Structure and properties of MoS2 films

MoS2 films with a thickness of about 500 nm were prepared onto silicon wafers and steel substrates by means of r.f. magnetron sputtering. The structure and the microstructure of the MoS2 films are essentially influenced by the coating parameter. X-ray diffraction measurements indicate that the atomic structure in the films is neither amorphous nor nanocrystalline in the sense of a three-dimensional lattice. Theoretical calculations of the interference functions I (2 theta) were performed, which are based on model structures consisting of two-dimensional MoS2 basal lattice planes stacked in a non crystallographic way in c-direction. There is no crystallographic correlation between atoms of neighboured layers at a distance of about 6.7 Angstrom. TEM investigations reveal the nanostructured state of the MoS2 films with a grain size between 3 and 10 nm The comparison with the measured X-ray diffraction curves shows that the model reproduces the typical behaviour of the experimental curves. The number and quality of stacking of the a-b basal lattice planes into c-direction as well as the extension of ordering the atoms within the a-b-planes are the parameters influenced by preparation conditions. Variation of grain size and correlation lengths is within the range of a few nanometers. The friction properties of the films are found to correlate with these structural parameters. The best values of friction coefficent and number of revolutions in cratch tests were obtained for the MoS2 films with highest degree of order in the structure. Thermal treatment leads to growth of the nanocrystals. The formation of the three-dimensional lattice starts at temperature > 750 degrees C and reaches 20 nm in size at 900 degrees C.