Controllable Growth of Lead-Free All-Inorganic Perovskite Nanowire

Jun 23, 2019 - Controllable Growth of Lead-Free All-Inorganic Perovskite Nanowire Array with .... of the as-fabricated CsSnBr3 NW array photodetector ...
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Article Cite This: J. Phys. Chem. C 2019, 123, 17566−17573

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Controllable Growth of Lead-Free All-Inorganic Perovskite Nanowire Array with Fast and Stable Near-Infrared Photodetection Mingming Han,†,‡,∇ Jiamin Sun,†,‡,∇ Meng Peng,§ Ning Han,∥ Zhihao Chen,⊥ Dong Liu,‡,⊥ Yanan Guo,‡ Shuai Zhao,‡ Chongxin Shan,# Tao Xu,¶ Xiaotao Hao,⊥ Weida Hu,§ and Zai-xing Yang*,†,‡ †

Shenzhen Research Institute of Shandong University, Shenzhen 518057, China School of Microelectronics, and ⊥School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China § State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China ∥ State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China # Henan Key Laboratory of Diamond Optoelectronic Materials and Devices, School of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, China ¶ SEU-FEI Nano-Pico Center, Key Lab of MEMS of Ministry of Education, Southeast University, Nanjing 210096, China

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‡

S Supporting Information *

ABSTRACT: Low-dimensional all-inorganic metal halide perovskites have been demonstrated as excellent building blocks for high-performance optoelectronic devices. Although many progresses have been achieved in low-dimensional all-inorganic perovskites, the substitution of toxic Pb is urgent for further optoelectronic applications. Here, we present the growth of lead-free all-inorganic CsSnX3 (X = Cl, Br, and I) perovskite nanowire (NW) arrays on a mica substrate by a solid-source chemical vapor deposition method. All of the lead-free all-inorganic CsSnX3 perovskite NW arrays epitaxially grow on the mica substrate to form equilateral triangles. The band gaps of the as-prepared CsSnX3 perovskite NW arrays decrease from 1.84 to 1.34 eV with X changes from Br to I. The high crystallinity is confirmed by the strong photoluminescence (PL) emission peaks and uniform twodimensional PL mapping images. In the end, the as-prepared high-quality CsSnI3 perovskite NW array is then configured into a near-infrared photodetector for the first time, exhibiting fast rise and decay time constants of 83.8 and 243.4 ms, respectively. All of the results present an important advance in the field of low-dimensional all-inorganic perovskites.

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INTRODUCTION

developed well for the controllable growth of low-dimensional perovskites.15−19,21−25 For example, Li et al. reported a solution method of solvothermal to realize the shape and phase control of low-dimensional CsPbX3 (X = Cl, Br, and I) nanocrystals.24 Furthermore, Y phase CsPbxSn1−xI3 alloy NWs were prepared by a simple solution-phase technology by Lei et al., with tunable band gap and electrical conductivity.25 In case of vapor-phase synthesis, chemical vapor deposition (CVD) and van der Waals epitaxy growth technologies have been adopted for the precise control over morphology, composition, and crystallinity of low-dimensional perovskites.15,17−19 Generally, a two-step vapor-phase synthesis

In the past decade, metal halide perovskites have attracted great research attention in solar cell technology, demonstrating an amazing fast progress in power conversion efficiency (PCE).1−14 Until now, the highest PCE of 23.7% is achieved for the perovskite solar cells,14 attributing to their high absorption coefficient, low density of defect states, broad light absorption spectra, and long diffusion length.11−13 These advantages also make the low-dimensional perovskites of nanowires (NWs) and nanosheets (NSs) as research stars in the past 5 years, exhibiting promising potential applications in next-generation optical and optoelectronic devices.15−19 As same as the exploration of other new functional materials, controllable growth of high-quality low-dimensional perovskites was full of challenges at the beginning stage.20,21 Vaporphase and solution-phase synthesis approaches have been © 2019 American Chemical Society

Received: April 9, 2019 Revised: June 21, 2019 Published: June 23, 2019 17566

DOI: 10.1021/acs.jpcc.9b03289 J. Phys. Chem. C 2019, 123, 17566−17573

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The Journal of Physical Chemistry C

Figure 1. Controllable growth of lead-free all-inorganic perovskite NW arrays. The optical images of the as-grown (a) CsSnCl3, (b) CsSnBr3, (c) CsSnI3, and (d) CsSnBrI2 NW array; (e) growth rates of CsSnX3 NWs; and (f) EDXS spectra of as-grown NW arrays. All the scale bars are 100 μm.

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EXPERIMENTAL SECTION Controllable Growth of Lead-Free All-Inorganic CsSnX3 NW Arrays. The lead-free all-inorganic CsSnX3 NW arrays studied here are synthesized by employing a solid-source CVD method in a three-zone horizontal tube furnace. The diagram of the growth setups can be found in Figure S1. Quartz tube with a diameter of 25 mm is used for the growth of CsSnX3 NWs. The solid source of SnX2 powders is located at the upstream zone, and CsX powders are located at the midstream zone. The growth substrates of freshly cleaved fluorophlogopite mica [KMg3(AlSi3O10)F2] are located at the downstream zone. The distances between the precursors of SnX2 and CsX and the mica substrate are 38 and 16 cm, respectively. Argon (99.999% purity) is used as the carrier gas to transport the vapor-phase precursors. Prior to heating, the pressure of the quartz tube is pumped to 6 × 10−3 Torr and then the growth chamber is purged with 50 standard cubic centimeter (sccm) Ar for 20 min. As soon as the fresh mica slice is heated to a stable growth temperature of 200−260 °C, SnX2 and CsX powders are heated to 220−350 and 610−670 °C in 7 min, respectively. With the same carrier gas flow, all of the growth pressures are 0.16 Torr. After the growth of 20 min, the source and substrate heater are stopped together and cooled to room temperature under the Ar flow. Material Characterization. The surface morphologies of the as-grown lead-free all-inorganic CsSnX3 perovskite NW arrays are examined using an optical microscope (Nikon Eclipse). Elemental identifications are performed using energydispersive X-ray spectroscopy (EDXS) attached to a scanning electron microscope (Nova NanoSEM 450, FEI Company). The crystal structure and crystallinity of the products are verified by X-ray diffraction (XRD, D8 ADVANCE, Bruker). PL spectra followed by two-dimensional mappings are measured by using a Raman spectrometer (inVia Reflex, Renishaw) with 532 nm laser. Photodetection Device Fabrication and Photoelectrical Property Measurements. The photodetection devices are fabricated directly on the growth substrate of mica. A stainless steel grid is used as the shadow mask and attaches to the NW arrays. Then, the electoral contact electrodes of 2 nm Ti and 50 nm Au thin films are deposited by electron beam evaporation. For keeping away from water and oxygen, the asfabricated photodetection devices spin with the protection layers of polymethyl methacrylate (PMMA).

process was needed for the growth of low-dimensional organic−inorganic perovskites.26,27 Namely, inorganic precursors with controlled low-dimensional nanostructures were prepared first,28 followed by a second conversion reaction process with an inorganic source.26 On the other hand, with higher stability and better crystallinity and performance, the attractive low-dimensional all-inorganic perovskites have also been prepared by vapor-phase synthesis technology.29,30 In fact, since Park et al. first prepared CsPbX3 NWs by chemical vapor transport method in 2016,31 low-dimensional allinorganic perovskites have taken over the stage from organic−inorganic hybrid perovskites gradually and dominated quickly the optoelectronic applications of solar cells, photodetectors, lasers, light-emitt ing diodes , and so forth.17,22,29,32−39 Although many progresses have been achieved in lowdimensional all-inorganic perovskites, the substitution of toxic Pb is urgent for the further optoelectronic applications.35,40 Fortunately, Sn, Ge, Bi, Sb, and so forth have been proved as nontoxic replacement elements in all-inorganic perovskites.35,41−44 However, it is rare to prepare low-dimensional lead-free all-inorganic CsSnX3 (X = Cl, Br, and I) perovskites during a vapor-phase synthesis process in the literature.35 Significantly, with the substitution of Pb by Sn, the band gap of CsSnX3 can decrease to 1.3 eV, indicating the born channel material for near-infrared photodetectors.25 In this work, leadfree all-inorganic CsSnX3 NW arrays are successfully prepared on mica in a three-zone CVD furnace. The composition, diameter, and length of CsSnX3 NWs are controlled well by source kinds, source mass, source temperature, and growth time. In the photoluminescence (PL) measurements, the asprepared lead-free all-inorganic CsSnBr3−xIx (x = 0, 2, 3) NW arrays show very strong and sharp PL spectra, with broad tunable band gaps from 1.34 to 1.84 eV. Importantly, with a narrow band gap of 1.34 eV, the as-prepared CsSnI3 NW arrays are used first as the channel material for near-infrared photodetectors, displaying an impressive photodetection performance of high stability and fast response with the rise and decay time constants of 83.8 and 243.4 ms, respectively, under 940 nm infrared illumination with a bias voltage of 0.1 V. All of the results indicate the potential applications of the asprepared high-quality lead-free all-inorganic perovskite NW arrays for next-generation photodetection, light-emitting diodes, and energy harvest. 17567

DOI: 10.1021/acs.jpcc.9b03289 J. Phys. Chem. C 2019, 123, 17566−17573

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The Journal of Physical Chemistry C With a band gap of 1.84 eV, the CsSnBr3 NW array photodetection device is illuminated under the lasers of 445, 532, 635, and 730 nm, and the corresponding photoelectrical performances are characterized by a standard Lakeshore electrical probe station and a Keithley 4200 semiconductor analyzer. The power of the incident irradiation can be tuned from 2.55 to 50.96 mW cm−2. On the other hand, with a narrow band gap of 1.3 eV, the CsSnI3 NW array photodetection device is demonstrated first as a room-temperature near-infrared photodetector in this work. In order to measure the near-infrared photoresponse, the CsSnI3 NW array photodetector is wire-bonded into 24-pin chip carriers and then evaluated in detail at a number of discrete wavelengths from 450 to 940 nm with the same light intensity of 50 mW mm−2. All the photodetector measurements are taken in a vacuum environment.

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RESULTS AND DISCUSSION Mica is a universally acknowledged excellent substrate for the controllable growth of low-dimensional nanostructures,

Figure 3. Optical property of the as-prepared lead-free all-inorganic perovskite NW arrays. (a) PL spectra of the as-grown CsSnBr3−xIx (x = 0, 1, 2, and 3) NW arrays. (b−d) Two-dimensional PL mapping images of CsSnBr3, CsSnBrI2, and CsSnI3, respectively.

Figure 2. Possible growth mechanism of the as-prepared lead-free allinorganic perovskite NW array. (a−c) SEM images of CsSnBr3 NW array grown at 260 °C for 1, 2, and 3 min, respectively. (d) Possible growth schematic illustration of CsSnX3 NW arrays. All the scale bars are 5 μm.

Figure 4. Near-infrared photodetection performance of as-fabricated CsSnI3 NW array photodetector. (a) Spectral response of the photodetector at a number of discrete illumination wavelengths from 475 to 940 nm under a light intensity of 50 mW mm−2 and at a bias voltage of 0.1 V; (b) time−response curves of CsSnI3 NW array photodetector irradiated at 940 nm with respect to the light intensities; (c) rise time and decay time constants; (d) responsivity and detectivity of CsSnI3 NW array photodetector at various light power intensities.

especially in the present of hot research fields of perovskites and transition-metal dichalcogenides, owing to its atomic flatness, surface inertness, together with the hexagonally arranged in-plane lattice characteristics.45−47 In this work, mica is also adopted as a growth substrate for the synthesis of lead-free all-inorganic CsSnX3 perovskite NW arrays in a threezone horizontal tube furnace. During this vapor growth process, the pressure of growth chamber, the heating temperature, the flow of carrier gas, the kinds and mass of source materials can be controlled accurately. With the optimal growth conditions, lead-free all-inorganic CsSnX3 perovskite NW arrays can be successfully prepared on mica here. In fact, a glass slice and a Si wafer with 50 nm SiO2 are also used for the growth of CsSnBr3 NW array, using the same optimal growth conditions of CsSnBr3 NWs grown on mica. As shown in Figure S2, CsSnBr3 perovskite NWs are found to be difficult to be grown on these growth substrates. The optical images of the as-grown CsSnCl3, CsSnBr3, CsSnI3, and CsSnBrI2 NW arrays are shown in Figure 1a−d, respectively. Obviously, all of the as-

grown lead-free all-inorganic CsSnX3 perovskite NWs grow uniformly on the mica substrate. The cross sections of the asprepared CsSnBr3 and CsSnI3 NWs can be found in Figure S3. With threefold alignment symmetry, all of the CsSnX3 (X = Br and I) NWs form equilateral triangles. This phenomenon is also found in the controllable growth of all-inorganic CsPbX3 (X = Cl, Br, and I) perovskite NWs.15,18,48 Furthermore, the growth rates of the as-grown CsSnX3 NWs can be deduced from the length statistics of more than 300 NWs. As shown in Figure 1e, the growth rates of CsSnCl3, CsSnBr3, CsSnI3, and CsSnBrI2 are 1.24, 1.33, 1.55,, and 1.44 μm min−1, respectively. Anyway, the lengths of the as-grown lead-free all-inorganic perovskite NWs are all longer than 30 μm, being benefit to the fabrication of NW array-based optoelectronic 17568

DOI: 10.1021/acs.jpcc.9b03289 J. Phys. Chem. C 2019, 123, 17566−17573

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The Journal of Physical Chemistry C Table 1. Photodetection Performances of All-Inorganic Perovskite-Based Photodectors perovskites

wavelength (nm)

responsivity (mA/W)

detectivity (jones)

response times rise/decay time

CsSnI3 NW array Cs3Sb2Cl9 NR CsPbI3 NW array CsPbI3 NW CsPbI3 NW array CsPbCl3 MWs CsPbI3 NRs CsPbBr3 NW CsPbI3 NW array CsPbBr3 1D arrays CsPbBr3 NP CsSnBr3 NSs CsBi3I10 TF CsPbBr3 NWs network CsPbBr3 MC CsPbBr3 MR

940 410 630 740 530 405 405 405 halogen light 470 442 442 650 421 400 420

54 3.616 × 106 1.294 × 106 4.489 × 106 350 14.3 2.92 × 106 2.4 × 106 6.7 1.377 × 106 3400 640 2.18 × 104

3.85 × 105 1.25 × 106 2.6 × 1014 7.9 × 1012 1.64 × 1010

83.8 ms/243.4 ms 130 ms/230 ms 0.85 ms/0.78 ms

6 × 107

1013

5.17 × 1013 1.58 × 108 7.5 × 1012 1.93 × 1013

3.212 ms/2.511 ms 0.05 ms/0.15 ms 0.252 ms/0.3 ms 292 ms/234 ms 21.5 μs/23.4 μs 0.6 ms/0.9 ms 19 μs/24 μs 330 μs/380 μs 100 ms 0.5 ms/1.6 ms 8 ms/8 ms

Idark/Ion 6.3 μA/6.8 μA 0.21 μA 0.1 pA/8 nA 29 nA/90 A 0.01 nA/31 nA

bias (V) 0.1 0.9 5 1 2 1 5

0.813 nA/89 nA 1.5 5 1 2.5 3 3

refs this work 63 64 65 66 67 22 36 68 69 70 71 72 73 74 75

Notes: NW is nanowire, NR is nanorod, MW is microwire, NP is nanoplatelet, NS is nanosheet, TF is thin film, MC is microcrystal, MR is microrod.

group semiconductor NWs.51−53 As the used source mass increases up to 0.1 g, two-dimensional CsSnBr3 NSs begin to appear owing to the expected lower nucleation rate.15,54,55 However, this result will help to the controllable growth of high-quality two-dimensional lead-free all-inorganic perovskite NSs in the near future. Beyond the source mass, the source temperature of CsBr powder is also studied in Figure S5. Similar to the above discussed results, with a low source temperature, the vapored source is insufficient, resulting in a low density of thin CsSnBr3 NWs. Otherwise, the vapored source is excessive at a high source temperature, contributing to the formation of thick CsSnBr3 NWs with heavy coatings. In the end, the growth time is also found to be a significant element in the controllable growth of lead-free all-inorganic CsSnX3 perovskite NW arrays. Shown in Figure 2a−c are the SEM images of CsSnBr3 prepared within 1, 2, and 3 min with the optimal source mass and temperature. In detail, with a growth time of 1 min, short triangular NWs and triangular pyramids can be observed on the substrate. When the growth time is prolonged, the NWs show longer length and thicker diameter. The triangular pyramids can only be found on the mica when the growth time is shorter than 3 min, plausibly resulting from the incomplete nucleation of CsSnBr3 samples. Fortunately, these triangular pyramids disappear as the growth time up to 3 min. In a word, by adopting an optimal growth condition of 0.05 g SnBr2 and 0.05 g CsBr powders evaporated at 240 and 630 °C for 20 min, high density and uniform CsSnBr3 NW array can be prepared on the mica substrate. On the basis of the above discussion, a possible growth mechanism of lead-free all-inorganic CsSnX3 perovskite NW arrays can be proposed here, as shown in Figure 2d. At a high temperature, SnX2 and CsX decompose as Sn, X, and Cs atoms first. With the carrier gas, atoms of Sn, Br, and Cs come together to nucleate at the surface of mica. Generally, the surface of a freshly cleaved mica is expected to be atomically smooth and free of dangling bonds, promoting the epitaxial growth of large-scale and high-quality crystals.15,35 Meanwhile, owing on the anisotropy lattice mismatch between the mica substrate and the perovskite materials, the asdeposited CsSnX3 samples can be grown unrestrictedly along a

devices. The typical EDXS (Figure 1f) is employed to characterize the compositions, demonstrating that the asprepared CsSnX3 NW arrays are composed of Cs, Sn, and X (X = Cl, Br, and I) with a molar ratio of ∼1:1:3, in good line with the stoichiometry of CsSnX3. Furthermore, the cubic phase of as-grown CsSnX3 NW array is verified by XRD, as shown in Figure S1. The CsSnBr3 NW array holds the cubic phase in 15 days, with a decreasing XRD intensity. However, in case of CsSnI3 NW array, the intensity of its XRD signal decreases to zero in 6 h. For the alloy system of CsSnBr2I NW array, its XRD signal will disappear after 30 h, along with the increase of lattice space in the process of phase transitions. Obviously, because of the easy oxidation of Sn2+ to Sn4+,35,49,50 the stability of all-inorganic CsSnX3 (X = Cl, Br, and I) perovskites needs to be improved in the near future. In short, uniform and ordered lead-free all-inorganic CsSnX3 perovskite NW arrays with precise compositions are successfully prepared by the simple CVD technique. For guiding the controllable growth of other kinds of leadfree all-inorganic perovskite NW arrays, it is necessary to explore the growth mechanism of the as-prepared CsSnX3 NW arrays. On the basis of the results of Figure 1, all of the asprepared CsSnX3 NW arrays have similar crystal phase and growth rate, and hence, the growth mechanism can be concluded from one kind of the as-prepared CsSnX3 NW array. In this case, the influences of source kinds and mass, source temperature, and growth time are investigated in detail on the morphology, density, diameter, and length of CsSnBr3. For keeping the stoichiometry of CsSnX3, the source mass of used SnBr2 and CsBr powders is fixed at a ratio of ∼1:1. As shown in Figure S4, the source mass indeed plays an important role in the morphology, density, diameter, and length of CsSnBr3. With insufficient source masses, CsSnBr3 nanoparticles and thin NWs with low density and large length distribution are observed in the optical images. On the other hand, with an excessive source mass of 0.07 g, thick CsSnBr3 NWs with heavy coatings are obtained, attributing to the sidewall growth of NWs. This phenomenon is caused by the excessive source vapor pressure during NW growth process, which is commonly observed in the CVD growth of III−V 17569

DOI: 10.1021/acs.jpcc.9b03289 J. Phys. Chem. C 2019, 123, 17566−17573

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The Journal of Physical Chemistry C certain crystal direction, resulting in the growth of NWs.35 Furthermore, the long and uniform CsSnX3 NWs can be prepared with a longer growth time. After the successful growth of lead-free all-inorganic CsSnX3 perovskite NW arrays, their band gap structure and crystallinity quality should be further studied for exploring their potential optoelectronic applications. As shown in Figure 3a of PL spectra, the emission peaks of CsSnBr3 and CsSnI3 NW arrays are located in the visible and near-infrared region of 675 and 926 nm, respectively, corresponding to the band gaps of 1.84 and 1.34 eV. These band gaps are in good line with the previous reported values.35 Compared to CsPbX3 perovskite NW arrays,15,17,19,23 the as-grown CsSnX3 perovskite NW arrays show narrower band gaps down to 1.34 eV, expanding the future optoelectronic applications of perovskites from visible to near-infrared region. Alloying is a good way for tuning the band gaps of requisite semiconductors.17 In this case, by varying the halide composition ratio of Br/I in CsSnX3 perovskite NW arrays, the emission peaks are tuned successfully between 675 and 926 nm. Beyond the band to band emissions, there are emission peaks of defect states in the PL spectra, as shown in Figure S6. The dark-field optical emission image of CsSnBr3 NW can be found in Figure S7, showing bright red emission spots at the tips of NW under a laser of 532 nm, indicating the promising application as lasers.17,34,35 Furthermore, as shown in the two-dimensional PL mapping image, the equal brightness demonstrates the high uniformity of chemical composition in CsSnBr3 (Figure 3b), CsSnBrI2 (Figure 3c), and CsSnI3 (Figure 3d) NW array. It is worth to point out that the strong and uniform PL emission of the as-grown CsSnX3 perovskite NW arrays is similar to the result of CsPbX3 perovskite NW arrays in the literature,15,17 indicating the promising applications in next-generation photodetectors, photovoltaic cells, light-emitting diodes, and lasers. In a word, high-quality lead-free all-inorganic CsSnX3 (X = Br and I) perovskite NW arrays are successfully prepared by a simple CVD method, demonstrating tunable band gaps from visible and near-infrared region. Actually, the optical stability is very important for the optoelectronic applications of lead-free all-inorganic perovskite NW array. Fortunately, their optical stability can be improved easily by covering a protection layer of PMMA, insulating from air and preventing from the phase transitions. As shown in Figure S8, the PL properties of the CsSnBr3−xIx (x = 0, 1, and 3) NW array covered with or without PMMA are studied in detail. Without the protection layer of PMMA, the CsSnBr3 NW array shows a similar stability compared to that of the NWs covered with PMMA. However, without the protection layer of PMMA, the PL intensities of CsSnBr2I and CsSnI3 NW arrays decrease to zero in 23 and 30 h, respectively. In contrast, with the protection layer of PMMA, these phase transition processes can prolong to 35 h and 6 days for CsSnBr2I and CsSnI3, respectively. In a word, the protection layer of PMMA can help to improve the optical stability of allinorganic CsSnBr3−xIx (x = 0, 1, and 3) NWs. With tunable band gaps from 1.84 to 1.34 eV, the as-grown high-quality lead-free all-inorganic CsSnX3 (X = Br and I) perovskite NW arrays are the potential excellent candidates for high-performance photodetectors in the visible and nearinfrared region. In this case, CsSnX3 photodetectors are fabricated directly on the mica substrate by a simple shadow mask process. For preventing from the phase transitions, a protection layer of PMMA is spun on the as-fabricated CsSnX3

(X = Br and I) photodetectors. Furthermore, the photodetection performance of the as-fabricated CsSnX3 (X = Br and I) photodetectors are measured in a vacuum environment. The optical images and schematics of the as-fabricated photodetectors can be found in Figure S9. With a band gap of 1.84 eV, photodetector fabricated by CsSnBr3 NW array is illuminated by lasers of 445, 532, 632, and 730 nm. The corresponding optoelectronic performance can be found in Figure S9. For expect, with an illumination laser wavelength larger than the band gap of the as-prepared CsSnBr3 NW array (675 nm), no photocurrent can be detected. In this case, the 632 nm laser is selected to illuminate the photodetector for better understanding the photosensing behavior of the CsSnBr3 NW array. The photocurrent linearly increases with the enhancement of incident intensity. Also, the photodetector shows good repeatability with a high detection photocurrent of 1.95 μA. The high detection photocurrent is vital for practical photoelectric devices, benefiting from the high-quality NW array. In fact, with the NW array alignment in the channel, a CsSnI3 NW array photodetector also shows a high detection photocurrent of 6.8 μA under the illumination of 940 nm laser. Compared to the visible light photodetection, infrared photodetection plays a more important role in night vision, military missile tracking, medical imaging, industry defect imaging, environmental sensing, exoplanet exploration, and so forth.56,57 With a narrowest band gap among all-inorganic perovskites, CsSnI3 NW array is one of the optimal candidates for nearinfrared photodetection. The near-infrared photodetection performance of the as-fabricated CsSnI3 NW array photodetector is shown in Figure 4. Figure 4a exhibits the response of the device at a number of discrete illumination wavelengths from 475 to 940 nm under a light intensity of 50 mW mm−2 and at a bias voltage of 0.1 V. Obviously, the maximal responsivity is obtained at a wavelength of 940 nm, and hence, the 940 nm laser is selected to illuminate the photodetector. Figure 4b shows the time−response curves of a CsSnI3 NW array photodetector illuminated at 940 nm with different light intensities. The photocurrents increase with the increase of the light intensity from 1.85 to 26.69 W mm−2 at a bias voltage of 0.1 V. It is worth pointing out that the as-built photodetector shows an excellent stability and reproducibility. Beside the high photocurrent and good repeatability, the photodetector also exhibits a fast response time. In general, the time of photocurrent increase from 10 to 90% of peak value and vice versa is defined as the rise and decay time constants, respectively.28,58−60 As shown in Figure 4c, the rise time and decay time can be identified as 83.8 and 243.4 ms. The high respond speed benefits from the high-quality, less surface states and trap centers of the as-prepared CsSnI3 NW array. Furthermore, responsivity (R) and detectivity (D*) of the photodetector can be obtained at different power intensities as presented in Figure 4d. R is defined as the photocurrent generated per unit power of incident light on the effective area of a photodevice and D* reflects the sensitivity of photodetector. They can be calculated by the following equations, respectively. R= 17570

I − Ioff ΔI = on L light PA DOI: 10.1021/acs.jpcc.9b03289 J. Phys. Chem. C 2019, 123, 17566−17573

The Journal of Physical Chemistry C D* =

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(Af )1/2 A =R R / In 2eIoff

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

where ΔI is the difference between the photocurrent and the dark current, Llight is the incident light intensity, P is the light power density, A is the effective area of the detector, f is the electrical bandwidth, In is the noise current, and e is the elementary charge.49−52,59−62 As a result, responsivity and detectivity of the CsSnI3 NW array photodetector can be calculated as 54 mA W−1 and 3.85 × 105 jones, respectively. The values of R and D* can be further improved by constructing the devices in an antisymmetric electrode or top-gated field effort transistor and so forth for decreasing the dark current of the as-fabricated NW array photodetector. As shown in Table 1 are the photodetection performances of lowdimensional all-inorganic perovskites. Obviously, the asfabricated CsSnI3 NW array photodetector is the first nearinfrared detector in the literature, exhibiting fast rise and decay time constants of 83.8 and 243.4 ms, respectively.

ORCID

Ning Han: 0000-0001-9177-6792 Tao Xu: 0000-0001-5436-0077 Xiaotao Hao: 0000-0002-0197-6545 Weida Hu: 0000-0001-5278-8969 Zai-xing Yang: 0000-0002-9040-8805 Author Contributions ∇

M.H. and J.S. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Science Technology and Innovation Committee of Shenzhen Municipality (grants JCYJ20170307093131123), the National Key R&D Program of China (no. 2017YFA0305500), Taishan Scholars Program of Shandong Province, Shandong Provincial Natural Science Foundation, China (grant ZR2017MF037), and the “Outstanding young scholar” and “Qilu young scholar” programs of Shandong University.

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CONCLUSIONS In conclusion, lead-free all-inorganic CsSnX3 (X = Cl, Br, I) perovskite NW arrays are successfully prepared by controlling the source kinds, source mass, source temperature, and growth time on freshly cleaved mica substrate in a solid-source CVD process. Because of the reconstructed reciprocal lattice relation, CsSnX3 NWs epitaxially grow on the mica substrate to form equilateral triangles with the growth direction of [100]. High crystallinity of the as-prepared lead-free all-inorganic CsSnX3 NW arrays is confirmed by PL spectra. At the same time, the as-grown CsSnI3 perovskite NW array shows a narrow band gap of 1.34 eV, expanding the future optoelectronic applications of perovskites from visible to near-infrared region. In the end, the as-grown CsSnI 3 perovskite NW array is employed in a room-temperature near-infrared photodetector for the first time, showing high stability and fast rise and delay times of 83.8 and 243.4 ms, respectively. All of the results reveal the technological potency of the as-prepared lead-free all-inorganic CsSnX3 NW arrays for next-generation high-performance 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/acs.jpcc.9b03289. Growth schematic diagram and XRD patterns of asgrown CsSnX3 NW arrays, SEM images of the asprepared CsSnBr3 samples by using glass and SiO2 as growth substrates, SEM images of the cross section of the as-prepared CsSnBr3 and CsSnI3 NWs, optical images of CsSnBr3 NW array grown with different source masses, optical images of the CsSnBr3 NW array grown at different CsBr heating temperatures, fullwavelength range PL spectra of CsSnX3 NW arrays and mica substrate, dark-field optical emission image of CsSnBr3 NW, PL spectra of CsSnBr3−xIx (x = 0, 1, and 3) NW array covered with a protection later of PMMA or not, and the photodetection performance of the asfabricated CsSnBr3 NW array photodetector (PDF) 17571

DOI: 10.1021/acs.jpcc.9b03289 J. Phys. Chem. C 2019, 123, 17566−17573

Article

The Journal of Physical Chemistry C

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