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

Abstract

We investigate valley exciton dynamics in MoSe2 monolayers in polarization-and time-resolved photoluminescence (PL) spectroscopy at 4K. Following circularly polarized laser excitation, we record a low circular polarization degree of the PL of typically ≤ 5%. This is about 10 times lower than the polarization induced under comparable conditions in MoS2 and WSe2 monolayers. The evolution of the exciton polarization as a function of excitation laser energy and power is monitored in PL excitation (PLE) experiments. Fast PL emission times are recorded for both the neutral exciton of ≤ 3 ps and for the charged exciton (trion) of 12 ps.Monolayers (MLs) of the transition metal dichalcogenides (TMDCs) MoS 2 , MoSe 2 , WS 2 and WSe 2 are semiconductors with a direct bandgap in the visible region [1][2][3]. Their optical properties are dominated by excitons, strongly Coulomb-bound electron hole pairs [4][5][6][7][8][9][10][11][12]. 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 . 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.So ideal conditions for valley polarization generation in PL experiments need to be investigated also for ML MoSe 2 . This material is very promising for valley index manipulation [24,25] with bright, well separated emission lines for neutral (X 0 ) and charged excitons (trions T) [11] shown in Fig. 1, which allows to investigate the valley physics for these complexes individually at low temperature. In time-integrated PL experiments at 4K we record essentially unpolarized emission P c 0 following excitation as close as 100 meV above the A-exciton with a σ + polarized laser. One possible origin for the low PL polarization would be efficient depolarization with a typical time τ s much shorter than the PL emission time τ . Our time resolved measurements show PL emission times τ in the ps range, just as in the case of ML MoS 2 [26] and WSe 2 [27]. This hints at either faster polarization relaxation in MoSe 2 in the sub-picosecond range or inefficientDet. optical polarization generation due to anomalies in the bandstructure. MoSe 2 ML flakes are o...

Exciton states in monolayer MoSe 2 : impact on interband transitions

“…The higher energy peak (FWHM=10 meV) at 1.667 eV has previously been attributed to the neutral A-exciton X 1s A [22,42,43]. At 1.633 eV we record the trion emission (T) corresponding to a binding energy of 34 meV, in agreement with previous measurements [22,[42][43][44].…”

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

“…This leads to non-zero steady state valley polarization in MoSe 2 , which has never been observed before under non-resonant excitation. [38][39][40] 3 …”

Graphene against Other Two-Dimensional Materials: A Comparative Study on the Basis of Photonic Applications

“…Two-dimensional materials present photoluminescence, which is defined as the light emission from any form of matter after the absorption of photons. Two-dimensional materials, such as graphene [15,16] [24,25], and phosphorene [26], have been researched as photoluminescent materials. Luminescence can be achieved thanks to the reduction of thickness of materials, since it produces a transition from indirect band gap to a direct band gap.…”