Chemically derived graphene was synthesized by the modified Hummer's method 13, a well-known approach to produce monolayered graphene via liquid-phase exfoliation of graphite oxide. Natural graphite powder (SEC Carbon SNO-30) was washed in H₂SO₄ and K₂S₂O₈, and oxidized in KMnO₄ and H₂SO₄. After centrifugation, the resulting graphite oxide was exfoliated into graphene oxide (GO) by ultra-sonication (100 W, 30 min, 60°C). Then, a GO aqueous dispersion was produced by centrifugation and dialysis to neutralize a pH. The morphology of GO synthesized by this procedure was characterized by Raman spectroscopy (excited by 532-nm Ne laser) 14, optical microscope, scanning electron microscope, and atomic force microscope (in tapping mode using Si tips).

A reduction step of GO into graphene plays an essential role to determine the electrical properties of the resulting graphene films. GO was reduced as follows: GO was dispersed in aqueous solution containing N₂H₄, a strong reductant, with NH₃ to adjust pH 15. This was reacted in 95°C water bath for 1 h, and the color of dispersion changed from brownish color to gray. Finally, the solvent of reduced graphene oxide (RGO) dispersion was replaced by N,N-dimethylformamide (DMF) using an evaporator. RGO can be dispersed well in many kinds of organic solvents including DMF, while it is easily aggregated in aqueous solution because of its low electrostatic repulsion force. A RGO sample deposited on Au (10 nm)/SiO₂ (90 nm)/Si substrate was prepared for the evaluation of the reduction state by x-ray photoelectron spectroscopy (monochrome Al Kα X-ray).

Our graphene films were deposited on glass substrates (Corning7059) by a spray-coat method at a substrate temperature of 200°C in an atmosphere containing the solvent vapor. The thickness of the films was controlled by varying the spray amounts. The optical transmittance was measured in the wavelength range from 250 to 2500 nm, and the sheet resistance was measured by van der Pauw method.

Doping graphene via charge transfer by TCNQ molecules was carried out as follows. First, 0.01 g of TCNQ powder (>98.0%, Tokyo Chemical Industry Co. Ltd., Tokyo, Japan) was dissolved into 5 ml of DMF solvent. It is expected that TCNQ molecules in DMF are radicalized 16. Then, 5 ml of RGO dispersion and radicalized TCNQ in DMF were mixed and stirred for 1 week at room temperature. The color of mixture solution changed from yellow-green to orange. This RGO-TCNQ mixture dispersion has been very stable for over a few months, and no clear evidence of aggregation was observed.

Large GO flakes (over 30 × 30 μm²) were present in the GO aqueous dispersion as shown in Figure 2a. The surface morphology of these flakes was measured to be atomically thin (0.4 nm) two-dimensional (2D) structure using AFM as shown in Figure 2b,c, indicating the presence of monolayer of GO. In addition, a Raman peak shift and peak shape of second-order two phonons process peak at 2700 cm^-1, referred to as the 2D band, which indicates about 25% of GO flakes were single layer of carbon as demonstrated in our previous article 14.

The carbon 1s core level XPS spectra of GO, RGO, and graphite samples were shown in Figure 3. From the semi-quantitative analysis by XPS, the relative amount of oxygen containing functional groups in each sample was estimated. Peak separation was carried out for all samples after Shirley background was subtracted. The relative ratios of each component consisted of aromatic rings (284.6 eV), C-OH (286.5 eV), C-O-C (287.0 eV), and O = C-OH (288.3 eV) are summarized in Table 1. Oxygen-containing functional groups decreased from around 50 to around 25% of all components after reduction process. Such a low concentration of oxygen-containing functional groups is comparable to the RGO reduced by high-temperature annealing 17.

Figure 4a shows photograph of fabricated graphene films on glass substrates at various spray volumes. SEM images of fabricated graphene films revealed them to be continuous and uniform (Figure 4b). Figure 5a shows the optical transmittance spectra of these fabricated graphene films, and the transmittance decreased for all wavelength ranges as the spray volume increased. Optical and electrical properties are summarized in Figure 5b. Sheet resistance of minimum spray volume sample was too high to be measured by our analyzer. The graphene films obtained in this study had a sheet resistance as high as 1 × 10⁶ Ω/square with a transparency of 88% at 550 nm. Such a sheet resistance was the lowest obtained compared with previously reported chemically derived graphene films as deposited 618. Post-annealing treatment was expected to improve the performance of our graphene films due to removal of residual solvent and oxygen-containing functional groups on RGO. Actually, Becerril et al. 19 obtained the highest performance in chemically derived graphene films through high-temperature annealing in vacuum. However, no post-annealing treatment on our graphene films was conducted, since the focus was on an inexpensive fabrication procedure without any vacuum systems.

The SEM images of individual doped graphene flakes indicate RGO flakes maintaining 2D structures after interaction with TCNQ molecules in liquid phase as shown in Figure 6a. Continuous and uniform film morphology of the doped graphene films was confirmed by SEM images as shown in Figure 6b.

Figure 7a shows optical transmittance spectra of doped and undoped graphene films at the same spray volumes. Except for an appearance of slight adsorption around 500 nm, spectrum did not change dominantly after doping. Transmittance (at 550 nm) as a function of sheet resistance of doped and undoped graphene films is summarized in Figure 7b. Owing to carrier doping from TCNQ, the sheet resistance drastically decreased by two orders of magnitude without degradation of optical transparency. To the best of our knowledge, such drastic doping effects have never been achieved until now 20. However, the estimated sheet carrier concentrations were 9.96 × 10¹⁰ and 1.17 × 10¹² cm^-2 for the undoped and doped graphenes, respectively. These estimated values are similar to the reported values by Coletti et al. 21. They modified the carrier concentration of monolayer epitaxial graphene on SiC by one order of magnitude by deposition of tetrafluoro-TCNQ. In short, the better doping effect cannot be interpreted only by accelerated charge transfer induced by radicalized TCNQ molecules in DMF solvent. Further it is necessary to consider other factors such as improvement of film stacking or percolation effect.