Growth and Transfer of Nanowires with High Density and Aspect-Ratio Onto Flexible Substrates

Abstract

Nanowire-based devices have gained tremendous interest because of their potential in fabricating devices with ultra-high energy conversion and storage, such as photovoltaics, thermoelectrics, capacitors, batteries, spintronics, and piezoelectrics. Since the device performance is often directly proportional to the active surface area, fabricating vertically aligned nanowire arrays with high density and aspect ratio is important to maximize their efficiencies beyond the current limit. More importantly, many of these devices require substrates with specific properties of flexibility, transparency, and light weight. Thus, the key challenge lying ahead is not only to improve methods to maximize surface area of nanowires but also fabricate them onto a desired substrate. Currently, researchers have not been able to produce nanowire devices with maximum density. Many methods struggle with difficulties in obtaining catalytic seeds with small diameter and maintaining nanowires with small diameter during growth due to the high surface energy associated with substrates. While templated approaches provide an excellent way of demonstrating nanowire density control, it is difficult to form and remove the back electrode and integrate these nanowires into useful devices. In this study, we focused on using the anodized alumina oxide (AAO) as a template for nanowire deposition and then transferring the vertically aligned nanostructure onto a transparent flexible substrate. AAO provides maximum density with its closely-packed hexagonal pore arrangement and, by carefully tuning the experimental conditions, the pore diameter and interpore distance can be precisely controlled. To demonstrate the transfer process, nanowires with various dimensions and materials, including Au, Ag, Cu, Pt, and ZnO, were fabricated using electrodeposition. These embedded nanowires were then lifted off and transferred onto a polydimethylsiloxane (PDMS) substrate. During the AAO etching and drying process, an electrostatic repulsion (ESR) technique was applied to prevent aggregation and keep the nanowires vertically aligned. The current-voltage (IV) response before and after nanowire transfer were measured and compared to demonstrate the efficiency of the transfer process. As a result, nanowires were fabricated on a flexible substrate with densities and aspect-ratios of 1011/cm3 and 1000:1, respectively. This technique provides a new route of fabricating large-scale, cost-effective flexible optoelectronic devices. Keywords: Dye sensitized solar cell (DSSC), ZnO nanowires, solar cell, alumina template, electrodeposition, flexible electronics