Multiple exciton generation in nano-crystals revisited: Consistent calculation of the yield based on pump-probe spectroscopy

Multiple exciton generation (MEG) is a process in which more than one exciton is generated upon the absorption of a high energy photon, typically higher than two times the band gap, in semiconductor nanocrystals. It can be observed experimentally using time resolved spectroscopy such as the transient absorption measurements. Quantification of the MEG yield is usu- ally done by assuming that the bi-exciton signal is twice the signal from a single exciton. Herein we show that this assumption is not always justified and may lead to significant errors in the estimated MEG yields. We develop a methodology to determine proper scaling factors to the signals from the transient absorption experiments. Using the methodology we find modest MEG yields in lead chalcogenide nanocrystals including the nanorods

In situ etching for total control over axial and radial nanowire growth

We report a method using in situ etching to decouple the axial from the radial nanowire growth pathway, independent of other growth parameters. Thereby a wide range of growth parameters can be explored to improve the nanowire properties without concern of tapering or excess structural defects formed during radial growth. We demonstrate the method using etching by HCl during InP nanowire growth. The improved crystal quality of etched nanowires is indicated by strongly enhanced photoluminescence as compared to reference nanowires obtained without etching

In situ XAS study of the local structure and oxidation state evolution of palladium in a reduced graphene oxide supported Pd(II) carbene complex during an undirected C–H acetoxylation reaction

In situ X-ray absorption spectroscopy (XAS) investigations have been performed to provide insights into the reaction mechanism of a palladium(II) catalyzed undirected C–H acetoxylation reaction in the presence of an oxidant. A Pd(II) N-heterocyclic carbene complex π-stacked onto reduced graphene oxide (rGO) was used as the catalyst. The Pd speciation during the catalytic process was examined by XAS, which revealed a possible mechanism over the course of the reaction. Pd(II) complexes in the as-synthesized catalyst first go through a gradual ligand substitution where chloride ions bound to Pd(II) are replaced by other ligands with a mean bond distance to Pd matching Pd–C/N/O. Parallel to this the mean oxidation state of Pd increases indicating the formation of Pd(IV) species. At a later stage, a fraction of the Pd complexes start to slowly transform into Pd nanoclusters. The mean average oxidation state of Pd decreases to the initial state at the end of the experiment which means that comparable amounts of Pd(0) and Pd(IV) are present. These observations from heterogeneous catalysis are in good agreement with its homogeneous analog and they support a Pd(II)–Pd(IV)–Pd(II) reaction mechanism

On the transformation mechanism of K2Ti4O9 to TiO2(B) and formation of microvoids

TiO2(B), a potential catalyst support, has been synthesized from a support precursor, K2Ti4O9. High resolution electron microscopy revealed that the 32% volume loss in this transformation is achieved by formation of facetted microvoids, around 10 nm in size, maintaining the morphology and size of the original crystals. A structural mechanism, including a hypothetical structure for a K2Ti 2O5 intermediate, is suggested.TiO2(B), potentiellement support de catalyseur, a été préparé à partir d'un précurseur K2Ti4O9. La microscopie électronique haute résolution a revélé que la perte en volume de 32% lors de la transformation est compensée par la formation de microcavités, d'une taille moyenne de 10 nm, ce qui préserve la morphologie et la forme du cristal original. Un mécanisme structural, proposant une structure pour un intermédiaire K2Ti2O5, est suggéré

In-situ manipulations and electrical measurements of III-V nanowhiskers with TEM-STM

A scanning tunnelling microscope (STM) mounted in a sample holder for a transmission electron microscope (TEM), a TEM-STM, have been used for in-situ electrical measurements of semiconductor nano whiskers. The device enables measurements and manipulations of nano structures while observing them in a TE