Effect of substrate temperature on MAPLE deposition of synthetic eumelanin films.

Eumelanin is an important pigment almost ubiquitous in animals and plants exhibiting interesting charge transport capabilities. Its poor solubility in common solvents represents a severe limitation for preparing thin films. It was recently demonstrated that eumelanin films can be successfully deposited with the MAPLE (Matrix Assisted Pulsed Laser Evaporation) technique starting from a frozen water suspension, using infrared laser radiation. The low laser absorption of ice together with the high absorption of eumelanin suggests that the target ablation is due to laser energy absorbed by the eumelanin molecules, followed by thermal energy transfer, and ejection of ice/water/vapor containing undamaged eumelanin molecules and supramolecular structures. Here, we report on the deposition of eumelanin thin films on substrates at different temperatures eventually followed by in-situ annealing. Structural characterization (UV-VIS, FTIR, AP-MALDI) confirms that the deposited films maintain the characteristics of the eumelanin biopolymer. Morphological characterization (AFM) shows that surface roughness increases with increasing substrate temperature during MAPLE deposition, but is not influenced by annealing. Preliminary electrical characterization shows that eumelanin films seem to obey Ohm’s law without evidence that charge injection from gold electrodes is affected by the presence of significant energy barriers. Moreover, charge transport is drastically reduced in vacuum, even if the phenomenon is at least partially reversible.