A nanoscale study of the structure and electrical response of Sepia eumelanin

Abstract

Eumelanin, the brown-black member of the melanin biopigment family, is a prototype material for sustainable (green) organic electronics. Sepia eumelanin (Sepia) is a type of biosourced eumelanin extracted from the ink sac of cuttlefish. Electron microscopy and scanning probe microscopy images of Sepia show distinguishable near spherical granules with diameters of about 150-200 nm. We have recently reported on predominant electronic transport in printed films of Sepia formulated inks including the (insulating) binder Polyvinyl-butyral (PVB). In that work, we proposed that inter-granular percolative transport, observed for micrometric interelectrode distances, is promoted by the confining action of the PVB binder on the Sepia granules. Considering that inter-granular transport implies intra-granular transport, in this work we proceeded to a nanoscale study of Sepia granules by High Resolution Atomic Force Microscopy (HR-AFM) and Conductive-AFM (c-AFM). We have observed protrusions on the surface of the Sepia granules, suggesting sub-granular structures compatible with the hierarchical development of Sepia, as proposed elsewhere. For films of Sepia formulated inks deposited on gold-coated substrates, c-AFM revealed, for the very first time, a nanoscale electrical response. Nanoscale studies provide the key to structure-property relationships in biosourced materials strategic for sustainable organic electronics.

Fig. 1. Redox forms of eumelanin building blocks. –R is –H for 5,6-dihydroxy indole (DHI) and –COOH for 5,6-dihydroxy indole-2-carboxylic acid (DHICA).

Fig. 2. AFM images of drop cast Sepia on gold-on-SiO₂: (a) 1.2 μm × 1.2 μm-sized and (b) 400 nm × 400 nm-sized (corresponding to the square in (a)). (c) Histogram showing the distribution of the diameters of the Sepia granules.

Fig. 3. (a) 400 nm × 400 nm AFM image of Sepia drop cast on Au-on-SiO₂, (b) relative height of the surface profile associated with line C1 on the granule in (a).

Fig. 4. AFM images of Sepia-PVB films: (a) 3 μm × 3 μm-sized, (b) 1 μm × 1 μm-sized and (c) current image corresponding to the AFM image in (b).