Understanding and controlling the growth of carbon nanowalls

Sebastian Tigges, University of Duisburg-Essen, Duisburg, Germany
Axel Lorke, University of Duisburg-Essen, Duisburg, Germany
Nicolas Wöhrl, University of Duisburg-Essen, Duisburg, Deutschland

Carbon nanowalls (CNWs; also called vertically oriented graphene nanosheets) exhibit exceptional thermal as well as electrical conductivity. Their surface area can be controlled by adjusting process parameters during growth. This makes them especially attractive for application in sensors and energy applications (i.e. fuel cells), where a large, chemically active surface area is necessary for optimum efficiency. Extensive research has been conducted to understand the complex mechanisms contributing to the growth of CNWs[1]. The most important challenge is proper understanding of growth mechanisms to directly control morphology and structure.

Here, we investigate different CNW structures that are obtained by tuning process parameters such as temperature and substrate bias, as well as flow rate of the precursor gas and pressure at low temperatures (~350°C) in a inductively coupled plasma-enhanced chemical vapour deposition system (ICP-PECVD). Three distinct CNW structures of varying morphology and inter-wall distance are obtained. The CNWs are characterized by scanning electron microscopy (SEM) and Raman spectroscopy. SEM results demonstrate a high aspect ratio of individual walls with heights of several microns and widths of a few nanometres. Raman spectroscopy shows significant variation in both defect density and defect type, depending on morphology. Additionally, scanning Auger microscopy is used to spatially resolve the chemical composition of a typical CNW structure. Plasma characterization is done by optical emission spectroscopy and from these measurements it is shown how the formation of different CNW structures is depending on the residence time of the complex precursor molecule used as carbon source in the process. From these observations a simple growth model is derived.

[1] Bo, Z., Yang, Y., Chen, J., Yu, K., Yan, J., Cen, K., 2013. Plasma-enhanced chemical vapor deposition synthesis of vertically oriented graphene nanosheets. Nanoscale 5, 5180–5204.

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