Raman and optical spectroscopy of eumelanin films

Melanin obtained from the liver of Rana esculenta L., was isolated from melanosomes and deposited as thin film on quartz substrate, in order to perform Raman, absorption and photoluminescence measurements at room temperature. The Raman spectrum was analysed by considering the contribution of the vibrational modes from different functional groups of the melanin structure. The absorption and photoluminescence measurements support the model that melanin consists of nano-aggregates of oligomeric structures rather than extended heteropolymer. An optical gap value of about 0.6 eV was estimated by considering the Tauc model. The largest size group of clusters mainly contribute to determine the optical gap value, whereas the PL emission is due to groups of clusters which are selectively pumped.