Controlled Synthesis of Lead-Free and Stable Perovskite Derivative

Department of Chemical Engineering and Materials Science and Department of Physics and Astronomy, University of California—Irvine, Irvine, Californi...
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Controlled Synthesis of Lead-Free and Stable Perovskite Derivative Cs2SnI6 Nanocrystals via a Facile Hot-Injection Process Aifei Wang,† Xingxu Yan,† Mian Zhang,† Shibin Sun,† Ming Yang,† Wei Shen,† Xiaoqing Pan,†,‡ Peng Wang,*,† 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, People’s Republic of China ‡ Department of Chemical Engineering and Materials Science and Department of Physics and Astronomy, University of CaliforniaIrvine, Irvine, California 92697, United States S Supporting Information *

ABSTRACT: Colloidal nanocrystals of lead halide perovskites have recently received great attention due to their remarkable performance in optoelectronic applications (e.g., light-emitting devices, flexible electronics, and photodetectors). However, the use of lead remains of great concern due to its toxicity and bioaccumulation in the ecosystem; herein we report a strategy to address this issue by using tetravalent tin (Sn4+) instead of divalent lead (Pb2+) to synthesize stable Cs2SnI6 perovskite nanocrystals. The shapes of assynthesized Cs2SnI6 nanocrystals are tuned from spherical quantum dots, nanorods, nanowires, and nanobelts to nanoplatelets via a facile hot-injection process using inexpensive and nontoxic commercial precursors. Spherical aberration corrected scanning transmission electron microscopy (Cs-corrected STEM) and simulation studies revealed a well-defined face-centered-cubic (fcc) perovskite derivative structure of Cs2SnI6 nanocrystals. The solution-processed Cs2SnI6 nanocrystal-based field effect transistors (FETs) displayed a p-type semiconductor behavior with high hole mobility (>20 cm2/(V s)) and high I-ON/I-OFF ratio (>104) under ambient conditions. We envision that this work will pave the way to produce new families of high-performance, stable, low-cost and nontoxic nanocrystals for optoelectronic applications.

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morphologies, such as zero-dimensional (0D) spherical quantum dots and nanocubes, one-dimensional (1D) nanorods and nanowires, and two-dimensional (2D) nanoplatelets.18−24 Despite the significant developments in lead halide perovskite nanocrystals, it is now highly desirable to replace lead, due to its high toxicity and bioaccumulation in the ecosystem, with some environmentally benign metal to synthesize metal halide perovskite nanocrystals with comparable optical and optoelectronic performance.25−29 There have been efforts to replace lead with nontoxic elements, such as Sn, Ge, Bi, and Sb. Among them, an Sn-based inorganic perovskite (CsSnI3) thin film has been successfully synthesized and used as the hole-transporting material in solar cells.27,30−32 More recently, divalent tin based CsSnX3 perovskite nanocubes have also been reported using trioctylphosphine (TOP)-assisted colloidal synthetic methods.26 Unlike CsSnX3, which needs to be handled under an inert atmosphere, it is expected that the use of tetravalent tin (Sn4+) instead of divalent tin (Sn2+) could address the stability

etal halide perovskites, with the advantages of facile processing, low cost, and superior optoelectronic properties, have recently emerged as a new class of semiconductors with great potential for a variety of applications, including high-efficiency photovoltaic cells, light-emitting diodes, lasers, and photodetectors.1−17 Key to this success is the development of various solution-based synthetic techniques to control the morphology and composition or restrict the physical dimensions of the perovskite crystallites to a few nanometers.3−7 In particular, CsPbX3 (X = Cl, Br, I, or their mixtures) nanocrystals display size- and composition-dependent photoluminescence (PL) emissions from 410 to 700 nm, characterized by ultranarrow emission line widths of 12−42 nm and high PL quantum yields over 90%, which are comparable to or even better than those of well-developed Cd chalcogenide nanocrystals. These fascinating characteristics have raised widespread interest in both the scientific and industrial communities to develop new types of colloidal nanocrystals of metal halide perovskites.18 Concerning lead-based perovskites, various chemical synthetic approaches have so far been reported to synthesize nanocrystals with sizes in the range of several nanometers to tens of nanometers and with variable © 2016 American Chemical Society

Received: April 3, 2016 Revised: October 25, 2016 Published: October 25, 2016 8132

DOI: 10.1021/acs.chemmater.6b01329 Chem. Mater. 2016, 28, 8132−8140

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Figure 1. Synthesis of Cs2SnI6 nanocrystals: (A) schematic of procedures for the controlled synthesis of perovskite Cs2SnI6 nanocrystals (left panel) and photograph of the as-prepared Cs2SnI6 samples under UV light (right panel); (B−F) TEM images of Cs2SnI6 nanocrystals with different shapes (the inset of B gives an HRTEM image of Cs2SnI6 spherical quantum dots); (G) particle size distribution histogram corresponding to (B); (H−J) length−diameter ratio histograms corresponding to (C−E) (the insets to G−J give imaginary lines showing the best-fit Gaussian distributions).

acid and oleylamine in octadecene (ODE) at 220 °C to generate crystalline Cs2SnI6 nanocrystals. Notably, no toxic phosphines, such as tri-n-octylphosphine (TOP) and tributylphosphine (TBP), were used during the synthesis of the nanocrystals. Upon injection of the cesium oleate precursor, a white product was initially observed which later turned dark brown to indicate the formation of nanocrystals. During the course of the reaction, samples were successively collected and cooled to room temperature in an ice−water bath. It was observed that, with the prolongation of reaction time, the fluorescence of the product changed and deepened from bright red to dark red under 365 nm UV light (Figure 1A). In contrast to unstable CsSnX3 nanocrystals, made from SnX2 precursors, the tetravalent Cs 2 SnI 6 nanocrystals, made from SnX 4 precursors, were found to be quite stable under ambient conditions. Moreover, the synthesis of CsSnX3 nanocrystals requires the use of toxic tri-n-octylphosphine (TOP) as a coordinating solvent and the product needs to be purified in an

issue, as Sn4+ is more stable against oxidation in comparison to Sn2+.29,33 Recently, Lee et al. produced a Sn-based perovskite Cs2SnI6 film with higher air stability and good solar cell efficiency.27 To the best of our knowledge, little success has been achieved so far in the preparation of lead-free tetravalent tin Cs2SnI6 nanocrystals. Here we report the first synthesis of well-structured lead-free and stable Cs2SnI6 nanocrystals with different morphologies via a facile phosphine-free hot-injection approach using inexpensive and nontoxic commercial precursors. These Cs2SnI6 nanocrystals are favorable for stable, low-cost, earth-abundant, less toxic or nontoxic, environmentally friendly next-generation solution-processed optoelectronic devices. Using a modification of Protesescu’s CsPbX3 nanocrystal protocol, we synthesized single-crystalline Cs2SnI6 nanocrystals of difference shapes by a facile hot-injection method under airfree conditions, as shown in Figure 1. Typically, cesium oleate reacted with tetravalent tin(IV) iodide in the presence of oleic 8133

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Figure 2. Structural characterizations of Cs2SnI6 nanobelts: (A) large-area TEM image of the nanobelts; (B) atomic resolution STEM-HAADF image of Cs2SnI6 nanobelts; (C) typical EDS mapping with corresponding STEM-HAADF image and elemental maps of Cs, Sn, and I; (D) crystal structure of Cs2SnI6 (cubic, space group Fm3m, a = 11.65 Å), which was obtained by removing half of the Sn atoms at intervals; (E, G) low-voltage atomic resolution STEM-HAADF analysis of the Cs2SnI6 structure and STEM-HAADF images viewed along the [001] and [211] directions, respectively, acquired with an FEI Titan at 60 kV (see more details in the Experimental Section); (F, H) corresponding atomic models (top) and simulated STEM-HAADF images (bottom) viewed along the [001] and [211] directions, respectively, using QSTEM simulation software (see more details in the Experimental Section). In (F), the green balls represent the column containing both Sn and I atoms along the [001] direction, and the purple and blue balls represent Cs and I, respectively. In (H), the pink balls represent the mixture column of Cs and I atoms along the [211] direction, and the gray and purple balls represent Sn and I, respectively.

argon-filled glovebox (O2, H2O < 1 ppm),26 while our Cs2SnI6 nanocrystals were obtained in an oven-dried Schlenk flask under ambient conditions. Samples at different time points were taken and imaged with TEM and HRTEM, as shown in Figure 1. At the initial stage (e.g., 1 min), the shapes of the samples were found to be spherical quantum dots with an average diameter of ∼2.5 nm (see Figure S1 in the Supporting Information). Then nanorods were the dominant products after 5 min of reaction. With the passage of time, the nanorods progressively grew into nanowires and the length/diameter ratio increased from 3 to 28 after a 10 min reaction period. After 30 min of reaction time, the nanowires assembled side by side and finally nanobelts were produced. A similar nanobelt formation process has already been observed by Deng and co-workers in Sb2O3 nanowire and nanobelt synthesis.34 Nanobelts can be transformed into nanoplatelets (NPLs) with a thickness of around 8 nm after 60 min of reaction, as shown in an AFM image (Figure 1F and Figures S2 and S3 in the Supporting Information). The

HRTEM images corroborated the single-crystalline Cs2SnI6 nanostructures, and no phase transformation was observed with an extension of reaction time, as can be seen in Figure S4 in the Supporting Information. We take Cs2SnI6 nanobelts as a model sample for property and detailed structural analysis. Parts A and B of Figure 2 show typical TEM and atomic resolution STEM-HAADF images of the Cs2SnI6 nanobelts. According to the TEM images, nanobelts have an average width of about 30 nm, a thickness of 6−8 nm, and a length of several micrometers (Figure S3 in the Supporting Information). The HRTEM images indicated the single-crystalline nature of Cs2SnI6 nanobelts and length along the [111] direction. Figure 2C suggests a homogeneous distribution of tin, cesium, and iodide in the nanobelts. During microscopic investigations we found Cs2SnI6 nanobelts to be sensitive under the incident beam, and as a result the structure of the nanobelts was damaged after one frame of the STEMHAADF image. A similar phenomenon was also previously observed in methylammonium lead iodide perovskite struc8134

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Figure 3. (A) X-ray diffraction (XRD) patterns of Cs2SnI6 nanobelts (black) measured under ambient conditions and the standard pattern of bulk Cs2SnI6 (red). (B) Survey X-ray photoelectron spectrum (XPS) of the Cs2SnI6 nanobelts measured under ambient conditions. (C, D) Highresolution XPS spectra and peak fitting for Sn 3d and I 3d, respectively. The two peaks are from the spin−orbit splitting corresponding to electrons from the 3d5/2 and 3d3/2 states.

tures.35 Time-dependent TEM analysis also aided in investigating the stability of Cs2SnI6 nanostructures, and no change in the morphology of perovskite was observed within 1 week in an ambient environment (∼28 °C with 60% relative humidity) (Figure S5 in the Supporting Information). In order to avoid electron beam damage, we used spherical aberration corrected scanning transmission electron microscopy (Cs-corrected STEM) at a low voltage of 60 kV. Figure 2D depicts a ball-and-stick model of the antifluorite structure of Cs2SnI6 (space group Fm3m, a = 11.65 Å). From a structural viewpoint, Cs2SnI6 crystallizes in the face-centeredcubic (fcc) structure and the unit cell is composed of four [SnI6]2− octahedra at the corners and the face centers and eight Cs+ cations at the tetragonal interstitials. The Cs2SnI6 structure is a perovskite derivative and is obtained by removing half of the Sn atoms at each center of the [SnI6] octahedron at intervals (i.e., the compound can also be written as CsSn0.5I3, with a structure consisting of isolated SnI64− octahedra).29 Figure 2E,G and Figure S6 in the Supporting Information illustrate STEM-HAADF images from two orientations of the nanobelts, which are consistent with Cs2SnI6 structure (JCPDS Card No. 51-0466). The interplanar distance of 2.91 Å in Figure 2E matches well with that of the (004) plane in Cs2SnI6 (d(004) = 2.911 Å). Thus, Figure 2E is acquired along the [001] direction by a Weiss zone law. Similarly, the interplanar distances of 4.12 and 3.36 Å in Figure 2G match with those of the (0−22) and (2−2−2) planes in Cs2SnI6 (d(0−22) = 4.117 Å

and d(2−2−2) = 3.362 Å). Thus, Figure 2G is acquired along the [211] direction by a Weiss zone law. Figure 2F,H shows the simulated STEM-HAADF images, which matched the experimental results in Figure 2E,G, respectively. Due to identical atomic numbers of Sn, I, and Cs atoms (ZSn = 50, ZI = 53, and ZCs = 55), the contrast variations among different atomic columns are rather low in the STEM-HAADF images. Notably, the apparent difference between bright column positions in the STEM-HAADF image (Figure 2G) and the atomic model (Figure 2H) is along the [211] direction. Such contrast delocalization during STEM-HAADF imaging is due to the highly defocused probe with a volcano shape,36,37 which is confirmed with the simulated HAADF image (Figure S7 in the Supporting Information). From all the analyses above, the nanobelts were confirmed to be of Cs2SnI6 perovskite structure. The crystal structure of the Cs2SnI6 nanobelts was further characterized by powder X-ray diffraction (Figure 3A). XRD data indicate that the phase structure of the Cs2SnI6 nanobelts adopts the perovskite structure, and the obvious diffraction peaks can be indexed at 2θ = 26.49, 30.69, 43.95, 50.06, 54.56°, which correspond to diffractions from the {222}, {004}, {044}, {226}, and {444} planes. Moreover, these results revealed that Cs2SnI6 crystallizes in the cubic Fm3̅m(225) space group under ambient conditions (JCPDS No. 51-0466), which is consistent with the TEM and STEM observations in Figure 2. Additionally, with respect to the stability of Cs2SnI6, XRD analysis has been performed as a function of time; no obvious 8135

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Figure 4. (A) UV−vis absorption and photoluminescence (PL) emission spectra of the as-synthesized Cs2SnI6 nanobelts measured in mother liquor under ambient conditions. The inset to (A) gives a photograph of the colloidal solution of the nanobelts under UV light (365 nm). (B) Threedimensional excitation−emission matrix (EEM) fluorescence spectroscopy of the Cs2SnI6 nanobelts in mother solution.

Figure 5. (A) Schematic illustration of the field effect transistor device configuration using Cs2SnI6 nanobelts as the channel materials measured under ambient conditions. (B) Linear plot of the I−V characteristics at a drain-source voltage (Vds = 10 mV). Black and red lines are positive scan and negative scans, respectively. The inset to (B) gives the Ids−Vgs curve on the natural logarithmic scale.

temperature (>240 °C), only bulk crystals were produced. When investigating the influence of ligands, we found that when oleic acid was used as the sole ligand (without oleylamine), no Cs2SnI6 nanocrystals were obtained, whereas the exclusive use of oleylamine (without oleic acid) only generated bulk Cs2SnI6 crystals. Thus, we assume that oleylamine acts as a complexing agent for Sn4+ ions, while oleic acid plays a role in suppressing Cs2SnI6 nanocrystal growth. These results implied that both oleic acid and oleylamine are required in the formation process of Cs2SnI6 nanobelts, suggesting an oleic acid/oleylamine cooperatively controlled crystallization mechanism.40,42,43 The optical properties of as-synthesized Cs2SnI6 nanobelts were investigated by UV−vis absorption and photoluminescence (PL) spectroscopy under an ambient atmosphere (Figure 4A). The PL emission spectrum of Cs2SnI6 shows a peak around 620 nm (2.0 eV) with full width at half-maximum of 49 nm (0.16 eV), which suggested a significant blue shift due to quantum confinement effects in comparison with bulk Cs2SnI6 crystals with a band gap of 1.3 eV,27,29 thus indicating the formation of Cs2SnI6 nanocrystals. The Cs2SnI6 nanobelts were further analyzed using three-dimensional excitation−emission matrix (EEM) fluorescence spectroscopy (Figure 4B). The PL emission peak displays no shift with a change in the excitation

reduction in the intensity of diffraction peaks was observed at different points of time (∼28 °C with 60% relative humidity). Moreover, we also did not find any phase transition related peaks within 1 week (Figure S8 in the Supporting Information). Thus, the stability of Cs2SnI6 is found to be significantly higher than that of Sn2+ perovskite compounds.26,30,38 The surface chemistry of Cs2SnI6 was investigated by X-ray photoemission spectroscopy (XPS). The presence of tin and iodine is clearly discernible in survey XPS spectra (Figure 3B−D and Figures S9 and S10 in the Supporting Information). In high-resolution XPS spectra of tin, the 3d5/2 region showed a single peak at a binding energy of 486.6 eV. Favorably, the higher oxidation state (+4) of tin in our nanocrystals probably contributes to the air stability of Cs2SnI6 nanobelts in an ambient environment. A comparison with the literature, the use of an Sn(IV) salt as the precursor, and XRD and XPS measurements indicate that our product most likely consists of Sn(IV).26,39−41 In order to better understand the growth mechanism of the Cs2SnI6 nanobelts, we investigated the effect of reaction temperature and organic ligands on the morphology of the product. We found that ∼220 °C is an appropriate temperature for the nucleation and growth of Cs2SnI6 nanocrystals. At lower temperature (104). According to the literature, the well-studied PV perovskite CH3NH3PbI345 is reported to have p-type conduction, while the thin film29 of Cs 2SnI 6 exhibited n-type conduction. In addition, the polycrystalline pellet of Cs2SnI6 annealed at 200 °C yielded an n-type semiconductor with a high electron mobility of 310 cm2/(V s) and a carrier concentration of ∼1 × 1014 cm−3.27 Here, the transconductance, gm = dIds/dVgs, was acquired by fitting the linear region of the Ids versus Vgs curve. The fieldeffect mobility of this device was estimated on the basis of equations in the literature.46,47 The calculated hole mobility was found to be 20.2 cm2 (V s), while the calculated hole concentration was 9.1 × 1018 cm−3 (see more details in the Experimental Section). These devices can be stored under ambient conditions for at least 2 weeks without significantly changing the performance. Moreover, the hole mobility of our nanobelt-based FET device is much higher than those of the recently reported PbSe, CdSe/ZnS, and ZnTe colloidal nanocrystal based devices48−50 and is comparable to the best hole mobility of the organic FETs.51 In summary, we developed a facile solution-phase hotinjection approach for shape-controlled synthesis of lead-free and stable single-crystalline perovskite derivative Cs2SnI6 nanocrystals, such as spherical quantum dots, nanorods,

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EXPERIMENTAL SECTION

Chemicals. Cs2CO3 (Aladdin, 99.9%), tetravalent tin(IV) iodide (Aladdin, 99.0%), octadecene (ODE, Aladdin, 90%), oleic acid (CH3(CH2)7CHCH(CH2)7COOH, 90%, Aladdin, AR), oleylamine (OLA, Aldrich, 70%), hexane (99.9%, Sinopharm Chemical Reagent Co., Ltd., China), and toluene (99.9%, Sinopharm Chemical Reagent Co., Ltd., China) were purchased and used without further purification. Preparation of Cs−Oleate. Cs2CO3 (0.814 g) was loaded into a 100 mL three-neck flask along with octadecene (10 mL, ODE) and oleic acid (2.5 mL), dried for 1 h at 120 °C, and then heated under N2 to 150 °C until all of the Cs2CO3 reacted with oleic acid. Since Cs oleate precipitates out of ODE at room temperature, it has to be preheated (e.g., 100 °C) before use. Synthesis of Cs2SnI6 Nanocrystals. In a typical synthesis, 7 mL of ODE and 0.18 mmol of SnI4 were loaded into a three-neck flask and degassed under vacuum for 1 h at 100 °C. Prior to injection, a mixture of OLA (0.5 mL) and (OA) 0.5 mL was heated to 120 °C under vacuum for 1 h. The temperature was then elevated to 220 °C and 0.4 mL of as-prepared Cs oleate solution (0.4 M) was swiftly injected with vigorous stirring. The color of the solution instantaneously changed from transparent red to dark brown, indicating the formation of nanocrystals. After a specific reaction time (e.g., 1, 5, 10, 30, and 60 min), the reaction mixture was cooled by an ice−water bath. The crude solution was then purified with a mixture of toluene and hexane (1/1 v/v) under ambient conditions. No argon-filled glovebox was needed during the synthesis and purification processes. Atomic-Resolution STEM-HAADF. Atomic resolution STEMHAADF imaging was performed using a double aberration corrected FEI Titan3 G2 60-300 instrument at Nanjing University, Nanjing, People’s Republic of China. To alleviate beam damage, a low accelerating voltage of 60 kV was also employed. The semi-angle of the probe-forming aperture was about 18 mrad. A 0.134 nm spatial resolution can be routinely achieved. The inner and outer semi-angles of the HAADF detector were about 46 and 154 mrad, respectively, and the acquisition time was chosen to be 10 μs per pixel. The images shown in the main text were filtered through standard Wiener deconvolution to remove the background noise for a better display (see raw images in Figure S6 in the Supporting Information). STEM Simulations. STEM-HAADF image simulations were calculated using the multislice based simulation software QSTEM. All of the input parameters, including probe size, convergence angle, and collection angles of the HAADF detector, were set according to the experimental conditions. The probe array was 400 × 400 pixels with 0.1 Å resolution. For the simulation of the [001] direction as shown in Figure 2E, the sample thickness and defocus were 14 and 15 nm, respectively. For the simulation of the [211] direction as shown in 8137

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Chemistry of Materials Figure 2G, the sample thickness and defocus were 9 and −25 nm, respectively. A source size of 0.1 nm was used. For allof the simulations, no aberrations were taken into account. Field Effect Transistor Devices. Solution-processed field effect transistors were fabricated by drop-casting the concentrated Cs2SnI6 nanocrystal solution onto a clean silicon wafer with a 500 nm SiO2 layer and prepatterned gold as the electrodes. All of the measurements were carried out in an air atmosphere with 50% humidity under ambient conditions. The transconductance gm = dIds/ dVgs was acquired by fitting the linear region of the Ids versus Vgs curve. The field-effect mobility of this device can be estimated on the basis of the equation46,47 μ=

L gm WCoxVds

ACKNOWLEDGMENTS

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REFERENCES

(1) Stoumpos, C. C.; Kanatzidis, M. G. The Renaissance of Halide Perovskites and Their Evolution as Emerging Semiconductors. Acc. Chem. Res. 2015, 48, 2791−2802. (2) Zhao, Y.; Zhu, K. Organic-inorganic hybrid lead halide perovskites for optoelectronic and electronic applications. Chem. Soc. Rev. 2016, 45, 655−689. (3) 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. (4) Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 338, 643−647. (5) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, 395−398. (6) Zhou, H.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z.; You, J.; Liu, Y.; Yang, Y. Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, 542−546. (7) Nie, W.; Tsai, H.; Asadpour, R.; Blancon, J.-C.; Neukirch, A. J.; Gupta, G.; Crochet, J. J.; Chhowalla, M.; Tretiak, S.; Alam, M. A.; Wang, H.-L.; Mohite, A. D. High-efficiency solution-processed perovskite solar cells with millimeter-scale grains. Science 2015, 347, 522−525. (8) Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I. Solvent engineering for high-performance inorganic−organic hybrid perovskite solar cells. Nat. Mater. 2014, 13, 897−903. (9) Park, Y. S.; Guo, S.; Makarov, N.; Klimov, V. I. Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots. ACS Nano 2015, 9, 10386−10393. (10) 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. (11) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348, 1234−1237. (12) Etgar, L.; Gao, P.; Xue, Z.; Peng, Q.; Chandiran, A. K.; Liu, B.; Nazeeruddin, M. K.; Graetzel, M. Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells. J. Am. Chem. Soc. 2012, 134, 17396− 17399. (13) Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Graetzel, M. Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, 316−319. (14) Snaith, H. J. Perovskites: The Emergence of a New Era for LowCost, High-Efficiency Solar Cells. J. Phys. Chem. Lett. 2013, 4, 3623− 3630. (15) Dar, M. I.; Abdi-Jalebi, M.; Arora, N.; Moehl, T.; Graetzel, M.; Nazeeruddin, M. K. Understanding the Impact of Bromide on the Photovoltaic Performance of CH3NH3PbI3 Solar Cells. Adv. Mater. 2015, 27, 7221−7228.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b01329. Additional AFM, HRTEM, and TEM images, XPS, XRD, PL decay, UV−vis, PL spectrum, and Ids−Vgs curves at different Vds results (PDF)

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We appreciate helpful discussions with Prof. Jing Kong (Massachusetts Institute of Technology). This work was supported by the Thousand Talents Program for Young Researchers, the National Basic Research Program of China (Grant No. 2015CB654901), the National Natural Science Foundation of China (Grant Nos. 51502130, 11474147), the Natural Science Foundation of Jiangsu Province (Grant Nos. SBK2015043303, BK20151383), the Shuangchuang Program of Jiangsu Province, the International Science & Technology Cooperation Program of China (Grant No. 2014DFE00200), and Fundamental Research Funds for Central Universities.

where the channel length L is 5.5 mm, the channel width W is 2.5 mm, the capacitance Cox (Cox = ε0εr/d, ε0 = 8.854 × 10−12 F m−1, εr for SiO2 is 3.9, and d is the thickness of SiO2, which is 500 nm), and Vds = 200 mV. The calculated mobility (μ) of the device is 20.2 cm2 V−1 s−1. The hole concentration in the nanoribbon can be estimated by nh= σ/eμ, the conductivity σ = RS/L, where R = (0.2 V)/(1.0 × 10−6 A) = 0.2 × 106 Ω, S is the vertical cross-section area (=2.5 mm × 100 nm = 2.5 × 10−6 cm2), L is the length between the electrodes (5.5 mm), e is the charge of an electron (1.6 × 10−19 C), and μ is the calculated hole mobility (20.2 cm2 V−1 s−1). The calculated hole concentration is 0.91 × 1019 cm−3. Characterization Details. Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 F20 electron microscope operating at 200 kV. The low-voltage high-resolution STEM investigation was carried out on a double aberration corrected TitanTM cubed G2 60-300 S/TEM equipped with Super-XTM technology. The available point resolution is about 0.1 nm at an operating voltage of 60 kV. X-ray powder diffraction (XRD) was measured with a Bruker AXS D8 X-ray diffractometer equipped with monochromated Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was performed using an achromatic Al Kα source (1486.6 eV) and a double pass cylindrical mirror analyzer (ULVACPHI 5000 VersaProbe). Fourier transform infrared (FTIR) spectra were recorded with a Varian 3100 FTIR spectrometer at room temperature. Ultraviolet and visible absorption (UV−vis) spectra were recorded with a Shimadzu UV-3600 plus spectrophotometer under ambient conditions. The photoluminescence (PL) spectra from the mother liquor of Cs2SnI6 nanocrystals were obtained with a Horiba PTI QuantaMaster 400 steady-state fluorescence system or with a homemade fiber fluorimeter system from Thorlabs operating under ambient conditions.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Z.D.: [email protected]. *E-mail for P.W.: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 8138

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Efficiency in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2016, 120, 11777. (32) Chung, I.; Lee, B.; He, J.; Chang, R. P. H.; Kanatzidis, M. G. Allsolid-state dye-sensitized solar cells with high efficiency. Nature 2012, 485, 486−489. (33) Xiao, Z.; Zhou, Y.; Hosono, H.; Kamiya, T. Intrinsic defects in a photovoltaic perovskite variant Cs2SnI6. Phys. Chem. Chem. Phys. 2015, 17, 18900−18903. (34) Deng, Z.; Chen, D.; Tang, F.; Meng, X.; Ren, J.; Zhang, L. Orientated Attachment Assisted Self-Assembly of Sb2O3 Nanorods and Nanowires: End-to-End versus Side-by-Side. J. Phys. Chem. C 2007, 111, 5325−5330. (35) Yuan, H.; Debroye, E.; Janssen, K.; Naiki, H.; Steuwe, C.; Lu, G.; Moris, M.; Orgiu, E.; Uji-i, H.; De Schryver, F.; Samorì, P.; Hofkens, J.; Roeffaers, M. Degradation of Methylammonium Lead Iodide Perovskite Structures through Light and Electron Beam Driven Ion Migration. J. Phys. Chem. Lett. 2016, 7, 561−566. (36) Mkhoyan, K. A.; Maccagnano-Zacher, S. E.; Kirkland, E. J.; Silcox, J. Effects of amorphous layers on ADF-STEM imaging. Ultramicroscopy 2008, 108, 791−803. (37) Lupini, A. R.; Pennycook, S. J. Localization in elastic and inelastic scattering. Ultramicroscopy 2003, 96, 313−322. (38) Leijtens, T.; Eperon, G. E.; Noel, N. K.; Habisreutinger, S. N.; Petrozza, A.; Snaith, H. J. Stability of Metal Halide Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1500963. (39) Deng, Z.; Peng, B.; Chen, D.; Tang, F.; Muscat, A. J. A New Route to Self-Assembled Tin Dioxide Nanospheres: Fabrication and Characterization. Langmuir 2008, 24, 11089−11095. (40) Xu, X.; Zhuang, J.; Wang, X. SnO2 Quantum Dots and Quantum Wires: Controllable Synthesis, Self-Assembled 2D Architectures, and Gas-Sensing Properties. J. Am. Chem. Soc. 2008, 130, 12527−12535. (41) Marshall, K. P.; Walton, R. I.; Hatton, R. A. Tin perovskite/ fullerene planar layer photovoltaics: improving the efficiency and stability of lead-free devices. J. Mater. Chem. A 2015, 3, 11631−11640. (42) Bu, W.; Chen, Z.; Chen, F.; Shi, J. Oleic Acid/Oleylamine Cooperative-Controlled Crystallization Mechanism for Monodisperse Tetragonal Bipyramid NaLa(MoO4)2 Nanocrystals. J. Phys. Chem. C 2009, 113, 12176−12185. (43) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Rare-earth oxide nanopolyhedra, nanoplates, and nanodisks. Angew. Chem., Int. Ed. 2005, 44, 3256−3260. (44) Xu, P.; Chen, S.; Xiang, H.-J.; Gong, X.-G.; Wei, S.-H. Influence of Defects and Synthesis Conditions on the Photovoltaic Performance of Perovskite Semiconductor CsSnI3. Chem. Mater. 2014, 26, 6068− 6072. (45) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole diffusion lengths > 175 μm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967−970. (46) Deng, Z.; Cao, D.; He, J.; Lin, S.; Lindsay, S. M.; Liu, Y. Solution Synthesis of Ultrathin Single-Crystalline SnS Nanoribbons for Photodetectors via Phase Transition and Surface Processing. ACS Nano 2012, 6, 6197−6207. (47) Yin, Z. Y.; Li, H.; Li, H.; Jiang, L.; Shi, Y. M.; Sun, Y. H.; Lu, G.; Zhang, Q.; Chen, X. D.; Zhang, H. Single-Layer MoS2 Phototransistors. ACS Nano 2012, 6, 74−80. (48) Zhang, J.; Chen, P.-C.; Shen, G.; He, J.; Kumbhar, A.; Zhou, C.; Fang, J. p-Type Field-Effect Transistors of Single-Crystal Zinc Telluride Nanobelts. Angew. Chem., Int. Ed. 2008, 47, 9469−9471. (49) Rosen, E. L.; Buonsanti, R.; Llordes, A.; Sawvel, A. M.; Milliron, D. J.; Helms, B. A. Exceptionally Mild Reactive Stripping of Native Ligands from Nanocrystal Surfaces by Using Meerwein’s Salt. Angew. Chem., Int. Ed. 2012, 51, 684−689. (50) Woo, J. Y.; Lee, S.; Lee, S.; Kim, W. D.; Lee, K.; Kim, K.; An, H. J.; Lee, D. C.; Jeong, S. Air-Stable PbSe Nanocrystals Passivated by Phosphonic Acids. J. Am. Chem. Soc. 2016, 138, 876−883. (51) Wang, C. L.; Dong, H. L.; Hu, W. P.; Liu, Y. Q.; Zhu, D. B. Semiconducting pi-Conjugated Systems in Field-Effect Transistors: A

(16) 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. (17) Tian, Y.; Merdasa, A.; Unger, E.; Abdellah, M.; Zheng, K.; McKibbin, S.; Mikkelsen, A.; Pullerits, T.; Yartsev, A.; Sundstrom, V.; Scheblykin, I. G. Enhanced Organo-Metal Halide Perovskite Photoluminescence from Nanosized Defect-Free Crystallites and Emitting Sites. J. Phys. Chem. Lett. 2015, 6, 4171−4177. (18) 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 (CsPbX(3), X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692−3696. (19) 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. (20) 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. (21) Akkerman, Q. A.; Motti, S. G.; Srimath Kandada, A. R.; Mosconi, E.; D’Innocenzo, V.; Bertoni, G.; Marras, S.; Kamino, B. A.; Miranda, L.; De Angelis, F.; Petrozza, A.; Prato, M.; Manna, L. Solution Synthesis Approach to Colloidal Cesium Lead Halide Perovskite Nanoplatelets with Monolayer-Level Thickness Control. J. Am. Chem. Soc. 2016, 138, 1010−1016. (22) 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. (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 CH3NH3PbX3 (X = Br, I, Cl) Quantum Dots: Potential Alternatives for Display Technology. ACS Nano 2015, 9, 4533−42. (24) Huang, H.; Susha, A. S.; Kershaw, S. V.; Hung, T. F.; Rogach, A. L. Control of Emission Color of High Quantum Yield CH3 NH3 PbBr3 Perovskite Quantum Dots by Precipitation Temperature. Adv. Sci. 2015, 2, 1500194. (25) Hao, F.; Stoumpos, C. C.; Guo, P.; Zhou, N.; Marks, T. J.; Chang, R. P. H.; Kanatzidis, M. G. Solvent-Mediated Crystallization of CH3NH3SnI3 Films for Heterojunction Depleted Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137, 11445−11452. (26) Jellicoe, T. C.; Richter, J. M.; Glass, H. F.; Tabachnyk, M.; Brady, R.; Dutton, S. E.; Rao, A.; Friend, R. H.; Credgington, D.; Greenham, N. C.; Bohm, M. L. Synthesis and Optical Properties of Lead-Free Cesium Tin Halide Perovskite Nanocrystals. J. Am. Chem. Soc. 2016, 138, 2941−4. (27) Lee, B.; Stoumpos, C. C.; Zhou, N.; Hao, F.; Malliakas, C.; Yeh, C.-Y.; Marks, T. J.; Kanatzidis, M. G.; Chang, R. P. H. Air-Stable Molecular Semiconducting Iodosalts for Solar Cell Applications: Cs2SnI6as a Hole Conductor. J. Am. Chem. Soc. 2014, 136, 15379− 15385. (28) Lee, S. J.; Shin, S. S.; Kim, Y. C.; Kim, D.; Ahn, T. K.; Noh, J. H.; Seo, J.; Seok, S. I. Fabrication of Efficient Formamidinium Tin Iodide Perovskite Solar Cells through SnF2−Pyrazine Complex. J. Am. Chem. Soc. 2016, 138, 3974−3977. (29) Saparov, B.; Sun, J.-P.; Meng, W.; Xiao, Z.; Duan, H.-S.; Gunawan, O.; Shin, D.; Hill, I. G.; Yan, Y.; Mitzi, D. B. Thin-Film Deposition and Characterization of a Sn-Deficient Perovskite Derivative Cs2SnI6. Chem. Mater. 2016, 28, 2315−2322. (30) Chung, I.; Song, J. H.; Im, J.; Androulakis, J.; Malliakas, C. D.; Li, H.; Freeman, A. J.; Kenney, J. T.; Kanatzidis, M. G. CsSnI3: Semiconductor or metal? High electrical conductivity and strong nearinfrared photoluminescence from a single material. High hole mobility and phase-transitions. J. Am. Chem. Soc. 2012, 134, 8579−87. (31) Kaltzoglou, A.; Antoniadou, M.; Kontos, A. G.; Stoumpos, C. C.; Perganti, D.; Siranidi, E.; Raptis, V.; Trohidou, K. N.; Psycharis, V.; Kanatzidis, M. G.; Falaras, P. Optical-Vibrational Properties of the Cs2SnX6(X = Cl, Br, I) Defect Perovskites and Hole-Transport 8139

DOI: 10.1021/acs.chemmater.6b01329 Chem. Mater. 2016, 28, 8132−8140

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Chemistry of Materials Material Odyssey of Organic Electronics. Chem. Rev. 2012, 112, 2208− 2267.

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DOI: 10.1021/acs.chemmater.6b01329 Chem. Mater. 2016, 28, 8132−8140