A monolayer of molybdenium disulfide (MoS2) has a unique and strong photoluminescence (PL) in the visible wavelength because of its direct bandgap at 1.8 eV (~680 nm). This unique characteristic makes monolayer MoS2 a good candidate for optoelectronic application. This is unlike bulk MoS2 with a few layers of MoS2, which is an indirect bandgap semiconductor with reduced PL.
Raman spectroscopy is a powerful technique to assess the number of layer of 2D materials such as in graphene and MoS2. In MoS2, the peak separation of in-plane S-Mo-S vibration (E12g mode) and out-of-plane vibration of S atoms (A1g mode) can be used to estimate the number of layers [1,2]. We measured the MoS2 samples using uRaman from Technospex on a Nikon-Ci body. The uRaman module consists of a compact narrow line width laser (532 nm excitation and spectra bandwidth < 1 MHz). Using diamond as a reference, the system spectral resolution was measured to yield ~ 8 cm-1. Nikon 100X objective was used to measure the MoS2 samples (model – TU Plan Fluor 100X, NA 0.9). The exfoliated MoS2 sample was prepared by peeling a bulk MoS2 using Scotch tape. By performing Raman spectroscopy on the exfoliated sample (Figure A), we have identified 6 different locations having different peak separations, indicating the presence of 6 different layers of MoS2 (Figure B). The two spectra fingerprint, E12g and A1g bands, are observed at 383 and 401 cm-1 at location 1. As the number of layers increases, these two peaks become more separated, reaching 380 and 405 cm-1 at location 6 (Figure B and C). The Raman peak separation is contributed by the change in chemical interactions within the MoS2 layers. As the layer number increases, the MoS2 layers experience higher van der Waals force and long range Columbic interlayer interactions, which contributed to the blue shift in E12g mode and red-shift in A1g peak.
Acknowledgement- We thank Mr. Calvin Pei Yu Wong (NUS) for providing the MoS2 sample for measurement.
Reference:  ACS Nano 2010, 4, 2695;  Adv. Funct. Mater., 2012, 22, 1385.