The size of its band gap determines the suitability of a material for use in different applications, for example, in computers or in solar cells. In the case of artificial two-dimensional (2D) materials, such as graphene or single-layer (SL) transition metal dichalcogenides (TMDCs), electronic properties, including the band gap, are drastically different from their parent compounds where dimensionality is not reduced. The electronic properties of 2D materials do not only depend on the material but also on its environment, for example, the substrate it is placed on. By changing the dielectric properties of the substrate or the carrier concentration in the material, the band gap size can be modified. Besides control of the band gap, control of the spin- and valley-degrees of freedom has been suggested as a new type of tuning knob for carrier dynamics and SL TMDCs, such as SL MoS2 and WS2, are particularly promising candidates for new spin- and valleytronic devices. This is due to the braking of inversion symmetry in their crystal lattices, a strong spin-orbit coupling, and a direct band gap at the K and K’ valleys in their electronic structure. In order to have information about ultrafast carrier dynamics and valley-degrees of freedom, both time- and angular-resolution is necessary. This can be achieved by means of time- and angle-resolved photoemission spectroscopy (TR-ARPES). Recently, this type of experiments became possible with the arrival of ultrafast high harmonic laser sources with high repetition rates. The XUV laser source at the Artemis laboratory at the Central Laser Facility can produce the high photon energies necessary to access the K (K’) point; the point of the Brillouin zone at which a band gap might be observed. Therefore, the main topic of this talk will be the results obtained by TR-ARPES on epitaxially grown SL MoS2 on Au(111) and graphene, and WS2 on Ag(111). Technical requirements, the experimental system and the growth procedures will also be presented.
 

Voditelji seminara IF-a: Nataša Vujičić i Damir Starešinić