Journal of Molecular Structure 1031 (2013) 194–200

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Journal of Molecular Structure

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

Effect of ligands on the photoluminescence properties of water-soluble PbSquantum dots

Yaxin Yu, Kexin Zhang, Shuqing Sun ⇑Department of Chemistry, Tianjin University, Tianjin 300072, PR China

h i g h l i g h t s

" PbS QDs were synthesized in an aqueous route with three different ligands: TGA, MPA, and L-Cysteine as the capping molecule." The capping ligands and pH value plays a crucial role in determining the growth rate and optical properties of PbS QDs." The synthesized L-Cysteine capped-PbS QDs were demonstrated appropriate stability against H2O2 etching.

a r t i c l e i n f o

Article history:Received 11 September 2012Received in revised form 5 October 2012Accepted 5 October 2012Available online 16 October 2012

Keywords:Capping ligandPbS quantum dotsAqueous synthesisLuminescence characteristics

0022-2860/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.molstruc.2012.10.016

⇑ Corresponding author. Tel.: +86 13920690912; faE-mail address: s625827@163.com (S. Sun).

a b s t r a c t

In this study, we report the synthesis of PbS quantum dots (QDs) using an aqueous route with three dif-ferent ligands: thioglycolic acid (TGA), 3-mercaptopropionic acid (MPA), L-Cysteine (L-Cys) as the cappingmolecule. Fourier transform infrared (FT-IR) spectrometry analysis demonstrates that PbS QDs arecapped with these three capping ligands through the coordination. The influences of various experimen-tal variables, on the growth rate and luminescent properties of the obtained QDs have been systemati-cally investigated. Experimental results indicate that the capping ligands and pH value plays a crucialrole in determining the growth rate and optical properties of obtained PbS QDs. Furthermore, the stabilityof L-Cys-capped QDs was compared with the TGA and MPA capped QDs were investigated. Experimentalresults indicate that the L-Cys-capped QDs show better chemical stability than those capped by TGA andMPA. The synthesized L-Cys-capped PbS QDs were characterized by high-resolution transmission electronmicroscopy (HRTEM) and X-ray diffraction (XRD), the results indicate that the QDs were about 8 nm insize and dispersed well with a rock salt crystalline structure.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Colloidal semiconductor nanocrystals, also known as quantumdots (QDs) have been the focus of special consideration due to theirunique fundamental properties and potential application [1–3].They are suitable for applications such as optoelectronic devices[4], light-emitting devices (LEDs) [5], and biological labels [6]. Upto date, many kinds of QDs with different size, shape, and compo-sition have been extensively investigated for not only their theo-retical fundaments but also their practical applications [7–9]. Asan important branch of QDs materials, the near-infrared emitting(NIR) QDs with emission range of 700–1300 nm are becomingincreasingly attractive in the last 5 years [10–12]. This is becauseabsorption and scattering by biological tissues is much reducedin this spectral window compared to the visible range. This allowspenetration of excitation light and fluorescence photons deep into

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biological samples, allowing fluorescence imaging depths on theorder of centimeters. Another advantage lies in the limited auto-fluorescence from tissues, resulting in improved signal-to-noise ra-tio and sensitivity [13].

As one of the important narrow band gap IV–VI semiconductormaterials, PbS QDs, in particular, show increasing promise due totheir large exciton Bohr radius (18 nm) and small bulk band gap(Eg = 0.41 eV), leading to a pronounced quantum size effect andoptical activity in the near-infrared (NIR) spectral regimes[12,14]. These properties make the PbS QDs as a good candidatein various promising applications in the field of photoelectronicand biological devices [15].

To date, two successful routes for synthesis of highly photolu-minescence PbS QDs have been developed, namely the organome-tallic method and water-based synthesis with thiols as cappingligands [16–18]. For the organometallic method, Hines and Scholes[16] synthesized PbS QDs with excellent optical properties throughthe organometallic route at high temperature. Although this routeis great success, there are some inherent limitations includinghigh-cost, unfriendly, and rigorous experimental conditions.

Y. Yu et al. / Journal of Molecular Structure 1031 (2013) 194–200 195

Moreover, the as-prepared QDs are generally capped with hydro-phobic ligands, and hence cannot be directly employed in bioappli-cations. Even though these QDs which capped with hydrophobicligands can be successfully transferred into aqueous solutionthrough ligand substitution [19], they are quite complicate withseveral drawbacks. For example, it is very difficult to efficientlymaintain the QDs stability and emission quantum yield (QYs) onthe transfer process [20]. In parallel with the success of organome-tallic routes, aqueous routes have been developed to directly pre-pared QDs in aqueous solution with water soluble ligands ascapping reagents. Compared with organometallic routes, aqueoussynthesis is reagent-effective, less toxic, and more reproducible[21,22]. Furthermore, the synthesized QDs often show improvedwater-stability and biological compatibility. Recently, water-solu-ble PbS QDs with different thiols as capping agent has been at-tracted interest, Deng et al. [18] reported a route to synthesis ofwater-soluble PbS QDs stabilized with dihydrolipoic acid, and themaximum photoluminescence QYs was approximately 10% underoptimized conditions. Zhao et al. [23] reported the synthesis ofPbS QDs in aqueous medium, by using a mixture of 1-thioglyceroland dithioglycerol as the stabilizer and have shown that the photo-luminescence intensity QYs of PbS QDs was from 7% to 10%.

Among the different kinds of stabilizers used for the synthesisof QDs, the natural biomolecules such as L-Cysteine (L-Cys) havebeen attracted great attention, because it is an aminophenol of hu-man body without any cytotoxin. Currently, CdSe [20], CdS [24],ZnS [25], CdSe/CdS [26], and CdTe [27] QDs were synthesized byusing L-Cys as capping ligand. Among these aqueous QDs, differentmolar ratio of ligands to monomers (mental ions) plays an impor-tant role in the photoluminescence properties. Therefore, differentligands could lead to different surface structures and effect photo-luminescence properties of QDs. However, the understanding ofhow ligands effect on the optical properties of PbS QDs is stilllacking.

In this paper, we reported a facile method to synthesized PbSQDs via aqueous approach by using three kinds of capping ligandsincluding L-Cys, TGA, and MPA, respectively. Under the same syn-thesis conditions (i.e. reaction temperature, pH value, and the mo-lar ratio of Pb2+/S2�/ligands), the photoluminescence emissionintensity of L-Cys-capped QDs was highly than that of eitherTGA- and MPA-capped QDs. The result suggests that L-Cys mole-cules have a significant value for high photoluminescence PbSQDs synthesized in the aqueous solution. Furthermore, the stabil-ity of the L-Cys-capped QDs was compared with the TGA- andMPA-capped QDs. The experimental results indicate that the L-Cys-capped QDs show better chemical and thermal stability thanthose capped by TGA and MPA.

2. Experimental

2.1. Chemicals

L-Cysteine (99%), thioglycolic (TGA, 99%), lead acetate trihydrate(99%), sodium sulfide nonhydrate (99%), and other routine chemi-cals were purchased from Tianjin Guangfu Fine Chemical Researchinstitute Inc. 3-mercaptopropionic acid (MPA, 99%) was purchasedfrom Sigma–Aldrich. All of the reactants were used without anyfurther purification. The water used in all experiments was deion-ized to a resistively of 18.2 MX cm.

2.2. Preparation of water soluble PbS QDs with different kinds ofligands

The present water-soluble PbS QDs were synthesized directly inan all-aqueous procedure as described as follow: Firstly, 30.3 mg

(0.25 mmol) of L-Cys was dissolved in 5 ml of deionic water in a100 ml three-necked flask, air in the system was pumped-off andreplaced with N2 for 30 min. The pH value of the solution was ad-justed to 9.0 by dropwise addition of 1.0 M NaOH with string. Then,lead acetate trihydrate solution (1 ml, 0.1 M) was introduced intothe solution of L-Cys with strong magnetic stirring. The white pre-cipitate was produced which indicates that the formation of thePb–L-Cys complex in the solution. Finally, a solution of Na2S(2 ml, 0.015 M) was slowly injected into mixture solution with syr-inge. The solution instantly turned dark-brown, indicated the for-mation of PbS QDs. The resulting solution mixture was heated to100 �C and refluxed for different time. Then it was cooled to roomtemperature to terminate the growth of PbS QDs. The samples withdifferent ligands like TGA and MPA were synthesized in the samemanner.

2.3. Characterization

The pH value of a solution was measured by a PHS-25 pH metermade by Shanghai Jinmai Company. All photoluminescence spectraof QDs suspension were measured using a Shimadzu RF-5301 fluo-rescence spectrometer with both the excitation and the emissionslit width set at 2 nm. From the excitation spectra, the wavelengthfor the optimal emission intensity was identified first, and the PLemission spectra were then obtained using the optimal excitationwavelength. PL spectra were taken at the excitation wavelengthkex = 620 nm. UV–vis spectra were obtained on a Shimadzu UV-3101 PC spectrometer with the slit width of 2 nm. To conductinvestigations in high-resolution transmission electron microscopy(HRTEM), the QDs were deposited from dilute aqueous solutionsonto copper grids with carbon support by slowly evaporating thesolvent in air at room temperature. The HRTEM images and en-ergy-dispersive X-ray analysis (EDX) were acquired using a JEOL100 CX-II transmission electron microscopy equipped with an Fal-con EDS system operating at an acceleration voltage of 120 kV.Powder X-ray diffraction (XRD) was obtained by wide-angle X-ray scattering, using a BDX3300 X-ray powder diffractometerequipped with graphite monochromatized Cu Ka radiation(k = 1.54178 Å). And the sample was deposited on a piece of Si(100) wafer. FT-IR spectra were recorded on a Magna-560 spectro-photometer. PL quantum yields of PbS QDs in water were calcu-lated by comparing their integrated emission to that of a solutionof cypate in aqueous DMSO (20%) solution. Fluorescence quantumyields (relative values) of samples were calculated according to thefollowing expression:

Yu ¼ Ys� FuFs� As

Auð1Þ

where the subscripts Ys is the fluorescence quantum yield to thereference, Fs and Fu are the integral fluorescence intensity respec-tively refer to the reference and the sample, As is the absorbanceat the excitation wavelength of the sample and Au is the refractiveindex of the solvent.

3. Result and discussion

3.1. Effect of capping ligands on FT-IR spectra

The extracted powder was used to record FT-IR measurementsto identify the functional groups on the surface of three differentcapping ligands stabilized PbS QDs. FT-IR spectra of TGA-, MPA-,and L-Cys-capped PbS QDs are shown in Fig. 1. Fig. 1a display char-acteristics band of TGA-capped QDs at 3412 cm�1 is ascribed to thestretching vibration of �OH, the absorption bands occur at1403 cm�1 is attributed to the stretching vibration of the C–O

Fig. 1. The FT-IR spectra of (a) TGA-, (b) MPA-, and (c) L-Cys-capped PbS QDs.

196 Y. Yu et al. / Journal of Molecular Structure 1031 (2013) 194–200

bond, and the spectra of TGA-capped QDs show the peak at2922 cm�1 that can be attributed to the stretching vibration ofthe C–H bond. However, the stretching vibration of the S–H bond(2560 cm�1) are not observed in Fig. 1a indicates that TGA com-bines on the surface of the PbS QDs with concomitant loss of theS–H bond. Fig. 1b shows the FT-IR spectrum of MPA-capped PbSQDs. From Fig. 1b, the absorption bands occur at 655 and1232 cm�1 are due to C–S and C–O stretching vibration, respec-tively. The peaks at 1384 cm�1 and 1566 cm�1, are because of sym-metric, asymmetric vibrations of �COO. The peak at 2924 cm�1 canbe attributed to the asymmetric vibration of the �CH2 bond. Theabsence of S–H stretching bands around 2560 cm�1 indicates thecapping molecule is bonded to surface lead through Pb–S chemicalbond by cleaving its hydrogen bond. Fig. 1c presents the samplePbS QDs capped with L-Cys. The characteristics peaks of S–H arenot seen in the L-Cys-capped PbS QDs, which indicated that theS–H bond was cleaved and formed a new S–Pb bond between L-Cys and PbS QDs. In addition, it can be observed that there is a shiftof the stretching vibration of the carboxyl group in the L-Cys from1575 cm�1 to 1585 cm�1, which indicates the formation of thecovalent bands between carboxyl group of L-Cys and the Pb atomon the surface of the QDs. Moreover, the broad peak at3412 cm�1 indicates the banding vibration of N–H of the �NH2

moiety on the L-Cys capped PbS QDs, it could be ascribe to the basi-fication of NH3

+ group of L-Cys during the synthesize [28].

3.2. Photoluminescence (PL) spectra of PbS QDs with different ligands

The success of our approach strongly depended on optimalexperimental conditions. In this experimental, we selected threekinds of thiols ligands such as TGA, MPA, and L-Cys as capping re-agents to synthesize PbS QDs in the aqueous solution. In threekinds of ligands synthetic systems, reaction conditions are thesame, i.e., the molar ratio of the Pb/ligands/S was fixed at1:2.2:0.3 under reflux at 100 �C while changing the refluxing timeand pH value.

3.2.1. Dependence of growth rate and size-distribution of PbS QDs ondifferent ligands

Fig. 2 shows the typical evolutions of PL emission spectra of PbSQDs with the use of three kinds of capping reagents (TGA, MPA,and L-Cys). The PL spectra for all samples were measured underthe same measurement conditions. The PL spectra of TGA-cappedPbS QDs were shown in Fig. 2a. From Fig. 2a, with the reflux pro-ceeding, the PL emission wavelengths of QDs are shifts from741 nm to 810 nm, which indicates the QDs grow to large size.

With refluxing time increasing, the PL intensity of QDs increasesand reached a maximum value at 20 min, the corresponding PLquantum yield is only about 12.9%. Further heating to 120 min,the PL intensity of QDs started to decrease gradually. Fig. 2b pre-sents typical evolutions of PL emission spectra of MPA-PbS QDs.With MPA as the capping ligands, the PL emission wavelengthsshift from 734 to 805 nm after refluxing from 10 to 240 min. Withheating of mixture solution, the PL intensity of QDs system isgreatly improved and reached a maximum value at 20 min, thecorresponding PL quantum yield reaches 15%. It is noted that themaximum value of PL intensity of MPA capped-QDs is higher thanthe one capped with TGA. Fig. 2c presents the typical evolutions ofPL emission spectra of L-Cys capped PbS QDs prepared in the aque-ous phase. Under refluxing, the trends of PL emission wavelengthare similar with those of TGA- and MPA-capped PbS QDs. As shownin Fig. 2c, the PL emission wavelengths are from 719 to 801 nmafter refluxing from 10 to 420 min. The PL intensity of QDs in-creases and reached a maximum value at 30 min, the largest PLquantum yield reaches 18%. Although the redshifts of PL emissionwavelength would be observed in the PL spectra of different QDswhen refluxing time went on, the PL quantum yield of L-Cys cappedPbS QDs is higher than that of other two PbS QDs obviously.

Fig. 2d shows the PL emission wavelengths of three kinds of li-gands capped PbS QDs versus refluxing time. The growth of PbSQDs in the system was demonstrated by a red-shift of the PL emis-sion wavelength the process of thermal treatment. There was a fastred-shift (69 nm in 110 min) for TGA-capped PbS QDs in the con-trol experiment. While for L-Cys-capped PbS QDs, there is onlyabout 44 nm red-shift in 110 min refluxing. The experimental re-sults show that the growth rate of TGA-capped PbS QDs was fasterthan that of MPA- and L-Cys-capped QDs. As the size of PbS QDs getlarger, the above trend is observed more clearly.

Fig. 2e shows the PL emission intensity of three ligands cappedPbS QDs versus refluxing time. As summarized in Fig. 2e, the PLemission intensity of the resulting PbS QDs strongly depends onthe capping agent used. With the introduction of different thiol li-gands (TGA, MPA, and L-Cys) to the reaction solution, these PbSQDs exhibited an increase in their PL intensity with refluxing timeand reached the maximum value at 20 min, 20 min, and 30 min,respectively. Prolonged reaction time can enhance the PL intensitydue to reduce the nonradiative combination by minimizing thesurface defects of QDs [29]. However, with further refluxing, thePL intensity of QDs started to decrease gradually. It might be thereason that the thiols ligands gradually decomposed with overextending of refluxing time. In addition, we noted that the PL emis-sion intensity of QDs with L-Cys was higher than MPA and TGA, it isdue to the amine terminal group of L-Cys which different from MPAand TGA. These structural differences may be used to elucidate thevital effect of L-Cys on synthesize of PbS QDs. Strong hydrogenbonding between amine terminal groups of neighboring surfacecapped QDs may facilitate the coalescence of QDs, decreasing thedangling bonds at QD surfaces to prevent nonradiative recombina-tion at surface sites. In addition, besides the decrease of surfaceunpassivated atoms, the amine group may also give more tight sur-face organic shell which could prevent the quencher diffusing tothe defects sites on the QD surface [30]. Hence, the resulting PbSQDs capped with L-Cys would show more favorable fluorescenceemission, compared to use of TGA and MPA.

3.2.2. Effect of pH value on PL intensity of PbS QDsThe pH value is another important factor that strongly influ-

ences the optical performance of the as-prepared PbS QDs. In thisstudy, we selected out five kinds of pH values from 7.5 to 9.9 with1.0 M NaOH aqueous solution while keeping the other variables,the effect of the pH value on the optical properties of PbS QDs withthree capping ligands (TGA, MPA, and L-Cys) was explored, and the

Fig. 2. The typical evolutions of PL emission spectra of PbS QDs with the use of three kinds of capping reagents: (a) TGA (b) MPA (c) L-Cys, and (d) The PL emissionwavelengths of three kinds of ligands capped PbS QDs versus refluxing time. (e) The PL emission intensity of three ligands capped PbS QDs versus refluxing time.

Y. Yu et al. / Journal of Molecular Structure 1031 (2013) 194–200 197

obtained result is shown in Fig. 3. As summarized in Fig. 3, withincreasing pH value, the PL intensity of as-prepared QDs was grad-ually increased, and the QDs show the strongest PL intensity (at pH8.5, 9.0, and 9.9), then decays with a further increase of pH value.Under the condition of alkaline pH value condition, thiols ligandcan form thiolate ion through deprotonation process. Due to thestrong binding capability, the thiols ligand can form stable com-plexes with free metal ions Pb2+. Under the condition of higherpH value, the Pb-thiols complexes can exist in molecular state,which could easily coverage on the surface of the PbS QDs andthe QDs with larger size would be formed [31]. Therefore, moretrap sites on the QDs surface will be removed, resulting in the

improvement of the PL intensity [32]. However, too fast growthrate is not benefit for the PL properties of QDs due to the existenceof non-radiative recombination centers in the growth process [33].This can explain our experimental results that a further increase ofpH value, the PL intensity of PbS QDs solution started to decrease.

In additions, PL intensity of L-Cys-capped QDs reached a maxi-mum value when the pH value is 9.0. If using pH 8.5, the synthe-sized TGA- and MPA-capped PbS QDs displays a relatively highPL intensity. Meanwhile, it is observed that the maximum valueof PL intensity of L-Cys-capped QDs was higher than that of MPA-and TGA-QDs. A previous report had proved that the thiols-endof mercaptoacetic acid is the anchor on the nanocrystals, while

Fig. 3. The PL emission intensity of three ligands (TGA, MPA, and L-Cys) capped PbSQDs versus pH value.

Fig. 5. Powder X-ray diffraction patterns of as-synthesized PbS QDs capped with L-Cys.

198 Y. Yu et al. / Journal of Molecular Structure 1031 (2013) 194–200

the carboxylic acid acts as a secondary coordinator [34]. However,the structural of L-Cys is different from MPA and TGA. It containingamine group, which have a noticeable effect on the PL intensity ofQDs.

3.2.3. PL stability of PbS QDs under H2O2 etchingWe did the H2O2 etching experiments to investigate the differ-

ence in chemoxidation stability of PbS QDs capped with differentligands including TGA, MPA, and L-Cys. The tolerance of differentthiols ligands capped PbS QDs against oxidation by H2O2 usingthe literature method [31]. In the H2O2 etching experiment, thesynthesized PbS QDs with identical optical density (0.1) wasloaded in a quartz PL cuvette and 10 ll of 1% H2O2 solution wasadded to this cuvette with stirring under ambient conditions atroom temperature. The PL spectra were measured after differentintervals of time. Fig. 4 shows the temporal evolutions of PL emis-sion peak wavelength and intensity corresponding to each PbS QDscapped with different ligands against H2O2 etching. As shown inFig. 4a, with prolonging etching time, the PL emission wavelengthsof three QDs shifted to shorter wavelength, which indicate the sizeof QDs were decreased [29]. In addition, the shift extent of TGA-and MPA-capped QDs is much more pronounced compared withthat capped with L-Cys. For instance, the PL emission peak of PbSQDs capped with TGA and MPA bore a blue-shift of �40 nm, while

Fig. 4. The temporal evolutions of PL emission peak wavelength (a) and intensity (b) corragainst H2O2 etching.

for L-Cys capped QDs the blueshift value is only 10 nm. Due to thebandgap emission peak position directly reflecting the particle size,the more pronounced blue-shift of emission peak in PbS QDs aremore sensitive to H2O2 etching and are less stable compared to L-Cys capped PbS QDs. From Fig. 4b, when H2O2 was adding intothe sample, there was a sharp decrease of PL intensity in all threesamples. After 20 min of etching time, there was nearly no lumi-nescence in the sample which capped with TGA and MPA, whilethere was still 40% of the PL intensity for the L-Cys-capped sample,which indicated that the L-Cys capped PbS QDs do improved thestability against H2O2 oxidation.

3.3. Microstructural characterization

As discussed above, highly luminescent QDs can be synthesizedwhen L-Cys chosen as capping ligands. It exhibited higher PL inten-sity and stronger PL stability. In order to characterize the crystalstructure and morphology of L-Cys-QDs, the X-ray diffraction(XRD) and high-resolution transmission electron microscopy(HRTEM) was employed. The phase structure of the synthesizedPbS QDs was characterized by XRD, which is shown in Fig. 5. It isobvious that all of the peaks in the powder XRD patterns have beenindexed to the crystalline PbS in a cubic structure. For a compari-son, the relative intensities of the diffraction peaks from the stan-dard card (JCPDS No. 05-0592) were labeled on the bottom of Fig. 5

esponding to each PbS QDs samples capped with three ligands (TGA, MPA, and L-Cys)

Fig. 6. (a) Typical HRTEM image of as-synthesized PbS QDs capped with L-Cys. (b) The HRTEM image of PbS QDs with lattice resolved are also presented.

Fig. 7. EDX spectrum of as-synthesized PbS QDs capped with L-Cys.

Y. Yu et al. / Journal of Molecular Structure 1031 (2013) 194–200 199

as well. It is observed that all the peaks are very much prominent.The diffraction peaks are broader than those of bulk sample, inaccordance with their smaller crystalline domains. Furthermore,no peaks of impurities were detected in XRD pattern, indicatingthe high purity of the sample. The crystallite size estimated fromthe Debye–Scherrer formula [35]

D ¼ kk=b cos h ð2Þ

Herein D is the average crystalline size of QDs, k is a constant (shapefactor, about 0.9), k is the X-ray wavelength (1.5405 Å), b is the fullwidth at half maximum (FWHM) of the main diffraction line, and his the diffraction angle. The calculated result indicates that themean crystalline size of PbS QDs sample is approximately as 7.4 nm.

Fig. 6a shows the obtained HRTEM image of L-Cys-capped PbSQDs under the above-mentioned experimental conditions. Theoverview HRTEM image illustrates the good size distribution ofPbS QDs with an average size of 8.0 nm diameter, which is consis-tent with the results from HRTEM image. Fig. 6b reveals well-re-solved lattice fringes, demonstrating that the QDs are singlecrystalline with high crystallinity, the clear lattice planes markedby white lines and arrows.

Further, we carried out energy dispersive X-ray spectroscopy(EDX) measurements for L-Cys-capped PbS QDs. Fig. 7 depicts therepresentative EDX analysis of the PbS QDs. Only Pb and S peakswere detected and the Cu peak was attributed to the substrate.Herein, EDX analysis also indicated the presence of a small quan-tity of C and O relating to the carbon supporting membrane. The

strong peaks for Pb and S in the spectrum confirmed the formationof the PbS QDs without any impurity. Based on the calculation ofthe peak areas, the atomic ratio of Pb:S was 70:30, which was veryclose to the stoichiometry of PbS.

4. Conclusions

In summary, we successfully synthesized three kinds of thiolsligands such as TGA-, MPA-, and L-Cys-capped PbS QDs in aqueoussolution. In comparison of the synthetic selectivity of three ligands,it is found that the growth rate and luminescent properties of PbSQDs are dependent on the type of capping ligands. The L-Cys-capped PbS QDs exhibit the highest PL intensity at the same exper-imental conditions. The influence of pH on the PL intensity of QDswas investigated. The stabilities to chemoxidation on luminescentproperties of PbS QDs were improved that L-Cys capped PbS QDsshow better chemoxidation stability than those capped by TGAand MPA.

Acknowledgement

This work is supported by Key Project of Tianjin Sci-Tech Sup-port Program (No. 08ZCKFSH01400).

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