Fig. 3: Momentum-indirect Q-valley excitons in WSe2 and WS2.
From: Strain fingerprinting of exciton valley character in 2D semiconductors
a PL spectra in a 1L-WSe2 at selected strain values. Above a critical strain value εcrit ≥ 0.35%, a strain-independent feature (XKQ, red Gaussian) emerges on the higher energy side of \({{{{\rm{X}}}}}_{{{{\rm{KK}}}}}^{0}\) (blue Gaussian). b Extracted emission energy of \({{{{\rm{X}}}}}_{{{{\rm{KK}}}}}^{0}\) (blue) and XKQ (red) in 1L-WSe2 vs. strain. The solid lines are the theoretically predicted emission energy of the KK and the hybrid KK-KQ excitons. The inset depicts the hybridization scenario and the corresponding emission. c Simulated energies of the KK and KQ excitons vs. position on a line cut across the membrane for two different values of εcenter. A strain inhomogeneity, ε(x = ± 1 μm) ≈ 0.9 × εcenter, causes spatially dependent resonance condition EKK = EKQ. A KQ exciton diffuses (anti-funnels) and emits light from the position within the membrane where the resonance conditions are achieved. d False color map of d2PL/dE2 vs. strain in 1L-WS2 highlighting individual excitonic peaks. Blue and purple lines indicate the KK and ΓQ excitons, respectively. The energy difference between a pair of KK peaks and a pair of ΓQ peaks corresponds to the binding energy of charged excitons (trions). e Ratio of the areas under the trion and exciton peaks vs. VG for KK (blue) and ΓQ (purple) excitons in weak strain regime (ε ≤ 0.2%). For VG ≥ + 20 V, this ratio for KK and ΓQ excitons increases sharply suggesting that the Fermi level has entered the CB. f Extracted exciton peak positions vs. strain and corresponding strain gauge factors for KK, ΓQ, and KQ excitons in a 2L-WSe2.
