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Temperature-dependent photoluminescence from chemically and thermally reduced graphene oxide Tran Viet Cuong, Viet Hung Pham, Eun Woo Shin, Jin Suk Chung, Seung Hyun Hur, Eui Jung Kim, Quang Trung Tran, Hoang Hung Nguyen, and Paul A Kohl Citation: Applied Physics Letters 99, 041905 (2011); doi: 10.1063/1.3616142 View online: http://dx.doi.org/10.1063/1.3616142 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/99/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Photocatalytic water splitting to hydrogen production of reduced graphene oxide/SiC under visible light Appl Phys Lett 102, 083101 (2013); 10.1063/1.4792695 Graphene hydrogenation by molecular hydrogen in the process of graphene oxide thermal reduction Appl Phys Lett 102, 071910 (2013); 10.1063/1.4793484 Raman studies of chemically and thermally reduced graphene oxide AIP Conf Proc 1512, 1262 (2013); 10.1063/1.4791511 Oxygen density dependent band gap of reduced graphene oxide J Appl Phys 111, 054317 (2012); 10.1063/1.3694665 Photoluminescence and band gap modulation in graphene oxide Appl Phys Lett 94, 111909 (2009); 10.1063/1.3098358 This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to IP: 169.230.243.252 On: Mon, 29 Sep 2014 06:11:34 APPLIED PHYSICS LETTERS 99, 041905 (2011) Temperature-dependent photoluminescence from chemically and thermally reduced graphene oxide Tran Viet Cuong,1 Viet Hung Pham,1 Eun Woo Shin,1 Jin Suk Chung,1 Seung Hyun Hur,1 Eui Jung Kim,1,a) Quang Trung Tran,2 Hoang Hung Nguyen,2 and Paul A Kohl3,b) School of Chemical Engineering and Bioengineering, University of Ulsan, Daehak-ro 102, Nam-gu, 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 School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 303320100, USA (Received 17 June 2011; accepted July 2011; published online 27 July 2011) Temperature-dependent photoluminescence (PL) of graphene oxide (GO) reduced with hydrazine and heat has been measured to investigate the effect of reduction type on the bandgap of the reduced GO Nitrogen functionalities formed in the hydrazine-treated GO were responsible for a strong localization of carriers that caused in a fluctuation in PL peak position with temperature The intensity of C-OH peak was relatively low in the heat-treated GO, indicating that raising temperature facilitated the removal of hydroxyl groups, resulting in larger sp2 domain size and C 2011 American Institute of Physics [doi:10.1063/1.3616142] smaller bandgap energy V Although graphene with two-dimensional sheet of sp2-hybridized carbon becomes promising material, graphene-based applications are still limited since defect-free graphene exhibits zero bandgap.1 Recently, many attempts have been made to open the bandgap of graphene For example, Wang and co-workers reported photoluminescence (PL) from graphene fluoride.2 Nourbakhsh et al.3 opened the bandgap of graphene by using oxygen plasma-treatment, and Loh et al.4 explored PL from chemically modified graphene such as graphene oxide (GO) or reduced GO whose electronic structure was modified GO reduction is considered as an efficient method because this approach is a simple way to vary the size, shape, fraction of the sp2 domains, its edge structure, and chemical modification of its interior or edge, which enables us to modulate the bandgap of graphene In addition, this method is often based on the solution process that is inexpensive, facile, scalable, and high yield for massproduction Despite the above advantages of GO reduction to open the optical gap of graphene, the effect of reduction type on the optical bandgap of the reduced GO is not still clearly understood It is believed that a better understanding of the reduction mechanism helps open up an opportunity for tailoring the bandgap of graphene In this work, temperature-dependent PL was measured from GO reduced by two different processes: hydrazine and thermal treatment Temperature-dependent PL spectra can be effectively used to elucidate the emission origin.5 X-ray photoelectron spectroscopy (XPS) study was also performed to investigate the effect of reduction type on the chemical structure of the reduced GO Thermally reduced graphene (TRG) film was prepared by reducing GO film in a quartz tube at 700 C and 10À6 Torr for h The GO film was obtained by spin-coating GO a) Author to whom correspondence should be addressed Electronic mail: ejkim@ulsan.ac.kr b) Electronic mail: paul.kohl@chbe.gatech.edu 0003-6951/2011/99(4)/041905/3/$30.00 dispersion on a quartz substrate at 4000 rpm for 15 s The GO dispersion was synthesized via a modified Hummers method from expanded graphite which was prepared by microwave-assisted thermal expansion of graphite (Grade 1721, Asbury Carbon).6 Chemically reduced graphene (CRG) film was prepared as follows: the GO dispersion was reduced with excess hydrazine monohydrate at room temperature to form graphene dispersion The use of excess hydrazine makes the dispersion basic to provide a high charge density on the resulting graphene sheets After h reduction, the graphene dispersion was diluted 75 times with water/ ethanol (80:20 v/v) and then homogenized by ultrasonication for The diluted solution was finally sprayed on a preheated quartz substrate to obtain the CRG thin film using an airbrush system with N2 as a carrier gas The prepared films were characterized using an XPS Thermal Scientific Instrument with a monochromatic AlKa radiation (hk ¼ 1486.6 eV) The PL spectra was taken from 10 K to 300 K on a SpectraPro-300i Monochromator (Acton) using He-Cd laser source with a wavelength of 325 nm and an output of 10 MW power Figure shows the temperature-dependent PL spectra of CRG and TRG films The PL spectra of the reduced GO thin films are quite similar in shape to what has been observed from nanostructured amorphous carbon and GO suspensions,7,8 which are attributed to the recombination of electron-hole pairs localized within small sp2 carbon clusters embedded within a sp3 matrix and to the presence of oxygen-containing functional groups, respectively It is worthwhile to note that the peak position of the CRG sample is blue-shifted with fluctuation as the temperature increases from 10 K to 300 K, while that of the TRG sample remains almost unchanged This result indicates that a carrier localization effect in sp2 clusters plays a dominant role in emission peak shift over temperature-induced bandgap shrinkage effect that causes a red-shift of the PL peak It appears that the carriers generated in the TRG sample are not 99, 041905-1 C 2011 American Institute of Physics V This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to IP: 169.230.243.252 On: Mon, 29 Sep 2014 06:11:34 041905-2 Cuong et al Appl Phys Lett 99, 041905 (2011) FIG (Color online) PL spectra taken at temperatures from 10 K to 300 K for (a) CRG and (b) TRG thermalized, possibly due to a strong carrier localization effect Our interesting observation suggests that the optical emission from the reduced GO may result from the recombination of electron-hole pairs in localized electronic states originating from various structure-related defects, rather than from band-edge transitions as is the case in typical semiconductor materials In addition, at room temperature (300 K), the peak of the CRG film is located at a lower wavelength than that of the TRG film A possible explanation would be that TRG has a larger sp2 domain than CRG, resulting in a smaller bandgap due to a relatively weak carrier confinement The PL results in Fig reveal that the shape and shift of PL emission peak depend largely on reduction type Accordingly, the bandgap of the reduced GO can be tuned by varying sp2 domain size or by introducing various functional groups The model structures of GO, CRG, and TRG are skematically illustrated in Fig 2, which is based on the theoretical work by Gao et al.9 It is known that there exist four kinds of oxygen functionalities in GO (Lerf-Klinowski model): epoxide (-O-) and hydroxyl (-OH) located in the basal plane of GO are the major components and carbonyl (-C¼O) and carboxyl (-COOH) distributed at the edges of GO are minor A noticeable difference between hydrazine and thermal reductions are as follows: In the case of hydrazine reduction, nitrogen functionalities are formed such as aminoaziridine and hydrazone, which may be responsible for a strong localization of carriers due to the incorporation of nitrogen In the case of thermal reduction at a high temperature of 700 C, hydroxyl groups attached to the interior and edge of an aromatic domain are almost eliminated from GO Thus, the TRG film reduced at high temperature may have larger graphitic crystallite size, i.e., larger sp2 domain size, than the CRG film The ratio of the D peak intensity to the G peak intensity (ID/IG) in the Raman spectra is known to vary inversely with in-plane graphitic crystallite size (La) which is given as La ¼ 4.4 (IG/ID).10,11 Our previous work revealed that the ID/IG ratio of TRG (1.41) was smaller than that of CRG (1.48),12 indicating that the sp2 domain size of TRG (3.12 nm) is larger than that of CRG (2.97 nm) From the PL spectra taken at room temperature in Fig 1, the optical gap of the TRG sample (1.99 eV) was found to be lower than that of the CRG sample (2.23 eV) Our results confirm that a larger sp2 domain results in a smaller optical gap An XPS analysis below was performed to verify the PL and Raman interpretations FIG (Color online) Schematic illustration of the structures of GO, CRG, and TRG This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to IP: 169.230.243.252 On: Mon, 29 Sep 2014 06:11:34 041905-3 Cuong et al FIG (Color online) XPS spectra of GO, TRG, and CRG: (a) C1s peaks and (b) O1s peaks Figure exhibits the C1s and O1s XPS spectra of GO, TRG, and CRG The C1s XPS spectrum of GO consists of three separated peaks related to oxygen functionalities such as hydroxyl, epoxide, and carbonyl After thermal or hydrazine reduction, the oxygen functionalities attached to the interior of an aromatic domain in GO are significantly removed, which is indicated by a dramatic decrease in peak intensity A peak at 285.9 eV corresponding to carbon-nitrogen bond appeared in the C1s spectrum of the CRG sample The appearance of this peak originates from the conversion of epoxide into aminoaziridine and of carbonyl into hydrazone when hydrazine is used as a reducing agent.13 The presence of these nitrogen functionalities can explain the incorporation of nitrogen into the reduced GO The incorporated nitrogen is likely to act as an active defect site to capture carriers at nonradiative recombination centers, thus resulting in a strong localization of carriers.14,15 This result is well consistent with the peak fluctuation in the PL spectra of the CRG sample in Fig 1(a) Combined PL and XPS results suggest that CRG has a strong carrier localization effect compared to TRG because of the presence of nitrogen functionalities Fig 3(b) illustrates the deconvolution of the O1s peaks for GO, TRG, and CRG The O1s peak is composed of three distinct peaks assigned to (1): O¼C–OH, (2): C¼O, and (3): C–OH groups The intensity of the C¼O peak is found to be much greater than that of the C–OH peak for the TRG sample This signifies that most of the carbonyl groups are not removed with thermal treatment, while the hydroxyl groups are almost removed In the case of CRG, the intensity of the C–OH peak is comparable to that of the C¼O peak, indicating that a Appl Phys Lett 99, 041905 (2011) considerable amount of C–OH and C¼O groups still remain in the CRG sample A shake-up satellite peak observed in the O1s XPS spectra of the CRG sample results from p!p* transition.16 The XPS results favourably support the proposed structures of CRG and TRG in Fig Hydroxyl groups located at the edge of an aromatic domain are readily removed from GO with heat treatment, resulting in lager sp2 domain size, while the hydrazine-mediated reduction forms nitrogen functionalities Our observations are in good agreement with previous reports by other groups.9,13 In summary, the temperature-dependent PL from CRG and TRG has been investigated Our results demonstrated that optical emission from the reduced GO results from recombination of electron-hole pairs in localized electronic states, rather than from band-edge transitions The bandgap of the reduced GO can be modulated by chemical or thermal reduction because the bandgap can be tuned by varying sp2 domain size or by incorporating various functional groups The nitrogen functionalities such as aminoaziridine and hydrazone formed in hydrazine-treated GO are responsible for a strong fluctuation in the local band gap In the case of heattreated GO, hydroxyl groups located at the edge of an aromatic domain are eliminated from GO, resulting in larger sp2 domain size and smaller bandgap T Ohta, A Bostwick, T Seyller, K Horn, and E Rotenberg, Science 313, 951 (2006) B Wang, J R Sparks, H R Gutierrez, F Okino, Q Hao, Y Tang, V H Crespi, J O Sofo, and J Zhu, Appl Phys Lett 97, 141915 (2010) A Nourbakhsh, M Cantoro, T Vosch, G Pourtois, F Clemente, M H Van der Veen, J Hofkens, M M Heyns, S D Gendt, and B F Sels, Nanotechnology 21, 435203 (2010) K P Loh, Q Bao, G Eda, and M Chhowalla, Nature Chem 2, 1015 (2010) M Liao, Z Feng, S Yang, C Chai, Z Liu, J Yang, and Z Wang, Solid State Commun 121, 287 (2002) W S Hummers and R E Offeman, J Am Chem Soc 80, 1339 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Davis, and J F Moulder, Handbook of Xray Photoelectron Spectroscopy (PerkinElmer Corporation, Minnesota, 1978) This article is copyrighted as indicated in the article Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions Downloaded to IP: 169.230.243.252 On: Mon, 29 Sep 2014 06:11:34 ... 2011) Temperature-dependent photoluminescence (PL) of graphene oxide (GO) reduced with hydrazine and heat has been measured to investigate the effect of reduction type on the bandgap of the reduced. .. groups.9,13 In summary, the temperature-dependent PL from CRG and TRG has been investigated Our results demonstrated that optical emission from the reduced GO results from recombination of electron-hole... emission from the reduced GO may result from the recombination of electron-hole pairs in localized electronic states originating from various structure-related defects, rather than from band-edge transitions
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