Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals

The fundamental properties and ultimate performance limits of organolead trihalide MAPbX3 (MA = CH3NH3 +; X = Br– or I–) perovskites remain obscured by extensive disorder in polycrystalline MAPbX3 films. We report an antisolvent vapor-assisted crystallization approach that enables us to create sizable crack-free MAPbX3 single crystals with volumes exceeding 100 cubic millimeters. These large single crystals enabled a detailed characterization of their optical and charge transport characteristics. We observed exceptionally low trap-state densities on the order of 109 to 1010 per cubic centimeter in MAPbX3 single crystals (comparable to the best photovoltaic-quality silicon) and charge carrier diffusion lengths exceeding 10 micrometers. These results were validated with density functional theory calculations

Electron tunneling characteristics on La0.7Sr0.3MnO3 thin-film surfaces at high temperature

We report on the electron tunneling characteristics on La0.7Sr0.3MnO3 (LSM) thin-film surfaces up to 580 °C in 10[superscript −3] mbar oxygen pressure, using scanning tunneling microscopy/spectroscopy (STM/STS). A thresholdlike drop in the tunneling current was observed at positive bias in STS, which is interpreted as a unique indicator for the activation polarization in cation-oxygen bonding on LSM cathodes. Sr-enrichment was found on the surface at high temperature using Auger electron spectroscopy, and was accompanied by a decrease in tunneling conductance in STS. This suggests that Sr-terminated surfaces are less active for electron transfer in oxygen reduction compared to Mn-terminated surfaces on LSM.United States. Dept. of Energy (Office of Fossil Energy Award No. DE–NT0004117

Electron Transfer From a Semiconductor to a Metal and Its Implication on Photocatalysis for Hydrogen Production

Charge transfer from or to a metal deposited on an oxide semiconductor is central to photocatalysis. To probe into this phenomenon, the effect of gold coverage on the chemical state of Ti cations, upon photoexcitation of rutile TiO2(110) single crystal, was investigated. Photocatalytic reaction of gas phase ethanol (a hole scavenger) on TiO2(110) and Aux/TiO2(110) resulted in the formation of Ti3+ cations. Increasing the Au coverage led to a gradual decrease of these Ti3+ cations. Under the investigated reaction condition, the “quasi” total consumption of these reduced states was found at a ratio of Au atoms to reacted Ti3+ cations close to one: [Au][Ti+3]hν→1; this corresponded to about 0.50 at. % of Au/TiO2. The relationship, which is similar to that of hydrogen production rates, obtained on model and practical photocatalytic systems, suggests that the slow reaction rates, generally observed in photocatalysis, are intrinsic to the metal–semiconductor properties

Electron tunneling characteristics on La0.7Sr0.3MnO3 thin-film surfaces at high temperature

We report on the electron tunneling characteristics on La0.7Sr0.3MnO3 (LSM) thin-film surfaces up to 580 °C in 10[superscript −3] mbar oxygen pressure, using scanning tunneling microscopy/spectroscopy (STM/STS). A thresholdlike drop in the tunneling current was observed at positive bias in STS, which is interpreted as a unique indicator for the activation polarization in cation-oxygen bonding on LSM cathodes. Sr-enrichment was found on the surface at high temperature using Auger electron spectroscopy, and was accompanied by a decrease in tunneling conductance in STS. This suggests that Sr-terminated surfaces are less active for electron transfer in oxygen reduction compared to Mn-terminated surfaces on LSM.United States. Dept. of Energy (Office of Fossil Energy Award No. DE–NT0004117

Direct Visualization of a Gold Nanoparticle Electron Trapping Effect

A new atomic-scale anisotropy in the photoreaction of surface carboxylates on rutile TiO2(110) induced by gold clusters is found. STM and DFT+U are used to study this phenomenon by monitoring the photoreaction of a prototype hole-scavenger molecule, benzoic acid, over stoichiometric (s) s-TiO2, Au9/s-TiO2, and reduced (r) Au9/r-TiO2. STM results show that benzoic acid adsorption displaces a large fraction of Au clusters from the terraces toward their edges. DFT calculations explain that Au9 clusters on stoichiometric TiO2 are distorted by benzoic acid adsorption. The influence of sub-monolayers of Au on the UV/visible photoreaction of benzoic acid was explored at room temperature, with adsorbate depletion taken as a measure of activity. The empty sites, observed upon photoexcitation, occurred in elongated chains (2 to 6 molecules long) in the [11̅0] and [001] directions. A roughly 3-fold higher depletion rate is observed in the [001] direction. This is linked to the anisotropic conduction of excited electrons along [001], with subsequent trapping by Au clusters leaving a higher concentration of holes and thus an increased decomposition rate. To our knowledge this is the first time that atomic-scale directionality of a chemical reaction is reported upon photoexcitation of the semiconductor

Trapping shape-controlled nanoparticle nucleation and growth stages via continuous-flow chemistry

Continuous flow chemistry is used to trap the nucleation and growth stages of platinum-nickel nano-octahedra with second time resolution and high throughputs to probe their properties ex-situ. The growth starts from poorly crystalline particles (nucleation) at 5 seconds, to crystalline 1.5 nm particles bounded by the {111}-facets at 7.5 seconds, followed by truncation and further growth to octahedral nanoparticles at 20 seconds

A scalable synthesis of highly stable and water dispersible Ag-44(SR)(30) nanoclusters

We report the synthesis of atomically monodisperse thiol-protected silver nanoclusters [Ag-44(SR)(30)](m), (SR = 5-mercapto-2-nitrobenzoic acid) in which the product nanocluster is highly stable in contrast to previous preparation methods. The method is one-pot, scalable, and produces nanoclusters that are stable in aqueous solution for at least 9 months at room temperature under ambient conditions, with very little degradation to their unique UV-Vis optical absorption spectrum. The composition, size, and monodispersity were determined by electrospray ionization mass spectrometry and analytical ultracentrifugation. The produced nanoclusters are likely to be in a superatom charge-state of m = 4-, due to the fact that their optical absorption spectrum shares most of the unique features of the intense and broadly absorbing nanoparticles identified as [Ag-44(SR)(30)](4-) by Harkness et al. (Nanoscale, 2012, 4, 4269). A protocol to transfer the nanoclusters to organic solvents is also described. Using the disperse nanoclusters in organic media, we fabricated solid-state films of [Ag-44(SR)(30)](m) that retained all the distinct features of the optical absorption spectrum of the nanoclusters in solution. The films were studied by X-ray diffraction and photoelectron spectroscopy in order to investigate their crystallinity, atomic composition and valence band structure. The stability, scalability, and the film fabrication method demonstrated in this work pave the way towards the crystallization of [Ag-44(SR)(30)](m) and its full structural determination by single crystal X-ray diffraction. Moreover, due to their unique and attractive optical properties with multiple optical transitions, we anticipate these clusters to find practical applications in light-harvesting, such as photovoltaics and photocatalysis, which have been hindered so far by the instability of previous generations of the cluster

Synthesis of Copper Hydroxide Branched Nanocages and Their Transformation to Copper Oxide

Copper oxide nanostructures have been explored in the literature for their great promise in the areas of energy storage and catalysis, which can be controlled based on their shape. Herein we describe the synthesis of complex branched nanocages of copper hydroxide with an alternating stacked morphology. The size of the nanocages’ core and the length of the branches can be controlled by the temperature and ratio of surfactant used, varying the length from 85 to 232 nm long, and varying the core size from 240 to 19 nm. The nanostructures’ unique morphology forms by controlling the growth of an initial spherical seed, and the crystallization of the anisotropic arms. The Cu­(OH)₂ nanostructures can be converted to polycrystalline CuO branched nanocages and Cu₂O nanoframes. We show that the branched nanocage morphology of CuO has markedly superior catalytic properties to previous reports with CuO nanomaterials, resulting in a rapid and efficient catalyst for C–S coupling