Hexamethyldisilazane-triggered room temperature synthesis of hydrophobic perovskite nanocrystals with enhanced stability for light-emitting diodes

Journal of Colloid and Interface Science 552 (2019) 101–110

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Hexamethyldisilazane-triggered room temperature synthesis of hydrophobic perovskite nanocrystals with enhanced stability for lightemitting diodes Tan Li a, Yanxi Ding a, Shefiu Kareem a, Fen Qiao b,⇑, Ghafar Ali c, Changyan Ji d, Xiujian Zhao a, Yi Xie a,⇑ a

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology No. 122, Luoshi Road, Wuhan 430070, PR China School of Energy & Power Engineering, Jiangsu University, Zhenjiang 212013, PR China Nanomaterials Research Group, Physics Division, PINSTECH, Islamabad 44000, Pakistan d Hunan Provincial Key Laboratory of Fine Ceramics and Powder Materials, School of Materials and Environmental Engineering, Hunan University of Humanities, Science and Technology, Loudi 417000, PR China b c

g r a p h i c a l a b s t r a c t Hexamethyldisilazane tunes morphology and enhances stability of the perovskite nanocrystals by surface modification with hydrophobic ACH3 groups.

a r t i c l e

i n f o

Article history: Received 25 March 2019 Revised 2 May 2019 Accepted 14 May 2019 Available online 15 May 2019 Keywords: All-inorganic perovskite nanocrystals Hexamethyldisilazane Hydrophobicity Stability WLEDs

a b s t r a c t A novel facile room-temperature, hexamethyldisilazane (HMDS)-mediated strategy is demonstrated for the synthesis of all-inorganic perovskite colloidal nanocrystals (NCs). As a unique reaction-triggering and morphology-directing agent, HMDS is introduced for the first time to trigger the roomtemperature reaction for generating perovskite NCs with controlled morphology and optical properties. Particularly, the stability of the resulting NCs is greatly enhanced due to the surface modification by hydrophobic -CH3 groups from HMDS. The typical CsPbBr3 perovskite NCs films are highly stable without significant decrease in photoluminesence (PL) intensity after being exposed to 60% relative humidity for 720 h. Moreover, no noticeable change of phase and morphology occurs even after 100 days of exposure. The representative CsPbBr3 NCs are employed in a prototype white-light-emitting diodes (WLEDs) on 365 nm commercial GaN chip. The present strategy provides a facile and versatile route not only to control the morphology and optical properties of perovskites nanomaterials at room temperature but also enhance their stability, which will bring promising potential application for optoelectronics. Ó 2019 Published by Elsevier Inc.

⇑ Corresponding authors. E-mail addresses: [email protected] (F. Qiao), [email protected] (Y. Xie). https://doi.org/10.1016/j.jcis.2019.05.044 0021-9797/Ó 2019 Published by Elsevier Inc.

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1. Introduction Metal halide perovskite materials have found great potentials in solution-processed photovoltaics (PVs) application [1,2]. More recently, colloidal nanocrystals (NCs) of perovskite have attracted much attention due to their promising potential applications in the fields of photovoltaics (PVs) [3], laser [4–6], solar concentrator [7], photocatalysis [8,9], hydrogen evolution [10], detectors [11– 14] and light-emitting diodes (LEDs) devices [15–20]. In particular, all-inorganic perovskite NCs demonstrate many advantages such as high quantum efficiencies, cost-effectiveness, narrow emission width, high structural stability, compositional versatility and controllable optical properties [12,21–24]. The physicochemical properties of perovskite NCs can be tuned in a direct synthesis or postchemical transformation to adjust their morphology and chemical composition [25–30]. The previously reported direct synthetic methods of perovskite NCs involved either hot-injection or heatup procedures, which usually required high temperature (˃100 oC). Apart from this, these procedures generally involved the large quantities of insulating long-chain capping ligands to solubilize the precursors, tune the morphology and/or maintain the colloidal stability. Moreover, judicious purification and/or surface engineering steps are required to overcome the ligand-induced charge injection barriers and to obtain high-performance optoelectronic devices. All these factors limit the potential cost advantage and practical applications of perovskite NCs. In comparison to the abundant reports on more mature hightemperature synthesis of perovskite nanomaterials, room temperature (RT) synthesis, a low cost and easily scalable technique, remained less explored in perovskite NCs [31–38]. Generally, the widely-reported low-temperature synthesis of perovskite nanomaterials was based on the ligand-assisted reprecipitation protocol (LARP) [39], which involved the injection of the dissolved precursors into a non-coordinating solvent in the presence of coordinating ligands. Compared with hot-injection, this technique required a lower ligand concentration, but was generally severely limited by the low solubility of Cs precursor. It is important to mention that some of the RT procedures for perovskite NCs generation involve the pre-synthesis of Cs-oleate precursor at high temperature, which was similar to the hot injection route [31,34,37]. On the other side, the inferior stability of cesium lead halide perovskites (CsPbX3, X = Cl, Br, I) NCs under ambient environment (e.g. variable humidity, heat, and/or light) has been one of the main obstacles in the practical utilization of perovskite nanomaterials [10,15,40–43]. The long-term operation stability is challenging for the reliable perovskite-based optoelectronic devices [44]. Exposure of the colloidal perovskite NCs to ambient environment leads to diminishing photoluminescence quantum yield (PLQY) [45] and transformation of shape/phase [42,46]. Several strategies such as surface organic ligands techniques [40,41,47], inorganic passivation [48–51], appropriate choice of anti-solvent in washing process [42,52], and embedding the perovskite crystals in a hydrophobic matrix [53,54], have been explored to enhance the stability of perovskite nanomaterials. As a silane coupling agent with low surface energy, hexamethyldisilazane (HMDS) is usually utilized in fabricating superhydrophobic surfaces due to its ACH3 groups [55,56]. Herein, we propose for the first time a facile HMDS-assisted RT approach to prepare perovskite NCs (Scheme 1), in which the HMDS played three important roles: (1) triggering the reaction to generate perovskite NCs at RT; (2) tuning the morphology (i.e. nanospheres and nanocubes) and the optical properties of NCs; (3) modifying the NCs surface with hydrophobic ACH3 groups and thus enhancing their stability (Scheme 1). The typical HMDS-mediated NCs film exhibited water contact angle of 110° (Scheme 1), suggesting a hydrophobic property. This hydrophobic property enhanced the

Scheme 1. Schematic diagram illustrating the synthesis of CsPbBr3 nanoshperes and nanocubes by mediating the amount of HMDS at room temperature.

stability of the resulting NCs, thereby preserving the PL emission spectra and crystal structures of the perovskite specimens upon exposure to the ambient environment. We subsequently employed the CsPbBr3 NCs to fabricate white-light-emitting diodes (WLEDs).

2. Experimental section 2.1. Materials Lead(II) bromide (PbBr2, 99.0% trace metals basis), lead(II) chloride (PbCl2, 99.999% trace metals basis), lead(II) iodide (PbI2, 99.999% trace metals basis), cesium carbonate (Cs2CO3, 99.99%), trioctylphosphine (TOP, 90%) and hexamethyldisilazane (HMDS, 98%) were purchased from Aladdin, octadecene (ODE, technical grade, 90%), oleylamine (OM, 80–90%), oleic acid (OA, 90%) and anhydrous hexane from Macklin. All the chemicals were used as received without any further purification. 2.2. Room temperature synthesis of perovskite CsPbBr3 nanospheres and nanocubes The HMDS-modified perovskite CsPbBr3 NCs (denoted hereafter as HMDS-CsPbBr3) were synthesized using a modified version of our previously reported heat-up protocol [20,57]. In a typical synthesis of CsPbBr3 nanocubes, a mixture of PbBr2 (69 mg, 0.188 mmol) and Cs2CO3 (11 mg, 0.0337 mmol) was loaded in a 50 mL three-neck flask containing ODE (10 mL), HMDS (6 mL), OA (0.5 mL) and OM (1.0 mL) at RT. The mixture was put under vacuum and stirred for 30 min, followed by filling with Ar and continuing the reaction for 6 h. A light yellow semi-transparent solution was formed after 25 min of reaction. The resulting NCs were cleaned twice by adding hexane and centrifuging at high speed (10000 rpm, 10 min). The supernatant was discarded and the precipitated NCs were suspended in 0.5 mL hexane to form the colloidal dispersion for subsequent characterizations and use. Perovskite nanospheres were also prepared using the same procedure in the presence of small amount of HMDS (e.g. 2–3 mL).

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2.3. Transmission electron microscopy (TEM) The TEM images were acquired using a JEM 2100F (JEOL, Japan) microscope equipped with a field emission gun operating at 200 kV accelerating voltage. The samples were prepared by drop-casting NCs solution onto 300-mesh Cu grids covered by an ultrathin amorphous carbon film. Subsequently, the Cu grids were placed in a high vacuum pump station to evaporate the solvent and to preserve the NCs free from oxidation. 2.4. X-ray diffraction (XRD). The phase identification measurement of the as-synthesized NCs was carried out on a Bruker D8 Advanced X-ray diffractometer (Germany) equipped with a 1.8 kW CuKa ceramic X-ray tube, operating at 40 kV and 40 mA. The XRD patterns were collected in air at RT in Bragg-Brentano parafocusing geometry with a step size of 0.02°. The samples for XRD characterization were prepared by drop-casting a concentrated NC solution on a glass wafer and drying in an Ar-filled glovebox. The data analysis was carried out using X’PertHighscore Plus software from PANalytical. 2.5. Optical absorption spectroscopy Optical absorption spectra of NC solutions were collected with a UV-3600 UV-Vis-NIR spectrophotometer (Shimadzu, Japan) in the wavelength range of 300–900 nm. The samples wereprepared in an Ar-filled glovebox using hexane as solvent. The diluted NC solutions were prepared in 1 cm path length quartz cuvette with airtight screw cap. 2.6. Steady-State PL and PLQY measurements The sample preparation was identical to that in the optical absorption test. The steady-state PL measurements were conducted by exciting the samples with 355 nm and recorded on a fluorescence spectrophotometer (TM/QM/NIR, PTI, USA). PL spectra for PLQY measurement and calculation were recorded using fluorescence spectrophotometer equipment with an integrating sphere, by exciting the NC colloidal dispersion with 355 nm. The exciting light was coupled to an optical fiber connected to an integrating sphere in which a quartz cuvette with NCs solution was placed. The PLQY of the typical perovskite NCs was calculated according to the method described by Manna and coworkers [29]. Four data were collected for the calculation: sample emission (SEM), sample excitation (SEX), blank emission (BEM), and blank excitation (BEX), based on which the PLQY was estimated as formula

PLQ Yð%Þ ¼

SEM  BEM  100 BEX  SEX

2.7. Time-resolved PL The PL decay process of the representative perovskite NCs was acquired using a phosphorescence lifetime spectrometer (Edinburgh, FLS 980) equipped with a 375 nm, 60.8 ps pulse width laser and a time-correlated single-photon counting setup at RT. 3. Results and discussion 3.1. Room temperature synthesis of perovskite nanospheres and nanocubes We developed a facile RT method to control the morphology and optical properties of perovskite NCs using HMDS as a

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reaction-triggering and morphology-directing agent. In the absence of HMDS, the RT reaction of Cs2CO3 with PbBr2 didn’t form any perovskite NCs, even by adding more ligands (e.g. OM, OA) or by prolonging the reaction time. Interestingly, the addition of HMDS in the reaction enabled the formation of CsPbBr3 NCs. Particularly, the morphology and optical properties can be controlled by changing the amount of HMDS in the precursors. Sphere-like NCs with average diameter of around 3.9 nm were formed in the presence of 2 or 3 mL HMDS (Fig. 1a). The corresponding discernible PL emission at around 490 nm is provided in Fig. 2a (black curves). With the increase of HMDS in the precursor, the NCs evolved from nanospheres to nanocubes with an average edge size of 10.3 nm (Fig. 1b). More details of the morphological evolution and particle size distribution with increasing HMDS content are provided in Fig. 2a and b and Fig. S1-S3 of the Supporting Information (SI). As systematically illustrated in Fig. 2a, the PL peaks have a red1shift from 490 to 512 nm when the amount of HMDS is increased from 2 to 10 mL (see also Table S1 in the SI). As a result of the quantum confinement effect [58,59], the spherical quantum dots (QDs) displayed a blue shift in PL band compared with the CsPbBr3 nanocubes. The typical green emission of the CsPbBr3 nanocubes is consistent with that of perovskite NCs achieved at high temperature without HMDS [57]. Note that the full width at half-maximum (FWHM) of the emission peak in all samples is between 18.8 and 26.2 nm (Table S1 of the SI), indicating a narrow particle size distribution (see also Fig. S3). It has been found that the synthesis yield of the NCs increases with increasing amount of HMDS. Improving the synthesis yields is beneficial to scalability and the NCs obtained could be used in the LED device manufacturing. The XRD patterns of the various NCs are illustrated in Fig. 2b. The XRD patterns of nanocubes formed in the presence of relatively large amounts of HMDS (e.g. 4–6 mL) show strong and clear diffraction peaks, which can be assigned to the orthorhombic phase of perovskite CsPbBr3 (JCPDS No. 98-009-7851) [60,61]. However, besides the main orthorhombic peaks, two additional peaks located at 29.36 and 33.34° can also be observed from the nanospheres formed in the presence of 2–3 mL HMDS (Fig. 2b). These peaks can be indexed to the tetragonal CsPb2Br5 phase [62]. The crystal structure characteristic is further confirmed by the HRTEM analyese. The lattice spacing of 0.294 and 0.412 nm can be assigned to the (0 4 0) and (2 0 0) planes of the orthorhombic CsPbBr3 structure, respectively (Fig. 1c and d, and Fig. S2). PL decay lifetimes of the representative samples are measured and the decay curves could be well-fitted with tri-exponential function (Fig. 2c and d). The average lifetimes are thus determined to be 10.4 and 18.0 ns for the nanospheres and nanocubes, respectively. Overall, the quantum yield of the nanocubes is higher than that of nanospheres, as summarized in Table S1 of the SI. In addition to the amount of HMDS, the reaction time also affected the morphology and the optical properties of the resulting NCs. The particle size increases with prolonging the reaction time. Nanosheet-like NCs are formed when the reaction was conducted for 24 h (Fig. S4a and c in the SI). The XRD patterns of the resulting NCs indicate an obvious orthorhombic phase formation (Fig. S4d). As the reaction time increases from 6 to 24 h, a red shift from 510 to 515 nm in the PL emission is observed (Fig. S4e). Controlling the composition of CsPbX3 NCs has been an effective way to tune their optical properties. This could be realized by either a direct synthesis or ion exchange on a template of the assynthesized perovskite NCs [29,63]. We have noticed that no any perovskite NCs are formed when we completely replaced PbBr2 with PbCl2 or PbI2 in the precursors, which might be due to the

1 For interpretation of color in Fig. 2, the reader is referred to the web version of this article.

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Fig. 1. TEM (a,b) and HRTEM (c,d) images of the CsPbBr3 nanospheres (a,c) and nanocubes (b,d) achieved with different amounts of HMDS as dictated.

low activity of PbCl2 and PbI2. However, adding a small amount of PbCl2 or PbI2 to the precursors of Cs2CO3 and PbBr2 leads to a significant tunability of the optical properties (Fig. S6 in the SI). The optical tunability is due to the formation of halide-mixed perovskite NCs, akin to the previously reported halide-mixed NCs synthesized at high temperature [57,64]. It is important to mention that the present facile RT approach can be readily extended to large-scale synthesis. As a typical example, a scalability of CsPbBr3 nanocubes by an order of magnitude is demonstrated (Fig. 3a). The TEM image exhibit the formation of monodisperse nanocubes with average edge size of 10.5 nm (Fig. 3b), and the XRD pattern confirmed the formation of orthorhombic phase (Fig. 3d). The UV–Vis optical spectra (Fig. 3c) along with photographs of the as-formed crude dispersion and cleaned NC solution in hexane displayed bright green emission (insets of Fig. 3a,b) with a narrow emission line-width of 22.8 nm. These results are in good agreement with those reported in the small-scale synthesis. Hence it is finally proved that the pre-

sent synthetic approach can be extensible and reproducible, making it an ideal general protocol for NC engineering. To analyze the role of the capping ligand in the NCs, the Fourier transform infrared spectroscopy (FTIR) measurements of the typical NCs along with the various ligands were conducted. FTIR spectra of the nanosphere (2 mL HMDS) and nanocubes (6 mL HMDS) are shown in Fig. 4. For comparison, FTIR spectrum of the CsPbBr3 NCs formed at 110 °C in the absence of HMDS (i.e. in the presence of OM and OA) is also provided (Fig. S5b, black curve). The CAH bonds are observed in all NCs, indicating OA and OM as the capping ligands. The absorption band near 2956 cm1 corresponds to the stretching vibration of ACH3 group, and the absorption peak at 845 cm1 can be indexed to the SiACH3 [65]. Notably, the intensity of these peaks became stronger in the nanocubes compared to the nanospheres. The FTIR results confirmed that the functional methyl groups (i.e. ACH3) have been successfully modified on the surface of the CsPbBr3 NCs. We hypothesized that HMDS acts as a capping ligand like OM and OA, which might complexes some

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Fig. 2. (a) Steady-state absorption spectra (solid curves) and PL spectra (dash curves), and (b) XRD patterns of various CsPbBr3 NCs collected in the presence of different amounts of HMDS. The XRD stick patterns of standard CsPb2Br5 (JCPDS 00-025-0211) and orthorhombic phase CsPbBr3 (JCPDS No. 98-009-7851) were provided in panel (b) for comparison. (c,d) Time-resolved decays of the PL emission at 490 nm and the fitting curves of the typical perovskite nanospheres (c) and nanocubes (d), respectively.

of the Cs, Pb and Br precursors during the reaction and mediates the growth of nanoparticles, and consequently binds to the surface of the resulting NCs. The surface modification by methyl groups facilitates the protection of the resulting NCs from water molecules and thus improves the stability of NCs under ambient environment, which will be discussed in the following sections. 3.2. Stability investigation of the HMDS-mediated perovskite NCs The instability of perovskite nanomaterials was reported to as a result of uncontrollable physicochemical properties [42,66]. Interestingly, the introduction of HMDS not only triggered the reaction of Cs2CO3 with PbBr2 to generate perovskite NCs with controlled morphology, but also enhanced the stability of the final NCs due to the surface modification by hydrocarbyl groups. As shown in Fig. S7, although a decreasing trend of the emission intensity can be distinguished in all NC dispersions (in hexane) upon exposure to air and moisture (60% relative humidity), the PL intensity of the samples formed in the presence of a large amount of HMDS dropped much slower than that of NCs synthesized using less HMDS. Similar evolution of PL intensity over the HMDS amounts was also observed in the various NCs films (Fig. 5a, and Fig. S8). It can be observed that the PL intensity gradually decreased with prolonging the exposure time. The PL intensity of the film fabri-

cated by nanospheres (with 3 mL HMDS) almost vanished after 5 h of exposure to ambient condition (Fig. S8b). However, the intensity of nanocubes film (with 6 mL HMDS) dropped by only 21.6% after 720 h (i.e. 30 days) under similar conditions (Fig. S8e). To further examine the structural stability of CsPbBr3 NC films, we carried out XRD measurement before and after exposure to an ambient environment. The perovskite nanospheres evolved from mixed phases (Fig. 2b) to CsBr phase after 24 h of exposure (Fig. 5b, green, red and black curves), indicating the decomposition of the crystal structure. On contrary, the nanocubes formed in the presence of a large amount of HMDS (e.g. 5–6 mL HMDS) preserved their overall orthorhombic crystal structure even after 100 days of exposure under similar ambient conditions (Fig. 5b, blue and magenta curves). Furthermore, TEM results revealed that the nanospheres were destroyed upon exposure to air for 24 h (Fig. 5c), while the perovskite nanocubes were nearly intact even after 100 days (Fig. 5d). All these observations confirmed that the stability of CsPbBr3 NCs can be remarkably improved by efficient surface modification using HMDS. The enhanced stability is due to the intrinsic hydrophobic properties of the NCs modified by hydrocarbyl groups, preventing the diffusion of water molecules to the NCs. The hydrophobic property was evidenced by measuring the water contact angle on the corresponding NC film, as illustrated by the inset of Fig. 5d. It has been

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Fig. 3. Characterization results of perovskite CsPbBr3 NCs obtained via large-scale room temperature synthesis: (a) Photographs of large-scale-synthesized CsPbBr3 dispersion under normal indoor light. Inset provides the photograph of the as-formed dispersion under UV illumination (365 nm excitation wavelength). (b) TEM image, (c) optical absorption (black line) and PL spectrum (red line), and (d) XRD pattern of the large-scale CsPbBr3 NCs. Inset in panel b) reports the photographs of the NC solution (in hexane) under UV light illumination (365 nm excitation wavelength). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4. FTIR spectra of the perovskite nanospheres and nanocubes achieved in the presence of 2 and 6 mL, respectively.

reported that modification of perovskite materials with hydrophobic substances improved the stability, which can be summarized as follows: (1) Surface organic ligands techniques. This has been finalized by coating the NCs with tightly binding ligands during the direct synthesis [45] or by ligand replacement [41,47]. For example, polyvinyl pyrrolidone (PVP) was physically adsorbed on the surface of perovskite NCs to form a protective layer, leading to a highly stable specimens [67]. (2) Cross-linking the perovskite crystals in organic ligands [40]. This technique was reported by Manna and co-workers. Exposure of the colloidal perovskite NCs film to low X-ray flux resulted in intermolecular C@C bonding of the capping ligands, which covered the surface of NCs and thus protected the NCs. This transformation of ligand shell enabled the fabrication of stable and strongly fluorescent patterns over millimeter scale areas. (3) Embedding the perovskite crystals in a hydrophobic matrix [15,53]. Zhou et al. demonstrated an in-situ formation of MAPbX3 NCs embedded polyvinylidene fluoride (PVDF) composite films by separating the crystallization processes of PVDF and MAPbX3 NCs [53]. The perovskite NCs in the as-fabricated film

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Fig. 5. Stability of perovskite NCs films synthesized with different amounts of HMDS. (a) Time evolution of PL intensity of the perovskite films exposed to ambient condition. The average temperature and humid are 27 °C and 60%, respectively. (b) XRD patterns of films of the NCs synthesized in the presence of 2, 3 and 4 mL of HMDS, respectively, after stored under ambient condition for 24 h, and films of NCs synthesized in the presence of 5 and 6 mL of HMDS, respectively, after stored in an ambient condition for 100 days. (c-d) TEM images of the typical nanospheres exposed to air for 24 h (c) and nanocubes for 100 days (d), respectively. Insets in panels c-d) depict the photograph of water contact angle of the corresponding fresh NCs films.

were uniform in size due to the crystallization control via the interactions between MA+ and -CF2- group. The films exhibited enhanced PL properties and improved stability against water moisture and UV radiation. The encapsulation of colloidal CsPbBr3 NCs into other hydrophobic macroscale polymeric matrices such as poly(lauryl methacrylate) (PLMA) and polystyrene (PS) also enhanced water and light stability of the resulting perovskitepolymer composites [54]. Other strategies such as inorganic passivation by using inorganic metal salts or complex in a direct synthesis or posttreatment, have also been explored [48,49]. It was revealed that the simple inorganic passivation (e.g. ZnBr2, InBr3, CuBr2, PbBr2) could be implemented to effectively generate stable CsPbX3 NCs without drastic drop of PLQY, changes in size and morphology, and transformation of crystalline structures under ambient condition in 5 days [68]. Passivation via a post-treatment on the assynthesized perovskite templates with salts of inorganic ions (e.g. SnCl4, BiCl3, ZnCl2, AgNO3) was also an effective way to improve the stability [50,51].

3.3. Fabrication of WLEDs with perovskite NCs In view of the fabricating WLEDs, a GaN UV LED chip emitting at 365 nm was used. The as-synthesized perovskite NCs and other phosphors were uniformly mixed and dispersed in silicone resin and deposited on the LED chip. Two representative CsPbBr3 samples were selected for fabricating WLEDs: nanospheres mixing with blue emitting BaMgAl10O17:Eu, green (Sr,Ba)2SiO4:Eu and red (Sr,Ca)AlSiN3:Eu phosphors; nanocubes mixing with blue emitting BaMgAl10O17:Eu and red emitting (Sr,Ca)AlSiN3:Eu phosphors. The nanospheres were not stable, and consequently blue, green and red phosphors were added in the case of nanospheres-based device. Fig. 6a depicts the CIE chromaticity diagram plotted from the corresponding emission spectra of the nanospheres-based device. Fig. 6b illustrates the electroluminescence (EL) spectrum of the CsPbBr3-based WLED driven by a 180 mA working current. At 180 mA, the CIE coordinates and the color rendering index (CRI) values of the WLED device were (0.4387, 0.3903) and 74.0, respectively. We summarized the CRI and correlated color temper-

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Fig. 6. (a) CIE chromaticity diagram of the fabricated WLED device based on CsPbBr3 nanospheres. Insets provide the digital photographs of the fabricated WLED device (B) and the LED lamp displayed at working condition (A). (b) EL spectrum of a WLED device fabricated with CsPbBr3 nanospheres, green (Sr, Ba)2SiO4:Eu and red (Sr,Ca)AlSiN3:Eu on a UV LED chip (k = 365 nm) under a working current of 300 mA. (c) Output spectra of WLEDs at different currents. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 7. (a) CIE chromaticity diagram of the fabricated WLED device based on green CsPbBr3 nanocubes. Insets provide the digital photographs of the fabricated WLED device (B) and the LED lamp displayed at working condition (A). (b) EL spectrum of a WLED device fabricated with blue BaMgAl10O17:Eu, green CsPbBr3 nanocubes and red (Sr,Ca) AlSiN3:Eu on a UV LED chip (k = 365 nm) under a working current of 300 mA. (c) Output spectra of WLEDs composed of blue BaMgAl10O17:Eu, green CsPbBr3 and red (Sr,Ca) AlSiN3:Eu phosphor, operating at different working currents. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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ature (CCT) of the white light generated by the devices at different drive currents (Fig. S9 in the SI). The relative intensity ratios of the three peaks of the output spectra of the WLED could be tuned by varying the diode currents (Fig. 6c). As the current increased from 60 to 300 mA, the blue intensity increased while the green intensity decreased. Correspondingly, the CCT was tuned in the range of 2713–2908 K, displaying typical warm white light. Similar to the nanospheres-based device, the nanocubes-based LEDs also revealed warm white display. Fig. 7a shows the CIE chromaticity diagram of the nanocubes-based WLED device. The EL spectrum of the warm WLED operated at 300 mA is shown in Fig. 7b. The evolution of the relative EL intensity of blue and green emissions over the currents was similar to the nanosphere-based device. As the drive current increased from 60 to 300 mA, the CIE color coordinates and CRI of the device were slightly varied from (0. 4535, 0.4185) to (0. 4550, 0.3770), and 90.6–81.6. The WLEDs emitted bright warm white light with CCT in the range of 2496– 2853 K upon the decrease of drive current from 300 to 60 mA. In comparison with the nanospheres-based WLEDs having CRI in the range of 70.3–79.6, the CRI values of the nanocubes-based WLED lamps have been significantly improved (i.e. high up to 90.6). Furthermore, the external quantum efficiency (EQE) of the nanocubes-based WLEDs reached to 1.76%, much higher than that of the nanospheres-based device (Fig. S9). 4. Conclusion In summary, we developed a facile and highly effective synthetic protocol to generate perovskite NCs with tunable shape and size at RT using a unique capping ligand (i.e., HMDS). Spherical CsPbBr3 NCs were achieved by adding a small amount of HMDS (e.g. 2–3 mL) into the precursors. The shape of CsPbBr3 NCs was tailored from nanospheres to nanocubes in the presence of a large amount of HMDS (e.g. more than 5 mL). FTIR analysis revealed that the resulting NCs were successfully modified by ACH3 groups from HMDS. The binding of methyl groups on NCs surfaces led to the hydrophobicity, which consequently enhanced the stability of the resulting NCs and NCs films upon exposure to ambient condition. By properly selecting the precursors and optimizing the synthetic parameters, the present unique HMDS-mediated approach can be extended to the synthesis of other perovskite NCs as well. This technique will help the researchers not only control the morphology and optical properties but also improve their stability, thus allowing the resulting NCs to have promising potential for WLEDs. Acknowledgements This work is supported by the National Undergraduate Innovation Program (No. 20171049701020), the Chutian Scholars Program (2014), the National Natural Science Foundation of China, China (No. 51461135004) and Scientific Leadership Training Program of Hubei ([2012]86). We thank Dr. Xiaoqing Liu and Dr. Chunhua Shen (Center for Materials Research and Analysis, Wuhan University of Technology) for their help in TEM and XRD characterization.. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.05.044. References [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc. 131 (2009) 6050–6051.

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