Exciton states in monolayer MoSe ₂ : impact on interband transitions

Abstract

We combine linear and non-linear optical spectroscopy at 4 K with ab initio calculations to study the electronic bandstructure of MoSe2 monolayers. In 1-photon photoluminescence excitation (PLE) and reflectivity we measure a separation between the A-and B-exciton emission of 220 meV. In 2-photon PLE we detect for the A-and B-exciton the 2p state 180 meV above the respective 1s state. In second harmonic generation (SHG) spectroscopy we record an enhancement by more than 2 orders of magnitude of the SHG signal at resonances of the charged exciton and the 1s and 2p neutral A-and B-exciton. Our post-Density Functional Theory calculations show in the conduction band along the K − Γ direction a local minimum that is energetically and in k-space close to the global minimum at the K-point. This has a potentially strong impact on the polarization and energy of the excitonic states that govern the interband transitions and marks an important difference to MoS2 and WSe2 monolayers.

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

“…These findings shown in Fig. 2 are in stark contrast to the results reported for PLE measurements in MoS 2 [19,26] and WSe 2 [10], plotted for comparison in the same figure. For ML MoS 2 a circular polarization degree above 40% was reported for laser excitation about 100 meV above the neutral Aexciton [19].…”

“…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.…”

“…For ML MoS 2 a circular polarization degree above 40% was reported for laser excitation about 100 meV above the neutral Aexciton [19]. For ML WSe 2 a global maximum for valley polarization and coherence was found 140 meV above the neutral A-exciton transition, which is interpreted as the excitation of the excited exciton level 2s [10]. It is therefore very surprising to see for laser excitation energies as close as 100 meV no measurable valley polarization in MoSe 2 .…”

“…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].…”

“…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.…”