Reexamination of structures, stabilities, and electronic properties of holmium-doped silicon clusters HoSi _n (n = 12–20)

Abstract

The total energies, growth patterns, equilibrium geometries, relative stabilities, hardnesses, intramolecular charge transfer, and magnetic moments of HoSi _n (n = 12–20) clusters have been reexamined theoretically using two different density functional schemes in combination with relativistic small-core Stuttgart effective core potentials (ECP28MWB) for the Ho atoms. The results show that when n = 12–15, the most stable structures are predicted to be exohedral frameworks with a quartet ground state, but when n = 16–20, they are predicted to be endohedral frameworks with a sextuplet ground state. These trend in stability across the clusters (gauged from their dissociation energies) was found to be approximately the same regardless of the DFT scheme used in the calculations, with HoSi₁₃, HoSi₁₆, HoSi₁₈, and HoSi₂₀ calculated to be more stable than the other clusters. The results obtained for cluster hardness indicated that doping the Ho atom into Si₁₃ and Si₁₆ leads to the most stable HoSi _n clusters, while doping Ho into the other Si _n clusters increases the photochemical sensitivity of the cluster. Analyses of intracluster charge transfer and magnetic moments revealed that charge always shifts from the Ho atom to the Si _n cluster during the creation of exohedral HoSi _n (n = 12–15) structures. However, the direction of charge transfer is reversed during the creation of endohedral HoSi _n (n = 16–20) structures, which implies that Ho acts as an electron acceptor when it is encapsulated in the Si _n cage. Furthermore, when the most stable exohedral HoSi _n (n = 12–15) structures are generated, the 4f electrons of Ho are virtually unchanged and barely participate in intracluster bonding. However, in the most stable endohedral HoSi _n (n = 16–20) frameworks, a 4f electron does participate in bonding. It does this by transferring to the 5d orbital, which hybridizes with the 6s and 6p orbitals and then interacts with Si valence sp orbitals. Meanwhile, the total magnetic moments of the HoSi _n (n = 16–20) clusters are considerably higher than those of HoSi _n (n = 12–15). Interestingly, the endohedral HoSi₁₆ and HoSi₂₀ clusters can be viewed as the most suitable building blocks for novel high-density magnetic storage nanomaterials and for novel optical and optoelectronic photosensitive nanomaterials, respectively.