Abstract
Two-dimensional (2D) group-VI transition metal dichalcogenide (TMD) semiconductors, such as MoS2, MoSe2, WS2 and others manifest strong light matter coupling and exhibit direct band gaps which lie in the visible and infrared spectral regimes. These properties make them potentially interesting candidates for applications in optics and optoelectronics. The excitons found in these materials are tightly bound and dominate the optical response, even at room temperatures. Large binding energies and unique exciton fine structure make these materials an ideal platform to study exciton behaviors in two-dimensional systems. This review article mainly focuses on studies of mechanisms that control dynamics of excitons in 2D systems – an area where there remains a lack of consensus in spite of extensive research. Firstly, we focus on the kinetics of dark and bright excitons based on a rate equation model and discuss on the role of previous ‘unsuspected’ dark excitons in controlling valley polarization. Intrinsically, dark and bright exciton energy splitting plays a key role in modulating the dynamics. In the second part, we review the excitation energy-dependent possible characteristic relaxation pathways of photoexcited carriers in monolayer and bilayer systems. In the third part, we review the extrinsic factors, in particular the defects that are so prevalent in single layer TMDs, affecting exciton dynamics, transport and non-radiative recombination such as exciton–exciton annihilation. Lastly, the optical response due to pump-induced changes in TMD monolayers have been reviewed using femtosecond pump–probe spectroscopy which facilitates the analysis of underlying physical process just after the excitation.
Highlights
Thin transition metal dichalcogenides (TMDs) are one of the important subclasses of two-dimensional (2D) materials with a wide range of physical properties and potential applications
We review the intrinsic and extrinsic factors that control exciton dynamics and transport in 2D semiconducting TMDs
Based on theoretical calculations[49,51,56] and experiments,[57,58] it is concluded that Mo and W-based TMDs have bright and dark exciton ground states respectively due to reversal of spins in the conduction band
Summary
Thin transition metal dichalcogenides (TMDs) are one of the important subclasses of two-dimensional (2D) materials with a wide range of physical properties and potential applications. TMDs and related vdW heterostructures possess weak dielectric screening and strong geometrical con nement which give rise to an extraordinarily strong Coulomb interaction, presenting a new paradigm for fundamental exciton physics It opens the door for probing fascinating many-particle phenomena, such as formation of different types of excitons including optically allowed and forbidden dark excitons as well as spatially separated interlayer exciton states.[40,41,42,43,44,45] They exhibit binding energies in the range of 0.5 eV, which is one to two orders of magnitude larger than in conventional materials, such as in GaAs.[7,8,9,10,46] In these materials, excitonic features being very stable, dominate the optical response and non-equilibrium dynamics. We nally conclude giving a touch on the remaining challenges in this eld
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