Electronic, excitonic and magnetic properties of two-dimensional heterostructures

Two-dimensional (2D) crystals such as graphene or transition metal dichalcogenides (TMDCs) are a fascinating class of quantum materials. These compounds are obtained isolating the single atomic sheets that normally form bulk layered crystals, and the reduced dimensionality endows them with unique physical properties. These peculiar characteristics have made them a very interesting research topic, with potential for fundamental discoveries but also technological applications. This thesis focuses on the behaviour of semiconducting two-dimensional materials of the TMDC family, which possess unusual spin- and valley-dependent optical and transport characteristics. The aim is to engineer their properties and build 2D heterostructures to investigate multiple quantum degrees of freedom (charge, spin and valley) for applications in energy-efficient information technologies.

To this end, the first part of the dissertation explores the layered material PtSe2 for its potential as a channel material for nano electronic devices. By studying charge transport as a function of material thickness, temperature and carrier concentration, one finds that PtSe2 allows for multiple kinds of band-structure engineering. First, a metal-semiconductor transition driven by interlayer interaction is observed. Further magneto-transport studies show the presence of layer-dependent, defect-induced magnetism. These results show the potential of nanoscale engineering to obtain designed physical properties in 2D materials.

The second part of the thesis instead deals with indirect excitons in 2D heterostructures as a promising platform to realize new computing devices. First, a room temperature excitonic switch is demonstrated, taking advantage of the large exciton binding energies found in TMDCs. The work then focuses on the control of valley-polarized excitons, demonstrating the possibility to electrically control the polarization, energy and brightness of interlayer excitons. Building on top of these two results, an excitonic switch for currents of valley-polarized excitons, i.e. a valleytronic transistor, is realized. These results highlight how excitonic devices could be a capable platform for computing schemes based on the pseudo-spin degree of freedom.