L-Type Ligand-Assisted Acid-Free Synthesis of CsPbBr3 Nanocrystals

May 22, 2019 - L-Type Ligand-Assisted Acid-Free Synthesis of CsPbBr3 .... spectra, SEM images, J–V curves, lifetime components of different samples ...
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Letter Cite This: Nano Lett. 2019, 19, 4151−4157

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L‑Type Ligand-Assisted Acid-Free Synthesis of CsPbBr3 Nanocrystals with Near-Unity Photoluminescence Quantum Yield and High Stability Qixuan Zhong,‡ Muhan Cao,‡ Yafeng Xu,‡ Pengli Li, Yong Zhang, Huicheng Hu, Di Yang, Yong Xu, Lu Wang,* Youyong Li, Xiaohong Zhang,* and Qiao Zhang*

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Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University 199 Ren’ai Road, Suzhou 215123, Jiangsu People’s Republic of China S Supporting Information *

ABSTRACT: X-type ligands, for example, the pair of oleylamine (OAm) and oleic acid (OA), have been widely used to prepare CsPbX3 nanocrystals (NCs). However, the proton exchange between coordinated OAm and OA may induce the detachment of ligands, resulting in poor performance after cleaning or long-time storage. Herein, density functional theory calculations predict that primary amines (L-type ligands) can stabilize a PbBrx-rich surface and yield a trap-free material with fully delocalized valence band maximum and conduction band minimum states, which can significantly improve the photophysical properties and stability of CsPbBr3 NCs. Along this prediction, a room-temperature reprecipitation method using L-type ligands (OAm, n-octylamine, or undecylamine) as the sole capping ligand has been developed to synthesize high-quality CsPbBr3 NCs with near-unity photoluminescence quantum yield and dramatically improved stability against purification and water treatment. The enhancement can be attributed to the strong binding of unprotonated amines to lead atoms and the effective surface passivation provided by the resulted PbBrx-rich surface, which are highly consistent with the theoretical predictions. This work not only offers an approach to synthesize high-quality perovskite NCs but also provides an in-depth understanding of the surface modification of CsPbX3 NCs for practical applications. KEYWORDS: Acid-free, CsPbBr3, ligands, stability, near-unity photoluminescence quantum yield

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desirable to develop a method that can make CsPbX3 NCs with excellent photophysical properties, high stability, and clean surfaces in a simple and cost-effective way. Compared to the X-type ligands, L-type ligands, for example, primary amines, are neutral two-electron donors with a lone electron pair that coordinates with surface cations.14,20,21 Considering the crystal structure of CsPbX3 NCs, L-type ligands might be a good candidate to provide strong binding with surface lead atoms, leading to enhanced stability and excellent photophysical properties. Herein, by using an R-NH2 as the L-type ligand, we demonstrate that the L-type ligand brings a trap-free material with fully delocalized valence band maximum (VBM) and conduction band minimum (CBM) states using theoretical calculations. As a result, the L-type ligand may improve the photoluminescence (PL) and stability. Furthermore, for the first time, we report an acid-free roomtemperature reprecipitation method for the synthesis of highquality CsPbBr3 NCs, in which an L-type ligand (OAm, octylamine or undecylamine (UAm)) is used as the sole capping ligand. It can be found that the strong binding

ver the past several years, all-inorganic perovskite cesium lead halide (CsPbX3, X = Cl, Br, or I) nanocrystals (NCs) have attracted much attention due to their excellent photophysical properties and promising applications in optoelectronics and photovoltaics.1−4 Despite recent progress in making high-quality CsPbX3 NCs, how to achieve practical applications of such NCs is quite challenging. For example, although surface engineering has proven to be a feasible strategy to improve the photophysical properties and stabilities of CsPbX3 NCs,5−12 it is still difficult to prepare CsPbX3 NCs with near-unity photoluminescence quantum yield (PLQY) and high stability in a simple and effective manner.8,10,13 Capping ligands are indispensable for stabilizing colloidal NCs.14,15 The most widely used capping ligand for colloidal CsPbX3 NCs is the pair of oleic acid (OA) and oleylamine (OAm),12,16−18 an X-type ligand that contributes one-electron to the metal−ligand bond.19−21 However, the proton exchange between coordinated OAm and OA may induce the detachment of ligands, resulting in more trap sites and poor optoelectronic performance after cleaning or long-time storage.22−24 On the other hand, without sufficient cleaning, excessive capping ligands can act as an insulator layer and block the surface charge transfer, deteriorating their performance in practical applications.25−27 It is therefore highly © 2019 American Chemical Society

Received: April 22, 2019 Revised: May 18, 2019 Published: May 22, 2019 4151

DOI: 10.1021/acs.nanolett.9b01666 Nano Lett. 2019, 19, 4151−4157

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atoms, especially for the PbBrx-terminated surface with CH3NH2 passivating ligand (Figure 1c,d), while the holes mainly delocalized on the PbBrx layer.32 Therefore, our theoretical results demonstrated that the R-NH2 passivating ligand preferred to bind with the PbBrx-rich surface and might yield a trap-free bandgap with fully delocalized VBM and CBM states, which may stabilize the CsPbBr3 NCs and enhance the luminescence efficiency. To verify the hypothesis, a room-temperature reprecipitation method using OAm as the sole capping ligand is developed. In a typical synthesis, CsBr and PbBr2 were dissolved completely in ultradry OAm and dry dimethylformamide (DMF) first, followed by a rapid injection of the solution into dry toluene. The colorless solution became green immediately, as shown in Figure 2a. Ten s later, the product was separated from the solution through centrifugation (details shown in Experimental Section). From the TEM image and particle size distribution histogram (Figures 2b and S2), monodisperse CsPbBr3 nanocubes with an average edge length of 17.1 nm were prepared. The X-ray diffraction (XRD) pattern in Figure 2c indicated that the obtained products could be indexed as an orthorhombic CsPbBr3 phase (ICSD no. 01-072-7929).17,33,34 From the high-resolution TEM (HRTEM) image (inset in Figure 2b), a lattice spacing of 0.42 nm could be observed clearly, which was consistent with the (110) plane of orthorhombic CsPbBr3.17,35 The as-prepared OAm-CsPbBr3 NCs in solution showed a bright green fluorescence (Figure 2d) with a sharp absorption peak located at 502 nm and a PL emission peak at 514 nm with a full-width wavelength at halfmaximum (fwhm) of 20 nm. In addition, the OAm-CsPbBr3 NC film showed similar optical properties. It should be mentioned that both of the solution and film showed a PLQY of 100 ± 1.3%, suggesting few defects in the crystal. This result matched well with our theoretical prediction that the product is a trap-free material with fully delocalized VBM and CBM states. Figure 2e represents the time-resolved PL decay curve of OAm-CsPbBr3 NCs, suggesting an average lifetime of ca. 25.5 ns. As a reference, OAm and OA capped CsPbBr3 (OAm/ OA-CsPbBr3) NCs were also synthesized through the same method.36 As shown in Figure S3, the PLQY and PL lifetimes were 90% and 12.4 ns, respectively, which were consistent with previous reports.17,36 From Table S1, an about 10 ns (τ1) component could be attributed to excitonic recombination, which increased to 71% in OAm-CsPbBr3 NCs. In addition, the fast decay 2.43 ns (τ3) vanished upon the L-type ligand passivation, suggesting that the trap states resulting from surface defects were eliminated. The change in PL lifetime component was highly consistent with the enhancement in PLQY, indicating the decrease of surface defects and increase in radiative recombination rates.30,37−39 X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDS) measurements were carried out to study the surface properties of as-prepared NCs. To avoid the disturbance from free ligands or salts, the NCs were washed thoroughly. The full XPS spectrum was shown in Figure S4. As depicted in Figure 3, the binding energy at 738.7 and 724.6 eV can be assigned to Cs 3d 3/2 and Cs 3d 5/2, respectively. The Pb 4f core-level signal can be fitted to Pb 4f 5/2 and Pb 4f 7/2 at 143.4 and 138.5 eV, respectively.30,40 The single peak at 68.5 eV was observed for Br ions. Based on the XPS analysis, the atomic ratio of Cs:Pb:Br was calculated to be 1.00:1.72:3.72 (Figure 3e). The ratio of Cs:Pb:Br = 1.00:1.73:3.82 obtained from the EDS

between the nonprotonated L-type ligand and surface Pb cations can lead to the formation of a lead bromide (PbBrx)rich surface, which further confirmed the theoretical predictions. Because of the strong binding between L-type ligand and Pb as well as the resulted PbBrx-rich surface, the asobtained CsPbBr3 NCs showed a PLQY of 100 ± 1.3% and high resistance against multistep purification and polar solvent (water) treatment. In order to identify a potential surface passivation mechanism when the L-type ligand was used as the sole capping ligand, the density functional theory (DFT) calculations were performed. To simplify the calculations, we used CH3NH2 instead of a long-chain ligand to demonstrate the surface passivation.12 Two possible surfaces were considered here. One was a CsBr-terminated surface with Br vacancy (Figure S1a,b), which was considered as a surface trap in previous reports,12,28−30 and the other one was a PbBrxterminated surface (Figure S1c,d) ,which was reported to be generated after surface treatment.7,31 The nonprotonated CH3NH2 as a L-type ligand has the potential to bind the surface of uncoordinated lead atoms.20,21 As a result, two types of passivation structures were compared accordingly. For the CsBr-terminated surface passivated by the CH3NH2 ligand, the binding energy of CH3NH2 ligand with a lead atom on the Brvacancy site was calculated to be 0.72 eV with a N−Pb bond length of 2.70 Å. In contrast, for the PbBrx-terminated surface passivated by a CH3NH2 ligand, the binding energy was calculated to be 0.87 eV with a N−Pb bond length of 2.58 Å, which was much stronger than that on the CsBr-terminated surface. It indicated that the nonprotonated L-type ligand strongly bound with lead atoms, leading to the formation of a PbBrx-rich surface. The charge density distributions of VBM and CBM for these two types of passivation structures were calculated and plotted in Figure 1. The VBM and CBM charge density represented the holes and electrons distributions, respectively. The electrons were fully delocalized on the lead

Figure 1. Charge density plots of (a) VBM and (b) CBM for the CsBr-terminated surface with a CH3NH2 passivating ligand on Br vacancy site and (c) VBM and (d) CBM for PbBrx-terminated surface with a CH3NH2 passivating ligand bound with the surface Pb atom. Cs, Pb, and Br atoms are shown as green, gray, and orange, respectively. 4152

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Figure 2. (a) Schematic illustration of the acid-free room-temperature reprecipitation method. (b) TEM image of as-synthesized OAm-CsPbBr3 NCs. Inset in (b) shows the corresponding HRTEM image and the scale bar is 2 nm. (c) XRD pattern of OAm-CsPbBr3 NCs (red) and standard pattern of orthorhombic CsPbBr3 (ICSD no. 01-072-7929). (d) Absorption (black) and PL (red) spectra of OAm-CsPbBr3 NCs solution (solid) and film (dash). Inset in (d) shows the photograph of OAm-CsPbBr3 NCs film (left) and dispersed in toluene (right) under UV light (λ = 365 nm). (e) Time-resolved PL lifetime of OAm-CsPbBr3 NCs.

measurement (Figure S5) was in good agreement with the XPS and our computational calculation results, both of which revealed a PbBrx-rich surface of OAm-CsPbBr3 NCs. The PbBrx-rich surface here could sufficiently passivate the surface. As a result, the OAm-CsPbBr3 NCs exhibited a near-unity PLQY. In comparison, the atomic ratio of Cs:Pb:Br was about 1.00:1.10:4.11 obtained from a XPS spectrum of OAm/OACsPbBr3 NCs (Figure S6), indicating a Br-rich surface, which displayed a significantly weaker passivation effect and a relatively lower PLQY (90%). The N signal was also detected by the XPS characterization. For OAm-CsPbBr3 NCs, there are two distinct N split doublets, as highlighted in Figure 3d. One peak at 400.1 eV can be indexed as a −NH2 group from nonprotonated OAm, while another one at 402.1 eV can be fitted to protonated amine groups (ammonium, −NH3+).7 The protonated amine groups −NH3+ may be produced in the presence of moisture in air during the spin coating and transferring prior to measurement, because OAm cannot be protonated in the dry (nonprotonated) solvent. On the contrary, only one single peak at 402.0 eV could be detected for OAm/OA-CsPbBr3 NCs (Figure S7), elucidating the existence of protonated amines only, which came from the OAmH+ original proton exchange of OAm and OA (Figure 4a).41 1 H- and 13C-nuclear magnetic resonance (NMR) measurements have been carried out to clearly identify the different binding motifs on the surface of OAm- and OAm/OACsPbBr3 NCs (Figure 4). From the 1H NMR spectrum of OAm-CsPbBr3 NCs, α1-CH2 1H resonances of the nonprotonated OAm appeared at 2.64 ppm. The α1-CH2 13C resonances of OAm located at 42.24 ppm confirmed that the

Figure 3. XPS spectra of (a) Cs 3d, (b) Pb 4f, (c) Br 3d, and (d) N 1s regions for OAm-CsPbBr3 NCs. (e) Table displaying relevant element ratios from XPS and EDS analyses.

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trap sites.5,41,46 In contrast, the coordination between nonprotonated OAm with Pb atoms was stronger (from DFT results), leading to the formation of a PbBrx-rich surface. Combined with the surface atomic ratio (Cs:Pb:Br = 1.00:1.72:3.72 for OAm-CsPbBr3 NCs and Cs:Pb:Br = 1.00:1.10:4.11 for OAm/OA-CsPbBr3 NCs), the schematic illustrations of OAm- and OAm/OA-CsPbBr3 NCs were proposed, as shown in Figure 4d,e. To verify the impact of OAm, OAm-CsPbBr3 NCs with different sizes were obtained by adjusting the concentration of OAm. As shown in Figure S9, the size of OAm-CsPbBr3 NCs can be tuned with the variation of OAm concentration. As expected, OAm-CsPbBr3 NCs of different sizes with near 100% PLQY were obtained, confirming that sole OAm can provide effective surface passivation for CsPbBr3 NCs. To further prove the proposed mechanism, other L-type ligands, noctylamine (OctAm), and UAm, were used as the sole capping ligand to prepare CsPbBr3 NCs (details in the Experimental Section), respectively. As shown in Figures S10−13, highquality CsPbBr3 NCs with near-unity PLQY were successfully obtained. Not limited to the decrease of the surface trap and excellent optical property with near 100% PLQY, OAmCsPbBr3 NCs were expected to show a higher stability than OAm/OA-CsPbBr3 NCs due to the strong binding between OAm and surface Pb atoms as well as the resulting PbBrx-rich surface. Proper purification of NCs is necessary to remove excess ligands because they will deteriorate the device performance by acting as an insulating layer.23,25,47 However, insufficient ligands may cause severe aggregation. Since the L-type ligand (OAm) bound strongly on the surface of CsPbBr3 NCs, the asprepared NCs should have a higher stability against multistep purification. To confirm this hypothesis, OAm/OA-CsPbBr3 and OAm-CsPbBr3 NCs were purified by hexane through the same procedure (details in the Experiment Section). TEM images and photographs of the samples at different washing stages are shown in Figure 5. After the second washing, OAm/ OA-CsPbBr3 NCs displayed serious aggregation and very weak emission. The green emission was completely quenched after washing for 3 times (Figure 4a-d). In comparison, OAmCsPbBr3 NCs exhibited much a higher stability during purification. From the TEM images and size distribution histograms (Figures 5e,f and S14), these NCs could maintain their original shape and size after being washed for four times. The optical properties including PL intensity and PLQY of OAm-CsPbBr3 NCs displayed a negligible decrease after cleaning (Figures 5 and S15). The enhanced stability of OAm-CsPbBr3 NCs arose from the strong binding energy between OAm and Pb atoms (0.87 eV obtained from computational calculations) and resulting PbBrx-rich surface. The species and density of surface ligands were measured by Fourier transform infrared spectroscopy (FTIR). As expected, the intensity of ligands decreased dramatically after each purification step (Figure S16 and S17). After washed for four times, almost no peaks could be recognized for both samples in the FTIR spectra, suggesting a very clear surface. Thermogravimetric analysis (TGA) measurement was carried out to quantitatively determine the residual of ligands. As shown in Figure 5i,j, only 4.7 wt % residual was detected for OAmCsPbBr3 NCs after washed for 4 times. In comparison, the OA/OAm system has a 14.1 and 30 wt %48 residual ligand for the reprecipitation and hot-injection methods, respectively, indicating the powerful protection effect of OAm even at low

Figure 4. (a) Schematic illustration of protonation of OAm into OAmH+ in the presence of OA. Selected regions of (b) 1H- and (c) 13 C NMR spectra of OAm-CsPbBr3 NCs (red) and OAm/OACsPbBr3 (black). Schematic of (d) PbBrx-rich surface of OAmCsPbBr3 NC and (e) Br-rich surface of OAm/OA-CsPbBr3 NC.

nonprotonated OAm was the only capping ligand on OAmCsPbBr3 NCs.42 From NMR results, the surface OAm was nonprotonated, which had lone pair electrons that could coordinate with Pb cations.43 By contrast, the downfield shift (from 2.64 to 2.87 ppm) of α2-CH2 1H resonances and opposite shift (from 42.24 to 39.74 ppm) for α2-CH2 13C resonances in OAm/OA-CsPbBr3 NCs could be detected due to the generation of −NH3+ in OAmH+, which then coordinated with the surface Br atoms.10,41 Nuclear Overhauser effect NMR spectroscopy (NOESY) was employed to confirm the binding ligands, as shown in Figure S8a. Only zero-quantum coherence artifacts, which were typically recognized from the antiphase (positive and negative) contribution, were shown in cross-peaks. Different from pure OAm, intense and negative NOE cross peaks of OAm-CsPbBr3 NCs appeared in Figure S8b, indicating the binding of OAm on the surface of CsPbBr3 NCs.19,44,45 From the above analysis, we could conclude that the main difference between these two examples was determined by the form of OAm (protonated for OAm/OA- and nonprotonated for OAm-CsPbBr3 NCs). Another distinct difference was the atoms they coordinated with. In the OAm/OA system, the proton exchange between amine and acid (OAm + OA ⇌ OAmH+ + OA−) led to the formation of oleylammonium (OAmH+) ions that connected to Br anions, while carboxylate ions (OA−) offered one electron and coordinated with Pb cations.19,41 When the proton-exchange reaction further continued (OAmH+ + OA− ⇌ −COOH3NCH2−), ammonium carboxylate (−COOH3NCH2−) was obtained.19,42 With the loss of the lone pair of electrons, the ligands would detach from the surface, resulting in poorer stability and more surface 4154

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20 min and completely disappeared in 30 min. From the PL spectra and normalized intensity curves (Figures S20a and S21), the PL intensity decreased rapidly and dropped to zero at 60 min. In a striking contrast, the OAm-CsPbBr3 NCs show a much higher stability against water treatment. As shown in Figure 6b, the bright green emission was maintained during the whole process. As shown in Figure S21, the PL intensity of asprepared OAm-CsPbBr3 NCs increased slightly in the first 15 min, caused by UV light illumination, and then dropped gradually. After treated for 60 min, 80% of the initial PL intensity was reserved, further confirming the significantly enhanced stability of OAm-CsPbBr3 NCs. This acid-free method was a general and versatile method for the synthesis of CsPbX 3 NCs with different halide components. Various OAm-CsPbX3 (X = Br, Br/Cl, Br/I) NCs were obtained (details in the Experimental Section) by controlling the proportion of halogens in the precursor solution, as shown in Figures S22 and S23. The emission can be readily tuned from blue to red with the PL peak in the range of 449−644 nm. The as-prepared CsPbX3 NCs exhibited the same orthorhombic phase with CsPbBr3 (Figure S22c). The corresponding PLQY ranged from 20% to 100% (Table S2), which was still higher than that of OAm/OA-CsPbX3 with the same components shown in Table S3.17,36 It should be pointed out that CsPbCl3 NCs cannot be obtained because of the very low solubility of CsCl and PbCl2 in the solvent. For OAm-CsPbI3 NCs, the obtained product was an orthorhombic phase (“yellow” phase) at ambient temperature, displaying poor PL emission. In conclusion, through DFT calculations, we predict that the L-type ligand has a promising potential in reducing surface defects and improving the stability of CsPbBr3 NCs. Experimentally, a facile acid-free room-temperature reprecipitation method was employed to prepare high-quality CsPbBr3 NCs with near-unity PLQY and high stability by using a nonprotonated L-type ligand (OAm, octylamine, or UAm) as the sole ligand. Different from traditional X-type ligand (the pair of OAm and OA) capped CsPbBr3 NCs, the assynthesized OAm-CsPbBr3 NCs exhibited strong coordination between the OAm and Pb atoms, resulting in a PbBrx-rich surface, which further confirmed the DFT prediction. As a result, the CsPbBr3 NCs exhibited fewer nonradiative trap sites, enhanced PLQY, clean surface, and higher stability against multiple purifications and water treatments. This work is important because it not only developed a simple method to prepare high-quality CsPbX3 NCs but also provided better a understanding of the significant role of ligands in the metal halide perovskite field. Experimental Section. Materials. Cesium bromide (CsBr, 99.999%), lead bromide (PbBr2, 99.999%), oleic acid (OA, 90%), oleylamine (OAm, 80−90%), n-octylamine (OctAm, 99%), and undecylamine (UAm, 99%) were purchased from Alfa Aesar. Dimethylformamide (DMF, 99%) and hexane (95%) were purchased from Sigma−Aldrich. Toluene (99.95%) was purchased from Fisher. OAm, DMF, and toluene were dried prior to use. Synthesis of OAm-CsPbBr3 NCs. PbBr2 (0.4 mmol, 0.1468 g), CsBr (0.4 mmol, 0.0851 g), and 2.4 mL of dry OAm were added to 10 mL of dry DMF. The mixture was stirred at room temperature until a complete dissolution. 0.5 mL precursor solution was swiftly injected into 10 mL of dry toluene (or mixture containing 5 mL of hexane and 5 mL of toluene). The

Figure 5. TEM images of (a−d) OAm/OA-CsPbBr3 NCs and (e−h) OAm-CsPbBr3 NCs after purification for (a, e) one, (b, f) two, (c, g) three, and (d, h) four times. The inset images are the corresponding photographs under UV light (λ = 365 nm). (i) TGA and (j) corresponding magnified curves of OAm- and OAm/OA-CsPbBr3 NCs.

density. Due to the clean surface of OAm-CsPbBr3 NCs, the OAm-CsPbBr3 NCs film is expected to have a good carrier transport property. A uniform and compact film of OAm/OAand OAm-CsPbBr3 NCs could be obtained with a thickness of 40 and 36 nm, respectively (SEM images shown in Figure S18). The OAm-CsPbBr3 NCs film showed a much higher conductivity than that of OAm/OA-CsPbBr3 NCs film (Figure S19), which can be ascribed to the high density of long-chain ligands on the surface of OAm/OA-CsPbBr3 NCs. The resistance of CsPbBr3 NCs against water treatment was tested, in which 2 mL of hexane solution was put on the top of 1 mL of water in a cuvette, as shown in Figure 6. For OAm/ OA-CsPbBr3 NCs, the green emission became very weak after

Figure 6. Photographs showing the resistance of different samples against water treatment: (a) OAm/OA-CsPbBr3 NCs and (b) OAmCsPbBr3 NCs. Upper layer, hexane; bottom layer, water. 4155

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containing 7 atomic layers in a 2 × 2 supercell. A vacuum layer larger than 15 Å was used to avoid interaction between periodic images.

colorless solution turned green immediately. After 10 s, the reaction was terminated by centrifugation. Purification Process. The crude colloidal solution was first centrifuged at 11,000 rpm for 15 min at 10 °C. Then the precipitate was collected and dispersed in hexane. This process was repeated several times during the following purification process. Synthesis of OctAm-CsPbBr3 and UAm-CsPbBr3 NCs. This procedure is similar to the synthesis of OAm-CsPbBr3 NC, but OAm was replaced by octylamine and UAm, respectively, in the precursor solution. Synthesis of OAm-CsPbX3 NCs. In a mixture of 2.4 mL of OAm and 10 mL of DMF, 0.4 mmol PbX2 and 0.4 mmol CsX (details shown in Table S4) were dissolved to prepare the precursor solution. Then, 0.5 mL precursor solution was quickly added into the dry toluene. After 10 s, the crude colloidal solution was collected by centrifugation. Synthesis of OAm/OA-CsPbX3 NCs. This procedure is similar to the synthesis of OAm-CsPbX3 NC but in the presence of OA (1.0 mL) in the precursor solution. Fabrication of Film. CsPbBr3 NCs in hexane were spincoated on a quartz plate (for PLQY test) or ITO glass (conductivity test) at 3000 rpm for 60 s. Characterization Methods. UV−vis absorption spectra were recorded in a range of 300−800 nm by using an Evolution 220 spectrophotometer in transmission mode. The PL spectra were obtained by a FLUOROMAX-4 spectrofluorometer equipped with a xenon lamp. PLQY was determined by using an integration sphere incorporated into a FLUOROMAX-4 spectrofluorometer. The PL lifetime measurements were taken by using a HORTB-FM-2015 spectrofluorimeter. Powder XRD was performed using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.54056 Å). TEM images were collected by a TECNAI G2 F20 transmission electron microscope with an accelerating voltage of 200 kV and a Gatan SC200 CCD camera. FTIR spectra of the samples were recorded with a Bruker Vertex 70 spectrometer in a range from 4000 to 400 cm−1. XPS spectra of the as-prepared samples were measured by Shimadzu ULTRADLD using Mg Ka radiation from an X-ray source operated at 8.0 kV and 30 mA. The photographs were taken using a Canon 80D SLR camera. TGA measurement was carried out on a Mettler Toledo Switzerland thermogravimetric analyzer. 1H- and 13C NMR spectra were recorded on a Bruker Advance III HD-400 MHz spectrometer equipped with a BFO smart probe. For the conductivity test, CsPbBr3 NCs film on ITO glass was baked at 40 °C for 2 min and then transferred into a vacuum thermal evaporator with a pressure below ∼1.0 × 10−6 mbar chamber to deposit Ag cathodes. The conductivity of obtained films was collected by a Keithley 2400 sourcemeter. The thickness of films was performed using an alpha-SE ellipsometer (J. A. Woollam Co.) in the spectral range of 380−800 nm. Computational Method. All calculations were performed in the Vienna ab Initio Simulation Package (VASP)49−51 using the projector augmented wave (PAW) method to account for core−valence interactions.52 The Perdew−Burke−Ernzerhof functional53 of DFT including the D3 van der Waals correction was used to account for London dispersion interactions.54 The kinetic energy cutoff for plane wave expansions was set to 400 eV, and the geometric structures were converged to within 3 × 10−2 eV/Å for maximal components of forces. The surface slab model was constructed on the basis of orthorhombic CsPbBr3

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.9b01666. Simulation details, optical properties, particle size distribution histograms, TEM images, EDS spectrum, XPS spectra, NOESY NMR spectra, time-dependent PL spectra, FTIR spectra, SEM images, J−V curves, lifetime components of different samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Youyong Li: 0000-0002-5248-2756 Xiaohong Zhang: 0000-0002-6732-2499 Qiao Zhang: 0000-0001-9682-3295 Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21673150, 21703146) and the Natural Science Foundation of Jiangsu Province (BK20180097). We acknowledge the financial support from the 111 Project, Collaborative Innovation Center of Suzhou Nano Science and Technology (NANO−CIC), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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