Controlled Synthesis of Composition Tunable Formamidinium Cesium

Article pubs.acs.org/cm

Controlled Synthesis of Composition Tunable Formamidinium Cesium Double Cation Lead Halide Perovskite Nanowires and Nanosheets with Improved Stability Chuying Wang,† Yukang Zhang,† Aifei Wang, Qian Wang, Haiyan Tang, Wei Shen, Zhe Li, and Zhengtao Deng* Department of Biomedical Engineering, College of Engineering and Applied Sciences, Nanjing National Laboratory of Microstructures, Collaborative Innovation Center of Chemistry for Life Sciences, Nanjing University, Nanjing, Jiangsu 210093, P. R. China S Supporting Information *

ABSTRACT: Lead halide perovskites with well-defined morphology have attracted attention for their unique properties as a promising new class of semiconductor materials in photovoltaics and optoelectronics. However, controlling morphologies and compositions of perovskite nanostructures with improved stability, especially for double cation lead halide perovskite, still remains a challenge. Here, we demonstrate a colloidal synthetic approach for direct synthesis of stable singlecrystal formamidinium (FA) cesium double cation lead halide FA0.33Cs0.67PbBr3−xIx (0 ≤ x ≤ 3) perovskite nanostructures with controllable morphology over a wide range of halide compositions, without using a previous anion-exchange process. The presence of FA alloyed in the A site for the pure cesium lead halide perovskite structure can stabilize the nanocrystals while delivering a better balance between structure and composition. On the basis of the FA0.33Cs0.67PbBr3−xIx alloy perovskite system, we achieved nanowires and nanosheets with high yield of various halide compositions by tuning the ligand participating in the reaction. These new perovskite nanostructures with well-defined morphology and compositions demonstrated improved stability and fluorescent and optoelectronic properties, which may allow them to find application as replacements for conventional semiconductor nanostructures in nanoscale devices.

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INTRODUCTION Nanocrystals with well-defined morphology usually show some unique properties and thus have become a promising new class of materials for future commercial applications.1−8 Halide perovskites, AMX3 (A = MA+, FA+, Cs+; M = Pb2+, Sn2+; X = Cl−, Br−, I−), have recently emerged as promising semiconductor materials, thanks to their unique properties, such as broad chemical tunability, narrow emission wavelength, excellent charge-transport properties, and limited charge recombination and so on.9−17 Since CH3NH3PbI3 was demonstrated in dye-sensitized solar cells in 2009, there have been great advances in solar cells, light-emitting diodes, and lasers.18−20 For early methylammonium (MA, CH3NH3+) lead halide perovskite, MAPbX3 (X = Cl, Br, I), there is an intrinsic lack of stability to both high temperature and water, due to the methylammonium cation.21,22 To improve the stability of hybrid perovskite, a strategy is doping or fully substituting the methylammonium cation with less reactive cations such as organic formamidinium (FA, HC(NH2)2+), cesium, or mixed cations.23−25 Increasing the complexity of A cation in the perovskite system can also increase the entropy of the system and improve the stability of ordinary unstable materials.24,26−28 © 2017 American Chemical Society

Halide perovskite nanocrystals with well-defined morphology may be a promising system due to their properties. For early methylammonium lead halide perovskites, many advances on morphology control have been achieved by solution deposition of a mixture of CH3NH3X and PbX2, such as CH3NH3PbI3 nanowires, CH3NH3PbBr3 quantum wires, (C8H17NH3)2(CH3NH3)2Pb3(IxBr1−x)10 nanorods, and so on, and anion exchange is also applied to expand the range of perovskite nanocrystal compositions.29−31 Cesium lead halide perovskites with well-defined morphology and wide-ranged composition can also be synthesized by the similar approaches of solution deposition and anion exchange, such as CsPbBr3 nanowires, CsPbCl3 nanowires, and CsPbX3 (X = Cl, Br, I) nanowires.32−35 The morphology of cesium lead halide perovskite nanocrystals by colloidal synthesis, at the same time, has also rapidly extended to 1D nanowires, 2D nanosheets, and nanorods.36−41 However, compared with the approach of solution deposition, previous colloidal synthesis usually needs anion exchange to expand the composition of Received: November 14, 2016 Revised: February 14, 2017 Published: February 21, 2017 2157

DOI: 10.1021/acs.chemmater.6b04848 Chem. Mater. 2017, 29, 2157−2166

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Chemistry of Materials

Figure 1. Schematic illustration for the formation of perovskite nanowires and nanosheets.

nanosheets seem to become translucent under a 3.5 kV electron beam, which indicates an ultrathin thickness. A further characterization on the thickness of as-synthesized nanosheets was done by atomic force microscope (AFM), shown in Figure 4. From the height profiles (shown in Figure 4C,D), we estimated a 4.2 nm thickness for the nanosheet sample. According to previous work on CsPbBr3 nanocrystals, the thickness of a monolayered nanosheet was estimated to be around 3 nm.39 Considering that, compared with pure CsPbBr3, alloy FA0.33Cs0.67PbBr3−xIx has a larger lattice, due to the larger ion radii of FA and iodide, we tend to think that the thickness we estimated approaches monolayer nanosheets. Furthermore, the approaches of both nanowires and nanosheets of perovskites with all compositions have been achieved by just tuning the amount of OA and OAm. We found, however, that morphology cannot be well controlled when OAm becomes a major ligand for iodide-rich perovskite samples. For these samples, a new morphology between 1D and 2D, nanotube, occurred as shown in Figure S3. Compared with perovskite nanowires and nanosheets, the size of these nanotube samples in all three dimensions is so large after a same reaction time, which reflects that the growth of these nanotubes was out of control. X-ray diffraction (XRD) of both nanowires and nanosheets are shown in Figure 5A,B. As there is no standard PDF card for the crystal phase of quinary FA0.33Cs0.67PbBr3−xIx, orthorhombic CsPbBr3 (ICSD 97851, a = 8.207 Å, b = 8.255 Å, and c = 11.759 Å) was applied for further studies. It can be easily observed that there is an obvious shift in the XRD pattern while bromide is gradually substituted for iodide, which indicates a larger lattice, due to a change of anion radii. The XRD pattern is not only determined by the composition but is also closely related to the morphology of samples. There is an extra peak at around 11° for nanosheets, which cannot be observed in the same composition with other morphologies such as bulk crystals or nanowires. The extra peak at around 11° is usually thought to be a characteristic for 2D perovskite structure, and they are originated from the diffraction of the X-rays with the (00n) facets of the 2D perovskite crystal.31,42 The main diffraction peak of all of these perovskites, the (220) and peak at around 30° was selected and fitted by Gaussian function,

nanocrystals, which may cause a worse performance for nanowires and nanosheets. Here, a colloidal synthetic approach of stable single crystal FA0.33Cs0.67PbBr3−xIx perovskite with controllable morphology over a wide range of halide compositions is reported (shown in Figure 1). We compared FA-contained perovskites with pure cesium perovskites and found that a better balance among structure, composition, and stability can be delivered after cesium is partially substituted for FA. On the basis of this, the well-defined morphology of nanowire or nanosheet can be observed; after just adjusting the ligand in the reaction, the FA0.33Cs0.67PbBr3−xIx alloy nanostructure shows quite different optoelectronic response, which can expand its practical applications.

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RESULTS AND DISCUSSION Morphology. In our approach, perovskite nanocrystals were synthesized under a ligand system that consists of oleic acid (OA), oleylamine (OAm), and bis(2-ethylhexyl)-amine (BEHA). The morphology of these perovskites can be synthesized by tuning the ligand system, which is actually the amount of OA and OAm that participate into the synthesis process. Besides, we also found that different morphologies can be better distinguished and selected while BEHA participates in the reaction (shown in Figure S1). BEHA is herein necessary for optimizing morphology control in this system. The shape and size of as-synthesized perovskite nanocrystals were examined by scanning electron microscope (SEM) for nanowire and nanosheet samples, shown in Figures 2, 3, and S2, respectively. As shown in Figures 2 and S2A, diameters of the nanowire samples vary from 15 to 300 nm, and lengths of these samples are at least several micrometers, which delivers a very high aspect ratio for all of these nanowire samples. As for nanosheets shown in Figures 3 and S2B, the lateral size of these samples varies from 2 to 4 μm. The SEM images indicate that there is a morphology of octagon for all of these nanosheet samples. All of the samples show a smooth surface, which might imply low defect density. Both samples of nanowire and nanosheet perform a high yield according to the low magnification SEM images. We observed that in the low magnification SEM images of nanosheet samples as-synthesized 2158


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Figure 2. Morphology characterization of FA0.33Cs0.67PbBr3−xIx nanowires. (A−E) Nanowires of different compositions with x = 0, 0.5, 1, 2.5, 3, respectively; insets show the enlarged images of the corresponding single nanowires.

shown in Figure 5C,D, for further assessment on the relationship between lattice constant and composition of the perovskites. The relationship, shown in Figure 5E,F, reflects a linear dependence between the lattice distance and composition of the perovskites for both nanowires and nanosheets,

Figure 3. Morphology characterization of FA0.33Cs0.67PbBr3−xIx nanosheets. (A−E) Nanosheets of different composition with x = 0, 0.5, 1, 1.5, 2, respectively; insets show the enlarged images of the corresponding single nanosheets. 2159


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Figure 4. AFM analysis of FA0.33Cs0.67PbBr1.5I1.5 nanosheets: (A) multiple nanosheets and (B) single nanosheet. Height profiles (C) obtained from the image in (B) along the white line (D). Height distribution based on (C). The peak centered at 0 nm corresponds to the mica substrate.

Figure 5. Structure characterization of FA0.33Cs0.67PbBr3−xIx nanocrystals. The XRD for perovskite nanowires (A) and nanosheets (B) in different compositions and fitting (220) peak for nanowires (C) and nanosheets (D). Lattice constant from XRD diffraction peaks plotted as functions of composition of iodide for (E) nanowires and (F) nanosheets.

through a direction perpendicular to (210) plane. For nanosheets, at the same time, situations become relatively simple. We found that only (110) and (11̅0) planes can be seen, which may indicate that the [001] direction is forbidden by the ligand system during the growth process of nanosheets. The fast Fourier transforms (FFTs) of HRTEM images, shown in Figure 6E−H, clearly indicate a linear decline in the lattice constants of FA0.33Cs0.67PbBr3−xIx alloys with an increasing iodide concentration. The growth section models of perovskites were shown in Figure 6I,J, respectively.

indicating the phase transition and instability were suppressed over the whole range of compositions. In order to develop a better knowledge of the selection of morphology, several nanowire and nanosheet samples were analyzed by high-resolution transmission electron microscopy (HRTEM), shown in Figure 6A,B, for nanowires, and Figure 6C,D, for nanosheets, respectively. It can be observed that perovskite nanowires and nanosheets are grown through quite different lattice planes. Both the (210) and (220) lattice planes can be seen in nanowires, and both nanowires were grown 2160


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Figure 6. HRTEM images for different perovskite (A) FA0.33Cs0.67PbBr0.5I2.5 nanowires, (B) FA0.33Cs0.67PbI3 nanowires, (C) FA0.33Cs0.67PbBr1.5I1.5 nanosheets, and (D) FA0.33Cs0.67PbBrI2 nanosheets, respectively, and their FFT images (E−H). The models of the growth sections of (I) nanowires and (J) nanosheets are shown, respectively.

Figure 7. Optical characterizations of FA0.33Cs0.67PbBr3−xIx nanocrystals. Optical absorption (dashed lines) and PL spectra (solid lines) for (A) FA0.33Cs0.67PbBr3−xIx nanowires and (B) FA0.33Cs0.67PbBr3−xIx nanosheets. Normalized PL intensity of CsPbBr0.5I2.5 and FA0.33Cs0.67PbBr0.5I2.5 nanowires in solvents made of (C) hexane−ethyl acetate and (D) hexane−acetone.

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Chemistry of Materials Band Gap and Optical Properties. The optical properties of as-synthesized perovskite nanocrystals are shown in Figure 7, including PL emission and absorbance spectra (shown in Figure 6A,B, respectively). As the iodide concentration increases, both PL emission and absorbance spectra present a red shift, and the PL emission peak shifts from 520 nm, for a pure bromide composition, to 685 nm, for a pure iodide composition. For the further study on the optical properties of the FA0.33Cs0.67PbBr3−xIx (0 ≤ x ≤ 3) alloy nanocrystals, timeresolved PL decay spectra were measured (shown in Figure S4 and Table S1) and fitted by a biexponential decay model, reflecting the surface site recombination (fast component) and the bulk recombination (slow component), respectively.30,32,33 Band gap energies (Eg) of perovskite samples are related to the ratio of their halide components, as shown in Figure S5 and Table S2. For the alloy APbBrzI3−z perovskite samples, it is often considered that there is a binomial relationship between Eg(z) and z and can be written as eq 1 z Eg (z) = E Iodide + (E Bromide − E Iodide − b) × 3 ⎛ z ⎞2 +b×⎜ ⎟ ⎝3⎠ (1)

Figure S8A) deviates seriously from the linear dependence, which indicates a poor balance on the reactivity between bromine and iodine. This problem, however, can be alleviated after FA was added into the system; as such a relationship (shown in Figure S8B) matches the linear dependence much better in the FA−cesium lead halide system. Significantly, we observed that the stability of perovskite structure was improved after cesium was partially substituted for FA. To confirm these observations, two samples with similar band gap, shape, and synthesis approach, one from cesium lead halide system and the other from an FA−cesium lead halide system were selected for further testing (shown in Figure S9). Alloy semiconductor nanocrystals usually show a worse monochromaticity due to their mixed composition, and this phenomenon can be observed in quantum dots, nanowires, and nanosheets.47−49 That is the reason why the FA−cesium lead halide sample shows a larger fwhm. A stability test of CsPbBr0.5I2.5 and FA0.33Cs0.67PbBr0.5I2.5 NCs was done by irradiating continuous wave (cw) type UV−vis laser (420 nm) for up to 2 h. Both perovskite samples were dispersed in a mixed solvent that contains hexane, to ensure dispersion of both samples and polar solvents, and both ethyl acetate and acetone were selected to break the perovskites. Their stability was evaluated by PL emission spectrum (3 min per count). A PL emission peak shift can be observed in the test of all the samples, shown in Figure 7C,D, due to a solvation process in ethyl acetate or acetone. As shown in Figure 7C, for the stability in hexane−ethyl acetate, the PL fluorescence of the FA0.33Cs0.67PbBr0.5I2.5 sample can be observed during the whole period of the test, and its relative intensity finally decreased to 70%. The relative intensity of the CsPbBr0.5I2.5 sample, at the same time, finally decreased to 2.2% and nearly without any PL fluorescence after the test. As shown in Figure 7D, for the stability in hexane−acetone, the CsPbBr0.5I2.5 sample was broken immediately and lost almost all PL fluorescence (less than 2%). The FA0.33Cs0.67PbBr0.5I2.5 sample performs much better and finally remains at 5.3% relative intensity after the test. Besides, it can be seen that there is a fluorescent enhancement at around 30 min during the test for the FA0.33Cs0.67PbBr0.5I2.5 sample, in both hexane−ethyl acetate and hexane−acetone, which may relate to a recrystallization process under continuous irradiation. Meanwhile, during the test of the CsPbBr0.5I2.5 sample, an extra PL emission peak below 630 nm is shown during the test in both solvents, indicating a change in its composition or structure. To synthesize nanocrystals with the designed morphology, such as nanowires and nanosheets, all of the precursors are finely balanced and stable enough for better control. For the lead halide perovskite system, it is much more challenging to synthesize pure CsPbI3 with designed morphology, as shown in Figure S10A, and as-synthesized CsPbI3 nanowires show a poor morphology, in both aspect ratio and yield and are hard to control during the reaction process. After cesium was partially substituted by FA, as shown in Figure S10B, the morphology of the perovskite sample was obviously improved and a nanowire sample with much better aspect ratio and yield and can be synthesized directly. Electrical Transport and Photocurrent Measurements. To investigate electrical transport and photocurrent properties, two perovskite samples, a nanowire sample and a nanosheet sample, were selected for comparison. Both samples were prepared by dropping sample solutions directly onto prepatterned gold electrodes, and the gap between the

where EIodide and EBromide are the bandgaps of pure APbI3 and APbBr3, respectively, and b is the so-called bowing parameter that accounts for the effects of composition disorder on the conduction and valence band edges.43 During our measurement, we found that for FA0.33Cs0.67PbBrzI3‑z (0 ≤ z ≤ 3) nanocrystals, it is quite different for such a relationship of nanowires and nanosheets. As shown in Figure S6A, the bowing parameter is (−0.22 ± 0.52) eV for the nanowire alloys. That is, there seems to be a combination of positive and negative bowing, which might be due to other differences among these nanowire alloys such as ligand and so on, and making a sigmoidal curve instead of a parabola curve. The bowing parameter is −0.54 eV for nanosheets, as shown in Figure S6B. Formamidinium−Cesium Alloy. Orthorhombic CsPbI3 is usually in δ-phase, instead of perovskite phase, and presents a much worse performance.33,44 Whether an AMX3 perovskite structure can be sustained is usually assessed by a Goldschmidt tolerance factor, calculated as eq 2 t=

RA + RX 2 (RM + RX)

(2)

where R stands for the ionic radii, and it is often considered that an AMX3 perovskite structure can be well sustained if the tolerance factor is between 0.8 and 1.0.26,27,44,45 For CsPbI3, t is around 0.8, indicating that cesium cation is a little bit small, and thus we need another cation with larger radii at the A site, such as FA. If cesium is partially substituted for FA, we could deliver a better balance between the structure and the composition. For synthesis of multinary colloidal nanocrystals, it is commonly known that the reactivity of all precursors needs to be finely balanced to obtain small quantum-confined NCs with controlled composition.46 In the inorganic lead halide perovskite alloy system, however, it is hard to balance the reactivity of its halide anion. The quantitative elemental composition of the alloy is analyzed by XPS, shown in Figure S7 and Table S3, and the reactivity of halide components was assessed by the relationship between the iodide contained in the samples and iodide precursors contained in the system. For pure cesium lead halide system, this relationship (shown in 2162


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Figure 8. I−V characteristics of (A) FA0.33Cs0.67PbBr1.5I1.5 nanosheets and (B) FA0.33Cs0.67PbBr0.5I2.5 nanowires under dark and light conditions, and a fit of the I−V curve for the Ohm regime and TFL regime to (C) FA0.33Cs0.67PbBr1.5I1.5 nanosheets and (D) FA0.33Cs0.67PbBr0.5I2.5 nanowires. (E) Isd−Vgs characteristics of a typical nanosheet or nanowire device under dark and light conditions.

electrodes was 1 mm. Under electric field, current−voltage characteristics under dark and illumination conditions are quite different from each other in the measurement for both samples (shown in Figure 8A,B). Under a dark condition, it can be observed that there is high resistance in both samples, and the current−voltage characteristics show an insulator-like response. The conductance of both samples increases by 102 times after illumination, and the photoconductance of both samples exhibits a nonlinear increase at a higher field. That is, the

conduction of samples follows Ohm’s law with a linear current−voltage curve under low field, and when the threshold trap-filled limit voltage (VTFL) is reached, the conduction transfers into another model and falls into the TFL mechanism. It is worth noticing that these conduction mechanisms can only be resolved in semiconductors with low defect level. Figure 8C,D shows the trap-filled limit voltage of both samples, and a VTFL less than 0.5 V can be measured. The VTFL is at such a low 2163


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synthesis of other double or triple cation perovskite nanocrystals like AMX3 (A = FA+, MA+, Rb+, Cs+; M = Ge2+, Sn2+; X = F−, Cl−, Br−, I−, or combinations thereof) as well.

level, also indicating a low density of defect states, that typically causes an increase in VTFL level. Further studies on these properties were investigated by measuring the current−voltage characteristics under a condition of gate voltage varying from −2 to 2 V, and the transport (IDS−VGS) characteristics of the samples for a drain voltage at 2 V are shown in Figure 8E. A decreased drain current was measured while increasing the source voltage for both samples and in both the dark and the illumination situation, and the drain current can be measured while the gate voltage is in both negative and positive regions. Both samples also show a photoresponse and present an enhanced drain current under illumination conditions. For semiconductor nanocrystals, the electrical transport properties are related to their morphologies, and a tendency that can be observed is that lead halide perovskite nanocrystals with better aspect ratio can perform better electrical transport properties.50−52 It can also be observed that perovskite nanocrystals with different morphology perform different aggregation approaches. As shown in Figure S11, 2D structures such as nanosheets (Figure S11A) usually aggregate and form many islands that leave plenty of high resistance areas. 1D structures such as nanowires (Figure S11B), at the same time, are much easier to aggregate and form network connections and bundles that contain scores of nanowires in each one, to enhance its conductance. In all of our measurements on the electrical transport and photocurrent properties, a substantial difference related to morphology is that the nanowire sample shows a much better conductance, increasing by 102 times, than the nanosheet sample. Such a big improvement can be not only caused by the difference of composition between both samples but also may be related to the difference of their morphologies and their aggregation approaches.

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EXPERIMENTAL SECTION

Chemicals. Chemicals used include cesium carbonate (99%, Aladdin), formamidine acetate (FAAc, 99%, Aladdin), lead bromide (99%, Aladdin), lead iodide (98%, Aladdin), sodium bromide (99%, Shanghai Lingfeng Chemical Reagent Co. Ltd.), sodium iodide (99.5%, Aladdin), 1-octadecene (1-ODE, 90%, Aladdin), oleic acid (OA, AR, Aladdin), oleylamine (OAm, 80−90%, Aladdin), bis(2ethylhexyl)amine (BEHA, 99%, Aladdin), hexane (AR, Sinopharm Chemical Reagent Co. Ltd.), ethyl acetate (AR, Nanjing Chemical Reagent Co. Ltd.), and acetone (AR, Sinopharm Chemical Reagent Co. Ltd.). All chemicals were used as received without any further purification. Preparation of Cs−Oleate and FA−Oleate Solution. A total of 1.96 g of Cs2CO3 or 1.25 g of FAAc was loaded into a 3-neck flask with 6 mL of OA and 24 mL of 1-ODE, degassed and under vacuum for around 40 min at 125 °C, until all Cs2CO3 or FAAc reacted with OA. Synthesis of FA0.33Cs0.67PbBr3−xIx (0 ≤ x ≤ 3) Nanocrystals. A total of 14 mL of ODE and an amount of 0.6 mmol of PbX2 (shown in Table S4, for nanowires, and Table S5, for nanosheets, respectively) were loaded into a 3-neck flask and degassed under vacuum for 45 min at 125 °C. 5 mL dehydrated ligand (certain amount of OA, OAm, and BEHA) were injected at 125 °C. After the injection, the solution would turn clear. Then the temperature was raised to 160 °C, and 0.67 mL of as-prepared Cs−oleate solution and 0.33 mL of as-prepared FA−oleate solution were quickly injected. After 8 min, the reaction mixture was injected into hexane directly. Isolation and Purification of FA0.33Cs0.67PbBr3−xIx (0 ≤ x ≤ 3) Nanocrystals. The as-synthesized NCs were separated by centrifugation at 12500 rpm for 12 min at room temperature, washed twice with a mixed solvent (hexane and toluene), and dispersed into hexane or toluene for further use. Characterization. Ultraviolet and visible absorption (UV−vis) spectra were recorded with a Shimadzu UV-3600 plus spectrophotometer at room temperature. PL emission spectra were irradiated under 420 nm UV−vis light and measured with a Horiba PTI QuantaMaster 400 steady state fluorescence system at room temperature. The topography of the nanosheet was investigated using a Bruker Multimode 8 atomic force microscope working in automode. Scanning electron microscopy (SEM) was performed on a ZEISS ULTRA55 electron microscope operating at 3.5 kV. Transmission electron microscopy (TEM) and high-resolution TEM were performed on a FEI Tecnai G2 F20 electron microscope operating at 200 kV. X-ray diffraction (XRD) measurements were employed a Rikagu Ultima III X-ray diffractometer equipped with Cu Kα radiation (λ = 1.541841 Å). X-ray photoelectron spectroscopy (XPS) measurements were performed using an achromatic Al Kα source (1486.6 eV) and a double pass cylindrical mirror analyzer (ULVACPHI 5000 Versa Probe). Current−voltage characteristics were obtained by a Keithley 2612B dual channel system source meter. The light source used for illumination in this report was a 300 W xenon lamp. The bottom gold contacts (100 nm) with 5 nm Cr base were fabricated on a 500 nm SiO2-coated Si chip by electron beam lithography. The fluorescence decay processes were recorded with time-correlated single-photon counting (TCSPC) technique on a system provided by PicoQuant (PicoHarp 300) equipped with a 400 nm laser and a time-correlated single-photon counting system at room temperature. Time-resolved PL decay curves were fitted to biexponential (shown in eq 3) decay curves of

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CONCLUSIONS In summary, we demonstrated the direct colloidal synthesis of stable, morphology tunable, formamidinium cesium double cation lead halide FA0.33Cs0.67PbBr3−xIx (0 ≤ x ≤ 3) perovskite nanostructures over a wide range of halide compositions. In our system, FA was drawn in to stabilize the nanocrystal, especially for the iodide-rich sample, which usually shows poor optical property and stability compared to those of the previous cesium lead halide perovskite system. The well-defined morphology of the nanowire or nanosheet can be controlled simply by adjusting the ligand in the reaction. The growth tendency of the nanosheet and nanowire can be chosen by simply altering the ratio between OA and OAm. BEHA participating in the reaction can optimize the morphology in selectivity. This approach can be applied to FA0.33Cs0.67PbBr3−xIx alloy nanostructure with a wide range composition with the morphology of nanowire or nanosheet. Additionally, different FA0.33Cs0.67PbBr3−xIx nanocrystals show quite different optoelectric responses, which may be attributed to their composition, morphology, and aggregation approaches. These nanostructures with well-defined morphology perform quite differently in their fluorescent and optoelectric properties, which may expand their practical application for further use. We envision that the shape-controlled FA0.33Cs0.67PbBr3−xIx perovskite nanocrystals will find widespread use in applications, such as lasing, light-emitting diodes, photovoltaics, solar concentrators, and photon detection. By the suitable choice of sources and/or synthetic parameters, it is reasonable to expect that the present methods can be extended to the

⎛ t⎞ ⎛ t ⎞ A(t ) = A1 exp⎜ − ⎟ + A 2 exp⎜− ⎟ ⎝ τ1 ⎠ ⎝ τ2 ⎠ 2164

(3) DOI: 10.1021/acs.chemmater.6b04848 Chem. Mater. 2017, 29, 2157−2166

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b04848. Additional experimental data, HRTRM, XPS, and FTIR results, including Figures S1−S11 and Tables S1−S6 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(Z.D.) E-mail: [email protected]. ORCID

Zhengtao Deng: 0000-0002-8428-8982 Author Contributions †

C.W. and Y.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant No. 51502130), Natural Science Foundation of Jiangsu Province (Grant No. SBK2015043303), Thousand Talents Program for Young Researchers, Shuangchuang Program of Jiangsu Province, Fundamental Research Funds for Central Universities, and Nanjing University.

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