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. 2020 Mar 5;20(4):2387–2395. doi: 10.1021/acs.nanolett.9b05020

ZnSe/ZnS Core/Shell Quantum Dots with Superior Optical Properties through Thermodynamic Shell Growth

Botao Ji †,‡,§, Somnath Koley †,‡, Ilya Slobodkin †,‡, Sergei Remennik ‡, Uri Banin †,‡,*
PMCID: PMC7467768  PMID: 32134676

Abstract

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Epitaxial growth of a protective semiconductor shell on a colloidal quantum dot (QD) core is the key strategy for achieving high fluorescence quantum efficiency and essential stability for optoelectronic applications and biotagging with emissive QDs. Herein we investigate the effect of shell growth rate on the structure and optical properties in blue-emitting ZnSe/ZnS QDs with narrow emission line width. Tuning the precursor reactivity modifies the growth mode of ZnS shells on ZnSe cores transforming from kinetic (fast) to thermodynamic (slow) growth regimes. In the thermodynamic growth regime, enhanced fluorescence quantum yields and reduced on–off blinking are achieved. This high performance is ascribed to the effective avoidance of traps at the interface between the core and the shell, which are detrimental to the emission properties. Our study points to a general strategy to obtain high-quality core/shell QDs with enhanced optical properties through controlled reactivity yielding shell growth in the thermodynamic limit.

Keywords: ZnSe/ZnS, core/shell QDs, heavy-metal-free, thermodynamic, kinetic

Colloidal quantum dots (QDs) are exceptional fluorescence emitters manifesting continuous color-tunability and narrow emission line widths.1,2 These properties, combined with the solution-processability of QDs, are the basis for their widespread implementation as building blocks in optical and optoelectronic applications including in commercial displays, lasing, light-emitting diodes (LEDs), and bioimaging that require high photoluminescence (PL) quantum yields (QY) and photostability.3−9 For over two decades, the efficient strategy to achieve these stringent demands has been through the formation of core/shell structures.10,11 A type-I band alignment, in which the semiconductor shell material with a larger band gap straddles the core semiconductor band gap, is the typical architecture that confines both electron and hole in the core and alleviates nonradiative decay via trap sites at the QD surface.12 Typically, the lattice mismatch between the core and the shell semiconductors needs to be small, so that the epitaxial shell growth can take place to passivate the surface of the core effectively without inducing interfacial defects.12,13 Large lattice mismatch actually limits the ability to achieve epitaxial growth of a thick shell and often other architectures may form.14−17

Various approaches were introduced to achieve high quality shell growth. Among these are the growth of alloyed shells, and shells with graded composition designated to relax the strain upon increasing shell thickness.18−23 These are particularly well developed for the Cd-chalcogenide semiconductor QDs. However, considering heavy-metal-free Zn-chalcogenides, the choice of suitable high bandgap semiconductor shell materials becomes much more limited. For instance, in order to obtain highly fluorescent ZnSe QDs, ZnS is the typical choice as the shell material to form type-I band alignment (Figure 1a).24−27 This emphasizes the need for additional high-quality shell growth strategies.

Figure 1.

Figure 1

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(a) Potential energy scheme for ZnSe/ZnS core/shell QDs.27 (b) Schematic illustration of the controlled shell growth of ZnS on a spherical ZnSe QD. Stage I, initial ZnS shell growth produces ZnSe/ZnS QDs with high QY when below a critical thickness. Stage II, ZnSe QDs with thick ZnS shells grown in a kinetic regime display decreased QY versus maintained high QY for shell growth in a thermodynamic regime. The thermodynamic growth is realized by using Zn precursor with low reactivity. Dependence of absorption spectra (c) and emission spectra (PL QY indicated) (d) upon shell thickness (in monolayers, MLs) when using zinc oleate (molar ratio of Zn/oleic acid, 1/10) as shell precursor for thermodynamic growth. (e) TEM image of the 4.0 nm ZnSe core QDs. (f) TEM image of ZnSe/ZnS core/shell QDs with 4 MLs of ZnS shell.

We have recently shown that the use of zinc oleate with low reactivity (molar ratio of Zn/oleic acid shell precursor of 1/10) led to unique growth of a helical shell of ZnS on ZnSe nanorods under thermodynamic growth conditions.28 The helical shell morphology allows one to maintain high quality band gap emission from the rods even upon growth of thicker shells, consistent with avoidance of creation of defects at the core/shell interface, which leads to carrier trapping and quenching band gap emission. Lowering the Zn/oleic acid molar ratio to 1/6.3 (1/4) transformed the shell morphology of ZnS to islands-shell (flat-shell), as the growth conditions were changed gradually from thermodynamic into kinetic control. In the case of the islands-shell growth, growth of a wetting layer with a critical thickness of ∼3.3 MLs was identified before the islands appeared on the surface of the nanorods. The morphology of the islands-shell case also enables avoidance of interfacial traps, much better than the case of the flat-shell growth, but the helical-shell morphology offers the optimal effective interfacial passivation.

This prior investigation of the shell growth mechanism on nanorods revealed the advantageous pathway to avoid interfacial trap formation while using the slow thermodynamic growth conditions. We hence perform the present study to implement such strategy for deposition of a coherent thick shell on strongly confined spherical QDs in order to achieve heavy-metal-free core/shell QDs with superior optical properties. This is particularly challenging and important for the Zn-chalcogenides nanocrystals system considering the need for stable blue colloidal QD emitters which are required for next generation electroluminescent display applications.

Herein, we address the highly important issue of the effects of shell growth rate conditions on the resultant core/shell QDs. As identification of the shell morphology is less obvious for the shells on spherical QDs compared with rods, the effect of the shell material growth rate was largely overlooked thus far. We apply the insights gained from our investigation of shell growth rate about shell morphology on nanorods and study the effect of growth conditions of ZnS shell on the structure and optical properties of ZnSe/ZnS core/shell QDs as a model system. It is shown that the growth of a ZnS shell under thermodynamic conditions, realized by using shell precursors with low reactivity, yields significantly improved emission properties compared with core/shell QDs grown under kinetic control. Under kinetic growth conditions, beyond a critical thickness, the fluorescence of ZnSe/ZnS QDs is gradually quenched. However, with thermodynamic growth conditions a high PL QY is well maintained (Figure 1b). This is also revealed by single-dot measurements demonstrating that the fraction of “ON-time” is significantly higher in the thermodynamic growth case, even for QDs with similar PL QYs. This clearly indicates that thermodynamic growth conditions can be employed to efficiently passivate the interface and thus avoid the formation of interfacial defects in core/shell QDs. The approach may be applied to additional core/shell QD architectures especially when a thick shell is required for enhanced stability and resilience against environmental and solvent effects affecting the QD surface.

ZnSe core QDs were synthesized through coinjection of Zn and Se precursors into a hot solution of 1-octadecene and oleylamine (see Supporting Information).29 Feeding additional Zn and Se precursors results in larger ZnSe QDs (Figure S1). The synthesized core ZnSe QDs with different sizes displayed low PL QY (<1%) due to the incomplete surface passivation by the organic ligands, accentuated by the large band gap that encompasses in-gap surface trap states (Figure 1). After purification, the ZnSe core QDs were redispersed in a mixture of 1-octadecene, oleylamine, and oleic acid and the growth of the ZnS shell was performed by continuous injection of zinc oleate and 1-octanethiol at elevated temperature (310 °C).30 In order to control the growth regime, changing over from kinetic to thermodynamic limits, zinc oleate precursors with varying reactivity were utilized. For suppressed reactivity required for the thermodynamic regime, a large excess of oleic acid compared to zinc was used (molar ratio of Zn/oleic acid, 1/10) thus limiting zinc availability by increasing the solubility of zinc oleate.31

Figure 1c,d shows the evolution of the absorption and PL spectra during the ZnS shell growth on the ZnSe cores (diameter, ∼4.0 nm), respectively. For the ZnSe core QDs, the absorption manifests resolved transitions indicating their monodispersity, as confirmed by the corresponding TEM image (Figure 1e). When one monolayer (ML) of ZnS was deposited on the ZnSe cores, a slight red shift of the first excitonic transition is observed in the absorption spectrum and the emission peak shifts from 412 to 419 nm, accompanied by a significant increase of PL QY from <1% to ∼60%. The PL QY reached 85% as the shell thickness increased to 2 MLs and maintained high levels of at least 85% upon further ZnS shell growth. During this process, the absorption cross section at energies higher than the ZnS band gap (3.75 eV, 331 nm) gradually increased, further indicating the growth of the ZnS shell (Figure 1c). Meanwhile, the profile of the emission spectra remained nearly unchanged. The core/shell QDs with all shell thicknesses display a narrow photoluminescence peak with full width at half-maximum (fwhm) smaller than 18 nm. Correspondingly, the size distribution of ZnSe/ZnS QDs remains narrow during the entire shell growth process, as revealed by TEM images (Figures 1f and S2). X-ray diffraction (XRD) measurements show that ZnSe/ZnS QDs inherit the zinc-blende crystal structure of ZnSe QDs. Upon growth of the ZnS shell, all the diffraction peaks shifted to higher angles due to the smaller ZnS lattice constant compared with ZnSe, an indication of epitaxial shell formation (Figure S3).

The implications of the thermodynamic shell growth regime are emphasized upon comparison with ZnS shell growth on ZnSe QDs by using more reactive zinc oleate precursors. This was achieved by using zinc oleate with reduced excess of oleic acid (1/6.3 molar ratio of Zn/oleic acid, Figures S4 and S5 for TEM images and XRD measurements, respectively). Figure 2a presents the comparison of the growth yield of ZnS when using zinc oleate with the two different molar ratios of Zn/oleic acid. For the 1/6.3 case, the initial growth yield was larger until the shell thickness reached 3.1 MLs. At this point, the PL QY of the ZnSe/ZnS QDs reached the maximum value of ∼85%. Remarkably, this shell thickness is similar to the critical thickness identified for the growth of ZnS islands-shell on ZnSe nanorods (∼3.3 MLs).28 After this critical thickness, the PL QY in the 1/6.3 case started to decrease indicating the occurrence of interfacial strain-related defects, whereas in the 1/10 case the high QY was maintained upon further shell growth. Finally, the QY of ZnSe/ZnS QDs with 7 MLs of ZnS shell when using 1/10 molar ratios of Zn/oleic acid is significantly higher than that when using the molar ratio of 1/6.3, albeit analogous shell thickness is seen, and it is also difficult to identify a different morphology of the obtained ZnSe/ZnS QDs (Figure 2c,d). Additionally, we have compared the time-resolved PL decays for thick shell (∼7 MLs) QDs grown under kinetic and thermodynamic conditions. As shown in Figure S6, the PL decay in the case of thermodynamic particles appeared biexponential and was dominated by a ∼ 6.2 ns component, which is rational for type-I ZnSe/ZnS QDs along with a longer decay component (∼16.6 ns). In the kinetic growth, the particles exhibit an additional short component (∼1.4 ns) along with a 6.5 ns component and a significantly long component (∼30 ns). The appearance of an explicit short component in kinetic growth further emphasizes the abundance of interfacial traps, which is in line with the PL QY decrease.

Figure 2.

Figure 2

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(a) Dependence of ZnS shell thickness grown on 4.0 nm ZnSe core as a function of the volume of shell precursor solution added. Error bars are presented by measuring the size of ∼200 QDs in one typical synthesis. (b) Dependence of PL QY of ZnSe/ZnS core/shell QDs on ZnS shell thickness. Molar ratio of Zn/oleic acid: black, 1/6.3; blue, 1/10. The error bars are extracted from multiple syntheses. (c,d) TEM images of ZnSe/ZnS core/shell QDs with 7 MLs of ZnS shell when using 1/6.3 (c) and 1/10 (d) molar ratios of Zn/oleic acid. The inset in (d) shows corresponding image of ZnSe/ZnS QDs under UV illumination.

Increasing the Zn/oleic acid ratio even further to 1:4 leads to a more drastic quenching effect of the PL QY with shell thickness (Figures S7 and S8). Hence, the ZnS shell growth when using molar ratios of Zn/oleic acid of 1/10 is suggested to take place within a thermodynamic shell growth regime, whereas the 1/6.3 ratio and lower is within the kinetic regime as will be further established and clarified below.

Further indications of the improved optical properties of core/shell QDs grown in the thermodynamic versus the kinetic regimes were obtained from single QD fluorescence studies (see Supporting Information and Figure S9 for details). Figure 3 shows typical single QD emission intensity trajectories for both kinetic (black line) and thermodynamic (blue line) growth conditions of the final core/shell QDs with similar shell thickness of 7 MLs. A bimodal fluorescence intensity distribution is seen in both samples, manifesting single-QDs blinking behavior randomly transitioning between ON- and OFF-states. The average ON-time fraction measured over 60 QDs of each type is ∼37% for the sample with shell growth under kinetic conditions and increases significantly to ∼60% under thermodynamic growth conditions (Figure 3). A larger ON-time fraction is already observed for samples with 4 MLs shell thickness, where the kinetically grown core/shell QDs also still have a high QY equal to that of the thermodynamically grown core/shell QDs (28% versus 42% ON-time fraction, respectively, Figure S10). In this case, the significantly harsher condition for the QDs blinking measurement than ensemble PL QY measurements is inferred to be the main reason for the different ON-time fractions of ZnSe/ZnS QDs, although they possessed similar PL QY. First, QDs are prone to be oxidized under excitation although they are embedded within a polymer matrix. More importantly, the Auger ionization process becomes affected especially when single QDs are continuously excited.32 The thermodynamically grown ZnSe/ZnS QDs manifest higher ON-time fraction due to the optimized shell growth, clearly demonstrating the advantage of thermodynamic growth conditions.

Figure 3.

Figure 3

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Typical fluorescence intensity time traces for single ZnSe/ZnS QDs with 4.0 nm core in a polymer matrix under continuous wave excitation. (a,b) Time traces for ZnSe QDs with 7 MLs of ZnS shell in the kinetic regime (a) and corresponding ON-time fraction distribution (b). (c,d) Time traces of ZnSe QDs with 7 MLs of ZnS shell in the thermodynamic regime (c) and corresponding ON-time fraction distribution (d). The ON-time fractions from single QD were extracted by defining a threshold intensity (red dotted line) creating binary statistics. Sixty particles were measured for each sample to extract the distribution of ON-times.

QDs’ blinking is typically assigned to a stochastic occurrence of charging in the QDs, leading to a formation of trion states that manifest rapid nonradiative Auger recombination while the QD is in this OFF-state.33−37 The charging may occur at surface traps or at traps at the core/shell interface. The suppression of blinking via growth of the thick ZnS shell is ascribed to the reduced charging on the QD surface, and the clear superior optical performance under the thermodynamic growth conditions manifested at both ensemble and single-particle levels indicates the avoidance of core/shell interfacial traps in this case, which presents a hidden impeding factor for high optical quality in the thick shell limit essential also for stability.38

The merit of the thermodynamic growth regime was further demonstrated by growing a ZnS shell on larger ZnSe core QDs of 5.4 nm in diameter. Such larger cores shift the PL wavelength closer to the blue color desired for display application (450–460 nm). In larger cores, the ability to achieve high performance is further impeded by the reduced electron–hole overlap stemming from the difference in confinement of the electron and hole wave functions owing to mismatch in their effective masses (for the electron me* = 0.21m0, for the heavy hole mhh = 0.6m0 where m0 is the free electron mass).39 Larger cores also have larger absolute interfacial and surface areas introducing further traps. As shown in Figure 4a, ZnSe QDs with 8.6 MLs of ZnS exhibit a bright blue emission at ∼430 nm with a fwhm of ∼18 nm.

Figure 4.

Figure 4

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(a) Absorption (black) and emission spectra (red) of 5.4 nm ZnSe core with ∼8.6 MLs of ZnS shell grown in the thermodynamic regime. The inset shows a corresponding optical image of the ZnSe/ZnS QDs under UV illumination. (b) Dependence of the emission QY of ZnSe/ZnS core/shell QDs on shell thickness. ZnSe core: 5.4 nm. Molar ratio of Zn/oleic acid: black, 1/6.3; blue, 1/10. The shell thicknesses marked with stars are not precise since island growth appears in these points. Error bars of shell thickness and PL QY are obtained by measuring the size of ∼200 QDs in one typical synthesis and QY from multiple syntheses, respectively. (c,d) HAADF-STEM and high-resolution STEM images of ZnSe/ZnS core/shell QDs when injecting 12 mL of shell precursors, molar ratios of Zn/oleic acid: 1/6.3. (e) Overlay mapping of two elements for the single ZnSe/ZnS QDs in (c) based on EDS scan. (f,g) HAADF-STEM and high-resolution STEM images of ZnSe core with ∼8.6 MLs of ZnS shell as shown in (a), molar ratios of Zn/oleic acid: 1/10. (h) Overlay mapping of two elements for the single ZnSe/ZnS QDs shown in (f) based on EDS scan.

For the kinetic growth conditions (molar ratios of Zn/oleic acid, 1/6.3), the PL QY first increased rapidly and reached its maximum of ∼40% when ∼4 MLs of ZnS were grown, close to the critical thickness identified also for the case of the small ZnSe QDs (Figure 4b). Upon further shell growth, a sharp decrease in PL QY was observed, and the morphology of the ZnSe/ZnS QDs started to become irregular, an indication of the shell inhomogeneity (Figure S11). The final morphology manifested an islands-decorated shell, as clearly demonstrated in the high-angle annular dark-field (HAADF) scanning TEM (STEM) image (Figure 4c,d). The ZnSe cores can be easily distinguished from the ZnS shell due to the higher electron scattering intensity of Se atoms compared with S atoms. Energy dispersive spectroscopy (EDS) elemental mapping in STEM indicates that Zn is distributed throughout the QDs, whereas Se and S elements are distributed in the core area and the outer shell, respectively (Figure 4e). The islands-decorated shell resembled the architecture identified in our study of ZnSe/ZnS core/islands-shell nanorods. As reported before, this type of dislocation nucleation of islands can partially release the interfacial strain during the shell growth process.28 The actual thickness of the ZnS shell still increased as more shell precursor was added, leading to accumulation of strain energy until the formation of interfacial defects to relieve the strain, which are detrimental for the PL properties. The existence of interfacial defects was further verified by the time-resolved PL decays. As shown in Figure S12, the PL decay profile of ZnSe/ZnS QDs with islands-decorated shell in Figure 4c synthesized under kinetic conditions consists of an additional short time component (∼1.9 ns), compared to the QDs in Figure 4f synthesized under thermodynamic growth conditions. The appearance of an additional short time component indicates the abundance of nonradiative decay channels associated with interfacial defects. Therefore, islands-shell growth is still not the ultimate thermodynamic favored morphology, although islanding in planar growth is typically thought to be a thermodynamic product.40

For the thermodynamic growth regime (molar ratios of Zn/oleic acid, 1/10), PL QY was enhanced upon the initial ∼3 MLs growth of ZnS to be ∼40% and unlike kinetic growth, maintained this level even when the shell thickness reached ∼8.6 MLs (Figure 4b and S13). However, we were not able to increase the QY beyond this 40% level. The HAADF-STEM images reveal a significant fraction QDs with a well-defined shell structure (Figure 4f,g), which is further confirmed by Se and S elements distributed in EDX elemental mapping in STEM (Figure 4h). Unlike in kinetic growth, this sample displays a relatively regular shape (Figure 4f and S14), indicating that thermodynamic growth conditions enable the coherent shell growth in a more organized way over islands growth, thus resulting in more efficient avoidance of traps at the core/shell interface.

The blinking behavior of single QDs is also greatly improved for the thermodynamic growth conditions as shown in Figure 5 comparing single-QD emission time traces of the two samples with thick ZnS shell. The ON-time fraction of ZnS-islands-decorated core/shell QDs was ∼40%, decreasing from 47% for 4 ML shell thickness, indicating that the kinetic condition of shell growth displayed a negative effect on fluorescence properties upon thicker shell growth (Figure 5a,c and Figure S15). This can be attributed to the strain-induced interfacial defects that cannot be dissipated for relatively fast shell growth. In contrast, a curing effect was observed for thermodynamic growth where minimal blinking with ON-time fraction above 75% is observed for thick shells, increased from 60% for 4 ML shell thickness (Figure 5b,c and S16), emphasizing the clear advantages of thermodynamic growth conditions.

Figure 5.

Figure 5

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Representative time traces of the fluorescence intensity for ZnSe/ZnS QDs with 5.4 nm core and thick ZnS shell grown in (a) kinetic conditions and (b) thermodynamic conditions (ZnS: ∼8.6 MLs). The ON-time fractions from single QDs were extracted by placing a threshold (red dotted line) to define the binary statistics. (c) Distribution of ON-time fraction for kinetically (black) and thermodynamically grown ZnS shell from the blinking data. (d,e) Log–log plot of the probability density of the ON (red circles) and OFF (blue circles) states for the aforementioned QDs with a (d) kinetically and (e) thermodynamically grown shell. The black lines are the best fit to the data by truncated power-law distributions. Thirty particles were analyzed for each sample to extract the ON-times distributions.

The PL intermittency of single QDs was further characterized by examining the statistics of ON- and OFF-times calculated from the blinking traces of ZnSe/ZnS QDs with thick shell (Figure 5a,b). The probability density of the duration of ON and OFF states could be fitted by power-law distributions as Inline graphic, where m is the power-law exponent, describing the statistics of ON and OFF events (Figure S17).41−43 As shown in Figure 5d, QDs with a kinetically grown shell exhibit almost similar value of m for both ON and OFF events with an average value of ∼1.8. However, QDs with thermodynamic shell growth possess the value of 1.4 for mon and 2.0 for moff on average (Figure 5e). The steeper slope of the OFF-time distribution than that of the ON-time distribution indicates that PL blinking is dominated by short OFF events and long ON events.30,44,45 The lower value of mon associated with higher value of ON-time fraction in the case of QDs following thermodynamic limit unambiguously endorses the beneficial effect of the thermodynamic growth over the kinetic one on the optical properties.

As already indicated above, the ensemble PL QY of ZnSe/ZnS QDs with 5.4 nm ZnSe cores reached its maximum with a limit of ∼40% even for the thermodynamic growth conditions, whereas the ON-time fraction from PL time traces for single QDs was found to be very high. This indicates the presence of a substantial fraction of particles with very low emission efficiency, along with a fraction of strongly emissive QDs. Indications for the origins of this low emissive fraction arises from further analysis of the HAADF-STEM images of ZnSe/ZnS QDs with thick 8.6 MLs shell thickness. About thirty-five percent of the QDs exhibit an anisotropic shell morphology with ZnSe cores being off center (as marked in white circles in Figure S18). Careful inspection of ZnSe cores reveals that small 4.0 nm ZnSe cores are close to spherical whereas the larger 5.4 nm ZnSe cores clearly deviate from the close to spherical morphology (Figure S1). The nearly spherical shape of small ZnSe cores affords the efficient passivation of all of the surface of ZnSe cores at the beginning of shell growth in line with the maintained uniform morphology with a narrow size distribution (Figures S2 and S4). The facets are too small to support clear island growth. In contrast, a significant portion of 5.4 nm ZnSe cores already manifests anisotropic morphology, which infers to an indication of exposing different facets and/or stacking faults. Indeed, uniform shell growth on anisotropic nanocrystals such as nanorods is proven to be difficult due to the reactivity difference between the rod apexes and sides facets.46,47 At the initial stage of ZnS shell growth in this case, the preferential growth on some facets over the other and stacking faults formed along the core/shell interface promoted the inhomogeneous shell growth, resulting in inefficiently passivated exposed facets and edges of the cores acting as trap sites. Further uneven ZnS growth eventually results in the formation of an “earlike” shell on the different facets of zinc-blende cores.48 This implies partial coverage of the shell on ZnSe cores, originating from the very beginning of the shell growth and leading to inadequate surface passivation. These QDs with very low emission efficiency did not contribute to the ON-time fraction because they were hardly observed especially in the wavelength range of blue. This is consistent with the observed very high ON-time fraction (∼75% on an average) while the PL QY does not exceed 40%.

Note that the inadequate passivation that forms QDs with very low emission efficiency is different from interfacial traps between the core/shell of islands-decorated ZnSe/ZnS QDs obtained under kinetic growth regime. The islands-shell growth was derived from the interfacial strain whereas the particles with earlike structures under thermodynamic growth regime is a result of preferential growth toward some specific facets or axes. As revealed in Figure S19, thermodynamically grown ZnSe/ZnS QDs with earlike shell (marked in white dashed circles) are substantially different from QDs with islands-shell under kinetic conditions. In general, from the onset of shell growth on the large 5.4 nm cores for both kinetic and thermodynamic conditions, there is already a division into two families of core/shell QDs, one with isotropic shell growth and the second manifesting “ears” growth, leading to incomplete coverage. Correspondingly, from the onset the latter family of QDs does not show PL due to traps on the exposed ZnSe regions and hence, even upon further shell growth the maximal QY does not exceed 40%. In addition to this, the first family with isotropic shell growth shows a different behavior beyond the critical thickness depending on the growth regime. The homogeneous shell deposition on more isotropic 5.4 nm ZnSe cores in thermodynamic and kinetic growth regime gives rise to adequate passivation up to the critical thickness. Further, thicker ZnS shell is grown without inducing interfacial defects only under thermodynamic conditions, producing strongly emissive QDs showing suppressed blinking behavior. Indeed, better control to achieve homogeneous shell growth on the large cores was thus far unsuccessful and remains a challenge.

In summary, we successfully applied the principles of thermodynamic growth of the shell on core QDs to optimize optical properties of core/shell QDs, as presented in the synthesis of ZnSe/ZnS QDs, a Cd-free large bandgap QDs system. The thermodynamic growth realized by reducing the reactivity of shell precursors enables coherent thick shell to be deposited on core QDs without inducing interfacial defects. Under such conditions, we have successfully synthesized high-quality blue-emitting ZnSe/ZnS QDs with high ensemble PL QY and narrow emission line-width. Notably, ZnSe QDs (5.4 nm) with a thick ZnS shell (∼8.6 MLs) exhibit a strongly suppressed emission blinking with an average ON-time fraction of ∼75% which holds great promise in potential optoelectronic and biological imaging applications. We envision that the principles of the thermodynamic shell growth can be expanded to other QDs for the synthesis of high-quality core/shell QDs to avoid interfacial defects. More broadly, these insights provide a promising general strategy to obtain core/shell QDs with superior properties.

Acknowledgments

We thank Dr. Jiabin Cui from The Institute of Chemistry at the Hebrew University for the assistance in the materials characterization. The research has received funding from the Israel Science Foundation (ISF, Grant 1363/18) in the frame of the Alternative Fuels Program. S.K. acknowledges the support from the Planning and Budgeting Committee of the higher board of education in Israel through a fellowship. U.B. thanks the Alfred & Erica Larisch memorial chair.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.9b05020.

  • Additional details and figures (PDF)

Author Contributions

∥ B.J. and S.K. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

nl9b05020_si_001.pdf (4MB, pdf)

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