Stacking faults in hexagonal and rhombohedral MoS 2 crystals produced by mechanical operation in relation to lubrication

Abstract

The crystal structure of synthesized MoS 2 was observed by high resolution transmission electron microscopy to have a rhombohedral lattice of three sandwich S-Mo-S layers of ABC stacking uniformly spaced through 0.615 nm. An extra half-plane observed in the layer lattice image from an ultramicrotomed section of this crystal suggested the appearance of a stacking fault due to a Shockley partial dislocation as in the case of h.c.p. MoS 2 (molybdenite). The direction of the occurrence of transformation was reversed to that in the h.c.p. MoS 2 case (h.c.p. → rhombohedral) by missing one layer in the rhombohedral stacking and this transformation leads to the appearance of a stacking fault, slip, twinning and h.c.p. lattice formation in the slip regions, due to S-S glide. All these transformations have been observed in the well-determined orientation [2 1 1 0] parallel to the electron beam on an atomic scale. The slip mechanism was interpreted in terms of the fundamental unit common to f.c.c., h.c.p. and rhombohedral lattices, elucidating the origin of easy glide in the characteristic crystal structure of MoS 2. Mo-S glide which entails deformation of the MoS 2 trigonal prism was proposed to form an S vacancy.