Chemical Physics Letters 599 (2014) 63–67

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High fluorescence quantum efficiency of CdSe/ZnS quantum dotsembedded in GPTS/TEOS-derived organic/silica hybrid colloids

http://dx.doi.org/10.1016/j.cplett.2014.03.0160009-2614/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Fax: + 55 34 32394106.E-mail address: vivianepilla@infis.ufu.br (V. Pilla).

Lorena D.S. Alencar a, Viviane Pilla b,⇑, Acácio A. Andrade b, Dario A. Donatti a, Dimas R. Vollet a,Fábio S. De Vicente a

a Departamento de Física, Universidade Estadual Paulista UNESP, Av. 24A 1515, Rio Claro, SP 13506-900, Brazilb Instituto de Física, Universidade Federal de Uberlândia UFU, Av. João Naves de Ávila 2121, Uberlândia, MG 38400-902, Brazil

a r t i c l e i n f o

Article history:Received 1 February 2014In final form 6 March 2014Available online 13 March 2014

a b s t r a c t

The thermo-optical properties of CdSe/ZnS core–shell quantum dots (QDs) embedded in organic/silicahybrid colloids (organic/silica sols) were measured using the thermal lens (TL) technique. GPTS/TEOS-derived organic/silica hybrid colloids were prepared by a sol–gel method from the hydrolysis reactionof 3-glycidoxypropyltrimethoxysilane (GPTS) and tetraethylorthosilicate (TEOS) alkoxides. TL transientmeasurements were performed to study the effect of the CdSe/ZnS QDs (with three different sizes �2.4, 2.9 and 4.4 nm) embedded in GPTS/TEOS-derived organic/silica sols. The thermal diffusivity, the frac-tion thermal load and the radiative quantum efficiency (g) were determined. Fluorescence measurementscorroborate the TL results and high g values were obtained.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Organic/silica hybrid materials prepared by the sol–gel methodhave attracted considerable attention because they combine theadvantages of organic polymers, such as toughness and flexibility,with those of inorganic components, such as enhanced mechanicalproperties and optical quality [1, 2]. The sol–gel process can be anexcellent method to entrap nanoparticles inside a matrix due tothe simplicity of the sol–gel preparation, the chemical inertnessof the matrix, the tunable porosity achieved by changing theamounts of silica precursors and the mechanical stability [3].Moreover, the optical transparency offered by sol–gel materialsmakes them an excellent option for the development of opticalsensing phases [4]. Organic/silica hybrid materials are the moststudied and important class of hybrids due to their high applicabil-ity in several fields. For instance, quantum dots embedded in silicahave been employed to obtain the best analytical features foracetone [3] and Cu2+ ion sensing [5], white QD-LED fabrication[6], DNA and [7] urea detection [8], ammonia vapor sensing [9],drug targeting and cellular labeling [10].

Nanostructured semiconductors, or quantum dots (QDs), arehighly fluorescent semiconductor nanoparticles with interestingsize-dependent properties due to the quantum confinement effect[11,12]. These materials have the potential for a variety of newapplications as fluorescent labels for biomedical science, diodes,

lasers, photonic devices and sensor materials [13–15]. Core–shellQDs are formed by a combination of two different semiconductors,increasing the fluorescence quantum yield and nanocrystalstability [11,16]. The surface passivation and functionalization ofQDs are important methods that can improve their nanomaterialproperties [11,17].

The chemical environment used to suspend or encapsulate QDsamples can have an important influence on their properties,including their radiative quantum efficiency (g) and thermalparameters [18,19]. Wu et al. [7] reported increases of 20% in gvalues after encapsulating CdSe/ZnS QDs into silica shells, suggest-ing that silica-coated QDs are highly fluorescent. Wang et al. [20]observed an improvement of up to 35% in the g of CdSe/CdxZn1�xSQDs after embedding in SiO2. The effects on the luminescent prop-erties of coating QDs with silica were also presented by Qian et al.[21], who reported an increase in g from 10–20% to 80% from sil-ica-coated CdSe QDs. Because of this influence, it is necessary toperform thermo-optical characterizations on new systemscomprising the nanoparticles and their surrounding media todetermine their potential in practical applications.

The present Letter reports the photothermal spectroscopic char-acterization of type I QDs, [22] the cadmium selenide/zinc sulfide(CdSe/ZnS) core–shell QDs, embedded in GPTS/TEOS-derived or-ganic/silica sols prepared by a sol–gel process. The studies consid-ered the effect of the size of the CdSe/ZnS core–shell QDs. Thethermo-optical properties of the QD samples embedded in theorganic/silica matrix, such as their thermal diffusivity (D), fractionthermal load (u) and g, were determined and compared with the g

Table 1Quantum dot size, average emission wavelength <kem> and thermal diffusivity D ofCdSe/ZnS core–shell QDs embedded in GPTS/TEOS-derived organic/silica sols.

Sample Concentration(mg/mL)

Size(nm)

<kem>(nm)

D(10�3 cm2/s)

QD1 0.28 (2.4 ± 0.2) (504 ± 6) (0.94 ± 0.03)QD2 0.28 (2.9 ± 0.4) (537 ± 6) (0.96 ± 0.06)QD3 0.19 (4.4 ± 0.9) (590 ± 30) (0.97 ± 0.09)

64 L.D.S. Alencar et al. / Chemical Physics Letters 599 (2014) 63–67

results of QD suspended in toluene. g study of CdSe/ZnS QDsembedded in organic/silica sols is important due to the unknowneffect of the QD surface ligands and QD capping on thefluorescence properties. Higher g values were obtained for theQDs embedded in organic/silica sols prepared by the sol–gelprocess in comparison with QDs suspended in toluene.

2. Thermal lens (TL) technique theory

The TL effect [23–27] is caused by heat deposition via nonradi-ative decay processes after laser energy is absorbed by a sample ofthickness L. The thermally-induced distortion of the laser beamwhen it passes through the sample is described by the change inthe optical path length (S) (ds/dT = L–1 dS/dT), which produceslensing at the sample. The presence of such a thermal lens is de-tected by its effect on the propagation of a probe beam passingthrough the sample. The propagation of a probe laser beamthrough the TL causes the beam to either spread (ds/dT < 0) or fo-cus (ds/dT > 0), depending mainly on the sample’s temperaturecoefficients of electronic polarizability, stress and thermal expan-sion. In the case of liquid samples, ds/dT � dn/dT, where dn/dT isthe refractive index temperature coefficient.

The TL effect can be modeled by calculating the temporal evolu-tion of the sample temperature profile, DT(r, t), caused by theGAUSSIAN intensity distribution of the excitation beam. The variationof the on-axis intensity of the probe beam, I(t), can be calculatedusing diffraction theory [24] in the continuous wave (cw) excita-tion regime in the following form [25]:

IðtÞ ¼ Ið0Þ 1� h2

tan�1 2mV

½ð1þ 2mÞ2 þ V2�sc=2t þ 1þ 2mþ V2

!" #2

;

ð1Þ

where m = (wp/we)2 and wp and we are the probe and excitationbeam radius at the sample, respectively; V = z1/z0, z1 is the distancebetween the sample and the probe beam waist; z0 is the probebeam Rayleigh range; and I(0) is the on-axis intensity when t iszero. The characteristic heat diffusion time is the following:

sc ¼w2

e

4Dð2Þ

where D = K/qC is the thermal diffusivity (cm2/s), K is the thermalconductivity (W/cm K), q is the density (g/cm3) and C is the specificheat (J/g K).

In the dual beam (excitation and probe beams) mode-mismatched configuration, the transient signal amplitude isapproximately the phase difference, h, of the probe beam betweenr = 0 and r ¼

ffiffiffi2p

we , which is induced by the pump beam and givenby the following expression: h = �HPabs where Pabs = PeaLeff. In thiscase, Pe is the power of the excitation beam, a (cm�1) is the opticalabsorption coefficient at the excitation wavelength (ke), Leff =(1�e�aL)/a is the effective length and L (cm) is the sample thick-ness. The fluorescence quantum efficiency g (or quantum yield)can be obtained by determining the absolute nonradiative quan-tum efficiency u (or the fraction of absorbed energy converted toheat) and applying the normalized parameter H = �h/Pabs. Forsolutions, g is defined as follows [26,27]:

g ¼ ð1�uÞ hkemike

; ð3aÞ

and u ¼ HKkp

dn=dT

� �ð3bÞ

where kp is the probe beam wavelength and <kem> is the averageemission wavelength.

3. Experimental procedure

3.1. GPTS/TEOS-derived organic/silica sample preparation

The preparation of the GPTS/TEOS-derived organic/silica col-loids was performed from a hydrolysis reaction of GPTS and TEOSalkoxides in mutual solvent (ethanol) promoted by reflux and mag-netic stirring at 80 �C for 30 min. A HNO3 solution was slowlyadded dropwise to the alkoxide solutions (HNO3 as a catalyst andwater as a source for the hydrolysis), and the reactant mixtureswere refluxed at 80 �C for 24 h under mechanical stirring to pro-duce very stable organic/silica sols at acidic conditions (pH = 2).The nominal GPTS:TEOS:H2O:HNO3:ethanol molar ratio used inthe hydrolysis was 1:1:4:0.07:0.4. In this Letter, CdSe/ZnScore–shell QDs suspended in toluene were obtained from EvidentTechnologies in three different particle sizes (Table 1) at a concen-tration of 5 mg/mL. The CdSe/ZnS QDs were embedded in theorganic/silica hybrid sol matrix derived from the hydrolyzedGPTS/TEOS alkoxide mixture at room temperature. For samplepreparation, 0.3 mL of CdSe/ZnS QDs suspended in toluene weremixed with 5 mL of organic/silica sols, resulting in organic/silicasols doped with QDs at the concentrations presented in Table 1.The QDs and the organic/silica sol matrix were perfectly compati-ble and soluble in various organic solvents, such as toluene. A sam-ple of ink doped into the GPTS/TEOS-derived organic/silica sols wasprepared by mixing 0.04 mL of ink and 9 mL of organic/silica solsunder constant magnetic stirring. This non-fluorescent samplewas used as a reference for the TL measurements.

3.2. GPTS/TEOS-derived organic/silica samples characterization

The thermo-optical properties of the core–shell colloidal solu-tions were investigated using the TL method [23–27]. TL transientmeasurements were performed using the mode-mismatcheddual-beam (excitation and probe) configuration. A He–Ne laser(kp = 632.8 nm) was used as the probe beam, and an Ar+ laser(ke = 514.5 nm) was used as the excitation beam. The excitationand probe beam radii at the sample were measured to bewe = (24.5 ± 0.5) lm and wp = (123 ± 3) lm. The excitation beamwas modulated by a mechanical chopper. The absorption of theexcitation beam generated a TL heat profile and a time-resolved in-duced phase shift that was proportional to h. The transient curvewas obtained by the weak probe beam, which counter-propagatednearly collinearly with the excitation beam. The probe beamtransient signal was detected by a photodiode connected to anoscilloscope. The oscilloscope traces were transferred to acomputer via a USB port using an appropriate acquisition program.Details of the experimental arrangement can be found elsewhere[24,26]. For the TL measurements, the solutions were containedin 2-mm cuvettes.

Optical absorption and fluorescence spectra measurementswere carried out with sample solutions placed inside 10-mmquartz cuvettes at ambient temperature. The equipment used tocollect the UV–VIS optical absorption and fluorescence spectracomprised a Varian Cary 50 UV–VIS spectrophotometer and aVarian Cary Eclipse, respectively.

Figure 2. Fluorescence spectra for (a) ink doped into GPTS/TEOS-derived organic/silica sols and for CdSe/ZnS QDs embedded in GPTS/TEOS-derived sols: (b) QD1, (c)QD2 and (d) QD3 (Table 1, ke = 320 nm, 1-cm quartz cuvette). The inset shows thetypical emission band for QDs suspended in toluene.

Figure 3. Thermal lens transient signal for ink doped into GPTS/TEOS-derivedorganic/silica sols (ke = 514.5 nm, Pe = 0.5 mW). The values obtained from the curvefitting were h = (0.2571 ± 0.0004) rad and sc = (1.460 ± 0.006) ms (2-mm quartzcuvette).

L.D.S. Alencar et al. / Chemical Physics Letters 599 (2014) 63–67 65

4. Results

Figure 1 shows the absorption spectra of the pure GPTS/TEOS-derived organic/silica sols (a); the ink-doped GPTS/TEOS-derivedorganic/silica sols (b); and the CdSe/ZnS QDs with different sizes(QD1 (c), QD2 (d) and QD3 (e)) embedded in the GPTS/TEOS-derivedorganic/silica sols (Table 1 and 1-cm quartz cuvette). The insetshows a representation of the core–shell CdSe/ZnS QDs used andthe characteristic absorption of the QDs suspended in toluene. Thetypical absorption bands of CdSe/ZnS QDs were obtained, and theircorresponding transitions can be found in the literature [28,29]. Theparticle sizes of the QDs in solution were determined from theirabsorption spectra, [30] and the values are shown in Table 1. Thepure GPTS/TEOS-derived organic/silica sols presented no significantabsorption at visible wavelengths (Figure 1a). For the CdSe/ZnS QDsembedded in the GPTS/TEOS-derived organic/silica sols (for exam-ple, Figure 1d), the band centered at�542 nm is typical for QDs witha particle size of 2.9 nm. In addition, the absorbance spectrum ispresented for the ink-doped GPTS/TEOS-derived organic/silica solmatrix, which presented a band at �586 nm.

Fluorescence spectra of the QDs with different particle sizesembedded in the GPTS–TEOS alkoxide matrix, i.e., for the sampleslisted in Table 1, are shown in Figure 2. For comparison, Figure 2apresents the emission spectrum of the ink embedded in the GPTS–TEOS alkoxide matrix, which presents no significant emission bandin the visible range at intensities comparable to those obtained forthe QD-doped GPTS–TEOS matrix (Figure 2b–d). The average emis-sion wavelengths <kem> of the emission bands are presented in Ta-ble 1 for the GPTS–TEOS encapsulated CdSe/ZnS QDs. The <kem>values for the QDs embedded in toluene (Inset Figure 2) are 525,560 and 622 nm [31]. In this form, the volume fractions of theCdSe/ZnS QDs exerted considerable influence on the fluorescenceproperties of the GPTS–TEOS encapsulated CdSe/ZnS QDs. The posi-tions of the emission bands for the QDs are dependent on the con-centration and the matrix in which the QDs are embedded [23,32].

Figures 3 and 4 show the TL transient signals for the ink-dopedGPTS–TEOS and for the CdSe/ZnS QDs embedded in the GPTS–TEOSderived silica-organic hybrid colloids (QD1, Table 1). Fitting theseresults with Eq. (1), the parameters h and sc were determined.The thermal diffusivity, D, was determined for the dopedGPTS–TEOS sample using Eq. (2). The obtained D value for eachsample of CdSe/ZnS QDs embedded in the GPTS–TEOS alkoxidesfor the different QD sizes is presented in Table 1. The D values(Table 1) obtained are the average of at least 6 independent sets

Figure 1. Absorbance spectra for (a) pure GPTS/TEOS-derived sols, (b) ink dopedinto GPTS/TEOS-derived sols, and CdSe/ZnS QDs embedded in GPTS/TEOS-derivedsols: (c) QD1, (d) QD2 and (e) QD3 (Table 1 and 1-cm quartz cuvette). The insetshows the typical absorption band for the core–shell CdSe/ZnS QDs suspended intoluene.

Figure 4. Thermal lens transient signal for CdSe/ZnS QDs embedded in GPTS/TEOS-derived sols (QD1, Table 1, ke = 514.5 nm, Pe = 1.96 mW). The values obtained fromthe curve fitting were h = (0.1235 ± 0.0004) rad and sc = (1.59 ± 0.01) ms (2-mmquartz cuvette). The inset show h versus Pe for CdSe/ZnS QDs embedded in GPTS/TEOS-derived sols for (a) QD1, (b) QD2 and (c) QD3 (Table 1).

Figure 5. Radiative quantum efficiency (g) at ke = 514.5 nm for CdSe/ZnS QDsembedded in GPTS/TEOS-derived sols (closed circle), toluene (up triangle) [31] andaqueous solution [32] (open diamond).

66 L.D.S. Alencar et al. / Chemical Physics Letters 599 (2014) 63–67

of measurements. The thermal diffusivity is not dependent on theCdSe/ZnS QD size. The average value of the thermal diffusivity forthe ink-doped GPTS–TEOS alkoxides (Figure 3) isD = (1.01 ± 0.07) � 10�3 cm2/s. The D value obtained is in goodagreement with the results (Table 1) obtained for the QD-dopedGPTS–TEOS alkoxides, and the D value most likely is characteristicof the GPTS–TEOS alkoxides used. To the best of our knowledge,there are no results on the thermal diffusivity of GPTS/TEOS-de-rived organic/silica colloids in the literature.

The behavior of the TL transient curves for the ink- or QD-dopedGPTS–TEOS solutions (Figures 3 and 4) indicates that dn/dT is neg-ative, i.e., TL causes a defocusing of the probe beam in the far field.The inset of Figure 4 shows the linear trend of the transient signalamplitude h as a function of Pe for the three different QD sizes (Ta-ble 1). The normalized thermal parameters of the ink or QDsembedded in the GPTS–TEOS matrix, H = hi/Pabs, were determinedfrom the transient thermal lens measurements (where hi is theamplitude of the TL signal for the core–shell QDs (i = cs) or theink (i = ink) embedded in the GPTS–TEOS). To determine u, we as-sumed negligible fluorescence for the ink-doped GPTS–TEOS ma-trix used in the TL measurements, such that all of the absorbedenergy was converted into heat by the sample, i.e., uink = 1(gink = 0). By normalizing H by Hink and using Eq. (3a), we obtain:

u ¼ HHink

¼ 1� gke

< kem >

� �ð4Þ

In fact, the calibration of the TL technique was verified previ-ously using sample of ink suspended in water, and consideringuink = 1 (i.e., gink = 0). The values obtained for D = (1.49 ± 0.07) �10�3 cm2/s and dn/dT = �(0.85 ± 0.08) � 10�4 K�1 are in goodagreement with the published values for water solvent [33,34].

Applying Eq. (4) and using both values of <kem> and ke = 514.5nm, we calculated the u and g values for the GPTS–TEOS encapsu-lated CdSe/ZnS QDs. The values of u and g obtained by the TLmethod are presented in Table 2 for the QDs embedded in theGPTS–TEOS and QDs suspended in toluene. The values of g are bothhigher than that of QDs in toluene (QD1tol, QD2tol and QD3tol,Table 2) [31] and dependent on the sizes of the QDs. For compari-son, the g values of CdSe/ZnS QDs suspended in toluene andaqueous solutions are presented in Figure 5. The best g resultwas obtained for the smallest QDs (�2.4 nm). The dependence ofthe radiative quantum efficiency on the QD size has already beenreported by our research group for samples of CdSe/ZnS QDs sus-pended in toluene [31], biofluid or aqueous solutions [32], andthe result was attributed to quantum confinement effects (QCE)[12,35,36]. In the regime of the QCE, as the size of the quantumdot is reduced, the energy spacing of its atomic-like states in-creases beyond the available thermal energy, which inhibits thethermally induced depopulation of the lowest electronic states[37].

Table 2H, u and g results obtained for CdSe/ZnS QDs embedded in GPTS/TEOS-derivedorganic/silica sols, CdSe/ZnS QDs embedded in toluene (tol) [31] and ink doped intoGPTS/TEOS-derived organic/silica sols obtained by applying TL technique (atke = 514.5 nm).

Sample H(W�1)

u g

QD1 (230 ± 30) (0.10 ± 0.01) (0.9 ± 0.1)QD2 (470 ± 100) (0.20 ± 0.04) (0.8 ± 0.2)QD3 (900 ± 200) (0.38 ± 0.07) (0.7 ± 0.1)QD1tol (1900 ± 100) (0.29 ± 0.02) (0.73 ± 0.04)QD2tol (2300 ± 200) (0.36 ± 0.04) (0.71 ± 0.04)QD3tol (3100 ± 400) (0.47 ± 0.06) (0.64 ± 0.07)Ink (2400 ± 100) 1 –

5. Conclusions

The thermal lens (TL) technique was applied to determine thenonradiative quantum efficiency (u) and fluorescence quantumefficiency (g) of CdSe/ZnS QDs embedded in GPTS/TEOS-derived or-ganic/silica hybrid colloids for three different QD sizes (2.4, 2.9 and4.4 nm). We found that both u and g depend on the CdSe/ZnS QDsize, and that GPTS/TEOS-derived organic/silica hybrid colloids as amatrix for CdSe/ZnS QDs improved g compared with CdSe/ZnS QDssuspended in toluene. Fluorescence spectroscopy corroborated theTL results.

Acknowledgments

The authors are thankful to the Brazilian funding agenciesCNPq, FAPESP, FAPEMIG, FUNDUNESP and PROPP-UFU for thefinancial support for this work.

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