Engineering of excitonic and spin-valley properties in van der Waals heterostructures – VallEx

The choice of two-dimensional (2D) materials to engineer devices with atomically flat active regions is currently extending far beyond graphene to a wide range of new semiconductors, insulators, metals and superconductors. Taking advantage of each material’s properties, the stacking of diverse 2D layers to build van der Waals heterostructures, should open the way to a new class of devices with potential applications in optoelectronics, spintronics, flexible electronics or photovoltaics.
Among the various families of 2D materials, semiconductor transition metal dichalcogenide (TMDC) materials (MoS2, WS2, MoSe2, WSe2 and MoTe2) exhibit especially exciting properties when thinned down to one monolayer. In contrast to graphene, TMDC monolayers have a direct band gap yielding interesting electronic and optical properties in the visible and near infrared regions of the optical spectrum. In particular, they exhibit a strong light-matter interaction governed by very robust excitons from cryogenic to room temperature (Coulomb bound electron-hole pairs with binding energy of several hundreds of meV). This strong light-matter interaction can be used for optoelectronics applications (photodetectors, LEDs, solar cells, non-linear optics). Secondly, the interplay between crystal inversion symmetry breaking and strong spin-orbit coupling provides a unique access to control simultaneously two degrees of freedom for data processing and storage: the spin (up or down) and the electron momentum in k-space (K+ or K- valleys at the corners of the Brillouin zone). Thus, in addition to rich spin-valley physics, TMDC materials open the way to the development of spintronics or valleytronics devices provided they can exhibit long spin-valley lifetimes and convenient manipulation of these degrees of freedom.
The goal of this disruptive project is to develop precise tuning of the strong light matter interaction and spin-valley properties of TMDC monolayers by embedding them in innovative van der Waals heterostructures. Three tuning parameters that can easily be controlled in a functional device will be studied: the dielectric environment, the charge carrier density and the electric field.
We will first improve the quality of our van der Waals heterostructures (mainly the stacking of TMDC monolayers, graphene and hexagonal boron-nitride) by fabricating them in strictly controlled environment. Then, we will evaluate the potential of dielectric environment engineering to tune the energy and the oscillator strength of the excitonic states and the electronic band gap. In parallel, we will study the spin-valley relaxation mechanisms in TMDC MLs. Most of the existing studies in the field have focused on the spin-valley properties of optically bright but short-lived excitons. We will deal with longer lived excitations which are more promising for long spin-valley lifetimes (dark excitons, spatially indirect excitons and resident carriers). The optical signatures of these species are more difficult to access than the well-studied bright excitons and will require the fabrication of complex structures (charge tunable devices, type II heterostructures, samples for edge excitation). Finally we will study the effects of external and internal electric fields on the excitonic and spin-valley properties of TMDC monolayers. In particular, we aim to demonstrate the Stark effect and the Bychkov-Rashba effect which may pave the way to the development of optoelectronics and spintronics devices based on TMDC materials. In particular, we aim to demonstrate a proof of concept of a spin-valley memory working at room temperature.