Transfer of Flexible Arrays of Vertically Aligned Carbon Nanofiber Electrodes to Temperature‐Sensitive Substrates

Abstract

The unique physical, chemical, and mechanical properties of vertically aligned carbon nanotubes (VACNTs) and nanofibers (VACNFs) have led to their use in a variety of applications. VACNTs and VACNFs have demonstrated viability as field electron emitters, electrochemical probes, and biosensors. These demonstrations have been enabled by the ability to deterministically synthesize vertically aligned nanostructures using catalytically directed plasma-enhanced chemical vapor deposition (PECVD) processes, where a carbonaceous source gas is decomposed and precipitated on catalyst nanoparticles at elevated temperatures. Catalyst activation in plasma requires substrate temperatures usually in the range 600–700 °C. There have been several recent reports on carbon nanofibers grown at lower substrate temperatures. However, it still remains an open question whether the graphitization quality of such nanofibers is sacrificed. The impact of low-temperature synthesis on the resultant structure’s chemical, electrochemical, and mechanical properties also requires further exploration. In addition, the actual growth temperatures due to plasma heating could be considerably higher than those measured at the substrate heater. These temperatures still impose restrictions on the types of substrates that can be used, with silicon and fused silica often being the substrates of choice owing to their compatibility with relatively high-temperature processes. Substrates sensitive to high temperatures, plasmas, or certain gases, including most polymers, glasses, and conventional microelectronics (i.e., complementary metal-oxide semiconductors, CMOSs), are degraded and sometimes destroyed by the harsh growth conditions of vertically aligned carbon nanostructures. As an alternative, in a recent report, carbon nanotubes encapsulated within an epoxy polymer were released from their growth substrate and aligned, using micromanipulators, on a receptor substrate in a lowtemperature process. This technique offers the advantage of preserving receptor substrates from the harsh growth conditions of carbon nanotubes. However, the spatial dimensions defined by the deterministic growth of nanotubes from photolithographically defined catalyst sites are lost. The necessity of realigning the encapsulated carbon nanotubes on the receptor substrate by micromanipulators also reduces the efficiency of the process. We report on a method for growing VACNFs in a high-temperature (630 °C) direct-current PECVD process and subsequently transferring these nanofibers to new substrates. In brief, following high-temperature growth on silicon substrates, carbon nanofibers were partially embedded in a UV-crosslinked epoxy membrane and peeled from their original growth substrate. The membrane, featuring embedded highaspect-ratio nanofibers, was then aligned and mated with an array of individually addressable contact pads. This process provides intact nanofibers that can be transferred to essentially any planar surface, including those that would otherwise be destroyed by the harsh conditions imposed on the substrate during nanofiber synthesis. Transferred carbon nanofibers were shown to retain their high-aspect-ratio morphology and their viability as electrodes, as demonstrated by electrochemical analysis and gold electrodeposition. Nanofiber arrays were grown on silicon wafers from photolithographically defined nickel catalyst sites as previously described (Fig. 1C, process steps 1,2). Following growth, nanofibers were inspected in a Hitachi 4700S scanning electron microscope at an acceleration voltage of 10 kV using a mixed-detector mode. Figure 1A presents a representative reC O M M U N IC A IO N S