Abstract
Applications of plasma-produced vertically oriented graphene nanosheets (VGNs) rely on their unique structure and morphology, which can be tuned by the process parameters to understand the growth mechanism. Here, we report on the effect of the key process parameters such as deposition temperature, discharge power and distance from plasma source to substrate on the catalyst-free growth of VGNs in microwave plasmas. A direct evidence for the initiation of vertical growth through nanoscale graphitic islands is obtained from the temperature-dependent growth rates where the activation energy is found to be as low as 0.57 eV. It is shown that the growth rate and the structural quality of the films could be enhanced by (a) increasing the substrate temperature, (b) decreasing the distance between the microwave plasma source and the substrate, and (c) increasing the discharge power. The correlation between the wetting characteristics, morphology and structural quality is established. It is also demonstrated that morphology, crystallinity, wettability and sheet resistance of the VGNs can be varied while maintaining the same sp3 content in the film. The effects of the substrate temperature and the electric field in vertical alignment of the graphene sheets are reported. These findings help to develop and optimize the process conditions to produce VGNs tailored for applications including sensing, field emission, catalysis and energy storage.
Highlights
Vertical graphene nanosheets (VGNs) consist of interconnected 3D porous networks of vertically oriented graphitic sheets, which are aligned perpendicularly to the substrates, containing 3–12 graphene layers [1]
Case I: Influence of growth temperature We investigated the early-stage nucleation and growth of VGNs over a substrate temperature range of 600–800 °C under CH4/Ar gas environment for 30 min, while the plasma power and distance from the substrate were maintained at 320 W and
NG structures were not observed below 600 °C and this is explained by adverse etching of graphene by hydrogen radicals in the plasma, which dominates over the graphene growth at lower temperatures [46]
Summary
Vertical graphene nanosheets (VGNs) consist of interconnected 3D porous networks of vertically oriented graphitic sheets, which are aligned perpendicularly to the substrates, containing 3–12 graphene layers [1]. The interconnected vertical 3D network is anchored onto a nanometer-thick graphitic base layer grown on the VGNs–substrate interface Their unique properties such as large surface area, non-agglomerated structure, sharp edges, excellent thermal and electrical properties, thermal and electrochemical stability and ease in functionalization make VGNs promising candidates for a wide range of applications that include field emission, sensing, energy storage, metamaterials, biomedical and other devices [1,2,3,4,5,6,7]. Efficient utilization of VGNs depends on their effective surface area, which is determined by two major factors: (a) vertical sheet density and (b) intersheet spacing Enhancing these two factors improves the electron transfer kinetics and electrochemical properties [2,8]. Such self-organization can be realized by controlling the process parameters during growth
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