Palladium in the gap: Cluster assemblies with band edges localized on linkers

Cluster-assembled materials composed of [Pd ₂As ₁₄] ^4- polyvalent anions, where a palladium dimer links two As ₇ clusters, have been synthesized in zero- and two-dimensional architectures. Their dimensionality was controlled through the selection of the counterions, and their structures were characterized by single-crystal X-ray diffraction. Reflectance spectroscopy measurements indicate that the zero- and two-dimensional assemblies have band gaps of 1.35 and 1.15 eV, respectively, and are smaller than the 1.80 eV gap of the As ₇ ^3- cluster. Theoretical investigations reveal that the frontier orbitals of the cluster building blocks are located primarily on the Pd dimer. Further, these orbitals are embedded inside the intrinsic band gap of the As ₇ ^3- clusters, reducing the band gap energy with edge states that are localized on the Pd sites. It is suggested that such a periodic array of narrow localized states may lead to interesting transport and optical properties as the carriers may become localized at these sites.