Raman Microspectroscopy of MoS2 by uRaman

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 (E¹_2g mode) and out-of-plane vibration of S atoms (A_1g 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, E¹_2g and A_1g 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 E¹_2g mode and red-shift in A_1g peak.

Acknowledgement- We thank Mr. Calvin Pei Yu Wong (NUS) for providing the MoS₂ sample for measurement.

Reference: [1] ACS Nano 2010, 4, 2695; [2] Adv. Funct. Mater., 2012, 22, 1385.