Bulk dichalcogenides

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.

CDW in TaS2
CDW in TaS2
CDW in TiSe2
CDW in TiSe2
O and Ti defects in tiSe2
O & Ti defects in TiSe2
We study bulk TiSe2 which undergoes a commensurate CDW transition at around 200K. The origin of the CDW and a possible chiral nature of this ordered electronic phase are among the open questions we are investigating. To this end, we combine low-temperature scanning probe microscopy and spectroscopy and Density Functional Theory simulations (collaboration with David Bowler at UCL/LCN).

Copper intercalated TiSe2 hosts several other properties: the CDW exhibits a remarkable instability towards the formation of stripes at low Cu concentrations. Above x=0.4% copper content CuxTiSe2 becomes superconducting with a maximum critical temperature of 4K at x=0.8. The interplay between charge ordered phases and superconductivity is a key question in a number of superconducting systems, including high temperature superconductors. CuxTiSe2 offers a fantastic playground to explore these topics by means of scanning tunnelling microscopy and spectroscopy.

Further reading

Réalisation : Sur Mesure concept