NANO LETTERS
High-Quality Graphenes via a Facile Quenching Method for Field-Effect Transistors
2009 Vol. 9, No. 4 1374-1377
Y. B. Tang,† C. S. Lee,*,† Z. H. Chen,† G. D. Yuan,† Z. H. Kang,‡ L. B. Luo,† H. S. Song,† Y. Liu,†,‡ Z. B. He,† W. J. Zhang,† I. Bello,† and S. T. Lee*,† Center of Super-Diamond and AdVanced Films (COSDAF) and Department of Physics and Materials Science, City UniVersity of Hong Kong, Hong Kong SAR, China, and Functional Nano & Soft Materials Laboratory (FUNSOM), Soochow UniVersity, Suzhou, Jiangsu 215123, China Received October 6, 2008; Revised Manuscript Received January 21, 2009
ABSTRACT Single- and few-layer graphene sheets with sizes up to 0.1 mm were fabricated by simply quenching hot graphite in an ammonium hydrogen carbonate aqueous solution. The identity and thickness of graphene sheets were characterized with transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. In addition to its simplicity and scalability, the present synthesis can produce graphene sheets with excellent qualities in terms of sizes, purity, and crystal quality. The as-produced graphene sheets can be easily transferred to solid substrates for further processing. Field-effect transistors based on individual graphenes were fabricated and shown to have high ambipolar carrier mobilities.
Graphene, a two-dimensional (2D) honeycomb lattice of sp2bonded carbon atoms, has attracted much attention due to its peculiar properties.1-5 Charge carriers in graphene were found to obey a linear dispersion relation near the Fermi energy and behave as massless Dirac fermions,1-4 thus giving rise to many extraordinary properties such as the quantum Hall effect3,4 and ambipolar electric field effect.1 In addition, graphene exhibits nondispersive transport characteristics and electron mobility as high as ∼15000 cm2/(V s) at room temperature,1-3 which is much higher than most conventional semiconductor materials. Recently, much progress has been achieved in the fabrication of multi- or monolayer graphene sheets into nanoscale electronic and photonic devices including field-effect transistors,1,6-10 electromechanical resonators,11 and ultrasensitive sensors.12 Also, graphenes have been used as transparent electrodes in device applications taking advantage of their high conductivity, excellent transparency, and high chemical and thermal stability.13-15 To date, graphene sheets (monolayer or few layer) have been prepared by various methods, such as mechanical cleavage of graphite using adhesive tape,1-3 thermal graphitization of silicon carbide,5,16 chemical reduction of exfoliated graphite oxide,17,18 bottom-up organic synthesis,15 epitaxial growth of graphene on metal substrates,19 exfoliation of * Corresponding authors,
[email protected] and
[email protected]. † City University of Hong Kong. ‡ Soochow University. 10.1021/nl803025e CCC: $40.75 Published on Web 03/20/2009
2009 American Chemical Society
sulfuric acid-intercalated graphite in organic solvents,20,21 plasma-enhanced chemical vapor deposition,22 and so on. While the mechanical cleaving “Scotch tape method” is a simple way to produce few-layer graphene sheets,23 residues of adhesive tape25 and dispersion solvents, such as acetone and propanol,1 are inevitably left on the surfaces of the substrates and graphene sheets after drying,24,25 which may modify the intrinsic properties of the graphenes. Recently, Dai et al.26 have produced in organic solvents high-quality single-layer graphenes, which exhibit high electrical conductance. Their method consists of seven steps and needs about 4 days to allow the oleum molecules and organic solvents to fully insert into the graphitic layers.26 Here, we report a simple, fast (needing only 10 min), and low-cost process for the synthesis of high-quality graphene sheets without adhesive tape or organic solvents. In a typical process, a piece of highly ordered pyrolytic graphite (HOPG) was repeatedly bent to introduce tiny mechanical cracks. The stressed HOPG was rapidly heated to 1000 °C within 5 min and quickly quenched to room temperature in a bath of cool water containing 1.0 wt % ammonium hydrogen carbonate (NH4HCO3). During quenching the hot HOPG would decompose the NH4HCO3 and vaporize a large amount of water. After quenching, grayish supernatant was formed on the water surface (Figure S1a in Supporting Information) and scooped with a solid substrate, such as a SiO2-coated Si wafer, for further characterization or processing. In addition
Figure 2. (a) Raman spectra of monolayer graphene sheet and bulk graphite. Inset shows optical image of the monolayer graphene used for Raman study. (b) Zoom-in comparison of the 2D band of the monolayer graphene and bulk graphite.
Figure 1. (a) A tapping mode AFM image of a typical graphene sheet. (b) Height profile along the white line cut in (a), showing the thickness of the graphene is about 0.5 nm. (c and d) HRTEM images of monolayer and bilayer graphene.
to graphene sheets the supernatant also contained bulk graphite and small graphite flakes. However, the graphite materials have much weaker bonding to the SiO2/Si substrate than do the graphene sheets. As a result, rinsing of the sample with plenty of water was effective in removing the bulk graphite, small graphite flakes, and the NH4HCO3 vesicant, which is highly water soluble. Tapping-mode atomic force microscopy (AFM) measurements were preformed to measure the thickness of graphene; note that the minimal height at the edge of the graphene sheet was taken as the thickness. The Raman spectra were recorded at room temperature using an argon-ion laser of 633 nm and power of
