Materials Letters 64 (2010) 765–767 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / m a t l e t Optoelectronic properties of graphene thin films prepared by thermal reduction of graphene oxide Tran Viet Cuong a, Viet Hung Pham a, Quang Trung Tran b, Jin Suk Chung a, Eun Woo Shin a, Jae Seong Kim a, Eui Jung Kim a,⁎ a b Department of Chemical Engineering, University of Ulsan, Ulsan 680-749, South Korea Department of Solid State Physics, Faculty of Physics, Ho Chi Minh City University of Natural Sciences, 227 Nguyen Van Cu St., Dist 5, Ho Chi Minh City, Vietnam a r t i c l e i n f o Article history: Received 20 August 2009 Accepted January 2010 Available online 13 January 2010 Keywords: Graphene oxide Thin films Luminescence Optical materials and properties a b s t r a c t Graphene thin films have been prepared by thermal reduction of graphene oxide Raising the reduction temperature results in a red-shift of the G peak in Raman spectra The reduction temperature turns out to strongly affect the morphology of the prepared graphene film Photoluminescence (PL) results show that the band gap of graphene can be tuned by varying the reduction temperature The thermal reduction process has been optimized in an effort to minimize the formation of wrinkles/folds on the graphene surface leading to enhanced PL and Raman peak intensities and reduced electrical sheet resistance © 2010 Elsevier B.V All rights reserved Introduction Graphene has increasingly attracted attention owing to its fascinating physical properties including quantum electronic transport, tunable band gap, extremely high mobility, high elasticity, and electromechanical modulation [1] These unique properties hold great promise for potential applications in many technological fields such as nanoelectronics, sensors, nanocomposites, batteries, supercapacitors and hydrogen storage [2] In order to turn graphene applications into reality, one must fabricate the material in large-scale In the beginning, isolated graphene was prepared by the micromechanical cleavage of graphite crystals However, the low productivity of this method makes it unsuitable for large-scale use Recently, an alternative method for creating single graphene sheets starting from graphene oxide (GO) which demonstrated the possibility of low-cost synthesis and the fabrication of large-scale transparent films has been suggested Graphite can be oxidized to produce GO and then exfoliated to create stable aqueous dispersions of individual sheets After deposition, GO may be reduced to graphene either chemically or thermally [3] Although there are a number of reports on the synthesis of graphene films, very few studies have been done to systematically explore the effect of fabrication condition on the electrical and optical properties of graphene To the best of our knowledge, the photoluminescence (PL) spectra of graphene films have not been reported yet In this letter, we investigate the optoelectronic properties of graphene thin films prepared by thermal reduction of GO Especially, ⁎ Corresponding author Tel.: + 82 52 259 2832; fax: + 82 52 259 1689 E-mail address: ejkim@ulsan.ac.kr (E.J Kim) 0167-577X/$ – see front matter © 2010 Elsevier B.V All rights reserved doi:10.1016/j.matlet.2010.01.009 PL measurements were carried out to analyze the optical property of the films Experimental The graphene thin films investigated in this work have been obtained by thermal reduction of GO in a quartz tube To study the optoelectrical properties of the graphene film, the quartz tube was heated to temperatures ranging from 600 °C to 800 °C at 10− Torr for h The GO films were prepared by spin-coating GO paste-like dispersion on quartz substrate at 4000 rpm for 15 s The GO paste-like dispersion was produced via a modified Hummers method from expanded graphite which was prepared by the microwave-assisted thermal expansion of graphite (Grade 1721, Asbury Carbon) The surface morphology of the films was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) The structure and bonding configurations in the films were examined using a Laser Raman Spectrometer SPEX 1403 with a He–Ne laser at an excitation wavelength of 632.8 nm PL measurement was performed employing a SpectraPro-300i monochromater (Acton) The samples were excited by using a 325 nm He–Cd laser with an output of 10 mW power at room temperature The sheet resistance was determined by fitting current–voltage (I–V) curves obtained from two probes station using Keithley 4200 semiconductor character system (MS Tech) Results and discussion The SEM and AFM images of the graphene thin films prepared by thermal reduction of GO at three different temperatures are shown in 766 T.V Cuong et al / Materials Letters 64 (2010) 765–767 Fig The SEM images exhibit that the prepared graphene films are uniform with slight wrinkles/folds on their surface The thermal reduction process may reduce the contact area of GO with the quartz substrate Consequently, the graphene sheets tend to aggregate due to the attractive force between layers and an overall decrease in hydrophilicity, thus resulting in the formation of wrinkles/folds One can also clearly see graphene wrinkles/folds in AFM images The surface roughness is found to depend largely on the reduction temperature and the smooth surface is achieved for the sample reduced at 700 °C These results imply that the reduction temperature has a strong effect on the surface features Accordingly, the thermal reduction process has been optimized to obtain graphene thin films with a mirror-like surface Fig 2(a) shows the Raman spectra of GO and graphene thin films prepared by thermal reduction of GO at three different temperatures Two major peaks are observed referred to as D and G Two minor peaks named 2D located at ∼ 2655 cm− and S3 near 2906 cm− are also observed in the as-prepared GO sample and vanish in the postreduced samples This implies that the graphene films prepared by thermal reduction of GO have considerable defects In addition, the G band of the samples is red-shifted from 1594 cm− to 1572 cm− when the reduction temperature increases from 600 °C to 800 °C, which is consistent with the results reported by other groups [4,5] These results indicate that a red-shift of the G peak results from heat treatment during the thermal reduction process Raising the reduction temperature results in a red-shift of the G peak Initially, there are small sp2 clusters in the GO separated by an amorphous and highly disordered sp3 matrix, which forms a high tunnel barrier between the clusters During the heat treatment, the thermal energy facilitates clustering of sp2 phase forming connection between ordered rings and phase transition from amorphous to the two-dimensional nanocrystalline graphene This transformation results in a red-shift and broadening of the G band Note in Fig 2(a) that the sample reduced at 700 °C exhibits the maximum peak intensity The Raman peak intensity is found to correlate well with the surface roughness Until now, very little is known about the PL of graphene thin film, which is an important property for application in optoelectronic devices Fig 2(b) represents the PL spectra of graphene thin films prepared by thermal reduction of GO at three different temperatures It is somewhat surprising to note that the PL spectra for the thermally reduced GO films are quite similar to what is observed from nanostructured amorphous carbon For instance, in Fig 2(b), three broad visible peaks located at 512, 572, and 627 nm are bluer than previous reports [6,7] It is well known that the PL in amorphous carbon occurs due to the radiative recombination of electrons and holes in the band-tail states created by sp2 rich clusters However, the sp2 clusters in the thermally reduced GO films are small in size compared with highly disordered sp3 matrix, which creates a very Fig SEM and AFM images of graphene thin lms (2 àm ì àm) prepared by thermal reduction of GO at three different temperatures: (a)–(b) 600 °C, (c)–(d) 700 °C, and (e)–(f) 800 °C T.V Cuong et al / Materials Letters 64 (2010) 765–767 767 Fig Current–voltage curves of graphene thin films prepared by thermal reduction of GO sheet resistances of our graphene films prepared by thermal reduction of GO at 600 °C, 700 °C and 800 °C are found to be 14 kΩ, 5.7 kΩ and 7.8 kΩ, respectively, which are determined by a linear fitting of I–V curves Therefore, the reduction temperature has also a significant effect on the electrical property of the graphene film The sheet resistance becomes a minimum at 700 °C where the graphene film has smooth surface morphology A relatively high sheet resistance of our samples may result from extensive modification of the 2-dimensional crystal lattice in the preparation of the solution-based graphene film, which usually degrades electrical and thermal conductivities [8] Conclusions Fig (a) Raman spectra of GO and graphene thin films prepared by thermal reduction of GO at different temperatures, and the inset on the top right corner indicating the variation of G peak position with reduction temperature, (b) room temperature PL spectra of grapheme thin films prepared by thermal reduction of GO strong fluctuation in the local band gap Thus, sp2 clusters with a narrower gap are embedded in a sp3 matrix which acts as a tunnel barrier between them and causes a blue-shift in the PL spectra The sample reduced at 700 °C shows a stronger PL peak intensity with a broader full width at half-maximum (FWHM) than the samples reduced at 600 °C and 800 °C This broad and strong intensity at 700 °C may be related to the mirror-like surface morphology In fact, the films reduced at 600 °C and 800 °C have rougher surface and larger clustering than the film reduced at 700 °C as confirmed in SEM, AFM images, and Raman spectra Although the mechanism of light emission observed in the graphene film remains to be explored, a significant finding in this study indicates that thermal reduction of GO might result in strained, disordered graphene film, which enables us to potentially tune the band gap of graphene I–V curves of graphene thin films prepared by thermal reduction of GO at different reduction temperatures are depicted in Fig The In summary, GO thin film was thermally reduced to form graphene thin film Under heat treatment, the G peak in Raman spectra was redshifted and higher reduction temperature caused a more red-shift of the G peak The SEM and AFM results indicated that the reduction temperature has a strong effect on the surface morphology via the formation of wrinkles/folds The maximum PL and Raman peak intensities and the minimum electrical sheet resistance were achieved for the sample reduced at 700 °C The thermal reduction process was optimized to reduce the formation of wrinkles/folds which enables us to tune the band gap and improve the optoelectronic properties of graphene thin films Acknowledgement This work was supported by the EXCEED program of the University of Ulsan References [1] Ohta T, Bostwick A, Seyller T, Horn K, Rotenberg E Science 2006;313:951–4 [2] Geim AK, Novoselov KS Nat Mater 2007;6:183–91 [3] Stankovich S, Dikin DA, Piner RD, Kohlhaas KA, Kleinhammes A, Jia Y, et al Carbon 2007;45:1558–65 [4] Tung VC, Allen MJ, Yang Y, Kaner RB Nat Nano 2009;4:25–9 [5] Kudin KN, Ozbas B, Schniepp HC, Prud'homme RK, Aksay IA, Car R Nano Lett 2007;8:36–41 [6] Henley SJ, Carey JD, Silva SRP Appl Phys Lett 2004;85:6236–8 [7] Robertson J Diamond Relat Mater 1996;5:457–60 [8] Allen MJ, Tung VC, Kaner RB Chem Rev 2010;110:132–45  ... al / Materials Letters 64 (2010) 765–767 767 Fig Current–voltage curves of graphene thin films prepared by thermal reduction of GO sheet resistances of our graphene films prepared by thermal reduction. .. spectra of GO and graphene thin films prepared by thermal reduction of GO at different temperatures, and the inset on the top right corner indicating the variation of G peak position with reduction. .. represents the PL spectra of graphene thin films prepared by thermal reduction of GO at three different temperatures It is somewhat surprising to note that the PL spectra for the thermally reduced GO