SIZE QUANTIZATION AND SURFACE-STATES OF MOLYBDENUM SULFIDE CLUSTERS - A MOLECULAR-ORBITAL APPROACH

A projection procedure is described, as a tool to separately analyze the electronic states of surface and bulk regions of nanometer-sized particles. It allows to discuss surface states in terms of specific atoms, coordination numbers, oxidation states, and connectivities and it gives insight into the size quantization of clusters. The procedure is general and can be used for studying clusters up to respectable size, of different composition and different structure. Molecular orbital calculations on D-3h - (MoS2)(3n2)S-2(H2S)(6n), n = 1,2, ... clusters, cluster chains and the band structure of (2)(x)[MoS2] are reported. The HOMO and LUMO are separated by only about 0.1 eV in Mo12S38H24. Even this small splitting vanishes in the larger clusters. However, in the limiting infinite case, (2)(x)[MoS2], the expected semiconductor band gap evolves. Applying the projection procedure to the clusters we find that surface states, originating from the outermost molybdenum atoms and being of mainly 4d(x2-y2), 4d(xy), and 4d(z2) type with some S(3p) contributions, fill up the band gap and that the bulk shows the expected quantum-size behaviour.