Room-Temperature Synthesis of 2D Hexagonal Boron Nitride (h-BN

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Functional Inorganic Materials and Devices

Room-Temperature Synthesis of 2D Hexagonal Boron Nitride (hBN) Nanosheets Stabilized CsPbBr3 Perovskite Quantum Dots Yang Li, Liubing Dong, Nan Chen, Ziquan Guo, Ying Lv, Jianghui Zheng, and Chao Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20400 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Materials & Interfaces

Room-Temperature Synthesis of 2D Hexagonal Boron Nitride (h-BN) Nanosheets Stabilized CsPbBr3 Perovskite Quantum Dots Yang Li a, Liubing Dong b, Nan Chen c, Ziquan Guo c, Ying Lv d, Jianghui Zheng e, Chao Chen a,*1 a College b

of Energy, Xiamen University, Xiamen 361005, China

Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China

c Department

of Electronic Science, Xiamen University, Xiamen 361005, China

d College e School

of Materials, Xiamen University, Xiamen 361005, China

of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Sydney 2052, Australia

Abstract Poor stability always restricts the application of all-inorganic perovskite quantum dots (PQDs). Herein, 2D hexagonal boron nitride (h-BN) nanosheets were firstly utilized to stabilize CsPbBr3 PQDs through a facile heterogeneous nucleation-growth process at room temperature. In the synthesized h-BN/CsPbBr3 PQD nanocomposites, cubic CsPbBr3 PQDs adhere on h-BN nanosheet surfaces benefiting from the high specific surface area and abundant mesopores of the 2D nanosheets. The nanocomposites prepared at optimized loading of h-BN nanosheets and reaction time display good green-emitting performance with a narrow full width at half maximum of ~20.0 nm and high color purity of 92.0%. Unique 2D structure and excellent thermal

* Corresponding author. E-mail address: [email protected] (C. Chen). 1 ACS Paragon Plus Environment

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conductivity of h-BN nanosheets endow the h-BN/CsPbBr3 PQD nanocomposites with significantly enhanced humidity-stability and thermal-stability. The white light-emitting diodes (LEDs) assembled with the green-emitting nanocomposites, a blue chip and a commercial red phosphor possess a low correlated color temperature of 4190 K, a color-rendering index of 76 and a high luminous efficacy of 57 lm/W. Further, the color gamut of the synthetic white light based on blue-emitting h-BN/CsPbBr1.5Cl1.5 PQDs, green-emitting h-BN/CsPbBr3 PQDs and red-emitting h-BN/CsPbBr1.2I1.8 PQDs is 114% of the National Television System Committee standard. This work paves a new way for utilizing 2D nanomaterials to synthesize stable all-inorganic PQDs for white LEDs and displays. Keywords:

CsPbBr3

perovskite

quantum

dots;

2D

h-BN

nanosheets;

humidity-stability; thermal-stability; room-temperature synthesis; white LEDs

1. Introduction All-inorganic CsPbX3 (X=Cl, Br, I) perovskite quantum dots (PQDs) have drawn considerable attention in recent years. Outstanding electronic and optical properties, such as high photoluminescence quantum yield (PLQY), controllable bandgap and narrow full width at half maximum (FWHM),1-5 endow CsPbX3 PQDs with distinct advantages in white light-emitting diodes (LEDs) and backlight displays. For instance, to achieve a good performance for backlight displays, red phosphors and green phosphors with narrow emission gap are needed to couple with a blue-emitting diode chip to construct white LEDs with a wide color gamut.6 CsPbX3 PQDs can better satisfy above requirements due to their narrow FWHM (which is associated with 2 ACS Paragon Plus Environment

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quantum confinement effect of PQDs), higher color purity and wider color gamut, compared with conventional phosphors. Although these unique characteristics make CsPbX3 PQDs a promising prospect in optoelectronic devices, poor stability in humid environment, under high-temperature condition and when exposed to continuous light illumination always hinders their practical application.7,8 To enhance the stability of CsPbX3 PQDs, various inorganic and organic materials have been used as matrices to protect PQDs.9-16 During them, organic polymer matrices, such as ethylene vinyl acetate (EVA), polystyrene and poly(methyl methacrylate),9-11 are capable of preventing the contact between CsPbX3 PQDs and water vapor in air thus can effectively improve the humidity-stability of PQDs. Nevertheless, heat sensitive feature of these polymers make them almost impossible to optimize PQDs’ thermal-stability.9 Relatively, inorganic matrices including SiO2,8,12,13 zeolite,14,15 CaF2,16 and so on have the capability to enhance PQDs’ thermal-stability benefiting from their own superior thermal resistance property.17 In most cases, a hot-injection method was applied to synthesize inorganic matrices protected CsPbX3 PQDs. Whereas the reaction temperature during hot-injection processes is uncontrollable, leading to a poor batch-to-batch reproducibility and limited applicability in large-scale production.9,18,19 In very recent years, a room-temperature synthesis strategy of CsPbBr3 PQDs was developed.9,19 However, room-temperature synthesis of inorganic materials protected PQDs has scarcely been reported. Besides, 2D nanomaterials have rarely been explored to be utilized as inorganic matrices in the composites with PQDs, even though their unique 3 ACS Paragon Plus Environment


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physicochemical characteristics (e.g., nanosheet structure and high specific surface area) have made them widely used in many other photoelectric devices.20-22 In this paper, 2D hexagonal boron nitride (h-BN) nanosheets were firstly utilized to stabilize CsPbBr3 PQDs through a facile heterogeneous nucleation-growth process at

room

temperature.

Micro-morphology

and

composition

of

synthesized

h-BN/CsPbBr3 PQD nanocomposites were studied and found that in the nanocomposites, cubic CsPbBr3 PQDs adhered on h-BN nanosheet surfaces. The effects of h-BN content and reaction time on the luminescent properties of the nanocomposites were investigated. At optimized h-BN loading and reaction time, the nanocomposites displayed good green-emitting performance with a narrow FWHM of ~20.0 nm and high color purity of 92.0%. Stability evaluation indicated that unique 2D structure and excellent thermal conductivity of h-BN nanosheets endowed the h-BN/CsPbBr3 PQD nanocomposites with significantly enhanced humidity-stability and thermal-stability. In addition, the assembled white LEDs with the nanocomposites displayed a low correlated color temperature (CCT) of 4190 K, a color-rendering index (CRI) of 76 and a high luminous efficacy of 57 lm/W. Furthermore, the color gamut of the synthetic white light based on blue-emitting h-BN/CsPbBr1.5Cl1.5 PQDs, green-emitting h-BN/CsPbBr3 PQDs and red-emitting h-BN/CsPbBr1.2I1.8 PQDs was measured to be 114% of the National Television System Committee (NTSC) standard. This work is believed to pave a new way for utilizing 2D nanomaterials to synthesize stable all-inorganic PQDs for white LEDs and displays.

2. Experimental 4 ACS Paragon Plus Environment

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2.1 Materials N, N-Dimethylformamide (DMF) and toluene were purchased from Sinopharm Chemical Reagent Co., Ltd. Oleic acid (OA), oleylamine (OAm), PbCl2 (99.999%) and PbI2 (99.99%) were obtained from Aladdin Reagent Co., Ltd. PbBr2 (99.99%) and CsBr (99.9%) were produced by Xi’an Polymer Light Technology Corp. h-BN was supplied by Shanghai Macklin Biochemical Co., Ltd. Above materials were used as

received

without

further

purification.

Commercial

red

phosphor

of

(Sr,Ca)AlSiN3:Eu2+ and InGaN blue LED chip were supplied by Shenzhen Looking Long Technology Co., Ltd and EPI LEDs Co., Ltd, respectively. 2.2 Preparation of h-BN/CsPbX3 (X=Cl, Br, I) PQD nanocomposites h-BN/CsPbX3 (X=Cl, Br, I) PQD nanocomposite powders were synthesized at room temperature. In a typical process of synthesizing h-BN/CsPbBr3, 0.4 mmol of PbBr2 and 0.4 mmol of CsBr were dissolved in 10 mL of DMF with magnetic stirring, followed by the addition of OA (1 mL) and OAm (0.5 mL) as ligands to stabilize the DMF-PbBr2-CsBr precursor solution. Subsequently, 1 mL of the above solution was added to the solution containing h-BN (x g, x=0.1, 0.2, 0.3 or 0.4) and toluene (10 mL) under vigorous stirring for 1 min and then stood for 10~60 min at room temperature. Finally, the precipitate was washed with hexene and naturally dried in the air condition to get h-BN/CsPbBr3 PQD nanocomposite powder. For comparison, pure CsPbBr3 PQDs were synthesized when h-BN was not introduced in the system. To synthesize

h-BN/CsPbBr1.5Cl1.5

PQDs

and

h-BN/CsPbBr1.2I1.8

PQDs,

CsBr+PbCl2+PbBr2 (with molar ratio of 1: 0.75: 0.25) and CsBr+PbBr2+PbI2 (with 5 ACS Paragon Plus Environment


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molar ratio of 1: 0.1: 0.9) were used to replace CsBr+PbBr2 (with molar ratio of 1: 1) respectively in aforementioned preparation procedures. 2.3 Characterization Crystal structure of the as-synthesized h-BN/CsPbBr3 PQD nanocomposites were analyzed by X-ray diffraction (XRD) analyzer (model: Ultima-IV) with a Cu Kα (λ = 1.5418 Å) radiation. Micro-morphologies were observed by scanning electron microscopy (SEM; model: SUPRA 55) and high-resolution transmission electron microscopy (HRTEM; model: JEM2100). Element composition and their valence states were studied using X-ray photoelectron spectroscopy (XPS) technique. Fourier transform infrared (FTIR) spectra were recorded on a spectrograph (model: Nicolet iS5). Thermogravimetric (TG) measurements were carried out on a simultaneous thermal analyzer (model: MDTC-EQ-M35-01) in nitrogen atmosphere at a heating rate of 5

oC/min.

Nitrogen adsorption/desorption tests were performed on a

Brunauer-Emmett-Teller analyzer (model: ASAP 2020M+C) to determine the specific surface area of pure h-BN and h-BN/CsPbBr3 PQD nanocomposites. A fluorescence spectrophotometer (model: FLS980, Edinburgh Instruments) was applied to obtain the samples’

photoluminescence

(PL)

emission

spectra

and

PLQY.

Temperature-dependent PL spectra of the nanocomposites were measured using a charge-coupled device (model: Ocean Optics, USB2000+) and a heating/cooling system (model: Linkam Scientiﬁc Instruments, THMS600E). The temperature was raised from 30 to 100 oC and then decreased to 30 oC. A white LED was constructed using the synthesized h-BN/CsPbBr3 PQD nanocomposite powders, commercial red 6 ACS Paragon Plus Environment

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phosphor of (Sr,Ca)AlSiN3:Eu2+ and an InGaN blue LED chip. Optical properties of the LED including luminous efficacy, CRI and CCT were measured using an integrating sphere (HAAS-2000, Everfine, China) with a forward current of 20 mA; besides, electroluminescence (EL) spectra were recorded during a continuous lighting of the LED for 480 min (to exclude destabilizing factors caused by manual packaging process, the first EL spectrum was recorded after 10 min lighting).

3. Results and discussion 3.1 Morphology and component of h-BN/CsPbBr3 PQD nanocomposites Figure 1a shows digital photograph of the toluene solution after the addition of DMF-PbBr2-CsBr precursor solution. The solution shows intense green emission under a UV (365 nm) light. This is associated with the formation of CsPbBr3 PQDs through a supersaturated recrystallization process.9,19 By introducing h-BN nanosheets into the toluene solution before the addition of DMF-PbBr2-CsBr precursor solution, yellow-green powder in gram-scale can be obtained finally (pure h-BN powder is white), and the powder glows green when being irradiated using a UV light source (as displayed in Figure 1b), which indirectly demonstrates the formation of CsPbBr3 PQDs in the h-BN based nanocomposite powder. Meanwhile, such a phenomenon suggests that the existence of h-BN nanosheets does not make significant negative effects on the luminescence properties of CsPbBr3 PQD itself, even h-BN accounts for a very high weight fraction of the nanocomposite powder. In the XRD pattern of the powder in Figure 1c, characteristic diffraction peaks of cubic CsPbBr3 PQDs (PDF#54-0752, whose crystal structure is as illustrated in Figure 1d) are detected at 7 ACS Paragon Plus Environment


2θ = 21.5°, 30.6°, 54.5° and so on; and the diffraction peaks that appear at 26.6°, 43.7°, 75.5°and 81.9° correspond to h-BN phase (PDF#85-1068). To clearly discriminate CsPbBr3 PQD phase, we provided the enlarged XRD pattern in Figure S1 and also used Rietveld refinement method to analyze the XRD data (Figure S2). The refined pattern agrees very well with the experimental data. The Rietveld refinement converged to Rwp =7.95% and Rp = 4.61%, confirming a high reliability of the reﬁnement. Refined parameters of CsPbBr3 PQD phase are a=b=c=5.88 Å and α=β=γ=90o (Pm-3m space group). Besides, weight fractions of CsPbBr3 PQD and h-BN in the nanocomposite powder are 0.83% and 99.17% respectively, estimated by the Rietveld refinement. XPS was then used to analyze the composition of the nanocomposite powder (Figure 1e-g and Figure S3). The elements of Cs, Pb and Br, as well as B and N, are detected in the nanocomposite. High-resolution XPS spectra of Cs 3d, Pb 4f and Br 3d show that the binding energy of Cs 3d3/2, Cs 3d5/2, Pb 4f5/2, Pb 4f7/2, Br 3d3/2 and Br 3d5/2 is 738.2, 724.2, 142.3, 137.8, 68.6 and 67.6 eV respectively. These values are consistent with the figures for previously reported CsPbBr3 PQDs, further confirming the formation of CsPbBr3 PQDs in the h-BN based nanocomposite. 23,24 Micromorphologies of pure h-BN and the synthesized h-BN/CsPbBr3 nanocomposite powder were observed. As displayed in Figure 2a and Figure S4, the h-BN shows a sheet-like structure with lateral dimension of several hundred nanometers and thickness of less than 100 nm. Introduction of CsPbBr3 PQDs does not obviously change the micro-morphology of the h-BN based nanocomposite 8 ACS Paragon Plus Environment

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(Figure 2b). The very small size and low mass loading of CsPbBr3 PQDs make them hard to be directly observed in SEM image. Despite of this, energy dispersive X-ray spectroscopy (EDS) mapping in Figure 2c reveals a relatively even distribution of CsPbBr3 PQDs in the h-BN based nanocomposite. Through TEM observations (Figure 2d and Figure S5), we can see that dot-like CsPbBr3 PQDs scatteredly disperse on the h-BN nanosheet surfaces (e.g., for the h-BN nanosheets marked as 1, 2, 3 and 4) as a whole, as illustrated by the inset in Figure 2d, even though PQDs are unable to penetrate into the space between two tightly stacked h-BN nanosheets (e.g., the nanosheets marked as 5 and 6). In fact, due to the typical 2D structure, the h-BN nanosheets possess a relatively high specific surface area of 25.8 m2/g (Figure S6), which is higher than that of previously used CaF2 matrix.16 Such a high specific surface area as well as abundant mesopores (reflected by the dramatic increase of N2 adsorption volume at high pressure in N2 adsorption/desorption isotherm in Figure S6) are considered to provide sufficient sites for the nucleation and growth of PQD nanocrystals.13,16 Consequently, the formed CsPbBr3 PQDs scatteredly adhere on h-BN nanosheet surface as exhibited in Figure 2d, accompanying with the decrease of the specific surface area of h-BN/CsPbBr3 nanocomposite (18.8 m2/g). The possibly preserved organic components (e.g., DMF and ligands from the DMF-PbBr2-CsBr precursor solution) in the synthesized nanocomposite power are further investigated through TG analysis and FTIR. TG curves of pure h-BN and the synthesized h-BN/CsPbBr3 nanocomposite in Figure S7 suggest that DMF has completely evaporated, whereas the ligands of OA and OAm are preserved in the 9 ACS Paragon Plus Environment


as-prepared h-BN/CsPbBr3 nanocomposite possibly due to their high boiling points of 350~360 oC. As generally considered, the ligands are beneficial for stabilizing the PQDs and helping to control the PQD size.25,26 In the FTIR spectra (Figure S8), the strong absorption bands centered at 1369 cm-1 and 807 cm-1 are assigned to the h-BN nanosheets.27 Some small peaks in the range of 2840-2950 cm-1 are also detected in the FTIR spectrum of h-BN/CsPbBr3 nanocomposite, which are typical absorption bands for species with hydrocarbon groups. This demonstrates the existence of OA and OAm.28 3.2 Luminescence properties of h-BN/CsPbBr3 PQDs Figure 3a shows the PL emission spectra of the as-synthesized h-BN/CsPbBr3 PQD nanocomposites with various amounts of h-BN nanosheets under an excitation wavelength of 365 nm. Emission peaks can be observed at around 521 nm, corresponding to green emitting behavior. The amount of added 2D h-BN nanosheets barely influences the emission peak position (Figure 3b). Meanwhile, with the increasing amount of h-BN nanosheets in these h-BN/CsPbBr3 PQD nanocomposites, corresponding emission peak intensity dramatically increases at first and then marginally descends after reaching a maximum value at 0.2 g h-BN loading. This is because small amount of h-BN nanosheets cannot able to provide enough sites for the nucleation and growth of CsPbBr3 PQDs sites.29 In addition, for the nanocomposites with different h-BN loading of 0.1-0.4 g, their PL emission peaks possess a FWHM value around 20 nm (the mean of these four FWHM values; corresponding standard deviation is 0.7 nm), as summarized in Figure 3b. Taking the h-BN/CsPbBr3 PQD 10 ACS Paragon Plus Environment

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nanocomposite with 0.2 g h-BN nanosheets as an example, its Commission International De L’Eclairage (CIE) chromaticity coordinate and color purity is (0.122, 0.790) and 92.0% respectively (Figure S9). In addition, the effect of reaction time (that is the time from the addition of DMF-PbBr2-CsBr precursor solution into toluene/h-BN solution to the collection of h-BN/CsPbBr3 PQD powder) on the luminescence properties of synthesized h-BN/CsPbBr3 PQD nanocomposites was also investigated (Figure 3c and Figure S10). When the reaction time is 30 min, the synthesized nanocomposite exhibits the highest PL emission intensity. Both shorter reaction time (e.g., 10 min) and too long reaction time (e.g., 60 min) lead to an inferior luminescent performance. The possible reason is that the former results in insufficient growth of CsPbBr3 PQDs and the latter causes PQD aggregation on h-BN nanosheet surfaces.29 Therefore, unless otherwise noted, the studied h-BN/CsPbBr3 PQD nanocomposite in this work was prepared with 0.2 g h-BN addition and 30 min reaction time. Furthermore, PLQY of the h-BN/CsPbBr3 PQD nanocomposite is measured to be 55.7% (Figure 3d), higher than the figures for NH4Br/CsPbBr3 PQD powders prepared by physical absorption method and EVA/CsPbBr3 PQD films.9,30 The high PLQY value of our h-BN/CsPbBr3 PQD nanocomposite is attributed to the fact that the 2D h-BN nanosheets provide enough self-assembly sites for CsPbBr3 PQDs and inhibit their uncontrollable growth to some extent (as discussed in Figure 2d).16,29 The lead halide perovskite nanocrystal structure of CsPbBr3 PQDs will degrade when the PQDs are exposed to moist environment, thus poor humidity-stability of 11 ACS Paragon Plus Environment


pure CsPbBr3 PQDs is one main limiting factor for their practical application.16,31 Humidity-stability of our designed h-BN/CsPbBr3 PQD nanocomposite was examined by keeping the nanocomposite in humid air (with ~80% humidity) and its PL emission behavior was evaluated every few hours. PL intensity retention vs time curve is plotted in Figure 4a. The PL intensity improves by ~10% in the first 16 h, which is probably due to the adsorption of water molecular on PQD surfaces to form perovskite hydrate that can passivate surface defects and improve the PL intensity of PQDs18,32 With prolonged exposure time, the PL intensity of the h-BN/CsPbBr3 PQD nanocomposite declines and then maintains at ~72% retention after about 110 h. Such a humidity-stability is much better than that of pure CsPbBr3 PQDs and CaF2 hierarchical nanospheres protected CsPbBr3 PQDs as exhibited in Figure 4a,16 demonstrating the distinct advantage of utilizing 2D h-BN nanosheets to protect CsPbBr3 PQDs. It seems paradoxical that the porous structure of h-BN allows diffusion of species for the formation of CsPbBr3 PQDs but h-BN also can protect CsPbBr3 PQDs from water molecules. In fact, as illustrated in Figure S11, during the synthesis of h-BN/CsPbBr3 PQD nanocomposites, vigorous stirring makes the h-BN nanosheets disperse equably in the solution and CsPbBr3 PQDs formed on h-BN nanosheet surface through a heterogeneous nucleation-growth process; while in the obtained h-BN/CsPbBr3 PQD nanocomposite powder, the nanosheets stack together. Enhanced humidity-stability of h-BN/CsPbBr3 nanocomposite powder is attributed to the following points. (i) The formed CsPbBr3 PQDs can be passivated by inorganic 12 ACS Paragon Plus Environment

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matrices

(i.e.,

h-BN

nanosheets

herein).8,16,30

(ii)

Stacked

nanosheets

in

h-BN/CsPbBr3 PQD nanocomposite powder prevent water molecules from attacking the PQDs.15 This point is also supported by the fact that amount of adsorbed water molecules in h-BN powder is negligible (as reflected by the TG curve in Figure S7). (iii) Considering that moisture-induced degradation of all-inorganic PQDs is always accompanied by increased size of PQD crystal grains,32,33 stacked h-BN nanosheets have spatial constraint effects on the grow up and decomposition of PQDs. (iv) The ligands of OA and OAm are preserved in the as-prepared h-BN/CsPbBr3 PQD nanocomposite powder, and these hydrophobic ligands have been proved to passivate PQDs as water-resisting layer.32 In addition, owing to the degradation and agglomeration of PQD nanocrystals under thermal stress, the PL emission of CsPbBr3 PQDs will quench, and as a result, thermal-stability of CsPbBr3 PQDs is not satisfactory.16 For instance, when temperature increases from 30 to 100 oC and then recovers to 30 oC, the PL intensity of pure CsPbBr3 PQDs is only 35% of original state as previously reported.30 Fortunately, the synthesized 2D h-BN nanosheets/CsPbBr3 PQD nanocomposite shows a significantly enhanced thermal-stability (Figure 4b and Figure S12): its PL intensity recovers 74% after above heating-cooling process. At least the two following factors are considered to contribute to the relatively good thermal-stability of the nanocomposites. (i) 2D h-BN nanosheet is an excellent conductor of heat. This is favorable to alleviating thermal stress inside h-BN/CsPbBr3 PQD nanocomposites, thus to mitigate the degradation and aggregation of PQD nanocrystals under thermal 13 ACS Paragon Plus Environment


stress.16,20 (ii) As discussed in Figure 2, CsPbBr3 PQDs adhere on h-BN nanosheet surfaces, and these “wall-like” nanosheets, as well as the relatively strong van der Waals force between nanomaterials (i.e., PQD nanocrystals and 2D nanosheets), prevent the random movement of PQDs. 3.3 Optical properties of h-BN/CsPbX3 PQD nanocomposites A white LED was constructed based on our green-emitting h-BN/CsPbBr3 PQD nanocomposite powder, a blue chip and a commercial red phosphor of (Sr,Ca)AlSiN3:Eu2+. EL spectrum at a driving current of 20 mA of the assembled white LED is displayed in Figure 5a (inset shows a photograph of the lighting LED). In the EL spectrum, three emission peaks appear at 450, 521 and 625 nm, corresponding to blue emission, green emission and red emission respectively. The assembled white LED possesses a low CCT of 4190 K, a CRI of 76 and a high luminous efficacy of 57 lm/W with the CIE chromaticity coordinate of (0.376, 0.388) (CIE chromaticity diagram is given in Figure S13). As exhibited in Figure 5b, continuous lighting for 8 h only resulting in a 11.7% decrease of EL intensity, implying the high stability of our synthesized h-BN/CsPbBr3 nanocomposite. It is worth nothing that the declined EL intensity is a consequence of the following two factors: (i) illumination of blue light, which can induce agglomeration of PQDs;33,34 (ii) heat produced by the lighting LED (as discussed in Figure 4b, enhanced temperature causes a deteriorated PL performance of h-BN/CsPbBr3 nanocomposite). Therefore, EL intensity of the LED is expected to partly recover after cooling benefiting from the reversible thermal response of our h-BN/CsPbBr3 nanocomposite. 14 ACS Paragon Plus Environment

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The nanocomposites of h-BN/CsPbBr1.5Cl1.5 PQDs and h-BN/CsPbBr1.2I1.8 PQDs were also synthesized in the same way of preparing h-BN/CsPbBr3 PQDS. Their optical photographs under room light and UV light and PL emission spectra are exhibited in Figure 6a. h-BN/CsPbBr1.5Cl1.5 PQD nanocomposite shows blue-emitting performance with PL emission peak at ~456 nm, and h-BN/CsPbBr1.2I1.8 PQD nanocomposite shows red-emitting performance with PL emission peak at ~633 nm. As a consequence, the color gamut of the synthetic white light based on our blue-emitting h-BN/CsPbBr1.5Cl1.5 PQDs, green-emitting h-BN/CsPbBr3 PQDs and red-emitting h-BN/CsPbBr1.2I1.8 PQDs is calculated to be 114% of the NTSC standard (Figure 6b), better than that of phosphor LEDs and some CdSe quantum dots-based LEDs (as summarized in Table 1).35-42 These results suggest that the h-BN/CsPbX3 (X=Cl, Br, I) PQD nanocomposites based white LEDs have potential application in backlight source for display devices.

4. Conclusions 2D h-BN nanosheets/CsPbBr3 PQD nanocomposites were synthesized through a facile heterogeneous nucleation-growth process at room temperature. In the synthesized nanocomposites, cubic CsPbBr3 PQDs scatteredly adhere on 2D h-BN nanosheet surfaces benefiting from the high specific surface area and abundant mesopores of these nanosheets. The nanocomposites display good green-emitting performance with a narrow FWHM of ~20.0 nm and high color purity of 92.0%, at optimized loading of h-BN nanosheets and reaction time. Unique 2D structure and excellent thermal conductivity of h-BN nanosheets endow the h-BN/CsPbBr3 PQD 15 ACS Paragon Plus Environment


nanocomposites with significantly enhanced humidity-stability and thermal-stability. The assembled white LEDs with the green-emitting h-BN/CsPbBr3 PQD nanocomposites, a blue chip and a commercial red phosphor possess a low CCT and high luminous efficacy. Besides, the color gamut of the synthetic white light based on blue-emitting h-BN/CsPbBr1.5Cl1.5 PQDs, green-emitting h-BN/CsPbBr3 PQDs and red-emitting h-BN/CsPbBr1.2I1.8 PQDs is 114% of the NTSC standard. This work is believed to pave a new way for utilizing 2D nanomaterials to synthesize stable all-inorganic PQDs for white LEDs and displays.

Associated content Supporting Information. The supporting information is available free of charge on the ACS Publications website. XRD patterns, XPS spectra, TEM images, FTIR analysis, FWHM, N2 adsorption/desorption isotherms, TG curves, CIE diagrams, thermal-stability measurements, FTIR analysis.

Acknowledgements Y. Li would like to thank the financial support from Chinese Scholarship Council (Grant No. 201806310048).

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Graphic abstract


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Figures

Figure 1. Photographs of (a) CsPbBr3 PQDs-toluene solution and (b) h-BN/CsPbBr3 composite powder. (c) XRD pattern of h-BN/CsPbBr3 composite powder. (d) Crystal structure of cubic CsPbBr3 PQDs. XPS spectra of (e) Cs 3d, (f) Pb 4f and (g) Br 3d for h-BN/CsPbBr3 composite powder.



Figure 2. SEM images of (a) pure h-BN and (b) h-BN/CsPbBr3 composite powder. (c) Areal distribution of N, Cs and Br elements in h-BN/CsPbBr3 composite powder. (d) HRTEM image of h-BN/CsPbBr3 composite powder (inset illustrates the dispersion of CsPbBr3 PQDs on h-BN nanosheet surfaces).


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Figure 3. (a) PL emission spectra and (b) corresponding FWHM and emission peak position of h-BN/CsPbBr3 nanocomposites with various amounts of h-BN nanosheets. (c) PL emission spectra of h-BN/CsPbBr3 PQD nanocomposites synthesized with different reaction times. (d) PLQY measurement of h-BN/CsPbBr3 PQD nanocomposite.



Figure 4. (a) Humidity-stability (the data for CsPbBr3 and CaF2-CsPbBr3 in a is from ref. 16) and (b) thermal-stability (the data for CsPbBr3 in b is from ref. 30) of h-BN/CsPbBr3 composite powder.


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Figure 5. (a) EL spectrum (inset: photograph of the lighting LED) and (b) EL intensity retention versus working time of the constructed while LED.



Figure 6. (a) PL emission spectra of h-BN protected CsPbBr1.5Cl1.5, CsPbBr3 and CsPbBr1.2I1.8 PQDs for blue, green and red-emitting respectively (insets: photographs of h-BN/CsPbBr1.5Cl1.5 and h-BN/CsPbBr1.2I1.8 in organic solutions and as powders). (b) The color gamut constructed based on our tricolor h-BN protected PQDs.


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Table 1. Color gamuts of various phosphors and quantum dots-converted white LEDs. Type

Phosphor

Cd-based quantum dots

Blue

Green

Red

Color gamut (NTSC1931)

Blue LED chip

Nitride phosphor

Nitride phosphor

126.0% a

Blue LED chip

Nitride phosphor

Nitride phosphor

96.4%

Blue LED chip

γ-AlON:Mn,Mg

K2SiF6:Mn4+

102.4%

36

Blue LED chip

β-SiAlON:Eu2+

:Mn4+

89%

37

Blue LED chip

β-SiAlON:Eu2+

85.6%

38 39-42

35

K2SiF6

:Eu2+

CaAlSiN3

ZnCdS/ZnS

CdSe@ZnS

CdSe@CdS@CdS

140% b

Blue LED chip

CdSe//ZnS/CdS ZnS

CdSe/CdS/ZnS/Cd SZnS

104.3%

38

Blue LED chip

SiO2-CsPbBr3/S DDA@ PMMA

K2SiF6:Mn4+

102%

37

114%

This work

PQDs h-BN/CsPbBr1.5Cl1.

h-BN/CsPbBr3

h-BN/CsPbBr1.2I1.

5 a With b

Ref.

8

lens and filter.

Theoretical value based on the ZnCdS/ZnS, CdSe@ZnS and CdSe@CdS@CdS quantum dots

that are separately reported in Refs. 40-42.


Room-Temperature Synthesis of 2D Hexagonal Boron Nitride (h-BN

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