Density functional studies of small silicon clusters adsorbed on graphite (0001) and diamond (100)

The structural and electronic properties of small silicon clusters (Siₙ with $n=2--7$) adsorbed on graphite (0001) and diamond (100) substrates are studied by density functional theory within periodic boundary conditions. A three-layer graphene slab with 60 carbon atoms in each layer is used to represent the graphite substrate. The diamond substrate is described by an eight-layer slab involving a p(8×4) surface cell. Side adsorption geometry of Siₙ on the substrates is considered. Maximum stability is encountered for particle site adsorption of the Siₙ clusters on the graphite surface and for bridge site adsorption on diamond. Weak covalent interaction prevails between the cluster and the graphite substrate, while much stronger bonding effects are observed for adsorption on diamond, leading to considerable structural deformation of the cluster. Partial density of states distributions are calculated for the Siₙ adsorbates. From this analysis, the clusters exhibit a distinct narrowing of their energy gaps upon deposition on the surfaces. This effect is found to be weak for the graphite and very substantial for the diamond substrate. In the latter case, it is attributed to the emergence of spectral lines close the Fermi energy of the cluster-substrate composite, reflecting sizable interaction between the p-electron subsystems of Siₙ and diamond.