Lattice vibrational spectra of nanocrystalline films of para-substituted benzene

We have obtained experimental low-frequency Raman spectra for para-dibromobenzene nanofilms of thickness ∼10 µm and ∼400 nm. With a decrease in the film thickness, the line frequencies are lowered, the linewidths increase, additional lines become more intense. We modeled the film structure by the molecular dynamics method and calculated the histograms of the lattice vibrational spectra by the Dean method. We found that as the film thickness decreases, the lattice parameters increase, the orientational disorder increases at its boundary. The structure of the studied film is similar to the structure of the para-dibromobenzene single crystal. In order to correctly interpret the lattice vibrational spectra, we need to take into account surface vibrations and the presence of vacancies in the structure.