Improvements to Modelling of Raman Scattering Intensity for Molybdenum Disulfide

Single layer molybdenum disulfide (MoS2) has attracted significant interest as a semiconducting two-dimensional material. In this work, ultrathin layers of MoS2 were exfoliated on geland wax substrates. Raman studies of the ultrathin layers of MoS2 were carried out to characterize the thickness. During Raman measurement, an anomaly occurs: the strongest Raman intensity appears at a finite thickness of the MoS2, a phenomenon also seen in graphite. A previous work [Y. Y. Wang et al., Appl. Phys. Lett. 92, 043121 (2008)] theoretically explained this unexpected phenomenon but with some questionable details. To improve their model, the complex index of refraction was properly used in the optical equations, and the assumption of optical interference of Raman signals from an infinitesimal source was added. This modified model successfully predicted the experimental results when applied to graphite and was then applied to MoS2. The simulations indicated that a constant complex index of refraction for MoS2 of 6.5-1.7i provided the best fit to published experimental results for MoS2 on SiO2/Si. This helped to estimate the thickness of ultrathin layers on gel. Seven layers MoS2 on gel showed the strongest Raman intensity according to this simulation.