Recent progress has been made on the synthesis and characterization of metal halide perovskite magic-sized clusters (PMSCs) with ABX3 composition (A=CH3NH3+ or Cs+, B=Pb2+, and X=Cl−, Br-, or I-). However, their mechanism of growth and structure is still not well understood. In our effort to understand their structure and growth, we discovered that a new species can be formed without the CH3NH3+ component, which we name as molecular clusters (MCs). Specifically, CH3NH3PbBr3 PMSCs, with a characteristic absorption peak at 424 nm, are synthesized using PbBr2 and CH3NH3Br as precursors and butylamine (BTYA) and valeric acid (VA) as ligands, while MCs, with an absorption peak at 402 nm, are synthesized using solely PbBr2 and BTYA, without CH3NH3Br. Interestingly, PMSCs are converted spontaneously overtime into MCs. An isosbestic point in their electronic absorption spectra indicates a direct interplay between the PMSCs and MCs. Therefore, we suggest that the MCs are precursors to the PMSCs. From spectroscopic and extended X-ray absorption fine structure (EXAFS) results, we propose some tentative structural models for the MCs. The discovery of the MCs is critical to understanding the growth of PMSCs as well as larger perovskite quantum dots (PQDs) or nanocrystals (PNCs).

Single-atom alloys (SAAs) are promising materials for heterogeneous catalysis due to their unique structure and electronic properties. SAAs have active sites narrowed down to the single-atom level, which combines the advantages of alloy materials and single-site catalysts. Given the unique structural feature of SAAs, their electronic properties can be more flexibly tailored than for their monometallic counterparts, which can be used to effectively control their catalytic activities. One interesting feature commonly observed for SAAs is the lower density of state (DOS) near the Fermi level than their bulk references. Comparing with results for their monometallic bulk reference, the most noticeable electronic property change in SAAs is the narrowing of the valence band, which gives them free-atom-like character. Moreover, the d-band position of both single atoms and their host metals can show a pronounced shift. These changes of electronic structure in SAAs could largely affect the adsorption behavior of adsorbates during the catalytic processes. Close examination of the relationship between electronic structure and catalytic activity can provide useful guidance for rational design of new catalysts with improved performance.

Ag nanostructures have a wide variety of uses in areas such as biological science and catalysis. Determination of the structural properties of Ag nanostructures can assist in the understanding of the mechanisms involved in these processes. This review provides a summary of recently published work with Ag nanostructures including very small Ag nanoclusters, larger Ag nanocrystals, and Ag nanoalloys. X-ray absorption spectroscopy (XAS) is used to elucidate structural and electronic information about the Ag nanostructures, which is then used to provide insight as to the antibacterial activities of the Ag nanostructures. Some unique features of our XAS analysis on Ag nanostructures include multiedge and multielement measurements, multishell fitting and wavelet transformation (WT) of extended X-ray absorption fine structure (EXAFS), and the correlation between X-ray absorption near edge structure (XANES) and density functional theory (DFT) modeling.

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Silver nanoclusters (NCs) are of significant interest owing to their interesting structural, electronic, and catalytic properties. Among these NCs, Ag25(SR)18 is particularly attractive due to its identical geometry as its Au counterpart, Au25(SR)18. Herein, we present the site-specific electronic properties of Ag25(SR)18 and Au25(SR)18 using X-ray spectroscopy experiments and quantum simulations. To overcome the final state effect observed in X-ray photoelectron spectroscopy (XPS), a unique method was developed to reliably analyze the charge transfer behavior of the NCs. Density functional theory calculations were combined with XPS to provide more insight into the electronic properties of the NCs. The differences in the XPS valence bands of these two NCs were further compared and interpreted using the relativistic effect. The first derivative of the X-ray absorption near-edge structure (XANES) spectrum was further used as a tool to sensitively probe the bonding properties of Ag25(SR)18.By combining the experimental XANES data and their site-specific quantum simulations, the large impact of the staple motif on the bonding properties of the NC was demonstrated. These findings highlight the unique electronic properties of each atomic site in Ag25(SR)18; the effective X-ray analysis techniques developed here can offer new opportunities for the sitespecific study of other NCs.