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CdSe quantum dots combined with [FeFe] hydrogenase mimics as molecular catalytic reaction centers based on earth abundant elements have demonstrated promising activity for photocatalytic hydrogen generation. Direct linking of the [FeFe] hydrogenase mimics to the quantum dots surface is expected to enhance the activity of the system by establishing close contact between the [FeFe] hydrogenase mimics and the light harvesting quantum dots supporting the transfer and accumulation of several electrons which are needed to drive hydrogen evolution. To circumvent the problem of limited colloidal stability upon covalent functionalization of the quantum dots under optimal pH conditions for hydrogen evolution, in this work, we report on the functionalization of quantum dots immobilized in a thin film architecture on a substrate with [FeFe] hydrogenase mimics by covalent linking via carboxylate groups as anchoring functionality to bind to the QD surface. The functionalization was monitored via UV/Vis absorption, photoluminescence, infrared (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) and quantified via micro X-ray fluorescence spectrometry (μXRF). The activity of the molecularly functionalized thin film was demonstrated and in dependence on the linker length TONs in the range of 360-580 (short linker) and 130-160 (long linker) were achieved. This work presents a proof of concept study showing the potential of thin film architectures of immobilized quantum dots as platform for light-driven hydrogen evolution and beyond.

Solar-driven photocatalytic generation of hydrogen from water is a potential source of clean and renewable fuel. Yet systems that are sufficiently stable and efficient for practical use have not been realized. Here, nanorod photocatalysts that have proven record activity for the water reduction half reaction were successfully combined with molecular metallocorroles suitable for catalyzing the accompanying oxidation reactions. Utilization of OH

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.

The combination of CdSe nanoparticles as photosensitizers with [FeFe]-hydrogenase mimics is known to result in efficient systems for light-driven hydrogen generation with reported turnover numbers in the order of 104-106. Nevertheless, little is known about the details of the light-induced charge-transfer processes. Here, we investigate the time scale of light-induced electron transfer kinetics for a simple model system consisting of CdSe quantum dots (QDs) of 2.0 nm diameter and a simple [FeFe]-hydrogenase mimic adsorbed to the QD surface under noncatalytic conditions. Our (time-resolved) spectroscopic investigation shows that both hot electron transfer on a sub-ps time scale and band-edge electron transfer on a sub-10 ps time scale from photoexcited QDs to adsorbed [FeFe]-hydrogenase mimics occur. Fast recombination via back electron transfer 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, respectively.

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.

Efficient light-stimulated hydrogen generation from top-down produced highly doped n-type silicon nanowires (SiNWs) with silver nanoparticles (AgNPs) in water-containing medium under white light irradiation is reported. It is observed that SiNWs with AgNPs generate at least 2.5 times more hydrogen than SiNWs without AgNPs. The authors' results, based on vibrational, UV-vis, and X-ray spectroscopy studies, strongly suggest that the sidewalls of the SiNWs are covered by silicon suboxides, by up to a thickness of 120 nm, with wide bandgap semiconductor properties that are similar to those of titanium dioxide and remain stable during hydrogen evolution in a water-containing medium for at least 3 h of irradiation. Based on synchrotron studies, it is found that the increase in the silicon bandgap is related to the energetically beneficial position of the valence band in nanostructured silicon, which renders these promising structures for efficient hydrogen generation.

Self-organized silver nanostructures were grown in porous Si/SiO2 matrix fabricated by ion track technology. The different silver nanostructures with shapes like “sunflowers”, “azalea” or “corn” were realized by applying wet-chemical electroless deposition. We show that reproducible self-organized silver “sunflower” like nanostructures provide a high enhanced Raman signal of Nile blue dye molecules. Signal enhancement for a few or even just a single silver “sunflower” is demonstrated by analyzing the surface-enhanced Raman signature of Nile blue dye molecules. According to this, the silver nanostructures can act as efficient surfaces for surface enhanced Raman spectroscopy as well as (bio)-sensor applications.

A cheap and facile wet-chemical 'silver mirror' reaction approach has been proposed for the silicon nanowires (SiNWs), formed by the metal-assisted wet-chemical etching, decoration with silver nanoparticles (AgNPs). Scanning electron microscopy studies show that AgNPs can be deposited uniformly on the surface of SiNWs by the variation of AgNO3 concentration and reaction time. Adopting the reaction conditions, a homogenous AgNPs shell is formed around the silicon nanowires. The resultant AgNPs@SiNWs nanostructures show significantly (up to 40%) higher light absorption in the near-infrared range (up to 1500 nm) in comparison to SiNWs without AgNPs. This approach opens a new perspectives for a cheap and fast preparation of AgNP shells around SiNWs. Our observation supports that synthesized nanostructures might have promising application potential in near-infrared photodetectors or as photocatalysts.

Deep level transient spectroscopy (DLTS) was performed on lowly n-doped silicon nanowires grown by metal-assisted wet chemical etching (MaWCE) with silver as the catalyst in order to investigate the energetic scheme inside the bandgap. To observe the possible diffusion of atoms into the bulk, DLTS investigation was also performed on the samples after removing the nanowires. Two of the four energy levels observed in the nanowires were also detected inside the substrate. Based on these results and on literature data about deep levels in bulk silicon, some hypotheses are advanced regarding the identification of the defects responsible for the energy levels revealed.

Semiconductor–insulator–semiconductor heterojunction solar cells were prepared using atomic layer deposition (ALD) technique. The silicon surface was treated with oxygen and hydrogen plasma in different orders before dielectric layer deposition. A plasma-enhanced ALD process was applied to deposit dielectric Al2O3 on the plasma pretreated n-type Si(100) substrate. Aluminum doped zinc oxide (Al:ZnO or AZO) was deposited by thermal ALD and serves as transparent conductive oxide. Based on transmission electron microscopy studies the presence of thin silicon oxide (SiOx) layer was detected at the Si/Al2O3 interface. The SiOx formation depends on the initial growth behavior of Al2O3 and has significant influence on solar cell parameters. The authors demonstrate that a hydrogen plasma pretreatment and a precursor dose step repetition of a single precursor improve the initial growth behavior of Al2O3 and avoid the SiOx generation. Furthermore, it improves the solar cell performance, which indicates a change of t...

Deep level transient spectroscopy (DLTS) was performed on n-doped silicon nanowires grown by metal-assisted wet chemical etching (MaWCE) with gold as the catalyst in order to investigate the energetic scheme inside the bandgap. To observe the possible dependence of the level scheme on the processing temperature, DLTS measurements were performed on the nanowires grown on a non-treated Au/Si surface and on a thermally pre-treated Au/Si surface. A noticeable modification of the configuration of the energy levels was observed, induced by the annealing process. Based on our results on these MaWCE nanowires and on literature data about deep levels in bulk silicon, some hypotheses were advanced regarding the identification of the defects responsible of the energy levels revealed.

High-order harmonic generation (HHG) in crystalline bulk semiconductors originates from nonlinear intraband currents and interband transitions of the electrons driven by the strong mid-IR electric field. By reducing the size of a semiconductor crystal to nanoscale and making it smaller than the exciton Bohr radius in all three dimensions, quantum dots (QD) are formed with electronic structure distinctly different from the bulk. Spatial quantization of electron and hole motion leads to discrete energy levels instead of a band structure as in the bulk [1]. Therefore, QDs can be considered as artificial atoms and enable transition from atomic/molecular gases to bulk solids. It was predicted recently that the efficiency of HHG in these quantum confined structures is significantly enhanced in comparison to bulk, which is explained by a reduction of quantum diffusion as a result of the reduced amount of the available states [2]. Here, we report on experimental investigation of HHG by mid-IR femtosecond laser pulses in arrays of CdSe QDs as a function of the laser pulse intensity and polarization.

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.