Nanoplatelets made from MoS2 and WS2

Free clusters of the layered semiconductor materials of type MS2 (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 WnS m we find that platelet structures dominate for clusters larger than W10S30, the most stable species are W15S42 and W21S56. These platelets have a metallic character and are chemically inert making them interesting building blocks for future nanoelectronics. Carbon clusters and nanoparticles form a large variety of different structures such as fullerenes and nanotubes. These systems derive from the layered structure of bulk graphite avoiding dangling bonds at the edges by the formation of p-bonds, resulting in a hollow sphere structure. Similar structures might exist for other materials forming layered structures like the metal-chalcogenides MS2 (M =Mo, W). Indeed, for nanoparticles of these materials nanotubes and onion-like structures have been observed [1–9]. However, until now the structures of MS2 nanoclusters in the size regime of fullerenes are unknown. The observation of onion-like MS2 nanoparticles might be taken as an indication for the existence of MS2 fullerene structures. However, there are two aspects opposing this assumption: (i) The energy required to bend a single MS2 sheet is larger than the corresponding bending energy in graphite [10,11]. The MS2 sheets consist of a triple layer with the metal atoms in between two S layers. Therefore, larger structures might be similar to carbon, but for nanoclusters differences are likely to appear. * Corresponding author. Fax: +49 7531 885133. E-mail address: [email protected] (G. Ganteför). (ii) The edges of planar fragments of a MS2 sheet can be stabilized by additional S atoms. Therefore, the formation of S-stabilized platelets might be preferred over fullerene formation [11]. Such platelet structures might be of use for future nanoelectronic devices, because they are chemically rather inert and stable even at ambient conditions. In addition, they might exhibit a size-dependent HOMO–LUMO gap like other nanoparticles consisting of materials which are semiconducting in the bulk. Triangular MoS2 nanoplatelets on an Au substrate have been studied by scanning tunnelling microscopy [12–14]. However, since the method of generation from 2-dimensional metal islands exposed to H2S strongly prefers the formation of 2-dimensional structures, the most stable structure of free MS2 nanoclusters is still unclear. It could be platelets or fullerenes or other structures. We therefore studied clusters of MS2 theoretically and experimentally. Triangular metallic platelets are found to be the favored structure in the size regime from 10 to 100 MS2 units with a considerable excess of S atoms stabilizing the edges. Below about 10 MS2 units 3-D compact cluster structures are preferred. Larger clusters have the double and triple mass of these magic platelets indicating the