(Invited) Radical-Controlled Plasma Nano Processing for Green and Life Innovations
Authors: Hori Masaru 
Published: February 2020

Plasma nanosciences and technologies have been leading all industries with bringing about innovations in green and life fields. The non-equilibrium chemical reactions induced by the synergetic effect of ions and radicals in the plasma processing has enabled to realize the isotropic etching with a high aspect ratio, the synthesis of functional nanomaterials through the plasma assisted self-organization and the surface chemical modification and so on in low pressure plasmas. In precise etching processes, the critical size dimension for the nanoscale pattering will be below 1nm and then the control of radical based on reactions on the side wall in the fine pattern with a nanoscale in size is of crucial importance. Additionally, the precise control of radicals is also strongly demanded in nanomaterial plasma processing. Carbon nanomaterials such as nanotubes, nanographenes and nanowalls attracted much attentions. They were synthesized by the plasma assisted self-organization, which was controlled by ions and/or radicals. Recently, the atmospheric pressure plasma and the in-liquid plasma have been developed. These plasmas have been successfully applied not only to the nanomaterial processing in the green innovation but also to the medical and agriculture fields in the life innovation. In these extremely complicated reactions, spatiotemporal controlling of radicals is a key point. Therefore, the measurement of behaviors of radicals in plasma nanoprocesses and thus the control of radicals on the basis of measured results has become important.

We have been synthesizing a carbon nanowall, which is a two dimensional grapheme layers are standing vertically on the substrate, by the radical-controlled plasma processing. The structural control of carbon nanowalls was successfully performed by the radical injection to the plasma. Varieties of morphologies of carbon nanowalls were synthesized by controlling radicals and applied to fuel cell devices, catalysis devices and bio template and so on.

Furthermore, ultrahigh density plasmas in the atmospheric pressure and in the liquid produce high density radicals. We have synthesized nanographenes by using the in-liquid plasma with alcohols. These nanographenes were also applied to the fuel cell devices. The atmospheric pressure plasma has been exposed to various kinds of cancer cells resulting in killing them. We found that ovarian cancer cells were successfully killed selectively against normal cells by the atmospheric pressure plasma and/or the plasma activated medium. In the case of synthesis of the plasma activated medium, controlling of densities of O radicals together with N and NO radicals exposed to the liquid medium was important to produce the optimum chemical species in the medium, which will be effective to kill cancers in vitro and in vivo. The systematical control of radicals in the nanoscale space at the gas and the liquid phase is a key issue to develop the plasma medicine, too. Therefore, we have stressed on the importance of development of an autonomous control plasma manufacturing system in order to accelerate green and life innovations. This system has multi-monitors to detect chemical reactions in the gas, the surface and interface between the gas and liquid, and in the liquid. On the basis of integration of measured results, it will make the precise control of species, especially radicals in the low pressure, atmospheric pressure and in-liquid plasmas.

In this article, cutting edge plasma nanoprocesses with the radical-control in the plasma nanoprocesses for the synthesis of nanocarbons in the green innovation and with that in the plasma medical treatment of cancers in the life innovation are introduced and the future vision for the next generation’s plasma nanoprocessing will be presented.