Controlling the thickness and uniformity during growth of multilayer graphene is an important goal. Here we report the synthesis of large-area monolayer and multilayer, particularly bilayer, graphene films on Cu-Ni alloy foils by chemical vapor deposition with methane and hydrogen gas as precursors. The dependence of the initial stages of graphene growth rate on the substrate grain orientation was observed for the first time by electron backscattered diffraction and scanning electron microscopy. The thickness and quality of the graphene and graphite films obtained on such Cu-Ni alloy foils could be controlled by varying the deposition temperature and cooling rate and were studied by optical microscopy, scanning electron microscopy, atomic force microscopy, and micro-Raman imaging spectroscopy. The optical and electrical properties of the graphene and graphite films were studied as a function of thickness.

Strongly coupled bilayer graphene (i.e., AB stacked) grows particularly well on commercial "90-10" Cu-Ni alloy foil. However, the mechanism of growth of bilayer graphene on Cu-Ni alloy foils had not been discovered. Carbon isotope labeling (sequential dosing of (12)CH(4) and (13)CH(4)) and Raman spectroscopic mapping were used to study the growth process. It was learned that the mechanism of graphene growth on Cu-Ni alloy is by precipitation at the surface from carbon dissolved in the bulk of the alloy foil that diffuses to the surface. The growth parameters were varied to investigate their effect on graphene coverage and isotopic composition. It was found that higher temperature, longer exposure time, higher rate of bulk diffusion for (12)C vs(13)C, and slower cooling rate all produced higher graphene coverage on this type of Cu-Ni alloy foil. The isotopic composition of the graphene layer(s) could also be modified by adjusting the cooling rate. In addition, large-area, AB-stacked bilayer graphene transferrable onto Si/SiO(2) substrates was controllably synthesized.

Monolayer graphene was deposited on a Si wafer substrate decorated with SiO(2) nanoparticles (NPs) and then exposed to aryl radicals that were generated in situ from their diazonium precursors. Using micro-Raman mapping, the aryl radicals were found to selectively react with the regions of graphene that covered the NPs. The enhanced chemical reactivity was attributed to the increased strain energy induced by the local mechanical deformation of the graphene.

Au nanoparticles and films are deposited onto clean graphene surfaces to study the doping effect of different Au configurations. Micro-Raman spectra show that both the doping type and level of graphene can be tuned by fine control of the Au deposition. The morphological structures of Au on graphene are imaged by transmission electron microscopy, which indicate a size-dependent electrical characteristic: isolated Au nanoparticles produce n-type doping of graphene, while continuous Au films produce p-type doping. Accordingly, graphene field-effect transistors are fabricated, with the in situ measurements suggesting the tunable conductivity type and level by contacting with different Au configurations. For interpreting the experimental observations, the first-principles approach is used to simulate the interaction within graphene-Au systems. The results suggest that, different doping properties of Au-graphene systems are induced by the chemical interactions between graphene and the different Au configurations (isolated nanoparticle and continuous film).

Graphene has been grown on Cu at elevated temperatures with different carbon sources (gaseous hydrocarbons and solids such as polymers); however the detailed chemistry occurring at the Cu surface is not yet known. Here, we explored the possibility of obtaining graphene using amorphous-carbon thin films, without and with hydrogen gas added. Graphene is formed only in the presence of H(2)(g), which strongly suggests that gaseous hydrocarbons and/or their intermediates are what yield graphene on Cu through the reaction of H(2)(g) and the amorphous carbon. The large area, uniform monolayer graphene obtained had electron and hole mobilities of 2520 and 2050 cm(2) V(-1) s(-1), respectively.

University of Texas at Austin; NSF [CMMI 0900569]; China Scholarship Council; National Natural Science Foundation of China [60827004, 90921002, 60776066]The deposition effect of metals on graphene was studied by in situ field effect transistor (FET) measurements in high vacuum. Metals such as gold (Au), silver (Ag), and copper (Cu) were deposited onto clean graphene surfaces, followed by FET measurements. The results show that Ag and Cu cause a shift in the Fermi level in the graphene from the Dirac point into the conduction band while Au causes a shift into the valence band. The induced carrier concentration was estimated at 2-6 X 10(12)/cm(2). The shifts in the Fermi level of the graphene are explained by the different work functions of these metals. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3471396

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Integration of 2D membranes into 3D macroscopic structures is essential to overcome the intrinsically low stretchability of graphene for the applications in flexible and wearable electronics. Herein, the synthesis of 3D graphene films (3D-GFs) using chemical vapor deposition (CVD) is reported, in which a porous copper foil (PCF) is chosen as a template in the atmospheric-pressure CVD preparation. When the 3D-GF prepared at 1000 °C (noted as 3D-GF-1000) is transferred onto a polydimethylsiloxane (PDMS) membrane, the obtained 3D-GF-1000/PDMS hybrid film shows an electrical conductivity of 11.6 S cm −1 with good flexibility, indicated by small relative resistance changes (ΔR/R 0 ) of 2.67 and 0.36 under a tensile strain of 50% and a bending radius of 1.6 mm, respectively. When the CVD temperature is reduced to 900 °C (generating a sample noted as 3D-GF-900), the 3D-GF-900/PDMS hybrid film exhibits an excellent strain-sensing performance with a workable strain range of up to 187% and simultaneously a gauge factor of up to ≈1500. The 3D-GF-900/PDMS also shows a remarkable durability in resistance in repeated 5000 stretching-releasing cycles. Kinetics studies show that the response of ΔR/R 0 upon strain is related to the graphitization and conductivity of 3D-GF which are sensitive to the CVD preparation temperature.