Facile access to versatile fluorescent carbon dots toward light-emitting diodes

Cite this:https://www.360docs.net/doc/8813645439.html,mun .,2012,48,2692–2694Facile access to versatile ?uorescent carbon dots toward light-emitting diodes w

Xin Guo,Cai-Feng Wang,Zi-Yi Yu,Li Chen and Su Chen*

Received 12th December 2011,Accepted 17th January 2012DOI:10.1039/c2cc17769b

We report the facile synthesis of carbon dots with tunable

?uorescence from unzipping of photonic crystals and their application in LEDs,which may provide an insight into the creation of multifunctional carbon dots adapted for various applications such as in optoelectronics,sensing,or bioimaging.The emergence of ?uorescent carbon dots (CDs)opens an available avenue to generate novel quantum dots (QDs)with unique luminescence,photostability,low toxicity,chemical resistance and electronic transport properties.1Such CDs have received increasing attention,owing in part to their potential as a promising benign alternative to heavy metal-based semi-conductor QDs in applications such as optoelectronic devices,2biological labeling,3or bioimaging.4So far,a variety of CDs have been developed by using elegant strategies including arc discharge,5laser ablation,6electrochemical oxidation,7combustion/thermal oxidation,8supported synthetic,9or microwave routes,10where the CDs are ‘‘broken o?’’from a larger carbon structure

(graphite,cement or multiwalled carbon nanotubes)or formed

from molecular precursors (candles or gas soot,citrate salts,

saccharides or carbohydrates).Impressive as it is,the properties of these as-produced CDs on quantum yields (QYs)and multi-color ?uorescence,however,pale in comparison to traditional metal-based semiconductor nanocrystals.In most cases,CDs present simple blue photoluminescence (PL)and QYs less than 10%,limiting the range of their practical applications.Particularly,the use of CDs as a new phosphor for light-emitting diode (LED)devices still remains sparse.Much work is therefore still highly needed in this area,such as utilizing alternative starting materials,improving QYs of CDs,and ?nding e?ective strategy in practical applications.

Herein,we report the ?rst synthesis of multicolor ?uorescent

CDs from chemical unzipping of epoxy-enriched polystyrene (PS)photonic crystals via a simple one-step pyrolysis procedure (Fig.1).We pyrolyzed uniform spheral colloidal crystals to straightly attain narrow-size distributed CDs without further treatment.Interestingly,by simply varying the pyrolysis tempera-ture from 200,300,to 4001C,CDs with tunable blue,orange and

white emissions under a single-wavelength UV excitation were directly obtained.These as-prepared CDs display excellent PL properties with QYs up to 47%,as well as high solubility in water and most organic solvents,which are meaningful for further applications.This work o?ers several advantages for producing CDs:easy handling,without surface passivation or tedious separation process,and straightly attaining e?cient CD-based white-light emitters.More importantly,we explored blue,orange and white LEDs based on photoluminescence of such CDs for the ?rst time,providing a new potentiality and insight into new generation of environmentally friendly optoelectronic devices.The successful synthesis of CDs was performed using

poly(styrene-co -glycidylmethacrylate)(PS-co -PGMA)photonic crystals as the precursor.PS-co -PGMA photonic crystals were prepared by the surfactant-free emulsion polymerization according to a modi?ed literature method.11The photonic crystals have a well-ordered structure and a close-packed array (Fig.1B and Fig.S1,ESI w ),in which the PS-co -PGMA microspheres are highly monodisperse with an average diameter of 186nm (Fig.1C).The brilliant blue color observed by optical microscopy further con?rms the uniformity of the microspheres (inset in Fig.1B).12Fig.1D shows that PS-co -PGMA microspheres have well-de?ned core–shell structure.Inspired by breakup of graphene sheets and carbon nanotubes into smaller parts due to an unzipping mechanism that the aligned epoxy groups formed during oxidation cause a rupture of the underlying C–C bonds,13we proposed that the as-prepared PS colloidal crystal microspheres with an epoxy-enriched surface could be unzipped into CDs with desired size.We calcined PS-co -PGMA colloidal crystals for 2h at 200,300,and 4001C under carrier gas of nitrogen,respectively,to achieve the products CD 200,

Fig.1(A)Schematic representation of the formation of carbon dots from the pyrolysis of photonic crystals.(B)The cross-section SEM image and (C and D)TEM images of PS-co -PGMA core–shell photonic crystals.The inset in (B)is the optical micrograph of the PS-co -PGMA photonic crystal ?lm.

State Key Laboratory of Materials-Oriented Chemical Engineering,College of Chemistry and Chemical Engineering,Nanjing University of Technology,Nanjing 210009,P.R.China.

E-mail:chensu@https://www.360docs.net/doc/8813645439.html,;Fax:+86-25-83582258;Tel:+86-25-83582258

w Electronic supplementary information (ESI)available:Experimental details and additional characterization data.See DOI:10.1039/c2cc17769b

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D o w n l o a d e d b y C h a n g c h u n I n s t i t u t e o f A p p l i e d C h e m i s t r y , C A S o n 19 A p r i l 2012P u b l i s h e d o n 18 J a n u a r y 2012 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 2C C 17769B

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光子晶体的解压

电弧放电激光烧蚀

石墨

多壁碳纳米管蜡烛　气烟煤

柠檬酸盐糖类或碳水化合物

稀少

环氧富集聚苯乙烯

苯乙烯　－　甲基丙烯酸缩水甘油酯

CD 300,and CD 400,respectively.The evolution process of unzipping PS-co -PGMA microspheres into CDs was deduced by TEM measurements (Fig.S2,ESI w ).Signi?cantly,these products exhibit bright blue (CD 200),orange (CD 300)and white (CD 400)?uorescence under UV light,respectively.

The chemical identity of the CDs was investigated.Fourier transform infrared (FT-IR)spectra indicate the characteristics of C Q C aromatic ring,C Q O,and –OH stretching modes in the products (Fig.S3,ESI w ).These functional groups confer the CDs with excellent dispersibility in water and most polar

organic solvents such as ethanol,N ,N -dimethylformamide (DMF)and N -methyl pyrrolidone (NMP).The Raman spectrum shows peaks at 1590cm à1(G band)and 1350cm à1(D band),revealing the presence of both sp 2and sp 3hybrid carbons (Fig.S4,ESI w ).14Fig.2shows the high-resolution transmission electron microscopy (HRTEM)images of CDs.The as-prepared CDs are well dispersed in narrow size distributions with mean diameters of 2.42nm (CD 200),3.51nm (CD 300)and 4.46nm (CD 400)(Fig.S5,ESI w ).The excellent dispersion might be bene?ted from the uniformity of sphere-like structure of the precursor.Well-resolved lattice fringes with an interplanar spacing of 0.23,0.27and 0.34nm were observed for CD 200,CD 300,and CD 400,respectively,which are close to the (100),(020)and (002)di?raction facets of graphite carbon,respectively.15The above results suggest that the CDs might be composed of nanocrystalline cores of graphitic sp 2carbon atoms functionalized with peripheral carboxylic/carbonyl groups and sp 3carbon defects.

The as-prepared CDs exhibit interesting optical properties.The emission spectra of the CDs show a typical excitation dependent feature that the PL peak shifts to longer wavelength as the excitation wavelength gradually increases (Fig.3A and Fig.S7and S8,ESI w ),similar to those of previously reported CDs.3–6Surprisingly,under a single-wavelength excitation,the CDs obtained from di?erent pyrolysis temperature display obviously distinct color emission,as shown in Fig.3B and C.Under a 365nm UV light,CD 200,CD 300,and CD 400exhibit bright blue,orange and white ?uorescence (Fig.3C),respectively,with the corresponding maximum emission band at 379,464and 424nm (Fig.3B),respectively.Their QYs were determined to reach 47%(Table S1,ESI w ),at the top of their categories.Time-resolved photoluminescence measurements indicate that CD 200,CD 300,and CD 400have decay lifetimes of 6.94?0.05,10.80?0.05,and 10.95?0.05ns,respectively (Fig.3D and Fig.S9,ESI w ).The ?uorescence lifetimes of the latter two products are much larger than those of previously reported luminescent CDs,which is profound for their applications such as

in optoelectronic devices or optical imaging.We note that there is rare success in fabrication of CDs with white photoluminescence 16as well as multiple emission colors under a single-wavelength excitation.And our work provides high-quality CDs with tunable bright blue,orange and white emission simply by varying pyrolysis temperature,which might contribute an alternative convenient one-step way to prepare multicolor even full-color CDs with high PL e?ciency and long ?uorescence lifetime potentially useful in optoelectronics,sensory,and biomedical areas.

To provide insight into the PL characteristics of these CDs,several control experiments were carried out.The CDs obtained from PS microspheres show weak blue ?uorescence while the products pyrolyzed from PGMA have no appreciable ?uorescence (Fig.S10,ESI w ),implying that the PS microspheres play an important role in the formation of ?uorescent CDs.In addition,an insigni?cant shift in the PL peaks was observed for the samples derived from pyrolysis of PS microspheres at di?erent tempera-tures (Fig.S10,ESI w ),di?erent from the feature that variation of pyrolysis temperature gave tunable multicolor emission for CDs gained from core–shell PS-co -PGMA microspheres.In the core–shell colloidal crystals,the epoxy groups on the PS micro-spheres can serve as unzipping sites to induce the rupture of underlying C–C bonds and accelerate the cracking of PS spheres into CDs with controllable size.The tethered functional groups on the CDs (Fig.S3,ESI w )can cause the structural defects helping to generate energy traps that emit light when stimulated.6Electron spin resonance (EPR)spectroscopy reveals that the CDs possess a considerable distribution of defect sites generated by free radicals (Fig.3E).Therefore,it is speculated that the quantum size e?ect,pyrolytic formation of di?erent polyaromatic ?uorophores and surface defects contribute to photoluminescence of CDs.In particular,we noted that CD 400has relatively larger size distribution,and hence the e?ects from nanoparticles of di?erent sizes in the sample along with di?erent emissive sites on each CD may result in white ?uorescence.

For practical application,it is signi?cant to obtain CDs with high performance in the solid state.As shown in Fig.3C and Fig.S11(ESI w ),the condensed ?lms of CDs also display strong photoluminescence.There is a red-shift and broadening

Fig.2HRTEM images of (A and D)CD 200,(B and E)CD 300and (C and F)CD 400,respectively.

Fig.3(A)PL spectra of CD 300aqueous solution at di?erent excitation wavelengths (in 20nm increment starting from 300nm to 480nm).(B)PL spectra of CD 200(black),CD 300(red),and CD 400(blue)in aqueous solution (l ex =365nm).(C)Photographs of the corresponding CD samples in aqueous solution (top)and in condensed ?lm (bottom)under a UV lamp (365nm).(D)Time-resolved ?uorescence decay curves of CD 300powder (l ex =380nm,l em =470nm).(E)The EPR spectrum of CD 300powder recorded at room temperature.(F)PL spectra of solid CD samples stored under ambient conditions over time.

D o w n l o a d e d b y C h a n g c h u n I n s t i t u t e o f A p p l i e d C h e m i s t r y , C A S o n 19 A p r i l 2012P u b l i s h e d o n 18 J a n u a r y 2012 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 2C C 17769B

Ｎ，Ｎ－二甲基甲酰胺

of the emission peak for CD ?lms compared with that of CDs in aqueous solution,which might be caused by inner ?lter e?ects and energy transfer within an inhomogeneous distribution emitting species embedded in the solid state.16After being stored under ambient conditions for 6months,the solid samples showed no obvious change in PL spectra (Fig.3F),revealing the good stability of CDs.The excellent PL properties in terms of brightness and stability make the as-prepared benign CDs useful in various applications.

To make the best of these CDs with the virtues of multicolor PL,high QYs and low toxicity,we directly employed the as-prepared solid CDs as phosphors to construct LED devices.The devices were made by coating CDs encased in superglue on the prototype solid-state lighting unit comprised of a 370nm excitation light emitting chip.The emission spectra of these devices displayed in Fig.4A show two components,one from the chip centered at 370nm and the corresponding visible component in the range of 450–770nm emanated from CDs.We analyzed the emission spectra of CDs in the LED devices for their color emission by

CIE (Commission Internationale d’E

clairage)1931chromaticity coordinates.The coordinates were realized at (0.19,0.28),(0.45,0.44),and (0.34,0.37)for CD 200,CD 300and CD 400,respectively (Fig.4B).As shown in Fig.4C–E,the devices provide bright blue,orange,and warm white illumination with CD 200,CD 300and CD 400applied as the phosphors,respectively.These results demonstrate the success in LED fabrication based on high PL performance of CDs,which has not been reported previously.As we know,the development of LEDs continues to be a topic of intense interest because of their great potential in lighting and display,17and traditional semiconductor QDs have been considered as one of the most promising phosphors owing to their color tunability and high luminescence e?ciency.18However,QDs are usually produced in diluted solution with quite low QD content,and the isolation process often involves the tremendous energy consumption as well as the occurrence of aggregation and ?uorescence diminishing of QDs,along with the intrinsic disadvantages of high cost and toxicity,greatly limiting their practical applications.Our work develops phosphors of cost-e?ective CDs with advantages of QD-comparable PL e?ciency,photostability and lower toxicity,which might represent a new paradigm for the generation of phosphors from the general category of semiconductor QDs.

In summary,we have described here the ?rst synthesis of robust CDs from unzipping of epoxy-enriched polystyrene photonic crystals,where a single-step pyrolysis fashion occurs to produce blue-,orange-and white-emitting CDs by varying preparation temperature.Also,the as-prepared CDs are well-dispersed in various polar organic solvents and water,and their PL QYs could be up to 47%without further surface treatment.The successful creation of blue,orange and white LED devices by using these CDs as a new kind of phosphors may further widen the applicability of CDs.This new meth-odology of unzipping colloidal crystals may confer a powerful and general strategy for the creation of multifunctional carbon dots materials,and thus might be adapted for more various applications,for instance,in optoelectronics,sensing,and bioimaging ?elds.

This work was supported by the National Natural Science Foundation of China (21076103,21006046and 21176122),the Specialized Research Fund for the Doctoral Program of Higher Education of China (20103221110001),and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Notes and references

1Y.P.Sun,B.Zhou,Y.Lin,W.Wang,K.A.S.Fernando,P.Pathak,M.J.Meziani,B.A.Harru?,X.Wang,H.F.Wang,P.J.G.Luo,H.Yang,M.E.Kose,B.L.Chen,L.M.Veca and S.Y.Xie,J.Am.Chem.Soc.,2006,128,7756;S.J.Yu,M.W.Kang,H.C.Chang,K.M.Chen and Y.C.Yu,J.Am.Chem.Soc.,2005,127,17604.2L.Y.Zheng,Y.W.Chi,Y.Q.Dong,J.P.Lin and B.B.Wang,J.Am.Chem.Soc.,2009,131,4564.

3Q.Li,T.Y.Ohulchanskyy,R.L.Liu,K.Koynov,D.Q.Wu,A.Best,R.Kumar,A.Bonoiu and P.N.Prasad,J.Phys.Chem.C ,2010,114,12062.

4L.Cao,X.Wang,M.J.Meziani,F.S.Lu,H.F.Wang,P.J.G.Luo,Y.Lin,B.A.Harru?,L.M.Veca,D.Murray,S.Y.Xie and Y.P.Sun,J.Am.Chem.Soc.,2007,129,11318.

5X.Y.Xu,R.Ray,Y.L.Gu,H.J.Ploehn,L.Gearheart,K.Raker and W.A.Scrivens,J.Am.Chem.Soc.,2004,126,12736.

6Y.P.Sun,X.Wang,F.S.Lu,L.Cao,M.J.Meziani,P.J.G.Luo,L.R.Gu and L.M.Veca,J.Phys.Chem.C ,2008,112,18295.7H.Li,X.He,Z.Kang,H.Huang,Y.Liu,J.Liu,S.Lian,C.H.A.Tsang,X.Yang and S.-T.Lee,Angew.Chem.,2010,122,4532.8A. B.Bourlinos, A.Stassinopoulos, D.Anglos,R.Zboril,M.Karakassides and E.P.Giannelis,Small ,2008,4,455;H.P.Liu,T.Ye and C.D.Mao,Angew.Chem.,Int.Ed.,2007,46,6473.

9R.L.Liu,D.Q.Wu,S.H.Liu,K.Koynov,W.Knoll and Q.Li,Angew.Chem.,Int.Ed.,2009,48,4598.

10H.Zhu,X.L.Wang,Y.L.Li,Z.J.Wang, F.Yang and X.R.Yang,https://www.360docs.net/doc/8813645439.html,mun.,2009,5118.

https://www.360docs.net/doc/8813645439.html,nge,N.Metz,M.N.Tahir,F.Fleischhaker,P.Theato,H.-C.Schroeder,W.E.G.Mueller,W.Tremel and R.Zentel,Macromol.Rapid Commun.,2007,28,1987.

12L.L.Yan,Z.Y.Yu,L.Chen,C.F.Wang and S.Chen,Langmuir ,2010,26,10657;Z.Y.Yu,L.Chen and S.Chen,J.Mater.Chem.,2010,20,6182.

13Z.Y.Li,W.H.Zhang,Y.Luo,J.L.Yang and J.G.Hou,J.Am.Chem.Soc.,2009,131,6320.

14D.Y.Pan,J.C.Zhang,Z.Li and M.H.Wu,Adv.Mater.,2010,22,734.

15S.N.Baker and G.A.Baker,Angew.Chem.,Int.Ed.,2010,49,6726.

16F.Wang,M.Kreiter,B.He,S.P.Pang and C.Y.Liu,https://www.360docs.net/doc/8813645439.html,mun.,2010,46,3309.

17S.-N.Yin,C.-F.Wang,Z.-Y.Yu,J.Wang,S.-S.Liu and S.Chen,Adv.Mater.,2011,23,2915.

18S.Yang,C.-F.Wang and S.Chen,J.Am.Chem.Soc.,2011,133,8412;M.J.Bowers,J.R.McBride and S.J.Rosenthal,J.Am.Chem.Soc.,2005,127,15378.

Fig.4(A)Emission spectra of LED lamps.(B)Placement of the CD 200(a),CD 300(b)and CD 400(c)emission spectra on the CIE 1931chromaticity chart.(C–E)Photographs of the corresponding LEDs with CD 200,CD 300and CD 400applied as phosphors,respectively.

D o w n l o a d e d b y C h a n g c h u n I n s t i t u t e o f A p p l i e d C h e m i s t r y , C A S o n 19 A p r i l 2012P u b l i s h e d o n 18 J a n u a r y 2012 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 2C C 17769B