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Bottom up Stabilization of CsPbBr3 Quantum Dots-Silica Sphere with Selective Surface Passivation via Atomic Layer Deposition Qinyong Xiang, Binze Zhou, Kun Cao, Yanwei Wen, Yun Li, Zhaojie Wang, Chenchen Jiang, Bin Shan, and Rong Chen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03096 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018
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Chemistry of Materials
Bottom up Stabilization of CsPbBr3 Quantum Dots-Silica Sphere with Selective Surface Passivation via Atomic Layer Deposition Qinyong Xiang,‡a, b Binze Zhou,‡a Kun Cao,*a Yanwei Wen,c Yun Li,a Zhaojie Wang,c Chenchen Jiang,c Bin Shanc and Rong Chen*a a. State Key Laboratory of Digital Manufacturing Equipment and Technology, School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Hubei 430074, China. b. China-EU Institute for Clean and Renewable Energy, Huazhong University of Science and Technology, Hubei 430074, China. c. State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Hubei 430074, China. ‡: These authors contribute equally to this work. E-mail:
[email protected],
[email protected] ABSTRACT: All-inorganic perovskite quantum dots suffer from poor stability in humid and heat environment. In this article, CsPbBr3 quantum dots (CsPbBr3 QDs) are stabilized by coating nanoscale alumina on CsPbBr3 QDs-silica luminescent sphere (CsPbBr3 QDs-SLS) via atomic layer deposition (ALD). Utilizing the intrinsic reactivity differences towards precursors, the surface defect sites of CsPbBr3 QDs are selectively passivated. The inorganic alumina coating layers can effectively reduce the ion migration and crystal deformation of CsPbBr3 QDs. In situ quartz crystal microbalance (QCM) measurements show that organic ligands remain attached to the CsPbBr3 QDs surface during ALD coating process. NMR, XPS and first-principles calculations are performed to reveal the interaction strength between CsPbBr3 QDs-SLS and precursors. The surface passivation of alumina on CsPbBr3 QDs-SLS effectively stabilizes the QDs without reducing the photo luminescent quantum yield.
INTRODUCTION In the past few years, all inorganic CsPbX3 (X=Cl, Br, I) lead halide perovskite quantum dots (LHP-QDs) have attracted great attention due to their excellent photoelectricity properties including high photoluminescence quantum yields (PLQY), high absorption coefficient, high carrier diffusion length, narrow full width at half maxima (FWHM).1-6 With these unique properties, the LHP-QDs have wide potential applications in LED, backlight display, photo detector and lasing etc. 7-10 Despite excellent optical properties, the instability of LHP-QDs hinders their practical applications. When LHP-QDs are exposed to external environment, several factors such as humidity, heat and light can induce surface damage, phase deformation and aggregation, leading to the photoluminescence quenching.11-12 This problem has motivated researchers to investigate ways of improving LHP-QDs’ stability in long term usage.13-19 Several strategies for enhancing LHP-QDs’ stability, include surface engineering and physical confinement, have been proposed. For surface engineering, the ligands with strong interaction such as trioctylphosphine (TOP), didodecyl dimethyl ammonium bromide (DDAB) are utilized to replace the ligands with weak interaction such as oleic acid (OA), oleylamine (OAm).13-16 The ligands exchange method can passivate and preserve the
QDs surface. However, for photoluminescence (PL) applications, QDs usually suffer from high humidity, heat and photon flux, causing the exchanged ligands easily desorb from the surface, leading to surface damage and crystal phase deformation under such conditions. For physical confinement, coating methods have been developed, which are more effective to enhance the stability of LHP-QDs due to strong physical anchoring and separation effect. For example, LHP-QDs are embedded in barrier materials (silica, alumina and zirconium oxide etc.) that are capable of reducing aggregation of QDs and avoiding contact with external environment. 17-19 However, the process is usually based on hydrolysis or swelling process, where LHP-QDs are located at the shallow surface of the sphere supports and some surface sites of LHP-QDs remain unpassivated.18-19 The ligands of LHP-QDs may also get destroyed during the embedding process. Achieving controllable coating and surface passivation of LHP-QDs on an atomic level remain a challenging endeavor. Atomic layer deposition (ALD) is a gas phase thin film fabrication method based on self-limiting chemistry. Since the thickness of coating layers could be achieved by varying ALD cycles, it is a promising method to achieve sub-nanometer precision for surface passivation and coating.20-23 Our group has demonstrated the use of ALD in constructing nanotraps
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Scheme 1. Schematic diagram for fabricating CsPbBr3 QDs embedded in silica sphere (QDs-SLS) and selective surface passivation via ALD of CsPbBr3 QDs silica luminescent sphere (SALS). around Pt nanoparticles (NPs) to improve their anti-sintering ability.24 Recently, we have further developed facet-selective ALD recipe where intrinsic surface energy differences are utilized in controlling deposition sites selectivity. Both these methods demonstrate the capability of ALD in atomic scale decoration of metallic NPs, effectively improving NPs’ structural stability and protecting volatile sites. 25-27 Similar technique could be a befitting method for QDs passivation. Several groups have investigated the application of ALD method in QDs stabilization. 28-30 For example, Yin et al. were the first to utilized ALD to improve the photo and thermal stabilities of chalcogenide QDs.29 Loiudice et al. utilized ALD method to stabilize CsPbBr3 QDs with alumina coating.30 While the method is beneficiary to the overall stability, it is important to study the surface growth mechanism and improve the optical performance of ALD coated CsPbBr3 QDs. The performance degradation is primarily caused by ion migrations or surface damages during ALD processes, which becomes only more critical for LHP-QDs, as LHP-QDs are more sensitive to environment than Cadmium based QDs. Slightly aggregation is also observed during thermal ALD coating process. 28Thus, it is of great importance to develop a benign and compatible ALD process tailored to the uniform encapsulation and stabilization of LHP-QDs. In this article, we report the development of a mild low-temperature ALD method for defect elimination and surface coating of CsPbBr3 QDs. This protection method can preserve CsPbBr3 QDs without damaging surface ligands and decreasing the optical performance. As shown in Scheme 1, the CsPbBr3 QDs are firstly dispersed onto silica spheres. Then ALD is carried out to coat alumina on the composite QDs sphere, while keeping the deposition temperature to as low as 50 ºC. As the ligands with long carbon chains on QDs have inert –CH3 end groups, the precursors (Trimethylaluminum, TMA) are selectively chemisorbed on the unsaturated surface groups and initiate the subsequent alumina growth. In this way, unprotected sites (without long chain protection) are selectively passivated with alumina and the ligands are kept intact. The composite QDs structures not only improve the initial PLQY value but also exhibit excellent stability in polar solvent, light and heat environment.
EXPERIMENT SECTION 1. Synthesis of CsPbBr3 QDs The CsPbBr3 QDs are synthesized according to a hot injection method.31 Typically, 15 mL of octadecene (ODE), and PbBr2 (0.2 g) are loaded into a 250 mL four-neck flask, after 10 min N2 flow treatment, the mixtures are degassed at 120 ºC for 60 min. Then 1.5 mL of OA, 3 ml of OAm are added into the four-neck flask and heated to 170 ºC under N2 flow. Then, 0.55 mL Cesium Stearate (CsSt) solution (0.15 M in ODE, 80 ºC) is injected into the flask. After 5 s reaction, the mixture is cooled by the ice-water bath. The resultant QDs are precipitated by 40 mL of ethyl acetate and separated via centrifugation (5000 RPM, 15 min). Then, the precipitate is dispersed in a toluene solution for synthesizing CsPbBr3 QDs-SLS. 2. Synthesis of CsPbBr3 QDs-SLS
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500μL TMOS is introduced into a 250 ml three-necked flask containing 40 mL of colloidal CsPbBr3-QDs toluene solution with sealing plugs. The sealed three-necked flask is placed in air environment with the temperature of 25 ºC and Relative humidity (RH) of 45%. After stirring 19 h, the CsPbBr3 QDs-SLS (precipitates) are collected through centrifugation at 5000 rpm for 15 min, and then the mixture is dried under vacuum for 6 h with a temperature of 25 ºC. The CsPbBr3 QDs-SLS is obtained by grinding the dried solid in an agate mortar (0.106 mm). 3. ALD coating processes Alumina films are fabricated in specially designed rotary fluidized ALD reactor. Before ALD coating, the CsPbBr3 QDs-SLS are sieved through a 140 mesh screen (0.106 mm) to get rid of large agglomerated CsPbBr3 QDs-SLS. 100 mg of sieved CsPbBr3 QDs-SLS is added in the sample holder. Trimethylaluminum (TMA) and H2O are used as precursors. N2 (99.9999% pure) is used as both fluidizing gas and carrier gas for precursors doses and purge. The fluidizing rotation speed is set at 200 rpm to disperse CsPbBr3 QDs-SLS during reactions to ensure uniform coating on the powder samples. The process of each ALD deposition cycle contains TMA pulse (t1), N2 purging (t2), H2O pulse (t3) and N2 purging (t4). The optimized time sequence is set at t1=10 s, t2=120 s, t3=10 s and t4=120 s. 4. Characterization The UV/vis absorption spectra are recorded with a lambda 35 UV/vis spectrophotometer (PerkinElmer Co.). The photoluminescence (PL) spectra and fluorescent lifetime are measured with time resolved fluorescence spectrometer (Edinburgh instruments FLS-980, 365 nm Xe lamp). The PLQY is measured using a barium sulfate coated integrating sphere attached to the FLS-980. The photoluminescence (PL) decay is measured by a FP-6500 (Jasco Co.) fluorescence spectrometer equipped with a 365 nm Xe lamp. XRD measurements are performed at 40 KV and 40 mA using a Cu Ka radiation (λ=1.5406 Å). The samples are scanned from 10°
