Doping graphene films via chemically mediated charge transfer
© Ishikawa et al; licensee Springer. 2011
Received: 6 September 2010
Accepted: 31 January 2011
Published: 31 January 2011
Transparent conductive films (TCFs) are critical components of a myriad of technologies including flat panel displays, light-emitting diodes, and solar cells. Graphene-based TCFs have attracted a lot of attention because of their high electrical conductivity, transparency, and low cost. Carrier doping of graphene would potentially improve the properties of graphene-based TCFs for practical industrial applications. However, controlling the carrier type and concentration of dopants in graphene films is challenging, especially for the synthesis of p-type films. In this article, a new method for doping graphene using the conjugated organic molecule, tetracyanoquinodimethane (TCNQ), is described. Notably, TCNQ is well known as a powerful electron accepter and is expected to favor electron transfer from graphene into TCNQ molecules, thereby leading to p-type doping of graphene films. Small amounts of TCNQ drastically improved the resistivity without degradation of optical transparency. Our carrier doping method based on charge transfer has a huge potential for graphene-based TCFs.
Transparent conductive films (TCFs) are a class of extremely important components of modern technology for applications such as optical devices and solar energy utilization . Indium tin oxide (ITO) is the most widely used material as TCFs; however, the high cost and the limited supply of indium, a rare-earth metal, have become a serious concern. Thus, alternative materials with high transparency and low electrical sheet resistance comparable to ITO are required. During the last decade, a number of materials, such as conducting polymer films  or nanostructured thin films  have been proposed as alternatives to ITO. Recently, carbon nanotubes have also shown high potential as the replacement material of ITO; however, their cost performance remains an issue .
Meanwhile, graphene, a single atomic layer of carbon, has attracted greater attention as an alternative material of TCFs because of its high electrical conductivity and transparency . In addition to its superb properties, graphene-based TCFs could also be cost-competitive if produced via a chemical production method. Therefore, we focused on developing an inexpensive chemical fabrication procedure in liquid phase without any vacuum systems.
The problem of high resistivity of chemically derived graphene-based TCFs  still remains to be resolved. Up to now, several types of carrier doping of graphene have been demonstrated including boron- or nitrogen-substitutional doping [7, 8], deposition of alkali metal atoms , adsorption of gaseous NO2 , and charge transfer from conjugated organic molecules [11, 12]. However, controlling the carrier type and concentration of dopants in graphene films is challenging, especially for fabrication of p-type films. With a view to improving the electrical properties of graphene-based TCFs, we propose a novel carrier doping method based on charge transfer from conjugated organic molecules. It is anticipated that liquid phase chemical interaction between graphene and conjugated organic molecules induces a high doping efficiency.
Synthesis of graphene
Chemically derived graphene was synthesized by the modified Hummer's method , 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) , 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 . 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).
Fabrication of graphene films
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 films
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 . 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.
Results and discussion
Characterization of GO and graphene
Relative ratio of all components for each sample
O = C-OH (%)
Doping graphene films
The authors developed a new and inexpensive fabrication method of chemically derived graphene-based TCFs and demonstrated a huge potential of doping effect via charge transfer by conjugated organic molecules. All of the fabrication steps including the reduction of GO and carrier doping were carried out in liquid phase. Therefore, this novel method proposed in this study does not require any vacuum system and is suitable for quantity synthesis. Furthermore, chemically derived graphene combined with the above doping technique could be a potential alternative to conventional transparent conductive materials.
indium tin oxide
reduced graphene oxide
transparent conductive films
This study was conducted as part of the Tokyo Tech Global COE Program on Evolving Education and Research Center for Spatio-Temporal Biological Network based on a grant from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. The natural graphite powder used in this study was donated by SEC Carbon Ltd.
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