Carrier Recombination Processes in GaAs Wafers Passivated by Wet Nitridation.

As one of the successful approaches to GaAs surface passivation, wet-chemical nitridation is applied here to relate the effect of surface passivation to carrier recombination processes in bulk GaAs. By combining time-resolved photoluminescence and optical pump-THz probe measurements, we found that surface hole trapping dominates the decay of photoluminescence, while photoconductivity dynamics is limited by surface electron trapping. Compared to untreated sample dynamics, the optimized nitridation reduces hole- and electron-trapping rate by at least 2.6 and 3 times, respectively. Our results indicate that under ambient conditions, recovery of the fast hole trapping due to the oxide regrowth at the deoxidized GaAs surface takes tens of hours, while it is effectively inhibited by surface nitridation. Our study demonstrates that surface nitridation stabilizes the GaAs surface via reduction of both electron- and hole-trapping rates, which results in chemical and electronical passivation of the bulk GaAs surface.

Electron Transfer Mediated by Iron Carbonyl Clusters Enhance Light-Driven Hydrogen Evolution in Water by Quantum Dots.

Photocatalytic water splitting has become a promising strategy for converting solar energy into clean and carbon-neutral solar fuels in a low-cost and environmentally benign way. Hydrogen gas is such a potential solar fuel/energy carrier. In a classical artificial photosynthetic system, a photosensitizer is generally associated with a co-catalyst to convert photogenerated charge into (a) chemical bond(s). In the present study, assemblies consisting of CdSe quantum dots that are coupled with one of two molecular complexes/catalysts, that is, [Fe2 S2 (CO)6 ] or [Fe3 Te2 (CO)9 ], using an interface-directed approach, have been tested as catalytic systems for hydrogen production in aqueous solution/organic solution. In the presence of ascorbic acid as a sacrificial electron donor and proton source, these assemblies exhibit enhanced activities for the rate of hydrogen production under visible light irradiation for 8 h in aqueous solution at pH 4.0 with up to 110 µmol of H2 per mg of assembly, almost 8.5 times that of pure CdSe quantum dots under the same conditions. Transient absorption and time-resolved photoluminescence spectroscopies have been used to investigate the charge carrier transfer dynamics in the quantum dot/iron carbonyl cluster assemblies. The spectroscopic results indicate that effective electron transfer from the molecular iron complex to the valence band of the excited CdSe quantum dots significantly inhibits the recombination of photogenerated charge carriers, boosting the photocatalytic activity for hydrogen generation; that is, the iron clusters function as effective intermediaries for electron transfer from the sacrificial electron donor to the valence band of the quantum dots.

Photostability and Photodegradation Processes in Colloidal CsPbI3 Perovskite Quantum Dots.

All-inorganic CsPbI3 perovskite quantum dots (QDs) have attracted intense attention for their successful application in photovoltaics (PVs) and optoelectronics that are enabled by their superior absorption capability and great photoluminescence (PL) properties. However, their photostability remains a practical bottleneck and further optimization is highly desirable. Here, we studied the photostability of as-obtained colloidal CsPbI3 QDs suspended in hexane. We found that light illumination does induce photodegradation of CsPbI3 QDs. Steady-state spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and transient absorption spectroscopy verified that light illumination leads to detachment of the capping agent, collapse of the CsPbI3 QD surface, and finally aggregation of surface Pb0. Both dangling bonds containing surface and Pb0 serve as trap states causing PL quenching with a dramatic decrease of PL quantum yield. Our work provides a detailed insight about the correlation between the structural and photophysical consequences of the photodegradation process in CsPbI3 QDs and may lead to the optimization of such QDs toward device applications.

Photostability of the Oleic Acid-Encapsulated Water-Soluble Cd x Se y Zn1-x S1-y Gradient Core-Shell Quantum Dots.

Composite systems where quantum dots (QDs) are combined with other nanomaterials (e.g., gold nanorods) in aqueous solutions have attracted broad attention-both for their potential in applications and for studies of fundamental processes. However, high-quality QDs are typically prepared in organic solvents, and the transfer of QDs to an aqueous phase is needed to create the desired QD composites. Photostability of the transferred QDs-both the steady-state and photo-induced dynamic properties-is essential for studying the processes in the composites and for their applications. We present a detailed study of the photostability of aqueous Cd x Se y Zn1-x S1-y gradient core-shell QDs obtained by various approaches using linker exchange and surfactant encapsulation. Beside the steady-state photoluminescence (PL) emission stability, we also study changes in the PL decay. From the variety of the studied samples, the water-soluble QDs encapsulated by a double layer of oleic acid show superior properties, that is, stable PL emission and PL decay under continuous light or pulsed-laser light irradiation. We demonstrate that the double-layer encapsulation of QDs can be used to create QDs-metal nanoparticle composites.

Light-Induced Ion Rectification in Zigzag Nanochannels.

Ion transport through nanoporous systems has attracted broad interest due to its crucial role in physiological processes in living organisms and artificial bionic devices. In this work, a nanochannel system with a zigzag inner surface was fabricated by using a two-step anodizing technique. The rectification performance of the zigzag channels was observed by I-V measurement in KCl solution. Unlike channels with asymmetric geometry, the mechanism was analyzed based on the "point effect" of charge distribution and "shape effect" of the zigzag channel. The current rectification ratio decreases from nearly 3.0 to 1.0 when the KCl concentration increased from 0.1 mM to 100 mM. The fabrication of different nanopore systems and exploration of novel mechanisms will help to develop biomimetic membranes for practical applications.