Transition metal dichalcogenides (TMDs) are layered materials with the general chemical formula of MX2, where M is a transition metal (e.g. Mo, Ti, Ta, V) and X is a chalcogen (S, Se, Te). Each slab is composed of a covalently bonded sandwich of a sheet of metal atoms and two hexagonal planes of the calcogen atoms. The adjacent layers are held together by week Van der Waals interactions to form the bulk crystal in different polytopes which vary in stacking orders and metal atom coordination.
This chemically versatile family of materials spans the entire range of electronic structures, from insulator to metal, and hosts a number of interesting properties such as charge density wave (CDW) modulations, orbital ordering and superconductivity. These materials are the subject of intense studies both in bulk and exfoliated few layer forms.
Similarly to graphene, transition metal dichalcogenides can be exfoliated to few layer thin forms. The reduced thickness results in confinement and dimensional crossover that drive changes in various physical properties. The chemical diversity of this family allows the tuning of these properties.
Unfortunately, many of these materials are very sensitive to ambient conditions impeding the exploration of this new and fascinating parameter space. To overcome this difficulty we have developed a method of mechanical exfoliation in ultra-high vacuum (UHV). This technique is easily adaptable to any UHV system and allows preparing and studying air sensitive nanoflakes in situ.
Our investigation of in-situ exfoliated flakes of VSe2 reveals a non-monotonic thickness dependence of the CDW transition temperature. We developed and applied a novel method to extract the local critical temperature based on the real space charge modulation measured by STM.