Valley polarization and intervalley scattering in monolayer MoS₂

Abstract

Submitted for the MAR13 Meeting of The American Physical Society Valley polarization and intervalley scattering in monolayer MoS 2 G. KIOSEOGLOU, University of Crete, A.T. HANBICKI, M. CURRIE, A.L. FRIEDMAN, D. GUNLYCKE, B.T. JONKER, Naval Research Lab-Single layer MoS 2 is a prime candidate material for implementing valleytronics because minima in the bandstructure at inequivalent K points of the Brillouin zone can be independently populated, thus making the valley index a potential state variable for information processing. Light of a particular helicity populates only one of the two K-valleys (either K or K') resulting in a strong emission at around 1.9 eV associated with a direct transition. We use energy and helicity dependent optical pumping to analyze the coupling of the valley and spin indices to the depolarization of emitted light. The circular polarization of the photoluminescence is very high for photoexcitation near the bandgap, and has a power-law decrease as the photo-excitation energy increases. We identify phonon-assisted intervalley scattering as the primary spin relaxation mechanism and present a model of depolarization that explains the wide variation in values for the optical polarization reported in the literature. Our results elucidate the basic processes that control the unique properties of this material and should help to realize future valleytronic applications. This work was supported by core programs at NRL and the NRL Nanoscience Institute.

Excitonic Linewidth Approaching the Homogeneous Limit in MoS2 -Based van der Waals Heterostructures

“…3(a) and 4(d). This is a major improvement on the usually reported emission linewidth of ≈50 meV at low temperature [9,14,16,[29][30][31][32][33][34]. The high-quality van der Waals heterostructures [35] we investigate here allow us to access new information on their optical and spin-valley properties.…”

Well separated trion and neutral excitons on superacid treated MoS2 monolayers

“…For example, at temperatures between 4 and 100 K ML MoSe 2 shows only sharp X and T emission (FWHM 10 meV) separated by about 30 meV [3]. In contrast, the PL of as-exfoliated or CVD grown ML MoS 2 over the same temperature range is characterized by a broad exciton peak (FWHM 60 meV) at an energy between 1.88 and 1.9 eV [11][12][13][14], which stems from overlapping T and X contributions as well as from broad low energy defects-related emission.…”

Polarization and time-resolved photoluminescence spectroscopy of excitons in MoSe2 monolayers

“…For comparison, the results obtained on ML MoS2 (upward triangle) by Kioseoglou et al [19] and on ML WSe2 (downward triangle) by Wang et al [10] in PLE measurements at 4K for samples on Si/SiO2 are shown, with typical experimental errors of ±1.5%. As the origin of the energy axis the neutral A-exciton emission is taken for each material, respectively.…”

“…These chiral optical selection rules leading to strong valley selectivity are expected to be a common feature for MoS 2 , MoSe 2 , WS 2 and WSe 2 . High values of the order of 50% for the circular polarization P c of the stationary photoluminescence (PL) emission corresponding to successful valley polarization have been reported in MoS 2 [17][18][19]22], WSe 2 [10, 20] and WS 2 [23], albeit with very different dependences on laser excitation energy i.e. on the excess energy of the initial excitation compared to the exciton emission energy.…”

“…TMDC MLs have emerged as very promising materials for optical, electronic and quantum manipulation applications [13,14]. In TMDC MLs crystal inversion symmetry breaking together with the strong spin-orbit (SO) interaction leads to a coupling of carrier spin and k-space valley physics, i.e., the circular polarization (σ + or σ − ) of the absorbed or emitted photon can be directly associated with selective carrier excitation in one of the two non-equivalent K valleys (K + or K − , respectively) [15][16][17][18][19][20][21]. These chiral optical selection rules leading to strong valley selectivity are expected to be a common feature for MoS 2 , MoSe 2 , WS 2 and WSe 2 .…”