Surface-enhanced resonance Raman scattering of MoS2 quantum dots by coating Ag@MQDs on silver electrode with nanoscale roughness

Abstract

Molybdenum disulfide quantum dots (MQDs) possess unique optical and electronic properties owing to their typical microstructures, such as two-dimensional (2D) honeycomb structure, quantum confinement, and surface passivation. However, the microstructures that have been experimentally investigated are far from being sufficient evidence for clearly understanding the electronic and optical mechanism due to the lack of effective detection methods. Here, pure MQDs were fabricated through the laser ablation of bulk MoS2 along the orientation parallel to the MoS2 layers, which is essential for surface-enhanced Raman scattering (SERS) measurement. Surface-enhanced resonance Raman scattering (SERRS) is introduced to dramatically enhance the Raman signals of MQDs by coating Ag@MQDs on a silver electrode with nanoscale roughness and highlights much more abundant Raman peaks corresponding to the MQD microstructures of the hexagonal crystalline domains, passivation electron-donating groups, defects, and disorders than those highlighted by normal Raman scattering (NRS) and resonant Raman scattering (RRS). Furthermore, owing to the adsorption of MQDs via oxygen-containing passivation groups on the silver surface, SERRS highlights more vibrations associated with passivation structures than Fourier transform infrared (FTIR) spectra that are usually applied to provide crucial evidence for the existence of passivation groups. Abundant Raman peaks derived from the defects and disorders of MQDs are also observed. Additionally, several second-order Raman peaks that are related to the hexagonal structure of MQDs can also be observed and are induced by defects, disorders, and adsorption on the silver surface. This work indicates that SERRS has advantages over high-resolution transmission electron microscopy (HRTEM), FTIR, NRS and RRS in characterizing the microstructures of MQDs.