Effect of sequential oxidation on the electronic structure of tungsten clusters

Using photoelectron spectroscopy and first principles molecular orbital calculations, we report the first observation of an abrupt change in the electronic structure of W4O m (m6 6) clusters at m 1⁄4 5 which is well below the bulk stoichiometric composition of tungsten oxide (WO3). The signature of this onset is established from an anomalous increase in the vertical detachment energies (VDE), adiabatic electron affinities (AEA), and the energy gaps between highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO). These changes are also accompanied by the cleavage of 50% of the W–W bonds and localization of electron charge density in W4O5 cluster. Atomic clusters, which constitute a new phase of matter intermediate between atoms and solids, have attracted considerable interest from researchers over the past three decades. One of the reasons for this continued interest is that clusters can illustrate the evolution of bulk properties one atom at a time. In small clusters the electrons occupy discrete molecular energy levels that are well separated from each other. In addition, a significant gap exists between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). As clusters grow, these separations, particularly near the HOMO, become increasingly small. Eventually the energy levels of the cluster would resemble the energy bands of the corresponding solid. The most often asked question in this regard is: When is a metal a metal? Arguably, this is a difficult and sometimes ambiguous question since the answer depends on how one defines a metal or a semiconductor atomic cluster. Nevertheless several investigations have been carried out in Hg [1] and Mg [2,3] clusters to address the issue of a change in the electronic structure from insu* Corresponding author. Fax: +1-804-828-7073. E-mail address: [email protected] (B.K. Rao). lating to metallic behavior as a function of size. One could argue that when the HOMO–LUMO gaps become small compared to the temperature, the cluster would exhibit metallic behavior. However, as has been seen in Mg clusters [2,3], the HOMO–LUMO gaps open and close with electronic shell closure. Thus, one cannot conclude that a smaller cluster with nearly vanishing HOMO–LUMO gap is metallic while the larger cluster with a non-zero HOMO–LUMO gap is like a semiconductor. Although it is difficult to clearly label a small atomic cluster as a metal or a semiconductor, there are signatures that distinguish between clusters composed of metal or semiconductor elements. For example, the HOMO–LUMO gaps of clusters of metallic elements tend to be smaller than those of semiconductor elements. Similarly, the electron density distribution of clusters of metallic elements do not show the same degree of directional bonding as is evident in clusters of covalently bonded atoms. In addition, clusters of metallic elements exhibit many low lying isomers (a consequence of flexible bonding characteristics) while the numbers of such isomers of covalently bonded elements are few. Thus based upon these identifying features, one