Fabrication of graphene films with high transparent conducting characteristics
© Ma and Zhang; licensee Springer. 2013
Received: 29 July 2013
Accepted: 13 October 2013
Published: 23 October 2013
We present a study on the transparent conducting characteristics of graphene-based films prepared by means of rapid chemical vapor deposition. The graphene films were grown on quartz slides with a CH4/Ar mixed gas under a constant flow at 950°C and then annealed at 1,000°C. It was found that the graphene films present excellent electrical conductivity with high transparency. The conductivity is up to 1,240 S/cm, the sheet resistance is lower than 1 kΩ/sq, and the transparency is well over 85% in the visible wavelength range of 400 to 800 nm, showing that the graphene films have very low resistivity and superior transparency and completely satisfy the need for transparent conductors. These properties can be used in many applications, such as transparent conductor films for touch panels.
61.48.+c, 78.67.Pt, 68.37.Hk, 68.65.Ac
A transparent conducting (TC) electrode is a key component in various optoelectronic devices, such as liquid crystal displays (LCDs), solar cells, organic solar cells, organic light-emitting diodes (OLEDs), etc. [1–4]. Indium tin oxide (ITO) is widely used as a transparent conducting electrode for these devices, but it is costly and shows poor transparency in the blue and near-infrared light ranges, instability in the presence of acids or bases, and susceptibility to ion diffusion into the substrate [5, 6]. Graphene exhibits an excellent carrier electronic mobility property [7, 8] and high transparency for visible and near-infrared spectra. Moreover, it is abundant in source and cheap in price, nontoxic, and harmless to people and environment. It can be adopted as a transparent conducting electrode in optoelectronic devices [9, 10]. For example, Wu et al. reported graphene as a TC electrode for organic LED . Also, Gan et al. and Ye et al. reported CdSe nanoribbon (NR)/graphene Schottky solar cells [12, 13].
In using graphene as a TC electrode, it is very important to deposit a large-scale uniform graphene film on Si and other substrates. Graphene has been deposited in various approaches, such as chemical vapor deposition (CVD) , metal-based epitaxy [15, 16], and other technologies [17, 18]. Recently, there have been reports on noncomposite reduction of graphene oxide (GO) into graphene using chemical routes and high-temperature annealing [19, 20]. It allows uniform and controllable deposition of reduced graphene oxide thin films with thicknesses ranging from a single monolayer to several layers over large areas. However, it causes some drawbacks, such as five- and seven-membered ring topological defects, which will bring down the electric conductivity of graphene. CVD has been successfully used to synthesize large-scale, conductive, and transparent graphene films from catalytic reactions that can be transferred onto arbitrary substrates [9, 11]. For example, large-area graphene or few-layer graphene films on metal substrates such as Ni and Cu by CVD technology [21, 22] have been reported. Since the graphene film is commonly placed on SiO2 and other transparent insulators in fabricating optoelectronic device architectures, graphene films on Ni or Cu must be transferred to SiO2 and other transparent insulator substrates, which may perplex the preparation process and technique of devices. In this work, the objective of our research was to fabricate large-area graphene films on SiO2 substrates and investigate their conductivity and transparency. Graphene on SiO2 can be easily used to make optoelectronic devices and freely transferred to other substrates by etching the SiO2 layer using HF. It is especially interesting for the purpose of constructing electrodes. Herein, we describe a simple and reproducible method to uniformly deposit a few layers of graphene films grown by CVD. We investigated the influence of deposition time and thickness on the transparent conducting characteristics: conductivity, sheet resistance, and transparency, of graphene films. It was found that the deposited large-scale, conductive, and highly transparent graphene films are suitable for use as constructing electrodes.
The graphene films were fabricated on quartz crystalline slides by a rapid CVD process. The growth system was composed of a large horizontal quartz tube furnace, a vacuum system, a gas meter, and an automatic temperature controller. Quartz crystalline substrates with a size of 15 × 15 × 2 mm3 were cleaned ultrasonically with a sequence of acetone, ethanol, and deionized water, and then they were blown with N2 to dry them and placed at the center of the furnace. Prior to deposition, the furnace was pumped to 10-2 Pa and heated to 300°C for 10 min to remove any water moisture. High-purity CH4 gas (99.999%) and Ar gas with a volume ratio of 1:10 were introduced into the reactive chamber at the same temperature (950°C). In the graphene deposition process, CH4 was initially decomposed to give a mixture of C and H2, and the C atoms were condensed on the quartz substrates to form graphene films while the working pressure was kept at 50 Pa. The growth process was carried out for 1 ~ 5 min, and then the samples were annealed at 1,000°C for 20 min. Finally, when the system had cooled down to room temperature, the samples were removed.
The morphology and structure of the samples were characterized by atomic force microscopy (AFM). The structure was analyzed by Raman spectroscopy, and the optical transparency was investigated by UV–vis spectroscopy (Shimadzu UV-3600, Kyoto, Japan). Finally, the conducting characteristics of the graphene films were evaluated by Hall effect measurement (HMS-3000, Ecopia, Anyang, South Korea).
Results and discussion
The graphene sample deposited for 5 min has a high transparency of over 85% in the visible wavelength range of 400 to 800 nm and a sheet resistance of 103 Ω/sq. These properties are much superior to those of GO films as transparent conductors. The high performance is attributed to the CVD technique that produced compact, large-area, uniform, and high-purity graphene films.
The transparent conducting properties of graphene films with different thicknesses were investigated. Ultrathin graphene films were deposited on quartz substrates by controlling a very low reactive flow rate and pressure of CH4 in the CVD technique. The transmission rate of the graphene films decreases with the thickness of the film, which is over 85% for the film of about 5 to 7 nm. The mobility and conductivity were found to rapidly increase up to their saturation values with the thickness of the film. The sheet resistance rapidly drops from 105 to 103 Ω/sq as the film thickness increases from 2 to 7 nm. The largest conductivity is up to 1,240 S/cm and the minimum sheet resistance is about 103 Ω/sq, showing that the graphene films have very low resistivity and completely satisfy the need for transparent conducting films.
This work was supported in part by the National Natural Science Foundation of China (no. 60976071) and the Scientific Project Program of Suzhou City (no. SYG201121).
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