Radical-assisted chemical doping for chemically derived graphene
© Ishikawa et al.; licensee Springer. 2013
Received: 12 November 2013
Accepted: 7 December 2013
Published: 19 December 2013
Carrier doping of graphene is one of the most challenging issues that needs to be solved to enable its use in various applications. We developed a carrier doping method using radical-assisted conjugated organic molecules in the liquid phase and demonstrated all-wet fabrication process of doped graphene films without any vacuum process. Charge transfer interaction between graphene and dopant molecules was directly investigated by spectroscopic studies. The resistivity of the doped graphene films was drastically decreased by two orders of magnitude. The resistivity was improved by not only carrier doping but the improvement in adhesion of doped graphene flakes. First-principles calculation supported the model of our doping mechanism.
Graphene, a single atomic layer of sp2 graphitic carbon, has received a lot of attention because of its attractive electromechanical properties and its potential applications for the ‘next-generation’ electronic devices [1–5]. Although mechanically cleaved graphene exhibits excellent electrical performance, such as a highest carrier mobility of over 200,000 cm2 · V-1 · s-1. The rate of production when using this mechanical exfoliation method is extremely limited. Therefore, there has been considerable impetus to discover a scalable production technique. Among the possible candidates, a chemical exfoliation method based on a liquid process is considered to now be well established. One of the greatest advantages of the chemical exfoliation method is that chemically derived graphene can be deposited or formed into films on any large-area substrate [7, 8]. Ease of modification and/or functionalization of the graphene are also reasons why the chemical method is widely accepted [9, 10]. Furthermore, it has been focused on as a new tunable platform for optical and other applications [11–14].
Carrier doping is a common approach to tailoring the electronic properties of semiconductor materials. Carrier doping can also dramatically alter the electrical properties of graphene. Although several techniques aimed at the carrier doping of graphene have been demonstrated, including boron- or nitrogen-substitutional doping [15, 16], the deposition of alkali metal atoms , and the adsorption of gaseous NO2, these doping methods have never achieved significant doping effects due to defect formation, inhomogeneous deposition, and the instability of gaseous species, respectively. Molecular doping, such as halide [19, 20] or polymer [21, 22], is a promising technique for pristine graphene films. However, effective doping method for chemically derived graphene has never been demonstrated. Tetracyanoquinodimethane (TCNQ) is well known as a powerful electron acceptor, and is expected to favor electron transfer from graphene into TCNQ molecules, thereby leading to p-type doping of graphene [23, 24]. Conjugated organic molecules such as these have been widely used in organic light-emitting diodes to improve device performance by controlling the hole injection barrier . Efficient doping of organic semiconductors, of carbon nanotubes, and of graphene has been demonstrated.
We demonstrate herein a novel carrier doping method for chemically derived graphene using radical-assisted conjugated organic molecules in the liquid phase. It is expected that liquid-phase chemical interactions between graphene and conjugated organic molecules induce high doping efficiency. Absorbance measurements provide direct evidence for charge-transfer (CT) interactions between graphene and radicalized TCNQ molecules in an organic solvent. Raman spectroscopy and ultraviolet photoelectron spectroscopy (UPS) have also been used to elucidate the effects of doping on doped graphene films, which showed improvements in resistivity of two orders of magnitude with highly stable doping effect. Previous attempts at carrier doping for chemically derived graphene have never decreased the resistivity by more than one order of magnitude . The doping mechanism of the chemical doping is investigated using first-principles calculation based on density functional theory. Our doping method is compatible with the wet production technique of chemical-exfoliated graphene. The doped graphene films can be formed by the all-wet process via the radical-assisted chemical doping method as demonstrated in this work.
Preparation and reduction of graphene oxide
Chemically derived graphene was synthesized using a modified version of Hummer's method, a well-known approach to producing monolayered graphene via the liquid-phase exfoliation of graphite oxide, as described previously in the literature . Natural graphite powder was donated by SEC Carbon Ltd. (Tokyo, Japan). All other chemicals were purchased from Kanto chemical Co. Ltd. (Sakado, Japan) and used directly without further purification. 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 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.
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. The mixed solution was reacted in a water bath at 95°C 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.
Doping and film fabrication
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. 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. 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 Raman spectroscopy was measured with a Jasco NRS-1000 (excited by a 532-nm green laser; Easton, MD, USA). Absorbance and transmittance spectra were obtained with Shimadzu SolidSpec3700 UV–vis by using a quartz cell for absorbance measurements. The sheet resistance was measured by van der Pauw method at room temperature in air. The presence of monolayered GO flakes in our synthesized GO aqueous solution was verified by atomic force microscope images by Raman peak shifts and by the peak shape of the second-order two-phonons process peak at 2,700 cm-1, referred to as the 2D band. The size of the flakes is up to 50 × 50 μm2. After liquid phase reduction by N2H4 and NH3, the solvent of the RGO aqueous solution was replaced by DMF using an evaporator. RGO can be dispersed well in many kinds of organic solvents including DMF, while it is easy to aggregate in aqueous solution due to its low electrostatic repulsion force. The conductivity and the Hall carrier mobility of individual monolayered RGO flakes were as high as 308 S · cm-1 and 121 cm2 · V-1 · s-1, respectively. Hall measurements were conducted in air at room temperature using Hall-cross geometry and Au/Ti electrodes.
The electronic structural analysis is carried out using the SIESTA3.1 code, which performs fully self-consistent calculations solving the Kohn-Sham equations . The Kohn-Sham orbitals are expanded using linear combinations of pseudo-atomics orbitals. The double-zeta polarized (DZP) basis set was chosen in this study. The calculations were done with the local density approximation (LDA), using the Ceperley-Alder correlation as parameterized by Perdew and Zunger . The electron-ion interaction was treated by using norm-conserving, fully separable pseudo-potentials . A cutoff of 200 Ry for the grid integration was utilized to represent the charge density. Two TCNQ molecules on and under the (4 × 4), (6 × 6), or (8 × 8) graphene supercell units were simulated for full relaxation of the systems. The Brilliouin zone was sampled by 20 × 20 × 1 k-points using the Monkhorst-Pack scheme for electronic properties calculations. It is necessary to ensure that the z axis of the periodic supercell (normal to the graphene surface) is large enough so that there is negligible interaction between the two graphene sheets. A distance of 170 Å along the z axis is found to be sufficient to ensure the energy convergence for configurations.
Results and discussion
Summary of calculation results for TCNQ/graphene charge transfer systems
4 × 4
6 × 6
8 × 8
4 × 4 both
6 × 6 both
8 × 8 both
Change transfer (e/molecule)
Sheet carrier conc. (1013 cm-2)
Absorption energy (kcal mol-1)
We developed a novel method for the carrier doping of graphene using radical-assisted conjugated organic molecules in the liquid phase. The absorbance data and the Raman spectra results indicated strong charge transfer interactions between RGO and TCNQ. The high doping efficiency of our method was demonstrated as an improvement in sheet resistance by two orders of magnitude, without degradation of the optical transparency. First-principles calculation predicted the model of our doping mechanism and the origin of high doping efficiency. Furthermore, the doping effect was quite chemically stable. The doped chemically derived graphene films fabricated by all-wet process have huge potential as an alternative material for transparent conductive films in low-cost and low-temperature processes.
This work 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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