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

We report the optical and vibrational properties of nanocrystalline MoS 2 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 MoS 2 layers was found to be ∼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 (∼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 MoS 2 . 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.