Few-nanometer-sized Î±-CsPbI3 Quantum Dots Enabled by Strontium

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Few-nanometer-sized #-CsPbI3 Quantum Dots Enabled by Strontium Substitution and Iodide Passivation for Efficient Red Light Emitting Diodes Ji-Song Yao, Jing Ge, Kun-Hua Wang, Guozhen Zhang, Bai-Sheng Zhu, Chen Chen, Qun Zhang, Yi Luo, Shu-Hong Yu, and Hong-Bin Yao J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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Journal of the American Chemical Society

Few-nanometer-sized α-CsPbI3 Quantum Dots Enabled by Strontium Substitution and Iodide Passivation for Efficient Red Light Emitting Diodes Ji-Song Yao1,2,†, Jing Ge1,3,†, Kun-Hua Wang1,2, Guozhen Zhang3,*, Bai-Sheng Zhu2, Chen Chen2, Qun Zhang1,3,*, Yi Luo1,3, Shu-Hong Yu1,2,4 and Hong-Bin Yao1,2,* 1Hefei

National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China 2Department of Applied Chemistry, Hefei Science Center of Chinese Academy of Sciences, University of Science and Technology of China, Hefei, Anhui 230026, China 3Department of Chemical Physics, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China 4Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei, Anhui 230026, China ABSTRACT: Cubic phase CsPbI3 quantum dots (α-CsPbI3 QDs) as a newly emerging type of semiconducting QDs hold tremendous promise for fundamental research and optoelectronic device applications. However, stable and sub-5 nm-sized αCsPbI3 QDs have rarely been demonstrated so far due to their highly labile ionic structure and low phase stability. Here, we report a novel strontium-substitution along with iodide passivation strategy to stabilize the cubic phase of CsPbI3, achieving the facile synthesis of α-CsPbI3 QDs with a series of controllable sizes down to sub-5 nm. We demonstrate that the incorporation of strontium ions can significantly increase the formation energies of α-CsPbI3 QDs and hence reduce the structure distortion to stabilize the cubic phase at the few-nanometer size. The size range from 15 down to sub-5 nm of as-prepared stable α-CsPbI3 QDs allowed us to investigate their unique size-dependent optical properties. Strikingly, the few-nanometer-sized α-CsPbI3 QDs turned out to retain high photoluminescence and highly close packing in solid state thin films, and the fabricated red light emitting diodes exhibited high brightness (1250 cd m−2 at 9.2 V) and good operational stability (L50 > 2 h driven by 6 V). The developed cation-substitution strategy will provide an alternative method to prepare uniform and finely size-controlled colloidal lead halide perovskite QDs for various optoelectronic applications.

INTRODUCTION

optoelectronic applications of CsPbX3 QDs11). One of the challenges towards obtaining purified, stable, few-nanometer-sized α-CsPbI3 QDs is that the cubic phase of CsPbI3 is a structurally metastable state which spontaneously transforms into the nonfunctional orthorhombic (δ) phase (Eg ≈ 2.82 eV) under ambient conditions.11, 22 Although attempts to stabilizing α-CsPbI3 QDs via complex purification and ligand passivation strategies have recently been reported,9, 12, 23 it is still hard to prepare purified and long-term stabilized α-CsPbI3 QDs with a series of sizes via an efficient way because of the highly labile ionic structure of CsPbI3.3 Much more challenges for preparation of stable, few-nanometersized α-CsPbI3 QDs are associated with the finite size effect and weak surface-bonding nature of α-CsPbI3 QDs. Smaller size of α- CsPbI3 QDs bears lower stability, given that they are more far from the equilibrium state with much higher surface energy.24 The soft basic nature of I– as well as highly ionic bonding in α-CsPbI3 QDs result in weaker QDs–ligand binding, further lowering the stability of α-CsPbI3 QDs.9 To practically stabilize the purified, few-nanometer-sized α- CsPbI3 QDs, a promising strategy is to increase the formation energy via atomic scale design. Fortunately, the

Cesium lead halide perovskite (CsPbX3, X = Cl, Br, or I) QDs, as a new type of semiconducting QDs for light emitting, exhibit high photoluminescence quantum yields (PLQYs), narrow emission bandwidth, reduced fluorescence blinking and bright triplet excitons.1-7 More attractively, these unique optical properties can be easily achieved in CsPbX3 QDs due to their high defect tolerance endowed by ionic lattices, unlike that the elaborate surface passivation is necessary for conventional II–VI QDs to acquire high PLQYs.1, 3, 8 Among various CsPbX3 QDs, the cubic phase cesium lead iodide perovskite QDs (α-CsPbI3 QDs), prized for their narrow direct band gap (Eg) of ca. 1.73 eV, have stimulated great interest in photovoltaics and red light emitting diodes (LEDs).9-17 To achieve highly efficient red perovskite LEDs18 and further interrogate the quantum confinement effect in this new type of QDs,19-21 preparation of uniform, stable, few-nanometersized α-CsPbI3 QDs with tunable optical properties is a prerequisite. Thus far, however, it remains rather elusive and challenging to facilely synthesize such α-CsPbI3 QDs with bright emission in the red spectral range (i.e., the so-called “perovskite red wall”—a long-standing obstacle for the 1

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added into a 25 mL 3-neck flask along with 5.0 mL of ODE and the obtained solution was degassed under vacuum for 15 min at 120 °C and then switched to be under nitrogen flow for 5 min. This degassing-refilling process was repeated for three times. Then, the dried OA and OAm (0.7 mL each, pre-heated at ~100 °C) were injected into above solution under nitrogen flow. 5 min later, the flask was put under vacuum again for 15 min until no gas bubble releasing from the solution. Then, the temperature of solution was increased to a desired temperature (100–170 °C) and 0.4 mL of Cs–OA solution was quickly injected. After 5 s, the reaction mixture was immediately cooled down to room temperature using an icewater bath. The Sr2+-substituted QDs were synthesized by using the same process except for the addition of three concentrations (20%, 40%, 60%, mole percentage of SrI2 relative to PbI2) of SrI2 and 0.087g of PbI2. For the control samples synthesized in excessive PbI2, the synthetic procedure was kept the same except for the addition of desired amount of PbI2. Synthesis of CsPbI3 QDs with TMSI. 0.076 g of Pb(CH3COO)2·3H2O and 0.019 g of CH3COOCs were loaded into a 25 mL 3-neck flask along with 5.0 mL of ODE and then degassed under vacuum for 15 min at 120 °C and then switched to nitrogen flow for 5 min. The above process was repeated for three times. After this, 1.0 mL of dried OAm and 0.3 mL of dried OA (pre-heated at ~100 °C) were injected into the solution under nitrogen flow. 5 min later, the flask was put under vacuum again for 15 min until no gas bubble releasing from the solution. Then, the solution was heated to a desired temperature and 85 μL of TMSI was rapidly injected into the solution. After 5 s, the reaction system was rapidly cooled down by using an ice-water bath. The CsPbI3 QDs with Sr(ac)2 were synthesized by using the same process except for the addition of various concentrations of Sr(ac)2. Purification of as-synthesized QDs. The obtained QDs in crude reaction solutions were precipitated out by the antisolvent ethyl acetate (its volume ratio to that of QD solution is 4:1) and then the mixture was centrifuged at 10000 round per minute (rpm) for 5 min. The obtained precipitate was dispersed into octane (5 mL) to form the QDs suspension. The above precipitation-dispersion process was repeated twice and the finally obtained QDs suspension in octane was stored under nitrogen atmosphere for the following characterizations and tests. It is worth noting that for the sub-5 nm CsPbI3 QDs synthesized at 130 °C, the QDs are difficult to be directly precipitated out by ethyl acetate at room temperature. Hence, the mixture of crude QDs solution and ethyl acetate (v/v, 1:4) was cooled in a freezer at 4 °C for 3 h at first and centrifuged at 10000 rpm for 3 min and then the obtained precipitate was dispersed into octane (5 mL). The second precipitation-dispersion process remained the same as the normal QDs treatment process. For the LED device fabrication, the obtained QDs in crude reaction solutions were precipitated by 10 mL of ethyl acetate with centrifugation at 10000 rpm for 5 min and then the QDs were dispersed into anhydrous octane (5 mL) and precipitated again by ethyl acetate (10 mL). The finally obtained QDs was dispersed into 5 mL of octane and filtered by a 0.2 µm poly (tetrafluoroethylene) filter. The filtered QDs suspension was diluted to be 10 mg mL−1 by octane for the LED device fabrication.

high plasticity of lattices in lead halide perovskites enables incorporation of different ions into the hosts to alter the formation energy of perovskite nanocrystals (NCs).1 In the CsPbBr3 QD system, the hetero-ions substitution or doping has been developed to improve the optical properties and stability of as-prepared CsPbBr3 QDs.25-27 For instance, Zou et al. demonstrated a valid route via Mn2+ substitution to stabilize the perovskite structure of CsPbBr3 QDs by enhancing their formation energy.27 As for α-CsPbI3 QDs, only very recently, Liu et al. showed that the alloyed CsSn1−xPbxI3 QDs could remain stable over months in air. However, the incorporation of Sn cations resulted in extremely low PLQYs of 0.3−3.0% for CsSn1−xPbxI3 (x = 0.4−0.8), which is obviously unsuitable for high-efficiency red LEDs.15 Herein, we present a highly effective strontiumsubstitution strategy to obtain few-nanometer-sized α-CsPbI3 QDs in a stable and purified colloidal state without using complex purification processes. We achieved the synthesis of a series of sizes of α-CsPbI3 QDs by adopting nontoxic, slightly smaller sized strontium cations (Sr2+, 118 pm in radius) to substitute Pb2+ (119 pm in radius) in the perovskite lattices. Our first-principles calculations confirmed that the substitution of Pb2+ by Sr2+ can enhance the formation energy of α-CsPbI3 QDs and reduce the structural distortion, making the cubic perovskite structure more stable. The size tunability (diameters from 15 down to 4.7 nm) of highly stabilized αCsPbI3 QDs provided an excellent platform for us to investigate the photoinduced exciton dynamics as well as the interplay between size-dependent quantum confinement effect and Sr2+-substitution effect by femtosecond and nanosecond time-resolved optical spectroscopy. Also, the uniform 5-nm-sized α-CsPbI3 QDs facilitated the fabrication of smooth films while maintaining the phase stability and high PLQYs (>40%) for several weeks. As a realistic outcome, the Sr2+-substituted α-CsPbI3 QDs yielded an external quantum efficiency (EQE) of 5.92% and improved operational lifetime of as-fabricated red LEDs.

EXPERIMENTAL SECTION Chemicals. Lead iodide (PbI2, Aladdin, 99.99%), 1octadecene (ODE, Aldrich, 90%), cesium carbonate (Cs2CO3, Afla Aesar, 99.99%), strontium iodide (SrI2, Alfa Aesar, 99.99%), octane (Alfa Aesar, anhydrous 99%), lead acetate trihydrate (Pb(CH3COO)2·3H2O, Aladdin, 99.998%), cesium acetate (CH3COOCs, Aladdin, 99.99%), strontium acetate (Sr(CH3COO)2, Aladdin, 99.97%), oleylamine (OAm, Aldrich, 70%), trimethylsilyl iodide (TMSI, Aladdin, 97%, with Cu as a stabilizer), oleic acid (OA, Afla Aesar, 90%), ethyl acetate (Alfa Aesar, anhydrous 99.8%), PEDOT:PSS solutions (Al4083 CLEVIOS), poly-TPD (Xi'an Polymer Light Technology Corp.), TPBi (Nichem). Synthesis of Cs–OA. 0.814 g of Cs2CO3, 40 mL of ODE, and 2.5 mL of OA were loaded into a 100 mL 3-neck flask and the mixture was dried for 1 h at 120 C under vacuum. Then, the mixture was heated to 150 C under nitrogen flow until that the Cs2CO3 powder was reacted with OA to form transparent solution. The obtained Cs–OA solution should be stored under nitrogen atmosphere and it would be preheated to 120 °C for the following QDs synthesis. Synthesis of CsPbI3 QDs with PbI2. 0.087 g of PbI2 was 2


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QDs morphology and nanostructure characterizations. The transmission electron microscope (TEM) analysis was high-angle annular dark field scanning transmission electron microscopy (HAADF–STEM) images were taken by a Talos F200X operating at accelerating voltage of 200 kV. The atomic force microscope (AFM) images were collected by using a Veeco DI Nano-scope V system with a tapping mode. QDs phase and composition characterizations. The energy dispersive X-ray spectroscopy (EDS) were collected on a Talos F200X with an energy dispersive detector. The powder X-ray diffraction (PXRD) pattern analysis was carried out by a Philips X’Pert PRO SUPER X-ray diffractometer using Cu Kα radiation (λ = 1.54178 Å). The elemental analysis was conducted by a PerkinElmer Optima 7300 DV inductively coupled plasma–optical emission spectrometer (ICP–OES). For this analysis, the purified QDs were digested in a 1% HNO3 and 1% HCl solution, which was further filtered through a syringe with 0.22 μm filter to remove non-soluble organics. The X-ray photoelectron spectroscopy (XPS) data was collected on QDs loaded by a silicon wafer by a Thermo ESCALAB 250 spectrometer equipped with a monochromatic Al Kr radiation source (1486.6 eV). QDs steady spectroscopy. The ultraviolet-visible (UV–vis) absorption spectra of QDs solutions were measured by using a PekinElmer in a transmission mode. The UV–vis absorption spectra of QD films (made by spinning QDs solutions on glass substrates) were collected by a SOLID3700 spectrophotometer. The PL spectra and PLQYs were collected on a Hamamatsu (C11347) absolute PLQY spectrometer, using 365-nm excitation on diluted toluene suspensions in a quartz cuvette at room temperature. To avoid the influence of different concentrations of QD solutions on PLQY measurement, we adjusted the concentrations to be an appropriate level, which is that the QDs solution was diluted in toluene until the 0.1 of UV–vis absorbance at the wavelength of 365 nm. QDs ultrafast spectroscopy. The femtosecond time-resolved TA spectroscopy characterizations were conducted on a Helios pump–probe system (Ultrafast System) under ambient conditions. The 400-nm pump (~20 nJ/pulse at the sample cell) was provided by the frequency-doubling of a Coherent Ti:sapphire regenerative amplifier output at 800 nm (pulse duration 35 fs). The white-light continuum probe (520–750 nm) was generated by focusing the 800-nm beam (split from the regenerative amplifier, ~0.4 μJ/pulse) onto a sapphire plate. The pump–probe time delay was controlled by a motorized optical delay line extending to ~8 ns. A routine cross-correlation procedure was adopted to determine the instrument response function (~100 fs). An optical fibercoupled multichannel spectrometer with a CMOS sensor was used to record the TA signals both temporally and spectrally. The TA data were further processed by a commercial software (Surface Xplorer). QDs time-resolved PL. The PL lifetime data were registered on an Edinburgh FLS920 fluorescence spectrometer using the technique of time-correlated single-photon counting. The 332-nm excitation was delivered by an EPLED-330 diode laser that features a pulse-width of 908.9 ps and a bandwidth of 8.8 nm, and the PL emission was monitored at maximum peaks for each sample. The PL kinetic traces were analyzed by an F900 software in the standard tail-fit mode, in which

performed on a Hitachi HT-7700 operating at accelerating voltage of 120 kV. The high resolution TEM (HRTEM) and the weighted residuals and χ2 values were used to evaluate the fitting quality. The samples were dissolved in toluene under ambient conditions. First-principles calculations. Electronic structures of QDs were computed using the PBE28 functional and plane-wave projector augmented wave (PAW)29-30 method with an energy cut-off of 500 eV as implemented in the Vienna ab initio simulation package (VASP).31-32 To investigate the impacts of size and composition on QDs, we firstly created 3×3×3 (27 units) and 4×4×4 (64 units) supercells of pristine CsPbI3 as two parent systems. Then we made appropriate substitutions of Pb with Sr to simulate 3.7%, 7.4% and 11.1% Sr-doping for CsPbI3. To adequately model the QDs, we isolated model systems with a vacuum layer of 20 Å (for the 27-units one) and 18 Å (for the 64-units one) in each of the three directions. The Monkhorst-Pack33 sampling scheme was used for generating the 1×1×1 k-point grids in the first Brillouin zone. All of the model systems were then fully relaxed until total energies converge at 1×10-4 eV and the root-mean-square residual force at 0.02 eV/Å. α-CsPbI3 QDs based LEDs fabrication and performance test. The ITO-coated glass substrates were cleaned by acetone under ultrasonication and dried with nitrogen flow. This cleaning process was repeated for three times. Then, these substrates were treated by UV−ozone for 15 min. After the surface treatment, the PEDOT:PSS solutions were spun onto the surface of ITO-coated glass substrates by a spin coater (chemat technology spin-coater kw-4A) at 4000 rpm for 60 s and annealed at 140 °C on a hot plate (chemat technology kw4AH) for 15 min under ambient conditions. A solution of Poly-TPD in chlorobenzene (8 mg/mL) was spin-coated on the PEDOT:PSS layer by the spin-coater at 2000 rpm for 60 s and annealed at 130 °C on the hot plate for 20 min under nitrogen atmosphere. The unsubstituted CsPbI3 and Sr2+substituted CsPbI3 QDs in octane were spin-coated onto the poly-TPD layer by the spin-coater at 2000 rpm for 60 s. Afterwards, 40 nm of TPBi layer, 1 nm of LiF layer and 100 nm of Al electrode were deposited by a thermal evaporation system equipped with a shadow mask under a high vacuum (~2 × 10−4 Pa). The device active area was 4 mm2. For the asfabricated LED devices performance tests, a Keithley 2400 SourceMeter was used to record the current versus voltage characteristics. The luminescence of the device was revealed by photon flux using a silicon photodiode, which was calibrated by a PR-670 spectra scan luminance-meter. The electroluminescent spectrum (EL) of the devices was collected using an Ocean Optics JAZ spectrometer. Other important parameters used to characterize LEDs were all calculated from the L–J–V and EL measurements under the assumption that the emission of the LEDs exhibits a Lambertian pattern. The measurements for devices were made at room temperature under nitrogen atmosphere.

RESULTS AND DISCUSSION As-synthesized typical 5-nm-sized Sr2+-substituted αCsPbI3 QDs. The TEM image (Figure 1a) displays the typical monodispersed 5-nm-sized α- CsPbI3 QDs synthesized via our developed Sr2+-substitution strategy (details in Experimental section). The synthesis of such large-quantity 3



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Size-tuning and stabilization mechanism of α-CsPbI3 QDs via Sr2+ substitution. To explore size tunability of αCsPbI3 QDs achieved by our developed Sr2+-substitution strategy along with thermodynamic pathways, we set up a series of reaction conditions, including the 0%, 20%, 40% and 60% concentrations (relative to lead iodide) of strontium iodide (SrI2) and the reaction temperatures at 100 oC, 130 oC, 150 oC and 170 oC. All of these reaction systems can produce NCs as revealed by TEM images in Figure S3. However, only the reaction systems marked out by the red lines in Figure S3 generated the pure cubic phase of CsPbI3 NCs after a simple extraction process from crude reaction solutions by using ethyl acetate as the antisolvent (Figure S4). In contrast, the products in the reaction systems outside of the red-linemarked region contain impurities (Figure S5). We also ruled out that the PXRD diffraction peaks come from impurities, including PbI2, CsI, and δ-CsPbI3, indicating that the cubic phase CsPbI3 were eventually decomposed into corresponding components after purification. A closer scrutiny revealed that the cubic crystallization of CsPbI3 preferred to occur at the reaction systems of higher concentrations of SrI2 and higher reaction temperatures. The atomic ratios of Sr/Pb in α-CsPbI3 QDs, determined by ICP−OES (Table S1), were found to be in the range of 1.8−4.9%, highlighting again the incorporation of Sr2+ in assynthesized CsPbI3 QDs. Moreover, careful examination of the PXRD patterns of as-obtained CsPbI3 QDs with different ratios of Sr/Pb (Figure S6) reveals that the (100) peaks of cubic phase CsPbI3 shift to higher angles upon the increase of Sr/Pb ratio, which further confirms that the Pb2+ ions were partially substituted by Sr2+ ions leading to the lattice contraction because of slightly smaller size of Sr2+ ions. The size distribution histograms in Figure 2a-f show that the average particle size decreased with increasing the SrI2 concentration and lowering the reaction temperature. The average particle size of α-CsPbI3 QDs obtained at 170 oC decreased from 14.06 to 7.33 nm (smaller than the effective exciton Bohr diameter of CsPbI3)4 with increasing the SrI2 concentration from 0% to 60% in the reaction systems. At the 60% concentration of SrI2, the size of pure α-CsPbI3 QDs was further reduced to 4.70 nm with decreasing the reaction temperature to 130 oC, which set the minimum size limitation in our developed reaction systems, given that the product was no longer the pure α-CsPbI3 once the reaction temperature was further lowered to 100 oC and the increase of SrI2 to 80% also cannot further reduce the size of α-CsPbI3 QDs (Figure S7 and S8). Figure 2g summarizes the size shrinking trend of pure α-CsPbI3 QDs in the series of SrI2 concentrations and reaction temperatures, showing that through controlling the Sr2+-substitution conditions we have prepared a series of sizes of α-CsPbI3 QDs in purified and stable colloidal state, a feat that has not been demonstrated previously in the literature.12,

and small size (5 nm) nanocubes is still a long-standing obstacle for red LED applications of CsPbI3 QDs according to the previous reports.14, 34-35 The magnified TEM images in Figure 1b confirmed their good uniformity and crystallinity. The typical HRTEM image of as-synthesized α-CsPbI3 QDs (Figure 1c) revealed a lattice distance of 0.61 nm, corresponding to that of (100) crystal Figure 1. Morphology characterizations of typical 5-nm-sized α-CsPbI3 QDs prepared with adding 60% SrI2 at 150 °C. a–c, TEM images of α-CsPbI3 QDs (a,b) with the corresponding HRTEM characterization (c). d–h, HAADF–STEM image of α-CsPbI3 QDs (d) and the corresponding elemental mapping of Cs (e), Pb (f), I (g) and Sr (h), demonstrating the presence of Sr in αCsPbI3 QDs. Scale bars: 200 nm (a), 20 nm (b), 2 nm (c), 20 nm (d–h). planes in the cubic-phase CsPbI3 QDs.9 To confirm the incorporation of Sr2+ ions into the α-CsPbI3 QDs, the spatial distributions of Cs, Pb, I and Sr species in as-obtained nanocubes were further revealed by HAADF−STEM (Figure 1d) and the corresponding elemental mapping (Figure 1e−h), all of which well overlap with each other, suggesting that Sr2+ cations have been homogeneously distributed among all of the QDs. Furthermore, the EDS spectrum analysis indicated a low Sr/Pb ratio of 0.08 (Figure S1) in as-synthesized Sr2+substituted CsPbI3 QDs, which may cause some small noise signals in Figure 1h, even where QDs are not presented. In addition, we conducted XPS characterizations to compare the Sr2+-substituted samples with the unsubstituted ones. The results (Figure S2a) revealed that both QDs contain Cs, Pb, I, C, N and O, in agreement with the previous reports.36-37 The detailed analysis (Figure S2b-d) showed that the spectra of Cs 3d, Pb 4f and I 3d in both samples can be well fitted with identical dominant peaks, indicating that the Sr2+ substitution did not distort the local binding environment.37 Importantly, two additional peaks (Figure S2e) from the Sr2+-substituted samples were observed at 135.5 and 133.8 eV, which can be assigned to the Sr 3d3/2 and 3d5/2, respectively.37 Moreover, the atomic ratio of Sr/Pb extracted from XPS is 4.5%, also evidencing the successful substitution of Pb2+ by Sr2+ in the α-CsPbI3 QDs.

14

To provide more insights into the effects of Sr2+ cation substitution and excess I- anion passivation on the size control, stability, and optical properties of as-obtained α-CsPbI3 QDs, we took two sets of control experiments, including the systems of acetate salts and excess PbI2 (details please see Experimental section). Firstly, we set up a series of reaction conditions in the system of acetate salts, including 0% strontium acetate (Sr(ac)2, ratio to lead acetate) at 170 oC, 40% 4


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Sr(ac)2 at 170 oC, 60% Sr(ac)2 at 170 oC, and 60% Sr(ac)2 at 150 oC in reaction precursors. The average particle size of αCsPbI3 QDs obtained at 170 oC did not show significant change with increasing the concentration of Sr(ac)2, but the average particle size decreased from 17.02 to 9.92 nm with

lowering the reaction temperature (Figure S9). The corresponding normalized PL spectra of samples (Figure S10) showed that the emission peak exhibited a blue-shift from 692 to 686 nm with reducing the size to around 9 nm. This means that the sizes of α-CsPbI3 QDs can

Figure 2. Particle size tuning and the formation energy of α-CsPbI3 QDs. a–f, Size distribution histograms and the corresponding TEM images of QDs under the synthetic conditions of 0% SrI2 at 170oC (a), 20% SrI2 at 170 oC (b), 40% SrI2 at 170 oC (c), 60% SrI2 at 170 oC (d), 60% SrI2 at 150 oC (e) and 60% SrI2 at 130 oC (f). The particle size of α-CsPbI3 QDs was controlled by varying the concentrations of SrI2 (relative to PbI2) and the reaction temperatures. Scale bars: 100 nm. g, Plot of the size shrinking trend for pure α-CsPbI3 QDs in the series of SrI2 concentrations and reaction temperatures. h, The calculated formation energies of αCsPbI3 QDs substituted with different Sr2+ contents. be reduced by lowering the reaction temperature without excessive I- anions but the size reducing trend is not as sharp as using excessive I- anions. Secondly, we set up a series of reaction conditions in the system of excessive PbI2, including the 0% excess PbI2 at 170 oC, 40% excess PbI2 at 170 oC, 60% excess PbI2 at 170 oC, and 60% excess PbI2 at 150 oC. The TEM images and corresponding size distribution histograms (Figure S11) indicate that the particle size of α-CsPbI3 QDs decreased from 14.06 to 8.67 nm with increasing the concentration of PbI2 from 0% to 60% and the size of αCsPbI3 QDs was further reduced to 7.86 nm with decreasing the reaction temperature to 150 oC. Meanwhile, the corresponding normalized PL spectra of the samples showed that the PL emission peak exhibited a blue-shift from 689 to 676 nm (Figure S12). These control experiments showed that the excessive I- anions along with lowering reaction temperature play a significant role to reduce the particle size of α-CsPbI3 QDs. In this scenario, it turned out that excessive I- anions can play a similar role as the influence of Br- anions in the growth of CsPbBr3 NCs based on the thermodynamic equilibrium to tuning the size of α-CsPbI3 QDs.38 Notably, the few-nanometer-sized α-CsPbI3 QDs can be well purified without inducing agglomeration by using ethyl acetate as the antisolvent in our synthesis, in stark contrast to the destabilization of ethyl acetate to CsPbI3 QDs reported in a previous work.9 This observation indicated that the

incorporation of Sr2+ significantly contributed to the formation of highly phase-stable α-CsPbI3 QDs. To understand such a stabilization effect, we calculated the formation energies per unit cell (∆Eform) of the α-CsPb1-xSrxI3 QDs (0 ≤ x < 12%) to evaluate the impact of Sr2+ substitution on the phase stability of perovskite structure. Note here that the formation energy was defined as the energy change during the formation of a QD from its isolated atoms.27 As shown in Table S2, the absolute value of ∆Eform decreased with decreasing the size of QDs from 2.5 nm (64 units) to 1.9 nm (27 units), accompanied by an increase of structure distortion with respect to the standard cubic structure. This result suggested that α-CsPbI3 QDs would become less stable as the size decreases by virtue of ultra-large surface-to-volume ratio. Meanwhile, the absolute value of ∆Eform experienced a monotonous increase with increasing the content of Sr2+ cations incorporated into the cubic unit cells of QDs, indicating that Sr2+-substituted α-CsPbI3 QDs can help stabilize the unsubstituted ones (Figure 2h). Likewise, the cell parameter of axis angle is closer to 90o with increasing the Sr2+ content, indicating that smaller B-site substitution will reduce the distortion extent of perovskite structure (Table S2). In short, the partial replacement of Pb2+ by the smaller Sr2+ can stabilize the α-CsPbI3 QDs through enhancing the formation energy and reducing the extent of structure distortion. 5



Spectroscopy characterizations of as-synthesized αCsPbI3 QDs. The finely controlled sizes of α-CsPbI3 QDs provided an excellent platform for us to interrogate the quantum confinement effect involved. It is worth noting that to correlate the Sr2+ substitution with QDs sizes the samples for the following comparison were denoted as atomic ratio of

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Sr2+-QDs size such as 1.8%-11.79 nm for the α-CsPbI3 QDs synthesized by using 20% SrI2 at 170 oC. As shown in Figure 3a,b, the optical properties of α-CsPbI3 QDs were correlated to their sizes. The corresponding normalized photoluminescence (PL)

Figure 3. Size-dependent optical properties of as-synthesized α-CsPbI3 QDs under different conditions. a, Normalized PL spectra of α-CsPbI3 QDs. b, (αhν)2 vs photon energy (eV) curves obtained from UV−vis absorption spectra of CsPbI3 QD films. c, d, Representative fs-TA spectra (excitation 400 nm) taken at several probe delays for the unsubstituted α-CsPbI3 QDs and the Sr2+substituted ones (i.e., 3.1% -5.38 nm), respectively. e, f, Characteristic decay-associated spectra for the unsubstituted α-CsPbI3 QDs and the Sr2+-substituted ones (i.e., 3.1% -5.38 nm), respectively. g, Schematic illustration of the photoinduced relaxation processes involved in the system. VB and CB stand for the valence and conduction bands, respectively. X1 and Xn denote the lowest and higher-lying excitonic states in the CB, respectively. The asterisk highlights the Sr2+-substitution-induced trap states in the CB. h, Variation trend of average PL lifetimes (blue) and PLQYs (black) versus the six sizes of α-CsPbI3 QDs. i, Schematic illustration of the size-dependent energy-band variation of α-CsPbI3 QDs, where X1(expt.) and X1(theo.) denote the experimentally observed and theoretically calculated lowest excitonic states, respectively. The bi-directional red and green arrows depict the experimentally observed and theoretically calculated band gaps, respectively. spectra of the samples showed that the PL emission peak exhibited a blue-shift from 689 to 658 nm (Figure 3a). Shrinking the size of α-CsPbI3 QDs from 14.06 to 4.70 nm turned out to broaden the band gap from 1.75 to 1.83 eV accordingly (Figure 3b). The entire UV-vis spectra were also provided to display the exciton absorption peaks of α-CsPbI3 QDs as shown in Figure S13. The size variation of α-CsPbI3 QDs was also correlated to their PLQY values as well (Figure S14). Also, as shown in Figure S15, the PLQY values of αCsPbI3 QDs obtained in the systems of acetate salts and the excessive I- anions both increased with increasing the concentrations of Sr(ac)2 and PbI2. This result indicates that the PLQY enhancement of α-CsPbI3 QDs in our synthesis comes from the synergistic effect of iodide passivation and strontium substitution. The theoretical band gaps associated with the quantum confinement effect due to size variation

were also calculated and all of the aforementioned trends are summarized in Table S3. The results indicated that the optical properties of α-CsPbI3 QDs were not precisely dependent on size effect as revealed by the theoretical calculations and meanwhile the PLQY values did not monotonously increase with reducing the QDs size. In this case, the effect of Sr2+ substitution on the optical properties of as-synthesized αCsPbI3 QDs should be taken into account. To provide more insights into the impact of quantum confinement and Sr2+ substitution on the optical properties (including photoexcitation and PL) of as-obtained α-CsPbI3 QDs, we resorted to time-resolved optical spectroscopy including femtosecond transient absorption (fs-TA) and PL lifetime characterizations of the samples. Firstly, the fs-TA spectroscopy characterization was performed on a representative Sr2+-substituted sample (3.1%-5.38 nm) in 6


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reference to the unsubstituted one. In the pump–probe configuration, the 400-nm pump was suited to photoinduce an interband transition for both samples and the white-lightcontinuum probe encompassed the wavelength range of 520– 750 nm (details in Experimental section). Figure 3c and 3d display the fs-TA spectra at several representative probe delays (i.e., from 5 ps to 7 ns) for the unsubstituted and substituted samples, respectively. Following the spirit of spectral analysis on the exciton dynamics involved in a similar inorganic cesium lead halide perovskite system (i.e., CsPbBr3),25, 39 we can readily attribute the spectral profiles of probe bleach (PB), labeled PB1 and PB2, to the ground-state bleach and hot-exciton-induced bleach, respectively, and link those of photoinduced absorption (PA), labeled PA1 and PA2, to the lowest excitonic state and higher-lying excitonic states, respectively. In view of the complex blending of these spectral profiles, a global analysis was employed to obtain the characteristic decay-associated spectra, as shown in Figure 3e and 3f. The corresponding time constants were extracted as follows: τ1 = 11.59 ± 0.04 ps, τ2 = 60.04 ± 0.15 ps, and τ3 >1 ns for the unsubstituted sample (Figure 3e), while τ1 = 10.00 ± 0.02 ps, τ2 = 43.06 ± 0.08 ps, and τ3 >1 ns for the substituted one (Figure 3f). These decay components describe the following three processes: intra-band hot-exciton relaxation (τ1), exciton trapping to the band-edge trap states (τ2), and exciton recombination (τ3), as depicted in Figure 3g. For the sake of clarity, we omitted here the hole-relaxation processes in the valence band (VB), which take place along with the electron-relaxation processes in the conduction band (CB). Apparently, the former two processes were accelerated for the substituted sample relative the unsubstituted one, indicating that the Sr2+-substitution in α-CsPbI3 perovskite did bring on new trap states within the CB band and near its band-edge1, 14, 36 (marked with an asterisk in Figure 3g) that promoted the state coupling involved in the relaxation processes. Despite the fact that our fs-TA characterization disclosed the promoted electron relaxation in the CB band caused by the Sr2+-substitution-induced trap states, it can hardly provide information about the band-edge PL kinetics that usually features a much longer lifetime. To this end, we resorted to nanosecond time-resolved PL characterization (details in Experimental section and the relevant data are collected in Figure S16 and Table S4). The average PL lifetimes versus the six sizes of α-CsPbI3 QDs are displayed in Figure 3h, in which the corresponding PLQY results (taken from Figure S14) are also given here for comparison. Obviously, the two sets of data exhibited a nearly reverse trend, which can be understood in the context of the interplay between sizedependent quantum confinement effect and Sr2+-substitution induced new electronic states as shown in Figure 3i. As is known, with reducing the particle size of α-CsPbI3 QDs, the exciton binding energy gets enhanced to accelerate the recombination rate of excitons and thus the PL lifetime decreases.40-41 This is commensurate with our observation of decreased PL lifetimes with reducing the particle size of αCsPbI3 QDs. Theoretically, the quantum confinement effect would levitate the lowest excitonic state (the energy level colored by green in Figure 3i) and in consequence the PLQYs would decrease rapidly with reducing the size due to the emergence of near-band-edge induced nonradiative trap states.20-21 However, our experimental results indicated that

the PLQYs improved with the reduction of particle size at first and then became saturated at smaller sizes. This unusual phenomenon can only be linked to the unique, highly ionic structure of perovskite, in which the Sr2+-substitution-induced band-edge states should be of radiative nature.1, 25 In other words, the combination of the band states of pristine α-CsPbI3 and the Sr2+-substitution-induced band states generates a new, lowest excitonic state at a lower energy level (colored by red in Figure 3i). In this scenario, the Sr2+ substitution in α-CsPbI3 QDs resulted in more efficient radiative recombination of excitons and thus the PLQY values were enhanced rather than suppressed. Meanwhile, the lowered energy level of the lowest excitonic state of Sr2+-substituted α-CsPbI3 QDs also illustrated the flatter variation of band gaps of α-CsPbI3 QDs in comparison to the calculated results (Figure S17), further confirming the Sr2+-substitution-induced modification of the band-edge electronic state structures in α-CsPbI3 QDs. The reduced PL lifetime and enhanced PLQY in our Sr2+substituted α-CsPbI3 QDs are beneficial for improving the recombination rate of charge carriers in as-fabricated red LEDs. Stability evaluation of as-synthesized α-CsPbI3 QDs. To evaluate the stabilities of as-synthesized α-CsPbI3 QDs for practical applications, we chose four typical QDs (i.e., 0%14.06 nm, 2.5%-10.10 nm, 4.9%-7.33 nm and 3.1%-5.38 nm) in both solution and solid-state thin-film forms. In the stability tests, all of the samples were stored under ambient conditions with the humidity of 30−40% at 20 ± 10 oC. Figure 4a shows the photos recorded at different time for unsubstituted αCsPbI3 QDs and Sr2+-substituted α-CsPbI3 QDs solutions. It was observed that the red-light emission of the unsubstituted sample solution under irradiation of UV light (365 nm, 6W) completely disappeared after 60-days. In contrast, the brightness of Sr2+-substituted sample solutions showed indiscernible decrease even after two-month storage under exactly the same conditions. As shown in Figure 4b, the Sr2+substituted α-CsPbI3 QDs retained a nearly constant PLQY with a record value of 80% even after storage for 60 days. However, the unsubstituted ones showed a decreased PLQY from the initial 78% to 53% after 30 days and finally to 1% after 60 days. The PXRD patterns of the samples after 60days storage in solution (Figure 4c) further confirmed that the unsubstituted ones were transformed to δ-CsPbI39 and in contrast the Sr2+-substituted ones maintained the cubic phase over two months. In addition, the TEM images (Figure S18) showed that the unsubstituted samples were reshaped to nanorods and nanosheets due to the phase transformation to δ-CsPbI3;42 whereas, the Sr2+-substituted samples still maintained the original, uniform, small-size nanocube morphology. Moreover, the Sr2+-substituted α-CsPbI3 QDs exhibited excellent photostability under sustained UV-light (365 nm, 6 W) irradiation at room temperature, with a test period from 30 min to 120 h. As shown in Figure S19, after 120 h the relative PL intensity of unsubstituted samples decreased to 1%. However, the Sr2+-substituted ones still maintained high stability and only decreased to 80% of the initial PL intensity, which indicates that the Sr2+-substituted α-CsPbI3 QDs exhibited not only better air stability but also photo stability in comparison to the unsubstituted ones. The solid thin film is the ideal, final state for the LED applications based on QDs, and generally the thin film of α-CsPbI3 is much 7



more unstable and would undergo immediate transformation to orthorhombic phase once exposed to ambient conditions.14 Thus, we further verified the stability of the α-CsPbI3 QDs films fabricated by spin-coating on glass substrates under ambient conditions. Figure 4d shows that the thin film of the unsubstituted sample gradually became non-fluorescent δ-

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CsPbI3. In contrast, the thin films of the Sr2+-substituted samples still exhibited bright red PL with well maintained high PLQYs (>40%) (Figure 4e) and the cubic phases as evidenced by the PXRD patterns (Figure 4f) even after 20day storage under ambient conditions, which is also more stable than any

Figure 4. Cubic phase and PL stability tests of as-synthesized CsPbI3 QDs in the forms of suspension (in toluene) and solid-state thin film. a, Photos of fresh and aged (after 60 days) CsPbI3 QDs suspensions under UV-light (365 nm) irradiation. From left to right, the samples were under the synthetic conditions of 0% SrI2 at 170oC, 40% SrI2 at 170 oC, 60% SrI2 at 170 oC, 60% SrI2 at 150 oC. b, PLQY values as a function of aged days for unsubstituted and Sr2+-substituted CsPbI3 QDs solutions. c, PXRD patterns recorded for the CsPbI3 QDs stored in solution for 60 days. d, Photos of fresh and aged (after 20 days) CsPbI3 QDs thin films under UV-light (365 nm) irradiation. From left to right, the samples were under the synthetic conditions of 0% SrI2 at 170oC, 40% SrI2 at 170 oC, 60% SrI2 at 170 oC, 60% SrI2 at 150 oC. e, PLQY values as a function of stored days for unsubstituted and Sr2+-substituted CsPbI3 QDs thin films. f, PXRD patterns recorded for the CsPbI3 QDs thin films stored for 20 days. g–j, AFM images of CsPbI3 QDs thin films under the synthetic conditions of 0% SrI2 at 170 oC (g), 40% SrI2 at 170 oC (h), 60% SrI2 at 170 oC (i), 60% SrI at 150 oC (j). Scale bars: 1 μm. 2 α-CsPbI3 QDs films reported elsewhere.9, 14 On the other hand, to evaluate the stability of as-synthesized α-CsPbI3 QDs more accurately, we compared the stability of two sets of control QD samples including the systems of acetate salts and excess PbI2 in solid-state thin-films under ambient conditions. The α-CsPbI3 QDs synthesized by using Sr(ac)2 maintained a high PLQY (>28%) even after storage for 16-days under ambient conditions (Figure S20). In contrast, the α-CsPbI3 QDs synthesized without Sr(ac)2 showed a decreased PLQY from initial 35% to 8% after 16-day storage. More impressively, as shown in Figure S21, the thin films of samples obtained by excessive PbI2 displayed a significant decrease of PLQY (< 15%) after 16-day storage under ambient conditions. This indicates that the excessive I- anions play a limited role on

improving the stability of α-CsPbI3 QDs in comparison to Sr2+-substitution. Therefore, the incorporation of strontium ions into α-CsPbI3 host can significantly stabilize the cubic structure of CsPbI3 and play a dominate role on the stabilization of α-CsPbI3 QDs obtained even in the system of acetate salts without excess I- anions. Therefore, the stability of α-CsPbI3 QDs can be attributed to the enhanced formation energy of cubic lattices of α-CsPbI3 with the assistance of Sr2+ substitution, which largely improved the resistance to humidity attack and photo-irradiation induced degradation. To further verify the advantage of small-size QDs for close packing in spin-coated thin films, the surface morphologies of as-fabricated α-CsPbI3 QDs films were characterized by AFM. All of the films exhibited low root-mean-square (RMS) 8


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Journal of the American Chemical Society α-CsPbI3 QDs resulted in more dense-packing films. α-CsPbI3 QDs-based red LEDs performance. To demonstrate the merit of Sr2+-substituted α-CsPbI3 QDs for LED devices, we fabricated red LEDs using as-fabricated QDs films as luminescent layers. As illustrated in Figure 5a, the LED

roughness and uniform density (Figure 4g–j), which are indispensable for constructing high-performance LED devices. It is worth noting that the RMS roughness of asfabricated α-CsPbI3 QDs films was gradually reduced from 3.1 to 2.2, 1.6 and 1.5 nm, corresponding to 0%-14.06 nm, 2.5%-10.10 nm, 4.9%-7.33 nm and 3.1%-5.38 nm, respectively, indicating that smaller sizes of monodispersed

10.10 nm, 4.9%-7.33 nm and 3.1%-5.38 nm). The voltage−luminance curves of as-fabricated LEDs are presented in Figure 5c. In terms of brightness, all of the devices based on Sr2+-substituted α-CsPbI3 QDs were much stronger than that based on unsubstituted α-CsPbI3 QDs in the entire driving voltage range. In particular, the device with the 4.9%-7.33 nm sample exhibited a highest brightness, the luminance of which reached a maximum of 1250 cd m−2 at the voltage of 9.2 V. This represents a high value achieved for the red LEDs based on CsPbI3 QDs to date (refer to Table S5). Moreover, the turn-on voltages (at a luminance of 1 cd m−2) of the LEDs with the size-reduced α-CsPbI3 QDs were 4.8, 4.0, 3.8 and 3.6 V, respectively, indicating that the Sr2+substitution-induced close-packing of small-size α-CsPbI3 QDs facilitated the charge injection into the emitting layers. Consequently, a high EQE value of 5.92% with a current density of 66.9 mA cm−2 has been achieved for the LED device based on the Sr2+-substituted CsPbI3 QDs (i.e., 3.1%5.38 nm), which is a factor of three higher than that of the unsubstituted CsPbI3 QDs-based LED (Figure 5d). The enhanced efficiency of the LED device based on Sr2+substituted α-CsPbI3 QDs should originate from the excellent film uniformity and high PL quantum efficiency. The operational instability is a challenging issue for metal halide perovskite LEDs (PeLEDs).43 We evaluated the operational stability of our fabricated devices under a driving voltage of 6.0 V and the EL emission intensity was recorded intermittently (Figure 5e). The luminance of the control device using unsubstituted α-CsPbI3 QDs dropped to half of the initial value after only 10 min; whereas, the half lifetime of Sr2+-substituted CsPbI3 QDs-based LED was significantly improved to 120 min and the device could be operated continuously for even 15 h. In addition, spectral instability is another critical problem for PeLEDs. As shown in Figure S23, the EL spectra of the LED based on Sr2+-substituted α-CsPbI3 QDs remained almost identical under the driving voltage of 6.0 V for 15 h, indicating high EL spectral stability of our developed α-CsPbI3 QDs-based PeLEDs.

Figure 5. Device characteristics of unsubstituted and Sr2+substituted α-CsPbI3 QDs-based red LEDs. a, Device structure of as-fabricated PeLEDs. b, EL spectra at various driving voltages. Inset: a photo of a working PeLED at a driving voltage of 6.0 V. c, Luminance versus driving voltage curves for LEDs based on α-CsPbI3 QDs synthesized under different conditions. d, EQEs of LEDs based on different αCsPbI3 QDs at different current density. e, Stability data for the devices based on the unsubstituted and Sr2+-substituted αCsPbI3 QDs tested at a constant driving voltage of 6.0 V. The luminances of the LEDs have been normalized to their initial values. device architecture is consisted of layered structure of ITO/PEDOT:PSS/poly-TPD/α-CsPbI3 QDs/TPBi/LiF/Al, in which ITO is the anode, PEDOT:PSS is the hole-injection layer, poly-TPD is the hole- transport/electron-block layer, TPBi is the electron-transport layer and LiF/Al is the cathode. The LED with the Sr2+-substituted sample of 3.1%-5.38 nm exhibited sharp electroluminescence (EL) peaks under various driving voltages at the wavelength of 678 nm with a full width at half maximum (FWHM) of 32 nm (Figure 5b), corresponding to Commission Internationale del’Eclairage (CIE) colour coordinates of (0.67, 0.26) (Figure S22), a feature quite ideal for red display applications. The EL emission peak of as-fabricated LED showed a little red-shift relative to the PL spectra of the corresponding α-CsPbI3 QDs in solution due to the inter-QDs interaction and Stark effect as previously reported.12 A photograph of the LED (inset in Figure 5b) also showed its bright red-light emission under a driving voltage of 6.0 V. To make more detailed comparisons, we examined the device performances using four typical αCsPbI3 QDs with different sizes (i.e., 0%-14.06 nm, 2.5%-

CONCLUSION In summary, we introduced Sr2+ cations into CsPbI3 hosts to stabilize the few-nanometer-sized α-CsPbI3 QDs via a simple yet effective hot-injection synthetic methodology with spontaneously improved photoluminescence properties of assynthesized QDs. We demonstrated that the few-nanometersized α-CsPbI3 QDs well maintained the cubic-phase stability and high PLQYs (>80%) for an impressive two-month-long period and the fabricated smooth solid thin film also retained cubic phase and high PLQYs (>40%) under ambient conditions for several weeks, which represented the most stable α-CsPbI3 QDs reported to date. More importantly, the few-nanometer-sized α-CsPbI3 QDs-based red LEDs exhibited a high brightness (1250 cd m−2), high external quantum efficiency (5.92%) and excellent operational 9



stability (two-hour half lifetime driven by 6 V). The combined features of Sr2+-substituted α-CsPbI3 QDs show great potentials for future stable and efficient red LED as well as high-efficiency photovoltaic devices.

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TEM images, XPS spectra, PXRD analysis, UV-vis absorption, PL spectrum, PL lifetime, Photostability test and Electroluminescence spectral stability.

AUTHOR INFORMATION

ASSOCIATED CONTENT

Corresponding Author

Supporting Information.

*E-mail: [email protected]

*E-mail: [email protected] *E-mail: [email protected] ORCID

(5) Raino, G.; Nedelcu, G.; Protesescu, L.; Bodnarchuk, M. I.; Kovalenko, M. V.; Mahrt, R. F.; Stoferle, T. Single cesium lead halide perovskite nanocrystals at low temperature: fast single photon emission, reduced blinking, and exciton fine structure. ACS Nano 2016, 10, 2485.

Hong-Bin Yao: 0000-0002-2901-0160 Qun Zhang: 0000-0002-5777-9276 Guozhen Zhang: 0000-0002-6116-5605

(6) Song, J. Z.; Li, J. H.; Li, X. M.; Xu, L. M.; Dong, Y. H.; Zeng, H. B. Quantum dot light-emitting diodes based on inorganic perovskite cesium lead halides (CsPbX3). Adv. Mater. 2015, 27, 7162. (7) Zhang, Q.; Yin, Y. D. All-inorganic metal halide perovskite nanocrystals: opportunities and challenges. ACS Cent. Sci. 2018, 4, 668. (8) Sun, Q.; Wang, Y. A.; Li, L. S.; Wang, D. Y.; Zhu, T.; Xu, J.; Yang, C. H.; Li, Y. F. Bright, multicoloured light-emitting diodes based on quantum dots. Nat. Photon. 2007, 1, 717.

Author Contributions †These authors contributed equally to this work. Notes The authors declare no competing ﬁnancial interest.

(9) Swarnkar, A.; Marshall, A. R.; Sanehira, E. M.; Chernomordik, B. D.; Moore, D. T.; Christians, J. A.; Chakrabarti, T.; Luther, J. M. Quantum dot-induced phase stabilization of α-CsPbI3 perovskite for highefficiency photovoltaics. Science 2016, 354, 92.

ACKNOWLEDGMENTS The authors acknowledge support from the Ministry of Science and Technology of China (No. 2014CB931800, 2013CB931800, 2014CB931702, 2016YFA0200602, 2016YFB0401701, 2018YFA0208702, 2017YFA0303500), the National Natural Science Foundation of China (No. 51571184, 21501165, 21875236, 21431006, 21573211, 21633007, 21421063, 51572128), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 21521001), the Users with Excellence and Scientific Research Grant of Hefei Science Center of the CAS (2015HSC-UE007, 2015SRG-HSC038), the Joint Funds from Hefei National Synchrotron Radiation Laboratory (Grant KY2060000111), the Chinese Academy of Sciences (No. KJZD-EW-M01-1, XDB01020000), and the Fundamental Research Funds for the Central Universities of China (WK2340000063, WK2060190085). We thank Professor Fengjia Fan for helpful discussion. We thank the support from USTC Center for Micro and Nanoscale Research and Fabrication.

(10) Sanehira, E. M.; Marshall, A. R.; Christians, J. A.; Harvey, S. P.; Ciesielski, P. N.; Wheeler, L. M.; Schulz, P.; Lin, L. Y.; Beard, M. C.; Luther, J. M. Enhanced mobility CsPbI3 quantum dot arrays for recordefficiency, high-voltage photovoltaic cells. Sci. Adv. 2017, 3, eaao4204. (11) Protesescu, L.; Yakunin, S.; Kumar, S.; Bar, J.; Bertolotti, F.; Masciocchi, N.; Guagliardi, A.; Grotevent, M.; Shorubalko, I.; Bodnarchuk, M. I.; Shih, C. J.; Kovalenko, M. V. Dismantling the "red wall" of colloidal perovskites: highly luminescent formamidinium and formamidinium-cesium lead iodide nanocrystals. ACS Nano 2017, 11, 3119. (12) Pan, J.; Shang, Y. Q.; Yin, J.; De Bastiani, M.; Peng, W.; Dursun, I.; Sinatra, L.; El-Zohry, A. M.; Hedhili, M. N.; Emwas, A. H.; Mohammed, O. F.; Ning, Z. J.; Bakr, O. M. Bidentate ligand-passivated CsPbI3 perovskite nanocrystals for stable near-unity photoluminescence quantum yield and efficient red light-emitting diodes. J. Am. Chem. Soc. 2018, 140, 562. (13) Lu, M.; Zhang, X.; Zhang, Y.; Guo, J.; Shen, X.; Yu, W. W.; Rogach, A. L. Simultaneous strontium doping and chlorine surface passivation improve luminescence intensity and stability of CsPbI3 nanocrystals enabling efficient light-emitting devices. Adv. Mater. 2018, http://dx.doi.org/10.1002/adma.201804691, 1804691.

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Few-nanometer-sized Î±-CsPbI3 Quantum Dots Enabled by Strontium

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