Bioinspired High‐Adhesion Metallic Networks as Flexible Transparent Conductors

Abstract

ITO replacements have been proposed and tested. These include conductive poly mers, [6,7] carbon materials (graphene [8,9] and carbon nanotubes [10,11] ), metal nano wires, [12][13][14] and metallic networks. [15][16][17][18] Among those, metallic networks with their good optoelectronic properties and excellent mechanical flexibility are the most promising.Previously, some of the coauthors of this work have developed metallic net works based on the cracking technology. [15,18,19] This technology employs thin films of a sacrificial material (deposited on a substrate), which cracks while drying. After metal deposition and liftoff of the sacrificial material, a continuous network of metallic nano/micro ribbons forms directly on the substrate. This technology produces excellent TCEs on solid substrates, outperforming ITO. While this technology is compatible with flexible substrates, adhesion remains a problem due to addi tional stresses experienced during substrate flexing. There are two failure mechanisms due to flexing: metal film fracture and delamination from the substrate. Fracture has been thor oughly studied in the context of tensile strain and bending etc. [15,18] Recent strategies to improve the nanowire networksubstrate adhesion include mechanical pressing, [20] plasmonic laser nanowelding, [21] nanosoldering, [22] additional solvent treat ment, [23] polymer encapsulation, and TiO 2 gel coverage. [24,25] These harsh processes cause typically some damage to the net work, and therefore, strategies for simultaneous improvement of adhesion while maintaining device performance remain a challenge. Recently, metallic films with nanopile interlocking have been fabricated, and have some potential to resolve this problem. [26] Here, we developed a highadhesion (HA) flexible TCE, based on a metallic crack network combined with the bamboo root idea. This HA network shows excellent optoelec tronic properties, combined with superb adhesion to flexible substrates. Figure 1a shows the rhizome structures on the bamboo roots, which through a characteristic root interlocking, provides exceptional mechanical support and stability for the bamboo plant. Figure 1b shows schematic of our highadhesion net work, with the dendritic nanowires resembling the rhizome structures (see the zoomin panel in Figure 1a). Figure 1c illus trates steps of our fabrication process, which begins with the spin coating of a polyimide (PI) layer on a silicon wafer (Step 1). After solidification of PI, a sacrificial crack mask (nail polish) is spin coated on the PI surface, and subsequently selfcracked during drying (Step 2). The cracking pattern is then copied onto PI film by using plasma etching (Step 3). The electroplating While metallic networks, proposed recently as a replacement for transparent conducting oxides, show superior optoelectronic performance and mechani cal flexibility, their adhesion to flexible substrates remains a problem. In this work, a high adhesion metallic network inspired by the ground-gripping functionality of bamboo roots is...

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