Qualitative Analysis of Mechanically Exfoliated MoS2 Nanosheets Using Spectroscopic Probes

The accurate and precise quantification of the number of layers of MoS2 nanosheets is highly desirable to accomplish the advanced optoelectronic device applications, which is a real challenge in cutting edge science. In recent years, multitudinous efforts have been put forward, but no certain protocol is available to validate the number of layers. In this aspect, we present various spectroscopic probes such as optical, Raman, and photoluminescence (PL) to quantify the number of layers in mechanically exfoliated MoS2 nanosheets. The contrast analysis of optical and scanning electron microscope (SEM) images has been performed to identify the number of layers. The Raman spectra indicate that E-2g(1) are shifted toward low-frequency and A(1g) peak in high-frequency direction during the transition from monolayer to bulk MoS2. The intensity, line width, and peak frequency of both Raman modes have shown the notable difference with the thickness variation. Moreover, the PL spectra show two prominent peaks at 679 and 638 nm, which corresponds to A and B excitons, respectively, for MoS2 monolayer. The monotonic decrease in energy difference of excitons from monolayer to few layers provides the new insight into the identification and validation of the number of layers. Further, the PL results are also supported by A exciton PL mapping of MoS2 nanosheets. Furthermore, the thickness of the different layers has been measured by the atomic force microscope (AFM). The obtained results legitimize that these spectroscopic probes are the metrological tool to identify the number of layers of MoS2 nanosheets to design the emerging next-generation 2D devices.