Trion Binding Energy Variation on Photoluminescence Excitation Energy and Power during Direct to Indirect Bandgap Crossover in Monolayer and Few-Layer MoS2

Abstract

The behavior of the trion and exciton photoluminescence (PL) under various laser excitation conditions, that is, energy and power, is studied in monolayer and few-layer (FL) chemical vapor deposition (CVD)-MoS2 flakes grown on SiO2/Si. The room-temperature μ-PL spectroscopy of the flakes shows that the A exciton at 1.86 eV is overlapped by the trion component. First, the thickness-dependent characterization of the flakes has been carried out. The trion spectral weight and dissociation energy are observed to increase with the number of layers, which is correlated with an increase in nonequilibrium electron density. We propose the presence of many-body effects explaining this unusual dependence in CVD-MoS2 on n-type SiO2/Si. In the power-dependent μ-PL measurements, the deconvolution of spectra for the exciton and trion bands shows a faster intensification of the trion component compared to the A exciton with increasing laser power. The trion binding (dissociation) energy varied from 28 to 33 meV in the monolayer and from 32 to 46 meV in FL MoS2, when increasing the power of excitation light. The phenomenon is found to be more prominent when increasing the energy of excitation from 2.33 to 3.06 eV. The enhancement of the trion binding energy leads to a strong intensification of the trion component. Thereby, the intense A exciton/trion PL band redshifts and becomes more asymmetric when increasing the power of excitation at both energies of 2.33 or 3.06 eV. Simultaneously, the B exciton contribution to the spectrum also increases. Such an enhanced formation of the trions and B excitons is explained by a rate of photogenerated electron–hole pairs, leading to a higher population of nonequilibrium electrons. In the measurements under a higher excitation energy of 3.06 eV, the PL redshift is more prominent because the trion component is more intense compared to the A band. This is explained by considering a higher absorption of MoS2 at higher energies.