Two dimensional materials for device applications

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDC) such as MoS2, WSe2, MoSe2, MoTe2, etc have emerged as brand new photonic materials due to their unique properties and multiple functions. TMDS's are atomically thin semiconductors of the type MX2 with a transition metal (M) atom and a chalcogen (X) atom (S, Se, Te). One layer of M atoms is sandwiched between two layers of X atoms. They are the part of the family of 2D materials, a name to emphasize their thickness. The discovery of graphene shows different properties emerge when a bulk crystal of macroscopic dimensions is thinned down to atomic layers. Like graphite, TMD bulk crystals are formed of monolayers bound to each other by Van der Waals attraction. Our research group exploits several interesting properties of TMDC's mentioned

below to investigate photocatalytic dye degradation, hydrogen evolution reaction (HER), supercapacitors and many other physical phenomena

(i) TMD monolayers MoS2, WS2, MoSe2, WSe2, MoTe2 have a direct band gap, and can be used in electronics as transistors and in optics as emitters and detectors.

(ii) The TMD monolayer crystal structure has no inversion center, which allows to access a new degree of freedom of charge carriers, namely the k-valley index, and to open up a new field of physics: valleytronics

(iii) The strong spin-orbit coupling in TMD monolayers leads to a spin-orbit splitting of hundreds meV in the valence band and a few meV in the conduction band, which allows control of the electron spin by tuning the excitation laser photon energy and handedness. The work on TMD monolayers is an emerging research and development field since the discovery of the direct bandgap and the potential applications in electronics and valley physics. TMDs are often combined with other 2D materials like graphene and hexagonal boron nitride to make van der Waals heterostructure. These heterostructures need to be optimized to be possibly used as building blocks for many different devices such as transistors, solar cells, LEDs, photodetectors, fuel cells, photocatalytic and sensing devices. Some of these devices are already used in everyday life and can become smaller, cheaper and more efficient by using TMD monolayers.