A Growth Mechanism for Free-Standing Vertical Graphene

Abstract

We propose a detailed mechanism for the growth of vertical graphene by plasma-enhanced vapor deposition. Different steps during growth including nucleation, growth, and completion of the free-standing two-dimensional structures are characterized and analyzed by transmission electron microscopy. The nucleation of vertical graphene growth is either from the buffer layer or from the surface of carbon onions. A continuum model based on the surface diffusion and moving boundary (mass flow) is developed to describe the intermediate states of the steps and the edges of graphene. The experimentally observed convergence tendency of the steps near the top edge can be explained by this model. We also observed the closure of the top edges that can possibly stop the growth. This two-dimensional vertical growth follows a self-nucleated, step-flow mode, explained for the first time.

Vertically Aligned Graphene Nanosheet Arrays: Synthesis, Properties and Applications in Electrochemical Energy Conversion and Storage

“…[48] Moreover, in the synthesis of VAGNAs without substrate biasing, a maximum height of ≈1 µm was usually observed (height saturation), which was considered to be due to the closure of the top edges of nanosheets with the prolonged growth duration (more details will be discussed below). [49] In this case, by prolonging the deposition time, the growth of VAGNA film will be accompanied with secondary nucleation. As a result, the pores formed by the surrounding vertical graphene nanosheets will be blocked by the top layer, leading to a structure similar to that of GFs.…”

“…In the case of catalytic metal substrates, e.g., Au, Cu and Ni, prior to nucleation, a ≈20-nmthick buffer layer was usually observed, e.g., amorphous and graphitic carbon, as shown in Figure 3a. [49,65] The amorphous layer was formed due to the catalytic effect of the metallic substrates to the carbon species when the time and temperature were insufficient for the crystallization of carbon. On the top of the buffer layer, the vertical graphene with open edges originated from the mismatches and curved areas of graphitic layer was suggested to be the nucleation sites for graphene nanosheets.…”

“…On the top of the buffer layer, the vertical graphene with open edges originated from the mismatches and curved areas of graphitic layer was suggested to be the nucleation sites for graphene nanosheets. [49] In contrast, for the strong carbide-forming Adv. Energy Mater.…”

“…Reproduced with permission. [49] Copyright 2014, American Chemical Society. b) Graphene sheets nucleate on silicon substrate with a heteroepitaxial relationship.…”

“…[67] The lateral size of graphene sheets increased almost linearly in the steady-state growth period (Figure 3d), which was suggested as a rate-limiting step. [32,49] It was demonstrated that VAGNA growth was through direct attachment of carbon species to the open edges of graphene nanosheets. [67] A continuum model based on the surface diffusion and mass flow was developed to describe the intermediate states of the steps and the edges of graphene.…”