Advancements in Plasma-Enhanced Chemical Vapor Deposition for Producing Vertical Graphene Nanowalls

Abstract

In recent years, vertical graphene nanowalls (VGNWs) have gained significant attention due to their exceptional properties, including their high specific surface area, excellent electrical conductivity, scalability, and compatibility with transition metal compounds. These attributes position VGNWs as a compelling choice for various applications, such as energy storage, catalysis, and sensing, driving interest in their integration into next-generation commercial graphene-based devices. Among the diverse graphene synthesis methods, plasma-enhanced chemical vapor deposition (PECVD) stands out for its ability to create large-scale graphene films and VGNWs on diverse substrates. However, despite progress in optimizing the growth conditions to achieve micrometer-sized graphene nanowalls, a comprehensive understanding of the underlying physicochemical mechanisms that govern nanostructure formation remains elusive. Specifically, a deeper exploration of nanometric-level phenomena like nucleation, carbon precursor adsorption, and adatom surface diffusion is crucial for gaining precise control over the growth process. Hydrogen's dual role as a co-catalyst and etchant in VGNW growth requires further investigation. This review aims to fill the knowledge gaps by investigating VGNW nucleation and growth using PECVD, with a focus on the impact of the temperature on the growth ratio and nucleation density across a broad temperature range. By providing insights into the PECVD process, this review aims to optimize the growth conditions for tailoring VGNW properties, facilitating applications in the fields of energy storage, catalysis, and sensing.

Keywords: carbon nanowalls (CNWs); graphene; plasma-enhanced chemical vapor deposition (PECVD); structural and morphological characteristics; vertical graphene nanowalls (VGNWs).

Figure 1

Normalized Raman spectra of VGNWs and carbon nanotubes (CNTs) grown on stainless-steel at different temperature. At temperatures around 750 °C, graphene nanowalls of sizes around 1 μm present the characteristic 2D peak of graphene and an I_D/I_G ratio lower than 1 indicating a lower number of defects [7]. (Reproduced with permission).

Figure 2

FE-SEM images of carbon nanostructures grown on stainless-steel (SS310) substrate by ICP-CVD from CH₄, corresponding to different processing temperatures: (a) Nucleation of carbon nanostructures on iron domains from the substrate at 575 °C. (b) Growth of a mixture of carbon nanotubes (CNTs) and graphene nanowalls (GNWs) at 700 °C. (c) Growth of GNWs of dimensions around 1 µm at 750 °C. (d) Formation of Cr particles segregated from the stainless-steel substrate (SS310) and their coalescence. Carbon nanostructures are not formed at 900 °C [7] (Reproduced with permission).

Figure 3

Plot of FWHM of 2D Raman peak versus growth temperature of VGNW’s prepared by ICP-CVD. (a) In colored bands there is a correspondence between FWHM (2D) and the number of monoatomic graphene layers. (b) Schematic representation of the process of formation of VGNWs growing from the edges. This process gives rise to the formation of graphene nanowalls that protrude from the surface of the substrate or by nucleation from CNTs, in seemingly random directions around the normal one to the substrate [7] (Reproduced with permission).

Figure 4

Quality of the graphene nanowalls as a function of the growth temperature on SS310 stainless steel [7]. (a) Evolution of the FWHM of the G peak with growth temperature, with a minimum in the temperature range of 650 °C to 750 °C. (b) Plot of I_D/I_G versus the growth temperature pointing to an opposite behavior of samples grown at lower temperature (blue and amber points) and the ones grown at high temperatures (red points). (c) Plot representing the FWHM(G) versus the intensity ratio I_D/I_G. In the case of disorder located at the edges, FWHM(G) is not correlated with I_D/I_G (flat dashed line) for points inside the bottom-left rectangle, which correspond to the graphene nanowalls having between 2 and 4 atomic layers. (d) Schematic representation of a scale of VGNW grown on SS310 substrate (Reproduced with permission).