Local modulation of excitons and trions in monolayer WS₂ by carbon nanotubes

Mixed dimensional van der Waals (vdW) heterostructures constructed by one-dimensional (1D) and two-dimensional (2D) materials exhibit extra degree of freedom to modulate the electronic and optical properties due to the combination of different dimensionalities. The charge transfer at the interface between 1D and 2D materials plays a crucial role in the optoelectronic properties and performance of the heterostructure-based devices. Here, we stacked single-walled carbon nanotubes (SWNTs) on monolayer WS₂ for a mixed dimensional vdW heterostructure, and investigated the local modulation of excitions and trions in WS₂ by SWNTs. Different directions of charge transfer between SWNTs and WS₂ are evidenced by the photoluminescence (PL) spectra of WS₂. The PL intensity can be either enhanced or weakened by individual SWNTs. In our work, the PL intensity of WS₂ is enhanced and the exciton peak position heterostructure is red-shifted about 3 meV due to the charge transfer from WS₂ to an individual SWNT (SWNT#1). The change of PL by another SWNT (SWNT#2) can not be well-resolved in far-field, but scanning near-field optical microscope (SNOM) measurements show that the PL intensity of WS₂ is weakened by the SWNT. The peak position of exciton is blue-shifted by ~ 1 meV while that of trion is redshifted by ~ 1 meV due to the charge transfer from the SWNT to WS₂. These results give insight into the charge transfer at the interface of SWNT/WS₂ heterostructure, and can be useful for design of optoelectronic devices based on mixed dimensional vdW heterostructures.