Structure, thermochemical properties, and growth sequence of aluminum-doped silicon clusters Si(n)Al(m) (n = 1-11, m = 1-2) and their anions.

A systematic examination of the aluminum doped silicon clusters, Si(n)Al(m) with n = 1-11 and m = 1-2, in both neutral and anionic states, is carried out using quantum chemical calculations. Lowest-energy equilibrium structures of the clusters considered are identified on the basis of G4 energies. High accuracy total atomization energies and thermochemical properties are determined for the first time using the G4 and CCSD(T)/CBS (coupled-cluster theory with complete basis set up to n = 3) methods. In each size, substitution of Si atoms at different positions of a corresponding pure silicon clusters by Al dopants invariably leads to a spectrum of distinct binary structures but having similar shape and comparable energy content. Such an energetic degeneracy persists in the larger cluster sizes, in particular for the anions. The equilibrium growth sequences for Al-doped Si clusters emerge as follows: (i) neutral singly doped Si(n)Al clusters favor Al atom substitution into a Si position in the structure of the corresponding cation Si(n+1)(+), whereas the anionic Si(n)Al(-) has one Si atom of the isoelectronic neutral Si(n+1) being substituted by the Al impurity; and (ii) for doubly doped Si(n)Al2(0/-) clusters, the neutrals have the shape of Si(n+1) counterparts in which one Al atom substitutes a Si atom and the other Al adds on an edge or a face of it, whereas the anions have both Al atoms substitute two Si atoms in the Si(n+2)(+) frameworks. The Al dopant also tends to avoid high coordination position.