Graphene, a single atomic layer of carbon with a hexagonal crystal structure, has been heavily investigated in the past decade for its many unique material properties. With its high electrical and thermal conductivity, near optical transparency, and high mechanical strength, graphene is considered a very promising 2D material for a broad range of applications, including energy storage, optical displays and sensors [1,2].
One challenge of working with graphene is to be able to process it without compromising its structural integrity. With relatively low applied powers of 10’s watts, our plasma cleaners are well-suited for treating multilayer or single layer graphene. In a series of application notes, we explore how researchers have applied plasma treatment to study and tune the various material properties of graphene.
EFFECT OF PLASMA OXIDATION ON GRAPHENE
A few research groups examined the effect of plasma oxidation and post-deposition annealing on the mechanical robustness and structural integrity of graphene. Their work explored how the graphene lattice was altered by these processes and offered insights on how these techniques can produce robust graphene for practical applications.
Hui et al. investigated plasma oxidation of chemical vapor deposited (CVD) graphene on Cu substrates. Unlike free-standing or weakly-bonded films, CVD graphene on substrates showed more robust response to oxygen plasma. Instead of attacking the carbon bonds as observed in free-standing graphene, the oxygen ions in CVD graphene oxidized the underlying Cu substrate such that it anchored the graphene to the oxidized Cu. CVD graphene was initially lightly doped with oxygen, followed by scavenging of oxygen and Cu oxidation through the graphene defect sites, further stabilizing the graphene layer. Eventually, the graphene underwent further transformation into reduced graphene oxide accompanied by complete oxidation of the Cu substrate. Oxidation of both Cu and graphene ultimately stabilized the structure. As a result, the CVD graphene-Cu system was able to withstand longer plasma treatment (hours rather than seconds).
In an earlier 2014 paper, Hui et al. also found that the plasma etch time required to completely remove CVD graphene from a Cu substrate increased with post-deposition anneal time. This was observed with air, O2, or N2 plasma. The resistance to plasma etching suggested improved adhesive strength of graphene to Cu at the interface. This suggested that annealing may improve the mechanical robustness of graphene while plasma etching may be one way to measure the mechanical strength of the graphene/Cu interface.
Ryu et al. compared UV-ozone and O2 plasma treatment to oxidize graphene. They found that both methods resulted in chemical reaction and attachment of oxygen atoms on the graphene surface, but O2 plasma also induced physical lattice distortion and defect formation. The number of observed topological defects increased with plasma treatment time, with partial lattice distortion and oxygen attachment taking place even after only 5 seconds of plasma exposure. The resulting defects caused strain in the C-C bonds, increasing chemical reactivity and structural instability such that oxygen easily attaches to these defect sites. As such, O2 plasma resulted in more effective surface oxidation and functionalization of graphene. Their work demonstrated that different oxidation techniques can yield different structural results in graphene.
Relevant Articles from Harrick Plasma Users
- Hui LS, Whiteway E, Hilke M and Turak A. “Synergistic oxidation of CVD graphene on Cu by oxygen plasma etching”. Carbon 2017 125: 500-508.
- Hui LS, Whiteway E, Hilke M and Turak A. “Effect of post-annealing on the plasma etching of graphene-coated-copper”. Faraday Discuss. 2014 173: 79-93.
- Ryu GH, Lee J, Kang D, Jo HJ, Shin HS and Lee Z. “Effects of dry oxidation treatments on monolayer graphene”. 2D Mater. 2017 4(2): 024011.
Supplemental References (Do not report using Harrick Plasma instruments)
 Allen MJ, Tung VC and Kaner RB. “Honeycomb Carbon: A Review of Graphene”. Chem. Rev. 2010 110(1): 132-145.
 Choi W, Lahiri I, Seelaboyina R and Kang YS. “Synthesis of Graphene and Its Applications: A Review”. Crit. Rev. Solid State 2010 35(1): 52-71.