The effects of residues introduced during the transfer of chemical vapor deposited graphene from a Cu substrate to an insulating (SiO 2) substrate on the physical and electrical of the transferred graphene are studied. X-ray photoelectron spectroscopy and atomic force microscopy show that this residue can be substantially reduced by annealing in vacuum. The impact of the removal of poly(methyl methacrylate) residue on the electrical properties of graphene field effect devices is demonstrated, including a nearly 2 Â increase in average mobility from 1400 to 2700 cm 2 /Vs. The electrical results are compared with graphene doping measurements by Raman spectroscopy. V

Field-effect transistors fabricated on graphene grown by chemical vapor deposition (CVD) often exhibit large hysteresis accompanied by low mobility, high positive backgate voltage corresponding to the minimum conductivity point (V(min)), and high intrinsic carrier concentration (n(0)). In this report, we show that the mobility reported to date for CVD graphene devices on SiO(2) is limited by trapped water between the graphene and SiO(2) substrate, impurities introduced during the transfer process and adsorbates acquired from the ambient. We systematically study the origin of the scattering impurities and report on a process which achieves the highest mobility (μ) reported to date on large-area devices for CVD graphene on SiO(2): maximum mobility (μ(max)) of 7800 cm(2)/(V·s) measured at room temperature and 12,700 cm(2)/(V·s) at 77 K. These mobility values are close to those reported for exfoliated graphene on SiO(2) and can be obtained through the careful control of device fabrication steps including minimizing resist residue and non-aqueous transfer combined with annealing. It is also observed that CVD graphene is prone to adsorption of atmospheric species, and annealing at elevated temperature in vacuum helps remove these species.

During chemical-vapor-deposited graphene transfer onto target substrates, a polymer film coating is necessary to provide a mechanical support. However, the remaining polymer residues after organic solvent rinsing cannot be effectively removed by the empirical thermal annealing in vacuum or forming gas. Little progress has been achieved in the past years, for little is known about the chemical evolution of the polymer macromolecules and their interaction with the environment. Through in situ Raman and infrared spectroscopy studies of PMMA transferred graphene annealed in nitrogen, two main processes are uncovered involving the polymer dehydrogenation below 200 °C and a subsequent depolymerization above 200 °C. Polymeric carbons over the monolayer graphitic carbon are found to constitute a fundamental bottleneck for a thorough etching of PMMA residues. The dehydrogenated polymeric chains consist of active CC bonding sites that are readily attacked by oxidative gases. The combination of Raman spectroscopy, X-ray photoemission spectroscopy, and transmission electron microscopy reveals the largely improved carbon removal by annealing in oxidative atmospheres. CO2 outperforms other oxidative gases (e.g., NO2, O2) because of its moderate oxidative strength to remove polymeric carbons efficiently at 500 °C in a few minutes while preserving the underlying graphene lattice. The strategy and mechanism described here open the way for a significantly improved oxidative cleaning of transferred graphene sheets, which may require optimization tailored to specific applications.

Only a small fraction of all available metals has been used as electrode materials for carbon-based devices due to metal-graphene interface debonding problems. We report an enhancement of the bonding energy of weakly interacting metals by using a metal-graphene-metal sandwich geometry, without sacrificing the intrinsic π-electron dispersions of graphene that is usually undermined by strong metal-graphene interface hybridization. This sandwich structure further makes it possible to effectively tune the doping of graphene with an appropriate selection of metals. Density functional theory calculations reveal that the strengthening of the interface interaction is ascribed to an enhancement of interface dipole-dipole interactions. Raman scattering studies of metal-graphene-copper sandwiches are used to validate the theoretically predicted tuning of graphene doping through sandwich structures.

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