Solid State Communications 150 (2010) 17741777

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Solid State Communications

journal homepage: www.elsevier.com/locate/ssc

Blue light emitting graphene-based materials and their use in generatingwhite lightK.S. Subrahmanyam, Prashant Kumar, Angshuman Nag, C.N.R. Rao Chemistry and Physics of Materials Unit, International Centre for Materials Science, New Chemistry Unit and CSIR Centre of Excellence in Chemistry, Jawaharlal Nehru Centre forAdvanced Scientific Research, Jakkur P.O., Bangalore 560 064, India

a r t i c l e i n f o

Article history:Received 29 June 2010Accepted 11 July 2010by E.V. SampathkumaranAvailable online 16 July 2010

Keywords:A. GrapheneB. Chemical synthesisD. Optical propertiesE. Luminescence

a b s t r a c t

It has been demonstrated that acid-treated graphene samples as well as reduced graphene oxide showfairly intense blue emission centered around 440 nm. Reduction of graphene oxide can be carried outeither chemically or by using different types of radiations. Blue emission from graphene-based materialscan be combined with the yellow emission from materials like ZnO to produce white light sources.

2010 Elsevier Ltd. All rights reserved.

1. Introduction

There have been a few reports in the literature stating thatcertain carbon species emit blue light upon excitation [14].Graphitic forms of carbon have rich chemistry withmany differentchemical functional groups present on the surface, particularly atdefect sites such as steps and edges [511]. In particular, oxygencontaining carboxyl, carbonyl, ether and such functional groupsare speculated to be present on graphitic surfaces [6,7,1115]. Inparticular, the presence of quinones is suggested to be the causeof blue emission. A possible origin of the blue photoluminescencein reduced graphene oxide (RGO) is the radiative recombinationof electronhole (eh) pairs generated within localized states. Theenergy gap between the and states generally depends onthe size of sp2 clusters [16] or conjugation length [17]. Interactionbetween nanometer-sized sp2 clusters and finite-sized molecularsp2 domains could play a role in optimizing the blue emission inRGO. The presence of isolated sp2 clusters in a carbonoxygen sp3matrix can lead to the localization of eh pairs, facilitating radiativerecombination of small clusters [18].

We have investigated the generation of blue light by graphene-basedmaterials and found that graphene samples oxidized by acidsas well as graphene oxide (GO) reduced by various means giveexcellent blue emission. In this article, we discuss the essentials ofthis aspect. Interestingly, we find that sunlight itself is sufficient toreduce GO while an excimer laser is even more effective. We have

Corresponding author. Tel.: +91 80 23653075; fax: +91 80 22082760.E-mail address: cnrrao@jncasr.ac.in (C.N.R. Rao).

also examined the role of ZnO in the photo-reduction of GO anddemonstrate how the blue emission from graphene samples can becombined with the yellow emission from the materials includinggraphene oxide and ZnO to generate white light.

2. Experimental details

Few-layer graphene samples were prepared by the thermalexfoliation of graphite oxide at high temperatures (EG) [19], bythe conversion of nanodiamond by heating in an inert atmosphere(DG) [19] and by the arc discharge of graphite electrodes in thepresence of H2 and He (HG) [20]. HG contains 23 layers whileEG and DG have 56 and 610 layers, respectively, as revealed byAFM measurements. Raman spectra of graphene samples showedthe characteristic D band (1320 cm1), G band (1570 cm1), Dband (1620 cm1) and 2D band (2635 cm1) [5,21]. The D andD-bands are defect-induced features. The G band corresponds tothe E2g mode of graphite and arises from the vibration of sp2bonded carbon atoms. The 2D band originates from second orderdouble resonant Raman scattering and varies with the numberof layers. The intensity of the 2D band is sensitive to dopingof graphene by holes or electrons [5,2124]. Graphene oxidewas obtained by prolonged sonication (1 h) of graphite oxide inwater medium to yield a brownish stable yellow solution. ZnOnanoparticles with an average size around 5 nm were preparedby the procedure described in the literature [25]. XRD patterns ofthe ZnO nanoparticles could be indexed on the hexagonal wurtzitestructure (space group: P63mc; a = 0.3249 nm, c = 0.5206 nm,JCPDS card no. 36-1451). The crystallite size of the nanoparticlescalculated from the X-ray line widths was consistent with the TEM

0038-1098/$ see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.ssc.2010.07.017

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K.S. Subrahmanyam et al. / Solid State Communications 150 (2010) 17741777 1775

Inte

nsity

(cps

)

400 500 600Wavelength (nm)

700 800

0.8

0.4

CIE

y

0.6

0.2

0.00.2 0.4 0.6

CIE x0.0 0.8

a

b

Fig. 1. (a) PL spectra of EG, acid-treated EG (FEG) and acid-treated HG (FHG). (b)CIE 1931 colour diagram showing the three points due to EG, FEG and FHG in theblue region.

results. ZnO nanoparticles show an absorption band at 360 nmwhich is blue-shifted relative to that of bulk ZnO due to quantumconfinement [26]. The particles exhibit a photoluminescence (PL)band centered around 550 nm.

Pristine graphene samples were treated with a solution ofconc. H2SO4 and conc. HNO3 in water (Table 1) under microwaveirradiation (2 min at 600 W) [19]. Before microwave treatment,the solution was sonicated for better mixing. We designate theacid-treated EG, DG and HG as FEG, FDG and FHG, respectively.A KrF excimer laser (248 nm, 5 Hz) was employed to irradiateaqueous solutions of GO taken in a quartz vial. The aluminummetalslit (beam shaper) which usually gives a rectangular beam wasremoved while laser reduction was carried out. This makes laserenergy almost uniform throughout the area where graphene oxideis present. It was observed that within half an hour of irradiationat 5 Hz reprate and at 300 mJ beam energy, GO turns black due tothe reduction. GO was also exposed to sunlight from the top sidein daytime. A mixture of GO and ZnO nanoparticles was irradiatedto cause reduction. Graphene oxide after reduction by irradiationwith an excimer laser or sunlight settles down at the bottom. Formaking an admixture of reduced graphene oxide (RGO) with ZnOnanoparticles, ZnO was sonicated separately in ethanol for 20 minand then added to the reduced graphene oxide solution in a dropby drop manner using a micropipette.

All the materials were examined by Raman spectroscopy. Themorphology of the samples was studied by transmission electronmicroscopy (TEM) with a JEOL JEM 3010. UVVis absorptionspectra were recorded using a Perkin-Elmer Lambda 900 UV/Vis/NIR spectrometer. Photoluminescence (PL) spectra were recorded

Fig. 2. (a) Photographs of (i) GO and laser-reduced GO at a beam energy of 300 mJwith (ii) 1000 and (iii) 18000 shots. (b) Photographs of (i) GO and GO exposed tosunlight for (ii) 2 and (iii) 10 h.

with a Perkin-Elmermodel LS55 luminescence spectrometer. Two-probe resistance measurements using digital multimeter werecarried out on solid samples.

3. Results and discussion

The as-prepared graphene samples exhibit weak blue emissioncentered around 400 nm upon UV excitation. On acid treatmentunder microwave irradiation (Table 1), the blue emission bandgets slightly red-shifted with the PL emission maximum (with en-hanced intensity) appearing around 435 nm as can be seen fromFig. 1(a). The nature of blue emission is quantified by the CIE chro-maticity diagram in Fig. 1(b). Another way to obtain blue emis-sion from graphene materials is via reduction of GO. GO itself ex-hibits brownish yellow emission with a band centered around 550nm [27,28]. Reduction of GO by chemical means or by radiationgives rise to blue-emitting species. The radiation can be ultravioletlight (Hg lamp), sunlight or a laser. GO upon prolonged irradiation(6 h) by sunlight gives rise to the same effect achieved by the ul-traviolet treatment for 40min. Laser irradiation requires only 10 or20 min depending on 5 or 10 Hz reprate. Laser irradiation not onlyreducesGO, but also fragments the graphene sheets into pieces. Re-duction of GO by laser radiation and sunlight can be observed visi-bly by the change in colour as shown in Fig. 2. Brownish yellow GOturns black after irradiation. Two-probe resistance measurementsshowed that the resistivity of GO (5000 Ohm cm) decreases to 550and 41 Ohm cm after reduction with sunlight and laser, respec-tively. This is expected as reduced GO would be more conductingthan GO.

Infrared spectra of the reduced samples show reduced inten-sities of the bands corresponding to OH ( 3418 cm1) and C=O(1640 cm1) compared to GO [27,29]. In Fig. 3(a), we show the PLspectra for yellow-emitting GO and the blue-emitting light sourcesachieved by irradiation with different light sources. In Fig. 3(b), we

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1776 K.S. Subrahmanyam et al. / Solid State Communications 150 (2010) 17741777

Table 1Conditions for acid treatment of graphene.

Graphene (5 mg) Weight (mg) HNO3 (ml) H2SO4 (ml) H2O (ml) Microwave treatment (s)

EG 5 4 4 8 2DG 5 4 4 8 2HG 5 4 4 8 2

Inte

nsity

(a.u.

)

400 500 600Wavelength (nm)

700 800

0.8

0.4

CIE

y

0.6

0.2

0.00.2 0.4 0.6

CIE x0.0 0.8

a

b

Fig. 3. (a) PL spectra of GO and reduced GO obtained by irradiation with laser(LRGO), ultraviolet light (URGO) and sunlight (SRGO). (b) The chromaticity diagramshows the nature of blue emission by the three RGOs.

showCIE chromaticity diagram for GO and the reducedGO samplesobtained by different means. Chemically reduced GO also showssimilar blue emission. The plausible mechanism for photo-assistedreduction of GO is as follows.

GO+ h GO (h+ e) (1)4h+ 2H2O O2 + 4H+ (2)4e+ GO+ 4H+ RGO+ 2H2O. (3)GO absorbs light generating electronhole pairs. Water acts as ahole scavenger yielding oxygen and protons, whereas the electronsare captured by the sp2 regions of GO. The captured electronstogether with the produced protons reduces the functional groupsin GO, extending the -electron network resulting RGO.

Blue emission from laser-reduced graphene oxide (LRGO) oracid-treated EG can be combined with the yellow emission fromgraphene oxide to achieve bluishwhite light (as shown in Fig. 4(a)).Sunlight-reduced graphene oxide (SRGO) and yellow-emitting GOalso yield bluish white light. The bluish white light obtainedfrom RGO + GO or FEG + GO is, however not stable as GO

a

b

Fig. 4. (a) PL spectra of GO and GO admixedwith 2 wt% of FEG and 37wt% of LRGO.(b) PL spectra of ZnO and ZnO admixedwith 28wt% of LRGO. Insets in figure (a) and(b) show the photograph of bluish white light emission after mixing LRGO with GOand ZnO, respectively.

gets reduced to graphene after a passage of time. It is, therefore,necessary to use an alternative material with yellow emission inplace of graphene oxide. For this purpose, we have employed ZnOnanoparticles which exhibit yellow emission centered around 550nm due to defects. Fig. 4(b) shows the bluish white emission fromLRGO+ ZnO nanoparticles. SRGO+ ZnO also give a similar bluishwhite light emission. It should be noted that acid-functionalizedgraphenes taken with ZnO nanoparticles, quench the defect-related yellow emission of ZnO.

Photo-assisted reduction of GO has been demonstrated re-cently, where nanocrystals of wide-bandgap semiconductors likeZnOwere used as sensitizers to absorb the UV light to generate theelectronhole pair, followed by the donation of the electron to GOto produce reduced GOZnO composite [3032]. We find that therole of ZnO is to act as a catalyst. While GO requires 6 h of UVirradiation to undergo reduction, the same extent of reduction isattained in40 min in the presence of ZnO nanoparticles. Clearly,reduction of GO using sunlight is the simplest and cheapest meansof obtaining blue-emitting reduced GO. This reduced species is su-perior to acid-treated graphene which tends to quench the photo-luminescence of ZnO.

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4. Conclusions

It is noteworthy that carbon-based materials such as reducedgraphene oxide or oxidized graphene give rise to blue emissionwhich is similar to inorganic materials such as gallium nitride.The blue emission from the graphitic materials can be exploited toproduce white light sources in combination with yellow emissionmaterials like ZnO.

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Blue light emitting graphene-based materials and their use in generating white lightIntroductionExperimental detailsResults and discussionConclusionsReferences