(Invited) A Rapid and Scalable Method for the Synthesis of Carbon Nanowalls
Authors: Mozetič Miran , Zaplotnik Rok , Primc Gregor , Vesel Alenka 
Published: February 2020

Carbon nanowall (CNW) materials [1] present useful properties related to the graphene based structure, the nano-microtopography of surface, the open architecture, high porosity and large specific surface area determined by the vertically oriented, interconnected walls with thickness in the nanoscale range and length/height in the micron scale. Various deposition procedures have been reported for carbon nanowalls, to mention a few of them: Microwave Plasma Enhanced Vapor Deposition [2], Radical Injection Plasma CVD [3], PECVD in pin-plate discharge geometry [4], downstream deposition in a low pressure RF plasma jet injected with acetylene in presence of hydrogen [5], radiofrequency PECVD applied to large area substrates [6]. The remarkable properties listed above make these materials very attractive for applications. Various reports pointed out to the potential of using CNW layers as large area supports for catalytic nanoparticles [7], stable and chemical resistant electrodes in electrochemical devices and batteries [8], in supercapacitors [9], almost total light trapping layers in energy devices [10] or electric field electron emitters in electrical devices [11]. Due to the enormous surface area these materials represent an effective sink for neutral plasma radicals and are currently among materials of highest coefficient for heterogeneous surface recombination [12].
All reported techniques for deposition of carbon nanowalls suffer from long deposition times as well as inability to deposit on large surfaces uniformly. The drawbacks of the current deposition techniques will be discussed. One limitation is the flux of carbon species from the gas phase to the surface. Low-pressure discharges operating in systems pumped by high-vacuum pumps do not allow for high fluxes, especially because the carbon nanowalls grow on surfaces by simultaneous treatment with atomic hydrogen that often represents the major partial pressure in processing chambers. Atomic hydrogen is essential for selective removal of loosely bonded carbon upon the deposition of nanowalls and few reports on omitting hydrogen have appeared in the literature. The extremely extensive recombination of hydrogen atoms to parent molecules on the growing nanowalls also represents a considerable risk of non-uniformity of deposited CNW film on a large scale. Continuous injection of atomic hydrogen from a separate source helps supplying lost H-atoms, but such a solution is not scalable to large substrates. From this point of view there is a need to decrease the hydrogen content in processing plasmas but the quality of the nanowalls decreases with decreasing H-atom fluxes. Namely, other morphological forms grow preferentially upon carbon deposition at reduced fluxes of hydrogen atoms. One solution of this problem is the application of powerful discharges, such as focused plasma devices, which allows for high ionization rate of carbon atoms. These devices, however, cannot assure for uniform deposition over large surfaces due to strong gradients involved at elevated pressures.
A novel, hydrogen-free technique for depositing carbon nanowalls will be presented. The technique allows for deposition of uniform CNW film at pressures in the mbar range thus overcomes the drawback of low-pressure deposition techniques.
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