Trion fine structure and coupled spin–valley dynamics in monolayer tungsten disulfide

Abstract

Monolayer transition-metal dichalcogenides have recently emerged as possible candidates for valleytronic applications, as the spin and valley pseudospin are directly coupled and stabilized by a large spin splitting. The optical properties of these two-dimensional crystals are dominated by tightly bound electron–hole pairs (excitons) and more complex quasiparticles such as charged excitons (trions). Here we investigate monolayer WS2 samples via photoluminescence and time-resolved Kerr rotation. In photoluminescence and in energy-dependent Kerr rotation measurements, we are able to resolve two different trion states, which we interpret as intravalley and intervalley trions. Using time-resolved Kerr rotation, we observe a rapid initial valley polarization decay for the A exciton and the trion states. Subsequently, we observe a crossover towards exciton–exciton interaction-related dynamics, consistent with the formation and decay of optically dark A excitons. By contrast, resonant excitation of the B exciton transition leads to a very slow decay of the Kerr signal.

Gate-Controlled Spin-Valley Locking of Resident Carriers in WSe2 Monolayers

“…This suggests monolayer WSe 2 is an excellent building block for hole spin/valley storage in more complex van der Waals devices [2]. Moreover, the very different dynamics in n-and p-type regimes argues against these long decays being due to optically-forbidden ("dark") neutral excitons [33], which are believed to exist in WSe 2 but which likely decay on shorter timescales [30]. We note, however, that putative charged dark excitons could in principle play a role in these gate-dependent studies.…”

Long valley relaxation time of free carriers in monolayer WSe2

“…* e-mail: xdcui@hku.hk On the experimental side, however, the valley/spin lifetime of free carriers remains some ambiguity. A spin resolved photocurrent measurements estimated the valley/spin lifetime in the range of 10 0 ∼ 10 2 nanoseconds in monolayer WS 2 , while optical pump-probe spectroscopy and time-resolved photoluminescence (PL) experiments gave a very short valley lifetime of several picoseconds [16][17][18][19][20][21][22][23] with a few exceptions where long valley lifetime of bound excitons were reported. [24][25][26] The huge discrepancy lies in that the excitonic effect is prevalent in optical responses of monolayer TMDs.…”

Temporal and spatial valley dynamics in two-dimensional semiconductors probed via Kerr rotation

“…However, the slow decay component of 70 ps for monolayer WSe 2 has a different origin than for monolayer MoS 2 . One possible explanation could be related to the existence of an intervalley dark exciton state [33][34][35], where a hole in the valence band of the +K valley is radiatively recombined with an electron in the conduction band of the -K valley upon optical excitation. However, as has been observed in previous PL measurements [33], an important indication of the existence of dark excitons is that the PL intensity increases as the temperature increases, which is the opposite trend to our previous observation [29].…”