Superior Photodetectors Based on All-Inorganic Perovskite CsPbI3

Jan 8, 2018 - Currently, one-dimensional all-inorganic CsPbX3 (X = Br, Cl, and I) perovskites have attracted great attention, owning to their promisin...
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Superior Photodetectors Based on All-Inorganic Perovskite CsPbI3 Nanorods with Ultrafast Response and High Stability Tao Yang,†,‡ Yapeng Zheng,†,‡ Zhentao Du,†,§ Wenna Liu,†,‡ Zuobao Yang,† Fengmei Gao,† Lin Wang,† Kuo-Chih Chou,‡ Xinmei Hou,*,‡ and Weiyou Yang*,† †

Institute of Materials, Ningbo University of Technology, Ningbo 315016, People’s Republic of China State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China § College of Materials Science and Engineering, Hunan University, Changsha 410082, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Currently, one-dimensional all-inorganic CsPbX3 (X = Br, Cl, and I) perovskites have attracted great attention, owning to their promising and exciting applications in optoelectronic devices. Herein, we reported the exploration of superior photodetectors (PDs) based on a single CsPbI3 nanorod. The as-constructed PDs had a totally excellent performance with a responsivity of 2.92 × 103 A·W−1 and an ultrafast response time of 0.05 ms, respectively, which were both comparable to the best ones ever reported for all-inorganic perovskite PDs. Furthermore, the detectivity of the PDs approached up to 5.17 × 1013 Jones, which was more than 5 times the best one ever reported. More importantly, the as-constructed PDs showed a high stability when maintained under ambient conditions. KEYWORDS: photodetectors, perovskites, nanorods, stability, detectivity and currently are a hot topic.18,19 For instances, most recently, Yang et al. reported CsPbBr3 microcrystal-based PDs with an ultrahigh responsivity up to 6 × 104 A·W−1 and fast response time of ∼1 ms;20 Shoaib et al. reported CsPbBr3 nanowire-based PDs with an ultrahigh responsivity of 4.4 × 103 A·W−1 and a fast response speed of 0.252 ms,21 suggesting their very promising applications and bright prospect. However, how to create perovskite PDs with totally superior performances, especially with a satisfactory stability to be used under air conditions, is still a great challenge. In the present work, single-crystalline all-inorganic CsPbI3 nanorods with high quality were synthesized in high yield via a solution process. Then, PDs based on individual CsPbI3 nanorods were constructed. The obtained PDs exhibited a totally superior performance with a responsivity of 2.92 × 103 A·W−1 and an ultrafast response time of 0.05 ms, which were both comparable to the best ones ever reported for all-inorganic perovskite PDs. Furthermore, the detectivity of the PDs approached up to 5.17 × 1013 Jones, which was more than

ince the discovery of carbon nanotubes,1 one-dimensional (1D) nanostructurs, such as nanowires, nanobelts, and nanotubes, have been attracting a great deal of interest over the past decades, due to their characteristics such as defectfree single crystals,2,3 high crystalline quality,4 large specific surface area, and Debye length comparable to their sizes.5 More interestingly, the conductive channel of 1D nanostructures could confine the active area of charge carriers and shorten the carrier transit time,6,7 making them advantageous for exploration of highly efficient nanodevices. Recently, the hybrid perovskites have attracted wide attention, due to their interesting applications in optoelectronic devices.8−10 However, the inevitable decomposition and volatilization of organic components within the hybrid perovskites render them with poor long-term stability.11,12 In comparison to the organohalide perovskites, the all-inorganic counterparts are more stable and recognized as a rising star; thus, they have attracted great interest due to their excellent charge transport properties13 and broad chemical tunability.14 It has been intensively reported that this class of materials has triggered wide and exciting applications in high-performance devices such as photovoltaic cells,15 light-emitting diodes,16 and lasers.17 Among them, the photodetectors (PDs) are considered as a representative one

S

© 2018 American Chemical Society

Received: November 18, 2017 Accepted: January 8, 2018 Published: January 8, 2018 1611

DOI: 10.1021/acsnano.7b08201 ACS Nano 2018, 12, 1611−1617

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of quartz in the XRD pattern come from the used substrate. These experimental results show that the as-synthesized products are high-quality single-crystalline perovskite CsPbI3 nanorods with a uniform diameter and length. The effect of reaction times, solvent amounts of OctAm, and reactant amount of PbI2 on the growth of CsPbI3 nanorods has been investigated, which has been discussed in the Supporting Information (Figure S6). It seems that the introduced reactant amounts play a fundamental role in the growth of high-quality CsPbI3 nanorods. Figure 2(a) shows a typical UV−vis absorption spectrum of the obtained product, indicating that the CsPbI3 nanorods have a direct band gap of ∼2.7 eV, with an absorbance peak at 405 nm. Based on these nanorods, the PD nanodevice is constructed, which is schematically illustrated in Figure 2(b). The two ends of a single nanorod are covered by a 30 nm Au film with a separation of 1 μm, as shown in Figure 2(c). The PD performances are then measured on a four-probe station in conjunction with a semiconductor characterization system, as shown in Figure 2(d). Figure 3(a) gives the typical current−voltage (I−V) characteristics of the CsPbI3 nanorod PD illuminated under light of 405 nm (3.06 eV) with an average power of 10.69 mW·cm−2 and dark conditions, respectively. It discloses that the dark current of the CsPbI3 nanorod PD is lower than 10 pA at a bias of 2 V. However, the photocurrent illuminated under 405 nm is 3 orders of magnitude higher than that under dark, which approaches 31 nA at the same voltage of 2 V. As shown in Figure 3(b), the symmetric and linear I−V curves verify that the ohmic contact has been established between the CsPbI3 nanorod and the electrodes. The sensitivity of the PDs is mainly characterized by the spectral responsivity (Rλ) and external quantum efficiency (EQE),22,23 which can be calculated by the following equations, respectively:

5 times the best one ever reported. More importantly, the PDs could be stable once maintained under ambient conditions, suggesting their very promising applications.

RESULTS AND DISCUSSION Figure 1(a) and Figure S1(a) in the Supporting Information are typical SEM images of the as-synthesized product under low magnifications, suggesting its high yield. Figure 1(b,c) and Figure S1(b−e) in the Supporting Information are their closer observations, showing that the resultant nanorods are highly pure in morphology with uniform distribution in diameter and length, which are averagely sized at ∼150 nm and ∼2 μm, respectively. Meanwhile, the diameter of the rods is highly uniform along the axial direction. Figure 1(d) shows a typical TEM image of a single CsPbI3 nanorod (the observations under different magnifications are shown in Figure S2, Supporting Information). The lower-left inset SAED pattern can be indexed to its orthorhombic phase, which is identical along the entire body, disclosing its singlecrystalline nature. The measured d spacing of ∼0.478 nm between two neighboring lattice fringes (Figure S3, Supporting Information) corresponds to the distance of (100) crystal planes. Both the TEM image and SAED pattern verify that the resultant nanorods grow along the [100] direction, which is schematically shown as the crystal structures in Figure S4 in the Supporting Information. The typical energy dispersive X-ray spectroscopy (EDX) (Figure 1(e)) reveals that the as-synthesized nanorods are composed of Cs, Pb, and I with a mole ratio of ∼1:1:3, in agreement with the stoichiometry of CsPbI3. The element mappings (Figure S5, Supporting Information) within a single rod present uniform spatial distributions of Cs, Pb, and I. The X-ray diffraction (XRD) pattern (Figure 1(f)) as well as the yellow color (the inset in Figure 1(f)) of the as-grown nanorods further confirms that they are orthorhombic perovskite CsPbI3 (JCPDS Card No. 18-0376). The detected signals

Rλ =

I − Ioff ΔI = on L light PA

(1)

Figure 1. (a−c) Typical SEM image of as-synthesized CsPbI3 nanorods under different magnifications. (d) Typical TEM image of a single CsPbI3 nanorod. The lower-left inset provides the SAED pattern. (e) Typical EDX spectrum of the nanorods. (f) Typical XRD pattern of the nanorods. The upper-left inset is a photograph of the CsPbI3 nanorods dispersed in hexane. 1612

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Figure 2. (a) Representative UV−vis absorbance spectrum of CsPbI3 nanorods. The inset is the plot of (αhν)2 vs hν. (b) Schematic diagram for the as-constructed PDs based on a single CsPbI3 nanorod. (c) Representative SEM image of the as-assembled PD with an individual nanorod. (d) Photograph showing the property measurements of the fabricated PDs conducted on a four-probe station.

Figure 3. (a) Typical logarithmic I−V and (b) I−V characteristics of the PDs under irradiation with 405 nm light (10.69 mW·cm−2) and in the dark. (c) Representative spectral response of the PDs with wavelengths ranging from 250 to 600 nm at a bias of 2.0 V. (d) Approximate EQE and detectivity of the PDs at different wavelengths.

EQE =

hc Rλ eλ

PD, it is very important for a PD to have a high conversion rate from photons to electrons/holes, namely, with a high EQE. Figure 3(c and d) provide the approximate Rλ and EQE of the PDs illuminated under 405 nm light at an applied voltage of 2 V, which can be up to 2.92 × 103 A·W−1 and 0.9 × 106 %, respectively. Notably, the responsivity can be comparable to the best one ever reported for the perovskite PDs, implying its high sensitivity.20,21

(2)

where ΔI is the difference between the photocurrent and the dark current, Llight is the incident light intensity, P is the light power, A is the effective area of the detector, h is Planck’s constant, c is the velocity of light, e is the electronic charge, and λ is the exciting wavelength. Since Rλ is proportional to the quantum yield of the 1613

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ACS Nano Table 1. Key Performances of Typical Perovskite PDs Ever Reported photodetector

bias (V)

rise/decay time (ms)

CsPbBr3 microparticles CsPbI3 nanocrystals films CsPbBr3 nanoparticles/Au anocrystals CsPb(Br/I)3 nanorod networks CsPbBr3 nanosheets CsPbBr3 thin films CsPbBr3 nanoarrays CsPbI3 nanoarrays CsPbBr3 nanosheets/carbon nanotubes CsPbBr3 nanoplatelets CsPbBr3 bulk single crystals CsPbBr3 bulk single crystals CsPbBr3 microcrystals CsPbBr3 nanowires CsPbI3 nanorods

10 1 2 8 5 6 5 1 10 1.5 0 5 3 3 2

1.8/1.0 24/29 0.2/1.2 680/660 0.019/0.025 0.43/0.318 0.0215/0.0234 292/234 0.016/0.38 0.6/0.9 0.23/0.06 0.069/0.261 0.5/1.6 0.252/0.3 0.05/0.15

responsivity (A·W−1)

detectivity (Jones)

ref

0.18

6.1 × 1010

0.01

1.68 × 109

27 31 25 19 34 29 26 35 28 30 32 33 20 21 this work

0.25 55 1 × 103 0.0067 31.1 34 0.028 2 6 × 104 4.4 × 103 2.92 × 103

9 × 1012 1.57 × 1012 7.5 × 1012 1.7 × 1011 1 × 1013 5.17 × 1013

Figure 4. (a) Reproducible on/off switching illuminated by 405 nm light with an intensity of 10.69 mW·cm−2 at a bias of 2 V. (b) Transient response under the illumination of a 405 nm light pulse chopped at a 100 Hz frequency. (c) Time−-response curves of a CsPbI3 nanorod PD irradiated at 405 nm with respect to the light intensities. (d) Relationship between the photocurrents and light intensities at a fixed incident-light wavelength of 405 nm.

where f is the electrical bandwidth and In is the noise current. In this case, once the dark current is dominated by the shot noise, D* can be expressed as

As shown in Figure 3(c), the spectral response of the PD has a discrimination ratio (i.e., among the wavelengths in the ranges of 250−380 nm UV light, 380−460 violet light, and 460−600 nm visible light) up to 3 orders of magnitude, suggesting that the as-built CsPbI3 nanorod nanodevice is a typical UV and blue light PD, in accordance with its band gap (Eg) of ∼2.7 eV (∼460 nm), as validated in Figure 2(a). As for a PD device, its detectivity can be calculated by D* =

D* =

(

)

(4)

where Ioff is the dark current and e is the elementary charge. Evidently, the dark current of the PD should be depressed as low as possible to distinguish very weak optical signals. To obtain a small Ioff, the semiconductor should have a low trap density and low thermal emission (recombination) rates with the desired

(Af )1/2 Rλ In

Rλ Ioff 1/2 2e A

(3) 1614

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light intensity, and θ is the exponent (0 < θ < 0.5), which determines the response of the photocurrent to the light intensity. By fitting the curve with the equation of I = αPθ, a θ of 0.39 is obtained (Figure 4(d)). Such a nonunity exponent suggests a complex process of electron−hole generation, recombination, and trapping within the CsPbI3 nanorod.38 In our case, the nanorod permits a much smoother charge flow within the single-crystalline channel, leading to the increased lifetime of the photogenerated carriers. Figure 5 shows the stability of the as-constructed CsPbI3 nanorod PDs, which is characterized by recording the photocurrent

good crystal quality to avoid current leakage during the operation. In the current case, the specific detectivity of the CsPbI3 nanorod PD illuminated with 405 nm light is calculated to be as high as 5.17 × 1013 Jones (cm·Hz1/2·W−1). From 250 to 450 nm, the ultrahigh detectivity of the PD device approaches 1013 Jones (at a bias of 2.0 V), which is more than 1 order of magnitude higher than that of a Si PD in the identical spectral region (i.e., 4 × 1012 Jones)18,24 and more than 5 times the best one ever reported for the perovskite PDs (see Table 1).19−21,25−35 The excellent performance of perovskite nanorod PDs is mainly attributed to the following reasons. First, the CsPbI3 nanorods have low recombination of charge carriers, low density of defects, and high absorption coefficient, which could result in a very strong photoelectric effect.21 Second, the single-crystal nanorod can provide a smooth and short path for carrier transfer, which significantly enhances the response speed. Third, the absorption coefficient of the perovskite can reach the order of 104 cm−1.18 Thereby, a few hundred nanometer layers of the material are required for light absorption.18 In comparison to the PDs as reported in Table 1, CsPbI3 nanorods with a diameter of ∼150 nm can absorb light with an energy smaller than Eg completely, which benefits the improvement in the detectivity. Figure 4(a) gives the time response of the PD devices, which is measured by periodically turning on and off the 405 nm light under air conditions at a bias of 2.0 V. The current clearly exhibits two distinct states when the light irradiation is on and off, respectively. It seems that the dark current is only 10 pA; nevertheless the photocurrent can be significantly enhanced to a stable value of 31.27 nA. It is worth pointing out that the as-built PD shows an excellent stability and reproducibility, which is evidenced by switching the light on/off for more than 200 cycles prior to testing under illumination. As a result, the switching ratio (SR) is calculated as SR =

Iph Idark

=

Ion − Ioff Ioff

Figure 5. Stability characteristics of the fresh PDs and after maintaining for 1 week under ambient conditions.

changes under ambient conditions. As for the fresh PDs, the photocurrents are highly stable under the irradiation of a 405 nm laser (10.69 mW·cm−2) for 24 h, which could remain almost the same as the initial value with a nearly ignorable fluctuation amplitude below 1%. More interestingly, as for the PDs after being preserved under ambient conditions for 1 week, their photocurrents monitored over another 24 h still behave steadily with nearly no reduction in the intensities, suggesting that our CsPbI3 perovskite PDs could be robust under air environments. The excellent stability of the present perovskite photodetectors is mainly attributed to the following reasons. First, in comparison to the organic−inorganic hybrid counterparts, the all-inorganic perovskites, such as cesium lead halides (CsPbX3) (X = Cl, Br, and I), tend to have a higher stability.34,39,40 Second, the high stability of the current photodetectors might be ascribed to the single-crystalline nature of the as-synthesized CsPbI3 nanorods.41−43

(5)

where Ion is the current measured by turning on the 405 nm light and Ioff is that measured by turning off the light. Accordingly, the SR can be ca. 3.13 × 103. To obtain the response time, the response speed is measured using a 405 nm continuous laser triggered with a pulse width of 100 Hz, as shown in Figure 4(b). The currents increase very sharply from one state to another, indicating its fast response to the light illumination. The response and recovery times, which are defined as the values needed for the dark current to reach 90% of the maximum photocurrent and vice versa down to 10%, are measured as ∼0.05 and 0.15 ms, respectively. Notably, the response time of 0.05 ms in the current case is around 10 and 5 times faster than those based on CsPbBr3 microcrystals (i.e., 0.5 ms)20 and CsPbBr3 single-crystalline nanowires (i.e., 0.252 ms)21 (see Table 1), evidencing the ultrafast response of the as-constructed perovskite PD device. Figure 4(c) shows the time−response curves of a CsPbI3 nanorod PD irradiated at 405 nm with respect to the light intensities, in which the photocurrents increase from 3.12 to 16.07 mW·cm−2 with the increase of the light intensity at a bias of 2 V. The stable photocurrent without evident fluctuation can increase drastically from 0.010 nA to 33.83 nA. Notably, the CsPbI3 nanorod PD exhibits a good repeatability and fast response, even after being subjected to the largest photocurrent of 33.83 nA for a long time. The dependence of the photocurrent on the light intensity can be expressed by a power law: I = αPθ,36,37 where α is a constant for a given wavelength, P is the

CONCLUSIONS In summary, we have demonstrated the exploration of superior PDs based on all-inorganic perovskite CsPbI3 nanorods with ultrafast response and high stability. The high-yield growth of high-qualified CsPbI3 nanorods with a uniform diameter of ∼150 nm and length of ∼2 μm has been accomplished via a facile solution process. The as-constructed PDs had a totally excellent performance with a responsivity of 2.92 × 103 A·W−1 and a response time of 0.05 ms, which were both comparable to the best ones ever reported for all-inorganic perovskite PDs. Furthermore, the detectivity of the PDs approached up to 5.17 × 1013 Jones, which was more than 5 times the best one ever reported. 1615

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ASSOCIATED CONTENT

More importantly, the as-constructed PDs had a high stability when maintained under ambient conditions. The present work might advance the exploration of perovskite PDs with totally excellent performance, which are promising to be practically applied as efficient optoelectronic devices.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b08201. Typical SEM, TEM, and HRTEM images of the as-synthesized CsPbI3 nanorods, crystal structural model and element mappings of CsPbI3 nanorods, and discussion on the effect of reaction times, solvent amounts of OctAm, and reactant amount of PbI2 on the growth of CsPbI3 nanorods (PDF)

EXPERIMENTAL METHODS Materials. Lead(II) iodide (PbI2, 99.999% trace metals basis), cesium carbonate (Cs2CO3, reagentPlus, 99%), 1-octadecene (ODE, technical grade, 90%), oleylamine (OAm, 70%), oleic acid (OA, 90%), hexanoic acid (HexAc, ≥99.5%), octanoic acid (OctAc, 99%), octylamine (OctAm, 99.5%), toluene (anhydrous), and hexane (anhydrous, 95%) were purchased from Sigma-Aldrich. All chemicals were used directly without further treatment. Preparation of Cesium Oleate Solution. In a typical process, 0.4 g of Cs2CO3 and 1.2 mL of OA were loaded into a 100 mL three-neck flask along with 15 mL of ODE, followed by degassing and drying under vacuum at 120 °C for 60 min. The obtained solution was then heated under N2 to 150 °C until the Cs2CO3 reacted with OA completely. Synthesis of CsPbI3 Nanorods. All of the procedures were conducted under ambient conditions. As a representive process, 8 mL of ODE, 96 mg of PbI2, 1 mL of OAm, and 0.2 mL of OctAm were loaded into a 100 mL three-neck flask and dried under vacuum for 45 min at 95 °C, leading to the formation of a clear solution. Subsequently, the resultant solution was heated to 120 °C under N2 and maintained there for 10 min. Then 0.6 mL of the Cs-oleate solution (prepared as described above) was swiftly injected into the flask and kept at 120 °C for 50 min to allow the growth of the perovskite nanostructures. After that, the solution was quenched immediately by an ice−water bath and subjected to centrifugation at 10 000 rpm for 3 min. Finally, the desired nanorod product was isolated by centrifugation at 7000 rpm for 3 min and redispersed in hexane/toluene for the following use. Device Fabrication and Measurements. The PD was constructed based on an individual CsPbI3 nanorod. First, the Si substrates covered with a 300 nm SiO2 layer were cleaned via sonication in sequence using a neutral detergent, deionized water, and acetone for 15 min, followed by treatment in a UV-ozone cleaner for 10 min. Then, the as-fabricated CsPbI3 nanorods were dispersed in ethanol, followed by dilution for obtaining a colorless solution. Subsequently, 2 μL of the resultant solution was subjected to spin-casting at 2000 rpm for 4 min on the Si substrates and drying at room temperature for 30 min. After that, 30 nm Au electrodes with ∼1 μm separation were deposited on the two ends of the nanorods by using photolithography, thermal evaporation, and a lift-off process sequentially. Structural Characterization and Photoelectric Property Measurement. The resulting products were characterized using field-emission scanning electron microscopy (S-4800, Hitachi, Japan), X-ray diffraction (D8 Advance, Bruker, Germany) with Cu Kα radiation (λ = 1.5406 Å), and high-resolution transmission electron microscopy (HRTEM, JEM-2010, JEOL, Japan) together with energy-dispersive X-ray spectroscopy. The UV−vis measurements of the obtained nanorods were performed on a UV−vis scanning spectrophotometer (U-3900, Hitachi, Japan). For measuring the absorbance spectra, the samples were first dispersed in a hexane/toluene mixture solution and then dropped on a slide glass. The electrical and optoelectronic measurements of the fabricated devices were conducted on a four-probe station in conjunction with a semiconductor characterization system (4200-CSC, Keithley, USA), whose data were recorded with a computer controlled by a Labview program. The as-assembled PDs were located in the center of the laser spot for measuring the performance, which was directed by an optical microscope attached to the four-probe station. The light source was a 500 W xenon arc lamp coupled to an Acton Research monochromator with order-sorting filters. The photocurrent was measured by fixing certain light wavelengths with adjustable light intensities (405 nm, Pmax = 200 mW, with a laser spot size of 4 mm in diameter), which were measured by an OAI-306 power meter. All experiments were performed at room temperature under ambient conditions.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (X.-M. Hou). *E-mail: [email protected] (W.-Y. Yang). ORCID

Weiyou Yang: 0000-0002-3607-3514 Author Contributions

T.Y. and Y.Z. contributed equalily to this work. X.H. and W.Y. conceived and directed the experiments. T.Y. and Y.Z. performed the experiments. T.Y., Y.Z., X.H., and W.Y. cowrote the manuscript. All the authors discussed the results and helped in the preparation of the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work was supported by National Natural Science Foundation for Excellent Young Scholars of China (Grant No. 51522402), National Natural Science Foundation of China (NSFC, Grant Nos. 51372122, 51372123, 51572133, 51672137, and 51702175), the Special Fund of the National Excellent Doctoral Dissertation (No. 201437), Zhejiang Provincial Nature Science Foundation (Grant No. LQ17E020002), and Natural Science Foundation of Ningbo Municipal Government (Grant No. 2016A610103 and 2016A610104). REFERENCES (1) Iijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56. (2) Huang, F.; Banfield, J. F. Size-dependent Phase Transformation Kinetics in Nanocrystalline ZnS. J. Am. Chem. Soc. 2005, 127, 4523− 4529. (3) Zheng, W.; Huang, F.; Zheng, R.; Wu, H. Low-dimensional Structure Vacuum-ultraviolet-sensitive (λ 175 μm in Solution-grown CH3NH3PbI3 Single Crystals. Science 2015, 347, 967−970. (14) Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I. Chemical Management for Colorful, Efficient, and Stable Inorganicorganic Hybrid Nanostructured Solar Cells. Nano Lett. 2013, 13, 1764− 1769. (15) Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395. (16) Tan, Z. K.; Moghaddam, R. S.; Lai, M. L.; Docampo, P.; Higler, R.; Deschler, F.; Price, M.; Sadhanala, A.; Pazos, L. M.; Credgington, D.; Hanusch, F.; Bein, T.; Snaith, H. J.; Friend, R. H. Bright Light-emitting Diodes based on Organometal Halide Perovskite. Nat. Nanotechnol. 2014, 9, 687−692. (17) Eaton, S. W.; Lai, M.; Gibson, N. A.; Wong, A. B.; Dou, L.; Ma, J.; Wang, L.-W.; Leone, S. R.; Yang, P. Lasing in Robust Cesium Lead Halide Perovskite Nanowires. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 1993−1998. (18) Dou, L.; Yang, Y. M.; You, J.; Hong, Z.; Chang, W. H.; Li, G.; Yang, Y. Solution-processed Hybrid Perovskite Photodetectors with High Detectivity. Nat. Commun. 2014, 5, 5404. (19) Tang, X.; Zu, Z.; Shao, H.; Hu, W.; Zhou, M.; Deng, M.; Chen, W.; Zang, Z.; Zhu, T.; Xue, J. All-inorganic Perovskite CsPb(Br/I)3 Nanorods for Optoelectronic Application. Nanoscale 2016, 8, 15158− 15161. (20) Yang, B.; Zhang, F.; Chen, J.; Yang, S.; Xia, X.; Pullerits, T.; Deng, W.; Han, K. Ultrasensitive and Fast All-inorganic Perovskite-based Photodetector via Fast Carrier Diffusion. Adv. Mater. 2017, 29, 1703758. (21) Shoaib, M.; Zhang, X.; Wang, X.; Zhou, H.; Xu, T.; Wang, X.; Hu, X.; Liu, H.; Fan, X.; Zheng, W.; Yang, T.; Yang, S.; Zhang, Q.; Zhu, X.; Sun, L.; Pan, A. Directional Growth of Ultralong CsPbBr3 Perovskite Nanowires for High-Performance Photodetectors. J. Am. Chem. Soc. 2017, 139, 15592−15595. (22) Hu, P.; Wen, Z.; Wang, L.; Tan, P.; Xiao, K. Synthesis of Few-layer GaSe Nanosheets for High Performance Photodetectors. ACS Nano 2012, 6, 5988−5994. (23) Ma, L.; Hu, W.; Zhang, Q.; Ren, P.; Zhuang, X.; Zhou, H.; Xu, J.; Li, H.; Shan, Z.; Wang, X.; Liao, L.; Xu, H. Q.; Pan, A. Roomtemperature Near-infrared Photodetectors Based on Single Heterojunction Nanowires. Nano Lett. 2014, 14, 694−698. (24) Gong, X.; Tong, M.; Xia, Y.; Cai, W.; Moon, J. S.; Cao, Y.; Yu, G.; Shieh, C. L.; Nilsson, B.; Heeger, A. J. H. High-detectivity Polymer Photodetectors with Spectral Response from 300 to 1450 nm. Science 2009, 325, 1665−1667. (25) Dong, Y.; Gu, Y.; Zou, Y.; Song, J.; Xu, L.; Li, J.; Xue, J.; Li, X.; Zeng, H. Improving All-Inorganic Perovskite Photodetectors by Preferred Orientation and Plasmonic Effect. Small 2016, 12, 5622− 5632.

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DOI: 10.1021/acsnano.7b08201 ACS Nano 2018, 12, 1611−1617