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 recently2,3 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 AuxM1-xTe2
and AgxM1-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
AuxM1-xTe2 composition, but
rather in a proper proportion of MTe2 and AuTe2
single crystals with a negligible amount of incorporated point
defects.