Size-tunable near-infrared PbS nanoparticles synthesized from lead carboxylate and sulfur

with oleylamine as stabilizer

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Nanotechnology 19 (2008) 345602 (9pp) doi:10.1088/0957-4484/19/34/345602

Size-tunable near-infrared PbSnanoparticles synthesized from leadcarboxylate and sulfur with oleylamineas stabilizerJincheng Liu, Huangzhong Yu, Zhonglian Wu, Wenli Wang,Junbiao Peng and Yong Cao

Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of SpecialFunctional Materials of the Ministry of Education, South China University of Technology,Guangzhou 510640, People’s Republic of China

E-mail: poycao@scut.edu.cn

Received 19 February 2008, in final form 15 June 2008Published 16 July 2008Online at stacks.iop.org/Nano/19/345602

AbstractHigh quality PbS nanocrystals are synthesized reproducibly through lead stearate and sulfurstabilized by oleylamine in a non-coordinating solvent. The morphology, crystalline form andphase composition of PbS nanocrystals are examined by transmission electron microscopy(TEM), high-resolution TEM, x-ray diffraction (XRD), energy-dispersive x-ray spectroscopy(EDS) and x-ray photoelectron spectroscopy (XPS). The as-synthesized PbS nanocrystals havestrong absorption and photoluminescence emissions in the near-infrared region. The size of PbSnanocrystals from 5 to 13 nm can be adjusted through the optimization of the synthesisconditions. The smaller PbS nanoparticles are obtained at the lower reaction temperature, lowerprecursor concentration, larger oleylamine quantity and larger lead precursor/sulfur ratio. Thebasic oleylamine enhances the reactivity of both lead stearate precursor and sulfur precursor inthe reaction.

S Supplementary data are available from stacks.iop.org/Nano/19/345602

1. Introduction

Since the first successful growth of high quality CdSenanocrystals [1], the reproducible and controllable synthesis ofcolloid semiconductor nanocrystals has attracted a great dealof interest due to their unique electrical, optical and magneticproperties. Over the past ten years, many different kinds ofnanocrystals [2] have been synthesized through organometallicapproaches based on the similar method for CdSe synthesis.

As an important branch of semiconductor nanocrystals,near-infrared nanocrystals are currently receiving widespreadattention due to their strong effect of quantum confinementand broad luminescence emission [3]. Lead sulfide (PbS) isan important direct gap semiconductor with a band gap of0.41 eV [4]. PbS nanocrystals have a large Bohr radius of20 nm composed of nearly equal contributions from electronand hole [5]. The broad bandgap value from 0.41 to 5 eVcan be approached when the mean size is smaller than the

Bohr radius [5]. PbS nanocrystals have potential applicationsin nonlinear optical devices [6], infrared detectors [7], displaydevices [8] and hybrid photovoltaic devices [9].

Monodisperse PbS nanocrystals were first synthesized inpoly(vinyl alcohol) (PVA) by Nenadovic et al in 1990 [10].Subsequently several groups reported the synthesis of PbSnanocrystals doped in glass [11]. In 2002, Cheon et al[12] reported the synthesis of PbS with different shapesfrom decomposing a single precursor at high temperatures.However, they were only able to obtain large PbS nanocrystals.The synthesis of colloid PbS nanocrystals utilizing anorganometallic approach was first reported by Scholes et al[13]. They synthesized PbS nanocrystals with a broadabsorption band from 800 to 2000 nm using lead oleate andbis (trimethylsilyl) sulfide (TMS) stabilized by oleic acid.With oleic acid as surfactant, Warner et al [14] succeeded insynthesizing very small PbS nanocrystals by using H2S as asulfur source. The reason for using high-reactive TMS and

0957-4484/08/345602+09$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

Nanotechnology 19 (2008) 345602 J Liu et al

H2S is that sulfur and lead carboxylate in the presence of oleicacid can react readily into bulk-like particles. In comparisonwith TMS and H2S, sulfur is inexpensive and environmentallyfriendly which is used extensively in the synthesis of metalsulfide nanocrystals. Hyeon et al [15] reported the synthesisof PbS nanocrystals capped by oleylamine of sizes from6 to 13 nm with lead chloride and sulfur. A similarsolventless approach to synthesize PbS nanocrystals stabilizedwith oleylamine was developed by the Ozin group [16]with a narrow full width at half-maximum (FWHM) of thephotoluminescence peak and a quantum yield (QY) of 40%.Alkylamine is an important weak ligand surfactant used widelyin the synthesis of semiconductor nanocrystals. Since adecomposing reaction with metal carboxylate typically occursat high temperatures in the organometallic route, alkylaminewas mostly used together with the metal chloride [15] ormetal alkyl precursor [17]. The lead alkyls are spontaneouslycombustible, flammable, poisonous and dangerous in thesynthesis of PbS nanocrystals. Furthermore, the residualchloride on the surface of PbS nanoparticles produced fromlead chloride precursors is a major drawback for its furtherapplications.

In this paper, we report an alternative approachto synthesize PbS nanocrystals with lead carboxylateand inexpensive environmentally friendly sulfur precursorsstabilized by oleylamine. During the synthesis process,the reaction rates of both lead carboxylate and sulfur areenhanced due to the presence of the basic oleylamine. Thesynthesis of PbS nanocrystals by this method can proceedat much lower reaction temperatures without decomposition.Accordingly, the reaction between lead carboxylate and sulfurbecome controllable to produce high quality near-infraredPbS nanocrystals. In addition, it is found that the meansize of PbS nanocrystals can be tailored by changing thesynthesis parameters such as oleylamine quantity, precursorconcentration, Pb/S ratios and reaction temperature. Thismethod is highly reproducible and economical for large-scalesynthesis of PbS nanocrystals for a variety of optoelectronicapplications.

2. Experimental details

2.1. Chemicals

Oleylamine (OA, 97%) and 1-octadecene (ODE, 90%) werepurchased from Aldrich. Erucic acid (technical grade85%, CH3(CH2)7CH=CH(CH2)11COOH) and behenic acid(C21H43COOH, 99%) were purchased from Alfa Aesar. Oleicacid (OLA, analytic agent), lead stearate (PbSt2, analyticagent), lead oxide (PbO, analytic agent, 99%) and sulfurpowder (S, analytic agent) were purchased from GuangzhouChemical Corporation. All the reaction agents were usedwithout any further purification. All solvents were purchasedfrom Guangzhou Chemical Corporation and used as received.

2.2. Synthesis of lead precursors

The lead erucic and lead behenic precursors were synthesizedwith PbO/erucic acid and PbO/behenic acid by refluxing in

xylene for one hour under Ar, respectively. The synthesizedprecursors were isolated by precipitation with methanol, andwashed with the mixed solvents of hexane and methanol twice.The precipitated precursors were vacuum-dried for 24 h beforeusing.

2.3. Synthesis of PbS nanocrystals

PbSt2 (0.5 mmol, 0.38 g) and 2 ml OA were added to thesolvent of ODE at room temperature and the resulting solutionwas heated to 150 ◦C under Ar for half an hour. Sulfur(0.25 mmol, 8 mg) was dissolved in 1 ml of OA underultrasonics for 30 min. The resulting sulfur solution wasinjected into the lead precursor complex solution at 70–220 ◦Cand aged at that temperature for 1 h (aging time kept to onehour for all experiments in this study). Ethanol (100 ml) wasadded to precipitate PbS nanocrystals. The precipitate wasretrieved by centrifugation, leaving behind deep blue coloredPbS nanocrystals. The synthesized PbS nanocrystals aredispersible in many organic solvents such as toluene, hexaneand octane.

2.4. Characterization of PbS nanocrystals

Transmission electron microscopy (TEM) and high-resolutionTEM (HRTEM) images were obtained using a JEOL 2010-H microscope (TEM) operated at 100 kV and 200 kV,respectively. The samples for TEM analysis were preparedby dropping dilute toluene solutions of PbS nanocrystalsonto carbon-coated copper grids. Powder x-ray diffraction(XRD) measurements were taken with Cu Kα radiation(k = 1.5418 A). FTIR spectra were recorded in the rangeof 4000–400 cm−1. The samples for FTIR calibrated bypolystyrene were prepared using the KBr technology. TheFTIR specimen of OA and the PbSt2 composite was takenfrom the reaction solvents at 180 ◦C, deposited with acetoneand then vacuum-dried before testing. EDS analysis wasperformed by the JEOL EPMA-1600 operated at 15 keV.XPS measurements were done by using a Kratos Axis Ultraspectrometer with a monochromic Al Kα source at 1486.6 eV,with a voltage of 15 kV and an emission current of 10 mA.The PbS nanocrystals were vacuum-dried at room temperaturefor 24 h before EDS and XPS analysis. Absorption spectrawere recorded by using a Lambda 900 Spectrophotometer,while photoluminescence spectra (PL) were measured by usinga Triax 320 fluorescence spectrophotometer manufacturedby Horiba–Jobin–Yvon Company with an InGaAs detectorexcited by a 6 W 808 nm laser diode (LD) at room temperature.

3. Results and discussion

Attempts to synthesize PbS nanocrystals from lead stearate(PbSt2) and sulfur (S) in the presence of oleic acid (OLA)ended in large particle sizes of about 500 nm. The synthesisreaction between PbSt2 and sulfur in the presence of OLAcannot occur under 180 ◦C. We found out that, by replacingOLA by oleylamine (OA) as stabilizer, the nucleation reactionbetween PbSt2 and sulfur can take place at temperaturesmuch lower than 180 ◦C. This demonstrates that OA can

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Figure 1. (a) Absorption spectra of PbS nanocrystals with tuning size from 5 to 9 nm in toluene solvent. (b) Band-edge absorption andphotoluminescence peaks for 5.6 nm PbS nanocrystals with OA (3 ml), ODE (10 ml), PbSt2 (0.5 mmol) and S (0.25 mmol) at 120 ◦C.

evidently enhance the reaction rate of lead stearate and sulfur.Consequently, the reaction of lead stearate and sulfur in thepresence of OA is controllable to synthesize high quality PbSnanocrystals at a lower temperature. We have shown that, byusing OA, PbS nanocrystals with small sizes down to 5.6 nmwith a narrow size distribution (±15%) can be acquired.

3.1. Synthesis of PbS nanocrystals

PbS nanocrystals have a large bulk exciton Bohr radius (20 nm)that allows for strong quantum confinement with relativelylarger crystals. As shown in figure 1(a), we were able toreproducibly obtain PbS nanocrystals with the first excitonabsorption peak from 1418 to 1780 nm.

Figure 1(b) shows the absorption spectrum and PLspectrum at room temperature of 5.6 nm PbS nanocrystalssynthesized with 10 ml ODE, 3 ml OA, 0.5 mmol PbSt2and 0.25 mmol S at 120 ◦C. The first exciton absorptionpeak is around 1467 nm, which is close to the data reportedby Murphy [18]. Compared with the absorption onset at3020 nm of bulk PbS materials a sizable blueshift occurs.Correspondingly, the PL emission maximum of 5.6 nm PbSnanocrystals is at 1506 nm with a full width at half-maximumof 150 meV. This indicates the low polydispersity (±15%)of PbS nanocrystals that can be realized without any post-synthesis size-selective precipitation according to the syntheticmethod proposed in this study.

Figure 2 shows the transmission electron microscopic(TEM) images of PbS with mean sizes of about 5.6 nm(figure 2(a)), 8.9 nm (figure 2(b)) and 11 nm (figure 2(d))prepared by PbSt2 and sulfur capped with OA, respectively.From figure 2(a), the shape of smaller PbS nanocrystalsprepared at 120 ◦C appears less smooth with a sharp edge.And from figures 2(b) and (d), the image of PbS nanocrystalshaving a larger mean size shows a smooth spherical shapeself-assembled into nanocrystal superlattices. Similar resultswere reported by Scholes [13], who observed that the largerPbS nanoparticles turned into spheres after heating while thesmaller ones remained angular. Cheon and his colleagues [12]reported that the shape of PbS nanocrystals was spherical whenusing the strong dodecanthiol ligand and cubic when using theweak dodecylamine ligand. They suggested that the strongdodecanthiol ligand blocked the growth of the [111] face and

the weak dodecylamine ligand enhanced the growth of the[111] face. However, in contrast, the weak ligand OA usedin this study leads to spherical nanocrystals. Additionally, theshape of smaller PbS nanocrystals is angular due to the favoredgrowth on the [111] face, and smoothed into spheres when itgrows into larger particles. The high-resolution transmissionelectron microscopic (HRTEM) image of PbS nanocrystalswith 8.9 nm demonstrates high crystallinity of the nanocrystals(figure 2(c)).

The size of PbS nanocrystals can be tuned by adjustingthe reaction temperature, the quantity of surfactant, the Pb/Sprecursor ratio and the precursor concentration. The size ofPbS nanocrystals is monitored by absorption spectra of PbSnanocrystals dispersed in toluene. As shown in figure 3(a),absorption peaks are blueshifted with decreasing synthesistemperature. This indicates that at lower reaction temperaturessmaller-size nanocrystals are obtained, similar to the resultsreported by Scholes [13]. After the temperature is loweredto the melting point of sulfur (117 ◦C), no evident absorptionpeak is observed, suggesting that the as-synthesized PbSnanocrystals have a large polydispersity. In addition, the factthat a large quantity of unreacted lead precursor for the reactionperformed at 70 and 90 ◦C indicates that the reaction rate attemperatures below 120 ◦C is very low. On the other hand,when the injection nucleation temperature is equal to or higherthan 220 ◦C, only the aggregated large PbS nanoparticlescan be obtained, because the reaction speed is too fast atthis temperature (see figure 1 in the supporting informationavailable at stacks.iop.org/Nano/19/345602).

The OA quantity is critical for preparation of highquality PbS nanocrystals. In our experiment, when the ratioof OA/PbSt2 is less than 6 (molar ratio) at the reactiontemperature of 120 ◦C, the as-prepared PbS nanocrystals areeasy to aggregate during the purification process. Figure 3(b)shows that the absorption spectra of PbS nanocrystals havean appreciable blueshift with increasing OA quantities in thereaction media. It indicates that smaller nanoparticles canbe obtained with larger OA quantities used. According tofigure 3(b), we also find that the absorption spectrum becomesbroader when 12 mmol OA used. It is noted that more OAused reduces the rate of ripening. Hence the size distributionof PbS nanocrystals becomes broader when more OA is usedin the synthesis of PbS nanocrystals. A similar observation

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Figure 2. TEM images of PbS nanocrystals of different sizes. (a) PbS nanocrystals synthesized with 3 ml OA, 10 ml ODE, PbSt2 (0.5 mmol)and S (0.25 mmol) at 120 ◦C. The PbS nanocrystals shown here have a lowest-energy excitonic transition at 1567 nm and an average diameterof 5.6 nm with a standard deviation of 0.54. (b) PbS nanocrystals synthesized with 3 ml OA, 10 ml ODE, PbSt2 (0.5 mmol) and S (0.5 mmol)at 180 ◦C. The PbS nanocrystals shown here have a lowest-energy excitonic transition at 1780 nm and an average diameter of 8.9 nm with astandard deviation of 0.41. (c) HTEM image of image (b). (d) PbS nanocrystals synthesized with 6 ml OA, 10 ml ODE, PbSt2 (1 mmol) andS (1 mmol) at 180 ◦C. The PbS nanocrystals shown here have an average diameter of 11 nm with a standard deviation of 0.49.

was reported by the Peng group [19] and they attributed thereduction of the growth rate to more alkylamine capped ontothe surface of nanocrystals.

The Pb/S ratio is crucial in controlling the size of PbSnanocrystals. At the same reaction temperature of 180 ◦C,PbS nanocrystals are synthesized at the Pb/S molar ratios of1:2, 1:1 and 2:1. Only large particles with a mean size of13 nm with large polydispersity can be obtained when the Pb/Sratio is 1:2 (figure 2 in the supporting information availableat stacks.iop.org/Nano/19/345602). As shown in figure 3(c),the first exciton absorption peaks are at 1780 and 1601 nm forthe Pb/S ratios of 1:1 and 2:1, respectively. A large blueshiftsuggests that the mean sizes of PbS nanocrystals decreaseevidently with the increase of the Pb/S ratio. This is similarto the previous results from Hyeon [15] for lead chloride asprecursor.

The concentration of lead precursors is also studied.As shown in figures 2(b) and (d), the average size ofPbS nanoparticles changes from 11 to 8.9 nm whenthe concentration of lead precursors changes from 0.1 to0.05 mol kg−1. This indicates smaller PbS nanocrystals canbe produced with the lower precursor concentration. Murphyet al [18] reported that 2 nm PbTe nanocrystals could besynthesized with long chain erucic acid when oleic acid is usedas the ligand. However, in our case when OA is used as the

stabilizer, lead erucic and lead behenic precursors yield onlylarger particles of about 13.6 and 14.1 nm (figures 4(a) and (b)),respectively. Further research of different lead carboxylates isin progress and will be reported in forthcoming reports.

3.2. XRD and EDS analysis

Powder x-ray diffraction (XRD) of the 8.9 nm PbSnanocrystals is shown in figure 5(a). The XRD patternreveals the cubic rock-salt structure of PbS without obviouscharacteristic reflection peaks from other impurities accordingto JCPDS card no. 78-1898 with the lattice parameter a =0.5918 nm. The broadening peaks are caused by the small sizeof PbS nanocrystals. The size of PbS nanocrystals calculatedfrom the Scherrer formula is 9.3 nm. This is close to themean size observed by TEM. The EDS spectrum (figure 5(b))indicates that a small quantity of C and O are present in PbSnanocrystals along with the Pb and S peaks. Nitrogen elementis not observed. The EDS analysis of PbS nanocrystals revealsthe ratio of lead and sulfur is close to 1:1.

3.3. XPS analysis

The photoelectron spectroscopy is used to study the chemicalenvironment and the electronic structure of PbS nanocrystals.

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Figure 3. (a) Absorption spectra of PbS nanocrystals with 10 ml ODE, 3 ml OA, PbSt2 (0.5 mmol) and S (0.25 mmol) at different reactiontemperatures. (b) Absorption spectra of PbS nanocrystals with different OA quantities at 10 ml ODE, PbSt2 (0.5 mmol) and S (0.25 mmol) at120 ◦C. (c) Absorption of PbS nanocrystals with different Pb/S ratios at 10 ml ODE, 3 ml OA at 180 ◦C.

Figure 4. (a) TEM image of PbS nanocrystals synthesized with ODE (10 ml), OA (3 ml), lead erucic (0.5 mmol) and S (0.25 mmol) at120 ◦C. The PbS nanocrystals shown here have an average diameter of 13.6 nm with a standard deviation of 1.23. (b) TEM image of PbSnanocrystals synthesized with ODE (10 ml), OA (3 ml), lead behenic (0.5 mmol) and S (0.25 mmol) at 120 ◦C. The PbS nanocrystals shownhere have an average diameter of 14.1 nm with a standard deviation of 1.31.

A typical photoemission spectrum of 8.9 nm PbS nanocrystalsis shown in figure 6(a). The Pb and S photoelectron lines fromthe nanocrystals can be detected along with the C and O peaksfrom the precursor. This is consistent with the EDS analysisresults. High-resolution Pb 4f and S 2p core-level spectra arerepresented in figures 6(b) and (c), respectively. The peaks at136.97 eV and 140.17 eV belong to the binding energy of Pb 4f7/2 and Pb 4f 5/2, respectively [20]. The doublet structure of S

2p is characterized by peaks centered at 160.24 and 161.29 eV.These binding energy values are close to those of the PbS bulkcrystal [21].

The Ozin group [16] estimated the lead-chloride-likeshell of 4 A existed in the surface of PbS nanocrystals.Although the chloride content at the nanocrystals’ surfacecan be reduced to a lower value by plasma-treating the PbSnanocrystal film [22], it will possibly produce defects in the

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Table 1. The nucleation experiments of PbS nanocrystals.

Experiment Precursors Injection solvents Nucleation temperature

Size of PbSnanocrystals(nm)

A PbSt2 1 mmol + OLA 2 mmol S 1 mmol/1 ml ODE >180 ◦C 500

B PbSt2 1 mmol + OLA 2 mmol S 1 mmol/1 mmol OA + 1 ml ODE 90 ◦C less nuclei and>180 ◦C second nucleation

500 and 15

C PbSt2 1 mmol + OLA 2 mmol S 1 mmol/9 mmol OA + 1 ml ODE 90 ◦C 10–20

b

Figure 5. (a) XRD patterns of 8.9 nm PbS nanocrystals. (b) EDS of8.9 nm PbS nanocrystals.

(This figure is in colour only in the electronic version)

solar cell device [23]. Weller and coworkers [20] reportedthat trioctylphosphine (TOP) ligands passivated the surface Ssites of PbS nanocrystals. The advantage of the synthesisapproach where lead stearate is used as the lead source andS/OA solvents as injection solvents without lead chlorideand TOP used is that PbS nanocrystals with a chloride-freeand phosphor-free fine surface chemical environment can beobtained.

3.4. The effect of OA on the synthetic system

The synthesis method of PbS nanocrystals in this study wastaken from the modified method for cadmium chalcogenidesproposed previously by Peng et al [24]. Scholes [13] reportedthat lead oleate and sulfur readily and easily reacted to formbulk-like particles. In our experiment, initially we tried to

synthesize PbS nanocrystals from PbSt2 and sulfur with anoleic acid (OLA) as stabilizer. We found out that only largePbS particles of about 500 nm could be obtained in thepresence of OLA (figure 7(a)). Then, we added OA into thereaction system from PbSt2, sulfur and OLA, and smaller PbSnanoparticles can be obtained (figure 7(c)).

For the purpose of investigating the nucleation andgrowth of PbS nanoparticles, we designed three comparativeexperiments as follows. (A) 1 mmol PbSt2 and 2 mmol OLAare placed into a flask with 10 ml ODE solvent and then heatedto 150 ◦C under Ar for one hour to form clear solutions. Thelead precursor solutions are then cooled to 90 ◦C. After theODE solution of sulfur (1 mmol S/1 ml ODE) is injected at90 ◦C, the reaction mixture is heated to 220 ◦C and aged at220 ◦C for 1 h. (B) Everything is the same as experiment Aexcept 1 mmol OA injected with sulfur precursor at 90 ◦C.(C) Everything is the same as experiment A except 9 mmolOA injected with sulfur precursor 90 ◦C.

For reaction A, the reaction solution slowly turns slightlydarkish at 180 ◦C during the temperature-rise period. Asshown in figure 7(a) and table 1, bulk-like PbS nanoparticlesof size about 500 nm can be observed. This suggests thatthe nucleation and growth reaction between PbSt2 and sulfurin the presence of OLA can only happen when the reactiontemperature is reaching 180 ◦C. In the presence of OLA, thelead and sulfur readily react into bulk-like nanoparticles. Inexperiment B, in the presence of OLA and OA, the solutionsturn darkish slowly at 90 ◦C and turn a deep dark color at180 ◦C during the temperature-rise period. Both smaller (about15 nm) and larger (about 500 nm) PbS nanoparticles can beobtained (see the TEM image in figure 7(b) and table 1).This indicates that two separated nucleation processes initiatedby OA (at 90 ◦C) and OLA (180 ◦C) occurs, respectively,when 1 mmol OA is added into system A (contains 2 mmolOLA) in the reaction system. This means 1 mmol OA cannotgenerate enough nuclei at 90 ◦C, and unreacted precursorsin the reaction mixture form new nuclei by OLA when thesystem is heated to temperatures up to 180 ◦C, resulting inlarge PbS particles. In experiment C, when 9 mmol OAand sulfur precursor are injected into the reaction flask, thereaction mixture becomes black immediately at 90 ◦C due tothe high reaction rate in the presence of a large quantity of OA.Figure 7(c) and table 1 show that only smaller PbS nanocrystalswith diameters of 10–20 nm are observed in experiment C. Thisclearly implies that nucleation is almost completed at 90 ◦Cand no new nuclei form in the subsequent heating and agingprocess. It can be concluded that OA plays a major role in the

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Figure 6. (a) XPS survey spectrum of PbS nanoparticles of 8.9 nm PbS nanocrystals. (b) High-resolution Pb 4f core-level spectrum of 8.9 nmPbS nanoparticles. (c) High-resolution S 2p core-level spectrum of 8.9 nm PbS nanoparticles.

PbS nanocrystal nucleation process at temperatures lower than180 ◦C.

Aminolysis will take place when the mixture of metalcarboxylate and oleylamine is heated to high temperatures.Some groups have reported the synthesis of metal oxidenanocrystals with this aminolysis route [25]. Since in oursynthesis system PbSt2 and OA are used as lead precursorand stabilizing agents, aminolysis might happen at hightemperatures. When slowly heating the PbSt2 and OA mixtureunder Ar up to 260 ◦C, the reaction mixture begins to turnturbid at 230 ◦C, indicating the aminolysis reaction starts. Thisindicates that no aminolysis happens when the synthesis of PbSnanocrystals is performed at temperatures below 230 ◦C.

Fourier-transform infrared spectroscopy (FTIR) is used forunderstanding the role of OA in the synthesis system. TheFTIR spectra in the region 2000–1000 cm−1 of pure OA,the mixture of OA and PbSt2 and pure PbSt2 are given infigures 8(a), (b) and (c), respectively. The bands at 1580–1650 cm−1 are attributed to the bending vibrating modesof –NH2 in OA (figure 8(a)) [26]. In the FTIR spectraof PbSt2, the bands (at 1512 cm−1 with a weak shoulderat 1537 cm−1 and 1413 cm−1, respectively) belong to the

COO− antisymmetric and symmetric stretching vibrationscomplexed with surface lead centers (figure 8(c)) [27]. Fromfigure 8(b), the broad band at 1520 cm−1 corresponds to theCOO− antisymmetric stretching vibrations, which are shiftedto higher wavenumbers relative to that of PbSt2. A new bandat 1639 cm−1 can be assigned to the coordination of amineto lead atom of PbSt2 [28]. The absence of any bands inthe acylamine stretching region (1650–1690 cm−1) confirmsthat no aminolysis reaction occurs at 180 ◦C [29], which isin good agreement with our observation in the experiment ofnucleation. The coordination interaction of PbSt2 and OAcan increase the reaction activity of the lead precursor withoutdecomposing PbSt2.

During the phosphine-free hot-matrix synthesis ofnanocrystals, Zou et al [30] proposed a dehydrogenationmechanism for formation of CdSe nanocrystals from cadmiumoleate and Se precursors in the presence of OLA andparaffin liquid. Yoshimura et al [31] proved that variousorganic solvents react with sulfur by liberating hydrogensulfide. They also found that the organic amines couldreduce the synthesis reaction temperature of nanocrystals bygenerating H2S more effectively in situ than the other tested

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a

c

b

Figure 7. (a) TEM image of PbS nanocrystals synthesized by nucleation experiment A. The PbS nanocrystals shown here have an averagediameter of about 500 nm. (b) TEM image of PbS nanocrystals synthesized by nucleation experiment B. The PbS nanocrystals shown here arecomposed of bulk-like particles (about 500 nm) and smaller PbS particles (about 15 nm). (c) TEM image of PbS nanocrystals synthesized bynucleation experiment C. The PbS nanocrystals shown here have diameters from 10 to 20 nm.

Figure 8. The FTIR spectra of (a) OA, (b) the mixture of OA andlead stearate, (c) lead stearate.

organics. In our synthesis, we also find that the presenceof OA could significantly reduce the nucleation temperature,which is similar to the results from Yoshimura [31].Between sulfur and organic amines, the replacement ofthe active methylene hydrogens with sulfur could produceH2S [32]. However, William et al [33] suggested H2S wasproduced by the homolytic scission of long sulfur chainsof polythiobisamines and amine polysulfides. We speculate

that the mechanism of formation of PbS could proceed viadehydrogenation of alkylamine and formation of H2S andsubsequent alkylaminesulfides, similar as reported in the workby Zou et al [30]. Further experiments are necessary to confirmthe step of H2S formation in our reaction.

4. Conclusions

It has been shown that high quality PbS nanocrystals withan adjustable size from 5 to 13 nm can be easily producedwith PbSt2 and sulfur stabilized by oleylamine in a hot-injection approach. The as-synthesized PbS nanocrystals havegood crystallinity and a good surface chemical environmentwith a narrow size distribution. The optimization ofreaction conditions such as synthesis temperature, precursorconcentration, the ratio of Pb/S and OA quantity areinvestigated in detail. Smaller PbS nanocrystals can beobtained at a lower reaction temperature, larger OA quantity,larger ratio of Pb/S and lower precursor concentration. OAenhances the reactivity of both lead precursor and sulfur in thesynthesis system. The possible mechanism has been discussed.

Acknowledgments

The work is financially supported by the NSFC project(no. 50433030) and the MOST project (no. 2002CB613404).The authors are also deeply grateful to Geoffrey A Ozin andLudovico Cademartiri for valuable discussions.

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