Tunable Photoluminescence from Graphene Oxide

Tunable Luminescence DOI:10.1002/ange.201200474 Tunable Photoluminescence from Graphene Oxide**

Chih-Tao Chien,Shao-Sian Li,Wei-Jung Lai,Yun-Chieh Yeh,Hsin-An Chen,I-Shen Chen,Li-Chyong Chen,Kuei-Hsien Chen,Takashi Nemoto,Seiji Isoda,Mingwei Chen,Takeshi Fujita, Goki Eda,Hisato Yamaguchi,Manish Chhowalla,and Chun-Wei Chen*

Graphene oxide(GO)is a graphene sheet modified with oxygen functional groups[1]in the form of epoxy and hydroxy groups on the basal plane and various other types at the edges.[1]It exhibits interesting steady-state photolumines-cence(PL)properties.[2–8]For example,low-energy fluores-cence in red to near infrared(NIR)wavelengths(from600–1100nm)has been detected for suspensions and solid thin films of as-synthesized GO.[2,3]In addition,broad lumines-cence from400to800nm from oxygen plasma-treated, mechanically exfoliated,single-layer graphene sheet has been reported.[4]Blue fluorescence with a relatively narrow bandwidth when excited with UV irradiation has also been detected from chemically reduced GO(rGO)and graphene quantum dots.[5,6]Recently,chemically modified GO or rGO with n-butylamine or Mn2+has also demonstrated PL emission at a range of energies.[9,10]A detailed explanation of the origin of such variable energy PL in GO has yet to be elucidated.This is partly because the sample preparation and reduction methods varied,making it difficult to compare the results.Herein,we have prepared GO suspensions that exhibit virtually all of the PL features observed by different groups,through careful and gradual reduction of the GO.The systematic evolution of the electronic structure and compre-hensive analysis of steady-state and transient PL along with photoluminescence excitation(PLE)spectroscopy measure-ments indicate that two different types of electronically excited states are responsible for the observed emission characteristics.

GO was synthesized using the modified Hummers method,[11]the details of which have been reported.[5]GO usually contains a large fraction of sp3hybridized carbon atoms bound to oxygen functional groups,which makes it an insulator.Reduction can be achieved chemically(e.g.hydra-zine exposure)or by thermal annealing in inert environ-ments.[12]Photothermal reduction of GO can be achieved by exposing GO samples to a Xenon flash in ambient condi-tions.[13]In this study,we prepared aqueous GO solutions and subjected them to steady-state Xe lamp irradiation(500W) with different exposure times of up to three hours.In contrast to reduction by an instantaneous flash,this method provides a controllable,gradual transformation from GO to rGO, allowing exploration of the PL evolution and emission mechanisms from as-synthesized GO to rGO.

The deoxygenation of GO after reduction was confirmed by X-ray photoelectron spectroscopy(XPS),as shown in Figure1.The C1s signals of the original GO can be deconvoluted into signals for the C=C bond in aromatic rings(284.6eV),CàO bond(286.1eV),C=O bond (287.5eV),and C(=O)àOH bond(289.2eV),in agreement with previous assignments.[14,15]Increased sp2carbon bonding with increased reduction time can be clearly measured,

which Figure1.C1s XPS spectra of GO at different photothermal reduction times.For the deconvoluted signals,C(O)OH(yellow),C=O(purple), CàC(cyan),and C=C(navy blue)were detected.

[*]C.T.Chien,S.S.Li,https://www.wendangku.net/doc/356428167.html,i,Y.C.Yeh,H.A.Chen,I.S.Chen, Prof.C.W.Chen

Department of Materials Science and Engineering,National Taiwan University

No.1,Sec.4,Roosevelt Road,Taipei,10617(Taiwan)

E-mail:chunwei@https://www.wendangku.net/doc/356428167.html,.tw

Dr.L.C.Chen

Center for Condensed Matter Sciences,National Taiwan University No.1,Sec.4,Roosevelt Road,Taipei,10617(Taiwan)

Dr.K.H.Chen

Institute of Atomic and Molecular Sciences,Academia Sinica

No.1,Sec.4,Roosevelt Road,Taipei,10617(Taiwan)

Dr.T.Nemoto

Institute for Chemical Research,Kyoto University

Uji,Kyoto,611-0011(Japan)

Prof.S.Isoda

Institute for Integrated Cell-Material Sciences,Kyoto University

606-8501Kyoto(Japan)

Prof.M.Chen,Dr.T.Fujita

WPI Advanced Institute for Materials Research,Tohoku University 2-1-1Katahira,Aoba-ku,Sendai,Miyagi,980-8577(Japan)

Dr.G.Eda,H.Yamaguchi,Prof.M.Chhowalla

Department of Materials Science and Engineering,Rutgers Uni-

versity

607Taylor Road,Piscataway,NJ08854(USA)

[**]This work is supported by the National Science Council,Taiwan for the project of“Core facilities of novel graphene based materials”.

(Project No.NSC98-2119-M-002-020-and

99-2119-M-002-012-).

Supporting information for this article is available on the WWW

under https://www.wendangku.net/doc/356428167.html,/10.1002/anie.201200474.

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was accompanied by decreasing peak intensities of the oxygen functional groups.The initial fraction of carbon atoms that were sp 2bonded (C =C)in as-synthesized GO was approx-imately 25%;this fraction gradually increases with exposure time.A maximum sp 2carbon fraction of roughly 69%was obtained for the sample that was irradiated for 3h,slightly lower than that achieved by chemical or ultra high vacuum (UHV)thermal reduction methods.[14,15]This difference is because the photothermal reduction used in this case is milder and allows for a gradual transition from GO to rGO.

The evolution of the corresponding normalized PL spectra of GO suspensions with photothermal reduction treatment time is shown in Figure 2a.The PL spectra were obtained by exciting the samples with a continuous wave

325nm He–Cd laser (Kimmon Inc.)and the excitation power of the laser was kept low,approximately 5mW.The typical measurement time for each spectrum was less than 1minute.No spectral shift was found under the acquisition conditions after 30minutes without photothermal reduction (see Sup-porting Information),indicating that the evolution of the PL is a real effect and not an artifact of the measurement system.The as-prepared GO samples showed broad PL,ranging from 400to 800nm.The PL peaks gradually move towards shorter wavelengths and narrower bandwidths with reduction,as a function of exposure time.The corresponding photographs of PL from GO solutions at different reduction times are shown in Figure 2b.The colors of PL emission can be gradually tuned from the original yellow-red in the as-synthesized GO solution to the blue in the rGO solution after reduction for 3h.The corresponding PL spectra of GO suspensions were deconvoluted into two Gaussian-like peaks,labeled as I P1and I P2,centered at different wavelengths

(Figure 3a–c).The peak positions and the full widths at half maximum (FWHMs)of I P1and I P2emission for GO and rGO samples are shown in Figure 3d,e.The PL of the as-prepared GO was dominated by the I P1peak centered at approximately

600nm and a small I P2emission peak centered at roughly 470nm.Incremental reduction by increasing exposure times leads to a gradual decrease in the I P1and a corresponding increase in I P2emission.The peak position of I P1was also shifted from 600nm for the as synthesized GO to 530nm after 3h of reduction.In contrast,the peak position of I P2only shifted slightly to shorter wavelengths with longer reduction time,and rGO exhibited I P2emission centered at approx-imately 450nm,accompanied by a very weak I P1emission.The increased fraction of I P2(versus I P1)of the integrated PL intensities with reduction time are shown in Figure 3f,which demonstrates a strong correlation with the increased sp 2fractions calculated from XPS measurements.Accordingly,the evolution of PL emission mainly arises from the varying I P1to I P2emission intensity ratios,as a result of the increase in the number of sp 2carbon atoms.

Next we performed PL excitation (PLE)and time-resolved photoluminescence (TRPL)spectroscopy to exam-ine the electron–hole recombination processes responsible for the I P1and I P2emissions.Figure 4a shows the two PLE spectra of the predominantly I P1emission (l emission =605nm)from the as-synthesized GO and the predominantly I P2

emission

Figure 2.a)Normalized PL spectra of the GO suspensions after differ-ent exposure times (0–180min)to photothermal reduction treatment.b)Photographs of tunable PL emission from GO at reduction times of 0min (yellow-red),75min (green)and 180min

(blue).

Figure 3.PL spectra of a)GO,b)rGO (reduction time =75min),and c)rGO (reduction time =180min)with two deconvoluted Gaussian-like bands of I P1and I P2emission.d)Peak positions and e)the

FWHMs of I P1and I P2as a function of reduction time.f)Correlation of I P2fractions (circles)with sp 2fractions (triangles;from XPS data)with reduction time.

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(l emission =454nm)from the rGO sample (reduced for 3h).To correct the absorption effect,we normalized the measured PLE spectra of GO and rGO samples by dividing their respective absorption coefficients at different excitation wavelengths.The I P1emission was found over a wide range of excitation wavelengths,from 300to 570nm.In contrast,a well-resolved absorption band with a narrow range of excitation wavelengths between 300and 350nm was observed for I P2emission in the rGO sample.Very distinct PLE spectra of I P1and I P2emissions indicate that the electron–hole recombination results from different types of electronically excited states for the two cases.Figure 4b,c show the corresponding transient PL decay curves.For the as-prepared GO sample,the dominant I P1emission shows a multi-expo-nential PL decay,which can be decomposed into three exponential components of 200,500,and 1400ps with various intensity ratios,yielding an average PL decay time of about 600ps.After photothermal reduction,the PL decay times for the I P1emission were found to decrease.The PL decay curves of I P2emission show very different transient behavior.The initial fast decay components of all the rGO samples show a similar temporal response of approximately 100ps,which is close to the instrumental response function (IRF)of the measurement setup,whereas the PL decay time of the long-lasting tail of I P2emission was found to increase with reduction time,reaching 1.5ns for rGO (180minute reduc-tion).The very different PLE and TRPL results also suggest that the luminescence of I P1and I P2emissions arises from two different types of electronically excited states in the hetero-geneous electronic structures of GO and rGO.

The atomic structure of GO has been proposed to be a graphene basal plane with a non-uniform coverage of the

oxygen-containing functional groups,resulting in sp 2carbon

clusters of a few nanometers isolated within a defective carbon lattice or the sp 3matrix.[12,16,17]First-principle calcu-lations have also suggested that a large fraction of the C atoms in the hydroxy and epoxy units are bonded to each other to form strips of sp 2carbon atoms.[18]In carbon materials containing a mixture of sp 2and sp 3bonding,the optoelectronic properties are mainly determined by the p and p *states of the sp 2sites,which lie within the s –s *gap.[18]Because p bonding is weaker and has lower formation energy,it is expected that a large number of disorder-induced localized states are in the two-dimensional network of as-synthesized GO,which consists of a large fraction of distorted carbon atoms attached to oxygen-containing functional groups.Because the interactions of the p states strongly depend on their projected dihedral angle,the structural disorder-induced localized states may be located in the band tail of the p –p *gap or lie deeper within the gap.As a result,optical transitions between these disorder-induced localized states may cause a broad absorption or emission band (see Supporting Information).The predominant broad I P1emis-sion band in the as-prepared GO arising from a wide-range of excitation energies may be mainly attributed to optical transitions from these disorder-induced localized states.During deoxygenation by reduction,the number of these disorder-induced states decreases so that the intensity of the I P1emission is diminished.Meanwhile,the reduction of GO leads to removal of oxygen-containing functional groups,and some of the carbon lattices in the originally distorted sp 2domains can form new graphitic domains of sp 2clusters.It has been suggested and experimentally demonstrated that the initially present sp 2domains in GO do not increase in size with reduction.[5,12,16]Instead,the reduction of GO usually leads to the creation of newly formed small sp 2clusters,which are smaller in size,but numerous.Because the free-energy cost for aromatic clustering would be considerable,[19]the disorder potential in the 2D network of GO or rGO is likely to oppose further clustering into large sp 2domains.These small sp 2clusters that create the isolated molecular states eventually percolate to mediate the transport of carriers by hopping.[5]

To further examine the atomic structure of these small sp 2clusters,we performed nanoscale morphology analyses of rGO using scanning tunneling microscopy (STM)and high-resolution transmission electron microscopy (HRTEM).Many small sp 2domains of a few nanometers in size were readily observed in the STM image (Figure 5a).The arrays of dark dots (shown by arrows)correspond to the crystalline conductive areas of small sp 2domains in rGO.The pseudo lattice size in these small sp 2clusters is around 0.4nm,larger than the typical lattice constant (0.245nm)of graphene resulting from the rotational moir?pattern between the sp 2domains in rGO and the highly ordered pyrolitic graphite (HOPG)substrate surface.[20]No atomically resolved STM image in the as-synthesized GO sample can be observed because of its highly insulating nature (not shown).Figure 5b shows the HRTEM image of the rGO sample.An image of one representative small sp 2cluster with a size of approx-imately 2nm consisting of about 50aromatic rings is

also

Figure 4.a)Measured (dash lines)and normalized (solid lines)PLE spectra of the I P1emission (l emission =605nm)from as-synthesized GO and the I P2emission (l emission =455nm)from rGO (reduction for 3h).The signal at about 375nm in the excitation spectrum of I P2emission is the Raman signal of water.The PL transient decay curves of b)I P1emission and c)I P2emission at different reduction times.

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shown in the inset.The electron–hole recombination among the confined cluster states originating from the small and isolated sp 2domains may lead to excitonic features such as those found in an organic light-emitting molecular semi-conductor,[21]yielding a narrower emission bandwidth and a well-resolved PLE spectrum as seen with the I P2emission band.Such excitonic behavior of I P2emission has been detected in single-walled carbon nanotubes (SWNTs)[22]because of the confined geometry but has not been detected in hydrogenated amorphous carbon (a-C:H),where a rather large disorder potential in a 3D network opposes clustering.[23]Blue-luminescent graphene quantum dots (GQDs)with sizes less than 10nm have been reported and the emission from the free zigzag edge sites was proposed as a possible explanation for the fluorescence.[6,24]However,we did not find laterally very small GO or rGO sheets in our samples that could be classified as free-standing GQDs (see Supporting Informa-tion).

Figure 6summarizes the mechanisms that explain the evolution of photoluminescence of GO with increased reduction.The original GO consists of numerous disorder-

induced defect states within the p –p *gap and exhibits a broad prominent PL spectrum centered at longer wavelengths,as shown in Figure 6a.After deoxygenation,the number of disorder-induced states within the p –p *gap decreases,and an increased number of cluster-like states from the newly formed small and isolated sp 2domains are formed,as shown in Figure 6b.The electron–hole recombination among these sp 2cluster-like states exhibits blue fluorescence at shorter wave-lengths with a narrower bandwidth.The tunable PL spectra of GO during reduction are therefore attributed to the variation of the relative intensity ratios of PL emission from two different types of electronically excited states,as a result of changing the heterogeneous electronic structures of GO and rGO with variable sp 2and sp 3hybridizations through reduction.

Experimental Section

For material characterizations,UV/Vis absorption spectra were obtained using a Jasco V570UV/Vis/NIR spectrophotometer.The PL spectra were obtained by exciting the samples using a continuous wave He–Cd laser at 325nm (Kimmon Inc.)and the emission spectra were analyzed with a Jobin–Yvon TRIAX 0.55m monochromator and detected by a photomultiplier tube and standard photocounting electronics.Time-resolved photoluminescence spectroscopy was performed with a time-correlated single photon counting (TCSPC)module (Picoharp 300,PicoQuant).A pulsed laser (372nm)with an average power of 1mW operating at 20MHz with duration of 70ps was used for excitation.Photoluminescence excitation (PLE)spec-troscopy was measured using a FluoroLog-3spectrofluorometer (Jobin–Yvon).XPS spectra were obtained using a VG Scientific ESCALAB 250system.Atomic force microscopy and scanning tunneling microscopy (Nano Scope IIIa,Veeco Inc.)were used to obtain the morphology and atomic-scaled images of GO or rGO deposited on HOPG substrates (ZYH grade,Advanced Ceramic Corporation)using mechanically cut Pt/Ir tips under

ambient

Figure 5.a)STM image of rGO with scan size of 15nm,scan rate of 20Hz,bias of 4.4mV,and current of 5pA.Arrows indicate small sp 2clusters aligned on the HOPG surface.b)High-resolution TEM image of the rGO.Inset:image of one representative sp 2

cluster.

Figure 6.Proposed PL emission mechanisms of a)the predominant I P1emission in GO from disorder-induced localized states.b)The predominant I P2emission in rGO from confined cluster states.

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conditions.High-resolution transmission electron microscopy (HRTEM)was performed using JOEL JEM-2100F TEM/STEM with double spherical aberration(Cs)correctors(CEOS GmbH, Heidelberg,Germany)to attain high-contrast images with a point-to-point resolution of1.4 .

Received:January17,2012

Revised:April11,2012

Published online:May23,2012

.Keywords:graphene·graphene oxide·luminescence·materials science·scanning-probe microscopy

[1]W.W.Cai,R.D.Piner,F.J.Stadermann,S.Park,M.A.Shaibat,

Y.Ishii,D.X.Yang,A.Velamakanni,S.J.An,M.Stoller,J.H.

An,D.M.Chen,R.S.Ruoff,Science2008,321,1815.

[2]X.M.Sun,Z.Liu,K.Welsher,J.T.Robinson,A.Goodwin,S.

Zaric,H.J.Dai,Nano Res.2008,1,203.

[3]Z.T.Luo,P.M.Vora, E.J.Mele, A.T.C.Johnson,J.M.

Kikkawa,Appl.Phys.Lett.2009,94,111909.

[4]T.Gokus,R.R.Nair,A.Bonetti,M.Bohmler,A.Lombardo,

K.S.Novoselov,A.K.Geim,A.C.Ferrari,A.Hartschuh,ACS Nano2009,3,3963.

[5]G.Eda,Y.Y.Lin,C.Mattevi,H.Yamaguchi,H.A.Chen,I.S.

Chen,C.W.Chen,M.Chhowalla,Adv.Mater.2010,22,505.

[6]D.Y.Pan,J.C.Zhang,Z.Li,M.H.Wu,Adv.Mater.2010,22,

734.

[7]K.S.Subrahmanyam,P.Kumar,A.Nag,C.N.R.Rao,Solid

State Commun.2010,150,1774.

[8]J.Lu,J.Yang,J.Wang,A.Lim,S.Wang,K.P.Loh,ACS Nano

2009,3,2367.

[9]Q.S.Mei,K.Zhang,G.J.Guan,B.H.Liu,S.H.Wang,Z.P.

Zhang,https://www.wendangku.net/doc/356428167.html,mun.2010,46,7319.[10]Z.X.Gan,S.J.Xiong,X.L.Wu,C.Y.He,J.C.Shen,P.K.Chu,

Nano Lett.2011,11,3951.

[11]M.Hirata,T.Gotou,S.Horiuchi,M.Fujiwara,M.Ohba,Carbon

2004,42,2929.

[12]K.P.Loh,Q.L.Bao,G.Eda,M.Chhowalla,Nat.Chem.2010,2,

1015.

[13]L.J.Cote,R.Cruz-Silva,J.X.Huang,J.Am.Chem.Soc.2009,

131,11027.

[14]S.Stankovich,D.A.Dikin,R.D.Piner,K.A.Kohlhaas,A.

Kleinhammes,Y.Jia,Y.Wu,S.T.Nguyen,R.S.Ruoff,Carbon 2007,45,1558.

[15]C.Mattevi,G.Eda,S.Agnoli,https://www.wendangku.net/doc/356428167.html,ler,K.A.Mkhoyan,O.

Celik, D.Mostrogiovanni,G.Granozzi, E.Garfunkel,M.

Chhowalla,Adv.Funct.Mater.2009,19,2577.

[16]C.Gómez-Navarro,J.C.Meyer,R.S.Sundaram,A.Chuvilin,S.

Kurasch,M.Burghard,K.Kern,U.Kaiser,Nano Lett.2010,10, 1144.

[17]K.A.Mkhoyan,A.W.Contryman,J.Silcox,D.A.Stewart,G.

Eda,C.Mattevi,https://www.wendangku.net/doc/356428167.html,ler,M.Chhowalla,Nano Lett.2009,9, 1058.

[18]J.Yan,L.Xian,M.Y.Chou,Phys.Rev.Lett.2009,103,086802.

[19]J.Robertson,Adv.Phys.1986,35,317.

[20]A.Hoshino,S.Isoda,H.Kurata,T.Kobayashi,J.Appl.Phys.

1994,76,4113.

[21]H.B?ssler,M.Gailberger,R.F.Mahrt,J.M.Oberski,G.Weiser,

Synth.Met.1992,49,341.

[22]S.M.Bachilo,M.S.Strano, C.Kittrell,R.H.Hauge,R.E.

Smalley,R.B.Weisman,Science2002,298,2361.

[23]J.Robertson,Phys.Rev.B1996,53,16302.

[24]J.Peng,W.Gao,B.K.Gupta,Z.Liu,R.Romero-Aburto,L.Ge,

L.Song,L.B.Alemany,X.Zhan,G.Gao,S.A.Vithayathil,

B.A.Kaipparettu,A.A.Marti,T.Hayashi,J.-J.Zhu,P.M.

Ajayan,Nano Lett.2012,12,844.

6770www.angewandte.de 2012Wiley-VCH Verlag GmbH&Co.KGaA,Weinheim Angew.Chem.2012,124,6766–6770