Figure 2: (a) PLE spectral map of MoSe2 with the excitation energy ranging from 2.3eV to 2.75eV. Lorentz fitting results of (a): PL intensity (c), PL peak energy and PL linewidth (d) are summarized as a function of excitation energy. (c), (d) are further divided into two regions based on the PL intensity. (b) The PL linewidth and PL energy peak (determined by Lorentz fitting) as a function of excitation intensity. The inset shows the 2D map of excitation intensity dependent PL spectra. The excitation-intensity-dependent PL is measured under an excitation of 2.33eV at 15K.
Figure 2 summarize our PLE and excitation-intensity-dependent PL data. The excitation energy ranging from 2.3eV to 2.75eV is set far beyond the A and B exciton energies to avoid resonant absorption. The excitation intensity is kept below \(100\mu W\) to minimize the local heating. The PL intensity across the excitation range primarily results from the corresponding excitation intensity profile (blue ball in Fig.2(c)) and the absorption coefficient (details in SI). At region I, the PL intensity decreases as the excitation energy increases primarily owing to the reduction of laser intensity (blue balls in Fig.2(c)). At region II, the PL intensity remains unchanged relatively and even slightly increases though the excitation intensity reduces with the increase energy, which may result from the boosted absorption in C band (more details in SI). The energy shift of A-1s exciton shows a consistent trend with PL intensity or exciton density, which also agrees well with our excitation intensity dependent PL result (Fig.2(b)). In Fig.2(b), the A-1s peak energy undergoes a slight redshift with the increase of exciton density under the excitation of 2.33eV accompanying with the linewidth broadening, which is consistent to the previous results 31. The redshift is attributed to the bandgap renormalization and Coulomb screening effect. Usually, as the excitation intensity increases, the electronic bandgap decreases owing to the bandgap renormalization from photocarriers 32, 33, 34, 35, whereas the Coulomb screening effect is enhanced owing to the increased exciton density, leading to the decrease of the exciton binding energy, and consequently results in the PL peak energy blueshift35. In monolayer MoSe2, the bandgap renormalization effect is larger than the Coulomb screening effect and therefore the PL peak undergoes a redshift as a function of excitation intensity. Figure 2(d) indicates that the excitation energy plays a more prominent role at low exciton density. Usually, a low-intensity excitation leads to narrower exciton PL linewidth on account of the less Auger-like exciton-exciton interaction26, 36 as elaborated in Fig.2(b). In region I, although the PL intensity or exciton density monotonically decreases with the increasing excitation energy, the PL linewidth almost linearly increases. It seems contradictory to our excitation intensity dependent PL results (Fig.2(b)) if only the exciton density induced linewidth variation is taken in account. We attribute this linewidth broadening to the acoustic-phonon assisted photoluminescence which we elaborate in the following section. In region II, the PL intensity remains flat and the acoustic-phonon assisted photoluminescence plays a solely role in broadening the linewidth. Hence, the PL linewidth in region II increases faster than in region I (the two red lines in Fig.2(d)). Meanwhile, the Raman scattering is exploited to monitor the lattice temperature under the excitation (\(\text{below\ }100\mu W\)), showing that the local heating is negligible and the local lattice temperature remains a constant in the excitation range (details in SI). The anomalous linewidth broadening in both regions I and II and the non-Lorentzian line shape of PL spectra are then attributed to the effective exciton temperature rise which activates the acoustic-phonon assisted photoluminescence process as demonstrated in Fig.3.