Study of the Structural and Electronic Properties of [Ti@Si16]n, [Sc@Si16K]n, and [V@Si16F]n (n ≤ 9) Aggregates from First Principles

Recent experimental and theoretical work has established that the ground state and low-lying energy isomers of endohedral M@Si-16 clusters (M = Sc-, Ti, or V+) have a nearly spherical cagelike form, an atomic-like closed shell electronic structure, and a large highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap of similar to 2 eV, which suggests the use of these clusters as basic units (superatoms) in the assembly of optoelectronic materials. As a first step in that direction, in this work are studied, by means of first-principles calculations, the trends in the formation of [Ti@Si-16](n), [Sc@Si16K](n), and [V@Si16F](n) aggregates as their sizes increase (n <= 9). We identify especially stable linear, planar, and three-dimensional patterns that can be used to grow low-dimensional periodical systems. When n >= 2, the aggregates with greater binding energy result from the bonding of n supermolecular units having D-4d symmetry for the M@Si-16 cage, instead of the Frank-Kasper symmetry ground state of the basic superatom. Particularly interesting aggregates are (i) [Ti@Si-16](n) rings when n >= 6, which can be grown as wires or nanotubes; (ii) rings and linear forms of [Sc@Si16K](n) aggregates having a rich variety of nearly degenerate isomers differing in the bonding site of K atoms and strongly varying electric dipole moments; and (iii) [M@Si16X](3m) wires (m = 1-3) formed by vertically stacking the [M@Si16X](3) starlike trimer with rotation of 60 degrees between consecutive trimer units, which show interesting magnetic configurations for M = V and X = F. The HOMO LUMO gap for the most favorable structure decreases with size, and the aggregates become nearly metallic when n <= 9.