Unconventional Reaction Phase Diagram for the Penetration Etching/Growth of Graphene Adlayers

Abstract

Cu has shown an advantage in growing monolayer graphene due to the very low C solubility and surface-mediated self-limiting growth, which hinders the growth of multilayer graphene. This work reports an unconventional penetration etching/growth of graphene adlayers tuned by oxygen beyond the self-limiting growth, supported by the C isotope labeling results. The effect of oxygen is nonmonotonic, i.e., with the increase of oxygen, graphene adlayers are etched without damaging the top layer, then shift to growth, and finally, all layers are etched. In addition, the reaction did not seem to reach equilibrium in the time range of the experiment but continued as if oxygen was increasing with respect to time. An oxygen-assisted exchange penetration model is proposed to interpret the growth mechanism. Oxygen etches the top shield layer, which is simultaneously healed by consuming the C species around adlayers and results in adlayer decomposition due to the break of equilibrium. Additionally, oxygen assists penetration of C from the gaseous agents into the shield layer for adlayer growth. The domination of one over the other depends on the concentration of oxygen, resulting in overall etching or growth. Finally, the synthesis of large-area monolayer and bilayer graphene films with good uniformity is demonstrated.

Achievements and Challenges of Graphene Chemical Vapor Deposition Growth

“…In bilayer or multilayer graphene synthesis, the growth methods are varied. Shen et al [87] reported that a certain amount of oxygen during growth [80] Copyright 2018, American Chemical Society. c) SEM micrographs of graphene grown at 100 °C with 100% full coverage.…”

“…Shen et al. [ 87 ] reported that a certain amount of oxygen during growth will tune the growth of adlayers beyond the self‐limited growth of the first layer. In this method, they achieve the growth of 90% coverage of uniform bilayer graphene.…”

Synthesis of Multilayer Graphene with Controlled C Supply

“…This explains that the thickness of graphene increases with the introduction of oxygen and decreases from the center to the edge. However, an overdosed oxygen may take C away by forming CO or CO 2 or may be other –C x O y H z species, [ 14 ] which also reduces the supply of C to the front side. As a result, the further increase of oxygen reduces the C supply at the center and the maximum C concentration moves away from the center, leading to the ring‐like distribution of graphene thickness (Figure 2a,b, the top three samples).…”

“…[7,8] When appropriate parameters are used, few-layer graphene (<10) can also be grown on Cu, normally in an inverted wedding cake mode, [9][10][11] and the C precursors to the layers under the top layer can be supplied by crossing graphene island edge side (side diffusion mode), [12] diffusing through the Cu foil from the other side (backside diffusion mode), [13] and penetrating the top layer assisted with oxygen. [14] When adding other elements such as Ni and Si to form alloys, few-layer graphene with even better uniformity and stacking order can be achieved. [15,16] However, the thickness of graphene is limited.…”

“…Although oxygen-assisted osmotic diffusion can continue to provide a C source after graphene covers the Cu substrates, the resistance increases significantly with the increase of layer number. [14] On the other hand, thick graphene films can be produced by C precipitation from metals such as Ni with high C solubility, but the uniformity is difficult to control and/or usually they are too thick, that is, over hundreds of nanometers. [17,18] Here, by separately tuning the gas atmosphere on the two sides of a Cu foil, multilayer graphene with unlimited growth was achieved.…”