The optical properties of monolayer transition-metal dichalcogenides (1L-TMDs) are predominantly governed by excitonic effects even in room temperature because of two-dimensional confined nature. As the result of strong coulomb interaction in 1L-TMDs, non-radiative exciton-exciton annihilation (EEA) is one of key influence to their light emission at nominal excitation density. Therefore, the modulation of EEA can help to make higher photoluminescence (PL) quantum yeild and develop optoelectric devices using 1L-TMDs.
Here, we observed reduced EEA rate in mechanically exfoliated monolayer tungsten disulfide (1L-WS2) by laser irradiation with improved light emission at the saturating optical excitation level. PL efficiency of 1L-WS2 in irradiated region increased with increasing the excitation intensity and finally it was 3 times higher at high excitation level compared to that in non-irradiated region, while the laser irradiated regions in 1L-WS2 have lower PL intensity at low excitation level than non-irradiation region. This kind of the excitation density dependence was confirmed by time-resolved PL measurement and EEA rate was reduced about 3 times by laser irradiation. Sulfur vacancies and lattice distortion might be formed by laser irradiation which can give rise to lower PL and shorter lifetime in laser irradiated region of 1L-WS2. But, we attribute these laser induced defects or adsorption of oxygen molecules in air to the origin of reduced EEA by hindering exciton diffusion. Our results could provide an idea for high performance opto-electric devices.
Two-dimensional (2D) transition metal dichalcogenides (TMDs) play important role for optoelectronic applications such as photovoltaics, photodetectors, and field-effect transistors (FETs). However, there are still limited by several problems such as structural defects during the chemical vapor deposition (CVD) growth process, poor photoluminescence (PL) quantum yield (QY) and deeply understanding of exciton dynamics of TMDs.
Recently, it was reported that treatment using the superacid bis (trifluoromethane) sulfonamide (TFSI) resulted in a PL QY near 100% in exfoliated 1L-MoS2 monolayers. One of main reason of improved PL QY is due to repair defects induced sulfur vacancies. however, the effects of these chemical treatments varied greatly depending on the synthesis method and the type of 1L-TMD; therefore, the exact origin of the emission enhancement is still challenge.
Here, we perform detailed optical characterization of TFSI and 7,7,8,8-tetracyanoquinodimethane (TCNQ) treaded with CVD-grown 1L-MoS2 by using near-field scanning optical imaging and spectroscopy with nanoscale spatial resolution (~80nm). NSOM is optical imaging technique beyond the diffraction limit using narrow aperture that has aperture size much less than the wavelength of light. We found that 1L-MoS2 of systematic variation of the spectral weights among neutral excitons, trions indicated that p-doping by TFSI treatment. However, the PL enhancement was attributed mainly to the reduction of structural defects caused by TFSI treatment. Our results suggest that 1L-MoS2 helps to clarify the mechanism by which chemical treatment enhances the optical properties of 1L-TMDs.
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