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.
The limited Photoluminescence (PL) quantum yield of monolayer Transition metal dichalcogenides (1L-TMDs) are surprisingly shown to increase up to ~ 100 % by defect passivation mechanism i.e. suppressing the exciton quenching caused by the structural defects by simple chemical treatment. However, the mechanism behind it is in veil due to lack of experimental results in atomic level. In this work, we carried out bis(trifluoromethane) sulfonimide (TFSI) treatment of 1L-MoS2 and 1L-WS2 with different defects domains grown by chemical vapor deposition (CVD) and found drastically enhanced PL intensity in case of 1L-MoS2 while about 5 fold enhancement in case of 1L-WS2. Similarly the Raman intensity of both 1L-TMDs were increased and the intensity ratio of 2LA(M) to A1g peaks for 1L-WS2 were increased in different defect domains after TFSI treatment which are the strong evidence of defect passivation. We directly observed the atomic healing of 1L-TMDs by TFSI molecules under scanning transmission electron microscopy (STEM) analysis of pristine and TFSI treated 1L-TMDs and found that about ~ 90 % sulfur vacancies of 1L-TMDs were filled after treatment. The direct anchoring of dissociated sulfur atoms from TFSI molecules to the sulfur vacancies of 1L-TMDs was found to be energetically favorable by density functional theory calculations. Our observation shed light on the mechanism of intriguing healing process of lattice defects of 1L-TMDs and suggests that 1L-TMDs can be made defect-free which widens and prompts the practical uses of 1L-TMDs in nanophotonics applications. Furthermore, correlated experimental results and details will be presented.