Small-Sized PbSe/PbS Core/Shell Colloidal Quantum DotsDiana Yanover,† Richard K. Capek,† Anna Rubin-Brusilovski,† Roman Vaxenburg,† Nathan Grumbach,†

Georgy I. Maikov,† Olga Solomeshch,‡ Aldona Sashchiuk,† and Efrat Lifshitz*,†

†Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute, Solid State Institute and ‡Electrical Engineering Departmentand Microelectronic Center, Technion-Israel Institute of Technology, Haifa 32000, Israel

*S Supporting Information

ABSTRACT: The work focuses on the synthesis of small-sized PbSe/PbS core/shell colloidal quantum dots with thecore diameter of 2−2.5 nm and the shell thickness of 0.5−1.0nm. The PbSe/PbS core/shell CQDs are chemically stableunder time-limited air exposure and have emission quantumefficiency of 60% at room temperature. The PbSe/PbS core/shell CQDs have a tunable absorption edge around 1 μm, largeexciton emission Stokes shift (∼150 meV), and small exchangeinteraction (∼1.5 meV). Theoretical calculations associate thementioned parameters to the small-size regime as well as to alift of band-edge degeneracy due to slight shape anisotropy.The specific parameters are of special interest in photovoltaicapplications.

KEYWORDS: small-sized nanocrystals synthesis, PbSe/PbS core/shell quantum dots, surface oxidation,band-edge temperature coefficient

■ INTRODUCTION

Lead chalcogenide (IV−VI) colloidal quantum dots (CQDs)are of great scientific interest because of the possibility of theirimplementation in many opto-electronic applications.1 SinceCQDs are characterized by a size-tunable narrow band gap(0.3−1.7 eV) with the broad band absorption profile rangingfrom near IR to UV,2−5 they are suitable for being used as lightharvesters in photovoltaic cells (PVCs). Furthermore, theelectron and hole effective masses of these CQDs are very small(me,h ≤ 0.1m0)

6 and thus both carriers have very similartransport properties and high degeneracy of electronic states.2

This results in a large carrier population, which is advantageousfor PVCs7 and gain8 devices.Besides, such CQDs have relatively long excited-state lifetime

(∼μs),9,10 which permits efficient charge extraction in PVCs11

and population saturation in optical switches.12 IV−VI CQDshave been recently discussed in relation to the concept ofmultiple exciton generation (MEG). The effect of MEG isproduced by two or more electron−hole pairs of a singleabsorbed photon with energy >2.7Eg. Although the issue is stillcontroversial,13,14 MEG is presumed to occur in PbSe-basedCQDs,13,15−22 which may provide the opportunity of increasingthe PVC power conversion efficiency beyond the Shockley−Queisser thermodynamic limit.23,24 There is some recentpractical evidence for the effect of MEG in PbSe-based opto-electronic prototype devices.25−29 Several studies have reportedthe integration of IV−VI CQDs into PVCs prototype devices indifferent configurations, including Schottky30,31 or CQDs-sensitized32,33 solar cells. The PbSe-based cells exhibited high

short-circuit current (JSC), while PbS-based devices exhibitedhigh open-circuit voltage (VOC).

34 Moreover, ultrasmall PbSeCQDs with the band gap energy of 1.3−2.3 eV show the powerconversion efficiency of 4−5%32 and are favorable for beingused in PVCs.32,35 Therefore, the current research should focuson the high chemical yield synthesis of small-sized IV−VICQDs that would have high chemical and photochemicalstability and a small number of carrier trapping sites despitetheir large surface-to-volume ratio.This study describes, for the first time, the synthesis and

characterization of small-sized PbSe/PbS core/shell CQDs withthe band-edge energy in the range of 1.1−1.4 eV. Previousstudies have shown the benefit of PbS-shell coating on largerPbSe cores with band-edge energies <1 eV and the advantage ofmodified PbSe/PbSexS1−x or PbSexS1−x/PbS heterostructures,involving an alloyed composition either in the core or in theshell.4,10,36 Increased sulfur content within the exterior part ofthe CQDs results in their considerable chemical and photo-chemical stability. Epitaxially grown PbS or PbSexS1−x shellshave an extremely small crystallographic mismatch (≤1.3%)with respect to crystalline PbSe cores, which eliminates core/shell interface defects.4,37 The implementation of alloyedCQDs, such as PbSexS1−x,

36,38 has already proved that thevalues of both Voc and JSC parameters are relatively large withrespect to those of cells based on pure PbSe and PbS CQDs.

Received: August 30, 2012Revised: October 30, 2012Published: November 6, 2012

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© 2012 American Chemical Society 4417 dx.doi.org/10.1021/cm302793k | Chem. Mater. 2012, 24, 4417−4423

Most important, the IV−VI core/shell heterostructuresexhibit a quasi-type-II electronic band alignment39 with partialcharge separation and, at the same time, sufficient delocaliza-tion of both carriers to the exterior surface, which allows chargeextraction. In addition, the variation of the composition and/orthe core-radius to shell-thickness ratio changes the band-edgeenergy, tunes the chemical potential with respect to the vacuumor the Fermi level of the electrode, and alters the density ofstates, thereby giving the possibility of pre-engineering theCQD properties to meet the demands of various applications.The recently developed PbSe/CdSe40 core/shell heterostruc-ture with the option of being coated with an extra layer of inertZnSe(S)41 as an exterior shell showed similar benefits; however,these CQDs are not included in the current study.The current study reports the adjusted procedure of

synthesizing small-sized PbSe/PbS core/shell CQDs, withultrasmall PbSe CQDs as core CQDs, and their crystallographicand composition properties. The procedure allows synthesizingthe CQD with appropriate emission quantum efficiency(∼60%). Furthermore, the absorption and emission propertiesof the obtained samples were investigated in a widetemperature range (4−298 K); information was obtainedabout their photochemical stability, reduction of the number ofcarrier trapping sites, and the absence of surface oxidation.

■ EXPERIMENTAL SECTIONList of Chemicals. Lead oxide (PbO; 99.9%), lead acetate

trihydrate (Pb(Ac)2*3H2O), selenium (99.99%), sulfur (99.99%),hexadecane (HDC; 99%), 1-octadecene (ODE; Tech.), oleic acid(OA; Tech. and ≥99%), acetonitrile (≥99.9%), diphenyl ether (DPE;Tech.), bis(trimethylsilyl) sulfide (TMS2S; Tech.), and 2,2,4,4,6,8,8-heptamethylnonane 98% (glassy solution) were purchased fromAldrich. Trioctylphosphine (TOP; 97%), and diphenylphosphine(DPP; 99%) were purchased from Strem. Tetrachloroethylene(TCE, spectroscopic grade) was purchased from Merck. Ethanol(absolute) and toluene (analytical) were purchased from Frutarom. Ifnot indicated otherwise, the highest-grade chemicals were usedwithout purification.Pure TOPSe Solution. TOPSe was prepared by mixing selenium

and TOP (2.5 g selenium per 1 mL of TOP) for at least 8 h at 100 °C.Then the unreacted selenium was separated by centrifugation, and theslightly yellow liquid was purified by vacuum distillation through a 40cm vigreux column. The colorless pure TOPSe was checked by 1HNMR and 31P NMR spectroscopy.Ultrasmall PbSe CQDs Synthesis. The reaction mixture,

consisting of 1.6 mmol PbO, 4.8 mmol OA, 6.4 mmol stearyl alcohol,and HDC (the total mass of the reaction mixture was brought to 8 gby adjusting the amount of HDC), was heated to 100 °C undervacuum in a 25 mL three-necked flask for 1 h. The particular injectiontemperature was adjusted under nitrogen atmosphere. Then theselenium precursor solution, containing 2.3 g of pure TOPSe, 0.95 g ofDPP, and HDC (the total volume of the injection solution being 4mL), was injected into the reaction mixture. The temperature wasreduced to the growth temperature of 70 °C. To quench the reactionand to perform the first precipitation, the flask was opened at standardatmosphere, and the reaction mixture was poured into a mixture ofethanol, acetonitrile, and toluene (volume ratio of 3:3:4, respectively).The CQDs were separated by centrifugation. It should be noted that,during the first precipitation, the CQDs were supposed to be protectedagainst oxidation by the unreacted phosphine. Therefore, for thesecond precipitation, the test tube was transferred to a nitrogen-filledglovebox and special precautions were taken to prevent the colloid orthe precipitate from coming into contact with oxygen in the next steps.The test tube was opened, and the supernatant was removed. Then theprecipitate was dissolved in hexane, and a 2-fold volume (relative tothat of the hexane) of acetonitrile was added. A two-phase systemformed, the CQDs being present in the hexane phase. Then a 4-fold

volume (relative to that of the hexane) of ethanol was added dropwise,which caused the disappearance of the second phase and theprecipitation of the CQDs. The mixture was kept aside for 16 h,and the precipitate was separated again by centrifugation. Then thesupernatant was removed, the precipitate was dried up under nitrogenatmosphere, and, finally, dissolved in hexane under nitrogenatmosphere.

Small-Sized PbSe/PbS Core/Shell Synthesis. The amount ofthe coating precursor was precalculated with regard for the amount ofPbSe CQDs and the desired number of PbS shell layers. In a typicalsynthesis, to obtain one monolayer of PbS shell, a solution of 1 × 10−3

mmol PbSe CQDs (diameter of 2.2 ± 0.5 nm) in hexane 0.13 mmolPb(Ac)2·3H2O (the amount necessary for a monolayer shell) and 1mmol OA were mixed with 5 mL of DPE in a 25 mL three-neckedflask and heated at 100 °C under vacuum for 1 h. The reaction mixturewas set under nitrogen, and the temperature was lowered to 70 °C.Then the PbSe CQDs (dissolved in hexane) were added to themixture, which was maintained at this temperature for another 15 minto evaporate the solvent. After that, 1.08 mL of the TMS2S-DPEsolution (100 μL of TMS2S were diluted in 4 mL of DPE; 0.13 mmolTMS2S) were injected dropwise into the reaction mixture to achievethe desired shell thickness. The reaction was completed after 10 min.Separation of the final core/shell CQDs from the reaction solution wasperformed by adding acetone and centrifuging the obtainedsuspension. The precipitate was redissolved in hexane, reprecipitatedby the second addition of acetone, and separated again bycentrifugation.

Temperature Stability of Ultrasmall PbSe CQDs. Thetemperature stability of small-sized PbSe CQDs (0.1 mM; 2 ± 0.5nm in diameter) was examined in the reaction mixture, at thetemperatures of above 60 °C by following the changes of theabsorption spectrum. Two nanometer PbSe CQDs dissolved in hexanewere injected into a 25 mL three-necked flask containing Pb-(Ac)2·3H2O, OA, and DPE (amounts necessary for the formation ofone shell, see the PbSe/PbS synthesis) to obtain the CQDconcentration of 0.1 mM after the evaporation of hexane. Thetemperature of the prepared solution was increased step-by-step, in10° increments, from 60 to 100 °C. After annealing for 30 min at eachtemperature, aliquots were taken for optical absorption measurements.

Characterization. High Resolution Transmission Electron Mi-croscopy (HR-TEM) images were taken with a Technai F20 G2

system operated at 300 kV. Samples for TEM measurements wereprepared on a carbon-coated copper grid using the spray technique tominimize contamination by organic solvents. X-ray Powder Diffraction(XRD) measurements were performed with a Philips XPertdiffractometer using the Cu Kα line. The samples were prepared bydepositing the CQDs onto a glass substrate. X-ray PhotoelectronSpectroscopy (XPS) was performed in a Thermo VG Scientific SigmaProbe fitted with a monochromatic X-ray Al Kα (1486.6 eV) source.The absorption spectra of CQDs in TCE solution were recorded usinga JASCO V-570 UV−vis-NIR spectrometer. Continuous-wave photo-luminescence (cw-PL) and time-resolved photoluminescence (PL)decay measurements were performed at various temperatures (4−298K) by inserting the CQDs (embedded in a glassy solution) in a Janisvariable-temperature cryogenic system. The CQDs were excited eitherby a continuous-wave Ar or by a pulsed YAG laser, and the emissionwas monitored by a Ge detector or a Hamamatsu photomultipliertube, both operating in the NIR spectral region.

PL Quantum Efficiency Measurements. The photolumines-cence quantum efficiency (PL QE) measurements at room temper-ature were performed on an integrating sphere system, based on theFS920 fluorimeter of Edinburgh Instruments Ltd. (U.K.), equippedwith a liquid nitrogen-cooled Ge photodetector and lock-inamplification. A solution of CQDs was placed inside an integratingsphere (Labsphere, Inc. IS-040-SL with UV−vis−NIR reflectancecoating), which was fiber-coupled to the fluorimeter and was excitedby monochromatic light of a Xenon lamp. The entire system responsewas normalized by a calibrated detector (Newport 818 IR) and amultifunction optical meter (Newport 1835C) in the 800−1700 nm

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region. The PL QE measurement technique was described by Friendet al.42

■ RESULTS AND DISCUSSIONA representative HR-TEM image of PbSe CQDs is shown inFigure 1A. It shows CQDs with the average diameter of 2.5 ±0.4 nm having a nearly spherical shape. The corresponding FastFourier Transform (FFT) pattern, displayed in the bottominset, confirms the fully crystalline structure of the CQDs. TheXRD results (Supporting Information, Figure S1) confirm thatthe PbSe CQDs have a rock-salt crystal structure. From the sizedistribution histogram shown in the top inset, it can be seenthat the size distribution of these CQDs is ∼15%.The ultrasmall PbSe CQDs were used as core CQDs for the

preparation of PbSe/PbS CQDs. Therefore, it was essential tomake sure that these cores retain their stability during thecoating process even at low temperature. Figure 1B shows theabsorption spectra of the PbSe CQDs dissolved in TCE. Thespectra were recorded after annealing for 30 min at eachtemperature. The dependence of the development of the firstexciton absorption band on the annealing temperature is shownin the inset. The first excitonic transition stays at a constantvalue up to 70 °C. Above 70 °C, the red shift of the firstexcitonic transition can be observed. The shift increases withtemperature and is indicative of the Ostwald ripening process.It should be noted that the temperature of 70 °C is substantiallylower than that usually used for coating larger PbSeCQDs.43−45 This fact can be attributed to the increase of theultrasmall CQD solubility at elevated temperatures, accordingto the Gibbs−Thomson relation.46 On the basis of thisobservation, the PbS shell coating onto ultrasmall PbSe CQDswas performed at 70 °C. The use of TMS2S as a sulfurprecursor made it possible to grow PbS shells with monolayerprecision. Diluted in DPE, TMS2S was added dropwise toprevent instantaneous supersaturation and co-nucleation of PbSCQDs.44

Purification of the PbSe CQDs after their preparation ruledout the possibility of selenium from the reaction mixture beingincorporated in the shell during the core/shell synthesis.Furthermore, the choice of low reaction temperature andTMS2S dilution should prevent interdiffusion of Se and S acrossthe core/shell interface. Hence, it can be assumed that theemployed conditions ensured the formation of a pure PbS shell.The formation of the PbSe/PbS core/shell CQDs was

confirmed by the correlation between the HR-TEM and theXPS measurements, assuming that the increase of the CQDvolume was proportional to the increase of the sulfur content.Hence, the relation[(rCS

3 −rC3 )/rC3 ] ≅ ((S)/(Se)) is used, whererC and rCS are the radii of PbSe core and PbSe/PbS core/shellCQDs, respectively.A representative HR-TEM image of PbSe/PbS core/shell

CQDs with the core overall diameter of 2.5 ± 0.4 nm and theshell thickness of ∼0.5 nm is shown in Figure 2A. The FFT

pattern and the size distribution histogram of the same CQDsare shown in the insets (bottom and top, respectively). Figure2A shows that the PbSe/PbS CQDs have the same structureand similar small size dispersion as the initial PbSe CQDs (seeFigure 1A). The representative XPS spectra of the PbSe/PbScore/shell and the PbSe core CQDs shown in Figure 2B provethe presence of both Se and S in the PbSe/PbS CQDs. Thesummary of the XPS elemental analysis of different core andcore/shell samples is given in the Supporting Information,Table S1. The dependence of the cation/anion ratio on thetotal diameter of the CQDs (Figure 2C) demonstrates theexcess of Pb in the ultrasmall cores (blue symbols). Thedeviation from the stoichiometry in the ultrasmall PbSe CQDsis relatively large, while in the PbSe/PbS core/shell CQDs (redsymbols) it is much smaller.Similar nonstoichiometric ratios reported recently by Dai et

al.,47 Smith et al.,48Moreels et al.,49 and Hughes et al.50 arecompared with the results of the present study in theSupporting Information, Figure S2. The nonstoichiometry inthe PbSe CQDs was suggested to be associated with the Pbcation-rich exterior surface of the CQDs being actually acompletely filled cationic shell,47 where the oxidation process isimpeded. The PbS shell coating process results in immediatefilling of anion vacancies and promotes further epitaxial growthof the shell.

Figure 1. (A) HR-TEM image of 2.5 ± 0.4 nm PbSe CQDs. Bottominset: the corresponding FFT pattern of the HR-TEM image. Topinset: Histogram of the CQD size distribution. (B) Absorption spectraof 2 ± 0.5 nm PbSe CQDs recorded after thermal annealing at 60−120 °C. Inset: dependence of the lowest absorption band on theannealing temperature.

Figure 2. (A) HR-TEM of 3.5 ± 0.5 nm PbSe/PbS CQDs. Top inset:histogram of the CQD size distribution. Bottom inset: thecorresponding FFT pattern of the HR-TEM image. (B) XPS spectraof PbSe/PbS (top) and of PbSe (bottom) CQDs. The assignment ofthe bands is shown in the panel. (C) Dependence of the cation/anionstoichiometric ratio on the CQD size in PbSe (blue) and in PbSe/PbS(red) CQDs.

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Figure 3 shows room temperature absorption (red lines) andcw-PL (blue lines) spectra of PbSe CQDs with the average

diameter of 2−2.5 nm and of the corresponding PbSe/PbScore/shell CQDs with the average diameter of 3.3−3.5 nm. Thecw-PL was recorded upon off-resonance excitation with an Ar+

ion laser. The size dependence of the cw-PL bands is indicativeof their association with the band-edge (1Se-1Sh) transitions.The cw-PL bands display a Stokes shift relative to the firstabsorption bands; the shift gradually increasing with thedecrease of the PbSe core diameter. Surprisingly, the Stokesshift of the emission in PbSe/PbS core/shell CQDs (top curvesin Figure 3) is smaller than that observed in the initial PbSecores (bottom curves in Figure 3). The Stokes shift was foundto be ∼140 meV in any core/shell CQDs with the averagediameter of ∼3.5 nm, which suggests that the shift depends onthe overall CQD diameter, while the influence of the core-to-shell ratio is negligible. Our previous theoretical calcula-tions37,39 showed that in the strong confinement regime, theband gap states lay above the core/shell interface potentialbarrier and are insensitive to the existence of the barrier(Supporting Information, Figure S3). So the carriers aredelocalized over the entire core/shell structure and, con-sequently, the overall diameter appears to be the main factorthat determines the decrease of the Stokes shift. The shift of140−460 meV is significantly larger than the one expected51,52

for spherical PbSe CQDs, which may be accounted for by (a)an exceptionally large exchange interaction in small-sized IV−VI CQDs; (b) the existence of the Franck−Condon shift,53,54

which can be manipulated by off-resonance excitation or/andsize distribution; (c) the lift of 4-fold degenerate valleys39,55,56

at the L-point of the Brillouin zone. Previous theoreticalcalculations estimated the existence of exchange splitting37,52,57

and the Franck−Condon shift53,54 in the range of 20−30 meV,which is smaller than the Stokes shift observed in the presentstudy (not shown here). Besides, the observed Stokes shift wasnot affected by a change in the excitation energy. Lifting the 4-fold degeneracy allows excitation to occur in one valley andemission to occur from another, which is observed innanostructures with a minor deviation from the sphericalshape, which may be undetectable in the TEM image.56 Mostlikely, a slight shape anisotropy of the PbSe cores is maintainedafter homogeneous covering of a PbS shell. A theoreticalapproach to the evaluation of the intervalley splitting both inPbSe and in PbSe/PbS CQDs is discussed below. While the off-

resonance excitation of an ensemble of CQDs is affected by thesize distribution, the theoretical approach shows close agree-ment with the Stokes shift energy found in the current study.The energy split created by the intervalley interaction at the L-point was estimated in the following way. The four-band k·penvelope-function model was adapted to the case of slightlyelongated nanoparticles. In addition to the shape anisotropy,the anisotropy of the band structure was also taken intoconsideration by differentiating between the longitudinal andtransverse components of the effective masses and themomentum matrix elements. The calculations were performedassuming that the CQDs were elongated in the ⟨110⟩crystallographic direction. The aspect ratio, a, was defined asthe ratio between the longer axis, L, and the shorter axis, D.The results of the calculations are summarized in Table 1.

On the basis of these results, it can be concluded that the 4-fold degeneracy of the direct L-point band gap is split into 2-fold degenerate states (ΔEL‑L being the energy of splitting)when each fold is doubly degenerate. The calculation resultsalso reveal the increase of the splitting energy with the increaseof the shape anisotropy. In particular, for CQDs having L = 2.1nm and D = 2 nm (a = 1.05), ΔEL‑L exceeds 100 meV. This isin agreement with the experimental results presented here.Figure 4A shows representative cw-PL spectra of PbSe

CQDs with the average diameter of ∼2 nm and of thecorresponding PbSe/PbS CQDs with the overall diameter of3.2 ± 0.4 nm, recorded at different temperatures under air-freeconditions. The spectra mainly comprise a single band althoughanother emission band occasionally appears in the cw-PLspectra, mostly at the energy above the main band, beingespecially pronounced at the lowest temperatures (SupportingInformation, Figure S4). The cw-PL spectra of PbS/PbSeCQDs presented in Figure 4A demonstrate a shift to higherenergies with increasing temperature, while the cw-PL spectraof PbSe CQDs hardly show any shift in a wide temperaturerange (see also Supporting Information, Figure S5). Thespectral shift of the band-edge emission with increasingtemperature is mainly due to lattice dilation or exciton−phonon interactions. The band gap energy (Eg) in bulk IV−VIsemiconductor materials is known to increase with increasingtemperature (i.e., the temperature coefficient of the band gapenergy is positive (dEg/dT > 0)), similar to other narrow bandgap materials.58,59 The band gap energy in IV−VI CQDs wasshown to increase with temperature for CQDs more than 2 nmin diameter, while for diameters <2 nm the band gap energydecreased with temperature.60,61 Thus, it can be suggested thatthe lack of a spectral shift of the PL band in the ∼2 nm-PbSeCQDs manifests the turning point from positive to negativetemperature coefficients of the band-edge energy. The

Figure 3. Absorption and PL spectra of the different PbSe and thecorresponding PbSe/PbS CQDs. The energy of the Stokes shift isshown next to the spectra.

Table 1. Dependence of the Inter-Valley Splitting Energy,ΔEL‑L (⟨110⟩ Direction), on the Aspect Ratio, a

L (nm) D (nm) a ΔEL‑L (meV)

2 2 1 722.1 2 1.05 1102.25 2 1.125 1562.5 2 1.25 2204 4 1 184.2 4 1.05 284.5 4 1.125 415 4 1.25 56

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corresponding core/shell CQDs with the larger overalldiameter of ∼3.5 nm37 do not show any distinct characteristicbehavior at the turning point. The temperature dependence ofthe PL integrated intensity (IPL) for PbSe and PbSe/PbS CQDsis shown in Figure 4B. The width of the open symbols in thefigure designates the experimental error. The IPL(T) profile ofPbSe/PbS CQDs exhibits a small increase of IPL at ∼20 K and asignificant increase at ∼150 K, reproducible in many samples.On exposure of the samples to air over a short period of time(∼30 min), this behavior is retained. The temperaturedependence of PbSe CQDs exhibits the enhancement of IPLat elevated temperature under air-free conditions (bluesymbols) and a consistent quenching of IPL upon exposure toair after 2−4 min (green symbols). In recent studies it wasfound that the PL intensity of oleic acid-capped PbSeCQDs50,62,63 decreased upon their exposure to air. Anotherstudy shows oxidation suppression of PbSe CQDs bysubstituting the oleic acid ligand for alkylselenide.50 Oxidationinduces carrier trapping, which is thermally activated (ordeactivated) above ∼150 K, that is, in the temperature rangewhich induces longitudinal-optical phonons (ELO = 17 meV64).Interestingly, carrier detrapping occurs in PbSe and PbSe/PbSCQDs above 150 K, which is supported by the value of PLquantum efficiency being above 59% for cores and 60% forcore/shells, as directly measured using the integrating spheretechnique.42 Despite the fact that the PL QE of core and core/shell CQDs in this study are nearly the same, the results shownin Figure 4B suggest that monolayer coverage of a PbSe core bya PbS shell gives sufficient protection from a fast-oxygenpenetration, thus, providing the PbSe/PbS CQDs withchemical and photochemical stability. It should be mentionedthat the values of PL QE at room temperature decreased after

exposure of all samples to air for hours. However, the short-term stability found here may be of importance in the time-limited processes used in the fabrication of CQD-based devices.The PL decay plots for PbSe and PbSe/PbS CQDs recorded

at various temperatures are presented in Figure 4C. The plotsshow a single-exponential and a biexponential behavior atelevated and low temperatures, respectively. The values ofweight-average lifetime, τ0 and the quantum efficiency, η, ateach temperature depend on the radiative (τrad) and non-radiative (τnrad) processes: 1/τ0(T) = 1/τrad(T) + 1/τnrad(T),η(T) = IPL(T)/I0 = τ0(T)/τrad(T), where I0 is the excitingphoton flux. By comparing the experimental values of IPL (300K) and the values of η(300 K) measured directly using theintegrating sphere technique,42 the calibration plot of IPL(T)was built (see Figure 4C). This allowed evaluating τrad at eachtemperature, as shown in Figure 4D. Normally, the core andthe core/shell CQDs have similar radiative lifetime if treated inan air-free environment. The dependence of the lifetime ontemperature in both PbSe and PbSe/PbS CQDs is accountedfor by the electron−hole exchange interactions, which causesplitting of exciton electronic states into bright and dark states.Emission from a dark state dominates the processes atcryogenic temperatures,56 which leads to lifetime values >10μs. In contrast to this, at elevated temperatures, both the brightand the dark states are populated, which results in the typicallifetime value of 6 μs at room temperature. For example, An etal.57 calculated the room-temperature value of τrad to be 5 μs for3 nm PbSe CQDs, and Moreels et al.65 measured the lifetime tobe 2 μs for 3.5 nm PbSe CQDs dispersed in TCE and 2.5 μs for3 nm PbSe CQDs dispersed in chloroform. This exceptionallylong lifetime can be associated with the relatively high dielectricconstant (ε∞ = 24) of the PbSe semiconductor or with possiblecontribution of intervalley mixing at the L-point of the Brillouinzone.57 The values of the bright-dark energy gap in spherical52

and in elongated56 CQD structures were found to be 10−30meV and <1 meV, respectively. The exchange splitting of 1−2meV obtained in the present study suggests that thermaloccupation of the bright state occurs at ∼20 K, which is in linewith the experimental evidence for an exceptionally longlifetime at temperatures <20 K. Moreover, the absolute value ofthe exchange splitting suggests the existence of a slight shapeanisotropy, which resembles the exchange splitting in rod-likestructures with an extremely small aspect ratio.In summary, the present work describes the procedure

adjusted for the synthesis of small-sized PbSe/PbS core/shellCQDs with the typical PL QE of 60% at room temperature.Carrying out the coating reaction at relatively low temperatures,as compared to the procedures of growing IV−VI CQDstypically used in the past, together with appropriately selectedreagents allowed to achieve the formation of PbS shells withcontrolled thickness. In the core/shell CQDs, the cw-PLintensity significantly increases with increasing temperature (upto room temperature), even on exposure to air over a shortperiod of time (∼30 min). These results suggest that thermallyactivated trapping, which is induced by the oxidation process atthe sites on the surface of CQDs, is suppressed. The small-sizedPbSe/PbS CQDs offer significant benefits for their implemen-tation in opto-electronic devices. They may be useful, inparticular, in CQD-based PVCs with suitable energy offset withrespect to the collecting electrodes, with demand for short-termair exposure and relatively long exciton lifetime for efficientcharge extraction.

Figure 4. (A) Representative PL spectra of 2 ± 0.5 nm PbSe (top) andthe corresponding 3.2 ± 0.4 nm PbSe/PbS (bottom) CQDs, measuredat different temperatures (indicated in the panel). (B) Plots of theintegrated PL intensity of air-free ∼2 nm PbSe CQDs (blue), thecorresponding air-free ∼3.2 nm PbSe/PbS CQDs (red), and ∼2 nmair-exposed PbSe CQDs (green) as a function of temperature. (C)Plot of the PL decay curves versus the measured time of ∼2 nm PbSeand ∼3.2 nm PbSe/PbS CQDs. (D) Plot of the evaluated radiativelifetime (see text) in ∼2 nm PbSe and the corresponding ∼3.2 nmPbSe/PbS CQDs as a function of temperature.

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■ ASSOCIATED CONTENT

*S Supporting InformationFigure of XRD of PbSe and PbSe/PbS CQDs. Table withadditional XPS data. Schematic plot of a PbSe/PbS electronicband structure. Figure of peak positions of PL bands of PbSeand PbSe/PbS CQDs. Figure of PL intensity of an air exposedPbSe CQDs. This material is available free of charge via theInternet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: ssefrat@tx.technion.ac.il.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors acknowledge support from the Israel ScienceFoundation (Projects No. 1009/07), and the European FP7self-assembled nanostructure system (SANS) project.

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