Morphology Evolution of Gradient-Alloyed CdxZn1âxSeyS1ây@ZnS

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Morphology Evolution of Gradient-Alloyed CdZn SeS @ZnS Core Shell Quantum Dots during Transmission Electron Microscopy Determination: A Route to Illustrate Strain Effects Jialun Tang, Sheng Huang, Zhaohan Li, Huaibin Shen, Zhao Lv, and Haizheng Zhong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12375 • Publication Date (Web): 01 Feb 2018 Downloaded from http://pubs.acs.org on February 3, 2018

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The Journal of Physical Chemistry

Morphology Evolution of Gradient-Alloyed CdxZn1xSeyS1-y@ZnS

Core Shell Quantum Dots during

Transmission Electron Microscopy Determination: A Route to Illustrate Strain Effects Jialun Tang,† Sheng Huang,† Zhaohan Li,‡ Huaibin Shen,‡ Zhao Lv,† and Haizheng Zhong*,† † Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, 5 Zhongguancun South Street, Haidian District, Beijing, 100081, China ‡ Key Laboratory for Special Functional Materials of Ministry of Education, Henan University, Kaifeng, 475004, China Corresponding Author * E-mail: [email protected]

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ABSTRACT. In this work, we reported the morphology evolution (formation of voids & size reduction) of gradient-alloyed CdxZn1-xSeyS1-y@ZnS quantum dots under electron irradiation during transmission electron microscopy observation. By investigating the correlations between shell gradients and morphology evolution, the formation of voids can be explained by the continuous electron irradiation induced atomic movement under interfacial strain. On the other hand, the size reduction can be attributed to the elastic scattering enabled sputtering of surface atoms. The as-formed voids of CdxZn1-xSeyS1-y@ZnS quantum dots with CdS rich cores are much larger than that of ZnSe rich ones and the sizes of voids decreased with the increasing of shell thickness. The comparison of the morphology evolution of CdxZn1-xSeyS1-y@ZnS core shell quantum dots with different composition gradients demonstrated that the size and shape of asformed voids illustrate the strain characteristics of shell gradient. This provides a guideline to understand the strain effects in gradient-alloyed core shell quantum dots through transmission electron microscopy measurement. We believe the deep insights into gradient-alloyed CdxZn1xSeyS1-y@ZnS

core shell quantum dots would push forward their optimization toward

commercialized light-emitting technology.


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1. INTRODUCTION In the last two decades, colloidal quantum dots (QDs) have been widely investigated as emissive materials for many light-emitting applications, and now have been successfully applied in the backlights for liquid crystal display (LCD) technology through a photoluminescence (PL) mechanism.1 Meanwhile, QD based light-emitting didoes (LEDs) based on electroluminescence (EL) mechanism are also on the dawn of commercialization.2 Although many materials have been explored, most of the efficient EL and PL devices still rely on the well-developed CdSe QDs.3-5 Especially, CdxZn1-xSeyS1-y@ZnS core shell QDs are prime candidates in current lightemitting applications.6 In conventional semiconductor devices, strain induced by lattice mismatch has been well known to affect the device performance.7,8 Accordantly, strain effects are more pronounced for core shell QDs than that of their bulk counterparts, which significantly influence their PL properties and stability.9-12 Alloyed CdxZn1-xSeyS1-y@ZnS core shell QDs with a chemical composition gradient, reduce the lattice mismatch as well as interfacial strain effects, providing alternative materials for light-emitting applications.13 For example, Yang et al. achieved optimized performance and stability of QD-LEDs by controlling the shell gradients.14 This was also confirmed in the laser15 and display exploration.16 Although the CdxZn1-xSeyS1y@ZnS

QDs are expected to show reduced strain effects, the shell gradients should be rational

designed to achieve non-blinking samples toward efficiency and stable QD-LEDs.17 From the viewpoint of materials optimization, there is urgent need to study the correlations between strain and shell gradients. Therefore, a rising question is how to determine the interfacial strain between core and shell in colloidal QDs. In particular, the shell gradients in alloyed QDs contain a strain distribution, which is more difficult for theoretical calculation and experimental determinations.18,19


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In bulk materials, the interfacial strain in heterostructures have been investigated by applying PL20 and Raman measurements.8,21 Although the spectroscopic measurements are convenient and accurate in micrometer scale, it is of great challenge to achieve spatial resolution at nanometer scale because of the limitation of light wavelength and focusing property. X-ray diffraction (XRD) could be effective to determine the strain effects in nanoscale materials, however, the analysis was usually limited by the intrinsic broaden peaks due to the small size and gradient compositions.22,23 This hindered the strain study in core shell QDs. High-resolution transmission electron microscopy (TEM), is a powerful technique to achieve atomic resolution. The strain effects in single component colloidal nanocrystals can be described and quantified using geometric phase analysis method.24,25 Unfortunately, this method cannot be adapted to analyze the gradient core shell QDs due to the lattice spacings difference. While it has also been known for some time that electron beam can cause considerable changes of nanomaterials.26-30 For example, Au nanoparticles were fabricated by exposing Au(I) alkanethiolate complexes under electron beam.31 Moreover, previous works also reported the transformation of metallic particles into metal/metallic oxide core shell nanoparticles or hollow metallic oxide particles through chemical reaction and/or nanoscale Kirkendall effects.32-34 In this work, we observed the morphology evolution of alloyed CdxZn1-xSeyS1-y@ZnS core shell QDs under electron beam during TEM observation. During the electron beam irradiation, the formation of voids and size decrease were observed for CdxZn1-xSeyS1-y@ZnS QDs and the morphology change varied with the composition and shell thickness. By correlating the morphology evolution process with the composition and structure of CdxZn1-xSeyS1-y@ZnS QDs, we developed a convenient method to illustrate the strain effects in alloyed core shell QDs through real-time TEM measurements. 2. EXPERIMENTAL AND COMPUTATIONAL METHODS


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2.1. Chemicals. cadmium oxide (CdO, 99%, aladdin), zinc acetate (Zn(Ac)2, 99.7%, macklin), sulfur (S, 99.5%, aladdin), selenium (Se, 99.9%, alfa aesar), tellurium (Te, 99.99%, alfa asear), trioctylphosphine (TOP, 90%, aladdin), oleic acid (OA, ≥90%, alfa aser), oleylamine (OLA, J&K), 1-octadecene (ODE, >90%, aladdin), dodecanethiol (DDT, 98%, alfa aesar), hexadecylamine (HDA, 90%, Aldrich), glacial acetic acid (analytical grade, Beijing Chemical Reagent Co., Ltd., China), methanol (analytical grade, Beijing Chemical Reagent Co., Ltd., China), acetone (analytical grade, Beijing Chemical Reagent Co., Ltd., China), toluene (analytical grade, Beijing Chemical Reagent Co., Ltd., China). All the reagents were used as received without further purification. 2.2. Synthesis of QDs. The CdxZn1-xSeyS1-y@ZnS QDs were synthesized by adapting Bae’s report.35 A fixed amount of CdO, Zn(Ac)2, OA and ODE were mixed in a three-neck bottle, heated and degassed for three times at 120 °C and then heated to 320 °C. At this temperature, a certain amount of Se and S dissolved in TOP was swiftly injected into the reaction flask. After 10 min, the precursor solution of Zn(Ac)2, OLA, DDT and ODE was dropwisely added into three-neck bottle for several times at time intervals of 7 min. Afterward, the reaction solution was cooled to ∼80 °C and precipitated by methanol, acetone and glacial acetic acid by three times, and then dried at 60 °C under vacuum to get powder. The CdxZn1-xS@ZnS QDs were synthesized following the protocols reported by Lee et al.36 The CdTe@CdSe QDs were synthesized according to the method of Chang et al.37 CdSe and CdS QDs were synthesized following Lee’s reports38 and ZnS QDs were synthesized using a modified method of Li et al.39 2.3. TEM characterization of QDs. The as-synthesized QDs were washed and precipitated for several times to get their powders which were dissolved in solvents. A diluted QDs solution was dropped on to a lacey carbon filmed grid, and dried under an infrared lamp for 5 min. TEM


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observations were conducted on a JEOL-2100F TEM at a voltage of 200 kV, magnification of 120 k，200 k and 400 k, current density of ~80 pA/cm2 under fixed beam flux without using any objective lens. TEM images were taken every 30 s to follow the morphology evolution of QDs. 2.4. Calculation of the vacancy formation energy of CdSe, CdS, ZnSe, ZnS. The vacancy formation energy (Ev) of CdSe, CdS, ZnSe and ZnS were calculated using LAMMPS code (metal unites).40 Stillinger-Weber potential was empirically designed to give the lowest energy for tetrahedrally bonded structures41 and the CdTeZnSeHgS0 potential was applied to model the atomic interactions.42 The key parameter and simulation results are summarized in table S1. 3. RESULTS AND DISCUSSION The CdxZn1-xSeyS1-y@ZnS QDs were synthesized using a modified method of Bae et al.35 For TEM imaging, a JEOL-2100F machine operating at 200 kV was used with same electron flux (usually at a magnification of 120 k, 200 k or 400 k, and current density of ~80 pA/cm2). We observed that the as-fabricated CdxZn1-xSeyS1-y@ZnS QDs samples show interesting morphology evolution under TEM electron beam irradiation. Figure 1 shows the TEM images of a typical CdxZn1-xSeyS1-y@ZnS QDs before and after electron beam exposure. With irradiation time prolonged, many voids appeared and gradually enlarged in alloyed CdxZn1-xSeyS1-y@ZnS core shell QDs. Meanwhile, the detailed analysis shows that the average size of CdxZn1-xSeyS1y@ZnS

also decreased by ~10% (Figure S1).


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Figure 1. The morphology evolution of composition gradient CdxZn1-xSeyS1-y@ZnS core shell QDs under electron beam irradiation with magnification of 400 k and current density of ~82 pA/cm2 for 8 min. It has been well-known that the formation of voids usually correlated with Kirkendall effect.43 The formation of hollow structure from metallic nanoparticle via Kirkendall effect usually resulted in size increase due to the diffusion of atoms from core to surface during oxidization or sulfuration.44 To clarify the evolution process, energy dispersive spectra (EDS) mapping was applied to study the composition distributions of the voids and the elemental maps of a typical QD is shown in Figure 2. It was found that the proportion of Cd, Zn and S in voids was significantly lower than that of Cd, Zn and S outside of voids, and the contents of Se are similar on both sides of voids’ edges. The appearance of voids correlated with the movement of Cd, Zn and S atoms, although the moving of Se atoms is uncertain. Based on EDS mapping results, no significant diffusion process was observed during the morphology evolution. Although the morphology evolution of alloyed CdxZn1-xSeyS1-y@ZnS QDs is very similar with the formation of hollow structures during TEM irradiation, the observed phenomenon cannot be explained to the well-recognized Kirkendall effect.


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Figure 2. EDS elemental maps of CdxZn1-xSeyS1-y@ZnS QDs before and after electron beam irradiation. The elemental maps taken from rectangle part for cadmium (green), using Cd Ledge; for zinc (red), using Zn K-edge; for sulfur (blue), using S K-edge; for selenium (yellow), using Se L-edge; the RGB maps by superposition of cadmium, zinc, sulfur and selenium maps. To further understand the formation of voids in gradient core shell QDs under electron beam, we firstly considered the interactions between high energy electrons and colloidal QDs (Scheme S1). According to the literature reported on bulk semiconductor, the energy transferred during an elastic or inelastic nuclear collision may be large enough to knock an atom from its lattice site.45 If the energy exceeds the threshold energy, the displacement of atomic nuclei to interstitial position forms Frenkel defects, and subsequently results in the formation of void. Meanwhile, the elastic scattering enables the sputtering of surface atoms from the specimen, resulting in the size reduction. In addition, the inelastic scattering usually introduces ‘collision’ process, which ends up as heat and temperature rises. Considering electron beam irradiation on CdxZn1-xSeyS1-y@ZnS core shell QDs, the atomic displacement and heating effects are expected to be more pronounced due to their reduced size and large surface to volume ratio.


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Due to the atomic displacement mainly accounts for the formation of voids, we then analyzed the possible influence of composition on the atomic displacement by comparing the energy transformed from electrons to an atom (EA) and Ev.46 Based on energy conservation and momentum conservation, the maximum EA transferred from an electron to an atom through elastic collision showed in Scheme S2, the maximum energy transfer can be calculated from the following equation,47

=

561

+ 2

where A is the weight of atom and ε=E/ (mc2), E is the original energy of electron in TEM with 200 kV accelerating voltage, m is the mass of electron and c is velocity of light. Therefore, the value of EA strongly depends on the atomic number and accelerating voltage of TEM. An elastic collision of electron with 200 kV accelerating voltage with an atom gives the value of maximum EA of 4.67 eV, 8.08 eV, 6.65 eV and 16.37 eV for Cd, Zn, Se, S respectively (Table S2). The Ev of CdSe, CdS, ZnSe and ZnS were calculated by LAMMPS code40 using the CdTeZnSeHgS0 potential function (see computational methods).42 Table S1 shows the calculated Ev of CdSe (Cd: 3.9 eV, Se: 4~5 eV), CdS (Cd: 4.4 eV, S: 5.5~5.5 eV), ZnSe (Zn: 3.3~3.6 eV, Se: 4~5 eV), ZnS (Zn: 3~3.6 eV, S: 3~8.5 eV) at room temperature. The values of EA are higher than that of its corresponding Ev for all the components in the as-fabricated QDs. Therefore, the formation of vacancies is much preferred for CdxZn1-xSeyS1-y@ZnS core shell QDs, corresponding to the results in Egerton’s report.45 As showed in Figure S2, no void was observed for CdxZn1-xSeyS1-y@ZnS core shell QDs when being heated at 600 °C. Therefore, the formation of voids should be mainly related to the atomic displacements under electron irradiation. Based on the above discussion, we proposed that the accumulation of vacancies


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under continuous electron beam irradiation induced the atomic movement under interfacial strain in the core shell QDs, showed in Scheme 1. This assumption is supported by the observed ringvoids in CdSe/ZnS core shell QDs (Figure S3) and observed size reduction effects (Figure S1). In addition, no void was observed in single component CdSe, CdS, ZnS QDs and CdxZn1-xSeyS1y

core after irradiation (Figure S4). This can be explained by the relaxed strain in composition

gradient CdxZn1-xSeyS1-y QDs. In the following, we illustrate how the interfacial strain affect the morphology evolution of CdxZn1-xSeyS1-y@ZnS QDs under electron beam irradiation.

Scheme 1. Scheme of the strain distribution in CdxZn1-xSeyS1-y QDs (a) and the formation mechanism of voids in CdxZn1-xSeyS1-y QDs under electron beam irradiation (b). Generally, for the core shell QDs with large lattice constant core and small lattice constant shell: the core is compressed in all directions, while the shell is compressed in the radial direction but stretched in the direction tangential to the interface.48 The strain in single component core shell QDs concentrates mainly in and nearby the interface.49 While for the alloyed core shell QDs, strain distributed homogeneous in whole dot (Scheme 1a). Vacancies irradiated by electrons moved under initial stress in gradient core shell QDs had opposite movement direction to the atoms driven by initial stress. Vacancies in the ZnS shell tending moved to interface and surface of QDs. Meanwhile, vacancies in gradient shell tending moved to


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the interface (Scheme 1b). To further illustrate the correlations between morphology change and interfacial strains, we studied the morphology evolution of QDs samples with varied composition and composition distributions. In Yang’ work, the QD-LEDs using CdxZn1-xSeyS1-y@ZnS QDs with more ZnSe than CdS as the emitting layer had better stability, higher efficiency and longer lifetime.14 The alloyed CdxZn1-xSeyS1-y@ZnS core shell QDs have gradient shell sandwiched between the Cd- and Serich core and the Zn- and S-rich outer shell. It has been found that the gradient shell distribution strongly determines the performance of QDs in EL devices. Here, we compared the influence of the shell composition on the morphology evolution in CdxZn1-xSeyS1-y@ZnS QDs under TEM observations. The shell compositions varied following a modified method45 and three typical samples were studied. The elemental contents of these samples were analyzed using X-ray Photoelectron Spectroscopy (XPS) measurements. In three samples, the contents of Zn were 24.9%, 26.6%, 31.6%, and contents of Se were 2.8%, 13.4% and 21.2%, respectively (Figure S5). These QDs are denoted as CdS-rich, medium and ZnSe-rich. The lattice mismatch between the ZnSe, CdS intermediate and the ZnS shell were 4.8% and 7.7%. As a result of low lattice mismatch, CdxZn1-xSeyS1-y@ZnS QDs with more ZnSe in the intermediate layer should have relaxed strain and less stress. Figure 3 shows the TEM images of CdxZn1-xSeyS1-y@ZnS QDs with different composition before and after the irradiation under 200 keV electron beam at a magnification of 200 k and a current density of ~86 pA/cm2 for 4 min. The sizes of the formed voids are characterized by calculating the size ratios of formed voids and primary dots (see details in Figure S6). With the increasing of contents of ZnSe, the area ratio between formed voids and their corresponding original size decreased from 32.7%, to 21.3% and 15.9% (Table


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S3). This phenomenon is consistent with enhanced stability of ZnSe-riched sample in Yang’s work.14

Figure 3. The morphology evolution of CdxZn1-xSeyS1-y@ZnS core shell QDs with different component, CdS-rich, medium and ZnSe-rich under electron beam for 4 min (magnification of 200 k, current density of ~86 pA/cm2). The strain relaxes and diminishes along with increasing dimension of core and thickening of shell, reported by Chen et al.50 We further studied the influence of shell thickness on the morphology evolution of CdxZn1-xSeyS1-y@ZnS QDs under electron beam irradiation. The CdxZn1-xSeyS1-y@ZnS QDs with same CdxZn1-xSeyS1-y core but varied thicknesses of ZnS shells of 1.6 nm, 3.7 nm, 6.5 nm, 10.4 nm and 11.5 nm, were investigated. Figure 4 shows the resulting samples after irradiation at magnification of 200 k and current density of ~86 pA/cm2. The specific morphology evolutions at the same condition were shown in Figure S7 and the XRD patterns and spectral properties were shown in Figure S8. It is noticed that the area ratio between voids and QDs dropped from 50.4% to 25.2%, 14.9%, 13.3% to 0% (as shown in Figure 4 and Table S4). In accordance to the work of Smith and Chen, as the strain relaxed to shell and the


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distribution of strain expanded, the strain decreased with increasing shell thicknesses at the interface of QDs.19,50 This is consistent with the experiemntal results that core shell QDs with thick shell suppress trapping better than QDs with thin shell.51

Figure 4. The morphology evolution of CdxZn1-xSeyS1-y@ZnS core shell QDs with increasing shell thicknesses under electron beam for 4min (magnification of 200 k, current density of ~86 pA/cm2). The diameters of QDs with different shell thicknesses were marked in graphs. Besides, we also synthesized CdxZn1-xS@ZnS and CdTe@CdSe core shell QDs and studied their morphology evolutions under electron irradition.36,37 The formation of voids was also observed in CdxZn1-xS@ZnS QDs after irradiated at magnification of 200 k and current density of ~86 pA/cm2 for 4 min and CdTe@CdSe QDs for 8 min (Figure S9), in which the lattice mismatches of cores and shells are 7.7% and 6.6%, respectively. 4. CONCLUSIONS


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In summary, we observed the formation of voids and size reduction in gradient-alloyed CdxZn1-xSeyS1-y@ZnS QDs under electron beam during TEM observation. The formation of voids can be explained by the continuous electron beam irradiation induced the atomic movement under interfacial strain. While, the size reduction can be attributed to the elastic scattering enabled sputtering of surface atoms. By studying the influence of shell gradient on the morphology evolution under electron beam, it was found that the feature of strain effects in the core shell QDs can be derived and illustrated. The sizes of voids in CdxZn1-xSeyS1-y@ZnS QDs decreased with small lattice constant difference between core and shell, and decreased with the increasing of shell thickness. This provides a guideline to understand the strain effects in the gradient-alloyed QDs. We believe that the deep insights into strain effects would push forward the optimization of CdxZn1-xSeyS1-y@ZnS QDs toward commercialized light-emitting technology. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. Additional information about the mechanism of electron beam irradiation in QDs, the EDS mapping, morphology evolution, XRD patterns, UVvis absorption and PL emission spectra of CdxZn1-xSeyS1-y@ZnS core shell QDs. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Author Contributions All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors would like to thank Prof. R. S. Liu and Prof. S. Xu for providing quantum dots, as well as Prof. J. Zou, Prof. N. C. Gianneschi and Dr. D. Li for helpful discussions. This work was supported by the National Natural Science Foundation of China (Grant Nos. 61722502, 61735004). REFERENCES (1) Kovalenko, M. V.; Manna, L.; Cabot, A.; Hens, Z.; Talapin, D. V.; Kagan, C. R.; Klimov, V. I.; Rogach, A. L.; Reiss, P.; Milliron, D. J.; et al. Prospects of Nanoscience with Nanocrystals. ACS Nano 2015, 9, 1012-1057. (2) Shirasaki, Y.; Supran, G. J.; Bawendi, M. G.; Bulović, V. Emergence of Colloidal QuantumDot Light-Emitting Technologies. Nat. Photon. 2013, 7, 13-23. (3) Dai, X.; Zhang, Z.; Jin, Y.; Niu, Y.; Cao, H.; Liang, X.; Chen, L.; Wang, J.; Peng, X. Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum Dots. Nature 2014, 515, 96-99. (4) Dai, X.; Deng, Y.; Peng, X.; Jin, Y. Quantum-Dot Light-Emitting Diodes for Large-Area Displays: Towards the Dawn of Commercialization. Adv. Mater. 2017, 29, 1607022. (5) Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H. S.; Fukumura, D.; Jain, R. K.; et al. Compact High-Quality CdSe/CdS Core/Shell Nanocrystals with Narrow Emission Linewidths and Suppressed Blinking. Nat. Mater. 2013, 12, 445-451.


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Morphology Evolution of Gradient-Alloyed CdxZn1âxSeyS1ây@ZnS

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