Optical Materials 34 (2011) 6–11

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Optical Materials

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Stable CdS QDs with intense broadband photoluminescence and highquantum yields

Abhijit Mandal, Jony Saha, Goutam De ⇑Nano-Structured Materials Division, Central Glass and Ceramic Research Institute, Council of Scientific and Industrial Research, 196, Raja S.C. Mullick Road, Kolkata, India

a r t i c l e i n f o

Article history:Received 28 April 2011Received in revised form 2 July 2011Accepted 4 July 2011Available online 3 September 2011

Keywords:CdS QDsThiolactic acidPhotoluminescenceHigh quantum yield

0925-3467/$ - see front matter � 2011 Elsevier B.V. Adoi:10.1016/j.optmat.2011.07.019

⇑ Corresponding author. Tel.: +91 33 24733469/96xE-mail address: gde@cgcri.res.in (G. De).

a b s t r a c t

Aqueous synthesis of CdS quantum dots (QDs) using thiolactic acid (TLA) as a capping agent was reported.These QDs exhibited excellent colloidal and photostability over a span of 2 years and showed intensebroadband and almost white photoluminescence suitable for solid state lighting devices. The photolumi-nescence (PL) property of the aqueous CdS QDs is optimized by adjusting various processing parameters.The highest quantum yield (QY) achieved for TLA capped CdS QDs of average size 3.5 nm was �50%.Luminescence lifetime measurements of CdS–TLA QDs indicated longer lifetimes and a larger contribu-tion of the surface-related emission, indicating removal of quenching defects.

� 2011 Elsevier B.V. All rights reserved.

1. Introduction

Semiconductor nanoparticles or quantum dots (QDs) become arecent topic of investigations [1–3] mainly due to their inherentbroad photoluminescence (PL) applicable for the white light emit-ting devices (LED) [1,3–6]. For solid state lighting applications,white light emitting QDs with high QYs are necessary. Further, sin-gle emitting component as white light source is preferable becauseof better reproducibility, cost effective, and simple fabrication pro-cedures. So far, a few results have been reported on the synthesis ofwhite light emitting QDs [6–9]. Although some of the reported QDs[10] showed white light emission, their QYs were of the order of10–20% and not enough for practical applications. So, synthesisof QDs that have broadband and closely white photoluminescence(PL) with noticeable QY can be useful from application point ofview. Aqueous semiconductor quantum dots (QDs) are suitablecandidates because of their unique properties, including strongluminescence, long luminescence lifetime, size-dependent opticalproperties, broad absorption cross-section [11–15], and so on.CdS QDs emitting light in the visible range (400–700 nm) areundoubtedly among the most promising materials for the fabrica-tion of LEDs. Efforts are on improving the aqueous synthesis meth-od to tune the size of such semiconductor crystals and preventsurface defects using capping molecules [16,17]. So far, thioglycolicacid (TGA), mercapto ethanol, thioglycerol, and 3-mercaptopropi-onic acid (3MPA) were used as common capping agents [18]. Theorganic capping provides electronic and chemical passivation of

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surface dangling bonds, prevents uncontrolled growth and agglom-eration of the QDs, and allows QDs to be chemically manipulatedlike large molecules with solubility and reactivity determined bythe identity of the surface ligand [19]. In an effort to producehighly stable and high luminescent CdS QDs, we have used thiolac-tic acid (TLA) as a capping molecule [20]. TLA is expected to be amuch better capping agent compared to other similar moleculesreported in the literature [18] because of the presence of methylgroup. The methyl group (+I effect) attached with the secondarycarbon atom of TLA will increase the electron density on thiolategroup causing better nucleophilicity. The hydrophobic characterof the TLA will also help in preventing the agglomeration of CdSQDs after capping. This study revealed that the process variablessuch as pH, Cd/S, and TLA/Cd ratios strongly influence the lumines-cence behavior of CdS–TLA QDs. Further, TLA capped CdS QDs pre-pared in this route showed broadband emission with a full-widthhalf-maximum (FWHM) of about 125 nm with high quantum effi-ciency. This can make the QDs to be useful in white light genera-tion for sold state lighting application [21,22]. The opticalproperties of TLA capped CdS QDs with steady state and timeresolved measurements have been investigated.

2. Experimental methods

2.1. Materials

All the chemicals were the highest purity available and used asreceived. Milli-Q water (18.2 MX) was used throughout this study.

Table 1Photophysical properties of CdS–TLA QDs as a function of pH during preparation.Synthetic conditions are room temperature and Cd/S/TLA = 2:1:4.

pH Absorbance max(nm)

PL max peak(nm)

QY Average lifetime(ns)

11.0 375 530 0.10 11.310.0 365 515 0.15 17.1

9.3 365 512 0.19 19.28.0 365 505 0.45 31.57.6 360 505 0.38 29.76.5 360 495 0.31 26.13.0 350 480 0.05 7.3

2 4 6 8 10 120.0

0.1

0.2

0.3

0.4

0.5

Qua

ntum

Yie

ld (Q

Y)

pH

Fig. 1. Plot of PL quantum yield (QY) of TLA capped CdS QDs as a function of pHduring the preparation of QDs by maintaining the Cd/S/TLA ratio of 2:1:4. The datapoints are joined with a line for visual guide.

A. Mandal et al. / Optical Materials 34 (2011) 6–11 7

2.2. Synthesis of CdS QDs

Aqueous solution of cadmium acetate dihydrate, Cd(OOCCH3)2

�2H2O solution (0.04 M), and sodium sulfide, Na2S (0.02 M), was firstprepared. For the representative sample with a molar ratio of Cd/S/TLA = 2:1:4, required amount of TLA (Sigma–Aldrich) was first dis-solved in 50 ml of water and to this solution 4 ml of 0.04 MCd(OOCCH3)2 solution was poured slowly with constant stirring.The pH of the solution was then adjusted with 0.1 M NaOH in therange of 3–11. Then, calculated amount of 0.02 M Na2S solutionwas added drop wise to the above solution. The as prepared QDs pre-pared at around pH 8.0 was used to reflux at around 100 �C (open aircondensation) for about 4 h to promote the growth of CdS QDs. Fivefractions were collected at different time intervals (called 0, 0.5, 1, 2,and 4 h) during reflux. The obtained TLA capped CdS QDs were dis-persed in water and characterized.

2.3. Characterization

Absorption spectra were recorded with a UV–Vis spectropho-tometer (Cary 50, Varian) in the 250–550 nm range. Photolumines-cence (PL) emission and excitation spectra were recorded on aPerkin Elmer LS-55 spectrophotometer. For all spectral measure-ments, a 1-cm path length quartz cell was used. All PL spectra werecalibrated with the absorption values at the excitation wavelength.PL quantum yields (QY) were measured using rhodamine B inwater, according to the previously described methods [18,23].Transmission electron microscopic (TEM) measurements were car-ried out with a Tecnai G2 30ST (FEI) operating at 300 kV. TEM sam-ples were prepared by drop casting the diluted solution onto acarbon-coated grid. The luminescence decay measurements wereperformed by time correlated single-photon counting (TCSPC)method. All luminescence decay curves were measured at themaximum of the photoluminescence peak. These samples were ex-cited at 373 nm using a picosecond diode laser (IBH Nanoled-07) ata repetition rate of 1 MHz. The typical FWHM of the system re-sponse is about 200 ps. The luminescence decays were analyzedusing IBH DAS6 software. The quality of fit was judged in termsof weighted residuals and reduced v2 values.

3. Results and discussion

The experimental parameters were varied in order to optimizethe synthesis of best possible TLA capped Cds QDs (TLA–CdS).These parameters are variation of solution pH with monitoringthe PL intensities and quantum yields and variation of relative con-centrations of Cd, S, and TLA. These are discussed in the followingsections separately.

3.1. Effect of pH during the synthesis of TLA–CdS QDs

TLA stabilized CdS QDs were synthesized at different pH (3–11)values prior to Na2S addition. In this case, the experimental resultsare discussed using a representative composition Cd/S/TLA = 2:1:4.It was observed that the CdS QDs were dispersed uniformly in thesolution at a pH above 6.5. When the pH of the solution was de-creased close to 3.0, precipitation occurred. The PL studies at differ-ent pH values showed that the emission peak shifted to the bluewith decreasing pH (Table 1), which is consistent with previousfindings [24,25]. The systematic change of pH helped in findingout the optimal pH condition of the sample having highest PLintensity with longer life time (Table 1). Fig. 1 illustrates the effectsof pH of the precursor solution on the QY of TLA capped CdS QDs.Maximum QY was obtained by adjusting the pH value of Cd2+ solu-tion at around 8.0.

It can be noted here that at high pH, the SH groups of the cap-ping agents at the surface of the colloidal particles will be con-verted to S2� which will help to bind the excess Cd2+ ions,through S2�—Cd2+—OH� linkages [26]. Thus, higher pH is expectedto strengthen the linkages between TLA and Cd2+ on the surface ofQDs. The modification of –SH groups and the accumulation of ex-cess Cd2+ on the surface thus seem to destroy the sites where radi-ationless recombination of the charge carriers can occur. All theseeffects would help to improve the PL intensity of QDs at higher pH[26].

3.2. Influence of concentration of Cd ions

The Cd/S ratio is usually effective in tailoring the size of thenanocrystals. In this ternary system (Cd/S/TLA), the Cd concentra-tion [Cd] was varied at a reaction pH of 8 keeping all the otherreaction parameters constant. Considering the composition Cd/S/TLA = x:1:4, the x being the molar concentration of Cd2+ [Cd] wasvaried as 1, 2, and 4. In this way, the S/TLA ratio remained fixedat 4. With increasing Cd/S ratio from 1 to 4, a blue shift is observedboth in UV–Vis and PL spectra. The blue shift of the absorbanceedge indicates decrease in size of the CdS QDs. These findings areconsistent with previously reported results [24,27]. As [Cd] in-creases, a greater proportion of Cd atoms is available for surfacereaction with the capping agent TLA. An insoluble precipitate isformed in case of lower Cd/S (Cd/S < 1), whereas at the higherCd/S ratios (Cd/S > 4:1), particles that formed did not show appre-ciable luminescence. The spectral data indicate that the best

350 400 450 500 550 600 6500

50

100

150

200

250

300

350

Lum

in. I

nten

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(a.u

.)

Wavelength/nm

As preparedafter two year

of preparation

(a)

(b)

(c)

300 350 400 450 500

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

Wavelength/nm

0 h 0.5 h 1 h 2 h 4 h

400 500 600 700 800

Lum

in. I

nten

sity

(a.u

.)

Wavelength/nm

0 h 0.5 h 1 h 2 h 4 h

Fig. 2. UV–Vis (a) and PL emission (b) spectra of TLA capped CdS QDs in aqueoussolution at different reflux times. The spectra were recorded at room temperature(25 ± 1 �C). Inset of (b) shows digital photograph of different sized TLA capped CdSQDs in aqueous solution obtained at various reflux times of 0 h (A), 1 h (B), and 4 h(C) under excitation of 365 nm UV lamp. (c) PL spectra of freshly prepared CdS–TLAQDs and after 2 years of preparation.

8 A. Mandal et al. / Optical Materials 34 (2011) 6–11

quality TLA capped CdS QDs (TLA–CdS) was obtained at the Cd/Sratio of 2, i.e., in the composition of Cd/S/TLA = 2:1:4. This is possi-bly due to the quality of the nanocrystal structure itself and bettersurface passivation of CdS QDs. Another possible reason is thatthere is a possibility to form a layer of Cd2+ thiol complex at higherCd2+/S ratio.

3.3. Influence of concentration of TLA

In order to check the effect of TLA concentration [TLA] on PLefficiency, five different batches of TLA capped CdS QDs were pre-pared at room temperature and maintaining the solution pH ataround 8.0. Here, in this composition of Cd/S/TLA = 2:1:y, the y[TLA] was varied as 1, 2, 4, 8, and 16 by fixing the Cd/S ratio at 2.On the basis of the PL studies, we found that the value of y = 4(i.e., TLA:Cd = 2) was best to obtain the highest emission intensity.At y = 1 and 2 (TLA:Cd = 0.5 and 1), the PL intensities were muchlower than that of y = 4 (TLA:Cd = 2), because of insufficient cover-age of the CdS QDs by TLA. At relatively lower concentration of TLA,there is a fair chance to form clusters of CdS QDs, and as a result,poor dispersion of CdS QDs occurred. Due to this, the emissionintensity as well as quantum efficiency was reduced. On the otherhand, at higher y values of 8 and 16 (TLA:Cd = 4 and 8), too manyTLA molecules in the suspension resulted in lowering of lumines-cence intensity. It is known that the use of too high concentrationof capping agent could cause the decrease in QDs size which inturn deteriorate the PL properties [13,18].

From the previous systematic studies, we found an optimizedmolar composition of Cd/S/TLA = 2:1:4 and a solution pH of 8 inobtaining the best quality TLA–CdS QDs having intense PL, lifetime, and QYs. So, detailed characterization of these TLA–CdSQDs has been made and presented in the following sections.

3.4. Photophysical properties of TLA–CdS QDs

The aqueous dispersion of TLA–CdS QDs synthesized using theoptimized composition of Cd/S/TLA = 2:1:4 and at pH 8.0 were sub-jected to heat treatment in refluxing condition at �100 �C. Theserefluxing studies were undertaken in order to improve the PL effi-ciency of CdS QDs. Formation and growth of the CdS QDs withincreasing reflux time were monitored by absorption and photolu-minescence spectroscopy. Fig. 2a shows the evolution of absorp-tion spectra of the TLA capped CdS QDs (TLA–CdS) with respectto the refluxing time. The as prepared sample showed a band atabout 365 nm corresponding to the smallest CdS ODs (�3 nm)which gradually shifted to longer wavelengths as the particlesgrew in size due to the heating under reflux condition. Fig. 2bshows the corresponding size-dependent red shifting of lumines-cence maxima from 505 nm to 650 nm observed with respect toreflux time. Inset of Fig. 2b shows digital photograph of TLA cappedCdS QDs in aqueous solution obtained at various reflux times of 0 h(A; greenish white), 1 h (B; yellow), and 4 h (C; reddish orange) un-der excitation of 365 nm UV lamp. The change of color suggests agradual growth of CdS QDs with respect to reflux time. This is inconsistent with the UV–Vis and PL spectral data.

As prepared TLA capped CdS QDs showed the emission peak ataround 505 nm, while the absorption peak was at 365 nm. Thelarge Stokes shifted emission (around 140 nm) is attributed tothe recombination of charge carriers immobilized in traps of differ-ent energies. There are many possible trap state emissions eachwith different emission wavelength. All these emissions could con-tribute to the relatively broad luminescence spectra of TLA cappedCdS QDs. The envelope of PL band was found to broad with aFWHM of over 120 nm (Fig. 2b). The broad band emission suggeststhat the luminescence is due to the surface trap luminescence.The observed broad PL peak is commonly attributed to the

Table 2Absorption and photoluminescence (PL) maxima, quantum yield (QY), and average lifetime values of different sized TLA–CdS QDs synthesized at pH 8.0 obtained at differentrefluxing times. Quantum efficiencies were measured against commercial Rhodamine B, kex = 400 nm at room temperature.

Absorbance max (nm) Reflux time (h) PL max peak (nm) QY (%) Decay components (ns) Average lifetime (ns)

365 0 505 45 T1 = 6.0 (7.25), T2 = 37.7 (82.02), T3 = 1.1 (10.73) 31.5370 0.5 520 50 T1 = 4.3 (4.76), T2 = 39.2 (92.84), T3 = 0.9 (2.4) 36.6380 1 600 33 T1 = 6.0 (14.67), T2 = 33.4 (83.15), T3 = 1.4 (2.18) 28.7400 2 615 20 T1 = 2.3 (12.87), T2 = 24.8 (79.00), T3 = 1.4 (8.13) 20.0415 4 650 12 T1 = 2.0 (20.09), T2 = 20.1 (50.61), T3 = 0.7 (29.3) 10.8

A. Mandal et al. / Optical Materials 34 (2011) 6–11 9

recombination of charged carriers trapped in the surface states andis related to the size of CdS QDs.

Without compromising the broadband luminescence property,we achieved promising QYs (�45–50%) from these TLA cappedCdS QDs (Table 2) synthesized by using such a simple wet chemicalmethod. The broadband luminescence property with high quan-tum efficiency (Fig. 2b; Table 2) of these QDs can be applicablefor white light emission. It can be noted that the colloidal solutionsof TLA capped CdS QDs prepared in this method are stable and do

Fig. 3. TEM images of representative TLA–CdS QDs: (a) as prepared (averageparticle size 3.2 nm) and (b) after refluxing for 1 h (average particle size 5 nm).

not change their optical properties for years at room temperature.We checked the photostability of as prepared CdS QDs by takingthe PL spectra of sample (Fig. 2c) which was prepared over 2 yearsbefore. Interestingly, we found that the spectra do not changenoticeably upon aging (see Fig. 2c). The methyl group having + I ef-fect attached with the secondary carbon atom of TLA helps to formbetter linkage with the CdS QDs, and the hydrophobic character ofmethyl group prevents agglomeration of QDs in aqueous medium.Thus, the overall stability of the systems increases enormouslywith their original high PL characteristics. It is noteworthy herethat for comparison we have also prepared the TGA capped CdSQDs following similar experimental conditions. This TGA–CdS sys-tem showed less PL efficiency, and we found significant decrease inthe PL intensity along with precipitation in less than a year. So, ourstudy confirmed better passivation of the CdS surface by TLA withmuch enhanced stability.

The average particle size of the CdS QDs can be determinedfrom the position of the absorption edge (Fig. 2a), by using thewell-known relation between particle size and absorption edge[28]. The absorption edge (ke) can be converted into the corre-sponding particle size by using Henglein’s empirical equation thatcorrelates the

R CdSð Þ ¼ 0:1= 0:1338� 0:0002345keð Þnm ð1Þ

where R and ke are the diameter (R) of the particles and absorptionedge, respectively. The ke values were evaluated from the intersec-tion of the sharply decreasing region of the spectra with the base-line [28]. In case of as prepared sample (Fig. 2a), ke was found to

20 40 60 80

102

103

104

4 h2 h

0.5 h

1 h0 h

Lum

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(a.u

.)

Time/ns

Fig. 4. Luminescence decay curves of CdS–TLA QDs in aqueous solution at roomtemperature obtained at different reflux times. Samples were excited at 375 nm,and fluorescence decay data were collected at their corresponding emissionmaxima.

10 A. Mandal et al. / Optical Materials 34 (2011) 6–11

be 440 nm, and putting this value to the Eq. (1), the R value was cal-culated to be 3.2 nm. Representative TEM micrographs TLA cappedCdS QDs with a Cd/S/TLA ratio of 2:1:4 are shown in Fig. 3a and b.The observed QDs were more or less spherical in shape and haveclear lattice fringes (See inset of Fig. 3a and b). TEM image of theas prepared CdS QDs (Fig. 3a) showed existence of spherical parti-cles of diameter �3–3.5 nm. This is consistent with the results ob-tained by Henglein’s empirical equation. TEM image of onerepresentative sample obtained after 1 h refluxing (Fig. 3b) showedgrowth of QDs to about 5 nm in diameter. The R value (4.7 nm) cal-culated using the corresponding ke (Fig. 2a) also found to be wellconsistent with the TEM result.

Fig. 4 shows the luminescence decay behavior of CdS QDs of dif-ferent size obtained by varying the reflux time. The decays weremonitored at their corresponding luminescence maxima. All decaycurves for the TLA capped CdS QDs showed multiexponential innature, which was observed for different kinds of II–VI nanomate-rials [29,30]. The decay components and average life time valuesare presented in Table 2. This multiexponential kinetics is thought

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400 nm420 nm625 nm

(a)

(b)

Fig. 5. (a) Photoluminescence decay curves of as prepared CdS–TLA QDs in aqueoussolution at room temperature obtained at various wavelengths (400 to 625 nm).Samples were excited at 375 nm. (b) Plot of average fitted decay time for the tracesfrom (a) versus monitoring wavelength. The data points are joined for visual guide.

to be caused by the presence of trap states. The faster initial decay(Fig. 4; Table 2) reflected the emission of QDs following a non-radi-ative manner. The longer decay components were associated withthe surface-related radiative recombination of carriers [10]. Thedistribution of decay times can be explained through multiexpo-nential decays due to the variation of the non-radiative decay ratescaused by trap states [29–32]. In our case, values of longer decaytimes were found to systematically increased up to a certain refluxtime (i.e., in conjunction with PL efficiency) indicating a delocaliza-tion of electrons by the photoexcitation and involvement of surfacestates in their recombination. Longer decay time implies a betterremoval of quenching defects, possibly from uncoordinated S2�

sites, from the surface of CdS QDs capped with TLA, resulting in ahigh QY.

We also probed the decay at different emission wavelengths(not only on the luminescence peak) to check the average decaytime if changes appreciably. Luminescence decay at differentwavelengths was analyzed using triexponential decay kinetics,and the results are presented in Fig. 5a. The fitted decay times atvarious wavelengths are summarized in Fig. 5b. It has been foundthat average decay time even at 400 nm region is over 20 ns, and at625 nm, it is around 27 ns (Fig. 5b). The average decay time in the450–600 nm is relatively remaining unaffected. From this point ofview, TLA capped CdS QD prepared by our method could be reallyuseful to get the white light with sufficient quantum efficiency aswell as longer average decay time.

4. Conclusions

Water-soluble CdS QDs were prepared through an environmentfriendly colloidal route using TLA as a capping agent. We observedthat TLA is a much better capping agent compared to other similarmolecules reported in the literature. TLA capped CdS QDs preparedin this work are relatively monodisperse, showed remarkable pho-tochemical and longer colloidal stability over a span of 2 years, andshowed intense broadband (almost white) photoluminescencewith high quantum yield (�50%) suitable for solid state lightingdevices and other applications. Long average lifetime of CdS–TLAQDs can be attributed to the effective removal of carrier-quenchingdefects, correlating well with a better quantum efficiency com-pared to the CdS QDs prepared by using other capping agents.

Acknowledgement

DST, Govt. of India is thankfully acknowledged for providing aproject under Nano Mission program. J.S. thanks CSIR for awardinga Junior Research Fellowship.

References

[1] M.A. Schreuder, K. Xiao, I.N. Ivanov, S.M. Weiss, S.J. Rosenthal, Nano Lett. 10(2010) 573.

[2] M.A. Schreuder, J.R. McBride, A.D. Dukes, J.A. Sammons, S.J. Rosenthal, J. Phys.Chem. C 113 (2009) 8169.

[3] Q. Dai, C.E. Duty, M.Z. Hu, Small 6 (2010) 1577.[4] D.R. Baker, P.V. Kamat, Langmuir 26 (2010) 11272.[5] A.H. Mueller, M.A. Petruska, M. Achermann, D.J. Werder, E.A. Akhadov, D.D.

Koleske, M.A. Hoffbauer, V.I. Klimov, Nano Lett. 5 (2005) 1039.[6] M.J. Bowers, J.R. McBride, S.J. Rosenthal, J. Am. Chem. Soc. 127 (2005) 15378.[7] A. Nag, D.D. Sarma, J. Phys Chem. C 111 (2007) 13641.[8] W. Chung, K. Park, H.J. Yu, J. Kim, B. Chun, S.H. Kim, Opt. Mater. 32 (2010)

515.[9] J. Zhang, L. Sun, C. Liao, C. Yan, Solid State Commun. 124 (2002) 45.

[10] B.A. Harruff, C.E. Bunker, Langmuir 19 (2003) 893.[11] A.L. Rogach, S.V. Kershaw, M.G. Burt, M. Harrison, A. Kornowski, A. Eychmuller,

H. Weller, Adv. Mater. 11 (1999) 552.[12] D. Gerion, F. Pinaud, S.C. Williams, W.J. Parak, D. Zanchet, S. Weiss, A.P.

Alivisatos, J. Phys. Chem. B 105 (2001) 8861.[13] J.O. Winter, N. Gomez, S. Gatzert, C.E. Schmidt, B.A. Korgel, Colloids Surf. A. 254

(2005) 147.

A. Mandal et al. / Optical Materials 34 (2011) 6–11 11

[14] L.V. Trandafilovic, V. Djokovic, N. Bibic, M.K. Georges, T. Radhakrishnan, Opt.Mater. 30 (2008) 1208.

[15] N. Gaponik, D.V. Talapin, A.L. Rogach, K. Hoppe, E.V. Shevchenko, A. Kornowski,A. Eychmuller, H. Weller, J. Phys. Chem. B 106 (2002) 7177.

[16] H. Zhang, L.P. Wang, H.M. Xiong, L.H. Hu, B. Yang, W. Li, Adv. Mater. 15 (2003)1712.

[17] Y.S. Park, A. Dmitruk, I. Dmitruk, A. Kasuya, M. Takeda, N. Ohuchi, Y. Okamoto,N. Kaji, M. Tokeshi, Y. Baba, ACS Nano 4 (2010) 121.

[18] H. Li, W.Y. Shih, W-H. Shih, Ind. Eng. Chem. Res. 46 (2007) 2013.[19] D.V. Talapin, A.L. Rogach, E.V. Shevchenko, A. Kornowski, M. Haase, H. Weller, J.

Am. Chem. Soc. 124 (2002) 5782.[20] H. Zhang, D. Wang, H. Mohwald, Angew. Chem. 118 (2006) 762.[21] X. Lu, J. Yang, Y. Fu, Q. Liu, B. Qi, C. Lu, Z. Su, Nanotechnology 21 (2010) 115702.[22] Q. Dai, D. Li, J. Chang, Y. Song, S. Kan, H. Chen, B. Zou, W. Xu, S. Xu, B. Liu, G.

Zou, Nanotechnology 18 (2007) 405603.

[23] R.F. Kubin, A.N. Fletcher, J. Lumin. 27 (1982) 455.[24] N. Herron, Y. Wang, H. Eckert, J. Am. Chem. Soc. 112 (1990) 1322.[25] A. Mandal, N. Tamai, J. Phys. Chem. C 112 (2008) 8244.[26] L. Spanhel, M. Haase, H. Weller, A. Henglein, A. J. Am. Chem. Soc. 109 (1987)

5649.[27] H. Weller, Angew. Chem. Int. Ed. Engl. 32 (1993) 41.[28] M. Moffitt, A. Eisenberg, Chem. Mater. 7 (1995) 1178.[29] A.M. Kapitonov, A.P. Stupak, S.V. Gaponenko, E.P. Petrov, A.L. Rogach, A.

Eychmuller, J. Phys. Chem. B 103 (1999) 10109.[30] X. Wang, L. Qu, J. Zhang, X. Peng, M. Xiao, Nano Lett. 3 (2003) 1103.[31] S.F. Wuister, C. de MelloDonegá, A. Meijerink, J. Phys. Chem. B 108 (2004)

17393.[32] Y. Nosaka, K. Yamaguchi, H. Miyama, H. Hayashi, Chem. Lett. 17 (1988)

605.