CdS and

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Letter

Effect of Electronic Structure on the Stability of CdSe/CdS and CdSe/CdS/ ZnS Quantum Dot Phosphor Incorporated into Silica/Alumina Monolith Zhichun Li, Long Kong, Hua Sun, Shouqiang Huang, and Liang Li ACS Appl. Nano Mater., Just Accepted Manuscript • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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ACS Applied Nano Materials

Effect of Electronic Structure on the Stability of CdSe/CdS and CdSe/CdS/ZnS Quantum Dot Phosphor Incorporated into Silica/Alumina Monolith Zhichun Li,a Long Kong,a Hua Sun,a Shouqiang Huang,a and Liang Li a,b,* a

School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800

Dongchuan Road, Shanghai 200240, China b

Shanghai Institute of Pollution Control and Ecological Security, 1239 Siping Road，Shanghai

200092, China * Corresponding author. E-mail: [email protected].

ACS Paragon Plus Environment

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Abstract: In this letter, we prepare ultrastable QD based phosphors. Furthermore, we find that type I CdSe/CdS/ZnS QD are fair stable during the thermal annealing process of QD-SiO2/Al2O3 monolith (SAM) phosphor, but quasi type II CdSe/CdS QD are easy to lose their emission, possibly because the delocalized electrons to shell are more susceptible to be captured by the surface trap sites. The LED encapsulated with CdSe/CdS/ZnS QD-SAM phosphor on the blue LED chip can maintain approximately 90% of its initial intensity after 450 h, in contrast, the light intensity of reference LED using CdSe/CdS/ZnS QD decrease to approximately 41%.

Keywords: quantum dot, electronic structure, SiO2/Al2O3 monolith, quantum dot phosphors, stability


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Introduction Semiconductor quantum dots (QD) have garnered immense attention over past decades because of their unique size dependent optical properties, 1,2 such as narrowband emission, high quantum efficiency, color tunability, and high color purity. They show tremendous potential as light-emitting diode (LED) down-converters for lighting and displays.3-5 Several QD based LED products are already available on market through incorporating QD into polymer films that have excellent barrier properties (“on surface”) or sealing QD into a glass micro-tubes (“on edge”), in which the obtained shelf-life of QD LED could extend to thousands of hours.6 But for the lighting industry, one of the ideal application forms of QD as down converter should be “on chip”. However, the lifetime of QD “on chip” especially on high power blue LED chip is still far from satisfied with the corresponding threshold of LED industry.7 Therefore, the long-term stability of QD itself still arouses people’s concern. In the past decades, a lot of efforts have been made to overcome stability problems of QD. For example, QD-silica monolith,8 mixed QD-salt crystals,9,10 aluminum oxide (Al2O3) coated QD,11 and QD/polymer hybrid exhibited excellent stabilities.12 As mentioned above, the enormous progresses have been achieved to address stability issues of QD, but there is still much room for boosting long-term photostability and thermal stability of QD in harsh conditions. Another issue for the LED application of QD is the dispersion problem. QD generally cannot be directly dispersed into the widely used silicone resins because of their incompatible surface, which results in the aggregation of QD and follows by self-absorption and light scattering loss. In most of cases, it is imperative to perform surface treatments to make them dispersible in silicone resin. It will be very helpful for QD-LED field if QD phosphors could be directly used


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as same as the rare earth phosphors in the current encapsulation process of LED industry, i.e. simply mixing them into resins without surface pre-treatment. It has already been proved that the SiO2/Al2O3 binary coating can provide better protection than the individual coating with SiO2 or Al2O3 for vulnerable perovskite QD, such as CsPbBr3 QD.13 The dense SiO2/Al2O3 layer on the surfaces could be perfect thermal, moisture, and oxygen barriers for the protected objects. In principle, if QD are successfully incorporated into SiO2/Al2O3 monolith (SAM), the stability of QD should be dramatically improved. Here, we prepare ultrastable QD phosphors to address the two aforementioned issues (stability, dispersion). The core-shell CdSe/CdS (Figure S1) and CdSe/CdS/ZnS QD (Figure S2) were selected as research object, because they have been reported as classic QD systems with high PLQYs, high stability, and easy reproducibility. The preparation procedure of QD phosphors was also provided in supporting information. Meanwhile, the initial effort to synthesize QD-SAM was made on CdSe/CdS QD (ca. 6.2 nm) (Figure S3). To our surprise, the PLQYs of CdSe/CdS QD (ca. 6.2 nm) showed negligible changes during the sol-gel reaction process (Figure S4 B), which indicated that the optical properties of QD were well maintained. Thermal annealing treatment can boost the photostability of QD-silica monolith, which is also be used to strengthen the stability of CsPbBr3 perovskite QD monolith. Accordingly, the CdSe/CdS (ca. 6.2 nm) QD-SAM was annealed for 1 h at three different temperatures under vacuum condition. It was observed that the PL intensity of QD-SAM phosphor decreased after annealing (Figure S4 C). We suspected that the PL decrease might origin from the thin CdS shell which didn’t well protect QD. Therefore, we increased the thickness of CdS shell according to the previous reports (Figure S3).14 The larger CdSe/CdS (ca. 8.0 nm) QD-SAM phosphor indeed


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showed better stability than CdSe/CdS (ca. 6.2 nm) QD-SAM, but their PL still decreased after annealing. It is conceived that the PL quenching during the thermal annealing process is possibly related to the corresponding electronic structure of CdSe/CdS QD.

Figure 1. Time-resolved PL decays and the fitted curves for CdSe/CdS QD (ca. 6.2 and 8.0 nm), CdSe/CdS/ZnS QD solution (A) and their corresponding QD-SAM phosphor samples (B).

To prove our hypothesis, we did a parallel experiment for CdSe/CdS/ZnS QD (ca. 7.8 nm) (Figure S3). As expected, the PLQYs of CdSe/CdS/ZnS QD were well maintained during the process of sol-gel reaction and annealing (Figure S4 B, C). Generally, higher temperature can increase the extent of cross-linking, and the densified structure have a pronounced impact on the photostability. But, with further increasing the annealing temperature, the PL quenching became more and more serious especially at 150 °C. There is a tradeoff between annealing temperature and stability. Therefore, we chose 100 °C as the optimized annealing condition for the CdSe/CdS/ZnS QD-SAM. In order to further account for the aforementioned phenomenon of PL quenching, we performed time resolved PL measurements for the CdSe/CdS QD and CdSe/CdS/ZnS QD solution and their corresponding QD-SAM phosphor samples (Figure 1), which shown the traces of biexponential decay, with a fast component and a slow component. The fast component is


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assigned to the nonoradiative recombination, and the slow component corresponds to the radiative recombination.15 The average PL lifetimes for the CdSe/CdS QD (ca. 6.2 and 8.0 nm) and CdSe/CdS/ZnS QD solution were 27.1 ns, 42.0 ns, and 24.5 ns, respectively. More importantly, in the case of the CdSe/CdS (ca. 6.2 and 8.0 nm) QD-SAM phosphor, the obtained average PL lifetimes became shorter after thermal annealing, which were 19.1 ns and 36.2 ns, respectively. In contrast, the average PL lifetime for the CdSe/CdS/ZnS QD-SAM phosphor was 23.6 ns, which almost underwent no change. The observed distinct difference in average PL lifetime may be related to the different responses of their electronic structures to the surface damages.

CB CdSe

CdSe

VB

CB

VB CdSe (3.7 nm)

CB

CdS CdSe/CdS (6.2 nm) CB CdSe VB

CdS CdSe/CdS (5.2 nm)

VB

CdS CdSe/CdS (8.0 nm) ZnS CdS CB CdSe VB CdSe/CdS/ZnS (7.8 nm)

Figure 2. Schematic approximate energy band alignment of quasi-type II and type I electronic structure for CdSe/CdS QD and CdSe/CdS/ZnS QD. Figure 2 showed the approximate energy band alignment of quasi-type II and type I electronic structure for CdSe/CdS QD and CdSe/CdS/ZnS QD. Generally, the conduction band offset between CdSe and CdS is relatively small compared to the valence band offset.16Increasing the CdS shell thickness in the CdSe/CdS QD, the extent of the overlap between the electron and the


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hole wave functions gradually decrease, which allows the electrons to overcome the conduction band offset barrier, leading to a gradual transition from type I band alignment to a quasi-type II one. As a result, CdSe/CdS QD (ca. 8.0 nm) possess quasi-type II electronic structure, the electron is largely delocalized over the entire QD volume, while the hole is strongly confined within the CdSe core.17,18 The shell-localized electrons are more susceptible to the trap sites before they recombine with the holes in the CdSe core. Thermal annealing could damage the CdS surface of CdSe/CdS QD, and produced many trap sites on both the inner and outer interfaces of CdS shell, which may capture the delocalized electrons and lead to the PL quenching and the shorter PL lifetime for the CdSe/CdS QD. On the contrary, in the case of CdSe/CdS/ZnS QD, ZnS has a higher band offset compared with CdS, which provides a typical type I electronic structure for the CdSe/CdS/ZnS QD, where the electron and hole can be effectively confined into CdSe core. It is the reason that CdSe/CdS/ZnS QD are more insensitive to the surrounding environment and surface damages. XRD pattern of CdSe/CdS/ZnS QD-SAM phosphor exhibited three preferential diffraction peaks of the (111), (220), and (311) planes for CdSe/CdS/ZnS QD (Figure S5), and the peak position underwent no shift with different CdSe/CdS/ZnS QD contents in comparison with the pure QD, which indicated that the crystalline structure of CdSe/CdS/ZnS QD was well preserved. Figure S6 showed that the PL peak position of the CdSe/CdS/ZnS QD-SAM phosphor red-shifted because of the increasing reabsorption as CdSe/CdS/ZnS QD content increased.19 Interestingly, the CdSe/CdS/ZnS QD-SAM was transparent as shown in inset of Figure S4 A, indicating that CdSe/CdS/ZnS QD was evenly dispersed in the SAM without evident aggregation, which was further proved by TEM images (Figure S7 and S8), where the CdSe/CdS/ZnS QD existed as individual


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nanoparticles in QD-SAM. The CdSe/CdS QD-SAM phosphor showed a similar phenomenon (Figure S9), what’s more, it indicated that the instability of CdSe/CdS QDSAM QD phosphor had nothing to do with the aggregation, which may depended on the type II electronic structure for the CdSe/CdS QD. The SEM image of CdSe/CdS/ZnS QDSAM phosphor was shown in Figure S10. The size of CdSe/CdS/ZnS QD phosphors was 150-230 nm, which was similar to some conventional phosphors. Meanwhile, the average absolute PLQYs of CdSe/CdS/ZnS QD-SAM phoshpor was 55%, just slightly lower than the pure QD in solution form (62%). It indicates that incorporating into SAM and milling process didn’t significantly alter the optical properties of QD.

Figure 3. Thermal stability of the CdSe/CdS/ZnS QD, SiO2 coated CdSe/CdS/ZnS QD powders, and CdSe/CdS/ZnS QD-SAM phosphors at 100 °C (A). Photostability of corresponding samples sealed with optical adhesive on the LED chip (B). When QD are applied into the devices, such as LED, the thermal stability are critical under the high energy excitation source. For comparison, the SiO2 coated CdSe/CdS/ZnS QD was also annealed as control group. The thermal stability of CdSe/CdS/ZnS QD, milled SiO2 coated CdSe/CdS/ZnS QD, and milled CdSe/CdS/ZnS QD-SAM phosphor in the powder forms were tested in an oven at 100 °C and 150 °C. Figure 3A showed that CdSe/CdS/ZnS QD-SAM phosphors possessed excellent thermal stability compared with pure QD and SiO2 coated QD at 100 °C. Owing to the lack of protective layer, the pure CdSe/CdS/ZnS QD quickly quenched and


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turned dark red color due to the thermal oxidation of QD surface. It seemed that SiO2 individual protective layer also improved the thermal stability of CdSe/CdS/ZnS QD, but the formed SiO2/Al2O3 binary protective layers could better insulate QD from the thermal oxidation and etching, and endowed CdSe/CdS/ZnS QD with better thermal stability. Meanwhile, CdSe/CdS/ZnS QD-SAM phosphors also exhibited improved thermal stability at 150 °C (Figure S11). In addition, we tested photostability of CdSe/CdS/ZnS QD-SAM phosphors sealed in UVcured optical adhesive (Norland 61) on the blue LED chip (peak at 455 nm, Philips Lumileds Rebel, 3W), which was operated at 20 mA under ambient temperature (ca. 20 °C and relative humidity 50%-60%), the test setups were shown in Figure S12 and S13. Figure 3B showed that CdSe/CdS/ZnS QD-SAM phosphors manifested prominent photostability compared with the pure QD; its emission was maintained at approximately 90% of the initial intensity after 450 h of operation. In contrast, the emission from the pure QD and SiO2 coated QD decreased to approximately 41% and 49% of the initial intensity under the intense blue light irradiation, respectively. The improved photostability of CdSe/CdS/ZnS QD on the LED at high currents was attributed to the formed more compact SiO2/Al2O3 protective layer, which worked as a barrier effectively preventing oxygen and moisture from reaching the QD. In addition, we investigated the photostability of CdSe/CdS/ZnS QD-SAM phosphors without annealing treatment compared with pure QD. As shown in Figure S14, the photostability of CdSe/CdS/ZnS QD-SAM phosphors was better than pure QD, but it was far away from the performance of annealed samples, which indicated that the annealing treatment was a crucial process for boosting the photostability of QD-SAM. Furthermore, we compared the thermal stability of CdSe/CdS/ZnS QD-SAM phosphors with different QD contents (Figure S15), and the sample


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with 23 wt% QD showed the most stable thermal behavior. But further increasing the DBATES percentage would deteriorate the thermal stability of QD. It was possible that some more byproducts, such as water or sec-butyl alcohol, were released during the annealing process, which could damage CdSe/CdS/ZnS QD in the QD-SAM. Moreover, we tested photostability on the LED operating at 20 mA under 50 °C and 70% relative humidity. Figure S16 showed that the CdSe/CdS/ZnS QD-SAM phosphors were much less sensitive to the humid and hot environment than pure QD, the remnant PL emission of CdSe/CdS/ZnS QD-SAM phosphors was maintain about 90% after illumination for 300 h, whereas the value for the pure QD and SiO2 coated QD drastically decreased to 25% and 40% after 144 h, respectively. Again, it was confirmed that CdSe/CdS/ZnS QD-SAM phosphors possessed excellent photostability.

Figure 4. FTIR spectra of SAM and CdSe/CdS/ZnS QD-SAM phosphor with varied QD wt% (A). S 2p XPS spectra of the CdSe/CdS/ZnS QD-SAM phosphor, SiO2 coated CdSe/CdS/ZnS QD, and CdSe/CdS/ZnS QD after light irradiation (B). Figure 4A showed the FTIR spectra of the SAM and CdSe/CdS/ZnS QD-SAM phosphor with varied QD wt%. A broad band from 992 cm-1 to 1155 cm-1 corresponding to amorphous aluminum silicate (xAl2O3.ySiO2), 20 was presented in both CdSe/CdS/ZnS QD-SAM phosphor and SAM, which may be as a proof of existence of Al2O3 and SiO2 in


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the CdSe/CdS/ZnS QD-SAM. Particularly, this broad band became stronger with increasing the DBATES content. At the same time, the CdSe/CdS/ZnS QD-SAM phosphors, SiO2 coated CdSe/CdS/ZnS QD, and CdSe/CdS/ZnS QD were treated for 12 h under a blue LED module with 0.70 W/cm2 light intensity (peak at 450 nm, Philips Fortimo), which was used to monitor the chemical changes of S before and after illumination by X-ray photoelectron spectroscopy (XPS). As shown in Figure 4B, the peak of S 2p at 162.0 eV was attributed to ZnS.21 After light irradiation; a new strong peak emerged at higher energy of 169.0 eV, which was usually assigned to SO42- group.22 This peak might originate from the oxidation of sulfur. In contrast, the XPS spectrum of the CdSe/CdS/ZnS QD-SAM phosphors and SiO2 coated CdSe/CdS/ZnS QD showed almost no changes after light irradiation. The results indicated that SiO2 also protected CdSe/CdS/ZnS QD from oxidation. But, under higher light intensity and temperature (sealed in UV cured optical adhesive on the blue LED chip), the photostability of CdSe/CdS/ZnS QD-SAM phosphors was still better than SiO2 coated CdSe/CdS/ZnS QD. In summary, we developed a novel approach to prepare QD-SiO2/Al2O3 monolith without ligand exchange and any aid of catalyst, such as ammonia, which avoided any damages to QD and preserved PLQYs. In addition, our study proved that the electronic structure of QD played very important roles on preserving optical properties during thermal treatments of QD. Type I CdSe/CdS/ZnS QD are fair stable during the thermal annealing process of QD-SAM phosphor, but quasi type II CdSe/CdS QD are easy to lose their emission. The compact SiO2 and Al2O3 in the QD-SAM act as robust protective layer to prevent the attack of oxygen and moisture from the QD. Therefore, the CdSe/CdS/ZnS QD-SAM exhibited stronger photostability during long-term light and heat shock compared to the vulnerable pure QD. We believe that this strategy


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represents a universal approach to improve the reliability of nano-phosphors including QD in LED applications.

Supporting Information Detailed description of the synthesis method, UV-Vis, and PL spectra for CdSe/CdS QD and CdSe/CdS/ZnS QD, TEM and SEM images, XRD patterns etc. These information are available free of charge via the internet at http://pubs.acs.org.

Author Information E-mail: [email protected]. ORCID Liang Li: 0000-0003-3898-0641 Notes The authors declare no competing financial interest. Acknowledgement This work is supported by the Major National Science and Technology Special Project of Water Pollution Control and Remediation (2017ZX07202) and National Natural Science Foundation of China (21773155). References 1. Jun, S.; Jang, E. Bright and Stable Alloy Core/Multishell Quantum Dots. Angew. Chem. Int. Ed. 2013, 52, 679-682. 2. Wang, Y.; Yang, S. C.; Yang, H.; Sun, H. D. Quaternary Alloy Quantum Dots: Toward LowThreshold Stimulated Emission and All-Solution-Processed Lasers in the Green Region. Adv. Optical Mater. 2015, 3, 652-657


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CdS and

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