Shape-Controlled Synthesis of All-Inorganic CsPbBr3 Perovskite

Oct 7, 2016 - We developed a colloidal synthesis of CsPbBr3 perovskite nanocrystals (NCs) at a relative low temperature (90 °C) for the bright blue e...
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Shape-Controlled Synthesis of All-Inorganic CsPbBr3 Perovskite Nanocrystals with Bright Blue Emission Zhiqin Liang,†,‡ Suling Zhao,*,†,‡ Zheng Xu,†,‡ Bo Qiao,†,‡ Pengjie Song,†,‡ Di Gao,†,‡ and Xurong Xu†,‡ †

Key Laboratory of Luminescence and Optical Information, Beijing Jiaotong University, Ministry of Education, Beijing, 100044, China ‡ Institute of Optoelectronics Technology, Beijing Jiaotong University, Beijing, 100044, China S Supporting Information *

ABSTRACT: We developed a colloidal synthesis of CsPbBr3 perovskite nanocrystals (NCs) at a relative low temperature (90 °C) for the bright blue emission which differs from the original green emission (∼510 nm) of CsPbBr3 nanocubes as reported previously. Shapes of the obtained CsPbBr3 NCs can be systematically engineered into single and lamellar-structured 0D quantum dots, as well as face-to-face stacking 2D nanoplatelets and flat-lying 2D nanosheets via tuning the amounts of oleic acid (OA) and oleylamine (OM). They exhibit sharp excitonic PL emissions at 453, 472, 449, and 452 nm, respectively. The large blue shift relative to the emission of CsPbBr3 bulk crystal can be ascribed to the strong quantum confinement effects of these various nanoshapes. PL decay lifetimes are measured, ranging from several to tens of nanoseconds, which infers the higher ratio of exciton radiative recombination to the nonradiative trappers in the obtained CsPbBr3 NCs. These shape-controlled CsPbBr3 perovskite NCs with the bright blue emission will be widely used in optoelectronic applications, especially in blue LEDs which still lag behind compared to the better developed red and green LEDs. KEYWORDS: CsPbBr3 perovskite, blue emission, shape control, quantum dots, lamellar structures, nanoplatelets, nanosheets

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INTRODUCTION The past few years have witnessed the outstanding photovoltaic properties of hybrid organic−inorganic lead halide perovskite such as CH3NH3PbI3,1−3 and there has been rapid progress in boosting their power conversion efficiency from 3.8%4 to 22.1%.5 With the general formula of ABX3 (A = organic ammonium cation or inorganic metal cation, B = Pb2+ or Sn2+, and X = halide anion), these perovskite materials have also attracted great interest in other optoelectronic devices such as photodetectors,6−9 light-emitting diodes,10−13 and lasers.14−16 Meanwhile, colloidal nanocrystals (NCs) of lead halide perovskites represent the latest entries with simple synthesis approaches having appeared in the last two years, and most efforts are focused on the CH3NH3PbX3 (X = Cl, Br, I) NCs.17−23 In parallel, the research landscape has also extended to all-inorganic cesium lead halide perovskites NCs, which was triggered by the first synthesis of CsPbX3 nanocubes in early 2015.24 These CsPbX3 nanocubes present a high quantum yield up to 90%, narrow emission bandwidths, tunable photoluminescence (PL) emission spectra, and higher stability compared with CH3NH3PbX3, which makes them promising for optoelectronic applications.25−28 In the past year, more works focus on the synthesis of CsPbX3 perovskite nanocrystals and the tuning of their optical band gap by controlling the particle size or introducing different halide anions. As is well-known, except the particle size and the composition, the shape of colloidal NCs not only controls their © 2016 American Chemical Society

physicochemical properties but also determines their optical and electronic properties for a wide variety of applications. For example, in the best developed metal chalcogenide NCs, the evolution from binary II−VI NCs to ternary I−III−VI2 and quaternary I2−II−IV−VI4 NCs has paralleled a trend seen in diverse shapes,29−32 thereby producing various applications.33−35 In contrast, the study of inorganic perovskite NCs in terms of shape, composition, and PL properties lags far behind, with only a few papers focusing on it. Very recently, two works reported that the CsPbBr3 perovskite NCs could be engineered into well-defined morphologies with green emission,36,37 while another two works are initially engaged in the synthesis of quantum confined CsPbBr3 nanoplatelets with distinctive blue emission.38−40 NCs with blue emission have attracted more attention to enhance their efficiency, which is still a challenge for the LED applications. For CsPbBr3 perovskites nanocrystals, although some significant results have been achieved as mentioned above, either they are all original green emission regardless of the various shapes or they are only nanoplatelets with blue emission via a quantum confinement effect in the thickness. The systematic researches in the shape control of CsPbBr3 perovskite NCs with bright blue emission are still under exploration. Received: July 12, 2016 Accepted: October 7, 2016 Published: October 7, 2016 28824

DOI: 10.1021/acsami.6b08528 ACS Appl. Mater. Interfaces 2016, 8, 28824−28830

Research Article

ACS Applied Materials & Interfaces

quantum dots, 0.5 mL for CsPbBr3 nanoplatelets stacked face-to-face, and 0.2 mL for large 2D CsPbBr3 nanosheets) and OA (0.6 mL for CsPbBr3 single quantum dots, 0.3 mL for CsPbBr3 lamellar-structured quantum dots, 0.5 mL for CsPbBr3 nanoplatelets stacked face-to-face, and 0.8 mL for large 2D CsPbBr3 nanosheets) were injected into the reaction mixture at 120 °C under Ar2 (see Table 1). After solubilization of a lead halide salt, the temperature was decreased to 90 °C and the above-mentioned Cs-oleate solution (0.4 mL) was injected quickly. Ten seconds later, the reaction mixture was cooled by the ice−water bath. Characterization. Transmission Electron Microscopy (TEM). Low-resolution TEM images were carried out using a JEOL JEM1400 microscope equipped with a thermionic gun operated at an acceleration voltage of 100 kV. High-resolution TEM images and energy-dispersive X-ray (EDX) spectroscopy were taken on a JEOL JEM-2100F instrument operating at 200 kV. Samples were prepared by diluting the crude NCs solution in toluene (20 μL in 1 mL), and then dropping diluted NCs colloidal suspensions onto a copper grid coated with carbon film, followed by evaporation of the solvent. X-ray Powder Diffraction (XRD). XRD measurements were performed on a Bruker D8 Advance X-ray powder diffractometer equipped with Cu−Kα radiation (λ = 1.540 Å). Samples were prepared as follows: 2 mL of crude NCs solution was separated from the organic solvents by centrifugation at 8000 rpm for 10 min. The NCs were redispersed in 50 μL of toluene and then dropped on a 2 cm by 1 cm glass slide, followed by evaporation of the solvent. Optical Absorption Spectroscopy (Abs). UV−vis spectra were recorded on a Shimadzu UV-3101 PC spectrophotometer at room temperature. Samples were prepared by diluting 20 μL of the crude NCs solution in 1 mL of toluene, in 1 cm path length quartz cuvettes with airtight screw caps. Photoluminescence Measurements (PL). The steady-state PL emission spectra were recorded on a Hitachi F4500 fluorescence spectrophotometer with a Xe lamp coupled to a monochromator. Samples were the same as those using for absorption spectra. They were excited at 365 nm at room temperature. The absolute photoluminescence quantum yield (PLQY) was carried out on an FLS 980 spectrometer using a integrating sphere. The PL decay process was measured on a Horiba Fluorolog phosphorescence lifetime system equipped with a 373 nm, 45 ps pulse laser and a timecorrected single-photon counting (TCSPC) system at room temperature.

Here, we investigated shape-controlled CsPbBr3 perovskite NCs with bright blue emission at a relative low temperature (90 °C). With the assistance of different surfactants, the CsPbBr3 NCs were synthesized with various shapes, including zerodimensional (0D) quantum dots with single and lamellarstructured morphologies, two-dimensional (2D) nanoplatelets by face-to-face stacking, and 2D flat-lying nanosheets (Scheme 1). Because of the preferential absorption with particular crystal Scheme 1. Schematic Diagram of the CsPbBr3 Nanocrystals with Various Shapes by Using Different Amounts of Oleic Acid (OA) and Oleylamine (OM): (a) OA:OM = 0.6:0.3, (b) OA:OM = 0.3:0.7, (c) OA:OM = 0.5:0.5, (d) OA:OM = 0.8:0.2

facets for different surfactants, the growth rates of different crystal facets of CsPbBr3 NCs were influenced remarkably, which account for the various shapes of the obtained products. All of these diversely shaped CsPbBr3 perovskite NCs produce bright blue emission, which can be attributed to the strong quantum confinement effects. Such superior shape-controlled and PL-tunable merits make them favorable for optoelectronic devices, especially for blue LEDs, which still lag behind compared to the well-developed red and green LEDs.

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EXPERIMENTAL SECTION

RESULTS AND DISCUSSION The shape-controlled synthesis of CsPbBr3 perovskite NCs was performed by reacting Cs-oleate with PbBr2 in dry octadecene (ODE) containing long-chain capping ligands (oleic acid and oleylamine). Cs-oleate was injected into PbBr2 solution at a lower temperature of 90 °C relative to that described previously. The detailed synthesis is found in the Experiment Section. The sizes and shapes of CsPbBr3 perovskite NCs could be easily tailored by adjusting the ratio of OA to OM. We first prepared CsPbBr3 quantum dots by directly adding 0.6 mL of OA and 0.3 mL of OM. As Figure 1a,b indicates, this synthesis yields fairly monodisperse, spherical quantum dots with an average diameter of 2.4 nm (see the SI, Figure S1). The crystal structure was investigated by high-resolution transmission electron microscopy (HRTEM). As shown in Figure 1c, the HRTEM image reveals that a typical CsPbBr3 quantum dot is single crystalline. Clear lattice fringe with an interplanar

Materials. Cesium carbonate (Cs2CO3, 99.9%), lead(II) chloride (PbCl2, 99.999%), lead(II) bromide (PbBr2, 99.999%), lead(II) iodide (PbI2, 99.999%), oleic acid (OA, 90%), oleylamine (OM, 70%), and octadecene (ODE, 90%) were purchased from Sigma-Aldrich. Toluene (95%, anhydrous grade) was obtained from Beijing Chemical Reagent, China. All chemicals were used without further purification. Preparation of Cs-oleate. Cesium-oleate precursor solution was prepared following the previously reported approach.24 In brief, 0.4 g of Cs2CO3 and 1.2 mL of OA were loaded into a 3-neck flask along with 15 mL of ODE. The mixture was dried for 1 h at 120 °C, and then heated to 150 °C for another 30 min under Ar2 until Cs2CO3 was completely dissolved. Since Cs-oleate precipitates out of ODE at room temperature, it has to be preheated over 100 °C to make it soluble before usage. Synthesis of CsPbBr3 NCs of Diverse Shapes with Blue Emission. ODE (5 mL) and PbBr2 (0.069 g) were loaded into a 25 mL 3-neck flask and dried for 1 h at 120 °C. Dried OM (0.3 mL for CsPbBr3 single quantum dots, 0.7 mL for CsPbBr3 lamellar-structured

Table 1. Different Amounts of Surfactants OA and OM for Various Shapes of CsPbBr3 Nanocrystals

OM OA

CsPbBr3 quantum dots (single)

CsPbBr3 quantum dots (lamellar-structured)

CsPbBr3 nanoplatelets (stacked face-to-face)

CsPbBr3 nanosheets (2D sheets)

0.3 mL 0.6 mL

0.7 mL 0.3 mL

0.5 mL 0.5 mL

0.2 mL 0.8 mL

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DOI: 10.1021/acsami.6b08528 ACS Appl. Mater. Interfaces 2016, 8, 28824−28830

Research Article

ACS Applied Materials & Interfaces

Figure 1. Characterization of the CsPbBr3 quantum dots. (a and b) TEM images. (c) HRTEM image. The white circle represents the boundary of the atomic lattice of the typical quantum dot. (d) XRD pattern. Dashed rectangle indicates the split of the diffraction peaks at ∼30°. (e) Optical absorption (black line) and PL emission (blue line) spectra. (f) Two-dimensional excitation and photoluminescence map. (g) PL decay and fitted curves of the PL emission at 453 nm.

decay lifetime was also monitored. As shown in Figure 1g, the PL decay curve can be well-fitted with a triexponential function

spacing of 3.36 Å can be identified, which coincides with the (1̅11) planes of orthorhombic CsPbBr3. EDX spectra further confirm that the quantum dots are composed of Cs, Pb, and Br elements (see the SI, Figure S2). To further analyze the phase structure, X-ray diffraction (XRD) was characterized. Figure 1d shows the XRD pattern of the obtained CsPbBr3 quantum dots. It is worth to note that the diffraction peaks between cubic and orthorhombic CsPbBr3 are very similar except the fine distinction at ∼30°, as stated in the previous reports.27,41 For the spherical CsPbBr3 quantum dots, the peak broadening caused by the small size makes it difficult to determine the exact phase. Nevertheless, the CsPbBr3 bulk crystal is known to exhibit a cubic perovsktie structure in the highest temperature phase.42 Upon lowering the temperature to 90 °C, CsPbBr3 undergoes the phase transition from cubic to tetragonal, and then to orthorhombic phase. Thus, we infer that the obtained spherical CsPbBr3 quantum dots exhibit an orthorhombic phase. This result is consistent with the HRTEM data analysis. Figure 1e presents the UV−vis absorption and PL spectra of the CsPbBr3 quantum dots. The absorption spectra has an obvious exciton absorption peak at 435.5 nm (2.85 eV). Correspondingly, a sharp PL emission was detected at the peak of 453 nm (2.74 eV) with a narrow full width at the halfmaximum (fwhm) of 22 nm. The small Stokes shift (0.11 eV) suggests the PL emission of CsPbBr3 quantum dots originates from the bound exciton recombination. This phenomenon can be attributed to the strong quantum confinement effects because the average size of the obtained quantum dots is smaller than the exciton Bohr radius (3.5 nm) of CsPbBr3 bulk crystal. The PLQY was measured to be 50.41% (see the SI, Table S2). Figure 1f displays the two-dimensional photoluminescence excitation map, depicting PL intensity measured as a function of excitation wavelength for CsPbBr3 quantum dots. In this image, PL intensity is represented by the color in the specific excitation and emission wavelength. This result shows that the emission range persists with the change of excitation wavelength, reflecting that the obtained monodisperse nanocrystals are uniform CsPbBr3 quantum dots. PL

⎛ t ⎞ ⎛ t ⎞ ⎛ t⎞ A(t ) = A1 exp⎜ − ⎟ + A 2 exp⎜ − ⎟ + A3 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠ ⎝ τ3 ⎠

where A, A1, A2, and A3 are constants, t is time, and τ1, τ2, and τ3 represent the decay lifetimes corresponding to the intrinsic exciton relaxation, the interaction between excitons and phonons, and the interaction between excitons and defects, respectively. The average lifetime (τave) can be calculated as τave =

A1τ12 + A 2 τ2 2 + A3τ32 A1τ1 + A 2 τ2 + A3τ3

The obtained CsPbBr3 quantum dots exhibit three lifetimes (τ): τ1 of 2.18 ns accounting for 32.90%, τ2 of 5.03 ns accounting for 64.21%, and τ3 of 19.16 ns accounting for 2.89%, respectively, which reveals the high ratio of radiative recombination to nonradiative transitions. The average lifetime of 4.45 ns was derived (see the SI, Table S1), comparable to the CsPbBr3 nanocubes of 1−29 ns reacting at the temperature of 150 °C.24 By adjusting the amounts of OA and OM, while keeping other conditions the same as that of deep-blue emitting CsPbBr3 quantum dots, this synthesis selectively yields skyblue-emitting CsPbBr3 lamellar-structured quantum dots as shown in Figure 2. When adding 0.3 mL of OA and 0.7 mL of OM, we obtained CsPbBr3 lamellar structures with at least 400 nm in length as presented in Figure 2a. These CsPbBr3 lamellar structures have a strong self-assembling tendency, and at first glance, one-dimensional (1D) nanowires were formed. However, close-up of these lamellar structures suggested that the building blocks of the 1D nanowires were in fact the organic mesostructures with CsPbBr3 quantum dots loaded on them (see the SI, Figure S3). These organic mesostructures serve as a soft template that makes CsPbBr3 quantum dots grow and align along them, finally forming the CsPbBr3 lamellar structures. The average diameter of these CsPbBr3 28826

DOI: 10.1021/acsami.6b08528 ACS Appl. Mater. Interfaces 2016, 8, 28824−28830

Research Article

ACS Applied Materials & Interfaces

Figure 3. Characterization of the CsPbBr3 nanoplatelets by face-to face stacking. (a) TEM image. Red and blue rectangles represent the nanoplatelets standing edge-on perpendicular to the substrate and tilted with the substrate, respectively. (b and c) Illustrations of the perpendicular and tilted nanoplatelets. (d) XRD pattern. (e) Optical absorption (black line) and PL emission (blue line) spectra. (f) PL decay and fitted curves of the PL emission at 449 nm.

Figure 2. Characterization of the lamellar-structured CsPbBr3. (a) TEM image. (b) XRD pattern. (c) Optical absorption (black line) and PL emission (blue line) spectra. (d) PL decay and fitted curves of the PL emission at 472 nm.

quantum dots was calculated to 3.6 nm (see the SI, Figure S4). EDX spectra suggest that Cs, Pb, and Br three elements are included in the lamellar structures (see the SI, Figure S5). The XRD pattern in Figure 2b indicates the CsPbBr3 lamellar structures exhibit an obvious orthorhombic phase, discriminated by the split of the diffraction peak at ∼30° marked by a dashed rectangle. These prepared CsPbBr3 lamellar structures exhibit a band edge absorption at 448 nm (2.77 eV) and a narrow band edge emission peak at 472 nm (2.63 eV) as shown in Figure 2c. The blue shift relative to the emission of CsPbBr3 bulk crystal is ascribed to the quantum confinement effects because the size of the lamellar-structured CsPbBr3 quantum dots is comparable to the exciton Bohr radius (3.5 nm) of CsPbBr3 bulk crystal. Although the PL emission is predominantly composed of the band edge emission, a weak exciton absorption at 437 nm (2.84 eV) and its corresponding exciton emission at 446 nm (2.77 eV) were also observed. It might be caused by the smaller nanocrystals in the distribution, having different quantum confinement effects from the dominant CsPbBr3 lamellar structures. PL decay lifetime was measured as shown in Figure 2d. The decay curve can be well-fitted by a triexponential function, and the obtained average lifetime is 8.57 ns. The small proportion of long-lived lifetime (see the SI, Table S1) may be assigned to the nonradiative decay channels, implying that the obtained products have less lattice defects. While changing the amounts of OA and OM to 0.5 mL for each, we obtained nanoplatelets stacking face-to-face, as shown in Figure 3a. The areas marked by red and blue rectangles represent the nanoplatelets standing edge-on perpendicular to the substrate and tilted with the substrate, respectively, as illustrated in Figure 3b,c. The lateral dimension and thickness of the nanoplatelets are about 16 and 2.3 nm, respectively (see the SI, Figure S6). EDX spectra reveal that the nanoplatelets consist of Cs, Pb, and Br three elements (see the SI, Figure S7). The XRD pattern of the nanoplatelets in Figure 3d displays an obvious orthorhombic phase discriminated by the split of the diffraction peak at ∼30° marked by a dashed rectangle. As presented in Figure 3e, CsPbBr3 nanoplatelets reveal the

exciton absorption peak of 438 nm (2.83 eV) and PL emission peak of 449 nm (2.76 eV). This deep-blue emission originates from strong quantum confinement effects of the CsPbBr3 nanoplatelets because the thickness of the obtained nanoplatelets (2.5 nm) is much smaller than the exciton Bohr radius (3.5 nm) of CsPbBr3 bulk crystal. It is worth to note that there still exists another very weak absorption and PL emission in the CsPbBr3 nanoplatelets, which may be influenced by the relative weaker quantum confinement effects of overlapped adjacent nanoplatelets to that of the single nanoplatelet. The PL decay curve of the nanoplatelets can be triexponent fitted well as displayed in Figure 3f. The average lifetime is determined to be 4.33 ns (see the SI, Table S1). The shape of CsPbBr3 NCs can be further engineered to large square nanosheets lying flat on the substrate when adding 0.8 mL of OA and 0.2 mL of OM. Figure 4a shows the TEM image of CsPbBr3 nanosheets with an average lateral dimension of 75 nm. The HRTEM image in Figure 4b indicates the interplanar distance of the flat-lying nanosheets is 5.8 Å, consistent with the (001) crystal plane of orthorhombic CsPbBr3 bulk crystal. This suggests the growth direction bound for the flat facet. Figure 4d shows the clear lattice fringes through enlarging the area marked by the red circle in Figure 4b. As presented in Figure 4e, the fast Fourier transformation (FFT) pattern has clear spots corresponding to the lattice spacing, indicating good crystallinity of the obtained CsPbBr3 nanosheets. The EDX spectrum in Figure 4f confirms that Cs, Pb, and Br elements are included in the nanosheets, in which the atom ratio of the three elements was calculated to be around 1:1:3 (see the SI, Figure S8). Figure 4c shows XRD pattern of the CsPbBr3 nanosheets; all diffraction peaks are in good agreement with the standard XRD pattern of orthorhombic CsPbBr3 (PDF# 18-0364). We measured UV−vis absorption and PL emission of these perovskite nanosheets. As shown in Figure 4g, the absorption spectrum is dominated by a strong sharp excitonic absorption 28827

DOI: 10.1021/acsami.6b08528 ACS Appl. Mater. Interfaces 2016, 8, 28824−28830

Research Article

ACS Applied Materials & Interfaces

CsPbBr3 nanosheets relative to the exciton Bohr radius or large nanocrystals with bulklike optical properties. To clarify, it is challenging to separate these nanosheets with different thicknesses due to the similar size in thickness for 3, 4, and 5 monolayer CsPbBr3. What’s more, only a few scattered nanosheets are corresponding to the very weak PL emissions except that of 452 nm as shown in Figure 4g, which raises difficulty in separation. The 452 nm PL decay lifetime curve was monitored as shown in Figure 4h. The obtained average lifetime is calculated to 4.63 ns (see the SI, Table S1). The formation mechanism of various shapes of CsPbBr3 NCs, such as 0D single and lamellar-structured quantum dots, as well as 2D nanoplatelets by face-to-face stacking and flatlying nanosheets, can be explained by preferential absorption with particular crystal facets for different surfactants. Therefore, growth rates of different crystal facets are influenced remarkably, which results in various shapes. In the typical synthesis of CsPbBr3 NCs, OA and OM were selected as organic ligands. While adding 0.6 mL of OA and 0.3 mL of OM, OA coordinates with Pb2+ and OM serves as a capping ligand for Pb2+. Both of them play a cooperative role, which favors the isotropic sphere-like micelles growth. When the amounts of OM are increased, lamellar structures can be found. We hypothesize that the long-chain alkane and lead bromide form alternating layered structures. Thus, they serve as a soft template and then form lamellar structures. That is, close-up of these lamellar structures were actually the organic mesostructures with CsPbBr3 quantum dots loaded on them. These organic mesostructures serve as a soft template that makes CsPbBr3 quantum dots grow and align along them, finally forming the CsPbBr3 lamellar structures. This mechanism has ever been reported for the formation of CsPbBr3 nanoplatelets40 and wurtzite CdSe nanosheets.43 When increasing the amounts of OA, excess OA has a strong binding ability with (001) crystal facets of CsPbBr3 NCs. Therefore, growth rates of (001) crystal facets are slower than that of (100) and (010) crystal facets, which forms small and single sheets. It was worth to note that the final 2D nanosheets might be formed by the oriented attachment of these single sheets. As shown in Figure 4a, the nanosheets present missing corners and irregular boundaries (also see the SI, Figure S10). We infer that they are grown from small single sheets by the oriented attachment mechanism. The similar phenomenon has been observed in previous research work.44

Figure 4. Characterization of the 2D CsPbBr3 nanosheets. (a) TEM image. (b) HRTEM image. Two white lines represent the boundary of the atomic lattice of the nanosheets. (c) XRD pattern. (d) Magnified HRTEM image through enlarging the area marked by red circle in (b). (e) FFT and (f) EDX spectra corresponding to (d). (g) Optical absorption (black line) and PL emission (blue line) spectra. The inset reveals discrete PL emission peaks corresponding to the nanosheets with different number of unit cell layers (n). (h) PL decay and fitted curves of the PL emission at 452 nm.

peak at 438.5 nm (2.83 eV). Besides, two additional small, but discernible, shoulders at 478 and 514 nm were observed. The absorption amplitude of both shoulders is less than 5% of that of the main absorption peak at 438.5 nm. We ascribe the former absorption peak at 477 nm to the relative weak quantum confinement effects of different thicknesses of the single nanosheet or overlapped adjacent nanosheets to that of the single nanosheet, while the latter absorption peak at 515 nm is likely to arise from the nanosheets thicker than the exciton Bohr radius (3.5 nm). Correspondingly, the dominant PL emission peak lies at 452 nm, which may be assigned to thickness of 3 CsPbBr3 perovskite unit cell layers. Furthermore, an additional mixture of PL peaks at 478, 489, and 516 nm was also observed in spite of the weak emission intensities compared to the deep-blue emission at 452 nm. Given the narrow emission line widths of these additional peaks, it is unlikely that they are due to self-defects. We believe these successive emission peaks at 478 and 489 nm arise from CsPbBr3 nanosheets with 4 and 5 CsPbBr3 perovskite unit cell layers, respectively (see the inset of Figure 4g and SI, Figure S9). Moreover, the PL peak at 516 nm arises from thicker

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CONCLUSION In summary, we report a shape-controlled synthesis of CsPbBr3 perovskite NCs with single and lamellar-structured 0D quantum dots, as well as face-to-face stacking 2D nanoplatelets and flat-lying 2D nanosheets morphologies via tuning the amounts of OA and OM. The obtained CsPbBr3 NCs present bright sky-blue emission at 472 nm and deep-blue emission at ∼450 nm due to the strong quantum confinement effects. The prepared 0D CsPbBr3 quantum dots show a sharp PL emission detected at the peak of 453 nm (2.74 eV) with a narrow full width at the half-maximum (fwhm) of 22 nm. The lamellarstructured CsPbBr3 quantum dots yield sky-blue emitting at 472 nm. The obtained CsPbBr3 nanoplatelets show the PL emission peak of 449 nm (2.76 eV), and the CsPbBr 3 nanosheets show that the dominant PL emission peak lies at 452 nm. Besides, PL decay lifetimes have been monitored, ranging from several to tens of nanoseconds, which means the higher ratio of exciton recombination and less defects. The 28828

DOI: 10.1021/acsami.6b08528 ACS Appl. Mater. Interfaces 2016, 8, 28824−28830

Research Article

ACS Applied Materials & Interfaces

Optoelectronic Devices with High Stability. Adv. Funct. Mater. 2016, 26, 5903−5912. (10) Tan, Z.-K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright LightEmitting Diodes Based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (11) Kim, Y. H.; Cho, H.; Heo, J. H.; Kim, T. S.; Myoung, N.; Lee, C. L.; Im, S. H.; Lee, T. W. Multicolored Organic/Inorganic Hybrid Perovskite Light-Emitting Diodes. Adv. Mater. 2015, 27, 1248−1254. (12) Stranks, S. D.; Snaith, H. J. Metal-Halide Perovskites for Photovoltaic and Light-Emitting Devices. Nat. Nanotechnol. 2015, 10, 391−402. (13) Dohner, E. R.; Jaffe, A.; Bradshaw, L. R.; Karunadasa, H. I. Intrinsic White-Light Emission from Layered Hybrid Perovskites. J. Am. Chem. Soc. 2014, 136, 13154−13157. (14) Zhu, H.; Fu, Y.; Meng, F.; Wu, X.; Gong, Z.; Ding, Q.; Gustafsson, M. V.; Trinh, M. T.; Jin, S.; Zhu, X. Lead Halide Perovskite Nanowire Lasers with Low Lasing Thresholds and High Quality Factors. Nat. Mater. 2015, 14, 636−642. (15) Xing, G.; Mathews, N.; Lim, S. S.; Yantara, N.; Liu, X.; Sabba, D.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Low-Temperature Solution-Processed Wavelength-Tunable Perovskites for Lasing. Nat. Mater. 2014, 13, 476−480. (16) Sutherland, B. R.; Hoogland, S.; Adachi, M. M.; Wong, C. T.; Sargent, E. H. Conformal Organohalide Perovskites Enable Lasing on Spherical Resonators. ACS Nano 2014, 8, 10947−10952. (17) Fu, A.; Yang, P. Organic-Inorganic Perovskites: Lower Threshold for Nanowire Lasers. Nat. Mater. 2015, 14, 557−558. (18) Zhu, F.; Men, L.; Guo, Y.; Zhu, Q.; Bhattacharjee, U.; Goodwin, P. M.; Petrich, J. W.; Smith, E. A.; Vela, J. Shape Evolution and Single Particle Luminescence of Organometal Halide Perovskite Nanocrystals. ACS Nano 2015, 9, 2948−2959. (19) Jang, D. M.; Park, K.; Kim, D. H.; Park, J.; Shojaei, F.; Kang, H. S.; Ahn, J.-P.; Lee, J. W.; Song, J. K. Reversible Halide Exchange Reaction of Organometal Trihalide Perovskite Colloidal Nanocrystals for Full-Range Band Gap Tuning. Nano Lett. 2015, 15, 5191−5199. (20) Li, G.; Tan, Z.-K.; Di, D.; Lai, M. L.; Jiang, L.; Lim, J. H.-W.; Friend, R. H.; Greenham, N. C. Efficient Light-Emitting Diodes Based on Nanocrystalline Perovskite in a Dielectric Polymer Matrix. Nano Lett. 2015, 15, 2640−2644. (21) Wong, A. B.; Lai, M.; Eaton, S. W.; Yu, Y.; Lin, E.; Dou, L.; Fu, A.; Yang, P. Growth and Anion Exchange Conversion of Ch3nh3pbx3 Nanorod Arrays for Light-Emitting Diodes. Nano Lett. 2015, 15, 5519−5524. (22) Zhang, Q.; Ha, S. T.; Liu, X.; Sum, T. C.; Xiong, Q. RoomTemperature near-Infrared High-Q Perovskite Whispering-Gallery Planar Nanolasers. Nano Lett. 2014, 14, 5995−6001. (23) Zhang, F.; Zhong, H.; Chen, C.; Wu, X.-g.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. Brightly Luminescent and Color-Tunable Colloidal C H3 N H3 Pb X3 (X= Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015, 9, 4533−4542. (24) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (Cs Pb X3, X= Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692−3696. (25) Wang, Y.; Li, X.; Song, J.; Xiao, L.; Zeng, H.; Sun, H. AllInorganic Colloidal Perovskite Quantum Dots: A New Class of Lasing Materials with Favorable Characteristics. Adv. Mater. 2015, 27, 7101− 7108. (26) Yakunin, S.; Protesescu, L.; Krieg, F.; Bodnarchuk, M. I.; Nedelcu, G.; Humer, M.; De Luca, G.; Fiebig, M.; Heiss, W.; Kovalenko, M. V. Low-Threshold Amplified Spontaneous Emission and Lasing from Colloidal Nanocrystals of Caesium Lead Halide Perovskites. Nat. Commun. 2015, 6, 8056. (27) Song, J.; Li, J.; Li, X.; Xu, L.; Dong, Y.; Zeng, H. Quantum Dot Light-Emitting Diodes Based on Inorganic Perovskite Cesium Lead Halides (Cspbx3). Adv. Mater. 2015, 27, 7162−7167.

shape-controlled CsPbBr3 perovskite applications, especially for blue LEDs, are still lacking compared to the well-developed red and green LEDs. However, in view of the destructive effects to the morphology and optoelectronic properties in the isolation and purification processes, further investigations should first focus on the surface stability to avoid desorbing so as to be better applied to LEDs, lasers, and photodetectors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b08528. TEM images of the CsPbBr3 single quantum dots and lamellar-structured quantum dots, the diameter distribution of the CsPbBr3 single quantum dots, lamellarstructured quantum dots and nanoplatelets stacking faceto-face, EDX spectra of the four samples, HRTEM images of the irregular CsPbBr3 nanosheets, the multipeak fitting of the PL spectra for the 2D CsPbBr3 nanosheets, fitted decay lifetimes, and the PLQY of the four samples (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National High Technology Research and Development Program of China (863 Program) under Grant No. 2013AA032205 and the Fundamental Research Funds for the Central Universities (FRFCU) under Grant No. 2016JBM066.

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REFERENCES

(1) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395−398. (2) Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476−480. (3) Im, J.-H.; Jang, I.-H.; Pellet, N.; Grätzel, M.; Park, N.-G. Growth of C H3 N H3 Pb I3 Cuboids with Controlled Size for High-Efficiency Perovskite Solar Cells. Nat. Nanotechnol. 2014, 9, 927−932. (4) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (5) Nrel Efficiency Chart, 2016. www.nrel.gov/ncpv/images/ efficiency_chart.jpg. (6) Dou, L.; Yang, Y. M.; You, J.; Hong, Z.; Chang, W.-H.; Li, G.; Yang, Y. Solution-Processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Commun. 2014, 5, 5404. (7) Sutherland, B. R.; Johnston, A. K.; Ip, A. H.; Xu, J.; Adinolfi, V.; Kanjanaboos, P.; Sargent, E. H. Sensitive, Fast, and Stable Perovskite Photodetectors Exploiting Interface Engineering. ACS Photonics 2015, 2, 1117−1123. (8) Yakunin, S.; Sytnyk, M.; Kriegner, D.; Shrestha, S.; Richter, M.; Matt, G. J.; Azimi, H.; Brabec, C. J.; Stangl, J.; Kovalenko, M. V.; Heiss, W. Detection of X-Ray Photons by Solution-Processed Lead Halide Perovskites. Nat. Photonics 2015, 9, 444−449. (9) Li, X.; Yu, D.; Cao, F.; Gu, Y.; Wei, Y.; Wu, Y.; Song, J.; Zeng, H. Healing All-Inorganic Perovskite Films Via Recyclable Dissolution− Recyrstallization for Compact and Smooth Carrier Channels of 28829

DOI: 10.1021/acsami.6b08528 ACS Appl. Mater. Interfaces 2016, 8, 28824−28830

Research Article

ACS Applied Materials & Interfaces (28) Wang, Y.; Li, X.; Zhao, X.; Xiao, L.; Zeng, H.; Sun, H. Nonlinear Absorption and Low-Threshold Multiphoton Pumped Stimulated Emission from All-Inorganic Perovskite Nanocrystals. Nano Lett. 2016, 16, 448−453. (29) Han, W.; Yi, L.; Zhao, N.; Tang, A.; Gao, M.; Tang, Z. Synthesis and Shape-Tailoring of Copper Sulfide/Indium Sulfide-Based Nanocrystals. J. Am. Chem. Soc. 2008, 130, 13152−13161. (30) Ibáñez, M.; Zamani, R.; Li, W.; Cadavid, D.; Gorsse, S. p.; Katcho, N. A.; Shavel, A.; López, A. M.; Morante, J. R.; Arbiol, J.; Cabot, A. Crystallographic Control at the Nanoscale to Enhance Functionality: Polytypic Cu2 Ge Se3 Nanoparticles as Thermoelectric Materials. Chem. Mater. 2012, 24, 4615−4622. (31) Shavel, A.; Arbiol, J.; Cabot, A. Synthesis of Quaternary Chalcogenide Nanocrystals: Stannite Cu2 Znx Sny Se1+X+2y. J. Am. Chem. Soc. 2010, 132, 4514−4515. (32) Riha, S. C.; Parkinson, B. A.; Prieto, A. L. Solution-Based Synthesis and Characterization of Cu2 Zn Sn S4 Nanocrystals. J. Am. Chem. Soc. 2009, 131, 12054−12055. (33) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-Based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503− 6570. (34) Wada, T.; Nakamura, S.; Maeda, T. Ternary and Multinary CuChalcogenide Photovoltaic Materials from Cu in Se2 to Cu2 Zn Sn S4 and Other Compounds. Prog. Photovoltaics 2012, 20, 520−525. (35) Fan, F.-J.; Wu, L.; Yu, S.-H. Energetic I−I I I−V I2 and I 2−I I− I V−V I4 Nanocrystals: Synthesis, Photovoltaic and Thermoelectric Applications. Energy Environ. Sci. 2014, 7, 190−208. (36) Zhang, D.; Eaton, S. W.; Yu, Y.; Dou, L.; Yang, P. SolutionPhase Synthesis of Cesium Lead Halide Perovskite Nanowires. J. Am. Chem. Soc. 2015, 137, 9230−9233. (37) Sun, S.; Yuan, D.; Xu, Y.; Wang, A.; Deng, Z. Ligand-Mediated Synthesis of Shape-Controlled Cesium Lead Halide Perovskite Nanocrystals Via Reprecipitation Process at Room Temperature. ACS Nano 2016, 10, 3648−3657. (38) Song, J.; Xu, L.; Li, J.; Xue, J.; Dong, Y.; Li, X.; Zeng, H. Monolayer and Few-Layer All-Inorganic Perovskites as a New Family of Two-Dimensional Semiconductors for Printable Optoelectronic Devices. Adv. Mater. 2016, 28, 4861−4869. (39) Akkerman, Q. A.; Motti, S. G.; Kandada, A. R. S.; Mosconi, E.; D’Innocenzo, V.; Bertoni, G.; Marras, S.; Kamino, B. A.; Miranda, L.; De Angelis, F.; Petrozza, A.; Prato, M.; Manna, L. Colloidal Cesium Lead Halide Perovskite Nanoplatelets with Monolayer-Level Thickness Control by a Solution Synthesis Approach. J. Am. Chem. Soc. 2016, 138, 1010−1016. (40) Bekenstein, Y.; Koscher, B. A.; Eaton, S. W.; Yang, P.; Alivisatos, A. P. Highly Luminescent Colloidal Nanoplates of Perovskite Cesium Lead Halide and Their Oriented Assemblies. J. Am. Chem. Soc. 2015, 137, 16008−16011. (41) Li, X.; Wu, Y.; Zhang, S.; Cai, B.; Gu, Y.; Song, J.; Zeng, H. Cs Pb X3 Quantum Dots for Lighting and Displays: Room-Temperature Synthesis, Photoluminescence Superiorities, Underlying Origins and White Light-Emitting Diodes. Adv. Funct. Mater. 2016, 26, 2435− 2445. (42) MØLLER, C. K. Crystal Structure and Photoconductivity of Caesium Plumbohalides. Nature 1958, 182, 1436. (43) Son, J. S.; Wen, X. D.; Joo, J.; Chae, J.; Baek, S. i.; Park, K.; Kim, J. H.; An, K.; Yu, J. H.; Kwon, S. G.; Choi, S.-H.; Wang, Z.; Kim, Y.W.; Kuk, Y.; Hoffmann, R.; Hyeon, T. Large-Scale Soft Colloidal Template Synthesis of 1.4 Nm Thick Cdse Nanosheets. Angew. Chem. 2009, 121, 6993−6996. (44) Lv, L.; Xu, Y.; Fang, H.; Luo, W.; Xu, F.; Liu, L.; Wang, B.; Zhang, X.; Yang, D.; Hu, W.; Dong, A. Generalized Colloidal Synthesis of High-Quality, Two-Dimensional Cesium Lead Halide Perovskite Nanosheets and Their Applications in Photodetectors. Nanoscale 2016, 8, 13589−13596.

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