Colloquium : Excitons in atomically thin transition metal dichalcogenides

Abstract

Atomically thin materials such as graphene and monolayer transition metal dichalcogenides (TMDs) exhibit remarkable physical properties resulting from their reduced dimensionality and crystal symmetry. The family of semiconducting transition metal dichalcogenides is an especially promising platform for fundamental studies of two-dimensional (2D) systems, with potential applications in optoelectronics and valleytronics due to their direct band gap in the monolayer limit and highly efficient light-matter coupling. A crystal lattice with broken inversion symmetry combined with strong spin-orbit interactions leads to a unique combination of the spin and valley degrees of freedom. In addition, the 2D character of the monolayers and weak dielectric screening from the environment yield a significant enhancement of the Coulomb interaction. The resulting formation of bound electron-hole pairs, or excitons, dominates the optical and spin properties of the material. Here we review recent progress in our understanding of the excitonic properties in monolayer TMDs and lay out future challenges. We focus on the consequences of the strong direct and exchange Coulomb interaction, discuss exciton-light interaction and effects of other carriers and excitons on electron-hole pairs in TMDs. Finally, the impact on valley polarization is described and the tuning of the energies and polarization observed in applied electric and magnetic fields is summarized.

Strong Light–Matter Interactions Enabled by Polaritons in Atomically Thin Materials

“…In recent years, excitons—i.e., electron–hole pairs bounded through their mutual Coulomb interaction—in 2D materials like TMDCs have attracted a great deal of attention due to their singular optical properties . These, to a great extent, reflect the presence of 2D excitons with large binding energies (hundreds of meV), thereby making such collective excitations stable even at room temperature.…”

“…This is in stark contrast with excitons in conventional bulk 3D semiconductors which typically possess binding energies of the order of (where at T = 300 K). The reason for this order‐of‐magnitude difference lies in inherently reduced screening in two dimensions …”

“…As the low‐energy physics of atomically thin TMDCs is well‐described by an effective Dirac‐like Hamiltonian with finite mass, the valley degree of freedom can be used to manipulate the spin polarization of electrons—due to the sizable spin‐orbit coupling exhibited by such materials—using circularly polarized light . Pioneering experiments have shown that it is possible to inject excitons with highly polarized spins that could be utilized for valleytronics .…”

Chiral Coupling of Valley Excitons and Light through Photonic Spin–Orbit Interactions

“…The valley Hall effect in monolayer TMDCs is closely related to valley circular dichroism,84,85 where the carriers give rise to opposite Hall currents in opposite valleys 86–88. Under the illumination of circularly polarized light and an in‐plane electric field, populations of carriers with opposite signs will accumulate in +K and −K valleys of monolayer TMDCs, respectively, as shown in Figure 2a.…”

“…In early studies, it was observed that PL polarization retention is independent of the substrate 9. It approaches unity at low temperature9,85 but is significantly lower (≈40% for monolayer MoS 2 and ≈10% for WS 2 ) at RT 85–87. Later in 2016, Nayak et al reported a world record of valley polarization contrast of 35% for monolayer WS 2 , achieved by using near‐resonant below‐bandgap excitation and chemical vapor deposition (CVD) grown monolayer WS 2 92.…”