Optical Properties and Raman Studies of Partially Edge Terminated Vertically Aligned Nanocrystalline MoS2 Thin Film

The optical and vibrational properties of nanocrystalline thin films of MoS2, comprised of a mixture of edge terminated vertically aligned (ETVA) and (001)-oriented regions, on large insulating substrates are reported. From high resolution transmission electron microscopy (HRTEM), the average size of ETVA nanocrystals were ~5 nm and each nanocrystal consisted of only 3 to 5 monolayers of MoS2. The films were highly transparent (~80%) but the percent of transmittance decreased as the energy of the incident light approached to the band gap. Additionally, weak excitonic peaks were observed both in the absorption and transmission spectra. The room temperature Raman study showed that both the E12g and A1g modes were significantly broader, and a few additional Raman modes were observed when compared to bulk MoS2. The broadening of the A1g mode was analyzed using the phonon-confinement model and the calculated particle size was in good agreement with TEM observations. Moreover, the temperature coefficient of the A1g mode was estimated from the temperature dependent Raman studies.