Charge Carrier Transport in Individual Granules of Sepia Melanin

Sepia melanin, a naturally occurring pigment found in the ink sac of cuttlefish, has garnered significant interest for its intriguing electrical properties and potential as an organic semiconductor material. Sepia melanin exhibits unique characteristics, such as its moisture-dependent electrical response [1] and broadband optical absorption [2], making it a promising material for applications in organic electronics, energy harvesting, and bioelectronics. Sepia melanin exhibits properties of disordered organic semiconductors whose charge transport behavior is attributable to a combination of band-like and hopping transport mechanisms [3]. The electrical conduction in Sepia melanin is influenced by its complex hierarchical structure, which eventually leads to melanin nanoparticles ~150-200 nm-sized self-assembled into granules. These nanoparticles result from variety of arrangements of pi-pi stacked molecules [4].<br/>In this study, we explored the charge transport properties of Sepia melanin at the nanoscale, with the aim to fully harness Sepia melanin's potential for sustainable optoelectronic technologies.<br/>We focus our investigations on a nanoscale, inter-digitated planar geometry of patterns (inter-electrodes distance ~200-700 nm) as our goal is to detect signals generated by individual granules of Sepia melanin. Patterns are fabricated by Electron Beam Lithography.<br/>Findings from our ongoing study reveal the influence of structural disorder on charge carrier transport in Sepia melanin, including charge carrier localization and trapping effects as it is confirmed from current-voltage measurement. Also, Temperature-dependent electrical characterizations are helping us shed light on aspects related to hopping transport and de-trapping of charge carriers, such as calculating the activation energy.<br/><br/>Such effects represent opportunities for tailoring charge transport characteristics in Sepia melanin-based devices. We observed that Sepia melanin granules reveal their remarkable high electrical conductivity.<br/>Work is in progress to study the Electrochemical Impedance Spectroscopy (EIS) response in dry and wet atmosphere complemented by current-time measurement to confirm the nature of the charge carriers (electrons or ions or both).<br/><br/><br/>1. Wunsche, J., et al., <i>Protonic and electronic transport in hydrated thin films of the pigment eumelanin.</i> Chemistry of Materials, 2015. <b>27</b>(2): p. 436-442.<br/>2. Pullman, A. and B. Pullman, <i>The band structure of melanins.</i> Biochimica et biophysica acta, 1961. <b>54</b>: p. 384-385.<br/>3. Haneef, H.F., A.M. Zeidell, and O.D. Jurchescu, <i>Charge carrier traps in organic semiconductors: a review on the underlying physics and impact on electronic devices.</i> Journal of Materials Chemistry C, 2020. <b>8</b>(3): p. 759-787.<br/>4. Clancy, C.M. and J.D. Simon, <i>Ultrastructural organization of eumelanin from Sepia officinalis measured by atomic force microscopy.</i> Biochemistry, 2001. <b>40</b>(44): p. 13353-13360.