CdSe

Letter pubs.acs.org/JPCL

Optical and Electronic Properties of Nonconcentric PbSe/CdSe Colloidal Quantum Dots Gary Zaiats,§ Arthur Shapiro,§ Diana Yanover,§ Yaron Kauffmann,† Aldona Sashchiuk,§ and Efrat Lifshitz*,§ §

Schulich Faculty of Chemistry, Solid State Institute, Russell Berrie Nanotechnology Institute, Technion-Israel Institute of Technology, Haifa 3200025, Israel † Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa 3200025, Israel S Supporting Information *

ABSTRACT: Lead chalcogenide colloidal quantum dots are attractive candidates for applications operating in the near infrared spectral range. However, their function is forestalled by limited stability under ambient conditions. Prolonged temperature-activated cation-exchange of Cd2+ for Pb2+ forms PbSe/CdSe core/shell heterostructures, unveiling a promising surface passivation route and a method to modify the dots’ electronic properties. Here, we follow early stages of an-exchange process, using spectroscopic and structural characterization tools, as well as numerical calculations. We illustrate that preliminaryexchange stages involve the formation of nonconcentric heterostructures, presumably due to a facet selective reaction, showing a pronounced change in the optical properties upon the increase of the degree of nonconcentricity or/and plausible creation of core/shell interfacial alloying. However, progressive-exchange stages lead to rearrangement of the shell segment into uniform coverage, providing tolerance to oxygen exposure with a spectral steadiness already on the formation of a monolayer shell.

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multiple-exciton emission quantum yield (QY) over conventional PbSe,19 as well as reduction of nonradiative decay processes.14 Several groups explored the structural12 and optical20 properties of PbSe/CdSe CQDs with thick concentric shells around the cores.13,20 It is worth noting that the exchange procedure is facet-dependent; thus, it may produce a nonconcentric shell with respect to the center of the cores.12,18 Recently published work21 suggested that the nonconcentric structure of PbS/CdS CQDs might be responsible for appearance of double bands in the emission spectra of these heterostructures. In this work, we use numerical and experimental methods to understand the effect of core/shell nonconcentricity on electronic structure, on the optical properties, and on the stability of CQD heterostructures under ambient conditions. The research is based on the investigation of PbSe/CdSe CQDs with shells that were limited to the outermost layers of the CQD,11,13,14,22 so that the initiation of nonconcentricity and/or the evolution from nonconcentric to concentric could be followed. Furthermore, the carriers’ expected distribution predicts potential application of the investigated materials in various optoelectronic devices. Characteristic spectral and structural properties of PbSe/ CdSe CQDs are summarized in Figure 1. Figure 1A displays the absorbance (dashed curves) and photoluminescence (PL, solid

ead chalcogenide (PbSe, PbS) colloidal quantum dots (CQDs) are receiving significant attention due to their tunable optical properties over a wide spectral range, making them good candidates, in particular, for various applications operating in the near-infrared (NIR) spectral regime, such as, NIR-emitting diodes,1 photovoltaic devices,2,3 and cancer therapy.4 However, their use is hindered by their limited stability under ambient conditions.5,6 A few groups have attempted to overcome this problem through passivation of the exterior surface by inorganic shells.7 This was achieved either by epitaxial coating of a semiconductor core by another semiconductor shell8−10 (e.g., PbSe/PbS CQDs) or by a cationexchange process, involving diffusion of foreign cations from the outermost to the inner CQD layers (e.g., PbSe/CdSe and PbS/CdS CQDs).11−14 The energy band offset between the core and the shell energy levels has a crucial role in determining the optoelectronic properties of heterostructures. The core/shell CQDs mentioned above exhibit a quasi-type-II band interfacial alignment,8,9,15,16 where one charge carrier is confined to either the core or shell while the other one is delocalized over the entire CQD’s volume. However, special attention should be paid to the shell width, which, if extremely large, can reduce probability of charge extraction from the core, likely limiting the efficiency of CQDs-based photovoltaic cells.17 Cation-exchange is a promising synthesis route18 that allows formation of a wide variety of geometrical structures.12 PbSe/ CdSe CQDs that experienced Cd-for-Pb cation-exchange until formation of thick shells showed a significant increase in © 2015 American Chemical Society

Received: March 10, 2015 Accepted: June 9, 2015 Published: June 9, 2015 2444

DOI: 10.1021/acs.jpclett.5b00498 J. Phys. Chem. Lett. 2015, 6, 2444−2448

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Figure 1. (A) The absorbance (dashed) and PL (solid) spectra of (a) PbSe and PbSe/CdSe after (b) 10, (c) 15, (d) 20, and (e) 25 min of exchange reaction. (B) The XRD patterns of (a) PbSe and PbSe/CdSe after (b) 30 and (c) 60 min of exchange reaction. (C,D) Representative HRSTEM images of nonconcentric (C) and concentric (D) PbSe/CdSe CQDs. The dashed box outlines a magnified image of a selected CQD. The dashed yellow lines indicate the crystallographic facets, and the reaction direction is shown by the arrows.

peaks are enhanced in curve (b) with relatively narrow line width, associated with the existence of the CdSe (zb) phase. These signatures designate the formation of the CdSe domain, presumably grown in a specific facet of the parent cores, in agreement with the high-angle annular dark field highresolution scanning transmission electron microscopy (HAADF-HRSTEM) (see below) and with previous observations reported in ref 12. Further progress in a cation-exchange process (curve (c)) results in a reduction of the (202) peak intensity (relative to that of (200)) and a broadening of the (111) peak. The broadening may be attributed to the following effects: (i) rearrangement of the shell domains into a concentric thin epitaxial shell with respect to the core center. Indeed, the Scherrer equation, showing the relation between the crystallite domain size, D, (viz., shell width) and a full width at halfmaximum of the XRD diffraction line (β), with D = (Kλ)/(β cos(θ)) (K is a shape factor of crystallites, and λ is the wavelength of the diffracted photons), supports the formation of a thin shell; (ii) generation of an alloyed PbxCd1−xSe zone at the PbSe/CdSe interface either with a concentric or nonconcentric shell around the CQD’s central point. The XRD observations were further confirmed by a X-ray photoelectron spectroscopy (XPS), as given in the Supporting Information (SI), Table S3. Figure 1C displays a HAADF-HRSTEM image of PbSe/ CdSe CQDs, showing a distinguished contrast between cores and shells, originating from the large atomic weight difference between Pb and Cd ions. The inset shows a magnified image of a single CQD. The fast Fourier transform analysis of the latter image (shown in the SI, Figure S2C) designates that the core is imaged in the [011] zone axis, and the yellow dashed lines mark the identified {111} set of planes. The analysis of the

curves) spectra of parent PbSe CQDs with an average diameter of 4.1 nm (curve (a)) and of the corresponding PbSe/CdSe CQDs aliquots (curves (b−e)), extracted during an-exchange reaction at different reaction times. The total reaction time was limited to 60 min, aiming to follow the early stages of the exchange process and the formation of a relatively thin exchange zone, while a giant shell creation was excluded from the present study. The spectra in Figure 1A are dominated by excitonic transitions with energy E1Se−1Sh. These optical measurements show an initial blue shift of E1Se−1Sh in curves (b−d) with respect to that of the parent PbSe cores as a result of reduction of the core diameter. However, after reaction time progress, the spectral shift is detained, and after ∼25 min, a retracting red shift of ∼30 meV takes place (see curve (e) with respect to curve (d)). Figure 1B shows XRD patterns of the air-exposed parent PbSe (curve (a)) and PbSe/CdSe (curves (b,c)) CQDs, obtained after different periods of cation exchange. The vertical lines represent the XRD lines of bulk PbSe rock-salt (rs), CdSe zinc blende (zb), and Pb3O4 lattices, as indicated in the legend. The schematic representations next to the curves highlight the corresponding stages of the cation exchange process, as discussed below. The intensity of the XRD patterns in Figure 1B is normalized relative to the (200) peak of PbSe (rs) CQDs. The comparison between the CQD XRD patterns and those of the bulk materials suggests that the (202) peak in curve (a) is associated with the existence of a Pb3O4 phase in the XRD pattern of the parent PbSe CQDs. The intensity of this peak is reduced after a short exchange process (curve (b)) and totally absent in the XRD pattern of the core/shell CQDs after a long exchange process (curve (c)). Furthermore, the (111), (220), and (222) 2445

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probability distribution of the hole (plot (c)) is significantly shifted from Os with a displacement that is defined as Δ = |Os − Oc| (as marked on the figure), while that of the electron (plot (d)) is only mildly relocated. The nonconcentricity between the core and shell may change the degree of spatial and dielectric confinement of the carriers, as well as affect the carrier wave function overlap. Accordingly, related calculation, using the effective mass approximation (see calculation details in the SI), predicted a red shift of the band gap energy as well as possible reduction of the overlapping integral upon displacement of the core and shell centers with respect to the other. This red shift of the band gap energy is defined as ΔEg = Eg,conc − Eg,nonconc, where Eg,conc and Eg,nonconc are the band gap energies of concentric and nonconcentric CQDs, respectively, Figure 2B shows a numerical evaluation of the magnitude of ΔEg versus the value of Δ/W, where W corresponds to the difference between the core (Rc) and shell (Rs) radii. ΔEg is evaluated for CQDs with various Rc/Rs ratios, as indicated in the legend. In all cases, ΔEg increases with the increase of Δ, while this dependence has the steepest slope for the smallest CQDs (with Rs = 1.0 nm) and the smallest Rc/Rs value. The evaluations predict a red shift (5−40 meV) of the band edge energy on the generation of concentric core/shell CQDs. It should be noted that the values of Eg,conc and Eg,nonconc were evaluated for pure core and shell constituents, ignoring plausible formation of alloyed regimes.22 However, these quantities may deviate from the proposed values in the case of a Pb/Cd interdiffusion process across an interfacial layer, which might be induced during a prolonged exchange process. The existence of two competing processes, the blue shift of E1Se−Sh due to contracting of the parent core radius and the red shift due to increase of symmetry, may explain the mild change in E1Se−Sh upon the decrease of the core radius, as well as the final pull-back of E1Se−Sh toward the red spectral region in the absorption curves (d,e) shown in Figure 1A. It was previously shown6,9 that PbSe CQDs are prone to oxidation after an extremely short time of air exposure. Here, we examine the tolerance of the parent PbSe cores and the corresponding PbSe/CdSe core/shell CQDs to air exposure by following the steady-state spectra and time-resolved PL spectra at various temperatures. Representative steady-state PL spectra of parent PbSe cores with Rc = 2.0 nm and of PbSe/CdSe core/ shell CQDs with Rs/Rc = 2.0/1.8 or 2.0/1.6 are shown in Figure 3A−C, respectively. The experiments were carried out under inert conditions; however, the samples were previously exposed to air for 1 h. The spectra in all cases are comprised of a single excitonic band, exhibiting an energy red shift with the decrease of the temperature, as typically found in IV−VI semiconductors, as well as variation in the bands’ intensity (IPL). It should be noted that a minor dip in the PL spectra at 0.9 eV is an experimental artifact due to a ghost line. Figure 3D shows plots of IPL values versus temperature for the CQDs given in the legend. The plots were normalized with respect to one another at the lowest temperature (5 K). It is seen from panel (D) that IPL of the various CQDs retains a nearly constant value up to 150 K, beyond which major changes take place. The parent PbSe and PbSe/CdSe CQDs with intermediate shell coverage (with Rs/Rc = 2.0/1.8 nm), which were exposed to air, showed a drastic reduction of IPL values, presumably due to severe oxidation of the surface. The oxidized sites act as carrier traps, in agreement with the existence of a pronounced XRD peak related to Pb3O4, as shown in Figure 1B. In contrast, the PL intensity of PbSe/CdSe with Rs/Rc = 2.0/1.6 nm (exposed

discussed image suggests that the exchange reaction is initiated by replacing Cd with Pb atoms at {111} strained corners, and it sequentially proceeds in the ⟨111⟩ direction. It is worth noting that in small-sized CQDs, some of the cubic corners are truncated. Thus, a cation exchange might start only from one side of the nanoparticle, giving rise to a nonconcentric alignment of the shell with respect to the CQD center. As the reaction proceeds, there is evidence (Figure 1D) of the formation of a uniform shell surrounding the inner core, formed by topotaxial growth toward the CQD interior.18 Figure 2A presents the probability distribution (ψ2) of electrons and holes in concentric (a,b) and nonconcentric (c,d)

Figure 2. (A) Normalized probability distribution of the (a,c) hole and (b,d) electron in concentric (a,b) and nonconcentric (c,d) core/shell CQDs. (B) Effect of the core center shift from the shell center (Δ = | Os − Oc|) normalized relative to W = Rs − Rc upon the Eg shift of the nonconcentric structure relative to the concentric structure.

core/shell CQDs. The central points of a core (Oc) spheroid and of a shell (Os) spheroid are marked on plot (c) by the solid and open circles, respectively. The normalized probability densities of the hole (plot (a)) and the electron (plot b) have a concentric alignment with respect to point Os. The hole distribution reflects localization toward the CQD’s center, while the electron distribution seems to be delocalized over a larger volume (see the color-coded scales). This representation coincides with a quasi-type-II behavior, as previously predicted for PbSe/CdSe CQDs.13 In a nonconcentric CQD, the 2446

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Figure 3. PL spectra recorded at various temperatures of (A) PbSe with Rc = 2 nm, (B) PbSe/CdSe with Rs/Rc = 2.0/1.8 nm, and (C) PbSe/CdSe with Rs/Rc = 2.0/1.6 nm. (D) Plots of the PL integrated intensity of the samples presented in (A−C) versus temperature.

To summarize, a cation-exchange process was used for the generation of PbSe/CdSe heterostructures. The efforts were focused on CQDs that experienced Cd-for-Pb exchange only at the outermost layers. The results showed a blue shift of a band gap upon exchange, which is counterbalanced by a red shift, due to nonconcentricity at the preliminary stages of the exchange process. Prolonging the reaction time induced the formation of a thin concentric shell that provided the CQDs with a higher emission QY, longer radiative lifetime, and, important of all, tolerance to oxidation. The study shed light on the effect of the core/shell geometry on the CQDs’ optical properties.

to air), as well as that of control air-free PbSe CQDs, exhibits an enhancement above 150 K and up to room temperature (RT). This behavior suggests that despite the air exposure of the PbSe/CdSe CQDs, their oxidation has been mitigated by sufficient surface passivation, avoiding luminescence quenching. It is worth noting that the discussed core/shell CQDs consisted of extremely thin shells; the observations in Figure 3 support the efficiency of such shells for surface passivation. Figure 4 presents plots of the PL QY values, measured at RT, versus Eg of PbSe cores (blue symbols) and of PbSe/CdSe

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ASSOCIATED CONTENT

* Supporting Information S

Experimental materials and methods, COMSOL model details, HRSTEM images, and variation of the 1Se−1Sh transition with time. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.5b00498.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 4. Plot of PL QY versus Eg of PbSe and PbSe/CdSe as given in the legend. The inset shows PL decay curves of PbSe and PbSe/CdSe, as described in the legend.

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ACKNOWLEDGMENTS The authors thank Dr. Kamira Weinfeld for her assistance with XPS analysis. We acknowledge support of the Israel Council for High Education - Focal Area Technology (No. 872967), the Volkswagen Stiftung, (No.88116), and the NiedersachsenDeutsche Technion Gesellschaft E.V (No. ZN2916)

CQDs (red symbols). PbSe/CdSe CQDs were synthesized from PbSe with Eg = 1.07 eV. The PL QY values were measured by an integrating sphere, as discussed in the SI. It is seen from the figure that the PL QY of PbSe/CdSe CQDs exhibits a substantial increase after reaching Eg = 1.3 eV, accompanied by the growth of the PL lifetime, as seen in the inset. The indicated changes can be related to the following factors: (a) reduction of surface defect sites often found in pristine PbSe cores; (b) cation-exchange progress from a nonconcentric to a concentric core/shell structure (see Figure 2B);23 (c) formation of a minor alloying that would further improve the core−shell boundary crystallographic matching.22 In any event, it is important to note that the PL QY for the parent PbSe CQDs is typically 30−40% and exceeds 60% for well-passivated PbSe/CdSe CQDs and at the same time has an extremely thin shell (thinner than a single unit cell), as discussed in the present case.

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REFERENCES

(1) Sun, L.; Choi, J. J.; Stachnik, D.; Bartnik, A. C.; Hyun, B.-R.; Malliaras, G. G.; Hanrath, T.; Wise, F. W. Bright Infrared QuantumDot Light-Emitting Diodes through Inter-Dot Spacing Control. Nat. Nanotechnol. 2012, 7, 369−373. (2) Zhang, J.; Gao, J.; Church, C. P.; Miller, E. M.; Luther, J. M.; Klimov, V. I.; Beard, M. C. PbSe Quantum Dot Solar Cells with More than 6% Efficiency Fabricated in Ambient Atmosphere. Nano Lett. 2014, 14, 6010−6015. (3) Etgar, L.; Yanover, D.; Capek, R. K.; Vaxenburg, R.; Xue, Z.; Liu, B.; Nazeeruddin, M. K.; Lifshitz, E.; Graetzel, M. Core/Shell PbSe/ PbS QDs TiO2 Heterojunction Solar Cell. Adv. Funct. Mater. 2013, 23, 2736−2741. 2447

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Letter

The Journal of Physical Chemistry Letters (4) Ken-Tye, Y. Mn-Doped Near-Infrared Quantum Dots as Multimodal Targeted Probes For Pancreatic Cancer Imaging. Nanotechnology 2009, 20, 015102. (5) Sykora, M.; Koposov, A. Y.; McGuire, J. A.; Schulze, R. K.; Tretiak, O.; Pietryga, J. M.; Klimov, V. I. Effect of Air Exposure on Surface Properties, Electronic Structure, and Carrier Relaxation in PbSe Nanocrystals. ACS Nano 2010, 4, 2021−2034. (6) Chappell, H. E.; Hughes, B. K.; Beard, M. C.; Nozik, A. J.; Johnson, J. C. Emission Quenching in PbSe Quantum Dot Arrays by Short-Term Air Exposure. J. Phys. Chem. Lett. 2011, 2, 889−893. (7) Talapin, D. V.; Rogach, A. L.; Kornowski, A.; Haase, M.; Weller, H. Highly Luminescent Monodisperse CdSe and CdSe/ZnS Nanocrystals Synthesized in a Hexadecylamine−Trioctylphosphine Oxide− Trioctylphospine Mixture. Nano Lett. 2001, 1, 207−211. (8) Sashchiuk, A.; Yanover, D.; Rubin-Brusilovski, A.; Maikov, G. I.; Capek, R. K.; Vaxenburg, R.; Tilchin, J.; Zaiats, G.; Lifshitz, E. Tuning of Electronic Properties in IV−VI Colloidal Nanostructures by Alloy Composition and Architecture. Nanoscale 2013, 5, 7724−7745. (9) Yanover, D.; Vaxenburg, R.; Tilchin, J.; Rubin-Brusilovski, A.; Zaiats, G.; Č apek, R. K.; Sashchiuk, A.; Lifshitz, E. Significance of Small-Sized PbSe/PbS Core/Shell Colloidal Quantum Dots for Optoelectronic Applications. J. Phys. Chem. C 2014, 118, 17001− 17009. (10) Bae, W. K.; Padilha, L. A.; Park, Y.-S.; McDaniel, H.; Robel, I.; Pietryga, J. M.; Klimov, V. I. Controlled Alloying of the Core−Shell Interface in CdSe/CdS Quantum Dots for Suppression of Auger Recombination. ACS Nano 2013, 7, 3411−3419. (11) Pietryga, J. M.; Werder, D. J.; Williams, D. J.; Casson, J. L.; Schaller, R. D.; Klimov, V. I.; Hollingsworth, J. A. Utilizing the Lability of Lead Selenide to Produce Heterostructured Nanocrystals with Bright, Stable Infrared Emission. J. Am. Chem. Soc. 2008, 130, 4879− 4885. (12) Casavola, M.; van Huis, M. A.; Bals, S.; Lambert, K.; Hens, Z.; Vanmaekelbergh, D. Anisotropic Cation Exchange in PbSe/CdSe Core/Shell Nanocrystals of Different Geometry. Chem. Mater. 2011, 24, 294−302. (13) De Geyter, B.; Justo, Y.; Moreels, I.; Lambert, K.; Smet, P. F.; Van Thourhout, D.; Houtepen, A. J.; Grodzinska, D.; de Mello Donega, C.; Meijerink, A.; Vanmaekelbergh, D.; Hens, Z. The Different Nature of Band Edge Absorption and Emission in Colloidal PbSe/CdSe Core/Shell Quantum Dots. ACS Nano 2010, 5, 58−66. (14) Abel, K. A.; Qiao, H.; Young, J. F.; van Veggel, F. C. J. M. FourFold Enhancement of the Activation Energy for Nonradiative Decay of Excitons in PbSe/CdSe Core/Shell versus PbSe Colloidal Quantum Dots. J. Phys. Chem. Lett. 2010, 1, 2334−2338. (15) 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. (16) Park, Y.-S.; Bae, W. K.; Padilha, L. A.; Pietryga, J. M.; Klimov, V. I. Effect of the Core/Shell Interface on Auger Recombination Evaluated by Single-Quantum-Dot Spectroscopy. Nano Lett. 2014, 14, 396−402. (17) Neo, D. C. J.; Cheng, C.; Stranks, S. D.; Fairclough, S. M.; Kim, J. S.; Kirkland, A. I.; Smith, J. M.; Snaith, H. J.; Assender, H. E.; Watt, A. A. R. Influence of Shell Thickness and Surface Passivation on PbS/ CdS Core/Shell Colloidal Quantum Dot Solar Cells. Chem. Mater. 2014, 26, 4004−4013. (18) Rivest, J. B.; Jain, P. K. Cation-Exchange on the Nanoscale: An Emerging Technique for New Material Synthesis, Device Fabrication, and Chemical Sensing. Chem. Soc. Rev. 2013, 42, 89−96. (19) Cirloganu, C. M.; Padilha, L. A.; Lin, Q.; Makarov, N. S.; Velizhanin, K. A.; Luo, H.; Robel, I.; Pietryga, J. M.; Klimov, V. I. Enhanced Carrier Multiplication in Engineered Quasi-Type-II Quantum Dots. Nat. Commun. 2014, 5, 1−8. (20) De Geyter, B.; Hens, Z. The Absorption Coefficient of PbSe/ CdSe Core/Shell Colloidal Quantum Dots. Appl. Phys. Lett. 2010, 97, 161908/1−161908/3. (21) Zhao, H.; Liang, H.; Gonfa, B. A.; Chaker, M.; Ozaki, T.; Tijssen, P.; Vidal, F.; Ma, D. Investigating Photoinduced Charge

Transfer in Double- and Single-Emission PbS@CdS Core@Shell Quantum Dots. Nanoscale 2014, 6, 215−225. (22) Abel, K. A.; Fitzgerald, P. A.; Wang, T.-Y.; Regier, T. Z.; Raudsepp, M.; Ringer, S. P.; Warr, G. G.; van Veggel, F. C. J. M. Probing the Structure of Colloidal Core/Shell Quantum Dots Formed by Cation Exchange. J. Phys. Chem. C 2012, 116, 3968−3978. (23) Grodzinska, D.; Pietra, F.; van Huis, M. A.; Vanmaekelbergh, D.; de Mello Donega, C. Thermally Induced Atomic Reconstruction of PbSe/CdSe Core/Shell Quantum Dots into Pbse/Cdse Bi-Hemisphere Hetero-Nanocrystals. J. Mater. Chem. 2011, 21, 11556−11565.

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