Geometric and electronic properties of Si-atom doped Al clusters: Robustness of binary superatoms against charging

The geometric and electronic properties of silicon-atom-doped aluminum clusters, Al_nSi_m (n = 7-30, m = 0-2), were investigated experimentally. The size dependences of the ionization energy and electron affinity of Al_nSi_m show that the stability of Al_nSi_m is governed by the total number of valence electrons in the clusters, where Al and Si atoms behave as trivalent and tetravalent atoms, respectively. Together with theoretical calculations, it has been revealed that neutral Al₁₀Si and Al₁₂Si have a cage-like geometry with central Si atom encapsulation and closed electronic structures of superatomic orbitals (SAOs), and also that they both exhibit geometric robustness against reductive and oxidative changes as cage-like binary superatoms of Si@Al₁₀ and Si@Al₁₂. As well as the single-atom-doped binary superatoms, the effect of symmetry lowering was examined by doping a second Si atom toward the electron SAO closing of 2P SAO, forming Al₁₁Si₂. The corresponding anion and cation clusters keep their geometry of the neutral intact, and the ionization energy is low compared to others, showing that Al₁₁Si₂ is characterized to be, Si@Al₁₁Si as an alkaline-like binary superatom. For Al₂₁Si₂, a face-sharing bi-icosahedral structure was identified to be the most stable as dimeric superatom clusters.