Experimental and theoretical studies on inorganic magic clusters : M4X6 (M = W, Mo, X = O, S)

Studies using ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) demonstrate that M4X6 (M = W, Mo and X = O, S) clusters show large gaps (about 2 eV) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), indicative of their high stability and chemical inertness. In particular, W4O6 has a lower symmetry and a larger HOMO–LUMO gap than other hitherto discovered magic clusters. Although the similarity between the electronic structures of W4O6 and Mo4S6 may be regarded as an indication that both clusters have similar geometric structures, our detailed DFT-calculations reveal otherwise. This result implies that synergetic approach using theoretical and experimental methods are essential to shed light on cluster geometries. Geometric, electronic and chemical properties of nanostructures are often much different compared to their bulk counterparts. Among various nanoclusters, the interest on so-called magic clusters has been increasing since the discovery of C60 [1]. Magic clusters exhibit closed electronic shell configurations, resulting in large gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), high ionization potentials and low electron affinities, which are shown to be a prerequisite for chemical inertness and high stability of the clusters [1–15]. In the case of alkali metal-like clusters, the magic numbers with large HOMO–LUMO gaps can be explained by the electron shell model [14]. Besides these electronic properties, compact and highly symmetric structures are often observed for magic clusters [1–15] Therefore, magic clusters are regarded as potential candidates for building blocks of new cluster materials, i.e., * Corresponding author. Fax: +49753188 3888. E-mail address: [email protected] (Y.D. Kim). one may be able to synthesize cluster assemblies [16], in which original geometric structures of the free magic clusters remain unchanged. Well-known examples for magic clusters are C60 [1], Na8 [5], Al 13 [6,7], Sb4 [8], Au20 [9], and Au55 [10] and some of these magic clusters are actually shown to form cluster materials [8]. Recently, attempts to synthesize new cluster materials have been extended to inorganic complexes [17]. For example, fullerene-like structures consisting of MoS2 units were synthesized previously [18], and it was shown that Al13H exhibits a HOMO–LUMO gap of 1.4 eV [11]. In the present work, electronic and geometric structures of inorganic clusters consisting of 4 metallic and 6 non-metallic atoms (M4X6, M =W, Mo and X = O, S) are introduced. W4O6 clusters show a large HOMO– LUMO gap in the anion ultraviolet photoelectron spectroscopy (UPS), which is also confirmed by theoretical calculations. W4O6 shows a relatively low symmetry compared to other hitherto discovered magic clusters such as C60 and Au20. Also, for W4S6 and Mo4S6, large HOMO–LUMO gaps were found using UPS, and 1 2 3 4 5 0 W 4 O m Binding Energy (eV) In te ns it y (a rb .u ni ts )