Search

A study of the structural, optical and electrical properties of synthetic and natural melanin by means of X-ray diffraction, absorption and photocurrent techniques is reported. The model of the natural melanin film as a network of nano-aggregates of polymeric units based on the indolic structure is proposed to explain the X-ray diffraction results. The shape of the absorption spectra is similar to that of amorphous and disordered semiconductors, with a very strong, broad band UV and visible absorption and an optical gap value of about 0.5 eV. Photosensitivity to sun spectra has been demonstrated by photoconductivity measurements of synthetic melanin pellets under AM1 light source illumination.

Undoped CdSe and CdSe:Zn thin films have been grown on silicon substrate by using pulsed laser deposition technique. The electrical, structural and optical properties have been investigated. The films grow crystalline and highly oriented. Electrical measurements show that they are n-type doped. The reflectivity and photoluminescence are consistent and point out that the undoped CdSe film present excitonic features at low temperature, differently from CdSe:Zn films, whose spectral features are related to band–band transition. The luminescence efficiency of CdSe:Zn persists up to room temperature, whereas the luminescence of undoped CdSe is scarcely visible above 250 K.

The structural and optical properties of CdS x Se 1− x alloy films, deposited on Si(111)-oriented substrates by the laser ablation technique, have been investigated. The films grow highly oriented in the hexagonal phase and the lattice constant c of the ternary alloy follows Vegard's rule. The exciton line recombination is well resolved in the PL spectra of each film alloy from 10 up to 300 K. The temperature dependence of the excitonic energy indicates that both acoustic and optical phonons contribute to the thermal shrinkage of the band gap: in particular, the acoustic phonon contribution is larger in ternary compounds than in the binary ones.