We report here a new phenomenon in photoluminescence of C-60 doped in polystyrene. Under 488 nm cw laser radiation, the PL intensity of the sample was found to increase with time. With 10 h of irradiation, the PL signal strength increased by 10 times, comparable to that from porous Si. The peak of the PL was found to shift to high frequency as well. More detailed studies showed that such an irreversible change of the sample might be a result of the lowering symmetry of oxidized C-60 fullerene in the polystyrene. (C) 1996 American Institute of Physics

Ge, W. K.

Jian, Z.

Loy, M. M. T.

Wang, D. Z.

Xiao, X. D.

Zhang, C.

Zhang, Q. J.

王大志

Applied Physics Letters

Xiamen University Institutional Repository

Photoluminescence study of C60 doped polystyreneChun Zhang, Xudong Xiao, Weikun Ge, M. M. T. Loy, Wang Dazhi et al.  Citation: Appl. Phys. Lett. 68, 943 (1996); doi: 10.1063/1.116106 View online: http://dx.doi.org/10.1063/1.116106 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v68/i7 Published by the AIP Publishing LLC.  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 25 Jul 2013 to 210.34.4.209. This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsceofhechPhotoluminescence study of C 60 doped polystyreneChun Zhang, Xudong Xiao, Weikun Ge, and M. M. T. Loya)Department of Physics, Hong Kong University of Science and Technology, Hong KongWang Dazhi, Zhang Qijin, and Zuo JianDepartment of Materials Science and Engineering and Structure Research Open Lab, University of Scienand Technology of China,Hefei 230026, People’s Republic of China~Received 2 October 1995; accepted for publication 11 December 1995!We report here a new phenomenon in photoluminescence of C60 doped in polystyrene. Under 488nm cw laser radiation, the PL intensity of the sample was found to increase with time. With 10 hirradiation, the PL signal strength increased by 10 times, comparable to that from porous Si. Tpeak of the PL was found to shift to high frequency as well. More detailed studies showed that suan irreversible change of the sample might be a result of the lowering symmetry of oxidized C60fullerene in the polystyrene. ©1996 American Institute of Physics.@S0003-6951~96!00807-7#-emin-f1iomotunthpLmec-,0eeakesinnab-sC60 fullerenes, with the highest occupied molecular obital to lowest unoccupied molecular orbital~HOMO–LUMO! gap of about 1.9 eV,1 show characteristics of molecular semiconductors.2 Neutral C60 fullerenes in solid formexhibit very weak luminescence at room temperature.3 Thislow fluorescence quantum yield is the result of a forbiddHOMO–LUMO transition and the very efficient intersystecrossing to the excited triplet states of C60.4 The strongestluminescence peak~about 1.7 eV! always lies below the tran-sition forbidden HOMO–LUMO gap.5 Matuset al.6 gave aself-trapped exciton model to explain the red-shifted~1.9–1.7 eV! photoluminescence~PL! of pristine C60. When C60 isused as a dopant in a conducting polymer, Yoshinoet al.7–10found that the material exhibits various novel properties suas quenching of photoluminescence~PL! and electrolumines-cence~EL!, enhancement of photoconductivity, and photoduced absorption.11 All those can be explained by photoinduced charge transfer.In this letter, we present detailed PL study of C60 dopedin polystyrene. The PL spectrum from the fresh sampleroom temperature is similar to that of the pristine C60 with apeak at 1.7 eV. However, under 488 nm laser irradiationextended time~on the order of hours! in air, the sample ex-hibited a fluorescence spectrum blue shifted with a peak nat 1.9 eV, while the PL intensity increased by more thantimes. We interpret this effect as due to the photo-oxidatof C60 fullerene in polystyrene.To prepare thin-film samples of C60 doped polystyrene,small polystyrene~PS! pellets and purified C60 ~.99.5%!powder were mixed together with weight ratios ranging fro65:1 to 377:1 in toluene solvent, followed by evaporationthe solvent under ambient atmosphere at room temperaFor PL measurement a cw Ar1 ion laser, operating at 488 nmwith output power of 100 mW was used as the excitisource. The laser beam was not focused and had a radiu1 mm to avoid sample damage. The luminescence fromsample was dispersed by a half meter focal length monocmator and detected by a photomultiplier tube with the helpa lock-in amplifier.Figure 1 shows PL spectra of the polystyrene sama!Electronic mail: phloy@usthk.ust.hkAppl. Phys. Lett. 68 (7), 12 February 1996 0003-6951/96/68(7)Downloaded 25 Jul 2013 to 210.34.4.209. This article is copyrighted as indicated in the abstract.r-nch-atorow0nfre.gs ofhero-oflewith higher C60 doping level PS:C60565:1) during 10 h oflaser irradiation. Curve~a! represents the initial spectrum,taken before extended laser irradiation of the sample. The Psignal was weak but readily observable. The broad spectruis peaked at 730 nm~1.7 eV!, which is the same as that ofpristine C60.3 However, as the laser irradiation time in-creases, the PL intensity increases and the peak of PL sptrum shifts to a shorter wavelength. After 10 h of irradiationthe intensity increases by 10 times and the peak shifts to 68nm @curve ~f! in Fig. 1#. Saturation can be reached if longerirradiation time is used. As shown in Fig. 2 and curve~g! ofFig. 1, the spectral peak eventually shifts to 660 nm. Thabove change is irreversible and the final luminescencstrength is comparable to that of porous silicon.For samples with a lower C60 doping level ~e.g.,PS:C605184:1; 377:1), thesame effect was observed. Af-ter extended laser irradiation of the sample, the spectral peeventually blue shifts to 660 nm as the PL spectrum reachsaturation. However, the lower concentration of C60 results ina smaller increase in PL intensity compared to that shownFig. 2. The undoped polystyrene sample has also beechecked under the same experimental condition. It hasweak PL at 570 nm and no PL enhancement effect was oserved with long time irradiation.Previous studies12–14 showed that solid C60 is sensitiveto light in the UV-vis region. Under UV-vis irradiation in avacuum or inert atmosphere, C60 can open up the doublebonds and form a polymerized structure. Under simultaneouFIG. 1. Room-temperature PL spectra of C60 doped polystyrene under laserirradiation in different exposing times~a! 25 min,~b! 150 min,~c! 275 min,~d! 400 min,~e! 525 min,~f! 650 min,~g! 1150 min.943/943/3/$10.00 © 1996 American Institute of Physics Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissionsatofaotlyrtpt--nge-ueeofs-exposure to UV-vis light and oxygen, C60 undergoes a pho-toassisted reaction with molecular oxygen. Optical vibrtional mode studies indicate rapid diffusion of oxygen insolid C60 with assistance from UV-vis photons. It was alsfound that the presence of O2 suppresses the formation oC60 polymer. The photo-oxidation products are reportedintact fullerene cages such as C60On .To determine the role of oxygen in our case, we repethe experiment by placing the sample under inert gas (N2).As shown in Fig. 3, when monitoring the PL intensity at 66nm, the PL spectrum remained unchanged after 5 h of irra-diation. This result indicates the irreversible PL changeC60 doped polystyrene sample under laser irradiationin air isdue to oxidization of the C60.To further prove our point that the effect is photoinduced, we heated the sample to 150 °C for 5 h. Althoughcolor of the sample changed due to the oxidization of postyrene, no significant change in PL was observed. Differelaser sources were also employed to irradiate the sample.found that the irreversible PL change of C60 doped polysty-rene only occurs when the photon energy of exciting soulies in the absorption band of C603,15 ~see Table I!. Thiswavelength specificity of the phenomena also implies phooxidation of C60 in polystyrene as the mechanism responsibfor the change.Since the photoinduced sample is permanent, we toroom-temperature FTIR transmission spectra of the sambefore and after laser irradiation. The difference betweentwo spectra is shown in Fig. 4. Two broad bands at aboFIG. 2. Change in photoluminescence intensities and peak positionsC60 doped polystyrene under ambient atmosphere during long time lairradiation.FIG. 3. Change in photoluminescence intensities of C60 doped polysytreneunder ambient atmosphere and protecting inert gas during long time lairradiation.944 Appl. Phys. Lett., Vol. 68, No. 7, 12 February 1996Downloaded 25 Jul 2013 to 210.34.4.209. This article is copyrighted as indicated in the abstract. -osat0f-he-ntWeceo-leokleheut1100 and 1750 cm21 indicate that the irradiated sample hadadditional C—O and CvO bonds.16 The 1750 cm21 band isassigned as oxidized C60, in agreement with other work.12Accurate analysis of chemical components and structures of the final product of C60 doped polystyrene after longtime laser irradiation still needs further investigation. Herewe offer one possible mechanism accounting for the enhancement of PL from the sample. Normally, C60 HOMO-LUMO transition is forbidden due to the high symmetry(I h) of the structure.17 Under laser irradiation, C60 undergoesa photoassisted reaction with molecular oxygen in air to formC60On , with oxygen atoms attached to the fullerenecage.18,19 The lower symmetry of the oxidized C60–PS ad-duct strengthens the HOMO-LUMO transition (S1–S0) suchthat the fluorescence can compete with intersystem crossi(S1–T1). This also explains the blue shift of the lumines-cence spectrum.In conclusion, C60 doped polystyrene exhibits weak lu-minescence similar to that from pristine C60. After long timelaser irradiation of the sample, the fluorescence signal bcomes stronger by more than 10 times and the spectrum blshifts to 650 nm. We interpret such effect as a result of thlowering of symmetry of oxidized C60 fullerene in the poly-styrene.This work was supported by William Mong Solid StateClusters Laboratory and the Research Grants CouncilHong Kong.oferserTABLE I. Dependence of photon energy of exciting source of PL enhancement effect of C60 doped polystyrene.Excitingwavelength~nm! PL spectrum Notes488 Change Into the absorption band with a peakmaximum at 450 nm514 Change Into the absorption band with a peakmaximum at 450 nm633 Change Directly into the lowest energyabsorption band808 No change Out of the absorption band of C60812 No change Out of the absorption band of C60FIG. 4. FTIR transmittance difference spectra for C60 doped polystyrenebefore and after long time laser irradiation.Zhang et al.Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions.l.y.....1S. H. Yang, C. L. Pattiette, J. Conceicao, O. Cheshnovsky, and RSmalley, Chem. Phys. Lett.139, 233 ~1987!.2J. H. Weaver, J. L. Martins, T. Komeda, Y. Chen, T. R. Ohno, G. H. KroN. Troullier, R. E. Haufler, and R. E. Smalley, Phys. Rev. Lett.66, 1741~1991!.3C. Reber, L. Yee, J. McKiernan, J. I. Zink, R. S. Williams, W. M. Tong, DA. A. Ohlberg, R. 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A. Zakhidov, Synth.Met. 70, 1317~1995!.11K. Isobe and K. Kaneto, Synth. Met.70, 1405~1995!.12P. C. Eklund, A. M. Rao, P. Zhou, Y. Wang, K.-A. Wang, G. T. Hager, andJ. M. Holden, Mater. Sci. Eng.19, 154 ~1993!.13J. H. Callahan, S. W. Mcelvany, and M. M. Ross, Int. J. Mass SpectromIon Proc.138, 221 ~1994!.14A. Dworkin, H. Szwarc, and R. Ceolin, Europhys. Lett.22, 35 ~1993!.15S. Saito and A. Oshiyama, Phys. Rev. Lett.66, 2637~1991!.16X. Pecsok, P. C. Painter, J. R. Shelton, and J. L. Keonig, Rubber ChemTechnol.49, 1012~1976!.17R. C. Haddon, L. E. Brus, and K. Raghavachari, Chem. Phys. Lett.125,459 ~1986!.18M. Menon and K. R. Subbaswamy, Chem. Phys. Lett.201, 321 ~1993!.19Z. Slanina, F. Uhlid, S.-L. Lee, and L. Adamowicz, Fullerene Sci. Technol2, 7 ~1994!.945Zhang et al. Reuse of AIP content is subject to the terms at: http://apl.aip.org/about/rights_and_permissions

Photoluminescence study of C-60 doped polystyrene

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Photoluminescence study of C-60 doped polystyrene

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