Exciton and valley properties in atomically thin semiconductors and heterostructures

Two dimensional van der Waals (vdW) materials recently emerged as promising candidates for optoelectronic, photonic, and valleytronic applications. Monolayer transition metal dichalcogenides (TMD) are semiconductors with a band gap in the visible frequency range of the electromagnetic spectrum. Their unique properties include evolution from indirect band gap in bulk materials to direct band gap in monolayers, large exciton binding energy (few hundred meV), large absorption per monolayer (about 10%), strong spin-orbit coupling, and spin-valley locking. Moreover, two or more TMD monolayers can be stacked on top of one another to create vdW heterostructures with exciting new properties. Optical properties of semiconductors near the band gap are often dominated by the fundamental optical excitation: the exciton (Coulomb-bound electron-hole pair). Excitons in TMD monolayers (intralayer exciton) exhibit a large binding energy and a very short lifetime. The excitons in TMD monolayers are formed at the boundary of the Brillouin zone at the K and K' points. The time-reversal symmetry dictates that spins are oriented with opposite directions, leading to distinct optical selection rules for the excitons at these two valleys, a property known as the spin-valley locking. Valley polarization is often characterized by circularly polarized photoluminescence (PL). We show that the degree of valley polarization in a WSe2 monolayer depends on the degree of disorder evaluated by the Stokes shift between the PL and absorption spectra. Intrinsic valley dynamics associated with different optical resonances can only be evaluated using resonant nonlinear optical spectroscopy. We discovered exceptionally long-lived intra-valley trions in WSe2 monolayers using two-color, polarization resolved pump-probe spectroscopy. A different type of excitons (interlayer excitons) may rapidly form in TMD heterostructures with a type-II band alignment. Because of the spatial indirect nature, interlayer excitons have a much longer lifetime, which is tunable by the twist angle between the two layers. Especially, we discover that multiple interlayer excitons formed in a small twist angle heterobilayer exhibit alternating circular polarization - a feature uniquely pointing to Moiré potential as the origin. We assign these peaks to the ground state and excited state excitons localized in a Moiré potential and explain how the spatial variation of optical selection rule within the moiré superlattice can give rise to multiple peaks with alternative circular polarization. The twist angle dependence, recombination dynamics, and temperature dependence of these interlayer exciton resonances all agree with the localized exciton picture. Our results suggest the feasibility of engineering artificial excitonic crystal using vdW heterostructures for nanophotonics and quantum information applications.Physic