Graphene CVD: Interplay Between Growth and Etching on Morphology and Stacking by Hydrogen and Oxidizing Impurities

Abstract

The growth of high quality graphene layers by chemical vapor deposition (CVD) has been found to strongly depend on growth conditions with results varying greatly from one laboratory to another for nominally identical conditions. We report the results of a systematic investigation of the role of hydrogen and oxidizing impurities present in the gas feedstock during the growth and cooling stages in low-pressure CVD. First, we show that for a partial pressure of oxidizing impurities below 1 ppb, hydrogen is not required for graphene growth from methane. Second, we demonstrate that purified hydrogen does not etch graphene films at typical growth temperatures. Third, a flow of purified hydrogen during cooling counterbalances graphene etching by oxygen, thus protecting the films. Films grown under high purity conditions (low level of oxidizing impurities) exhibit a higher bilayer and multilayer coverage; Surprisingly some of these bi-and multilayer graphene islands are twisted with respect to the first graphene layer as revealed by hyperspectral Raman imaging. Overall, this growth behavior suggests a competitive action between film growth from the carbon precursors and etching by the oxidative species. Our results provide new fundamental insights on the graphene CVD growth, highlighting the important yet indirect role of hydrogen and its major influence on controlling the action of oxidizing impurities on nucleation and etching during the growth process.

Synthesis of Graphene Films on Copper Foils by Chemical Vapor Deposition

“…Thus hydrogen gas plays another important role by counterbalancing oxidation and, in fact, when the level of oxidizing impurities is very low, the presence of H 2 is not required for growing high-quality graphene layers. [19] This also explains the wide range of reported growth conditions in the literature.…”

“…In practice, oxidants in the gas feedstock, air leaks into the reaction chamber, and gas desorption from the chamber wall and substrate can also significantly impact graphene growth. [19] It is well known that hydrocarbons can react via pyrolysis or chain reactions even without any catalyst, as long as the temperature is high enough. For the case of CH 4 , gas-phase reaction provides CH 3 , H, H 2 , and C 2 H y (y = 1 to 6).…”

Molecular dynamics simulation of graphene on Cu (1 0 0) and (1 1 1) surfaces

“…While early experiments used mechanical exfoliation with Scotch tape to produce single or few layer thick graphene samples, this process is not well suited for industrial production. issues are the role of hydrogen in the gas mixture [2,3,4], the interaction of graphene with Cu under different orientations [5], the influence of temperature on growth [6] and the diffusion of C atoms and clusters during graphene growth. [7].…”

Hyperspectral Raman imaging using Bragg tunable filters of graphene and other low‐dimensional materials

“…Figure 4a shows a 130 × 130 μm Raman image of the G band at around 1590 cm À1 obtained from a heterogeneous graphene sheet grown by CVD on copper and then transferred on 100 nm SiO 2 /Si substrate. [29] Environmental effects and structural variations produce various changes in the Raman spectra, [30] giving rise to different components shown as colors in the hyperspectral image. The first is a shift of the G band due to charge-transfer doping from the water-oxygen redox couple at the interface between graphene and SiO 2.…”

“…[32] These features have been selected to define a color code in the image about the layer composition, thanks to a digital manipulation of the hyperspectral dataset using Spectral Angle Mapping. Using reference images obtained with our Low-Energy Electron Microscope, [29] the three reference spectra (endmembers) representative of the different components have been guessed in the image of Fig. 4a and color coded as follows: monolayer graphene in blue, non-resonant bilayer graphene in green and resonant bilayers in red.…”