Research Article

Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22801−22808

Two-Step-Enhanced Stability of Quantum Dots via Silica and Siloxane Encapsulation for the Long-Term Operation of - Emitting Diodes Yun Hyeok Kim, Hyunhwan Lee, Seung-Mo Kang, and Byeong-Soo Bae* Wearable Platform Materials Technology Center Department of and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

*S Supporting Information

ABSTRACT: Despite innovative optical properties of quantum dots (QDs) for QDs- converted light-emitting diodes (QD-LEDs), the vulnerability of the QDs, against heat and moisture, has been a critical issue for commercialization and long-term use. To overcome the instabilities, we fabricated a thermally and photostable QDs-embedded silica/siloxane (S-QD/siloxane) film by embedding QDs in silica and siloxane encapsulation through a two-step sol−gel reaction. S-QDs were stably dispersed in the oligo-siloxane resin with even a QD concentration of 5 wt % without aggregation. The two-step physical barriers of silica and siloxane acted to decrease the of QDs and improve the stability against heat and moisture [85 °C/5% relative humidity (RH), 85 °C/85% RH, and 120 °C/5% RH], light (50 and 100 mA), and chemicals (ethanol, HCl, and NaOH). Our S-QD/siloxane film was applied as a color-conversion material on a blue LED chip without additional solidification and encapsulation processes for red and white QD-LEDs, exhibiting a wider color gamut (107% in CIE 1931) compared to NTSC. These enhancements indicate that our S-QD/siloxane film is a suitable material for long-term operation of QD- enhanced films and QD-LEDs in next-generation displays. KEYWORDS: quantum dots, sol−gel reaction, white light-emitting diodes, thermal and photostability, long-term operation

1. INTRODUCTION on an LED chip. During the solidification process, hydro- White light-emitting diodes (WLEDs) are promising solid- phobic long-chain alkyl on QDs surface give rise to agglomeration of QDs and poor compatibility of QDs in the state lighting devices to replace conventional light sources such 14 as fluorescent lamps and incandescent bulbs because of their polymer matrix. Second, an encapsulation process is essential ffi 1,2 after the combination step because QDs are vulnerable against high e ciency and long lifetime. To commercially fabricate 15−17 3+ 3+ heat, moisture, and chemicals. Generally, ligands are the WLED, Y3Al5O12:Ce (YAG:Ce ), which is a yellow phosphor particle, on a blue LED chip is a general choice.3 easily detached from the surface of QDs during device Despite the cost-effective YAG:Ce3+ phosphor, the low color- fabrication and long-term use, that is, defects form and rendering index and broad full width at half-maximum aggregation occurs. The defects cause PL quenching and

See for options on how to legitimately share published articles. − oxidation, and the aggregation leads to reabsorption and (FWHM) (50 100 nm) of the phosphors make the WLED 18,19 Downloaded via KOREA ADVANCED INST SCI AND TECHLGY on July 31, 2019 at 09:33:11 (UTC). ffi show a large overlap in blue-green and green-red peaks.4 In degradation of quantum e ciency. Enhancing the addition, grain boundary light scattering and high sintering reliability and lifetime of QD-LEDs is a challenge for long- temperature (more than 1500 °C) are also the reasons for term operation. replacing the phosphor particles.5 One of the recent attempts to improve the incompatibility ffi -based colloidal quantum dots (QDs)− problemisligandexchange;thisprocessusesanity ff 20 polymer composites have aroused considerable interest for di erences between the and the surface of QDs. the color-converting materials as on-chip-type LED encapsu- Functionalized phenyl groups and oleic acid ligands on QDs fi 6−9 surface improved solubility in pyridine and poly(methyl lants and on-surface-type QD enhancement lms. This is 21,22 because QDs exhibit a wider color gamut, higher photo- methacrylate). However, ligand exchange has problems luminescence (PL QY), narrower FWHM, and that it changes the optical properties of the original QDs, − 23−25 size-dependent color emission, compared to phosphors.10 12 causing a drop in the PL QY. In addition, in terms of ff However, to commercialize the QDs-converted LEDs (QD- stability, ligand-exchanged QDs still su er from ligand LEDs), some critical issues should be solved during device fabrication. First, the QDs are dispersed in a polymer matrix Received: April 22, 2019 with a solvent.13 Therefore, a solidification process is a Accepted: June 3, 2019 prerequisite before combination of QDs−polymer composites Published: June 3, 2019

© 2019 American Chemical Society 22801 DOI: 10.1021/acsami.9b06987 ACS Appl. Mater. Interfaces 2019, 11, 22801−22808 ACS Applied Materials & Interfaces Research Article

Figure 1. Design concept of an S-QD/siloxane film. (a) Synthesis process and chemical structure of S-QD. (b) Sol−gel condensation reaction between MPTMS, DPSD, and S-QD. (c) Fabrication of an S-QD/siloxane film by UV-initiated free-radical polymerization between methacrylate groups of S-QD and oligo-siloxane resin. S-QD is chemically bonded to oligo-siloxane resin by radical polymerization and sol−gel condensation. Inset photographs are S-QD/oligo-siloxane resin and S-QD/siloxane film under 365 nm UV light. detachment and are vulnerable to oxygen and moisture due to In this report, we present QDs-embedded silica/siloxane (S- the lack of encapsulation with a physical barrier. Shell coating QD/siloxane) films along with silica growth and siloxane − with oxide materials (SiO2,Al2O3, and In2O3) on individual encapsulation using a simple sol gel reaction. The QDs were fi QDs, embedding of QDs in silica (SiO2), and ligand exchange xed in amorphous silica and chemically dispersed in oligo- have improved the long-term thermal stability and photo- siloxane resin, which solves aggregation problems in long-term stability.26,27 Among oxide materials, which protect the surface storage. In addition, no solidification process was needed due of QDs from oxidation and decrease exposure of inside toxic to the solvent-free composite. The dual improved S-QD/ , amorphous silica has been most investigated due siloxane film, compared to the reported OA-QD/siloxane film, to its high optical transparency, precise control of thickness, showed superior long-term operation under harsh heat (120 and hydrophilicity.28,29 However, the individual thick silica °C) and moisture [85% relative humidity (RH)] conditions, shell has critical limitations of high QD concentration in the chemicals (ethanol), and current bias (100 mA) without an QD−polymer composite and initial PL degradation. additional encapsulation layer. The S-QD/siloxane composite QDs-embedded silica, produced by ligand exchange and was then fabricated as a color-conversion material into white hydrolysis of tetraethyl orthosilicate (TEOS) or tetramethyl QD-LEDs, which exhibit a wider color gamut compared to orthosilicate (TMOS), has been widely studied, and improve- NTSC. This two-step-enhanced S-QD/siloxane proves to be ments to photostability have been reported.30,31 To prepare an appropriate candidate for QD-LEDs. QDs-embedded silica, the ligands on the surface of the QDs should be replaced by those having hydroxyl or alkoxy (Si− 2. RESULTS AND DISCUSSION OCH3) groups, which can react with hydrolyzed TEOS or The fabrication process of an S-QD/siloxane film, as a color- TMOS. For example, 3-mercaptopropyltrimethoxysilane, 3- conversion material, is shown in Figure 1 and the Experimental aminopropyltriethoxysilane, 5-aminopentanol, and 6-mercap- Section in the Supporting Information. We purchased CdSe/ tohexanol (6-MHOH) have been used as the exchange CdZnS (core/shell) QDs passivated by oleic acid and used 32−34 ligands. A stably dispersed QDs-silica monolith was them without further purification. First, we synthesized S-QD reported using 6-MHOH, and the QDs-silica monolith was by ligand exchange from oleic acid to 3-mercaptopropylme- 35 compatible with siloxane resin. Despite the enhancement of thyldimethoxysilane (MPMDMS) and sol−gel reaction with stability, the QDs-embedded silica has the problems of TEOS (Figure 1a). Due to the affinity difference to the surface precipitation when stored for a long time after physical of QDs, we simply exchanged the ligand from oleic acid with dispersion in resin. In addition, studies about thermal MPMDMS via stirring at room temperature. After formation of quenching problems over 100 °C, which is a temperature silica, we post-treated the silica with 3-methacryloxypropyl- under high driving current, are still limited due to the trimethoxysilane (MPTMS) to increase its dispersibility in the instability of QDs against heat and moisture. organic solvent and methacrylate-phenyl oligo-siloxane resin.

22802 DOI: 10.1021/acsami.9b06987 ACS Appl. Mater. Interfaces 2019, 11, 22801−22808 ACS Applied Materials & Interfaces Research Article

Figure 2. Synthesis and optical properties of S-QD powder. (a) Photographs, (b) SEM images, and (c) TEM and EDS mapping images of S-QD. (d) FT-IR at each step during synthesis of S-QD. (e) UV−vis absorption and PL spectra of OA-QD and S-QD. (f) Traces of PL QY of S-QD powder stored at room temperature and 85 °C/5% RH condition. The inset graph is the PL spectra of S-QD powder before and after 30 days aging at 85 °C/5% RH condition.

Figure S1 shows three different samples in chloroform solvent: high density (Figure 2c). The EDS analysis shows that the (a) without MPMDMS, (b) without MPTMS, and (c) silica matrix (Si: yellow and O: blue) is formed by sol−gel synthesized S-QD. The modified QDs without MPMDMS reaction, and the QDs (Cd: green, Zn: violet, and Se: cyan) are are not enclosed in silica (Figure S1a) because oleic acid has dispersed without aggregation in the silica matrix. As shown in no reaction site with hydrolyzed TEOS. The un-methacrylate- Figure 2d, Fourier transform (FT-IR) analysis was functionalized QDs-embedded silica (without MPTMS) is carried out to confirm the synthesis and surface chemistry of S- incompatible with chloroform due to its hydrophilicity (Figure QD. After ligand exchange with MPMDMS silane (before S1b). However, the methacrylate-functionalized S-QD is well insertion of TEOS), the two carboxyl carbon peaks (COO−, dispersed in chloroform, that is, MPTMS plays an important 1550 and 1471 cm−1), ascribed to the oleic acid ligand, role in increasing the hydrophobicity of S-QD (Figure S1c). fi − −1 signi cantly decreased and the Si CH3 peak (1078 cm ) was S-QD was chemically and physically dispersed in meth- observed. The Si−CH peak is equivalent to the Si−CH of acrylate-phenyl oligo-siloxane resin in situ during sol−gel 3 3 36 neat MPMDMS silane. The oleic acid was exchanged with condensation (Figure 1b) and then stably dispersed in the MPMDMS with stirring for 1 day. After formation of S-QD by oligo-siloxane resin for 30 days. After that, we fabricated an S- − − fi sol gel reaction, the amorphous silica peak (1100 1000 QD/siloxane lm with the desired shape through UV radiation cm−1) was observed. In addition, hydroxyl (−OH, 3500−3000 using molds, without an additional binder (Figure 1c). The − − cm 1) and double carbon (CC, 1638 cm 1) groups were methacrylate-phenyl siloxane hybrid material has been used as formed. The peaks of −OH and CC groups indicate that the an LED encapsulant and passivation layer in LCD-TFT surface of S-QD is composed of hydroxyl and methacrylate because it exhibits high thermal decomposition temperature, high , and low WVTR compared to conven- groups. Figure 2e displays the (PL) and − tional polymers.37 39 To elucidate the improvement of the absorption spectra of QDs dispersed in chloroform. No PL dispersion, thermal, chemical, and photostability of the S-QD/ peak shift and degradation of absorption were observed after fi embedding QDs in silica. The PL and absorption of the QDs siloxane lm, oleic acid-capped QDs/siloxane (OA-QD/ ff − siloxane) films were fabricated according to the method in a were not a ected by the hydrolytic sol gel reaction for silica previous paper.40 formation. However, the PL QY was gradually decreased from Figure 2 shows representative data for the synthesis and 84 to 73%, as QDs were embedded in silica (Table S1). We luminescence properties of S-QD. The synthesized S-QD is measured traces of PL QY of S-QD powder at room stably dispersed in chloroform and can be stored in a powder temperature and 85 °C/5% RH condition (Figure 2f). This state; it emits a bright red light under 365 nm UV light (Figure is because storage in the powder state has a big advantage in 2a). Figure 2b provides an SEM image of S-QD; it is terms of volume and transportation as compared with the amorphous silica by sol−gel reaction of TEOS and its average dispersion state in a solvent. The PL QY of S-QD powder size is about 300 nm (Figure S2). Transmission decreases 6% at room temperature and 13% at 85 °C/5% RH microscopy (TEM) and energy-dispersive (EDS) condition for 30 days. S-QD is quite stable and can be stored analysis confirm the homogenous isolation of QDs in silica at in a powder state at room temperature.

22803 DOI: 10.1021/acsami.9b06987 ACS Appl. Mater. Interfaces 2019, 11, 22801−22808 ACS Applied Materials & Interfaces Research Article

Figure 3. (a) Photographs and (b) confocal images of OA-QD (left) and S-QD/oligo-siloxane (right) resins at QD concentrations of 3 wt %. (c) Two-dimensional (2D) images of the emission of the 3 wt % S-QD/oligo-siloxane film in the area of 400 μm2. (d) PL QY and (e, f) PL spectra of (e) OA-QD/siloxane film and (f) S-QD/siloxane film at various QD concentrations.

Figure 4. Thermal stability of OA-QD and S-QD/siloxane films. (a) Photographs of QD/siloxane films under 120 °C/5% RH conditions during 30 days. (b) Traces of PL QY as a function of aging time, (c) PL decay curves, (d) lifetime, and (e) evolution of lifetime components after 30 days aging of OA-QD and S-QD/siloxane films under various heat and moisture conditions.

Figure S3a shows the 29Si NMR spectra of S-QD/oligo- and hydroxyl groups of S-QD during sol−gel condensation. siloxane resin, which confirm synthesis of methacrylate-phenyl This phenomenon is visualized by confocal microscopy, shown oligo-siloxane resin with a degree of condensation of 85.3%. in Figure 3b. S-QD/oligo-siloxane resin (right) shows uniform The highly condensed resin, via simple sol−gel condensation dispersion of QDs, whereas QDs in OA-QD/oligo-siloxane for only 4 h, could be used as a matrix having thermal stability. resin (left) are highly aggregated at 3 wt % QD concentration. Despite sol−gel condensation, PL QY of S-QD/siloxane resin We fabricated the QD/siloxane films using only QD/oligo- was the same as that of S-QD, that is, siloxane encapsulation siloxane resins by radiating UV light. The QD/siloxane films maintained the optical property of S-QD. To improve the color could be obtained with various shapes and thicknesses using purity and color-conversion ability of the QD/polymer the molds, which is a key advantage of the material for composite, the high-concentration dispersion of QDs in the manufacturing. Figure S3b indicates FT-IR spectra of the S- polymer matrix is essential.41 Figure 3a shows OA-QD and S- QD/oligo-siloxane resin and S-QD/siloxane film. The double QD/oligo-siloxane resins with a concentration of 3 wt %. In carbon-bond (CC, 1637 cm−1) decreases by polymer- the case of OA-QD/oligo-siloxane resin, precipitation of QDs ization;42 the methacrylate groups in oligo-siloxane resin and is observed due to long alkyl chains of oleic acid and only S-QD were chemically cross-linked by free-radical polymer- physical dispersion of QDs in oligo-siloxane resin. In contrast, ization. The highly condensed oligo-siloxane resin and cross- S-QD maintains uniform dispersion in oligo-siloxane resin for linked methacrylate-siloxane co-networks bring about rigid 30 days by chemical bonding between oligo-siloxane bulks and superior thermal and mechanical properties. The

22804 DOI: 10.1021/acsami.9b06987 ACS Appl. Mater. Interfaces 2019, 11, 22801−22808 ACS Applied Materials & Interfaces Research Article

Figure 5. Photostability of direct encapsulated OA-QD and S-QD/siloxane (OA-LEDs and S-LEDs) on a blue LED chip. (a) Traces of PL QY as a function of aging time. (b) Photographs of OA-LEDs (left) and S-LEDs (right) under 50 mA before and after 30 days. (c) CIE color coordinates, (d) PL decay curves, (e) lifetime and color purity, and (f) evolution of lifetime components after 30 days of OA-LEDs and S-LEDs under 50 and 100 mA. uniform dispersion of S-QD is maintained after polymerization hand, the S-QD/siloxane film shows long-term thermal in siloxane hybrid material, confirmed by 2D images of the stability under conditions of 85 °C/5% RH and enhancement emission wavelength in the area of 100 μm2 (Figure 3c). of PL QY under 85 °C/85% RH. The S-QD/siloxane film is Because of reabsorption from aggregation of OA-QD in oligo- still stable against heat and moisture at an even higher QDs siloxane resin, a large red shift (13 nm) is obtained when the concentration. Especially, the improvement of thermal stability concentration of OA-QD is only 3 wt % (Figure 3e).43 is observed to be at its best under the condition of 120 °C/5% Especially, 5 wt % OA-QD/oligo-siloxane resin could not be RH. After thermal stability tests for 30 days, PL decay synthesized due to agglomeration of QDs during sol−gel dynamics of films was measured (Figure 4c). The PL decay condensation. However, the PL of the S-QD/ curves of all samples after 30 days aging were fitted using a siloxane film shows only a slight red shift (<3 nm) even when single- or bi-exponential decay function to calculate average τ QD concentration is as high as 5 wt % (Figure 3f). Due to the lifetimes ( av) and decay time components (Figure 4d,e): I(t) − τ − τ τ τ agglomeration problem of OA-QD when its concentration is = A0 + A1 exp( t/ 1)+A2 exp( t/ 2), where 1 and 2 refer to more than 3 wt %, we compared stabilities using samples with the decay time components of emission.45 The initial OA-QD 3wt%. and S-QD/siloxane films show the same PL decay curves and To evaluate the enhancement of the thermal stability by lifetimes, that is, silica formation, by hydrolytic sol−gel embedding QDs in silica, we monitored the optical properties reaction of TEOS, maintains the optical properties of OA- of QD/siloxane films under harsh conditions (85 °C and 120 QD. Among all curves after thermal aging, a dramatic drop of °C/5% RH, and 85 °C/85% RH). Figure 4a shows the the PL decay curves is observed in the OA-QD/siloxane film fi ° ° τ photographs of QD/siloxane lms under 120 C/5% RH under the condition of 120 C/5% RH and av decreased from fi τ τ conditions during 30 days. The S-QD/siloxane lm maintains 64.5 to 13.2 ns ( 1 = 38.47 ns, A1 = 12.2%, 2 = 11.67, and A2 = fi τ its red color, but the OA-QD/siloxane lm turns from red to 87.8%). The new nonradiative ( 2) recombination pathway of dark-brown as the time passes. The degradation of PL under the OA-QD/siloxane film indicates an increase of recombina- UV light is also observed (Figure S4). The bright red emission tion due to the interaction between and generated 46 τ of OA-QD/siloxane is changed to no emission after 30 days surface defects. However, no 2 is obtained and only a slight τ aging. Figure 4b indicates traces of PL QY during thermal decrease of radiative recombination ( 1) is found in the S-QD/ stability tests for 30 days. In a previous paper, the PL QY of the siloxane film. Results about the superior thermal stability of the 1 wt % OA-QD/siloxane film was maintained to its initial value S-QD/siloxane film agree well with the traces of PL QY. This under 85 °C/5% RH and increased under 85 °C/85% RH thermal stability may be derived from short silane ligands conditions by passivating trap states with water molecules.40,44 (lower organic portion) compared to oleic acid and fixed QDs Siloxane encapsulation was effective for the long-term thermal in the silica. In addition, using thermogravimetric analysis, the stability of QDs, compared to commercial polymers. However, S-QD/siloxane film is found to have delayed thermal when the QD concentration becomes 3 wt %, degradation of decomposition compared to that of neat methacrylate-phenyl PL QY is observed under the same conditions. On the other siloxane hybrid material (Figure S3c). Additional cross-linkage

22805 DOI: 10.1021/acsami.9b06987 ACS Appl. Mater. Interfaces 2019, 11, 22801−22808 ACS Applied Materials & Interfaces Research Article

Figure 6. (a) Normalized blue, green, and red spectra, (b) PL spectra, and (c) photographs of white QD-LEDs. CIE color coordinates of white QD-LEDs and individual color-filtered emission of our white QD-LEDs (red triangle) and NTSC standard (blue triangle) in (d) CIE 1931 and (e) CIE 1976 color space. and sol−gel condensation between S-QD and oligo-siloxane from 64.5 to 4.6 ns under 100 mA bias (Figure 5e,f). This lead to dense methacrylate-siloxane co-networks, resulting in a photoderived degradation of QDs in OA-LEDs is associated delay of thermal decomposition. with formation of nonradiative defect states on the surface or Figure 5 shows the photostability of QD/siloxane films, interface of the core/shell, referred to photo-oxidation,46 due directly encapsulated on a blue LED chip, with a QD to the weak affinity of the oleic acid ligand on QDs surface. In concentration of 3 wt %. The blue light, emitted from LED contrast, S-LEDs exhibit superior photostability as a slight τ chips, is completely converted to red light (Figure S5a). The decrease of av from 63.5 to 50.8 ns (50 mA, 30 days) and 44.9 τ PL spectra of the S-QD/siloxane-encapsulated LED (S-LED) ns (100 mA, 30 days); 1 shows the same tendency. In maintain their emission wavelength and increase the PL particular, color purity, which is an important factor for a intensity at forward bias currents from 10 to 100 mA (Figure color-conversion film, is also drastically reduced at current bias S5b). Compared to the OA-QD/siloxane-encapsulated LED in OA-LEDs (from 99 to 26% at 50 mA and 0.8% at 100 mA), (OA-LED) (Figure 5a), according to traces of PL QY, the whereas this is not the case in S-LEDs (from 99 to 96% at 50 photostability is considerably enhanced by embedding QDs in mA and 74% at 500 mA) (Figures 5f and S7).47 That is, silica silica. As shown in Figure 5b, the emission of OA-LEDs, under embedding acts as a barrier, enabling long-term preservation of 50 mA after 30 days, is a violet light that is close to blue. In QDs from harsh thermal and photo-operating conditions. In contrast, the pure red emission of S-QD is maintained after 30 addition, we measured the chemical stability of the S-QD/ days. Figures 5c and S6 show the Commission Internationale siloxane film in ethanol and in strong acidic (0.5 N HCl) and de l’Eclairage (CIE) color coordinates of QD-LEDs after strong basic (0.1 N NaOH) aqueous solutions, which cause a photostability tests: coordinates of OA-LEDs are greatly significant PL QY drop of QDs;19 the PL QY of the S-QD/ shifted from red to blue induced by thermal quenching and siloxane film is maintained for 30 days (Figure S8). That is, the photo-oxidation compared to those of S-LEDs, exhibiting no S-QD/siloxane film is suitable for device manufacturing. change (50 mA) and only a slight shift (100 mA). After To demonstrate our S-QD/siloxane film for white light- photostability tests for 30 days, we measured PL decay emitting diodes (WLEDs), we fabricated the white QD-LEDs dynamics (Figure 5d). In the case of OA-LEDs, short using a mixture of red and green colored S-QD/oligo-siloxane recombination (19.49 ns for 50 mA, 1.65 and 6.31 ns for resins on a blue LED chip. Each blue, green, and red light τ 100 mA, 30 days) and considerable drops of av are observed shows a narrow emission peak (Figure 6a), and the PL spectra

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Improved Performance from Multilayer Quantum Dot Light- Corresponding Author Emitting Diodes via Thermal Annealing of the Quantum Dot Layer. *E-mail: [email protected]. Adv. Mater. 2007, 19, 3371−3376. ́ ORCID (17) Shirasaki, Y.; Supran, G. J.; Bawendi, M. G.; Bulovic,V. Emergence of Colloidal Quantum-Dot Light-Emitting Technologies. Byeong-Soo Bae: 0000-0001-9424-1830 Nat. Photonics 2013, 7,13−23. Notes (18) Alivisatos, A. P. Semiconductor Clusters, Nanocrystals, and The authors declare no competing financial interest. Quantum Dots. Science 1996, 271, 933−937. (19) Noh, M.; Kim, T.; Lee, H.; Kim, C.-K.; Joo, S.-W.; Lee, K. ■ ACKNOWLEDGMENTS Quenching Caused by Aggregation of Water-Soluble CdSe Quantum Dots. Surf., A 2010, 359,39−44. This work was supported by the Wearable Platform Materials (20) Li, Z.-J.; Fan, X.-B.; Li, X.-B.; Li, J.-X.; Zhan, F.; Tao, Y.; Zhang, Technology Center (WMC) supported by a National Research X.; Kong, Q.-Y.; Zhao, N.-J.; Zhang, J.-P.; Ye, C.; Gao, Y.-J.; Wang, X.- Foundation of Korea (NRF) Grant funded by the Korean Z.; Meng, Q.-Y.; Feng, K.; Chen, B.; Tung, C.-H.; Wu, L.-Z. Direct Government (MSIP) (NRF-2016R1A5A1009926). Synthesis of All-Inorganic Heterostructured CdSe/CdS QDs in

22807 DOI: 10.1021/acsami.9b06987 ACS Appl. Mater. Interfaces 2019, 11, 22801−22808 ACS Applied Materials & Interfaces Research Article

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22808 DOI: 10.1021/acsami.9b06987 ACS Appl. Mater. Interfaces 2019, 11, 22801−22808