Photonics and optoelectronics of 2D semiconductor transition metal dichalcogenides

Abstract

Single-layer transition metal dichalcogenides (TMDs) provide a promising material system to explore the electron's valley degree of freedom as a quantum information carrier 1-3 . The valley degree of freedom in single-layer TMDs can be directly accessed by means of optical excitation 4-6 . The rapid valley relaxation of optically excited electron-hole pairs (excitons) 7-9 through the long-range electron-hole exchange interaction 10,11 , however, has been a major roadblock. Theoretically such a valley relaxation does not occur for the recently discovered dark excitons [12][13][14][15][16] , suggesting a potential route for long valley lifetimes 10 . Here we investigate the valley dynamics of dark excitons in single-layer WSe 2 by time-resolved photoluminescence spectroscopy. We develop a waveguide-based method to enable the detection of the dark exciton emission, which involves spin-forbidden optical transitions with an out-of-plane dipole moment. The valley degree of freedom of dark excitons is accessed through the valley-dependent Zeeman effect under an out-of-plane magnetic field. We find a short valley lifetime for the dark neutral exciton, likely due to the short-range electron-hole exchange 17,18 , but long valley lifetimes exceeding several nanoseconds for dark charged excitons.Single-layer transition metal dichalcogenides (TMDs, MX 2 : M=Mo, W; X=S, Se) are direct band-gap semiconductors with direct gaps located at the K and K' valleys of the Brillouin zone 19,20 . Both valence and conduction bands are spin-split at the two valleys by the strong spinorbit coupling. The exciton formed by Coulomb interaction from electron and hole of antiparallel spins is a bright exciton (optically active), and from electron and hole of parallel spins, a dark exciton (optically inactive). The bright exciton exhibits strong valley circular dichroism (i.e. each handedness of circularly polarized light couples only to one of the two valleys), which provides an effective means to access the valley degree of freedom 4-6 . However, the valley relaxation is very fast (order of 10 ps) for the bright neutral 7-9 and charged excitons 21-23 . The fast valley relaxation is attributed to the long-range electron-hole exchange interaction 10,11 , which mixes the two valley exciton states. On the other hand, intervalley scattering of the dark exciton would require a spin flip, which does not occur through the longrange exchange interaction 10 . A long-lived valley polarization of the dark exciton is thus possible. In tungsten-based TMDs the dark exciton has a lower energy than the bright exciton and has recently been shown long-lived 13,24 . Direct measurement of the valley lifetime of the dark exciton, however, remains challenging. This spin-forbidden exciton has an out-of-plane (OP) transition dipole moment 12,14,25 , making its detection difficult with conventional far-field optical techniques. In addition, unlike for the bright exciton, there are no valley-dependent optical selection rules for the dark exciton that can be utilized ...

Defect Healing and Charge Transfer-Mediated Valley Polarization in MoS2/MoSe2/MoS2 Trilayer van der Waals Heterostructures

“…[1][2][3][4][5][6][7][8][9][10] The bandgap of monolayer TMDCs occurs at the inequivalent (but degenerate) K and K' points of the hexagonal Brillouin zone. The broken inversion symmetry of a TMDC monolayer combined with the time reversal symmetry imposes opposite magnetic moments at the K and K' valleys.…”

Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

“…Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant interest due to their unique electronic and optical properties that arise from the emergent valley degree of freedom and the strong spin-orbit interactions (SOIs) [1][2][3] . These properties have also been actively explored for new electronics and optoelectronics applications [4][5][6][7][8][9][10][11][12][13][14][15] .…”

“…Understanding the spin-polarized conduction band structure is important for a wide array of spin-related phenomena in monolayer TMDs. Examples include the exciton optical selection rules 1,3,20,21 , exciton and spin lifetimes 18,[21][22][23] , and the spin and valley Hall effects 1,12,15 . A large ∆ !…”

“…The method relies on the optical selection rules that interband transitions in the dipole approximation are allowed only between bands of the same electron spin and valley 1,3,20 . We measure the energy shift of the absorption resonances and the PL Stokes shift while the Fermi level is continuously scanned from the upper valence bands to the upper conduction bands by dual local gates.…”

“…The main spectral features are the A and B excitons. They correspond to the optical transitions from the two spin-split valence bands to the corresponding conduction bands 1,3,20 [ Fig. 1(a)], modified by the strong e-h interactions [41][42][43][44][45] .…”