Highly Luminescent Dual-Color-Emitting Alloyed [Znx Sey ] Quantum

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Highly Luminescent Dual-Color-Emitting Alloyed [ZnCd SeS ] Quantum Dots: Investigation of Bimodal Growth and Application to Lighting Ching-Che Hung, Shih-Jung Ho, Chang-Wei Yeh, GuanHong Chen, Jin-Hua Huang, and Hsueh-Shih Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10182 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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The Journal of Physical Chemistry

Highly Luminescent Dual-color-emitting Alloyed [ZnxCd1-xSeyS1-y] Quantum Dots: Investigation of Bimodal Growth and Application to Lighting Ching-Che Hung, Shih-Jung Ho, Chang-Wei Yeh, Guan-Hong Chen, Jin-Hua Huang and Hsueh-Shih Chen* Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan (ROC). *

Email: [email protected]

ABSTRACT: Bimodal growth and application of highly luminescent alloyed [ZnxCd1-xSeyS1-y] quantum dots (QDs) with controllable dual emission peaks in one-pot synthesis is investigated in this study. The bimodal-sizedistributed alloyed [ZnxCd1-xSeyS1-y] QDs are grown from alloyed [ZnxCd1-xSey] seeds by introducing a monomer within the bimodal growth window during the seed growth. The synthesized bimodal alloyed [ZnxCd1-xSeyS1-y] QDs have a quantum yield as high as 95% under single wavelength excitation, where larger and smaller size groups individually contribute to 30% and 65%, respectively. The relative PL intensity and wavelength of the bimodal alloyed QDs can be adjusted by varying the growth time and the monomer concentrations. The dual-emitting QDs with color tunability can be directly applied to QD-converted light emitting diodes (LEDs), which need neither post-mixing nor color-adjusting process to vary the color axis and the color rendering index. This one-pot synthetic approach not only solves the mixing problem of QDs from different reaction processes, but also significantly reduces the synthetic time and ignores the color-adjusting process, offering much more efficient and reliable way for lighting and display in industry.

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■ INTRODUCTION Quantum dots (QDs) or nanocrystals (NCs) have drawn great attention for the last two decades because of their unique size-dependent properties and high luminescence efficiency. 1‒3 Due to their outstanding electrical and optical performances, they have been applied to a wide spectrum of fields, such as light-emitting diodes (LEDs),4‒9 solar cells,10,11 photodetectors,12 and biomarkers.13,14 Recently, researchers have made efforts to apply core/shell CdSe/ZnS or InP/ZnS QDs in lighting and display in order to improve the energy efficiency and increase the color gamut.15 In those applications, luminescence efficiency or quantum yield (QY), control of emission peak wavelength and width (i.e., average size/size distribution), and thermal stability are critical and need to be urgently improved. In general, highly efficient QDs could be prepared by surface overcoating with wide-band-gap inorganic materials such as ZnS shell or organic overcoats to form a core/shell structure. 16‒18 However, lattice mismatch between the core and the shell at the interface easily causes structural defects that lowers the QY of QDs. By introducing buffer layers like CdS or ZnSe to form a core/shell/shell or core/alloyed-multishell structure to reduce the lattice mismatch, the photoluminescence (PL) QYs of QDs can be improved.19‒23 Besides conventional core/shell QDs, alloyed QDs such as ZnCdSe are promising for their narrow emission bandwidth and feasibly adjusted wavelength solely by the composition that enables their ease in production. 24 They have been also found alloyed ZnCdSe QDs without shell overcoating have better charge injection compared to those core/shell CdSe/ZnS QDs.25 Moreover, alloyed CdSeS-based core/ZnS-based shell QDs with the composition gradient (ZnxCd1-xSeyS1-y) interface have also been reported.26 Unlike conventional alloyed QDs, the QDs still possess a quasi-core/shell structure with reduced interface lattice mismatch so they have better luminescence properties due to more effective conﬁnement of carriers in the alloyed cores. In such a system, the PL peak position and width are affected by both of the size and the composition of the QD cores, which can be varied by the ratio of Cd/Zn monomer concentration. In order to accurately control the particle size and the size distribution for QDs, it must be carefully taken into account the nucleation and the crystal growth stages. Typically, in the so-called hot injection method, the nucleation stage of stable embryos (i.e., nuclei, the smallest crystals in the equilibrium condition) and the growth stage are not overlapped too much. So, most of the nuclei can grow at the same starting point. The size focusing event, that is, smaller crystals grow relatively faster than larger ones based on the growth kinetics and automatically leads to a reduction in the size distribution.27‒29 On the other hand, size narrowing based on the thermodynamic equilibrium has also been reported. Due to high specific surface area of QD NCs, the surface energy plays a more important role over the volume free energy and dominates the total free energy of the crystal growth, and thus QDs are relatively more stable in certain surface morphologies such as closed shell, where the growing QDs preferentially form and thus the size deviation can be decreased. 30,31 The relatively more stable sizes for QDs have lower free energy or chemical potential, which is defined to be the chemical potential well in the QD crystal growth. By applying this chemical potential model, a one-pot method of preparing CdSe and core/shell CdSe/ZnS QDs with a controllable multimodal size distribution, in which average size and PL intensity of every size group can be individually controlled, has been reported.15,32 In a typical synthesis, a CdSe QD size group together with some magic-sized seeds were grown in the first stage, followed by thermal quenching to terminate the growth of the first size group and constrain them in the 2 ACS Paragon Plus Environment

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chemical potential well. Then, a series of injections of monomers are used to grow the magic-sized seeds individually, which finally generates CdSe QDs with bimodal size distribution and dual PL peaks. With a similar procedure, CdSe QDs with trimodal size distribution and triple emission peaks have also been prepared. In the present work, the one-pot strategy is modified to synthesize highly luminescent alloyed [ZnxCd1-xSeyS1y] QDs with bimodal size distribution and dual PL characteristics, which enables us to fabricate QDs-based white light emitting diodes (LEDs) by directly using as-synthesized alloyed QDs without any post color mixing process. This method significantly reduces the cost, simplifies the process, and improves the dispersion of two different QDs in a polymer. A model derived from the previous study has also been proposed to describe the growth.

■ EXPERIMENTAL SECTION Chemicals. Cadmium oxide (CdO), zinc acetate, selenium, oleic acid (OAc), 1-octadecene (ODE) were purchased from Aldrich. Sulfur and trioctylphosphine (TOP) were obtained from Alfa Aesar. All solvents were purchased from J. T. baker. Silicone or polydimethylsiloxane (PDMS) were obtained from Goal Bio. Synthesis of Bimodal Alloyed [ZnxCd1-xSe] QDs (Seeds) in One-pot (Control Experiment). 0.21 mmol of cadmium oxide, 4 mmol of zinc acetate, 12.6 mmol of oleic acid and 15 mL of 1-octadecene were placed in a 100 mL round flask filled with nitrogen and heated to 305 oC to form a Zn/Cd precursor. 0.117 mmol of Se powder dissolved in 0.3 mL of TOP (named: Se-precursor) was injected quickly into the reaction flask. After the injection, the reaction temperature was set to 300 oC for the [ZnxCd1-xSe] QD growth. During the growth, aliquots were taken from the reaction flask at varying time intervals. The final products were dispersed in toluene. Synthesis of Bimodal Alloyed [ZnxCd1-xSeyS1-y] QD in One-pot. For a typical synthetic procedure, [ZnxCd1-xSe] cores were first prepared with the same procedure as the control experiment. Then, a S-precursor prepared by 3.65 mmol of sulfur powder dissolved in 1.7 mL of TOP was injected into the reaction flask at 30 s after the initial Se-precursor injection. During the growth, aliquots were taken from the reaction flask at varying time intervals. The reaction was terminated by cooling the flask to room temperature. The final products were redispersed in toluene after the washing process. The PL wavelength and intensity of individual peaks of bimodal QDs were varied by the Se-precursor concentration (0.117, 0.233, 0.35 mmol dissolved in 0.3 mL TOP) and the S-precursor injection time (15 － 90 s after the Se-precusor injection). All other parameters such as the amount of OAc, ODE, TOP, reaction temperature and reaction time were kept the same. Examination of Chemical Potential Well. A series of experiments with different amount of S-precursor were conducted to examine the chemical potential well. The reaction conditions were same as the control experiment except additional S-precursor with different concentrations (3.65 mmol/ 1.82 mmol/ 0.91 mmol) dissolved in 1.7 mL trioctylphosphine were injected. A higher concentration S monomer provides more chemical energy for QDs to grow over the chemical potential well.



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Preparation of QD Film as Light Convertor. As-synthesized bimodal alloyed QDs were directly used or mixed with silicone. The QD-silicone mixture was cured at 120 oC under vacuum and a dried uniform freestanding film was obtained after one hour. The freestanding film was then attached to a blue light InGaN chip (λem = 450 nm) to form a light emitting diode (LED). Characterization. PL spectra of the samples were measured with an HORIBA FluoroMax-4. UV-Vis absorption spectra were collected on HITACHI U-3900. The fluorescent quantum yield (QY) of the QDs synthesized was estimated by comparing their fluorescence intensities with those of primary dye solution (R6G) at the same optical density. In order to measure the average size and size distribution of QDs, transmission electron microscopy (TEM) images of the QDs were taken by utilizing JEOL JEM-3000F. Those samples were prepared by dropping dilute toluene dispersion of QDs on a copper grid coated with an amorphous carbon film. Inductively coupled plasma-mass spectrometer (ICP-MS) data were collected using AGILENT 7500ce. All the samples were dissolved in 2% of HNO3 solutions. Powder X-ray diffraction (XRD) patterns were measured using Bruker AXS GmbH - D2 PHASER using Cu Kα radiation (1.5405 Å ).

■ RESULTS AND DISCUSSION The chemical potential plays a significant role in the crystal growth, especially for those magic-sized particles.33‒35 Take CdSe as an example, CdSe molecular clusters with the specific numbers of CdSe pairs such as 13, 17, 26, 35, 48, 69 and 72 are considered to be magic sizes.36 It has also been found that smaller CdSe nanocrystals with a closed-shell morphology exhibit a similar magic size effect due to the high specific surface area, which dominates the chemical potential of the QD growth.31,32 The magic-sized molecular clusters or nanocrystals are more thermodynamically stable than those with a size slightly deviated so they have a lower energy state, i.e., in a chemical potential well where they are confined by the chemical potential well.28,31,32 If QDs grow into a magic size with a corresponding chemical potential well, where is insufficient chemical energy provided by monomers, the QDs would be constrained in the lower energy state and stop growing in the crystal growth process. Those relatively stable states have been utilized to prepare either core or core/shell QDs with a bimodal or trimodal size distribution by a designed alternating thermodynamickinetic process.15,32 In the case of bimodal size-distributed CdSe QDs, size group A is grown in the first stage, followed by a quenching process preserving the size group in the chemical potential well throughout the subsequent processes. Then, size group B is selectively grown by a series monomer injections. By directing the thermodynamics-based growth to the kinetics-based regime, direct synthesis of multimodal-sizedistributed QDs with controllable multiple emission bands can be realized. However, for alloyed ZnxCd1xSexS1-x

QDs, it was found that the quenching process significantly affected the Zn-Cd-Se-S alloying and the QY was low (< 40%). In order to assure the Zn-Cd-Se-S alloying, we developed an isothermal process to prepare bimodal-size-distributed alloyed Zn-Cd-Se-S QDs with dual emitting colors, which have been directly applied to white LEDs. Synthesis of Alloyed [ZnxCd1-xSe] Seeds. For preparation of alloyed [ZnxCd1-xSe] seeds, Se-precursor is injected to a hot Cd/Zn precursor mixture. Figure 1a shows PL spectra of samples collected at different time 4 ACS Paragon Plus Environment

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intervals during the reaction. A sample collected just after the Se-precursor injection (~ 1 s) displays a broad PL band, indicating a broad size distribution of the seeds in the initial stage. Sample collected at 15 s shows dual PL peaks at ~ 460 and ~ 530 nm, which are confirmed to be from two different size groups of alloyed [ZnxCd1-xSey] seeds (named: A-aSeed and B-aSeed size groups, the exact composition will be shown later). A broad band in the long wavelength region (570－700 nm) is ascribed to the deep level emission from the defects generally observed for smaller II-VI nanocrystals. A ~ 30 nm red shift of the long wavelength peak (~ 526 nm) (i.e., A-aSeeds) for a sample collected at 30 s shows a typical QD growth event, while there is no obvious change in the short wavelength peak (i.e., B-aSeeds). In general, smaller nanocrystals should grow faster than larger ones, as shown by the so-called size focusing effect.27 However, if the nanocrystals are in a certain stable configuration such as magic size or morphology with a lower chemical potential state, the nanocrystals can preserve their size/morphology to a certain extent, which looks like they are confined in the chemical potential well, as observed for B-aSeeds, which exhibit no significant growth.31 For samples collected after 60 s, the long wavelength peak further red shifts to ~ 618 nm and finally stops at ~ 631 nm, while the short wavelength peak disappears ascribed to dissolution of unstable particles due to monomer consumption by larger particles, that is, Ostwald ripening in the final stage when monomer concentration becomes low. The PL QY of final [ZnxCd1-xSe] QDs samples estimated is generally between 10－30%. Optical absorption of samples collected at 15 s and 60 s are shown in Figure 1b. The first excitonic absorption peak of A-aSeeds shifts from 510 nm to 587 nm, demonstrating a growth event of the QDs. The optical absorption range of B-aSeeds, overlapping with that of A-aSeeds, would be between 400 to 450 nm in the initial growth stage (15 s) according to the PL emission. Figure 2 gives TEM images of two selected samples (15 s and 60 s), which clearly shows that QDs are in a bimodal size distribution ranging from 2 to 6 nm, while the 60 s sample exhibits a similar phenomenon ranging slightly broader from 2 to 7 nm. It is noted that in the 60 s sample the number of particles with the diameter between 3 to 4 nm significantly decreases compared with the 15 s sample, indicating dissolution of relatively instable particle as mentioned above. The number of particles in size groups 2－3 nm and 4－7 nm, on the other hand, increases as they are considered to be in a relatively more stable state. In brief, it has been found that alloyed [ZnxCd1-xSe] nanocrystals can develop into two size groups in the growth stage (15－90 s). B-aSeeds (i.e., small size group) are considered to be constrained in the chemical potential well and stay in the original state until Oswald ripening occurs, while AaSeeds (i.e., large size group) continuously grow throughout the reaction until equilibrium is established, as illustrated in Figure 3.



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Figure 1. (a) PL spectra from blank experiment [ZnxCd1-xSe] QDs (without S-precursor injection) grown from 0 s to 180 s. The PL curves are normalized. (b) UV-vis spectra of samples collected at 15 s and 60 s.

Figure 2. TEM images of [ZnxCd1-xSe] QDs collected at (a) 15 s and (b) 60 s after Se-precursor injection in the control experiment. The insets show number and diameter histograms.

Figure 3. A sketch of growth mechanism of [CdxZn1-xSe] seeds. After introducing a Se-precursor, seeds with a broad size distribution first form and then becomes a bimodal size distribution. Larger A-aSeeds continue to grow to red A-aQDs, while B-aSeeds cannot grow and stay in the original state until they eventually dissolve in the final stage. 6 ACS Paragon Plus Environment

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Hetero-epitaxial Growth of [ZnxCd1-xSeyS1-y] Layer on Bimodal [ZnxCd1-xSe] Seeds. To further improve the luminesce efficiency of the alloyed seeds, a S-precursor is introduced into the reaction mixture to form a higher bandgap passivation layer on the [ZnxCd1-xSe] seeds. However, the injection time of the Sprecursor is found to be rather critical since the extra S monomer can alter the QD growth and the size distribution, which has been found to easily turn the bimodal size distribution back into a monomodal one. As mentioned above, the seeds exhibit a bimodal size distribution at ~ 15 s after the initial monomer injection. It has been found that a windows between 15 to 90 s exists and allow the co-growth of the [ZnxCd1-xSeyS1-y] passivation layers for both size groups. If a S-precursor is introduced into the reaction system within the time window, the seeds can continue to grow with S incorporation. In the previous studies, we prepared bimodalsize-distributed CdSe/ZnS QDs using a different strategy based on a thermal quenching-kinetic ignition process, in which ZnS shell growth was carried out at two different elevated temperatures for every size groups. In that method, average size and amount of QDs of both size groups can be individually controlled by using the quenching temperature and the monomer concentration. In contrast to the present alloyed Zn-Cd-Se-S QD system, although the thermal quenching did preserve the bimodal size distribution, unfortunately, the QYs of the products are rather low. The reason is thought to be that the S-precursor is less reactive and defects may easily form when reacting with Zn/Cd/Se monomers on the [ZnxCd1-xSe] seeds at a lower temperature. Consequently, we tried to introduce the S-precursor into the reaction system without the thermal quenching to grow the high bandgap outer layer to improve the QY for both size groups. Figure 4a shows PL spectra of samples collected at different reaction time intervals. The [ZnxCd1-xSe] seeds with a bimodal size distribution are first grown by an initial Se-precursor injection, as shown by 15 s and 30 s curves (Figure 4a), both of which exhibit dual PL peaks. At ~ 30 s, a S-precursor is injected into the reaction system to involve S in the QD growth so the crystal composition transits to [ZnxCd1-xSeyS1-y] at the QD outer part (will be shown later). Figure 4a presents the PL peak position of B-Seeds red shifts from 469 nm (t = 30 s) to 567 nm (t = 180 s), while that of A-Seeds red shifts from 559 nm (t = 30 s) to 618 nm (t = 180 s). The final product contains AaQDs and B-aQDs size groups exhibiting dual PL characteristic. The PL QY of the bimodal [ZnxCd1-xSeyS1-y] alloyed QDs is generally between 80 to 95% and is estimated to be 95% for the current sample with optical density  0.01 under the 492 nm excitation, which is chosen to the maximum PL excitation intensity for BaQDs (Figure S1 & Figure S2). It is noted that A-aQDs and B-aQDs contributes ~ 30% and ~ 65% to the overall PL intensity respectively according to the PL peak areas. Typically, monomodal QDs with similar wavelengths synthesized by the same reaction system have QYs 96－99%. In addition, the optical density of the samples for the PL measurement in the present study is very low ( 0.01), so re-absorption of B-aQD by A-aQDs is expected to be a small extent. The QYs of individual size group after size separation are not estimated because the separation requires a poor solvent such as methanol, which unfortunately affects the QD optical properties to some extent. In a practical application such as lighting, all QDs generally need to be mixed together and individual QYs represent limited meaning since QDs have strong interaction, e.g., energy transfer or reabsorption. Optical Absorption. UV-vis spectra show similar results, as shown in Figure 4b. After an initial injection of Se-precursor, a prominent excitonic absorption peak at 509 nm together with a broad shoulder excitonic absorption peak appear at 410 nm (curve 15 s). After injecting S-precursor at ~ 30 s, the excitonic absorption peaks dramatically shift to longer wavelengths 586 and 536 nm assigned to A-aQDs and the B-aQDs 7 ACS Paragon Plus Environment


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contributions, respectively, as shown by curve 60 s. The final product possesses two prominent absorption peaks at 603 and 555 nm (t = 180 s).

Figure 4. (a) PL spectra from bimodal alloyed [ZnxCd1-xSeyS1-y] QDs collected at different reaction time intervals (15, 30, 60, 120, and 180 s) after the initial Se-precursor injection. Injection of S-precursor was carried out at 30 s just after the sampling. The PL intensities are normalized. Variation of the QY is shown in Figure S1. (b) UV-vis spectra of samples corresponding to 15, 60, and 180 s PL spectra. Incorporation of S turns A-aSeeds and B-aSeeds into larger nanocrystals with radially graded composition in the outer layer (final product named: A-aQDs and B-aQDs).

Figure 5. TEM images of the bimodal QDs collected at 15 s (a), 30 s (b), 60 s (c), 120 s (d), and 180 s (e). S-precursor was immediately injected after a sample collected at 30 s. Insets: size histogram of bimodal QDs and simulated curves of the size distribution. 8 ACS Paragon Plus Environment

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Evolution of QD Size by TEM and PL Analyses. In order to investigate the growth of bimodal alloyed [ZnxCd1-xSeyS1-y] QDs in the current one-pot system, TEM images of the samples collected at different reaction time intervals corresponding to the PL and UV spectra are presented in Figure 5. It shows that the alloyed [ZnxCd1-xSe] seeds collected at 15 s and 30 s possess two size groups (Figure 5a and b), which are consistent with the PL spectra. After introducing the S-precursor, the size distribution is still bimodal, though the average sizes of both size groups increases with growth time, as shown in Figure 5c-e and also exhibited by the PL spectra with dual emission peaks at 60 s, 120 s, and 180 s. The above PL and TEM results clearly indicates that the initially formed [ZnxCd1-xSe] seeds determine the dual PL emitting characteristic of final [ZnxCd1-xSeyS1-y] QDs. Figure 6a presents variations of size and PL wavelength of the individual size groups according to TEM images and PL spectra. It shows that both size groups have similar trends in the size and the PL wavelength, and both of them constantly increase with increasing reaction time. This fact infers that the S-precursor injection effects similar hetero-epitaxial [ZnxCd1-xSeyS1-y] growth mechanism for both AaSeeds and B-aSeeds toward A-aQDs and B-aQDs, respectively, and their outer hetero-epitaxial layer composition would be similar. ICP-MS Composition of Bimodal QDs. The composition variation along the radial orientation of QDs estimated by ICP-MS for the bimodal samples collected at the same reaction time intervals as PL/TEM is shown in Figure 6b. The initial [ZnxCd1-xSe] seeds possess average composition of Zn0.45Cd0.55Se. After introducing S-precursor, it appears that Cd becomes the majority, while the Zn ratio decreases, implying that formation of CdS is faster than that of ZnS. So more CdS first forms after the S-precursor injection until Cd exhausts up, while Zn gradually grows and becomes the major composition. For the outer layer, Zn and S elemental fraction gradually increase, showing a graded distribution of the composition. Finally, the alloyed [ZnxCd1-xSeyS1-y] QDs grown for 180 s have the composition of Zn0.82Cd0.18Se0.04S0.96, indicating the major composition at the QD outer part is ZnCdS, while central part is close to ZnCdSe. Strictly speaking, the QDs are neither so-called core/shell QDs, nor simple alloyed QDs. They are more like the 3D heterogeneous epitaxy of ZnCdSSe particles with composition varying in the radial orientation. Formation of the graded elemental distribution is attributed to the difference in the free energy barrier of the monomers, where Cd, Zn, Se, and S precursors have different reactivity and diffusion behavior to grow on the seeds. When the S-precursor is introduced, the CdS and ZnS would grow predominantly since high concentration S monomer provides more chemical potential for the crystal growth. The graded composition structure would be more beneficial to the QD luminescent properties because it reserves a high bandgap passivation layer but there is no obvious interface between CdSe and ZnS, which usually leads to lattice mismatch and dangling bonds resulting in the trap energy states. Therefore, the alloyed QDs can still effectively confine the electrons and the holes within the inner QD part that boosts the recombination in a more efficient way.



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Figure 6. (a) Variations of the diameter and the PL wavelength of bimodal QDs against the reaction time according to the TEM images and the PL spectra. The 30 s sample represents the [ZnxCd1-xSe] cores and the S-precursor was injected immediately after the sample collection. (b) Radial profiles of the composition of bimodal QDs are estimated according to the ICP-MS and TEM analyses. Ratios of Zn/Cd and Se/S as a function of the QD diameter are estimated for samples collected at 30, 60, 120, and 180 s. The calculation is based on the assumption of similar shell composition for both size groups as shown by above TEM/PL data. The mean diameter is an arithmetic average of both size groups and the outer compositions are estimated by subtracting the core contribution. Inset schematically shows the growth of bimodal QDs.

Figure 7. Sketch of the proposed growth mechanism of alloyed [ZnxCd1-xSeyS1-y] QDs with a bimodal size distribution.

Illustration of Bimodal Growth. The growth of alloyed bimodal [ZnxCd1-xSeyS1-y] QDs is schematically depicted in Figure 7. As mentioned above, [ZnxCd1-xSe] seeds with a bimodal size distribution first form in the initial stage after the first Se-precursor injection. Both of meta-stable B-aSeeds (small size group) and AaSeeds (large size group) are further driven to grow by a second S-precursor injection. The injected S monomers could react with remained Zn/Cd monomers in the presence of Se monomers so the growth of both A-aSeeds and B-aSeeds incorporating Zn, Cd, Se, and S elements are expected. Thus, alloyed [ZnxCd1-xSeyS1y] QDs with a bimodal size distribution (i.e., A-aQDs and B-aQDs) are finally obtained. In fact, the injected S monomers trigger the growth of meta-stable B-aSeeds that looks like they gain extra chemical energy to surpass the chemical potential barrier. 10 ACS Paragon Plus Environment

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Individual Size Group by Size Separation. According to the above results, the crystal size would play a relatively more important role than the composition for the QD PL characteristics. In order to further check the size effect, we attempted to separate two size groups by centrifugation together with a poor solvent for examining their PL properties. Figure 8a shows PL spectra of size-separated A-aQDs and B-aQDs, exhibiting that long-wavelength and short-wavelength PL peaks are indeed derived from the individual A-aQD and BaQD size groups, respectively. TEM images in Figure 8b and c further support that the size-separated A-aQDs and B-aQDs belong to the size groups at around 8.0 and 6.2 nm, respectively. Therefore, the QD size is a relatively more dominant factor in controlling the emission wavelength. On the other hand, A-aQDs and BaQDs are also analyzed by ICP-MS to evaluate contribution of the composition, as shown in Figure 9a. The chemical composition Cd/Zn/Se/S of separated A-aQD and B-aQD samples are 0.226/0.352/0.062/0.36 and 0.19/0.387/0.034/0.389, respectively, showing that their chemical compositions are rather similar, except more Se and S existing in A-aQDs and B-aQDs, respectively (Figure 9a). This is understandable as A-aQDs and BaQDs are from larger A-aSeeds and smaller B-aSeeds grown in the Se-rich environment in the initial stage, so the final A-aQDs possess more Se elements since the shell composition of both size groups are similar. This fact also implies that the chemical composition plays a relatively more important role in the initial PL wavelengths of A-aSeeds and B-aSeeds. The crystallography of size-separated A-aQD and B-aQD samples are examined by XRD, as shown in Figure 9b. The diffraction peaks of both size groups are similar and locate between the bulk zinc-blende CdSe and ZnS phase, which also demonstrate they have the same zinc-blende structure and a similar composition.

Figure 8. (a) PL spectra from bimodal alloyed QDs, individual A-aQDs, and B-aQDs after size separation by centrifugation. Bimodal alloyed QDs synthesized by injecting the S-precursor into the reaction mixture at 60 s (TOPS-Inj@60 s). TEM images of bimodal alloyed QDs (b), size-separated A-aQDs (c), and size-separated B-aQDs (d), which are corresponding to the PL curves in (a).



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Figure 9. (a) The chemical compositions of size-separated A-aQDs and B-aQDs analyzed by ICP-MS. (b) XRD patterns of bimodal alloyed [ZnxCd1-xSeyS1-y] QDs together with bulk zinc-blende CdSe and ZnS JCPDS cards. The XRD patterns show that both AaQDs and B-aQDs have the zinc-blende structure.

Thermodynamic Model of Bimodal Growth. A growth model has been proposed to address the cogrowth of two distinct QD size groups having different solubilities in the same reaction environment, where the growth conditions and final equilibrium were expected to be the same. First of all, non-uniform spatial distribution of the monomer concentration or chemical potential could be excluded as the reaction system is under vigorous mixing so it is unlikely that only two specific size groups can simultaneously grow. A more possible explanation is that the QDs encounter their meta-stable sizes or morphologies during the growth, arising from the so-called magic size or magic morphology such as the closed-shell morphology,37‒39 which is a lower energy state of nanocrystals in the growth.31 As mentioned above, the lower energy state is considered to be a chemical potential well for QDs in the crystal growth, comprising of the volume free energy and the surface free energy terms. Consequently, the surface composition and the crystal morphology should influence the crystal growth especially for nanocrystals with a high specific surface area. The QD chemical potential-size curve involving the chemical potential wells is manifested schematically in Scheme 1. The ∆𝜇 curve (dashed line), which is determined by the Gibbs-Thomson relationship, is the driving force of the monomers depositing on the QD surface. It elucidates that the crystal ∆𝜇 gradually decreases with increasing size from the conventional predication. If considering magic sizes and morphologies of a molecular cluster or crystal, the ∆𝜇－size growth curve would be modified with a series of the potential wells, as schematically shown by the solid curves below the dashed curve. The crystals become relatively stable in some certain magic sizes or morphologies, i.e., they are confined in the chemical potential well. Under this circumstance, the crystal growth rate is zero or very slow, unless there is sufficient monomer free energy provided for crystal growth to overcome the energy barrier. In the present reaction system, the monomers involve Zn, Cd, Se, and S, which form alloyed nanocrystals with a radially graded composition at the outer layer as shown above. An initial Se-precursor injection into the Zn-Cd precursor induces nucleation and growth that lead to alloyed ZnxCd1-xSe seeds (aSeeds) in a bimodal size distribution because some smaller nanocrystals (i.e., B-aSeeds) grow relatively slow when they situate in the chemical potential well under a lower monomer concentration, while larger ones (A-aSeeds) still continue to grow, which eventually generate two distinct size groups, as has been shown in Figure 1. This phenomenon 12 ACS Paragon Plus Environment

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infers that the remained monomer chemical potential after the nucleation process should be between the energy barrier of A-aSeeds (EA-aSeeds) and that of B-aSeeds (EB-aSeeds), i.e., EA-aSeeds < ∆𝜇𝑚𝑜𝑛𝑜𝑚𝑒𝑟 < EB-aSeeds. When an extra S-precursor injection provides sufficient ∆𝜇𝑚𝑜𝑛𝑜𝑚𝑒𝑟 for overcoming EB-aSeeds and EA-aSeeds, both AaSeeds and B-aSeeds can grow into larger nanocrystals. Therefore, the strategy to generate bimodal alloyed QDs is that the overall chemical potential must exceed the energy barrier of A-aSeeds and B-aSeeds in their chemical potential wells to trigger the growth.

Scheme 1. Illustration of growth mechanism of bimodal alloyed [ZnxCd1-xSeyS1-y] QDs. Growth mechanism of [ZnxCd1-xSe] seeds without S-precursor injection is shown in supporting materials.

Examination of Chemical Potential Well. In order to check the chemical potential well, we further use different amounts of S-precursor to investigate the QD growth. All the excises are carried out in the same condition except different concentrations of S-precursor used. As the monomer chemical potential is determined by concentration (i.e., 𝜇 = 𝜇0 + 𝑅𝑇 ln(𝑐 ⁄𝑐0 )), an additional S-precursor injection provides extra chemical energy 𝜇𝑆 for the crystal growth if 𝜇𝑆 is large enough. Figure 10a shows PL spectra of samples synthesized by using a Se-precursor injection to generate bimodal aSeeds (curves 15 s & 30 s), followed by various amounts of S-precursor (1, 0.5, 0.25, and 0 mmol S, denoted by TOPS1, TOPS0.5, TOPS0.25 and TOPS0, respectively). When there is no S-precursor injected, the dual PL peaks from bimodal-sized seeds finally turns back to a monomodal PL peak, which exhibits a classic QD growth event (curve S = 0). This result indicates that the larger A-aSeeds can grow to red QDs, while smaller B-aSeeds would finally dissolve in the final stage due to Ostwald ripening, as shown by Figure 10b. A similar result is observed when a small amount of S (e.g., S = 0.25 mmol) is introduced, though the final PL peak slightly broadens, showing that the QD size distribution is altered by the added S-precursor, but the smaller particles are still not observed in the final stage. Moreover, it is noticed that an introduced S-precursor also slightly blue shifts the PL peak position, which is ascribed to an increased bandgap caused by incorporation of S, as shown by Figure 10b. If there is 13 ACS Paragon Plus Environment


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more S-precursor (e.g., S = 0.5 mmol) injected into the reaction system, two individual PL peaks appear and both of them move to longer wavelengths in the final stage, indicating the extra S monomer can drive both size groups to grow into bimodal alloyed QDs. Larger amount of S-precursor (e.g., S = 1 mmol) has a similar result to 0.5 mmol S-precursor but leads to obvious blue shifts in both PL peaks, which are also attributed to the widened bandgap by S incorporation. Because the reaction condition is the same for all these experiments, the chemical potential ∆𝜇𝑚 can be considered to be the same at the injection moment of the S-precursor for all these experiments. When the extra chemical potential, e.g., 𝜇𝑆 , is high enough, both A-aSeeds and BaSeeds would obtain sufficient chemical potential to surpass the chemical potential wells and this continuously grow to A-aQDs and B-aQDs (i.e., cases TOPS1 and TOPS0.5). In contrast, if the introduced 𝜇𝑆 is too low, the overall chemical potential is not adequate for B-aSeeds to overcome the chemical potential well, resulting in monomodal PL (i.e., cases TOPS0.25 and TOPS0). These results also support the existence of higher chemical potential barrier for B-aSeeds, which could be in a relatively more stable state.

Figure 10. (a) PL spectra of bimodal QDs synthesized using various amounts of S (0, 0.25, 0.5, and 1 mmol, denoted by TOPS0, TOPS0.25, TOPS0.5 and TOPSe1) injected at 30 s after the initial Se-precursor injection, which led to bimodal size-distributed seeds (curve 15 s). (b) The sketch shows the bimodal QD growth in relation to the chemical potential in different situations.

Control Relative PL Intensity by Initial Se Monomer Concentration. On the basis of the proposed growth model, we attempted to control the relative PL intensity and wavelength for bimodal alloyed QDs by adjusting the growth time and concentration of injected precursors. First of all, the relative PL intensity is varied by the ratio of amounts of A-aSeeds and B-aSeeds via changing the initial Se monomer concentration. Since it is known that EA-aSeeds < EB-aSeeds, a higher initial Se monomer concentration would lead to more AaSeeds than B-aSeeds if EA-aSeeds < ∆𝜇𝑚𝑜𝑛𝑜𝑚𝑒𝑟 < EB-aSeeds. Then, a following S-precursor injection is used to drive both size groups of the seeds, i.e., A-aSeeds and B-aSeeds, to grow to A-aQDs and B-aQDs, respectively. Figure 11a presents PL spectra of bimodal alloyed QDs prepared with various amounts of an initial Seprecursor injection (0.117, 0.233, and 0.35 mmol Se, denoted by TOPSe1, TOPSe2, and TOPSe3, respectively). It exhibits that the relative PL intensity of A-aQDs to B-aQDs changes with different amounts 14 ACS Paragon Plus Environment

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of Se-precursor, while the PL wavelength roughly keeps at around 620 nm and 570 nm for A-aQDs and BaQDs, respectively. The QYs of the bimodal alloyed QDs ranges from 75 to 95% under single excitation wavelength between 470 to 492 nm. Control PL Wavelength of A-QDs. Moreover, the PL wavelength of A-QDs can be red-shifted by increasing the growth time of the seeds before the S-precursor injection. Figure 11b shows PL spectra of bimodal alloyed QDs prepared with a Se-precursor injection, followed by a S-precursor injection performed at various seed growth times (injected at 15, 30, 60, and 90 s, denoted by TOPS-Inj@15 s, TOPS-Inj@30 s, TOPS-Inj@60 s, and TOPS-Inj@90 s). TEM images of products are shown in Figure 11c-f. As the growth time of the seeds lengthens, size difference between two size groups increases so the size distribution tends to be bimodal. For instance, if the S-precursor is introduced shorter than 15 s (the seed growth time < 15 s), a final product is found to be in a monomodal size distribution because the remained Se monomer concentration is high and the both size groups are still growing. Additional injected S-precursor just speeds the growth of both size groups and the smaller size group may grow faster grow faster and join the larger size group that is so-called size focusing event. If the S-precursor injection is carried out late (i.e., in the dual-size window 15 －60 s), the small particles are not capable of joining the large size group since the size difference two size groups is too large, so two individual size groups will still exist in the final product, as shown in Figure 11b. Further late S-precursor injections lead to more distinctive bimodal size distribution, as shown in Figure 11cf. The PL wavelength of B-aQDs can be adjusted by the amount of S-precursor as has been shown above. Besides, the relative PL intensity is also changed with the S-precursor injection. As shown above, A-aSeeds can individually grow while B-aSeeds still remain in the original state after initial Se-precursor injection without the S-precursor injection. Consequently, when growth time of the bimodal seeds increases, more Cd, Zn, and Se monomers consumed by the A-aSeed growth, which results in insufficient S monomer concentration for growing B-aSeeds, so the relative PL intensity of B-aQDs decreases. The total QYs of those bimodal alloyed QDs in Figure 11 ranges from 65 to 95 %. Direct Use of As-prepared Bimodal QDs on White LEDs. As-prepared bimodal alloyed QDs are directly used to make a QD white light emitting diode (QD LED) without any QD mixing or color-adjusting process. The QDs are first prepared with a Se-precursor injection followed by another S-precursor injection carried out at different growth times of [ZnxCd1-xSe] seeds (15, 30, and 60 s) within the bimodal-size time window, producing different combinations of emission wavelength and intensity, as shown in Figure 12a. The color axes of three QD LEDs with as-synthesized QDs dispersed in silicone can directly show white light (0.330, 0.248) generated from complementary colors, warm white (0.390, 0.294) from three primary colors, and nearly pure white (0.314, 0.295), as shown in Figure 12b. The advantages of the bimodal QDs prepared by the one-pot approach are apparent. Recently, applications of QDs to lighting and displays have been significantly concerned.7,8,15 In our experience, optical properties of QDs in different polymers are significantly affected by the QD dispersion, which determines re-absorption and energy transfer among different QDs. However, we have found that QDs synthesized in various synthetic conditions exhibit different dispersivities in the same matrix due to unlike surface ligands, surface composition, and size difference. For example, red and green QDs have different degrees of dispersion in silicone or other polymers so mixing of red and green QDs with a selected polymer has been considered to be one of the key factors in QD-based 15 ACS Paragon Plus Environment


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LEDs. In the present study, the red and green emitting QDs are grown from the same reaction batch, producing bimodal alloyed QDs with two different colors in the same chemical environment. Consequently, their dispersion and the luminescence property are easily to be predicted. Moreover, the method also reduces the process complexity of color adjustment and the production cost of QD application for LEDs and other applications. 40 Therefore, this one-pot method, producing QDs with multiple emission bands, has been considered highly promising in practical applications and being introduced to mass production.

Figure 11. (a) PL spectra of bimodal alloyed QDs prepared with various amounts Se (0.117, 0.233, and 0.35 mmol, denoted by TOPSe1, TOPSe2 and TOPSe3), followed by a S-precursor injection at 30 s. The relative PL intensities of A-aQDs to B-aQDs with similar PL wavelengths can be varied by concentration of the initial Se-precursor injection. (b) PL spectra of bimodal alloyed QDs prepared from the bimodal seeds with different growth times (15－90 s), followed by a S-precursor injection. TEM images of QDs obtained from different seed growth times for (c) 15 s, (d) 30 s, (e) 60 s, and (f) 90 s.


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Figure 12. (a) Emission spectra of QD LED composed of an InGaN chip (λem = 450 nm) and as-synthesized bimodal alloyed QDs without post-mixing. PL properties of the QDs were controlled by the S-precursor injection times (15 s, 30 s, and 60 s). The assynthesized QDs were directly mixed with a commercial silicone to form freestanding light converting films, which were then placed on InGaN chips. The LEDs were operated at 10 mA. (b) Color axes of LEDs are shown in the CIE 1931 color space chromaticity diagram. Inset images show the device structure and a digit photo for the QD LED.

■ CONCLUSIONS One-pot synthetic method of preparing highly efficient alloyed [ZnxCd1-xSeyS1-y] QDs with controllable bimodal size distribution have been reported for the first time. The bimodal alloyed [ZnxCd1-xSeyS1-y] QDs possess QY as high as 95%, where the larger size group (A-aQDs) and the smaller size group (B-aQDs) contribute to 30% and 65%, respectively, under the excitation wavelength at 492 nm. The emission intensity and wavelength of the bimodal QDs can be adjusted by the growth time of the seeds and the following injected S-precursor concentrations. We also demonstrated QDs-based LEDs directly prepared from as-synthesized bimodal alloyed QDs without any QD mixing or color-adjusting process. Both the color axes and color rendering index of as-prepared white LEDs can be controlled by changing the synthetic parameters of the bimodal alloyed QDs during the synthetic process, which dramatically simplifies the fabrication process and can prevent QDs from the potential environmental contaminations. This method has been considered to be promising in preparing QDs for lighting and display applications.

■ ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. The QY variations of A-aQDs and B-aQDs during the growth, PL and PLE spectra, and sketch of growth mechanism of bimodal seeds without S-precursor injection are included.

■ ACKNOWLEDGMENT This work was supported by Ministry of Science and Technology (Taiwan) (contract nos. 102-2218-E17 ACS Paragon Plus Environment


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007-013, 104-2623-E-007-007-ET, and 105-2119-M-007-031).

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