8:00 PM - SB07.09.02
Bio-Sourced Eumelanin Pigments—Charge Transport Properties and Beyond
Manuel Reali1,Abdelaziz Gouda1,Clara Santato1
Organic electronic materials have been widely investigated as a valid complement to conventional electronic technologies, owing to their mechanical properties and large area printability 1,2. Green electronics recently opened new research avenues to use bio-sourced, biocompatible (possibly biodegradable) materials to limit the environmental impact of the electronics sector3. Within bio-sourced carbon-based materials, eumelanin, a black-brown conjugated biopigment, emerged as an excellent candidate for green electronics.
A rapid interest grew around eumelanin due to its fascinating physicochemical properties (e.g. broadband optical absorption4,5, metal-ion chelation6, mixed ionic/electronic conductivity7–9) and biocompatibility1,10. The conjugated sp2 backbone of eumelanin suggests that it would be a naturally occurring semiconductor. The amorphous semiconductor model (ASM) was proposed for the first time in the 70s after the reports on resistive switching behavior in wet melanin pellets11. Recent investigations on the role of adsorbed water on eumelanin charge carrier transport challenged the ASM by proposing mixed electronic/protonic and proton membrane models8,9,12. None of the current models can provide a comprehensive understanding of the charge carrier transport properties of eumelanin.
In this work we fabricated wet and dry sepia melanin pellets included between Cu and stainless-steel electrodes. We report for the first time the electrical response of dry eumelanin pellets. As opposed to previous studies that indicated dry eumelanin as an insulating material11,13 , dry pellets switch. Surprisingly, the electrical response of dry pellets features a quasi-linear, metallic-like behaviour. On the other hand, wet pellets show a reversible and reproducible resistive switching, in agreement with McGinness et al11.
Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), X-Ray Photoemission Spectroscopy (XPS) and Raman Spectroscopy did not reveal neither the formation of metallic filaments bridging the electrodes nor a phase transformation (i.e. amorphous to graphitic carbon) possibly induced by Joule heating. We can, therefore, exclude the formation of metallic filaments or graphitic carbon in the interpretation of the electrical response. These findings suggest that dry pellets would be predominant electronic conductors. Our results show that despite huge effort, certain fundamental aspects of eumelanin’s transport physics need to be radically reconsidered and confirm the tremendous potential of the biopiments for green electronics and bioelectronics.
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