Probing microstructures of molybdenum disulfide quantum dots by resonant Raman scattering

Abstract

Research on the photoluminescence (PL) mechanism of MoS2 quantum dots (MQDs) has entered into a new age that involves analyzing the complicated microstructures of MQDs that are presumably significant for PL emission. However, microstructures of MQDs have not been clearly observed and thoroughly identified by conventional detection techniques. In this work, pure MQDs were fabricated by pulsed laser ablation along the direction parallel to basal planes of the MoS2 crystal in deionized water to enable resonant Raman measurements. Resonant Raman scattering (RRS) that corresponds to microstructures of MQDs, especially defects and disorders at the edges and surfaces of MQDs, is obtained, which is distinctly different from that of bulk and monolayer MoS2 and has not been characterized in such a direct method by RRS spectroscopy. The highest intensity of the defect-induced LA(M) peak at approximately 217 cm−1, which is similar to the D band of graphene, indicates the existence of enormous defects and disorders. Furthermore, the LA(M) peak is split into a shoulder at 212 cm−1 and a peak at 217 cm−1 which are due to double resonance processes derived from defects on the edges and disorders in the planes, respectively. More resonant two-phonon Raman processes appear because of the strong electron-phonon coupling at resonance. In addition, the typical phonon modes of MoS2 and Raman-silent phonon modes are analyzed and identified. This work indicates that the features of microstructures of MQDs can be convincingly and experimentally characterized by RRS spectroscopy.