Shell Colloidal Quantum

Article pubs.acs.org/JPCC

Significance of Small-Sized PbSe/PbS Core/Shell Colloidal Quantum Dots for Optoelectronic Applications Diana Yanover, Roman Vaxenburg, Jenya Tilchin, Anna Rubin-Brusilovski, Gary Zaiats, Richard K. Č apek, Aldona Sashchiuk, and Efrat Lifshitz* Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute and Solid State Institute, Technion - Israel Institute of Technology, Haifa 32000, Israel ABSTRACT: Small-sized colloidal quantum dots (QDs) consisting of IV−VI semiconductors with the PbSe/PbS core/shell structure were synthesized by a specially developed wet-chemistry method. Their electronic properties were determined by the comparison of theoretical calculations with continuous-wave and transient photoluminescence measurements conducted at various temperatures. The results revealed the formation of quantum dots characterized by a tunable band gap around 1 μm, lifetime exceeding 4.0 μs at room temperature, photoluminescence quantum yield >60%, and resistance to oxidation for a relatively long period of time. The properties of the QDs vary with the core/shell architecture, which is beneficial for their optoelectronic applications.

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been proposed.57−59 An efficient passivation of PbSe QDs was achieved by the epitaxial growth of a PbS or CdSe shell on top of PbSe QDs, with the resulting formation of a core/shell heterostructure.9,28,60−63 Because the PbS and PbSe materials have similar crystallographic and dielectric parameters, they form a perfect crystalline heterostructure, exhibiting a relatively high PL quantum yield (QY),9 with a lifetime depending on the core-to-shell division.64 Regardless of the size, the photostability and chemical stability of IV−VI core/shell QDs are substantially improved as compared to those of the corresponding core QDs.65,66 The application of small-sized PbSe/PbS QDs in QD−TiO2 heterojunction PVCs showed power conversion efficiency as high as 5%. This result indicates that the QDs with the band gap energy in the range of 1.1−1.4 eV are of particular interest.17 The requirements for QDs to be used in any optoelectronic device include size uniformity, high-yield chemical synthesis, nearly defect-free structure, efficient absorption, and photochemical stability. The present work reports a synthesis procedure that provides small-sized PbSe/PbS QDs with high chemical yield. The electronic structure of the small-sized core/ shell QDs was calculated in the framework of the four-band k·p envelope function theory.22,67 The optical properties of the obtained QDs and their stability as a function of temperature and exposure to ambient conditions were examined by following their absorption and continuous-wave (cw-) and transient (tr-) PL spectra. The results are indicative of the formation of high-quality QD heterostructures, characterized by high absolute PL QY, exciton lifetime in the microsecond

INTRODUCTION For over a decade, lead chalcogenide (PbTe, PbSe, PbS) colloidal quantum dots (QDs) have been the focus of widespread scientific and technological interest.1−4 Lead chalcogenide semiconductors are characterized by a rock-salt crystal structure (space group Fm3m ̅ ), direct band gaps at four equivalent L-points in the first Brillouin zone, small electron and hole effective masses,5 a large dielectric constant,6 and a large effective exciton Bohr radius.7 In the nanosize regime, IV−VI QDs exhibit size-tunable direct band gap energy in the range of 0.3−1.7 eV, characterized by a broad-band absorption profile from the near-infrared (NIR) to the visible wavelength range.8−10 Moreover, these QDs can be processed via simple, low-cost, solution-based techniques, readily applicable to the fabrication of large-scale devices. The abovementioned properties of IV−VI QDs make them appropriate for use in NIR gain devices,9,10 photovoltaic cells (PVCs),11−17 Q-switches,18,19 thermoelectric systems,20−22 and biological markers.23−25 The synthesis procedures developed in the past decade made it possible to obtain lead chalcogenide nanostructures with controlled size and morphology,26−39 which laid the groundwork for studying their principal physical properties such as type of conductivity,4 density of states,40 electron−hole exchange interactions,41 radiative lifetimes,42 Auger relaxation, and multiple exciton generation.43−53 Important results were obtained in the cited works, but the chemical stability of IV−VI QDs under ambient conditions has been studied to a much lesser extent,54,55 although it may control most of the above-mentioned principal QD properties and be essential for their future applications. For example, the photoluminescence (PL) intensity was shown to decrease on the exposure of PbSe and PbS QDs to air.56 Various approaches aimed at reducing the oxidation effect, including controlled passivation of the surface by organic or inorganic ligands, have © XXXX American Chemical Society

Special Issue: Michael Grätzel Festschrift Received: January 15, 2014 Revised: May 17, 2014

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dx.doi.org/10.1021/jp500471s | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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sphere (Labsphere, Inc. IS-040-SL with UV−vis−NIR reflectance coating). The setup integrated all the light that entered the sphere (the excitation and the photoluminescence created in the sphere) through a highly scattering coating on the inside walls; the light was further focused onto a fibercoupled fluorimeter. The entire system response was normalized by a calibrated detector (Newport 818 IR) and a multifunctional optical meter (Newport 1835C) in the 800− 1700 nm region. The PL QY measurement technique was described by Friend et al.69 The PL QY η at any other temperature T lower than RT was calibrated based on the relative PL integrated intensity IPL at this temperature according to the following relation: η(T) = IPL(T)/I0, where I0 is the number of the absorbed photons, estimated here as the value of IPL at RT.

range, resistance against oxidation over a relatively long period of time, and the band gap energy (Eg) that can be tuned by changing the heterostructure architecture.

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EXPERIMENTAL METHODS Synthesis. The synthesis and characterization of small-sized PbSe and PbSe/PbS core/shell QDs were described previously.68 In brief, the preparation procedure included two steps. The first step was the preparation of small PbSe core QDs 2.0−2.5 nm in diameter. The main reaction mixture was composed of lead oleate (Pb(OA)2) and hexadecane (HDC). The selenium precursor solution, containing trioctylphosphine (TOP), diphenylphosphine (DPP), and hexadecane (HDC), was heated under vacuum for 1 h. Then, the selenium precursor solution was injected into the main reaction mixture and the obtained QDs were separated by centrifuging. At the second step of the procedure, the shell growth was carried out. Diluted bis(trimethylsilyl) sulfide (TMS2S) was added dropwise, at 70 °C, into the reaction mixture, containing PbSe QDs, Pb(OA)2, and diphenylether (DPE) and the obtained core/shell QDs were separated by centrifuging. Characterization. High-resolution transmission electron microscopy (HRTEM) images were taken with a FEI Titan 80−300 keV S/TEM. Samples for TEM measurements were prepared on a carbon-coated copper grid using the spray technique to minimize contamination by the organic solvents. Powder X-ray diffraction (XRD) measurements were performed with a Philips X-Pert diffractometer using the Cu Kα line. The samples were prepared by depositing the QDs onto a glass substrate. X-ray photoelectron spectroscopy (XPS) was used to determine the elemental composition of the examined QDs. XPS measurements were performed in a Thermo VG Scientific Sigma Probe fitted with a monochromatic X-ray Al Kα (1486.6 eV) source. The XPS spectra were collected from thin films of air-free and air-exposed (over 50 min) QDs, which were dropcasted onto clean gold substrates. The fine structure of the XPS peaks was resolved using XPSPEAK 4.1 software. The binding energy (BE) of the elements was corrected with respect to the C 1s (284.5 eV) line, and Shirley background was subtracted, while the XPS line shape was fitted to a Gaussian−Lorentzian sum function (GL = 15). The absorption spectra of the QDs in a tetrachloroethylene (TCE) solution were recorded at RT under air-free conditions using a JASCOV-570 UV−vis−NIR spectrometer. The cw-PL spectra and the tr-PL measurements were performed at various temperatures (4−300 K) by dispersing the QDs in 2,2,4,4,6,8,8heptamethylnonane, a glass-forming solution, and mounting the sample in a Janis variable-temperature cryogenic system. The QDs were excited either by a cw Ar+ or by a pulsed YAG laser, and the PL emission was monitored by a Ge detector or by a Hamamatsu photomultiplier tube, both operating in the NIR spectral region. The PL QY measurements at RT and under air-free conditions were performed by means of an integrating-sphere system, based on the FS920 fluorimeter, Edinburgh Instruments Ltd. (U.K.), equipped with a liquidnitrogen-cooled Ge photodetector and lock-in amplifier. The light from a Xenon arc lamp passed through a M300 monochromator (Edinburgh Analytical Instruments), and the resulting quasi-monochromatic beam with the profile centered at 886 nm was used to excite the QDs. The QD solution with the optical density in the range of 0.5−2 at 886 nm, in a 1 mm optical path-length cuvette, was placed inside an integrating

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RESULTS AND DISCUSSION QD Characterization. Figure 1A displays the HRTEM image of PbSe QDs with the average diameter of 2.5 ± 0.4 nm.

Figure 1. (A) HRTEM image of 2.5 ± 0.4 nm PbSe QDs and its FFT pattern (inset). (B) absorption spectra of the same QDs as in (A) and of the corresponding PbSe/PbS heterostructures with variable shell thickness and the total diameter as indicated in the legend. (C) HRTEM image of 3.5 ± 0.5 nm PbSe/PbS core/shell QDs and its FFT pattern (inset). (D) XRD patterns of PbSe (blue line) and PbSe/ PbS (green line) QDs indexed to the bulk rock-salt crystal structures of PbSe (blue vertical lines) and PbS (red vertical lines). All measurements were performed at RT and under air-free conditions.

The corresponding fast Fourier transform (FFT) pattern shown in the inset confirms full crystallinity of the QDs. Figure 1B displays the set of the absorption spectra of PbSe QDs and of the corresponding PbSe/PbS heterostructures with variable shell thickness; the total QD diameters are indicated in the legend. The growth of a PbS shell is characterized by a pronounced red shift of the PbSe/PbS absorption spectra with respect to that of the PbSe QDs. Figure 1C shows the HRTEM image of PbSe/PbS QDs with the core diameter of 2.5 ± 0.4 nm and the shell thickness of ∼0.5 nm; the corresponding FFT pattern is presented in the inset. Figure 1D displays the XRD patterns of PbSe (blue line) and PbSe/PbS (green line) QDs. The (111), (200), and (220) diffraction peaks appearing at the B



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originates from the surface lead ions bound to the oleic acid ligands (-O2CR). The binding energies (BEs) corresponding to the XPS peaks are summarized in Table 1. Figure 2C,D shows the XPS spectra of the same QDs as in Figure 2A,B after 50 min of air-exposure. Here, the 4f doublet was also resolved into two additional doublets, one of them corresponding to the lead−chalcogen bond (Pb−Se/S; pink) and the other to the lead−oxygen bond (Pb−O; dark green), which originates from the surface Pb ions bound to the oleic acid ligands (-O2CR) and also from the oxidized surface Pb ions, e.g., from PbO. The comparison of the green and the corresponding dark-green areas of PbSe and PbSe/PbS QDs, respectively, shows a ∼3fold increase of the PbO-related signal in the air-exposed QDs with respect to the air-free QDs. However, in the Se 3d spectrum (violet) of air-exposed PbSe QDs (see Figure 2E) there is no signal (whose expected location is indicated by the arrow in the panel) corresponding to Se4+ (SeO2, SeO32− species; ∼59 eV), which could arise from Se oxidation. This fact can be explained by high nonstoichiometry in small PbSe QDs, associated with the Pb cation-rich exterior surface of the QDs.68,70,71 The XPS spectrum of S 2s peak of air-exposed PbSe/PbS QDs (see Figure 2F) includes also the Se 2s peak (orange). Because the noise level is high, it is difficult to determine whether the signal of oxidized S 2s (∼234 eV) is present here. Our previous studies showed that the cation/anions ratio in 3.5 nm PbSe/ PbS QDs was ∼1.5, which makes it reasonable to assume that PbSO3 species may also exist on the QD surface.55,68 Theoretical Calculations. The electronic structure of the small-sized core and core/shell QDs was calculated in the framework of the four-band k·p envelope function theory,1 which describes the electronic properties of IV−VI semiconductors in the vicinity of the four equivalent L-points in the first Brillouin zone. Spin−orbit coupling and the band-edge effective mass parameters are adequately accounted for in the model. For convenience, the zero energy level is chosen for the vacuum level and the band-offsets in the case of PbSe/PbS QDs are chosen to be 0.155 and 0.025 eV for the conduction and the valence band, respectively. The radial variation of the material parameters is described by position-dependent effective masses and band-edge energies across the core/shell and the shell/ surrounding interfaces.67 The Hamiltonian is numerically diagonalized using the basis of spherical harmonics and spherical Bessel functions, yielding the single-particle energy levels and wave functions. A schematic drawing of the PbSe/PbS core/shell QD heterostructure is shown in Figure 3A (top). The scheme of the calculated electronic band structure for an exemplary small core/shell QD (3.5 nm) is shown in Figure 3A (bottom). The corresponding radial probability density (Ψ(r)2r2) of the charge carriers in the same QDs is displayed in Figure 3B. The steplike band alignment between the core and the shell (Figure 3A (bottom)) can be regarded as quasi-type II owing to the extremely small valence band offset. Such a situation permits the hole delocalization over the entire heterostructure, while

2θ values of 25.44°, 29.21°, and 41.80°, respectively, confirm the rock-salt structure of both the PbSe and PbSe/PbS QDs. The diffraction peaks of the corresponding bulk PbSe and PbS are designated by the vertical lines in the figure. On the basis of the XRD patterns, the average lattice parameters a in PbSe core and in core/shell PbSe/PbS QDs were calculated to be 6.15 ± 0.04 Å and 6.07 ± 0.03 Å, respectively. The comparison with the bulk values (a = 6.12 and 5.94 Å for PbSe and PbS, respectively) shows that the core/shell lattice parameter has an intermediate value between the bulk parameters of its constituents. Representative XPS spectra of air-free/air-exposed PbSe QDs (2.5 ± 0.4 nm) and of PbSe/PbS QDs (3.5 ± 0.5 nm) are shown in Figure 2. The 4f5/2 and 4f7/2 spin−orbit splitting of

Figure 2. (A, B) XPS spectra of Pb 4f peak of air-free PbSe and PbSe/ PbS QDs (black). The curve-fitted individual contribution of Pb−Se/S and of Pb−O bonding is labeled in pink and green, respectively. (C, D) XPS spectra of Pb 4f peaks of the same QDs as in panels A and B after 50 min of air-exposure. The curve-fitted individual contribution of Pb−Se/S and of Pb−O bonding is labeled in pink and dark green, respectively. (E) XPS spectra of Se 3d peak (violet) for air-exposed PbSe QDs. The location of oxidized Se (e.g., SeO2, SeO3−) is marked by an arrow. (F) XPS spectra of Se 2s and S 2s peaks for air-exposed PbSe/PbS QDs. The diameters of the examined PbSe and PbSe/PbS QDs were 2.5 ± 0.4 nm and 3.5 ± 0.5 nm, respectively.

the Pb 4f level in air-free PbSe and PbSe/PbS QDs (black) is shown in panels A and B of Figure 2, respectively. This 4f doublet was resolved into additional two doublets, one doublet corresponding to the lead-chalcogen bond (Pb−Se/S; pink) and the other to the lead−oxygen bond (Pb−O; green), which

Table 1. Binding Energies of the XPS Peaks of the Same PbSe and PbSe/PbS QDs as in Figure 2 air-free PbSe QDs PbSe/PbS QDs

air-exposed

Pb 4f7/2 (Pb−Se/S), BE

Pb 4f7/2 (Pb−O), BE

Pb 4f7/2 (Pb−Se/S), BE

Pb 4f7/2 (Pb−O), BE

Se 3d, BE

Se 2s, BE

S 2s, BE

138.42 137.18

139.38 138.16

138.67 137.31

139.61 138.55

227.80 −

− 227.80

− 224.58

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Figure 3. (A) Schematic drawing of the PbSe/PbS QD structure (top) and a typical energy band diagram for 3.5 nm QDs (bottom). (B) The calculated electron (blue) and hole (red) radial probability density functions in PbSe/PbS QDs. The vertical dashed and solid lines indicate the core and the shell boundary, respectively. (C) Dependence of the band gap energy Eg on the core radius and shell thickness of PbSe/PbS QDs of typical sizes studied in the present work.

the degree of the electron delocalization depends on the QD size. For the current study, it is important that the position of the lowest quantized electronic level largely exceeds the values of the conduction band offset, which should also permit the electron delocalization. However, the degree of the latter slightly deviates from that of its counterpart, the hole, as can be seen from the radial distribution presented in Figure 3B. The penetration of both charge carriers close to the QD exterior surfaces would enable their effective extraction in the PVC configuration. Figure 3C represents a plot of the calculated band gap energy Eg versus the core radius and the shell thickness. It can be seen that in relatively small QDs, Eg changes drastically with the core-to-shell ratio, which allows tuning this value as required. Spectroscopic Characterization. Representative RT absorption and cw-PL spectra of PbSe/PbS and the corresponding PbSe QDs of different diameters, recorded under air-free conditions, are presented in Figure 4A. The size dependence of the first absorption peak and of the PL band is indicative of their association with the (1Se−1Sh) band-edge transitions. It can be seen from the figure that the energy of this transition in the 3.5 nm PbSe/PbS QDs is higher than that in the 3.0 nm PbSe cores. This observation is indicative of the band gap energy dependence not only on the total size of the dot but also on the composition of the heterostructure, in accordance with the theoretical prediction (see Figure 4B). Figure 4B shows the absolute positions of the lowest quantized electron and hole energy levels in PbSe core (blue lines) and PbSe/PbS core/shell QDs (red dots), which were estimated using the following formulas: Ee (d) = χbulk + ηEconf.(d) for electrons and Eh(d) = χbulk − Egbulk − (1 − η) Econf.(d) for holes, where d is the QD diameter, χbulk the bulk PbSe electron affinity, Econf. (d) = EgQD (d) − Egbulk the total confinement energy, and η the fraction of Econf. acquired by the electron. The values of χbulk were taken from the cyclic voltammetry measurements;72 the blue line was evaluated using EgQD from the empirical sizing curve,70 while the red dots are based on the experimental data obtained in this work (some data are presented in Figure 4A). According to the four-band k·p model, in the PbSe/PbS QDs studied here, the total confinement energy is distributed equally between the electrons and the holes (which means that η = 0.5), while in the PbSe QDs η = 0.42. It can be seen from Figure 4B that the conduction-edge energy in the PbSe/PbS QDs is higher than that in the PbSe QDs of the same size, which results in the heterostructure with

Figure 4. (A) Absorption and cw-PL spectra of PbSe and PbSe/PbS QDs of various sizes as indicated in the legend. (B) Absolute positions of the lowest quantized levels in PbSe core and PbSe/PbS core/shell QDs of various sizes. (C) PL QY versus the optical band gap energy Eg for various sizes of PbSe and PbSe/PbS QDs. All measurements were performed at RT under air-free conditions.

an altered band gap energy with respect to the vacuum level. These results correlate well with our previous calculations;67 thus, it can be suggested that the absolute energy of the band D



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QDs is presumably associated with the formation of trapping sites due to surface oxidation.55,75 According to our results, a thin PbS shell of ∼1 nm is sufficient for protecting the core QDs from immediate oxidation. Thermally activated processes in the QDs were studied by following the cw- and tr-PL spectra with the variation of temperature. Representative cw-PL spectra of PbSe and of the corresponding PbSe/PbS QDs recorded in the temperature range of 4−300 K, under both air-free and air-exposure conditions, are shown in Figure 6A−D. The spectra are composed of a broad band that occasionally has an asymmetric shape and thus can be fitted by two Gaussians with the peak energy difference of ∼80 meV.76 Theoretical calculations reported previously77,78 indicate a degeneracy split due to the intervalley interaction caused by the shape anisotropy in PbSe

edges can be tuned with respect to the energy of the electrodes in various optoelectronic applications, thus improving the performance efficiency. The plots of PL QY versus Eg for PbSe and PbSe/PbS QDs are displayed in Figure 4C. The QY values increase with the increase of E g , which is in agreement with previous reports.55,73,74 Several mechanisms can contribute to this trend, namely, the oscillator strength of the interband transition, trapping by defect sites, and/or coupling to ligand vibrations. The oscillator strength in PbSe QDs was shown to decrease linearly with the increase of the QD diameter.71,73 The nonradiative pathway via the trapping process should limit the QY, mainly, in small-sized QDs, because of the large surface-to-volume ratio. It was also suggested that coupling to vibrational overtones of the surface ligands (e.g., the mode at 1.1 eV of oleic acid molecule73) may cause the PL QY loss. Despite the possible effect of the above-mentioned nonradiative pathways, our results revealed an increase of the PL QY for the smallest QDs, which suggests the dominance of the oscillator strength and the elimination of nonradiative pathways. The results presented in Figure 4C show that the PL QY values in PbSe/PbS QDs exceed those in the corresponding PbSe cores with the same Eg because of surface passivation of the PbSe core with the thin PbS shell. When the potential application of colloidal QDs in various optoelectronic applications is considered, the problem of the QD chemical and photochemical stability is of major concern. For this reason, we investigated the stability of the QDs obtained here by following the changes in the PL spectra of PbSe and PbSe/PbS QDs measured after exposure to air for a controlled period of time. Representative cw-PL spectra of 2.2 nm PbSe QDs and of 3.2 nm PbSe/PbS QDs are shown in Figure 5. The spectra were recorded at RT under air-free conditions (Figure 5A,B) as well as after 20 min of air-exposure (Figures 5C,D). It can be seen from Figure 5 that, as a result of air-exposure, the PL intensity of the PbSe QDs is reduced 30fold and the peak is blue-shifted by 20 meV, while the PL intensity and energy in the PbSe/PbS core/shell QDs remain nearly unchanged. The air-induced emission quenching in PbSe

Figure 6. (A−D) Cw-PL spectra of 2.2 nm PbSe and 3.3 nm PbSe/ PbS QDs recorded at RT under air-free conditions (panels A and B, respectively) and after 20 min of air exposure (panels C and D, respectively). The dependence of the normalized integrated PL intensity IPL on temperature for (E) 2.2 nm PbSe QDs, recorded under air-free conditions and after air exposure for 20 min, and (F) 3.3 nm PbSe/PbS QDs under air-free conditions and after air exposure for 20, 50, and 200 min.

Figure 5. Comparison of the cw-PL spectra of 2.2 nm PbSe and 3.2 nm PbSe/PbS QDs recorded at RT under air-free conditions (panels A and B, respectively) and after air exposure for 20 min (panels C and D, respectively). E



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QDs. In PbSe/PbS QDs, this shape anisotropy is largely compensated by the growth of a PbS shell and therefore is much less pronounced than in PbSe QDs (see Figure 1A,C). The temperature dependences of the PL integrated intensity IPL for the same QDs as in Figures 6A-D are presented in Figures 6E and 6F. The IPL values measured at each temperature were normalized with respect to the values obtained at RT with the PL QY of 60%. The IPL values obtained at RT for air-free PbSe and PbSe/PbS QDs are twice as large as those obtained at 5 K. The IPL values of PbSe/PbS QDs exhibit a gradual decrease with temperature rising from 5 to 25 K, followed by a plateau between 25 and 150 K, and, finally, by a substantial increase starting at ∼150 K and up to RT. The behavior of the IPL(T) curves obtained for air-exposed QDs and for those obtained under air-free conditions is different. It can be seen (Figure 6E) that in 2.2 nm PbSe cores which were exposed to air for 20 min, IPL deceases rapidly and is almost completely quenched at RT. Such behavior is presumably accounted for by thermal activation of the exciton transition into trapped states at ∼150 K, induced by surface oxidation. It should be noted that in 3.3 nm PbSe/PbS core/ shell QDs, the same time of air exposure (Figure 6F) did not lead to any changes in the PL intensity. The core/shell heterostructures became affected by oxidation only after airexposure for 50 min and more (see also the results for 200 min of air exposure, Figure 6F), which manifests itself by partial or complete PL quenching in the interval of 150 K to RT. This result implies that the surface oxidation process in PbSe/PbS core/shell QDs is slower than that in the core PbSe QDs. The effect of temperature on the optical transitions in QDs was further examined on various-sized PbSe QDs. Plots of the temperature variation of the PL peak energy EPL obtained under air-free (white background) or air-exposure (light orange background) conditions, are shown in Figure 7A (the QD size is indicated next to each curve). The data reveal the change of dEPL/dT from negative to positive values with the increase of the core diameter above 2 nm. A similar tendency was previously reported in the study of the absorption edge variation in PbSe QDs with temperature (dEg/dT).76,79 The observed changes correlated with the crystal dilation, phonon− electron interactions, and piezo-electric effect.1,79 The EPL(T) plots for exemplary 2.4 nm PbSe QDs were recorded both under air-free conditions (cyan symbols) and after exposure to air for 20 min (orange symbols, see the legend to Figure 7A). It can be seen that air exposure causes a negligible blue shift of the PL peak energy with the temperature increase. The observed effects can result from the oxidation-induced reduction of the PbSe core diameter to the critical size, where dEPL/dT changes its sign.80 Representative plots of the PL full-width halfmaximum (fwhm) versus the temperature, measured under airfree and air-exposure conditions (see the figure legend for details), are presented in Figure 7B. In general, PL fwhm is related to homogeneous effects inducing temperature dependence such as acoustic and optical phonon−electron coupling (as discussed elsewhere81,82) as well as to inhomogeneous effects such as size and shape dispersion.83 It can be seen from the curves in Figure 7B that the fwhm values in PbSe/PbS QDs (red symbols) are substantially smaller than those in the corresponding PbSe cores (black symbols), presumably because of the compensation of the shape inhomogeneity by the shell growth, as discussed above. The fwhm of air-exposed 2.4 nm PbSe QDs (green symbols) broadens because of the increase of the QD size dispersion. The temperature-dependent PL

Figure 7. (A) Plots of the PL peak energy EPL versus temperature for various sizes of PbSe QDs under air-free conditions and after airexposure for 20 min (orange). The QD diameters are indicated next to the curves. (B) Plots of the PL fwhm values versus temperature for PbSe and PbSe/PbS QDs under air-free conditions (black, purple, red) and for air-exposed PbSe (green) QDs. The QD diameters are indicated in the legend.

behavior provides evidence for the existence of trap states on the PbSe QD surface. These states, induced by oxidation, have distinct effect on steady-state PL at both 5 K and RT, which are eliminated by PbS shell coating. The nature of the exciton relaxation processes was further elucidated by tr-PL measurements. The PL intensity decay curves were obtained by exciting the sample at 1.17 eV and following the intensity variation over time. The low-power excitation ensured the formation of single excitons. The normalized PL decay curves for 2.2 nm PbSe and 3.5 nm PbSe/PbS QDs, recorded under air-free conditions at various temperatures, are presented in panels A and B of Figure 8, respectively. The curves are best fitted either to a single exponent, I(t) = A exp(−t/τ0) (where A is a pre-exponential factor and τ0 is the measured exciton lifetime) or to a double exponent, I(t) = A1 exp(-t/τ1) + A2 exp(−t/τ2), with the weighted decay time τ0 = A1τ1 + A2τ2. The existence of more than one decay component can be associated with a lift of the band-edge degeneracy, which is most pronounced in small cores because of their shape anisotropy. Figure 8C displays the plots of τRad versus the size of PbSe and PbSe/PbS QDs, obtained at RT. The data reveal larger radiative lifetimes in the core/shell QDs as compared to those of the corresponding cores.78 The relatively large τRad values are explained by the large dielectric constant of PbSe84 and by mixing of adjacent electronic-band minima.78 Such a long exciton lifetime is beneficial for charge extraction in the PVC application.

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CONCLUSIONS Great scientific and technological significance of colloidal QDs based on IV−VI semiconductors has lately inspired numerous investigations of their physical properties. However, until now, little attention has been paid to the question of the QD F



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Figure 8. Representative normalized tr-PL curves for PbSe (A) and PbSe/PbS (B) QDs recorded under air-free conditions at various temperatures (as indicated in the legend). (C) Dependence of τRad values on the size of PbSe (blue) and PbSe/PbS (orange) QDs at RT. (6) Romero, H. E.; Drndic, M. Coulomb Blockade and Hopping Conduction in PbSe Quantum Dots. Phys. Rev. Lett. 2005, 95, 156801. (7) Wise, F. W. Lead Salt Quantum Dots: The Limit of Strong Quantum Confinement. Acc. Chem. Res. 2000, 33, 773−780. (8) Harbold, J. M.; Du, H.; Krauss, T. D.; Cho, K.-S.; Murray, C. B.; Wise, F. W. Time-Resolved Intraband Relaxation of Strongly Confined Electrons and Holes in Colloidal PbSe Nanocrystals. Phys. Rev. B 2005, 72, 195312. (9) Brumer, M.; Kigel, A.; Amirav, L.; Sashchiuk, A.; Solomesch, O.; Tessler, N.; Lifshitz, E. PbSe/PbS and PbSe/PbSexS1−x Core/Shell Nanocrystals. Adv. Funct. Mater. 2005, 15, 1111−1116. (10) Pietryga, J. M.; Schaller, R. D.; Werder, D.; Stewart, M. H.; Klimov, V. I.; Hollingsworth, J. A. Pushing the Band Gap Envelope: Mid-Infrared Emitting Colloidal PbSe Quantum Dots. J. Am. Chem. Soc. 2004, 126, 11752−11753. (11) Nozik, A. J. Nanoscience and Nanostructures for Photovoltaics and Solar Fuels. Nano Lett. 2010, 10, 2735−2741. (12) Nozik, A. J. Quantum Dot Solar Cells. Phys. E (Amsterdam, Neth.) 2002, 14, 115−120. (13) Tang, J.; Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath, R.; Cha, D.; et al. Colloidal-Quantum-Dot Photovoltaics Using Atomic-Ligand Passivation. Nat. Mater. 2011, 10, 765−771. (14) Ip, A. H.; Thon, S. M.; Hoogland, S.; Voznyy, O.; Zhitomirsky, D.; Debnath, R.; Levina, L.; Rollny, L. R.; Carey, G. H.; Fischer, A.; et al. Hybrid Passivated Colloidal Quantum Dot Solids. Nat. Nanotechnol. 2012, 7, 577−582. (15) Ma, W.; Swisher, S. L.; Ewers, T.; Engel, J.; Ferry, V. E.; Atwater, H. A.; Alivisatos, A. P. Photovoltaic Performance of Ultrasmall PbSe Quantum Dots. ACS Nano 2011, 5, 8140−8147. (16) Barkhouse, D. A. R.; Debnath, R.; Kramer, I. J.; Zhitomirsky, D.; Pattantyus-Abraham, A. G.; Levina, L.; Etgar, L.; Grätzel, M.; Sargent, E. H. Depleted Bulk Heterojunction Colloidal Quantum Dot Photovoltaics. Adv. Mater. 2011, 23, 3134−3138. (17) Etgar, L.; Yanover, D.; Č apek, R. K.; Vaxenburg, R.; Xue, Z.; Liu, B.; Nazeeruddin, M. K.; Lifshitz, E.; Grätzel, M. Core/Shell PbSe/PbS QDs TiO2 Heterojunction Solar Cell. Adv. Funct. 2013, 23, 2736− 2741. (18) Brumer, M.; Sirota, M.; Kigel, A.; Sashchiuk, A.; Galun, E.; Burshtein, Z.; Lifshitz, E. Nanocrystals of PbSe Core, PbSe/PbS, and PbSe/PbSexS1‑x Core/Shell as Saturable Absorbers in Passively QSwitched Near-Infrared Lasers. Appl. Opt. 2006, 45, 7488−7497. (19) Sirota, M.; Galun, E.; Krupkin, V.; Glushko, A.; Kigel, A.; Brumer, M.; Sashchiuk, A.; Amirav, L.; Lifshitz, E. IV-VI Semiconductor Nanocrystals for Passive Q-Switch in IR. SPIE Proc. 2004, 5510, 9−16. (20) Wang, R. Y.; Feser, J. P.; Lee, J.-S.; Talapin, D. V.; Segalman, R.; Majumdar, A. Enhanced Thermopower in PbSe Nanocrystal Quantum Dot Superlattices. Nano Lett. 2008, 8, 2283−2288. (21) Loss, D.; DiVincenzo, D. P. Quantum Computation with Quantum Dots. Phys. Rev. A 1998, 57, 120−126. (22) Petta, J. R.; Johnson, A. C.; Taylor, J. M.; Laird, E. A.; Yacoby, A.; Lukin, M. D.; Marcus, C. M.; Hanson, M. P.; Gossard, A. C. Coherent Manipulation of Coupled Electron Spins in Semiconductor Quantum Dots. Science 2005, 309, 2180−2184.

chemical and photochemical stability. The current paper describes the synthesis of relatively small-sized (≤3.5 nm) PbSe/PbS QDs, a theoretical evaluation of their electronicband structure, and also a thorough investigation of their photochemical stability and steady-state and transient optical properties. The results reveal the following QD characteristics: (1) The band-edge energy can be tuned by varying the core-toshell division. (2) The charge carriers are delocalized over the entire core/shell heterostructures. (3) The absolute band-edge energy in small-sized PbSe/PbS QDs is higher than that in PbSe QDs of the same size. (4) A thin PbS shell results in the PL QY enhancement and in the increase of the electron−hole recombination time. (5) A thin PbS shell provides the QDs with resistance against oxidation for a sufficiently long time and in the reduction of the fwhm of the emission bands. The abovementioned properties are highly advantageous for various QDbased optoelectronic applications such as gain and photonic and plasmonic devices, photodetectors, and the abovementioned PVCs, where efficient charge injection into electron-collecting layer (e.g., TiO2 or ZnO) and device processing under ambient conditions are especially important.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +972-4-829-3987. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank O. Solomeshch for assistance in the PL QY measurements. We acknowledge the support from the Israel Science Foundation (Projects 1009/07 and 1425/04), the USA-Israel Binational Science Foundation ( 2006-225), the Israel Ministry of Science (3-896), and the European FP7 SANS and Nanospec projects.

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