Exciton interactions in CdS nanocrystal aggregates in reverse micelleLi Cao, Yanming Miao, Zebo Zhang, Sishen Xie, Guozhen Yang, and Bingsuo Zou Citation: The Journal of Chemical Physics 123, 024702 (2005); doi: 10.1063/1.1904563 View online: http://dx.doi.org/10.1063/1.1904563 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/123/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Exciton polarizability and absorption spectra in CdSe/ZnS nanocrystal quantum dots in electric fields J. Appl. Phys. 114, 043709 (2013); 10.1063/1.4816559 Photoluminescence properties and energy transfer processes from excitons to Mn 2 + ions in Mn 2 + -dopedCdS quantum dots prepared by a reverse-micelle method J. Appl. Phys. 100, 094313 (2006); 10.1063/1.2363238 Strong enhancement of band-edge photoluminescence in CdS quantum dots prepared by a reverse-micellemethod J. Appl. Phys. 98, 083514 (2005); 10.1063/1.2106008 Photoluminescence properties of single Mn-doped CdS nanocrystals studied by scanning near-field opticalmicroscopy Appl. Phys. Lett. 87, 133104 (2005); 10.1063/1.2058228 Excitons and surface luminescence of CdS nanoribbons Appl. Phys. Lett. 84, 795 (2004); 10.1063/1.1644625

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Exciton interactions in CdS nanocrystal aggregates in reverse micelleLi Cao, Yanming Miao, Zebo Zhang, Sishen Xie, and Guozhen YangNanoscale Physics & Device Laboratory, Institute of Physics, CAS, Beijing 100080, China

Bingsuo Zoua�

Nanoscale Physics & Device Laboratory, Institute of Physics, CAS, Beijing 100080;Micro-Nano Technologies Research Center, Hunan University, Changsha 410082, China

�Received 3 December 2004; accepted 16 March 2005; published online 18 July 2005�

Here we report the formation and spectroscopic properties of cadmium sulfide �CdS� nanocrystalsystems: individual nanocrystal and CdS aggregates. The optical absorption and luminescencespectra of the aggregated CdS nanocrystals and individual nanocrystal show exciton aggregate andindividual exciton characteristics. Although it is not Bose–Einstein condensation, such aggregatedquantum dots �QDs� seem to supply us opportunity to study the interactions and condensation ofexcitons in multi-QDs system, not in the separated QDs system. © 2005 American Institute ofPhysics. �DOI: 10.1063/1.1904563�

I. INTRODUCTION

The issue of Bose–Einstein condensation �BEC� of exci-tons has been extensively studied, following the early sug-gestion by Keldysh and Kozlov.1 During the past ten years,coupled quantum wells have emerged as a promising systemfor experiments on Bose condensation of excitons, with nu-merous theoretical2–4 and experimental5–7 studies aimed atthe demonstration of this effect. Excitons in quantum dots�QDs� have not been taken into consideration due to the lim-ited space for many excitons. However, it is still possible toconstruct a solid structure composed of multi-quantum dots,in which the electron and hole occupy different dots, theseexcitons are correlated electrostatically in different spatialconfinement. Here we report the spectroscopic properties ofCdS QD systems, for example, individual nanocrystal andCdS aggregates. The optical absorption and luminescencespectra of the aggregated CdS nanocrystals and individualnanocrystal show exciton aggregate and individual excitoncharacteristics. Such idea can be borrowed from the molecu-lar aggregates, because the exciton in a QD can also be con-sidered as an exciton in conjugated molecule,8 whose aggre-gation has attracted much attention.9,10 In this field, Bawendiand co-workers9 observed the spectral redshift of the capedCdSe QD solid; Kotov and co-workers10 reported the CdTenanocrystal aggregate growth to nanowire. Although theseare not related to the BEC of exciton, the aggregated QDsseem to supply us opportunity to study the interactions andBEC of excitons in multi-QDs, not in the separated QDssystem.

Before this study, the stability and interfacial force ofcapped CdSe nanocystal aggregates have been concerned,11

however, their optical properties have not been studied. Theabsorption band redshift and blueshift in different silvernanocrystal aggregate have been reported due to the electro-dynamic interaction.12 For Quantum confined semiconductornanocrystal considered as a exciton, if its size is less than the

Bohr radius,13–15 a common interest is to study their optical,magnetic, and electric properties.16–19 CdS nanocrystal is atypical II-VI semiconductor �exciton Bohr radius is about2–3 nm�.20–22 Its optical properties and exciton confinementhave been well defined in dispersed matrix.23–26 So CdSnanocrystal has been a good benchmark for the study on theconfined exciton in QD.

Over the past years, several theories and experimentshave indicated the evidences of Bose effects or BEC of ex-citons in semiconductors,2,3,5,7,27,28 especially Cu2O,29,30

which has a parity-forbidden direct gap that means a rela-tively long excitonic lifetime. In fact the exciton BEC en-countered serious problem due to the exciton diffusion orexciton lifetime in a macroscopic system. Hence scientistshave turned their eyes on the confined system.3 Coupledquantum well has been chosen to be one of the candidates forstudying exciton BEC �Refs. 4, 5, 7, and 31� in which theelectron and the hole of one exciton stay in different wells,therein lead to a long lifetime for BEC.

The monodisperse nanocystals are interesting for theirdominant quantum confinement effect, which could lead tolong lifetime excitons. Many experimental results indicatethe surface trapping forbids the long-lived exciton and BECstudies. In fact suitable surface coating with inorganic andorganic layer may effectively confine the exciton in aQD.32,33 If several QDs aggregates with long-lived excitonsare prepared, may they show exciton BEC?

In this work, CdS QD and QD aggregates are preparedindependently, their absorption and photoluminescence spec-tra are studied and the aggregated system shows clear evi-dence of exciton interactions, which may be a useful systemfor exciton BEC studies.

II. EXPERIMENTAL SECTION

We studied the monodisperse CdS QD and CdS QD ag-gregates. The nanocrystals are prepared by controlling pre-cipitation in alcohol, the reactants are Cd�AC�2 and Na2S,whose solution concentrations are 10−2M. After centrifuga-a�Electronic mail: zoubs@aphy.iphy.ac.cn

THE JOURNAL OF CHEMICAL PHYSICS 123, 024702 �2005�

0021-9606/2005/123�2�/024702/6/$22.50 © 2005 American Institute of Physics123, 024702-1

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tion, these nanocrystals are capped with surfactants in tolu-ene under refluxing. The surfactant is oleic acid. After theseproducts are separated from the aqueous solution, refluxingin toluene for 3 h, and then the remnant water was removedafter distillation. The difference for the monodispersed andaggregated systems is the amount of oleic acid used. Theformer use 3 ml oleic acid �the amount of oleic acid is threetimes than the latter� for 200 mg CdS nanocrystals. Aftercapping the former produces transparent colorless CdS solu-tion �diluted solution�. The latter shows suspended solutionwith orange color and lower transparency than the former.The HRTEM �enlargement factor: 1.8�105� and TU-1901UV-visible absorption spectrometer and PTI-C-700 fluores-cence spectrometer are used to study their morphology, op-tical absorption, and emission properties.

III. RESULTS AND DISCUSSION

Figure 1�a� and 1�b� are the HRTEM images of CdSnanocrystal coated by oleic acid and aggregated CdS nano-crystals coated by oleic acid. All of the coated individualnanocrystal are monocrystals, which are separated by thesurfactant and the mean diameter of the monocrystals isabout 4 nm as shown in Fig. 1�a�. The nanocystals in coatedaggregated system are randomly packed together or con-nected with their own lattice orientation and packing statesas shown in Fig. 1�b�. The mean size of nanocrystals in theaggregated system is also around 4 nm, and the whole size ofthe aggregate is 15 nm around. The aggregation degree ofnanocrystals is determined by the amount of surfactant mol-ecules added. More surfactants could lead to less aggregation

of nanocrystals or to coat only one nanocrystal as Fig. 1�a�shown. In this process the amount of oleic acid is small,leading to reverse micelle with one pool containing averagefive to seven nanocrystals as illumination in Fig. 2. Clearlythe aggregation number is not uniform. There are surfactantmolecules �oleic acid� coating the aggregates, minimumamount of water and ethanol molecules may exist in reversemicelle pool to help stabilize the reverse micelle. The arrowsare labeled as the orientation of the lattice axis in each nano-crystals as indicated by HRTEM.

Figure 3�a� is the absorption spectrum of individual-nanocrystal organosol. It shows obvious characteristics ofone confined exciton in a nanocrystal. There are not anyinteractions between excitons in different nanocrystals andthe band edge absorption wavelength shown in Fig. 2�a� is390 nm around. Figure 2�b� presents the absorption spectrumof the aggregated CdS nanocrystals, which is different fromthe organosol of monodispersed CdS nanocrystals. This ab-sorption spectrum indicates the band edge at about 485 nm,corresponding to the nanocrystal which diameter is above 8nm. HRTEM observation �in Fig. 1�b�� proves that the diam-eter of the nanocrystals is about 4 nm, and no 8 nm nano-crystals exist in this system. Compared to the colorless CdSsolution, the aggregated solution shows low transparency

FIG. 1. �a� HRTEM picture of individual CdS nanocrystal capped by oleicacid and �b� CdS aggregates capped by oleic acid, inset are the high-resolution phase of selected areas.

FIG. 2. �a� The nanocluster aggregation in reverse micelle �arrow indicatethe axis of crystal lattice�; �b� the macroscopic exciton after photoexcitationof QD aggregates if their lattice axis are parallel.

024702-2 Cao et al. J. Chem. Phys. 123, 024702 �2005�

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and orange color, indicating strong absorption in the visiblespectral region. Therefore the bandedge shift from 390 nm to485 nm originates from interactions of 4 nm nanocrystals.Moreover, the huge difference between the measured absorp-tion bandedge, a “virtual band edge �about 2.6 eV�” and theintrinsic band edge �about 3.2 eV�, may have important valuein their properties and applications.

Figure 4�a� is the photoluminescence spectra and the ex-citation spectra of monodispersed CdS nanocrystal organo-sol. The band edge emission is at about 393 nm, and a broad-band at around 515 nm is from the surface state. The black orblue continuous curves indicate the excitation spectra for dif-ferent emissions �dashed line� at band edge and surface state,respectively in Fig. 4�a�. Generally, the surface state emis-sion appears around 530–650 nm in the literature, this blue-shift �to 515 nm� of surface state in this experiment shouldbe related to the oleic acid group on the surface of CdSnanocrystals. The excitation spectra represent the absorptioncontribution to the band edge emission. Figure 4�b� is thephotoluminescence spectra and excitation spectra of CdSnanocrystal aggregate solution for band edge emission. Theblack continuous curve indicates the excitation spectrum andthe black dashed curve is the emission spectrum, the high

excitation absorption intensity at near 385 nm in the excita-tion spectra is due to the resonant enhancement by the bandedge emission. The emission peaks are at about 393 nm �A�and 494 nm �B�. The peak A corresponds to the band edgeemission of the individual CdS nanocyrstal, which can beexplained that there are some individual and less sufficientlycoated CdS nanocrystals in the aggregated solution becauseof aggregation distribution, that is to say, individual nano-crystal and the aggregate coexist in this solution. Howeverthe peak B �494 nm� is just located near the “virtual band-edge” which is described above. Peak B is very wide, as isknown that the surface state cannot be removed because ofthe lacking of oil acid in aggregate system, so we deduce thatthere exist another band in the region of 400–490 nm besidesthe surface state emission at 515 nm. Figure 4�b� also indi-cates the fitted emission band of peak A and B, in which E isthe band edge emission, L is originated from the excitationline of 355 nm due to the light scattering, D at 475 nm, canbe assigned to the virtual band edge emission of nanocrystalaggregate, whose corresponding absorption spectrum isshown in Fig. 3�b�. S is surface state emission at 515 nm,same as that in Fig. 4�a�. Another phenomenon is that boththe emission peak D and S is strongly relative to peak A

FIG. 3. �a� Exciton absorption of one CdS nanocrystal �4 nm� in reversemicelle. �b� Absorption spectrum of CdS nanocrystal aggregated solution.

FIG. 4. �a� The photoluminescence and excitation spectra of individual CdSnanocrystal organosol. �b� The photoluminescence spectra at 355 nm exci-tation and excitation spectrum of aggregated CdS nanocrystal organosol at393 nm emission, and the green curves are the fitted curves for the emissionspectrum.

024702-3 Exciton interactions in CdS nanocrystals J. Chem. Phys. 123, 024702 �2005�

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recession as compared with those of in Fig. 4�a� because theoleic acid capping �confinement� is less sufficient in the ag-gregated system.

To check the origin of peak B, we measured the excita-tion spectra for the emission peak B and emission spectraunder 380 nm excitation �see Fig. 5�, and observed that theemission �494 nm� comes from the intrinsic electronic ab-sorptions �above 390 nm in energy� of independent nano-crystals, and only negligible portion of absorption in 390–480 nm contributes. This phenomenon is similar to that ofthe blue continuous line in Fig. 4�a�. Fitting results show alsotwo components, 475 nm and 515 nm, but relative intensityof 475 nm band to 515 nm band decreased as compared withthat by 355 nm excitation �Fig. 4�b��, the difference of spec-tral weight of D , S in Fig. 4�b� and D1 , S1 in Fig. 5 resultsfrom different excitation wavelengths. This change is inagreement with the well known concept that the band edgeexcitaiton may produce more population on the surfacestates. This also indirectly proved the correctness of B bandfittings. The single particles contribute to 515 nm emission,and the aggregates contribute to the 475 nm emission besides515 nm; however, the absorptions in the region of 400–455nm �Fig. 3�b�� for the coated aggregated nanocrystals do notcontribute to B band. Hence the absorption in 400–465 nmdoes not come from individual particles, but from the inter-actions of nanocrystals. Moreover the absorption at this re-gion mainly contributes to radiationless transitions due to theselection rule. Therefore, there exists an energy level at 475nm, which can be resonantly excitated and can be observedat the long-wavelength end of the excitation spectra. Theoccurrence of 475 nm emission band and the absorption bandshift �392 nm to 490 nm� come from the interactions ofneighboring nanocrystals.

The interaction model between Frenkel excitons can ex-plain the above phenomenon as proposed in reference.34

There are two conditions to realize this interaction. The firstis the binding energy of exciton is much larger than the ex-citon dissociation energy. For coated CdS nanocrystals thebinding energy is proportional to the reciprocal of particleradius, so this energy is large enough to overcome the ther-mal effect at room temperature. The second is that the dis-

tance of excitons is close enough for dipole-dipole to couple.Then two excitons interact to produce two levels, the popu-lation between two levels depends the orientation of twoexciton dipoles. In this case, maybe the lattice axis orienta-tions as shown in Fig. 1 determine the exciton dipole orien-tation. Because the lattice axes of two neighboring nanocrys-tals are not in parallel or head to tail alignments, the levelsplitting magnitude from neighboring nanocrystals should bevariable. Moreover the population of two levels, above theintrinsic band edge and below the band edge, will occur si-multaneously above and below the band edge after photoex-citation. The 475 nm emission should be the down excitonlevel �about 470 nm� after exciton interaction. Above thisband to the band edge of single particles there is no morelevels populated�400–460 nm�, and the excitons migrate be-tween nanocrystals due to energy transfer, so efficient emis-sion cannot be observed if excitating in this region. Besidesthis excitation phenomenon, the dipole-dipole interaction ex-ist in aggregate, quadrupolar interaction should be enhancedsignificantly. These aggregate may behaves with clear non-linear optical phenomenon.

In order to confirm the above consideration, it is impor-tant to detect the up level due to the exciton interactions.Figure 6 is the spectra of the upconversion photolumines-cence of the nanocrystal aggregate film under Xe lamp of250 W. The upconversion weak bands �P1 and P2� from theaggregated CdS appear when excitating at 476 nm line withincreasing intensity. There is no emission band for the indi-vidual particle film. It is interesting that one emission band isrightly at the position of intrinsic band edge �about 395 nm�of individual nanocrystals, and another band is at about 350nm. This 350 nm band can be assigned to the up level ofexciton interaction in aggregates, so it cannot be observed inthe individual nanocrystal system. The upconversion phe-nomenon is essentially the process of multiple-photon ab-sorption by the quadrupolar transition. We did not see suchband in the solution, but only in the aggregate film. The bandat about 395 nm should come from the unaggregated nano-

FIG. 5. The excitation and emission spectra of CdS aggregates at excitationof 380 nm, fitting data, and the excitation spectra for 494 nm emission.

FIG. 6. The upconversion photoluminescence spectra of CdS aggregatesfilm. The excitation wavelength is about 476 nm.

024702-4 Cao et al. J. Chem. Phys. 123, 024702 �2005�

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crystals in the aggregated system as described above. Thetwo peaks P1 and P2 are wide in Fig. 6, which can be attrib-uted to the strong self-absorption phenomena because of therelatively high concentration in CdS aggregate film. Thepeaks of 350 nm �in Fig. 6� and 475 nm �in Figs. 4�b� and 5�is up and down exciton levels due to exciton interactionswith oblique orientation of their dipoles, which agree withthe theoretical prediction in Fig. 7�a�. The P1 and P2 emis-sion bands are gradually enhanced when the excitation inten-sity increases. As is known, the upconversion emission in-tensity increases significantly with the excitation intensityincreasing. The fact in Fig. 6 also supports the existence ofexciton interactions.

IV. DAVYDOV SPLITTING OF EXCITON INTERACTION

The individual QD can be thought of as an exciton afterphotoexcitation. If two excitons are close to each other, theymay encounter dipole-dipole interactions. A simple two par-ticles system is considered in order to explain this kind ofelectronic dipole-dipole interaction as that of Frenkel exci-ton, similar to two molecules. Figure 7�a� gives the excitedband structure and transition dipole array relation of twonanocrystals bound by electrostatic interaction. Hypotheti-cally, �G and �E individually indicate the wave function of

the ground and excited states of single nanocrystal. When thenanocrystals exist in the form of aggregation, the wave func-tion of the ground state is unchanged and that of the exci-tated state may be complex. The wave function of the exci-tation state of the simple two particles system �particle A andparticle B� can be expressed by the quantum mechanics:

�E�A,B� = �1/2�1/2 � ��G

A�EB ± �E

A�GB� ,

where �GA and �E

A are individually the wave function of theground and excitation states of the nanocrystal A and �G

B and�E

B are that of the nanocrystal B. Before aggregation A and Bhave equal energy. However, the interaction of the two exci-tons induces the energies from two exictions are splitted intotwo levels, one larger than original level, another smallerthan original level. This is called Davydov splitting.

The absorption and fluorescence spectra of aggregatedCdS nanocrystals could be explained with Davydov splitting.The energy difference of the ground state and the excitationstate depends on the interaction of the transition dipole oftwo nanocrystals in Fig. 7�a�, and Fig 7�b� shows the excitonabsorption profiles of the aggregated nanocrystal solutioncontaining individual nanocrystals. The exciton in each CdSnanocrystal owns its own independent dipole. Two excitonsinteract to produce two levels. The two levels from Davydovsplitting of exciton interaction produce absorption and emis-sion band �around 475 nm and 350 nm� due to the interac-tions. The broadband optical spectra reflecte the oblique in-teraction of excitons with random aggregation number andorientation. The transition rules for the two spiltting levels tovalence band depend on the symmetry of excitons. In oursystem random orientation of exciton in different nanocrys-tals should be responsible for the transition probability of350 nm and 475 nm emissions. The more parallel orientationof excitons in our system clearly show more population onthe up level, and the down level absorption may accompanyefficient energy transfer. This energy transfer should be thecause that minor contributions of 400–475 nm absorptionsproduce 475 nm emissions. This fact indicates the possibleformation of large exciton composed of multinanocrystals,and their transitions depend on the orientations of excitons.

Excitons interaction in semiconductor nanostructures isimportant for laser emission processes and nonlinear opticalresponses. Above experimental results indicate a preliminaryevidence of exciton interactions between QDs, this interac-tion resembles the Frenkel excitons interactions in dye mol-ecules. However, in past no one report this phenomenon inWannier exciton systems. There are Wannier excitons in CdScrystals. Our results support the concept that QD can be con-sidered as an exciton and the coupling between excitons indifferent QDs may lead to big exciton or exciton condensa-tion. It is important that the distance between QDs should benear zero for easy exciton couplings. If the excitons in QDsare connected via head to tail with each other, they mayexhibit giant oscillator effect to lead to polaritons.35 There-fore the close packed QD’s wire may be a good choice forthe study of the exciton BEC. If we can control the axisorientations of nanocrystals in the same direction, it is pos-sible to produce a giant exciton with long lifetime and studythe excitons condensation.

FIG. 7. �a� The excitation band structure-transition dipole array relation ofbounded two nanocrystals; �b� the exciton absorption profiles of nanocrystalaggregates and individual nanocrystals.

024702-5 Exciton interactions in CdS nanocrystals J. Chem. Phys. 123, 024702 �2005�

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V. CONCLUSIONS

In conclusion, we have prepared aggregated CdS nano-crystals and individual nanocrystal with capped by oleicacid. Aggregation can be verified by HRTEM picture andUV-visible absorption. These aggregated nanocrystals showinteresting spectroscopic properties, different from the be-haviors of the individual QD. Spectra indicate that multiex-citons interaction between QDs lead to new band absorptionand emission below and above the intrinsic band edge. Theexciton interactions in QD aggregate also show obviousDavydov splittings due to the dipole-dipole interactions. Thisexample will help us to understand �1� the one-dimensional�1D� array of QD aggregate for laser emission36,37 andbistable optical devices,38 and �2� 2D or 3D aggregate forstudying exciton condensation.

ACKNOWLEDGMENTS

The authors thank National Natural Science Foundationof China �Term No. 20173073�, national 973 project ofMOST of China, and 985 fund of Hunan University for thefinancial support.

1 L. V. Keldysh and A. N. Kozlov, Zh. Eksp. Teor. Fiz. 54, 978 �1968��Sov. Phys. JETP 27, 521 �1968��.

2 H. Chu and Y. C. Chang, Phys. Rev. B 54, 5020 �1996�.3 Z. G. Koinov, Phys. Rev. B 61, 8411 �2000�.4 Y. Naveh and B. Laikhtman, Phys. Rev. Lett. 77, 900 �1996�.5 L. V. Butov and A. I. Filin, Phys. Rev. B 58, 1980 �1998�.6 V. V. Krivolapchuk, E. S. Moskalenko, and A. L. Zhmodikov, Phys. Rev.B 64, 045313 �2001�.

7 L. V. Butov, C. W. Lai, A. L. Ivanov, A. C. Gossard, and D. S. Chemla,Nature �London� 417, 47 �2002�.

8 S. Schmitt-Rink, D. A. B. Miller, and D. S. Chemla, Phys. Rev. B 35,8113 �1987�.

9 C. R. Kagan, C. B. Murray, and M. G. Bawendi, Phys. Rev. B 54, 8633�1996�.

10 Z. Y. Tang, N. A. Kotov, and M. Giersig, Science 297, 237 �2002�.11 H. Mattoussi, A. W. Cumming, C. B. Murray, and M. G. Bawendi, and R.

Ober, Phys. Rev. B 58, 7850 �1998�.

12 A. Taleb, V. Russier, A. Courty, and M. P. Pileni, Phys. Rev. B 59, 13350�1999�.

13 H. Htoon, H. B. Yu, D. Kulik, J. W. Keto, O. Baklenov, A. L. Holmes, B.G. Streetman, and C. K. Shih, Phys. Rev. B 60, 11026 �1999�.

14 L. E. Brus, J. Chem. Phys. 80, 4403 �1984�.15 U. Banin, G. Cerullo, A. A. Guzelian, C. J. Bardeen, A. P. Alivisatos, and

C. V. Shank, Phys. Rev. B 55, 7059 �1997�.16 K. Ramamoorthy, C. Sanjeeviraja, M. Jayachandranb, K. Sankaranaray-

ananc, P. Bhattacharyad, and L. M. Kukrejad, J. Cryst. Growth 226, 281�2001�.

17 J. Loffler, R. Groenen, J. L. Linden, M. C. M. van de Sanden, and R. E.I. Schropp, Thin Solid Films 392, 315 �2001�.

18 K. S. Weissenrieder and J. Muller, Thin Solid Films 300, 30 �1997�.19 L. B. Kong, F. C. Li, L. Y. Zhang, and X. Yao, J. Mater. Sci. Lett. 17, 769

�1998�.20 V. A. Fonoberov, E. P. Pokatilov, and A. A. Balandin, Phys. Rev. B 66,

085310 �2002�.21 T. Vossmeyer, L. Katsikas, M. Gienig, I. G. Popovic, K. Diesner, A.

Chemseddine, A. Eychmiiller, and H. Weller, J. Phys. Chem. 98, 7665�1994�.

22 F. Koberling, A. Mews, and T. Basche, Phys. Rev. B 60, 1921�1999�.

23 A. D. Yoffe, Adv. Phys. 42, 173 �1993�.24 Y. Wang and N. Herron, J. Phys. Chem. 95, 525 �1991�.25 R. L. Joseph, G. Ignacy, and J. M. Catherine, J. Phys. Chem. B 103,

7613 �1999�.26 P. Nandakumar, C. Vijayan, and Y. V. G. S. Murti, J. Appl. Phys. 91,

1509 �2002�.27 K. Johnsen and G. M. Kavoulakis, Phys. Rev. Lett. 86, 858 �2001�.28 D. Porras, C. Ciuti, J. J. Baumberg, and C. Tejedor, Phys. Rev. B 66,

085304 �2002�.29 J. L. Lin and J. P. Wolfe, Phys. Rev. Lett. 71, 1222 �1993�.30 D. Snoke, Science 298, 1368 �2002�.31 L. V. Butov, A. C. Gossard, and D. S. Chemla, Nature �London� 418,

751�2002�32 H. Jasmina, M. D. Nada, A. W. Gregory, and P. W. Gary, J. Am. Chem.

Soc. 124, 4536 �2002�33 S. Gorer, J. A. Ganske, J. C. Hemminger, and R. M. Penner, J. Am.

Chem. Soc. 120, 9584 �1998�34 Excitons, edited by E. I. Rashba and M. D. Sturge �North-Holland, Am-

sterdam, 1982�, Vol. 2.35 E. Hanamura, Phys. Rev. B 37, 1273 �1988�.36 L. Cao, B. S. Zou, C. R. Li, Z. B. Zhang, S. S. Xie, and G. Z. Yang,

Europhys. Lett. 68, 740 �2004�.37 V. Chernyak, T. Meier, E. Tsiper, and S. Mukamel, J. Phys. Chem. A

103, 10294 �1999�.38 V. A. Malyshev, H. Glaeske, and K.-H. Feller, J. Chem. Phys. 113, 1170

�2000�.

024702-6 Cao et al. J. Chem. Phys. 123, 024702 �2005�

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