SEM micrograph of the nanocrystalline graphene grown for 30 min directly on 300 nm SiO2.
(a) and (b) Plan-view TEM images of the graphene directly grown on SiO2/Si for 30 min. At the bottom of (a), a layered structure at the free-standing edge is seen, as graphene tends to roll up at free edges during transfer to TEM grids. In (b), the arrows indicate co-deposited nanographite. (c) A typical convergent beam electron diffraction pattern showing unique features from monolayer graphene. (d) A diffraction pattern showing signals from more than one domain, in correspondence with the nanocrystalline structure.
(Color online) (a) and (b) Optical images of the graphene thin films deposited directly on SiO2 (300 nm) from CH4 precursor during 30 and 60 min, respectively. In each micrograph, the left section is a transferred Cu-grown graphene for comparison of optical contrast. (c) Average contrast of the graphene images vs deposition time. The dashed line indicates the contrast of the Cu-catalyzed graphene for comparison.
(Color online) Raman spectra (514 nm, × 100 objective, 0.5 mW) of graphene grown by CVD. (a) Typical Raman signatures of Cu-grown graphene (transferred to 300 nm SiO2/Si substrate). (b) Raman spectra of nanocrystalline graphene deposited directly on 300 nm SiO2/Si for 30, 45, and 60 min. For all the samples, the G and 2 D spectral peaks are clearly observed. Curves have been shifted along the ordinate for clarity.
(Color online) (a) The two-probe I-V curves of devices made on samples with various deposition time. The sheet resistance Rs is calculated from the four-probe configuration. Inset: an AFM line scan on a device made from the 30-min-grown sample, showing a step height of ∼2 nm. (b) The field effect in the nanocrystalline graphene. The sheet resistances (normalized to Rs at zero Vg ) are plotted against the gate voltage. Inset: the optical micrograph of the device layout. The active area is 4 μm × 4 μm.
The magnetoconductance Δσ(B) at different temperatures indicated. The inset shows the temperature dependence of the zero-field resistance Rs . The dashed line indicates the quantum resistance RQ = e 2/h ≈ 25.8 kΩ. The solid line is the power-law fitting for T ≥ 50 K.
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