The edges of two-dimensional (2D) materials can exhibit special structure and thus distinctive properties differing from the interior, such as the modified photoluminescence emission, the improved electrocatalytic activity, and the enhanced nonlinear optical response. In this work, we report the observation of abrupt enhancement of Raman scattered light at edges of layered MoS₂, which is closely related to the polarization of the incident light. More importantly, the intensity of out-of-plane A_1g mode is enhanced much more obviously than the in-plane ${\mathrm{E}}_{2\mathrm{g}}^1$ mode. The unique optical effect at edges is also reflected on the alteration of Raman selection rule. The forbidden Raman modes are observed at edge region, and the polarization of Raman scattered light can be modulated by the edge structure, which differs obviously from that of the interior. We attribute these particular performances to the modulation of the intensity and distribution of the electric field for the incident and Raman scattered light at the edge. This work provides a systematic research on the intensity and polarization modulation of the Raman scattering at edges, which would be helpful for understanding the distinctive optical properties of the edge structure.

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.

Anisotropic two-dimensional (2D) materials exhibit lattice-orientation dependent optical and electrical properties. Carriers doping of such materials has been used to modulate their energy band structures for opto-electronic applications. Herein, we show that by stacking monolayer rhenium disulfide (ReS₂) on a flat gold film, the electrons doping in ReS₂ can affect the in-plane anisotropic Raman enhancement of molecules adsorbed on ReS₂. The change of enhancement factor and the degree of anisotropy in enhancement with layer number are sensitively dependent on the doping level of ReS₂ by gold, which is further confirmed by Kelvin probe force microscopy (KPFM) measurements. These findings could open an avenue for probing anisotropic electronic interactions between molecules and 2D materials with low symmetry using Raman enhancement effect.

Manipulating the polarization of light at the nanoscale is essential for the development of nano-optical devices. Owing to its corrugated honeycomb structure, two-dimensional (2D) layered black phosphorus (BP) exhibits outstanding in-plane optical anisotropy with distinct linear dichroism and optical birefringence in the visible region, which are superior characteristics for ultrathin polarizing optics. Herein, taking advantage of polarized Raman spectroscopy, we demonstrate that layered BP with a nanometer thickness can remarkably alter the polarization state of a linearly-polarized laser and behave as an ultrathin optical polarization element in a BP-Bi₂Se₃ stacking structure by inducing the exceptionally polarized Raman scattering of isotropic Bi₂Se₃. Our findings provide a promising alternative for designing novel polarization optics based on 2D anisotropic materials, which can be easily integrated in microsized all-optical and optoelectronic devices.