Article pubs.acs.org/cm
PbS/CdS Core/Shell Quantum Dots by Additive, Layer-by-Layer Shell Growth Laxmi Kishore Sagar,†,‡ Willem Walravens,†,‡ Qiang Zhao,§ André Vantomme,§ Pieter Geiregat,†,‡ and Zeger Hens*,†,‡ †
Physics and Chemistry of Nanostructures, Ghent University, 9000 Ghent, Belgium Center for Nano and Biophotonics, Ghent University, 9000 Ghent, Belgium § Instituut voor Kern-en Stralingsfysica, KU Leuven, 3000 Leuven, Belgium ‡
S Supporting Information *
ABSTRACT: We demonstrate additive shelling of PbS core quantum dots by CdS using room-temperature colloidal atomic layer deposition. Combining in-depth electron microscopy, absorption spectroscopy and elemental analysis, we present a structural model of the PbS/CdS core/shell quantum dots after each of the successive colloidal ALD cycles. In particular, we find that such core/shell quantum dots are cation-rich and we argue that this nonstoichiometry should be attributed to a Cd-excess at the outer surface. Analyzing various PbS/CdS core/shell QDs, we demonstrate that CdS shells have no effect on the energy of the PbS band-edge transition for PbS core QDs larger than ≈4 nm, a finding we support by effective mass calculations. In spite of this, CdS shell growth generally decreases the photoluminescence quantum yield of PbS/ CdS core/shell QDs as compared to PbS core QDs. This suggests that photoluminescence quenching is mainly caused by defects at the PbS/CdS interface.
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recombination rates through an alloyed core/shell interface,15 which is key to suppress blinking and facilitate optical gain.13,16−19 For most QDs emitting in the visible, including those based on CdSe or InP core nanocrystals, various shelling procedures have been developed. These typically involve the growth of a shell around an initially formed core QD either in a layer-by-layer fashion20 or by heterogeneous nucleation or seeded growth.21,22 A major advantage of such additive methods is their allowing for a precise control over the core/ shell geometry because the size of the original core is preserved and the amount of shell material added is predetermined. Applications in need of QDs active in the infrared, which include photovoltaics and photodetection, mostly use PbE (E = S, Se, Te) QDs.23−25 Here, the corresponding cadmium chalcogenide makes for almost strain-free PbE/CdE core/ shell systems where, for example, PbSe/CdSe yields a higher efficiency for multiple exciton generation than the corresponding core PbSe QDs.26 Additive shell growth has, however, proven difficult for PbE/CdE core/shell systems since exposing PbE QDs to cadmium leads to a rapid surface-mediated replacement of Pb by Cd in the initial PbE core nanocrystals.27,28 Although this so-called cation exchange reaction can be used to grow CdE shells inward through the progressive
INTRODUCTION Semiconductor nanocrystals feature size-dependent optical properties due to quantum confinement of conduction-band electrons and valence-band holes. Considering their formation, solution-based colloidal methods stand out because these enable a wide range of semiconductors to be synthesized as colloidal nanocrystals with precisely controlled sizes and shapes and a suitability for solution-based processing.1 Such colloidal quantum dots are hybrid materials composed of a crystalline inorganic core, which mainly determines the optoelectronic properties, capped by an organic shell of ligands that stabilize quantum dot dispersions by steric hindrance.2 These organic ligands, however, proved relatively poor at passivating midgap states at the quantum dot surface, thus rendering for example the quantum dot photoluminescence highly sensitive to external influences.3,4 Such issues could be overcome by terminating core quantum dots by an epitaxial inorganic shell5−7 and the resulting core/shell quantum dots are now widely used as color converting phosphors in lighting and displays or explored as optical gain medium.8−12 Very often, a shell adds more functionality to the final core/ shell system than a mere passivation of the core surface. In the case of the seminal CdSe/CdS core/shell system for example, the CdS shell may enhance photoluminescence quantum yields to 90% or more.13 However, it also reduces self-absorption of the photoluminescence by boosting the absorption at wavelengths shorter than 500 nm14 and slows down Auger © 2016 American Chemical Society
Received: June 29, 2016 Revised: September 14, 2016 Published: September 14, 2016 6953
DOI: 10.1021/acs.chemmater.6b02638 Chem. Mater. 2016, 28, 6953−6959
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PbS/CdS Synthesis. We made PbS/CdS core/shell quantum dots using a colloidal atomic layer deposition approach as initially developed by Ithurria et al.30 Typically, a solution of 4.5 mg of PbS cores in 1 mL of toluene and 100 μL of oleylamine was combined with 1.5 mL of formamide to form a liquid−liquid two phase system. To grow a single layer of CdS, 40 μL of ammonium sulfide (5 mg, 0.0733 mmol) was first added to this system, after which the two-phase solution was thoroughly shaken, the polar phase discarded and the nonpolar phase washed twice with formamide to extract residual ammonium sulfide. Next, 1.5 mL of formamide was added to make a two phase system once more, together with 60 μL of a formamide solution of cadmium acetate (1.6 mg, 0.0061 mmol). This solution was again shaken thoroughly, the polar phase discarded and the nonpolar phase washed twice to extract residual cadmium acetate. By repeating this S and Cd exposure, different shells can be grown one after the other. Although we typically grew 3 to 4 shells, nothing restricts a further continuation of the procedure to grow any number of shells desired. Before recording the absorption spectra, the samples were purified once with toluene/isopropyl alcohol to get rid of excess OLA and other organic impurities. Structural and Elemental Characterization. XRD samples are prepared by drop casting a layer of the desired core or core−shell material from a hexane:heptane (80:20) solution on a glass substrate. Measurements were performed on a ARL XTRA Powder Diffractrometer. Transmission electron microscopy (TEM) images were recorded on an aberration corrected JEOL 2200-FS operated at 200 kV. High angle annular dark field (HAADF) images were recorded in scanning TEM mode with a spot size of 0.7 nm and a camera length of 60 cm. The average composition of the core/shell QDs has been determined by Rutherford backscattering spectrometry using a 1.57 MeV 4He+ beam. The energy of the backscattered ions was measured with a standard PIPS detector positioned at a scattering angle of 165.2°. From the integrated intensity of the S, Cd and Pb signals, respectively, the amount of deposited materials (hence the QD composition) was directly determined according to the Rutherford equation for the scattering cross section. Optical Characterization. For UV−vis absorption spectroscopy, known amounts were taken from reaction aliquots or stock solutions of PbS core and PbS/CdS core−shell QDs dispersed in toluene and kept in a nitrogen-filled glovebox, dried under nitrogen atmoshpere, redispersed in tetracholoroethylene (TCE) and analyzed using a PerkinElmer Lambda 950 UV−vis-NIR spectrophotometer. The same samples were analyzed for photoluminescence using a FL920 Edinburgh Instrument spectrofluorometer using a 450 W xenon lamp for excitation and a Hamamatsu NIR PMT for detection.
exchange of Pb for Cd, the resulting PbX/CdX core/shell QDs have considerable drawbacks. Most notably, shell growth by cation exchange strongly increases sample heterogeneity since core/shell QDs treated in the same reaction mixture can differ in view of the size and shape of the remaining PbX core and its position in the core/shell QDs.29 Because these variations have all a different influence on the optoelectronic properties, this enhanced heterogeneity has hampered the detailed characterization of their physical properties and limits their value for applications, which often need monodisperse samples. Here, we demonstrate an additive, layer-by-layer method for the growth of PbS/CdS core/shell QDs based on room temperature colloidal atomic layer deposition.30 We find that successive exposure of preformed PbS QDs to Cd and S precursors, increases the QD diameter by ≈0.45 nm. On the basis of X-ray diffraction and high-resolution transmission electron microscopy, we attribute this to the layer-by-layer addition of a CdS shell. Importantly, opposite from cation exchange methods, the additive shell growth preserves the wellresolved first-exciton transition of the initial PbS core QDs. Monitoring its red shift as a function of layer thickness for differently sized PbS core QDs enables us to delineate the core/ shell boundary and map charge-carrier delocalization in PbS/ CdS QDs. More specifically, we find that PbS/CdS QDs should be seen as heterostructures containing a stoichiometric PbS core, even if that initial core QD has a cation-enriched surface. In addition, we demonstrate that delocalization of photoexcited carriers over the CdS shell only occurs for PbS/CdS QDs with core diameters smaller than ≈4 nm, a result that agrees with predictions based on effective-mass calculations. In spite of this type 1 core/shell structure, PbS/CdS QDs exhibit no systematic increase of their photoluminescence quantum yield with increasing CdS shell thickness. This suggests that defects at the PbS/CdS interface, possibly induced by the c-ALD process, are important nonradiative recombination centers.31
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EXPERIMENTAL SECTION
Chemicals. Lead oxide (PbO, 99.99%) and oleic acid (OA, technical 90%) were obtained from Alfa-Aeser. Tetracholoroethylene (TCE, 99.0%, ACS reagent), formamide (99.5%, ACS reagent), ammonium sulfide (20% solution in water), cadmium acetate dihydrate (CdAc2)·2H2O, 98%) and diethylene glycol dimethyl ether (diglyme, anhydrous, 99.5%) were obtained from Sigma-Aldrich. Oleylamine (OLA, 80−90%) was obtained from Acros Organics. All the solvents like toluene, methanol and isopropyl alcohol were purchased from VWR. PbS Synthesis. PbS cores quantum dots were prepared by following the procedure of Hendricks et al.32 In brief, Pb-oleate was prepared by heating up 1 mol of PbO and 2.1 mol of oleic acid in dodecane under vacuum. For the synthesis of PbS QDs, 0.6 mmol of Pb-oleate was taken in 9 mL of dodecane and flushed at 110 °C for 1 h. After that, depending on the envisaged core quantum dot size, temperature (95−160 °C) and reaction time were changed and the appropriate thiourea precursor (0.2 mmol) dissolved in 1 mL of diglyme was injected. The reaction was quenched using a water bath. The QDs were purified twice using toluene/methanol, isopropyl alcohol as the solvent/nonsolvent combination. 3.3 and 4.3 nm QDs were prepared by injecting the precursor N,N-diphenylthiourea at 95 °C with a growth time of 2 and 5 min respectively; 5 nm QDs were prepared by injecting the precursor N-(p-methoxyphenyl)-N-dodecylthiourea at 125 °C with a growth time of 2 min; 7.2 nm QDs were prepared by injecting the precursor N-iso-propyl-N-dodecylthiourea at 150 °C with a growth time of 2 min and 9.6 nm QDs were prepared by injecting the precursor N-n-hexyl-N-dodecylthiourea at 160 °C with a growth time of 3 min.
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RESULTS AND DISCUSSION Layer-by-Layer CdS Shell Growth. Additive CdS shell growth around core nanocrystals requires their being introduced in a reaction mixture containing at least a cadmium precursor, such as the cadmium carboxylates that are widely used to grow CdSe/CdS QDs.20,22 In the case of PbS core QDs, however, this induces a rapid replacement of Pb by Cd, which renders the intended growth of a CdS shell around a preformed PbS core all but impossible. Hence the wide use of superseded shell growth as the second best option, where good use is made of this cation exchange reaction to form PbS/CdS QDs by the inward growth of a CdS shell that replaces the outer PbS layers of the original QDs.27,29 On the other hand, it is known that Pb for Cd exchange is a strongly temperaturedependent process that hardly proceeds at room temperature.28 This suggests that the additive formation of PbS/CdS core/ shell quantum dots might be possible by reducing the reaction temperature, eventually to room temperature. Such room temperature CdS shell growth reactions have been proposed before by Ithurria et al. in a process called colloidal atomic layer deposition (c-ALD).30 Here, the successive exposure of CdSe 6954
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Figure 1. (a) Absorption spectra of PbS QD dispersions (black) prior to and (colored) after successive colloidal ALD cycles. In the right part of the graph, spectra have been normalized at the maximum of the λ1S−1S band-edge transition; in the left part of the graph, spectra have been normalized at 500 nm. (b) X-ray diffractograms of (red) the initial PbS QDs and (blue) the same QDs after 3 colloidal ALD cycles. The vertical red lines indicate the diffraction pattern of bulk PbS. The inset shows fits to the respective (220) reflections that attest the slight broadening and shift to larger angles of the diffraction peaks after colloidal ALD.
Figure 2. (a) Bright field TEM image of the initial 7.2 nm PbS QDs. (b) Same QDs as in panel a after 3 successive c-ALD cycles. (c, top) Size histograms of the 7.2 nm PbS QDs after 0, 1, 2 and 3 c-ALD cycles (red, orange, green and blue, respectively). The histograms have been offset vertically and horizontally for clarity where the gridlines extend the respective ticks on the horizontal axis. (bottom) The average QD diameter as a function of the number of c-ALD cycles. (d) HR-TEM image of a single QD after 3 c-ALD cycles. The real space image and the Fourier transform (inset) show a monocrystalline QD without lattice mismatches between core and shell. (e) HAADF-STEM image of the same QDs as in panels b and d indicating that the particles consist of a core with a high atomic number (brighter in image) surrounded by a homogeneous, concentric shell with a lower atomic number and a thickness of 1.50−1.75 nm. (f) Line scan combined with energy dispersive spectroscopy of a single core/shell QD confirming the presence of Pb in the center and Cd at the edges.
cycle leaves the (normalized) absorption spectrum unchanged whereas a step-like absorbance increase at wavelengths slightly below the CdS bulk band gap develops after the second c-ALD cycle. A first confirmation that successive CdS c-ALD cycles result in the formation of PbS/CdS core/shell QDs comes from the X-ray diffractogram recorded prior to and after 3 c-ALD cycles. As shown in Figure 1b, c-ALD cycling result in a slight combined broadening and shift of the diffraction peaks toward larger angles than those of the original PbS QDs, as expected for shelling with a material having, such as CdS, a slightly smaller lattice parameter. A more detailed picture of the outcome of successive CdS cALD cycles follows from a transmission electron microscopy (TEM) study. Figure 2a,b shows bright field TEM images obtained before and after 3 c-ALD cycles. These give clear evidence that the nanocrystal diameter increases without the additional formation of spurious CdS nanocrystals. Size histograms determined from TEM images of the original 7.2 nm PbS QDs and the same nanocrystals after each of 3 c-ALD cycles are given in Figure 2c. These demonstrate a progressive increase of the QD diameter dQD of ≈0.5 nm per cycle, see
nanocrystals to sulfur and cadmium precursors leads to CdS formation by a sequence of self-limiting half reactions, a process recently used to improve the size dispersion of CdS QDs.33 This inspired us to develop a similar process starting from PbS QDs dispersed in toluene, that where successively exposed to formamide solutions containing either ammonium sulfide or cadmium acetate as the respective S and Cd precursors. Figure 1a shows the absorption spectra of a dispersion of 7.2 nm PbS QDs before and after up to three successive CdS cALD cycles. The spectrum of the original PbS QD dispersion features a pronounced first exciton or band gap transition λ1S−1S, attesting the sample’s narrow size dispersion of ≈5%. The first deposition cycle induces a marked red shift of λ1S−1S and the higher energy feature (λ1P−1P) of the PbS absorption spectrum, whereas additional cycles leave λ1S−1S and λ1P−1P almost unchanged. Moreover, the different deposition cycles hardly broaden the λ1S−1S peak, whereas the λ1P−1P feature becomes even more pronounced. Similar observations, i.e., that surface treatment can affect the oscillator strength of specific electronic transitions, have been made for Cd-based systems.4 At short wavelengths on the other hand, the first CdS c-ALD 6955
DOI: 10.1021/acs.chemmater.6b02638 Chem. Mater. 2016, 28, 6953−6959
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Chemistry of Materials Figure 2c, a figure in reasonable correspondence with the CdS lattice parameter of 0.59 nm. In high resolution TEM, the nanocrystals obtained after 3 c-ALD cycles exhibit a single coherent lattice image. Although both materials have a different crystal structure, rock salt for PbS and zinc blende for CdS, they share an almost identical sublattice of sulfur atoms. Similar to what is seen with PbS/CdS, PbSe/CdSe or PbTe/CdTe core/ shell QDs made using cation exchange, we find that this sublattice can form one coherent structure across the entire core/shell nanocrystals.27,29 This core/shell structure shows up more clearly in a dark field scanning TEM high angle annular dark field (HAADF) image, which features a concentric structure of a bright core surrounded by a 1.0−1.5 nm dark ring, see Figure 2d,e, respectively. Such a structure is expected for PbS/CdS core/shell nanocrystals, an interpretation that is confirmed by elemental analysis along the diameter of such a nanocrystal. As indicated in Figure 2f, such a line scan shows Cd and Pb to be present at the edges and in the center of the nanocrystal, respectively. We thus conclude that the proposed c-ALD process leads to the formation of PbS/CdS core/shell quantum dots by additive shell growth. Locating the Core/Shell Interface. A property of PbS/ CdS core/shell quantum dots most convenient for analyzing their structure is that the size of the PbS core (dcore) can be derived from the peak wavelength λ1S−1S of the first exciton transition by means of the band gap vs size relation of PbS core-only QDs.34 We can thus immediately obtain a PbS core diameter from absorption spectra as the ones shown in Figure 1a. Figure 3 represents the thus calculated dcore as a function of
hole wave function in the shell. This interpretation, however, is unsatisfactory for the PbS/CdS system studied here. Delocalization leads to a red shift that gradually levels off with increasing shell thickness. This is markedly different from the behavior of PbS/CdS, where the red shift abruptly stops after the first c-ALD cycle and, more in general, it would preclude a one-to-one relation between core size and band gap transition as seen with PbS/CdS core/shell QDs.34 The increase of dcore by 0.8 nm as estimated from the shift of λ1S−1S after the first cALD cycle, however, corresponds reasonably well to the PbS lattice parameter of 0.59 nm. This suggests that a monolayer of PbS rather than CdS is in effect grown around the original core QDs in this first cycle. Although the formation of PbS during the first c-ALD cycle seems contradictory because no Pb is added during the process, it can be understood from the surface termination of the PbS core QDs. Similar to many metal sulfide and metal selenide nanocrystals, their surface will be terminated by excess cations as schematically shown in Figure 4.36 It has been explicitly shown in the case of CdSe QDs that removal or addition of such excess cations hardly influences the QD band gap,4 which implies that λ1S−1S should be seen as a characteristic of the stoichiometric core. From this point of view, the first c-ALD cycle has a double effect as represented in Figure 4. The ammonium sulfide added first reacts with the excess surface Pb2+ to form a stoichiometric PbS QD, hence the increase of dcore by approximately the PbS lattice parameter, whereas the subsequent exposure to cadmium acetate leads to a termination of the thus formed QD by excess Cd. The same sequence of events then forms the actual CdS shell as from the second cALD cycle as depicted in Figure 4c. Importantly, this interpretation is further supported by the variation of the absorption spectra after each c-ALD cycle shown in Figure 1. Here, the first c-ALD cycle only leads to a red shift of λ1S−1S without any qualitative change of the absorbance in the visible whereas the second c-ALD cycles has no effect on λ1S−1S yet leads to the appearance of CdS absorption at wavelengths shorter than the CdS band gap. To support this interpretation, we analyzed the composition of the original 7.2 nm PbS QDs and the concomitant PbS/CdS QDs after each of 3 successive c-ALD cycles using Rutherford backscattering spectrometry (RBS). Normalizing the different RBS spectra to the intensity of the lead backscattering signal at 1.46 MeV, Figure 5 shows that c-ALD leads to an additional Cd-related backscattering signal that progressively increases in intensity. Moreover, both the initial PbS core and all PbS/CdS core/shell QDs are cation rich, with an average (NCd + NPb):NS ratio of 1.25. After the first c-ALD cycle, on the other hand, a NPb:NS ratio of 1 is measured. This corroborates the hypothesis that this first cycle turns the original Pb-rich PbS QDs into larger, stoichiometric PbS QDs terminated by an excess of surface Cd, which is only transformed into CdS after the second c-ALD cycle. Importantly, these findings answer the more general question as to how the nonstoichiometry of a core/shell QD should be attributed to core and shell.34 Considering all possibilities, including the stoichiometric core/nonstoichiometric shell, the nonstoichiometric core/stoichiometric shell and any inbetween combinations, the observations made here indicate that such core/shell QDs should be seen as nanoheterostructures with a stoichiometric core, wrapped by a nonstoichiometric shell where nonstoichiometry comes from excess metal cations at the nanocrystal outer surface.
Figure 3. Variation of the PbS core diameter with changing colloidal ALD cycles for differently sized PbS quantum dots. Following ref 34, core diameters are estimated from the band-edge absorption maximum λ1S−1S using the sizing curve for PbS quantum dots.
the c-ALD cycle for four PbS QD samples with initial diameters of 7.2, 5.1, 4.3 and 3.2 nm, respectively. Especially for the three samples with the largest diameter, we find that dcore markedly increases after the first c-ALD cycle to stay put during the following cycles. Although this merely reflects the already discussed shift of the absorption spectra, the absence of any further red shift after the first c-ALD cycle confirms the point that for PbS/CdS core/shell QDs, λ1S−1S is determined by the diameter of the PbS core. More intriguing is the initial increase of the PbS core diameter, which amounts to ≈0.8 nm regardless of the initial core QD size. The underlying red shift of the first-exciton transition upon shell growth is not uncommon, typical examples include CdSe/CdS20 and InP/ZnSe core/shell QDs,35 and is typically attributed to a relaxation of quantum confinement by the delocalization of either the electron or the 6956
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Figure 4. (a−c) Cartoon representation of (a) an initial PbS core QD, including a Pb-surface excess and (b,c) the effect of the first 2 colloidal ALD cycles leading (b) to an enlarged PbS core with a Cd-rich outer surface and (c) the formation of a PbS/CdS core/shell structure with a Cd-rich outer surface. Note that the cartoon does not take the different crystal structure of core and shell into account.
Figure 6. Carrier delocalization in PbS/CdS (a) 2D map showing the probability to find both electron and hole in the PbS core, for varying core radius and shell thickness. The vertical dashed lines indicate the starting cores used in this work. (b) Probability to find the electron (blue) or hole (red) in the shell region of a core/shell system based on a 3.2 (solid) and 7.2 nm (dashed) core.
Figure 5. Background corrected RBS spectra of (light to dark) 7.2 nm PbS core QDs and PbS/CdS QDs after each of 3 successive c-ALD cycles, with the backscattering signals attributed to sulfur (green), cadmium (red) and lead (blue). All signals are normalized to the intensity of the respective lead backscattering at 1.46 MeV and a horizontal offset is used for clarity. The inset shows the NPb:NS (blue) and NCd:NS (red) ratios for each sample with the full line indicating the 1:1 stoichiometry and the dashed line the total (NPb + NCd):NS stoichiometry of 1.25:1 found as an average for all 4 samples. The error bars are mainly due to the uncertainty on the sulfur backscattering intensity as a result of its small scattering cross section.
smallest core size (dcore = 3.3 nm) used here, a drop that only stabilizes at a shell thickness of approximately 2 CdS monolayers. The larger the PbS core, the less pronounced this overlap reduction and the more rapid it turns shellthickness independent. This trend is clearly in line with the experimental observations, where we see no shift of λ1S−1S with increasing shell thickness, apart from the second ALD cycle for the smallest core QD. Figure 6b shows the probability, plotted on a log scale, to find the electron and hole in the shell region of the PbS/CdS system for the 3.2 and 7.2 nm cores. One sees that charge carrier delocalization comes mainly from the electron, which is linked to its smaller effective mass in CdS. Again, the figure confirms the more substantial electron spillover for the smaller core, in line with the reduced electron/hole overlap shown in Figure 6a. As such, this work confirms in a direct manner that PbS/CdS core/shell QDs show a type 1 band alignment with little charge carrier delocalization.34 For one thing, this implies that additive CdS shell growth could be an efficient strategy to enhance the photoluminescence quantum yield of PbS core QDs, while preserving their initial line width. Photoluminescence of PbS/CdS Quantum Dots. Figure 7a−d shows the photoluminescence (PL) spectra of PbS cores of four different sizes and the PbS/CdS core/shell QDs made
Charge-Carrier Delocalization in PbS/CdS Quantum Dots. A closer look at Figure 3 shows that for the dcore = 3.3 nm PbS QD sample, the calculated core diameter shows a slight but reproducible further increase after the second c-ALD cycle. This suggests that carrier delocalization may become more important in PbS/CdS for smaller PbS cores. This is a wellknown quantum mechanical effect, particles confined in finite potential wells show enhanced wave function spillover the more narrow the well, and we further explored the possibility of carrier delocalization by means of effective mass calculations on concentric, spherical PbS/CdS core/shell QDs. Figure 6a shows the results as a 2D map of the probability to find both electron and hole in the core of the core/shell system as a function of the core diameter and the shell thickness, where the vertical dashed lines indicate the respective diameters of the starting cores used in this work. One sees that for thicker CdS shells, this joint probability markedly decreases for core diameters below ≈4 nm. Hence, this joint probability shows a significant drop with increasing CdS shell thickness for the 6957
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in the shell, access of electrons to the CdS outer surface is therefore unlikely to limit the PLQY of CdS-shelled QDs made by c-ALD. This suggests that a more important limiting factor is the quality of the PbS/CdS interface. More specifically, the marked difference between processing under protective atmosphere and ambient conditions and the possibility to enhance the PLQY by thermal annealing suggests that, e.g., oxides, vacancies or stacking faults may be incorporated at the PbS/CdS interface during the c-ALD process. A similar observation was made in the case of CdSe nanoplatelets that were grown thicker using c-ALD.31 Moreover, this interpretation is in line with the leveling off of the PLQY after the second c-ALD cycle, when the CdS shell is most likely complete and further CdS growth no longer affects the PbS/CdS interface.
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CONCLUSION We have demonstrated the additive growth of CdS shells around PbS core QDs by implementing a room temperature colloidal ALD process. This approach prevents the additional sample heterogeneity that is induced by cation exchange reactions. As a result, PbS/CdS core/shell QDs can be formed that largely preserve their pronounced spectral features in absorbance and photoluminescence and whose band-edge transition covers a wavelength range from about 1.0 to 2.5 μm. Having such well-defined PbS/CdS QDs at hand, we show that such overall nonstoichiometric heteronanostructures should be conceived as composed of a stoichiometric core and a nonstoichiometric shell featuring surface-bound excess cations. In addition, we demonstrate that PbS/CdS core/shell QDs exhibit little charge carrier delocalization in the CdS shell, with only a minor red shift of the spectral features relative to core PbS QDs for core diameters smaller than ≈4.0 nm. Nevertheless, we have no evidence that CdS shell growth by colloidal ALD systematically enhances the photoluminescence quantum yield. Apart from an initial increase for larger PbS QDs, the PLQY drops during CdS shelling to a value that stays put after the second c-ALD cycle. This suggests that rather than the CdS outer surface, nonradiative recombination is mediated by defects at the PbS/CdS surface.
Figure 7. Normalized photoluminescence of PbS core QDs and the subsequent PbS/CdS core/shell QDs after photoexcitation at 650 nm for different core sizes (a) 3.3, (b) 5.2, (c) 7.24 and (d) 10 nm exposed to (red, blue, green) 1, 2 and 3 c-ALD cycles, respectively. The inset shows the evolution of the quantum yield relative to that of the core QDs as a function of c-ALD cycle.
thereof after successive c-ALD cycles. Importantly, these results are only obtained when colloidal ALD shelling is fully executed under protective atmosphere; the same processing under ambient conditions hardly yields any photoluminescence at all (see Supporting Information S1). In line with the change of the absorbance spectrum after each of the different cycles, only the first c-ALD cycle leads to a pronounced red shift in all cases with only a limited broadening of the originally narrow photoluminescence. Moreover, CdS shelling hardly affects the energy difference between the maximum of the exciton absorption and emission (Stokes shift, see Supporting Information S2). Using the PbS sizing curves, the initial red shifts again correspond to an apparent increase of the core diameter by ≈0.8 nm. The insets in Figures 7a−d represent the photoluminescence quantum yield (PLQY) of all PbS/CdS core/shell QDs, relative to that of their respective PbS core QDs. Similar to PbS/CdS core/shell QDs grown using cation exchange, it appears that CdS shell growth does not lead to a systematic increase of the PLQY. Only for the two largest PbS cores, an increased PLQY is obtained after the first colloidal ALD cycle. The second cycle, on the other hand, results in a PLQY systematically lower than the original PbS QDs and no further changes occur after the third cycle. As shown in the Supporting Information (see Supporting Information S3), annealing the PbS/CdS dispersions at 100 °C can markedly enhance the PLQY without inducing major changes in the absorbance and photoluminescence spectra. Opposite from the PbS/CdS QDs synthesized here, CdS shell growth by c-ALD around CdSe core QDs results in a progressive PLQY increase similar to what is achieved by high temperature SILAR procedures.30 Since CdSe/CdS QDs exhibit a considerable leakage of the electron wave function
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02638. Information on the photoluminescence under ambient and inert conditions, Stokes shift of PbS/CdS core/shell QDs, annealing of the core/shell interface (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Z. Hens. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Z.H. acknowledges support by the European Comission via the Marie-Sklodowska Curie action Phonsi (H2020-MSCA-ITN642656), the Belgian Science Policy Office (IAP 7.35, photonics@be), IWT-Vlaanderen (SBO-MIRIS) and Ghent University (GOA no. 01G01513) for funding. P.G. acknowledges the Flemish FWO for a postdoctoral fellowship. 6958
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DOI: 10.1021/acs.chemmater.6b02638 Chem. Mater. 2016, 28, 6953−6959
