Structural patterns at all scales in a nonmetallic chiral Au_133(SR)_52 nanoparticle

Structural ordering is widely present in molecules and materials. However, the organization of molecules on the curved surface of nanoparticles is still the least understood owing to the major limitations of the current surface characterization tools. By the merits of x-ray crystallography, we reveal the structural ordering at all scales in a super robust 133–gold atom nanoparticle protected by 52 thiolate ligands, which is manifested in self-assembled hierarchical patterns starting from the metal core to the interfacial –S–Au–S– ladder-like helical “stripes” and further to the “swirls” of carbon tails. These complex surface patterns have not been observed in the smaller nanoparticles. We further demonstrate that the Au133(SR)52 nanoparticle exhibits nonmetallic features in optical and electron dynamics measurements. Our work uncovers the elegant self-organization strategies in assembling a highly robust nanoparticle and provides a conceptual advance in scientific understanding of pattern structures

Origins of Multivalley Electronic Transitions in Hybrid Perovskites Revealed by Transient Spectroscopy

International audienceMapping complex electronic excitations to the intricate lattice structure of hybrid organic-inorganic perovskites is critical to understand charge separation and hot-carrier extraction, processes that dictate energy-conversion and optoelectronic technologies. Here, we highlight how the dipolar CH3NH3+ organic molecule interacts with the inorganic PbI6- octahedral cage to impact the multiband, multivalley electronic structure of this halide perovskite. This is achieved by tracking the transient broadband optical spectra while tuning the structural lattice of the hybrid perovskite via its reversible temperature-dependent phase transition (PT). These temperature-dependent optical snapshots, here captured at 5 ps, reveal exquisite details of those bands, reporting for the first time a degeneracy lifting in the tetragonal state at 2.6 eV that increases as the organic molecule rotational degrees of freedom are suppressed in the orthorhombic state. Plotting this dispersion relation, along with a symmetry analysis of the PT, we describe how the electronic states evolve from the tetragonal to orthorhombic phase and ascribe the splitting to the nearly degenerate transitions at the R and M points of the Brillouin zone. Furthermore, a zone folding in the orthorhombic state explains other salient features of our experiments, such as the emergence of other allowed transitions near 2 eV and a decrease in hot carrier lifetime

Quantifying Bulk and Surface Recombination Processes in Nanostructured Water Splitting Photocatalysts via In Situ Ultrafast Spectroscopy

A quantitative description of recombination processes in nanostructured semiconductor photocatalystsone that distinguishes between bulk (charge transport) and surface (chemical reaction) lossesis critical for advancing solar-to-fuel technologies. Here we present an in situ experimental framework that determines the bias-dependent quantum yield for ultrafast carrier transport to the reactive interface. This is achieved by simultaneously measuring the electrical characteristics and the subpicosecond charge dynamics of a heterostructured photoanode in a working photoelectrochemical cell. Together with direct measurements of the overall incident-photon-to-current efficiency, we illustrate how subtle structural modifications that are not perceivable by conventional X-ray diffraction can drastically affect the overall photocatalytic quantum yield. We reveal how charge carrier recombination losses occurring on ultrafast time scales can limit the overall efficiency even in nanostructures with dimensions smaller than the minority carrier diffusion length. This is particularly true for materials with high carrier concentration, where losses as high as 37% are observed. Our methodology provides a means of evaluating the efficacy of multifunctional designs where high overall efficiency is achieved by maximizing surface transport yield to near unity and utilizing surface layers with enhanced activity

Ultrafast optical snapshots of hybrid perovskites reveal the origin of multiband electronic transitions

International audienceConnecting the complex electronic excitations of hybrid perovskites to their intricate organic-inorganic lattice structure has critical implications for energy conversion and optoelectronic technologies. Here we detail the multiband, multivalley electronic structure of a halide hybrid perovskite by measuring the absorption transients of a millimeter-scale-grain thin film as it undergoes a thermally controlled reversible tetragonal-to-orthogonal phase transition. Probing nearly single grains of this hybrid perovskite, we observe an unreported energy splitting (degeneracy lifting) of the high-energy 2.6 eV band in the tetragonal phase that further splits as the rotational degrees of freedom of the disordered CH3NH3+ molecules are reduced when the sample is cooled. This energy splitting drastically increases during an extended phase-transition coexistence region that persists from 160 to 120 K, becoming more pronounced in the orthorhombic phase. By tracking the temperature-dependent optical transition energies and using symmetry analysis that describes the evolution of electronic states from the tetragonal phase to the orthorhombic phase, we assign this energy splitting to the nearly degenerate transitions in the tetragonal phase from both the R- and M-point-derived states. Importantly, these assignments explain how momentum conservation effects lead to long hot-carrier lifetimes in the room-temperature tetragonal phase, with faster hot-carrier relaxation when the hybrid perovskite structurally transitions to the orthorhombic phase due to enhanced scattering at the Γ point

Plasma-Corona-Processed Nanostructured Coating for Thermoregulative Textiles

A rapid increase in the atmospheric temperature has been reported in recent years worldwide. The lack of proper aid to protect from exposure to the sun during working hours has raised the number of sunburn cases among workers. It is important to promote productive workplaces without compromising safety and health concerns. In the present work, we report the low-temperature plasma (LTP)-assisted tailoring of the surface properties of fabrics to reflect IR radiation from the sun. The LTP technique can be adapted for thermally sensitive materials such as fabrics and textiles due to its lower working temperature range of 30 °C. We have modified various substrates such as commercially available fabric, regular, and boron nitride-incorporated electrospun PET surfaces with tetraethoxy orthosilicate (TEOS) plasma. TEOS plasma treatment can deposit a reactive plasma-polymerized silane nanolayer on the surface of these substrates. The plasma-processed silane nanolayer was systematically characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy, Keyence 3D-microscopic imaging, and transmission electron microscopy (TEM). From the SEM and TEM data, the size of the nanoparticles was observed in the range 100–200 nm. The thermal regulation coating thickness was examined with a Keyence 3D imaging technique. The IR reflection potential of the surface was analyzed by using an FLIR thermal imaging system. The data revealed that the plasma-modeled nanosurface shows higher reflective potential toward IR rays, and it seems to be cooler than the unprocessed surface by approximately 15 °C. The stability and efficiency of the plasma-modified electrospun nanolayer in water were satisfactorily examined with SEM and IR imaging. Taken together, these results suggest the excellent potential of plasma processing to develop IR reflective coatings

Edge States Drive Exciton Dissociation in Ruddlesden-Popper Lead Halide Perovskite Thin Films

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