit bilden die in dieser Arbeit vorgestellten Erkenntnisse die Grundlage für die Entwicklung von stabilen Halbleiter-Molekül-Hybridmaterialien und deren zukünftigen Einsatz in vollständig artifiziellen und kostengünstigen Systemen für die photokatalytische Wasserspaltung

Ultrafast Electron Transfer from CdSe Quantum Dots to a [FeFe]-Hydrogenase Mimic

The combination of CdSe nanoparticles as photosensitizers and [FeFe]-hydrogenase mimics is known to result in efficient systems for light-driven hydrogen generation. Nevertheless, little is known about the details of the light-induced charge-transfer processes. Here we investigate the timescale of light-induced electron transfer between CdSe quantum dots and a simple [FeFe]-hydrogenase mimic adsorbed on the surface of the quantum dot under non-catalytic conditions. Our time-resolved spectroscopic investigation shows that hot electron transfer on a sub-ps timescale and band-edge electron transfer on a sub-10-ps timescale occurs. Fast recombination is observed in the absence of a sacrificial agent or protons, which under real catalytic conditions would quench remaining holes or could stabilize the charge separation. <br /

Mapping emission heterogeneity in layered halide perovskites using cathodoluminescence.

Recent advancements in the fabrication of layered halide perovskites and their subsequent modification for optoelectronic applications have ushered in a need for innovative characterisation techniques. In particular, heterostructures containing multiple phases and consequently featuring spatially defined optoelectronic properties are very challenging to study. Here, we adopt an approach centered on cathodoluminescence, complemented by scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy analysis. Cathodoluminescence enables assessment of local emission variations by injecting charges with a nanometer-scale electron probe, which we use to investigate emission changes in three different systems: PEA2PbBr4, PEA2PbI4and lateral heterostructures of the two, fabricated via halide substitution. We identify and map different emission bands that can be correlated with local chemical composition and geometry. One emission band is characteristic of bromine-based halide perovskite, while the other originates from iodine-based perovskite. The coexistence of these emissions bands in the halide-substituted sample confirms the formation of lateral heterostructures. To improve the signal quality of the acquired data, we employed multivariate analysis, specifically the non-negative matrix factorization algorithm, on both cathodoluminescence and compositional datasets. The resulting understanding of the halide replacement process and identification of potential synergies in the optical properties will lead to optimised architectures for optoelectronic applications