Semiconductor properties of natural melanins.

Basic semiconductor characteristics of natural melanins isolated from bovine eye, human dark hair, and banana peel were obtained by means of the dc dark conductivity experiments and optical absorption measurements. The results were compared with results obtained for synthetic melanin. Specific conductivity in natural melanins is of the order 10(-11) omega -1 cm-1 and in synthetic melanin 10(-8) omega -1 cm-1. Thermal activation energies in the range 298-333 degrees K are eye melanin, 0.93 eV; hair melanin, 1.01 eV; banana melanin, 1.04 eV; whereas synthetic melanin has two values of activation energy: up to 311 degrees K, 0.1 eV; above 313 degrees K, 0.78 eV. Optical gaps are: in eye melanin, 1.73 eV; in hair melanin, 1.35 eV; in banana melanin, 1.55 eV; and in synthetic melanin, 1.40 eV. The observed differences between natural melanins and the synthetic one could be explained by either the presence of protein residues in natural melanins or the influence of the isolation method on their electrical properties.