Optical and Vibrational Studies of Partially Edge-Terminated Vertically Aligned Nanocrystalline MoS2 Thin Films

We report the optical and vibrational properties of nanocrystalline MoS2 thin films, which are comprised of stripe-like partially edge-terminated vertically aligned (ETVA) nano-size crystals dispersed in < 001 > oriented regions, grown on large insulating substrates. From high-resolution transmission electron microscopy experiments, the average grain size of ETVA MoS2 layers was found to be similar to 5 nm and consist of three to five monolayers. The ETVA and flat nanocrystalline regions were in equivalent proportions in the films. The films were highly transparent (similar to 80%), but the transmittance decreased as the energy of the incident light approached the band gap. Additionally, weak excitonic peaks were observed in both the absorption and transmission spectra. The room-temperature Raman study showed that the line shape for both E-2g(1) and A(1g) modes was significantly broader, and few additional Raman modes were observed in comparison to bulk MoS2. The broadening in the line shape of the A(1g) mode was analyzed using the phonon-confinement model, and the calculated grain size was in good agreement with transmission electron microscopy measurements. Moreover, the temperature coefficient of the A(1g) mode was determined from the temperature-dependent Raman studies.