Correlating structure and electronic band-edge properties in organolead halide perovskites nanoparticles.

After having emerged as primary contenders in the race for highly efficient optoelectronics materials, organolead halide perovskites (OHLP) are now being investigated in the nanoscale regime as promising building blocks with unique properties. For example, unlike their bulk counterpart, quantum dots of OHLP are brightly luminescent, owing to large exciton binding energies that cannot be rationalized solely on the basis of quantum confinement. Here, we establish the direct correlation between the structure and the electronic band-edge properties of CH3NH3PbBr3 nanoparticles. Complementary structural and spectroscopic measurements probing long-range and local order reveal that lattice strain influences the nature of the valence band and modifies the subtle stereochemical activity of the Pb(2+) lone-pair. More generally, this work demonstrates that the stereochemical activity of the lone-pair at the metal site is a specific physicochemical parameter coupled to composition, size and strain, which can be employed to engineer novel functionalities in OHLP nanomaterials.

Structural Changes in Monolayer Cobalt Oxides under Ambient Pressure CO and O2 Studied by In Situ Grazing-Incidence X-ray Absorption Fine Structure Spectroscopy.

We have used grazing incidence X-ray absorption fine structure spectroscopy at the cobalt K-edge to characterize monolayer CoO films on Pt(111) under ambient pressure exposure to CO and O2, with the aim of identifying the Co phases present and their transformations under oxidizing and reducing conditions. X-ray absorption near edge structure (XANES) spectra show clear changes in the chemical state of Co, with the 2+ state predominant under CO exposure and the 3+ state predominant under O2-rich conditions. Extended X-ray absorption fine structure spectroscopy (EXAFS) analysis shows that the CoO bilayer characterized in ultrahigh vacuum is not formed under the conditions used in this study. Instead, the spectra acquired at low temperatures suggest formation of cobalt hydroxide and oxyhydroxide. At higher temperatures, the spectra indicate dewetting of the film and suggest formation of bulklike Co3O4 under oxidizing conditions. The experiments demonstrate the power of hard X-ray spectroscopy to probe the structures of well-defined oxide monolayers on metal single crystals under realistic catalytic conditions.

Fabrication and characterisation of a silicon-borosilicate glass microfluidic device for synchrotron-based hard X-ray spectroscopy studies.

Some of the most fundamental chemical building blocks of life on Earth are the metal elements. X-ray absorption spectroscopy (XAS) is an element-specific technique that can analyse the local atomic and electronic structure of, for example, the active sites in catalysts and energy materials and allow the metal sites in biological samples to be identified and understood. A microfluidic device capable of withstanding the intense hard X-ray beams of a 4th generation synchrotron and harsh chemical sample conditions is presented in this work. The device is evaluated at the K-edges of iron and bromine and the L 3-edge of lead, in both transmission and fluorescence mode detection and in a wide range of sample concentrations, as low as 0.001 M. The device is fabricated in silicon and glass with plasma etched microchannels defined in the silicon wafer before anodic bonding of the glass wafer into a complete device. The device is supported with a well-designed printed chip holder that made the microfluidic device portable and easy to handle. The chip holder plays a pivotal role in mounting the delicate microfluidic device on the beamline stage. Testing validated that the device was sufficiently robust to contain and flow through harsh acids and toxic samples. There was also no significant radiation damage to the device observed, despite focusing with intense X-ray beams for multiple hours. The quality of X-ray spectra collected is comparable to that from standard methods; hence we present a robust microfluidic device to analyse liquid samples using synchrotron XAS.

Thermal polymorphism and decomposition of Y(BH(4))(3).

The structure and thermal decomposition of Y(BH(4))(3) is studied by in situ synchrotron radiation powder X-ray diffraction (SR-PXD), (11)B MAS NMR spectroscopy, and thermal analysis (thermogravimetric analysis/differential scanning calorimetry). The samples were prepared via a metathesis reaction between LiBH(4) and YCl(3) in different molar ratios mediated by ball milling. A new high temperature polymorph of Y(BH(4))(3), denoted beta-Y(BH(4))(3), is discovered besides the Y(BH(4))(3) polymorph previously reported, denoted alpha-Y(BH(4))(3). beta-Y(BH(4))(3) has a cubic crystal structure and crystallizes with the space group symmetry Pm3m and a bisected a-axis, a = 5.4547(8) A, as compared to alpha-Y(BH(4))(3), a = 10.7445(4) A (Pa3). Beta-Y(BH(4))(3) crystallizes with a regular ReO(3)-type structure, hence the Y(3+) cations form cubes with BH(4)(-) anions located on the edges. This arrangement is a regular variant of the distorted Y(3+) cube observed in alpha-Y(BH(4))(3), which is similar to the high pressure phase of ReO(3). The new phase, beta-Y(BH(4))(3) is formed in small amounts during ball milling; however, larger amounts are formed under moderate hydrogen pressure via a phase transition from alpha- to beta-Y(BH(4))(3), at approximately 180 degrees C. Upon further heating, beta-Y(BH(4))(3) decomposes at approximately 190 degrees C to YH(3), which transforms to YH(2) at 270 degrees C. An unidentified compound is observed in the temperature range 215-280 degrees C, which may be a new Y-B-H containing decomposition product. The final decomposition product is YB(4). These results show that boron remains in the solid phase when Y(BH(4))(3) decomposes in a hydrogen atmosphere and that Y(BH(4))(3) may store hydrogen reversibly.

Sandwiched confinement of quantum dots in graphene matrix for efficient electron transfer and photocurrent production.

Quantum dots (QDs) and graphene are both promising materials for the development of new-generation optoelectronic devices. Towards this end, synergic assembly of these two building blocks is a key step but remains a challenge. Here, we show a one-step strategy for organizing QDs in a graphene matrix via interfacial self-assembly, leading to the formation of sandwiched hybrid QD-graphene nanofilms. We have explored structural features, electron transfer kinetics and photocurrent generation capacity of such hybrid nanofilms using a wide variety of advanced techniques. Graphene nanosheets interlink QDs and significantly improve electronic coupling, resulting in fast electron transfer from photoexcited QDs to graphene with a rate constant of 1.3 × 10(9) s(-1). Efficient electron transfer dramatically enhances photocurrent generation in a liquid-junction QD-sensitized solar cell where the hybrid nanofilm acts as a photoanode. We thereby demonstrate a cost-effective method to construct large-area QD-graphene hybrid nanofilms with straightforward scale-up potential for optoelectronic applications.

A combined small- and wide-angle x-ray scattering detector for measurements on reactive systems.

A detector with high dynamic range designed for combined small- and wide-angle x-ray scattering experiments has been developed. It allows measurements on single events and reactive systems, such as particle formation in flames and evaporation of levitating drops. The detector consists of 26 channels covering a region from 0.5° to 60° and it provides continuous monitoring of the sampled signal without readout dead time. The time resolution for fast single events is about 40 µs and for substances undergoing slower dynamics, the time resolution is set to 0.1 or 1 s with hours of continuous sampling. The detector has been used to measure soot particle formation in a flame, burning magnesium and evaporation of a toluene drop in a levitator. The results show that the detector can be used for many different applications with good outcomes and large potential.