(Invited) Building Graphene Nanowalls with Plasma: Processing, Functionalization, and Challenges
Authors: Cvelbar Uros , Santhosh Neelakandan Marath , Filipič Gregor 
Published: February 2020

Plasmas find applications in various fields of materials science, also in the building of 2D graphene nanowalls (GNW), which is an emerging field of material synthesis with a number of challenges related to its processing. Synthesis of GNW in a controlled manner with plasma-enhanced techniques opened future pathways for the large-scale, rapid functionalization of graphene for advanced applications. Plasma-supported methods enhance the possibility of processing (synthesis and functionalization) during the surface interactions between substrate and GNW offer the possibility for surface post-treatment. Functionalization during the plasma reaction with chlorine and doping of graphene by nitrogen, oxygen, boron, leads to altering the properties of GNW such as the opening of band gaps, controlling thermal transport, increasing the conductivity, etc. Plasma-supported post-processing of graphene nanowalls has proved very effective in modifying graphene’s surface energy, which in turn resulted in significant changes in the structural and electronic properties of the material. All these pre- or post-processed graphene nanowalls with enhanced properties can be applied in almost all the fields related to the daily life like biological applications, sensing applications, electronic devices, or even energy storage. Among the important plasma-assisted methods for GNW synthesis, the most frequently used method is plasma-enhanced chemical vapor deposition (microwave assisted, inductively coupled, capacitively coupled PECVD), which can enable the grow GNW on the substrate even in the absence of metal catalyst [1]. The overall plasma processing of GNW is controlled by using specific gases, typically, CH4/H2 are used as the gas systems. Besides these gas systems, there are many others such as Ar/CH4/H2, Ar/C2H2/H2, Ar/C2H2/NH3, CH4/NH3, and C2H2/NH3 [2]. Functionalization or doping of GNWs is correspondingly an important aspect in the plasma synthesis. There is a large number of research works carried out in the doping process, such as doping with N2, O2, B, H2, Cl and so on. Of course, these results lead to the challenges regarding the plasma-assisted mechanisms and its processing.
This talk addresses the most important challenges associated with plasma-assisted mechanisms of GNWs dealing with the growth and doping of GNWs. From this point of view, it’s clear the importance of a gas mixtures and plasma’s properties. In these systems, plasma parameters including the densities of ions and radicals are regulated by the discharge parameters like power, gas mixture ratio, gas flow, and pressure, whereas the main challenge is connected to understanding the role of plasma species in growth and their efficient control for improving the quality and selectively modifying properties of the synthesized GNW. Substrate used for growth GNW is also important since it allows various things like tuning carrier mobility in synthesized structures or use of different and more aggressive environments, e.g. high temperatures. For these purposes, different substrates such as 2D sheets (h-BN, MoS2), dielectric substrates (Si, SiN, Al2O3, SiO2) or transitional metal substrates (Ni, Cu, Co) are applied. Another challenge is material doping and understanding the mechanism of plasma doping. Nitrogen functionalization and doping is one of the potential directions how to alter the electronic properties, the oxygen plasma treatment for example helps to enhance surface morphological properties and widening band gaps, etc. [3]. In this perspective, the talk will highlight the recent progress in the field of building GNW including the processing, functionalization, and future challenges that we have to address in GNW synthesis.
This work has received funding from European Union’s Horizon 2020 research and innovation program under grant agreement No. 766894.
Reference
[1] K. (Ken) Ostrikov, U. Cvelbar, and A. B. Murphy, “Plasma nanoscience: setting directions, tackling grand challenges,” J. Phys. D. Appl. Phys., vol. 44, no. 17, p. 174001, 2011.
[2] M. Li, D. Liu, D. Wei, X. Song, D. Wei, and A. T. S. Wee, “Controllable Synthesis of Graphene by Plasma-Enhanced Chemical Vapor Deposition and Its Related Applications,” Advanced Science, vol. 3, no. 11. 2016.
[3] A. Dey, A. Chroneos, N. S. J. Braithwaite, R. P. Gandhiraman, and S. Krishnamurthy, “Plasma engineering of graphene,” Appl. Phys. Rev., vol. 3, no. 2, 2016.