Studies using ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) demonstrate that M 4 X 6 (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, W 4 O 6 has a lower symmetry and a larger HOMO-LUMO gap than other hitherto discovered magic clusters. Although the similarity between the electronic structures of W 4 O 6 and Mo 4 S 6 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.

Au nanoparticles grown on mildly sputtered Highly Ordered Pyrolytic Graphite (HOPG) surfaces were studied using Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS). The results were compared with those of Ag nanoparticles on the same substrate. By varying the defect densities of HOPG and the Au coverages, one can create Au nanoparticles in various sizes. At high Au coverages, the structures of the Au films significantly deviate from the ideal truncated octahedral form: the existence of many steps between different Au atomic layers can be observed, most likely due to a high activation barrier of the diffusion of Au atoms across the step edges. This implies that the particle growth at room temperature is strongly limited by kinetic factors. Hexagonal shapes of Au structures could be identified, indicating preferential growth of Au nanostructures along the (111) direction normal to the surface. In the case of Au, XPS studies reveal a weaker core level shift with decreasing particle size compared to the 3d level in similarly sized Ag particles. Also taking into account the Auger analysis of the Ag particles, the core level shifts of the metal nanoparticles on HOPG can be understood in terms of the metal/substrate charge transfer. Ag is (partially) positively charged, whereas Au negatively charged on HOPG. It is demonstrated that XPS can be a useful tool to study metal-support interactions, which plays an important role for heterogeneous catalysis, for example.

Free clusters of the layered semiconductor materials of type MS 2 (M = Mo, W) are studied using mass and photoelectron spectroscopy and compared to theoretical predictions. In contrast to carbon fullerenes, these clusters prefer planar platelet structures. The dangling bonds at the edges are stabilized by excess S atoms. For W n S À m we find that platelet structures dominate for clusters larger than W 10 S 30 , the most stable species are W 15 S 42 and W 21 S 56. These platelets have a metallic character and are chemically inert making them interesting building blocks for future nanoelectronics.

Electronic and geometric structures of M x S y (M = W, Mo; x = 1, 2, 4; y = 1-12) clusters have been studied using density functional theory calculations, and compared to experimental photoelectron spectra. For the metal atoms, an uptake of up to six sulfur atoms has been observed, which can be explained by the bonding of S − 3 chains. A structural difference to the corresponding oxides is the preference of bridging sites for S, which might be the origin of the differences between the structures of bulk MO 3 and MS 2. For x = 1, 2 the HOMO-LUMO gaps vary irregularly. For x = 4, a large HOMO-LUMO gap has been found for y = 6, 7, and 8 and the W 4 S 6 and Mo 4 S 6 clusters have been found to be magic with an extraordinarily high stability.

The geometric and electronic structures of gas phase (M03S3)nS2 and (M0 3 S 3)1I_1(MoS4h clusters with n = 2, 3, 4, ... are studied experimentally using mass and photoelectron spectroscopy. The M0 3 S 3 units form one-dimensional chains with length 'n'. There are two possible types of terminations at the ends: a single S atom or a MoS 4 cluster. The experimental results are compared to calculations based on density-functional theory. Although clusters of this type have been known for decades as constituents of Chevrel phases, we here report their first gas phase synthesis for sizes up to n = 17. With increasing n, the gap vanishes extending such clusters to a conducting wire.