REACTION OF NOBLE METAL DEPOSITS WITH TRANSITION-METAL CHALCOGENIDE SURFACES

A. Prodan ¹, V. Marinkovia ¹, R. Gril ¹, S. W. Hla ²

¹Institute Jozef Stefan, Ljubljana, Slovenia
²SuperESCA Group, Sincrotrone Trieste, Padrichiano, Italy

Transition-metal dichalcogenides and ditellurides (MTe₂ with M = Mo, Nb, W) in particular represent a very useful family of substrates for various nucleation and growth studies. First, their Te-M-Te sandwiches are kept together by weak Van der Waals forces, causing their terminating surfaces to be rather inert and second, their structures can all be derived from the undistorted a-MoTe₂ one by changing the lattice parameters and deforming the MX₆ octahedra¹.

It was shown recently^2,3 that gold and silver atoms, evaporated onto a-MoTe₂ and b-MoTe₂, not only grow epitaxially on these substrates, but under certain conditions also react with the surface tellurium atoms. Since the average structure of AuTe₂ (calaverite) is practically isostructural with the one of -MoTe₂ and WTe₂, while silver preferably forms with tellurium Ag₂Te, it was speculated that these two noble metals will either form similar epitaxially grown surface deposits, whose growth will depend on the substrate surface corrugation, the temperature, and the lattice mismatch between the deposit and the substrate, or will form with the top substrate layer different reaction products, which may as well be epitaxially related to the substrate.

The samples were prepared in two ways, by in-situ evaporation of noble metals onto the ditelluride substrates and by growing Au_xM_1-xTe₂ and Ag_xM_1-xTe₂ (0 < x < 0.02) crystals by iodine transport reactions. It was shown by means of transmission electron microscopy and diffraction, by scanning microscopy with x-ray energy dispersive analysis, and by scanning tunneling microscopy that in case of evaporation isolated defects with noble metal atoms occupying transition metal positions in the Te-M-Te sandwiches were formed during the nucleation stages. During further growth extended two-dimensional surface layers were formed, which were finally covered by rather mobile noble metal agglomerates of a few nanometers in size. Contrary, it was shown that during crystal growth small additions of e.g. gold in the starting material did not result in single crystals with the Au_xM_1-xTe₂ composition, but rather in a proper proportion of MTe₂ and AuTe₂ single crystals with a negligible amount of incorporated point defects.

1. W. G. Dawson and D. W. Bullett, J. Phys. C: Sol. State Phys. 20, 6159 (1987).
2. S. W. Hla, V. Marinkovia, and A. Prodan, Surf. Sci., 356, 130 (1996).
3. S. W. Hla, V. Marinkovia, and A. Prodan, Surf. Sci., 377-379, 979 (1997).