Engineering electronic structures of Nb6Ix superatomic clusters by metal atom incorporation: a first-principles study

Akin to the conventional building blocks of solids, the elements in the periodic table, the iodine capped Nb6Ix (x = 12-18) octahedron cluster is a well-known "superatomic building block" for designing not only cluster assemblies like Prussian blue analogues and hybrid perovskites but also for new catalysts. Recent achievements within the realm of atomic clusters using experimental and theoretical approaches have shown synthesis and precise tuning of magnetic and electronic properties of small octahedron clusters via atom-by-atom substitution. In this work, state-of-the-art density functional theory (DFT) calculations highlight the feasibility of tuning the valence electron concentration (VEC) by endohedral doping of 3d-block transition metals (M = Sc-Zn) inside the core of the homo-iodide Nb6 octahedron. The energetics of various structural isomers imply that all other M atoms can be doped within the Nb6Ix cluster with high I atom concentrations, except Sc and Ti. In-depth analysis shows the cohesive energy per atom and the formation energy (FE) of the clusters correlate with various structural and electronic parameters of these clusters exhibiting electronic shell closures with 18 and 24 VEC, with a substantial HOMO-LUMO gap within the range of 0.72 to 1.76 eV using HSE06 hybrid functionals. Interestingly, the FE calculations demonstrate that the MNb6I18 cluster can be formed even under M-rich conditions, having robust formation stability. We further explain how the Gibbs free energy (ΔGH) of adsorbed hydrogen correlates with different d-band centres of Nb and M atoms to highlight the impact of electronic structure on the catalytic activity towards the hydrogen evolution reaction (HER). Thus, it is possible to achieve potential HER catalysts with very small ΔGH for VNb6I18 to CoNb6I18 comparable to the Pt(111) surface. This work provides the atomic structure, stability, and electronic properties of endohedral doped Nb6Ix clusters that can pave the way for various potential applications in catalysis, molecular electronics and spintronics, and for further experimental research.

Unraveling the Origin of Unusual Cs Atom Disorder in Cesium Octahedral Molybdenum Halide Cluster Compounds, Cs2[{Mo6Xi8}Xa6] (X = Cl and Br)

In this study, we investigate structural disorder and its implications in metal cluster (MC)-based compounds, specifically focusing on Cs2[{Mo6Xi8}Xa6] (X = Cl and Br). Utilizing synchrotron radiation X-ray diffraction, Fourier transform infrared spectroscopy, and luminescence measurements, we examined the incorporation of water molecules into these compounds and their effects on the crystal structure and optical properties. Our findings reveal that the presence of water molecules induces the lattice disorder, particularly the displacement of Cs atoms. Density functional theory calculations, including dispersion corrections (DFT-D), were employed to model superlattices incorporating varying positions and amounts of water molecules. The DFT-D results corroborated experimental data, indicating that water molecules notably impact the lattice structure by causing the Cs disorder without altering the fundamental trigonal arrangement of MC units. Our results reveal that the composition of the compounds, specifically the Cs/[{Mo6Xi8}Xa6] ratio, remains stoichiometric, regardless of the amount of water in their lattice. Luminescence spectroscopies confirmed that the water incorporation and the lattice disorder had little effect on the luminescence wavelength, but purification enhanced the luminescence efficiency. This study highlights the importance of understanding structural disorders in MC-based compounds for optoelectronic applications and demonstrates the utility of DFT calculations in exploring complex crystallographic phenomena.

Transparent functional nanocomposite films based on octahedral metal clusters: synthesis by electrophoretic deposition process and characterization

Transparent optical thin films have recently attracted a growing interest for functional window applications. In this study, highly visible transparent nanocomposite films with ultraviolet (UV)-near-infrared (NIR)-blocking capabilities are reported. Such films, composed of Mo6 and Nb6 octahedral metal atom clusters (MC) and polymethylmethacrylate polymer (PMMA), were prepared by electrophoretic deposition on indium tin oxide-coated glass (ITO glass). PMMA was found to improve both the chemical and physical stability of Mo6 and Nb6 MCs, resulting in a relatively homogeneous distribution of the clusters within the PMMA matrix, as seen by microstructural observations. The optical absorption spectrum of these transparent MC@polymer nanocomposite films was marked by contributions from their Mo6 and Nb6-based clusters (absorption in the UV range) and from the ITO layer on silica glass (absorption in the NIR range). Mo6@PMMA nanocomposite films also exhibited excellent photoluminescence properties, which were preserved even after exposure to 50°C at a relative humidity of 70% for one month. These films cumulate high transparency in the visible range with remarkable UV-NIR blocking properties and represent interesting candidates for functional glass application.