Transition metal dichalcogenides (TMDs) are layered materials with the general chemical formula 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 surrounded by two hexagonal planes of chalcogen atoms. Adjacent slabs are held together by weak van der Waals interactions to form bulk crystals in different polytypes 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 exfoliated few layer and bulk forms.
Thickness dependence of the CDW in 1T-VSe2
Our investigation of in-situ exfoliated flakes of 1T-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 from the real space charge modulation measured by STM on every terrace.
In-situ exfoliation device
Many transition metal dichalcogenides are very sensitive to ambient conditions. 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.