(Invited) Plasma Synthesis and Structural Control for Carbon Nanowalls for Novel Nano-Bio Devices
Authors: Kondo Hiroki , Hori Masaru , Hiramatsu Mineo 
Published: February 2020

Carbon nanowalls (CNWs) are one of carbon nanomaterials. They consist of graphene sheets standing vertically on substrates, so that they have self-aligned three-dimensional structures. Significant recent attention has been focused on the functionalities of the CNWs because of their unique morphologies and excellent electrical properties. For example, since they have large surface-to-volume ratios and very high aspect ratios, they are expected as catalyst supporting materials in fuel cells, field emitters, and various kinds of templates. In addition, high-density graphene edges on the top-region of CNWs promise expression of novel functionalities and applications.

  In this study, we have investigated controlled synthesis techniques of carbon nanowalls, which have suitable morphologies and electrical properties for each device. Density ratios of hydrogen (H) and fluorocarbon (CFx (x=1-3)) radicals are essential to determine the density and height of CNWs, because the H radicals enhance desorption of fluorine (F) atoms from the adsorbed CFx. Ar+ ion irradiation is also important for the nucleation and vertical-growth of graphene sheets. According to observation results of their initial growth using ellipsometry and scanning electron microscopy, not only nucleation but also vertical-growth of graphene sheets could occur only under Ar+ ion irradiation with suitable energy and flux for each process. For example, when Ar+ion energy is too much, no film deposition occers due to etching. On the other hand, since too low energy ion bombardment can not induce nucleation of nanographen, only continuous amorphous carbon film grows on the substrate surface. Furthermore,  when ion flux is too high, continuous amorphous carbon films grow too, since deposition rates of carbon related radicals much higher than growth rate of vertical graphene sheets.

  By using a supercritical fluid, high-density metal nanoparticles or metal oxide ones can be uniformly supported on the whole surface area of CNWs. For example, TiO2nanoparticles-supported CNWs efficiently desolve methylene blue molecules, which are typical test reagent for photocatalytic performance under UV irradiation. Pt nanoparticles-supported CNWs also show fine catalyst performance at cyclic voltammetry (CV) measurements. In addition, they also show very high durability at high potential cycle tests, which demonstrate start and stop operations of fuel cells. Furthermore, the CNWs also show multimodal and unique characteristic as cell culturing template. Preliminarily, the wide-range control of surface wettability of CNWs from superhydrophilic to superhydrophobic can be realized by the post-growth plasma treatments. Then, the dependence of the HeLa cell culturing rate and morphological change on the surface wettability of CNWs scaffolds was systematically evaluated. The cell culturing rates were significantly dependent on the CNW densities, although the surface wettability of the CNWs was not significantly dependent. Morphological changes of the cells were not significantly dependent on the density of CNWs. On the other hand, using the chemically surface-modified CNWs as electrodes, the CV characteristics were measured in PBS (PBS, pH 7) containing bovine serum albumin (BSA). Using the as-grown CNW electrode without Ar atmospheric pressure plasma treatment, weak oxidation and reduction peaks were observed in anode peak potential at 0.2 V and cathodic peak potential at -0.3 V, respectively. On the other hand, in the case of typical surface-oxidized CNW electrode, a broad oxidation peak was recorded with an anode peak potential of 0.2 V. A high peak reduction current was also observed in the cathodic peak potential at -0.75 V. In the case of surface-oxidized CNW electrode of low height, the CV profile exhibited small peak currents compared with the case using typical CNW-oxide electrode, due to the decrease of immobilized BSA on the reduced surface area. These results indicate that the high potentials of CNWs as a novel platform for nano-bio applications.