Crystalline structures of Sb4 molecules in antimony thin films.

Elemental antimony occurs in the solid bulk in a metallic as well as an amorphous modification.[1] Over a wide temperature range the gas phase of elemental antimony comprises almost exclusively of tetrameric molecules;[2] however, a solid-state modification of antimony consisting of Sb4 molecules, comparable to white phosphorus and yellow arsenic, is not known to date. This is explained by the fact that the Sb4 molecule is so unstable that even quenching with liquid nitrogen leads to the polymeric amorphous phase.[3] Herein we demonstrate that scanning tunneling microscopy (STM) can be used to identify unexpected crystalline ordered areas with nearly cubic symmetry in antimony thin films that are prepared under appropriate conditions from Sb4 molecules. The lattice constants of these structures are clearly different from the lattice constants of metallic a-antimony and are in agreement with the known structural data of tetrahedral Sb4 molecules.[4±6] The symmetry of the wellordered areas in the antimony films is also in accordance with the structures of the modifications of the lighter homologous elements arsenic and phosphorus which are likewise composed of tetrameric units (As4 and P4). The present STM data therefore indicate that a new allotropic modification of antimony composed of Sb4 molecules exists on the nanometer scale. Substrate surfaces on which such well-ordered antimony structures can be observed, are the (0001) basal plane of MoS2 and the (100) plane of the surface alloy AuSb2. Atomically resolved STM images of both substrates are depicted in Figure 1. Antimony thin films on various surfaces and their amorphous, nonmetallic structure have been described in the literature.[7,8] On theMoS2(0001) surface the condensation of Sb4 at a substrate temperature of 190 8C also results in macroscopically amorphous antimony films without apparent structural order. This can be seen from Figure 2a, which displays a two-monolayer-thick film. If such a film is investigated at the highest resolution, local, well-ordered areas (marked by arrows in Figure 2b) can be identified at coverages of 1±2 monolayers. These local structures always exhibit a simple, rectangular symmetry. In general no epitaxial