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Journal of Luminescence 128 (2008) 1948– 1951

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Journal of Luminescence

0022-23

doi:10.1

� Corr

Tel.: +8

E-m

(J.-Y. Ch

journal homepage: www.elsevier.com/locate/jlumin

pH-dependent aggregation and photoluminescence behavior of thiol-cappedCdTe quantum dots in aqueous solutions

Yu Zhang a, Lan Mi a, Pei-Nan Wang a,�, Jiong Ma b, Ji-Yao Chen b,�

a State Key Lab for Advanced Photonic Materials and Devices, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, Chinab Department of Physics, Fudan University, Shanghai 200433, China

a r t i c l e i n f o

Article history:

Received 9 November 2007

Received in revised form

30 May 2008

Accepted 2 June 2008Available online 7 June 2008

PACS:

68.65.Hb

78.67.Hc

Keywords:

Quantum dots

Photoluminescence

Aggregation

Lifetime

13/$ - see front matter & 2008 Elsevier B.V. A

016/j.jlumin.2008.06.004

esponding authors. Tel./fax: +86 2165642092

6 2165643084 (J.-Y. Chen).

ail addresses: pnwang@fudan.edu.cn (P.-N. W

en).

a b s t r a c t

pH-dependent aggregation of thiol-capped CdTe quantum dots (QDs) in solutions was observed with a

confocal microscope. The average size of the QD aggregates increased from 28 nm to 1.4mm as the pH

decreased from 12 to 3. The basic condition improved the dispersion of QDs while the acidic condition

caused the detachment of surface ligands, leading to the aggregation of QDs. A PL lifetime of 80 ns was

detected for QDs at pH from 12 to 7, while it was shortened to 57 and 34 ns at pH 5 and 3, respectively,

due to the formation of surface defects.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, water-soluble semiconductor colloidal quan-tum dots (QDs) have attracted worldwide attention due to theirgreat potentials in bioapplications [1–3]. It is worth noting,however, that colloidal QDs are a kind of metastable nanocrys-tal–ligand complex in solutions. Protonation will cause thedetachment of the ligands from the QD core surface [4] andsubsequently the formation of QD aggregates. In bioapplications,aggregation of QDs will cause difficulties for QDs being inter-nalized into cells because the QD size is important for theirtransport across cell walls, cell membranes and/or nuclear pores[5]. Hence, to learn the dependence of QD aggregation on theirenvironmental pH is essential to the bioapplication of colloidalQDs. On the other hand, the detachment of capping ligands willcause the formation of surface defects, leading to the decrease ofPL efficiency and shortening of PL lifetime.

Recently, precipitations of water-soluble QDs in solutions [4,6]and pH-dependent PL in living cells [7,8] were studied. Interactionof QDs with their environment was proved to play a crucial role in

ll rights reserved.

(P.-N. Wang);

ang), jychen@fudan.edu.cn

determining its solubility and PL properties. Moreover, it has beenshown in a numerous papers that controlling this aggregation onecan fabricate advanced 1D and 2D structures [9–12]. However,very limited information has been reported about the aggregationprocess and PL decay dynamics related to the environmental pH.In this work, pH-dependent stepwise aggregations of thiol-cappedCdTe QDs were directly observed, the size distributions of theaggregates were measured and their pH-dependent PL propertieswere studied.

2. Experiments

The water-soluble thiol-capped colloidal CdTe QDs weresynthesized using a previously reported method [13]. The corediameter of the ODs used in this work was about 3.5 nm. QDsolutions with desired pH were prepared by dropwise addition ofQD solution (1 mg/mL) into NaOH or H2SO4 solutions. All thesamples had the same QD concentration of 0.1 mg/mL.

Direct observation of the pH-dependent aggregation of QDswas performed with a laser scanning confocal microscope(Olympus, FV-300, IX71). The details for microscopic measure-ment can be found in our previous work [14]. Briefly, a 405 nmsemiconductor laser beam (Coherent, Radius 405–25) was focusedby a 60� objective of the microscope to a spot of about 1mm in

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Y. Zhang et al. / Journal of Luminescence 128 (2008) 1948–1951 1949

diameter. The excitation power density used in this work wasabout 3.7 kW/cm2. A 585–640 nm band-pass filter was used todiscriminate against any stray laser light. The samples formicroscopic measurement were prepared by dripping QD solu-tions on glass slides, respectively, and then sealed with cover slips.

The hydrodynamic diameter distributions of QD aggregates insolutions with different pH were measured by the method ofdynamic light scattering (DLS) (Malvern, Autoszer 4700). All themeasurements were performed at room temperature with anirradiation wavelength of 532 nm and a detection angle of 901. Itwas found that the size distribution varied within 20 min aftertitration. Therefore, all the measurements were performed morethan 30 min after the titrations were finished to allow the pH andthe reaction reach equilibrium. All the samples were shakenbefore measurement to achieve a uniform suspension.

The absorption spectra of QD solutions were recorded with aspectrometer (SHIMADZU, UV-3101 PC UV–VIS–NIR). The PLspectra and PL decay dynamics were measured with theaforementioned confocal microscope using the point-stay mode.However, using the confocal system to measure the lifetime, evenprecipitation of QDs occurred, the excitation light could befocused on the precipitates; therefore, the measured spectra andlifetimes were still reliable. To avoid the affection from the QDdensity fluctuation, a 10� objective was used to achieve a largerillumination area. The spectra were measured using a spectro-meter (Acton, spectropro 2150i) equipped with a liquid-nitrogen-cooled CCD (Princeton, Spec-10:100B LN). A 405 nm laser wasused as the excitation source. When measuring the PL decay, theexcitation source was replaced by a picosecond laser (PL2143A,EKSPLA) operating at 410 nm with a pulse width of 28 ps. The PLoutput was directly guided into a photomultiplier tube (PMT)

Fig. 1. PL micrographs of QD solutions with pH valu

(1P28, Hamamatsu) with a 590 nm long pass filter and the PLdecay was recorded using a digital oscilloscope (Tektronics,TDS3000). Since photo-induced PL enhancement, blue shift andquenching were previously observed and were attributed to thephotooxidation effect [14], each of the QD solution samples werebubbled with nitrogen gas for 30 min to purge away oxygenmolecules and then sealed between a glass slide and a cover slipunder nitrogen atmosphere.

3. Results and discussion

Similar to the situation previously reported by other group [4],the QDs were found to disperse uniformly in the aqueoussolutions with pH values from 12 to 7. Further decreasing thepH to 5, QD flocculates began to appear. When the pH reached alow value of 3, precipitation occurred. The pH-dependentaggregation of QDs was directly observed with a confocalmicroscope and the acquired PL micrographs are depicted inFig. 1, where aggregates could be observed at pH 5 (Fig. 1B) andlarge particles appeared at pH 3 (Fig. 1A).

To achieve quantitative evidences, the size distribution of QDaggregates against pH value was measured by DLS. A histogram ofthe size distributions is demonstrated in Fig. 2. In the bestdispersive case at pH 12, the average diameter is 27 nm and thesize distribution is very narrow. Although the diameter of thesemiconductor core was only 3.5 nm in this work, the thiol-capped QDs should have a much larger hydrodynamic size.However, the exact hydrodynamic diameter of a single thiol-capped QD is difficult to measure. When the pH varied from 12 to3, the average hydrodynamic diameter of the QD aggregates

es of 3 (A), 5 (B), 9 (C) and 12 (D), respectively.

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Fig. 2. Hydrodynamic diameter distributions of CdTe QD aggregates in solutions

with different pH. The inset shows the average diameters against pH values.

Fig. 3. Absorption spectra (A) and PL Spectra (B) of CdTe QDs in solutions with

different pH.

Y. Zhang et al. / Journal of Luminescence 128 (2008) 1948–19511950

increased from 28 nm to 1.4mm gradually and the size distributionbecame much broader as well.

These phenomena can be well explained by the nature of theligands as elucidated by Peng et al. previously [4]. The interactionbetween a ligand and the surface cations can be regarded as aspecial type of coordinating bond. When hydrogen ions are addedinto the system, the hydrogen ions will compete for the surfaceligands with the nanocrystals. Therefore, the detachment of theligands from the QD core surface can be considered as adisplacement reaction, which will destroy their dispersibilityand cause aggregations. Larger aggregates are formed as moredetachments of ligands occur. When the aggregates are largeenough, precipitation happens. On the contrary, when the pHranges in the basic conditions, the OH� ions are favorable to thecoordination between the ligands and the semiconductor coresurfaces and the dispersibility of QDs are promoted.

However, the bond between the ligand and the semiconductorcore surface is not strong, the dissociation and re-coordination ofligands to the core surface in a solution is a dynamic process. Theaggregates detected were a result of a dynamic equilibrium. Evenin a basic condition, the occasional aggregation of QDs is possibleand the OH� ions will promote the dispersibility of QDs, resultingin a pH-dependent average size. Although precipitation onlyoccurs when pH is below a certain value [4], the increase of theaverage size with the decreasing pH is a gradual process. It isreported [9–12] that the detachment of mercaptane in the form ofR–S� leads to the decrease of the overall negative charge of thenanoparticles. This reduces the mutual electrostatic repulsion ofnanoparticles countering the dipole–dipole attraction. Therefore,nanoparticles of CdTe were found to spontaneously aggregate,even reorganized into crystalline nanowires or free-floating sheetsupon controlled removal of the protective shell of organicstabilizer.

As QDs have large surface-to-volume ratios, both physical andchemical properties are particularly sensitive to the surfacestructure. Therefore, it is important to explore the pH-dependentphotophysical properties of QDs, such as PL spectroscopy,quantum efficiency and lifetime. It was reported that thefluorescence decreased nearly 80% from the original value(measured at neutral pH) under acidic conditions (pH ¼ 2–4),whereas increased by about threefold in basic solutions(pH ¼ 10–12) [15]. Susha et al. found that the PL intensity ofCdTe NCs showed an almost linear decrease from pH 6 down topH ¼ 4 [16]. On the contrary, it is reported that the fluorescenceintensity of the CdTe crude solution could be enhanced by

decreasing the pH value and the strongest PL occurred atpH ¼ 4.5 [17]. They revealed that this pH effect is related to theexcess cadmium ions in the nondialysed CdTe solution. Withaddition of thioglycolic acid, a shell consisting of cadmiumthioglycolic acid complexes was formed around the CdTe coreand the fluorescence intensity was therefore enhanced. They alsodemonstrated that the fluorescence intensity decreases as the pHwas decreased when the CdTe nanocrystals were stabilized by a1:1 mixture of thioglycolic acid and 2-mercaptoethanol. More-over, precipitation emerged when the pH was lower than 7.

The absorption and PL spectra of QDs under different pHconditions are depicted in Fig. 3. A remarkable decrease of theintensity and a slight red shift of the spectral peak were observedfor the PL spectrum at pH 3 as shown in Fig. 3B. The decrease ofthe PL intensities in acidic conditions can be easily understood. Asaforementioned, the detachment of the surface ligands gave riseto the defect states that lay within the band gap and reduced thequantum yield by providing alternative pathways of excited-staterelaxation [4]. As the pH decreased, more surface defects formed,resulting in a gradual weakening of the PL intensity.

The red shifts of the PL spectra were observed by other groupsand explained with two reasons [18–20]. The wave function of theelectrons may leak out of the dot and overlap to some extent withthe wave function of a neighboring dot in the aggregates becauseof the very small distance between them. The formation of thecollective electronic states due to electron overlap interactionsresults in the red spectral shifts compared to the isolated dots. Inthis case, the red shifts are observed in both absorption and PLspectra [18,19]. Another cause for spectral red shift due toaggregation is the energy transfer from the photoexcited smallerQDs (larger band gap due to quantum confinement effect) to the

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Fig. 4. PL decay curves of CdTe QDs in solutions with different pH.

Y. Zhang et al. / Journal of Luminescence 128 (2008) 1948–1951 1951

larger ones (smaller band gap) [20]. However, as shown in Fig. 3,the red spectral shift in the case of pH 3 occurred only in theemission spectrum and not in the absorption spectrum. Therefore,the red shift observed at pH 3 was most likely originated from theenergy transfer from the smaller QDs to the neighboring largerones.

However, it is difficult to quantitatively investigate the PLefficiency for aggregates. For example, the Rayleigh scattering isintensified with the increasing particle size, the un-uniformdistribution of QDs or aggregates in solutions causes theuncertainty of the absorption and PL intensities. To achieveconvincing information about the pH-dependent nonradiativerelaxation, an advisable way is to study the PL decay dynamics.The measured PL decay curves are shown in Fig. 4. Since the basicenvironment would not cause the detachment of surface ligands,the PL lifetimes under the basic conditions were expected to bethe same as in the neutral condition. The fitted lifetimes with pHfrom 12 to 7 are around 80 ns. Under acidic conditions, thelifetimes decreased dramatically to 57 and 34 ns at pH 5 and 3,respectively. According to the obtained lifetime data, we canconclude that the basic environment does not destroy the surfaceligands and no significant amount of surface defects was formed.The decrease of PL intensity as pH varied from 12 to 7 (Fig. 3B) wasmost probably due to the decrease of the absorption intensity asshown in Fig. 3A.

The PL decay dynamics has been studied previously for CdSeand CdTe QDs in solutions as well as for the single QDs. Moststudies were mainly focused on their size or temperaturedependence [21–26]. The reported QD lifetimes covered anextremely broad range between a few hundreds of picosecondto a few hundreds of nanosecond. Most of them were highlynonexponential [5,21,24,27,28]. However, it should be noted thatall the PL decays in this work were well fitted to be singleexponential. It is well known that the time-dependent PL intensitycan be expressed as I ¼ A exp(�E/(kr+knr)t), where kr and knr areradiative and nonradiative decay rates, respectively. In general

cases, the PL lifetime is inversely proportional to the transitionrate k ¼ kr+knr, which shows a single exponential decay.

4. Conclusion

The basic condition improved the dispersibility of QDs whilethe acidic condition promoted the aggregation of QDs. Althoughprecipitation only occurred when pH is below a certain value, theaverage size of the QD aggregates increased gradually with thedecreasing pH. The lifetime was shortened in acidic conditionsbecause the acidic environment caused the formation of surfacedefects, leading to the enhancement of nonradiative decay.

Acknowledgments

Financial supports from the National Natural Science Founda-tion of China (60638010, 10774027), Shanghai Municipal Scienceand Technology Commission (06ZR14005) and the GraduateInnovation Fund of Fudan University (EYH1206005) are gratefullyacknowledged.

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