Direct Vapor Growth of Perovskite CsPbBr3 Nanoplate

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Direct Vapor Growth of Perovskite CsPbBr3 Nanoplate Electroluminescence Devices Xuelu Hu,†,∥ Hong Zhou,†,∥ Zhenyu Jiang,†,∥ Xiao Wang,† Shuangping Yuan,† Jianyue Lan,‡ Yongping Fu,§ Xuehong Zhang,† Weihao Zheng,† Xiaoxia Wang,† Xiaoli Zhu,† Lei Liao,† Gengzhao Xu,‡ Song Jin,§ and Anlian Pan*,† †

Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, State Key Laboratory of Chemo/Biosensing and Chemometrics, School of Physics and Electronic Science, Hunan University, Changsha 410082, People’s Republic of China ‡ Suzhou Institute of Nano-tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China § Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: Metal halide perovskite nanostructures hold great promises as nanoscale light sources for integrated photonics due to their excellent optoelectronic properties. However, it remains a great challenge to fabricate halide perovskite nanodevices using traditional lithographic methods because the halide perovskites can be dissolved in polar solvents that are required in the traditional device fabrication process. Herein, we report single CsPbBr3 nanoplate electroluminescence (EL) devices fabricated by directly growing CsPbBr3 nanoplates on prepatterned indium tin oxide (ITO) electrodes via a vapor-phase deposition. Bright EL occurs in the region near the negatively biased contact, with a turn-on voltage of ∼3 V, a narrow full width at halfmaximum of 22 nm, and an external quantum efficiency of ∼0.2%. Moreover, through scanning photocurrent microscopy and surface electrostatic potential measurements, we found that the formation of ITO/p-type CsPbBr3 Schottky barriers with highly efficient carrier injection is essential in realizing the EL. The formation of the ITO/p-type CsPbBr3 Schottky diode is also confirmed by the corresponding transistor characteristics. The achievement of EL nanodevices enabled by directly grown perovskite nanostructures could find applications in on-chip integrated photonics circuits and systems. KEYWORDS: lead halide perovskites, CsPbBr3 nanoplates, electroluminescence, vapor growth

E

Low-dimensional metal halide perovskite nanostructures have recently emerged as a class of inexpensive semiconductors that have exceptional promise for nanophotonic and nanooptoelectronic applications, owing to their outstanding optical and physical properties including low nonradiative recombination rate, high PL quantum yield, and broadly tunable band gaps.21−31 The lead halide perovskites (APbX3, X is a halide anion and A is a cation) include the well-investigated organic− inorganic hybrid perovskites (such as CH3NH3PbX3) and the emerging all-inorganic perovskites (such as CsPbX3). The latter have attracted more attention recently32−50 because of their improved chemical stability toward heat and moisture compared to the organic−inorganic hybrid ones. Optically pumped lasing from CsPbX3 perovskite nanostructures with low thresholds and excellent photostability have been

lectroluminescence (EL) devices based on single semiconductor nanostructures are potential miniaturized light sources for on-chip photonic integration.1−4 EL in semiconductor nanostructures is usually achieved with a carefully designed p−n junction,5−11 where the injected electrons and holes radiatively recombine in the depletion region to generate photons. Various approaches for achieving nanoscale p−n junctions with highly effective electrical injection have been reported.5−15 However, a very complicated manufacturing process is generally needed to fabricate such elaborate junctions. Alternatively, EL can also be realized in a simple semiconductor−metal junction, based on semiconductor nanostructures like carbon nanotubes (CNT),16−18 CdSe nanowires,19 and atomic layered MoS2.20 However, these kinds of EL devices generally suffer from very low light emission efficiency due to severe nonradiative recombination of the involved semiconductor nanostructures. Therefore, semiconductor nanostructures with less defect states and higher efficient light emission are urgently desired to improve the performance of such EL devices. © 2017 American Chemical Society

Received: May 25, 2017 Accepted: September 18, 2017 Published: September 18, 2017 9869

DOI: 10.1021/acsnano.7b03660 ACS Nano 2017, 11, 9869−9876

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Figure 1. (a) Schematic illustration of the fabrication processes of a single EL nanodevice based on ITO/CsPbBr3 nanoplate/ITO configuration. (b) SEM image of an EL device, showing a vapor-phase grown CsPbBr3 nanoplate is bridged on two ITO electrodes. Inset is a zoomed-in tilted image of the nanoplate. (c) Corresponding EDX mapping of the device, showing uniform spatial distribution of Cs, Pb, and Br in the nanoplate, In, Sn, and O elements detected in the electrodes (ITO), and Si and O elements in the channel of the device (SiO2) underneath the nanoplate bridge. (d) Absorption (red line) and PL spectra (blue line) of the CsPbBr3 nanoplate. Inset is the fluorescence image of the nanoplate excited by a 488 nm laser.

Figure 2. (a) Real-color EL image and (b) EL spectroscopic mapping of a CsPbBr3 nanoplate EL device with a forward bias voltage of V = +5 V applied to the top electrode. (c) Real-color EL image with a higher applied voltage of +8 V. (d) EL spectra of the device collected at different forward bias, in comparison with the corresponding PL spectrum (dashed line). (e) Log-scale plot of current (blue line) and EL intensity (red line) as a function of bias voltage. The inset shows the log−log plot of EL intensity versus current.

ported, 38,39,46−49 EL devices based on single CsPbX 3 nanostructures, such as nanoplates or nanowires, have not been achieved, to the best of our knowledge. The main obstacle is that the CsPbX3 nanostructures can be easily dissolved or destroyed by the polar solvents required in the traditional nanofabrication process, which makes the device fabrication

demonstrated, making them promising candidates as nanoscale light sources.37,41−45 However, for practical applications in information and communication technology, electrically pumped light-emitting devices (LEDs) and lasers are highly desirable. Although efficient LEDs using CsPbX3 nanocrystal thin films as the luminescent layer have been re9870

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applied at the top electrode. Bright light emission can be observed near the contact between the CsPbBr3 nanoplate and a negatively biased electrode, which is further confirmed by the corresponding EL spectroscopic mapping of the device (Figure 2b). The device becomes much brighter when the applied voltage further increases to 8 V. Figure 2c shows the emission spot spreads out at a high bias voltage, and a strong waveguide emission can be observed due to the high-quality optical cavity within the nanoplate. It is should be noted that the green emission at 8 V is bright enough to be observed by the naked eye. The EL spectra of the CsPbBr3 device recorded at various applied voltages are shown in Figure 2d. The EL intensity increases significantly with increasing the bias voltage from 4 to 8 V, and the peak position slightly red-shifted from 530 to 532 nm. This slight peak shift might be attributed to the inevitable heating effect at high current injection.53 All EL spectral profiles recorded at various bias voltages are very similar to the PL spectrum of the CsPbBr3 nanoplate (Figure 2d), indicating that both EL and PL originate from the band-to-band emission of CsPbBr3. The narrow fwhm (∼22 nm) of the EL spectra without any parasitic emissions indicates high color purity of our EL device and the potential for better color rendering. Figure 2e shows the bias-dependent current and EL intensity of the nanoplate device. The nonlinear I−V characteristic of the device at low bias voltage indicates the presence of Schottky barriers.54 The current (blue curve in Figure 2e) increases by several orders of magnitude to ∼0.2 mA when the bias is increased to 8 V, demonstrating efficient carrier injection at a high bias voltage. The turn-on voltage (Vth) is found to be as low as ∼3 V, and the emission intensity increases by several orders of magnitude with increasing the voltage (red curve in Figure 2e). Inset in Figure 2e shows that the EL intensity increases with increasing the current. All results presented here were operated at voltages lower than 8 V to avoid any damage of the devices under high voltage. We further estimated the external quantum efficiency (EQE) of the device by comparing with the PL intensity of a reference sample measured with the same optical setup (Figures S3 and S4). Summarized from several EL devices, the typical EQE is found to be increased as a function of voltages with values of 0.1−0.2% at 8 V (Figures S4 and S5), which is higher than that of the previously reported nanostructured EL devices using Schottky junctions.17,19,20 Although similar EL phenomena have been observed in CdSe, MoS2, and other n-type semiconductors, these materials show relatively broad EL spectra and rather low EQE. Moreover, the EL emission positions in those devices are always observed near their positively biased contacts,20,21 which are contrary to the case of our device, suggesting a p-type semiconductor behavior of the CsPbBr3 nanoplate. To further elucidate the mechanism of the EL in our devices, we conducted scanning photocurrent microscopy (SPCM) measurements. In the SPCM setup (shown in Figure 3a), a 488 nm laser spot with diameter of 1−2 μm was scanned on the surface of the nanoplate, and the resulting photocurrent was recorded as a function of the laser position. Figure 3b shows the photocurrent distributions over the nanoplate with Vsd of +1.5 and −1.5 V, respectively. We observed that the photocurrent peak is near the right (left) electrode at +1.5 V (−1.5 V); namely, the photocurrent has the highest value near the positively biased electrode. The extracted linear scanned photocurrent distributions across the two electrodes (Figure 3c) show the photocurrent decays exponentially as a function

very difficult or the resulting devices exhibit poor contacts and performance.31,32,50 In this work, we have developed a direct growth of CsPbBr3 nanoplates on top of prepatterned indium tin oxide (ITO) electrodes and achieved efficient EL from these nanoplate devices with a low turn-on voltage of ∼3 V. The EL spectra of the device displayed a narrow EL peak centered at 530 nm with a narrow full width at half-maximum (fwhm) of 22 nm, which is similar to that of the corresponding PL spectra. Through complementary study of scanning photocurrent and surface electrostatic potential measurements, we conclude that EL can be ascribed to the formation of ITO/p-type CsPbBr3 Schottky barriers and the efficient carrier injection into the junctions. The realization of directly grown perovskite nanostructure EL devices could have applications in on-chip integrated photonics circuits and systems.

RESULTS AND DISCUSSION The fabrication process of the EL devices is schematically shown in Figure 1a. An ITO-coated glass substrate with patterned electrodes was first prepared by a conventional lithograph and etching method.51 Both the width of the electrodes and the channel length between the electrodes are about 10 μm, and the thickness of the electrodes is about 180 nm. After ultrasonic cleaning in acetone and deionized water, the substrate was moved into a chemical vapor deposition (CVD) system for CsPbBr3 nanostructure growth following the procedures described in our previous work.42 Through carefully controlling the growth condition, square or rectangular CsPbBr3 nanoplates with lateral dimensions of several to a few tens of micrometers and a thickness of 100−500 nm were readily grown. A typical scanning electron microscope (SEM) image (Figure 1b) shows the CsPbBr3 nanoplate is bridged across the tops of the two ITO electrodes, constituting a twoterminal device. The inset in Figure 1b confirms the nanoplate is well contacted with the ITO electrodes. Energy-dispersive Xray spectrum (EDX) mapping of the device (Figure 2c) shows uniform elemental distribution of Cs, Pb, and Br in the nanoplate and In, Sn, and O elements in the electrodes (ITO). Nanoplates are formed randomly on the substrate, whereas approximately 50% of nanoplates bridged across two ITO electrodes. More information about composition analysis and corresponding I−V characteristics of the CsPbBr3 nanoplates are also shown in Figures S1 and S2. These results demonstrated the successful fabrication of CsPbBr3 nanoplate two-terminal device through the direct vapor growth on the prepatterned ITO electrodes. The optical absorption and photoluminescence (PL) spectra of the CsPbBr3 plates are presented in Figure 1d. The absorption spectrum (red line) exhibits an abrupt absorption edge with an onset at about 538 nm, which is consistent with the reported result of bulk CsPbBr3.52 The PL spectrum (blue line) excited by a 488 nm continues wave (CW) laser shows a sharp single emission band with a peak center at 529 nm, right at the absorption edge of the nanoplate, and a narrow fwhm of 21 nm. Inset in Figure 1d is the real-color image of the nanoplate excited by the laser, showing bright and uniform green emission within the nanoplate. EL measurements of the CsPbBr3 devices were performed with a confocal microscope system, and the fabricated device was electrically stimulated by a Keithley 2400 source-measure unit. Figure 2a shows the real-color EL image of a typical CsPbBr3 nanoplate device with a bias voltage of V = +5 V 9871

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Figure 3. (a) Schematic illustration of the SPCM setup with the definition of the direction of applied positive voltage. (b) Scanning photocurrent mapping images of the device with positive (top, +1.5 V) or negative (bottom, −1.5 V) bias voltages. The excitation was a 488 nm laser with an intensity of 5 W/cm2. (c) Corresponding photocurrent line scans across the two ITO electrodes. (d) Illustration of the energy band diagram of the device with V = +1.5 V and the generation of photocurrent with the illuminations close to positive (top sketch) electrode and the negative (bottom sketch) electrode.

Figure 4. (a) AFM and (b) KPFM images of a CsPbBr3 nanoplate on ITO substrate and the corresponding height and potential profiles along the solid line. (c) Energy band diagram of the CsPbBr3/ITO junction in EL mode near the negatively biased electrode (−Vsd).

of ITO electrodes; therefore, the thermal equilibrium energy band diagram near negatively biased contact (the region of EL) can be expected, as shown in Figure 4c. The Ec, Ev, and Ef correspond to the conduction band, valence band, and the Fermi level of the CsPbBr3, respectively. Energy band bends downward at the CsPbBr3/ITO interface, with a built-in potential VD of ∼35 mV. After understanding the photocurrent distribution and the nature of contact barrier, we now explain the working mechanism of the EL device. In general, there are two mainstream views for the EL emitting near contacts within single nanostructure device. One is the hot carriers impact excitation EL mechanism where carriers near the inhomogeneities are accelerated by local electric fields to high energies to actively generate electron−hole pairs. The other is bipolar EL mechanism where the electrons and holes are injected simultaneously into the nanostructures.19 It is known that a relatively broad EL spectrum would be observed due to the inelastic scattering process in the impact excitation mechanism.18 However, the EL spectrum observed in our experiments has almost the same spectral line shape as that of the PL (Figure 3e), indicating that hot carrier impact excitation mechanism may not work for our CsPbBr3 nanoplate-based EL devices. When considering the ITO/p-type CsPbBr3 Schottky diodes which form near the contacts and the high injection current above the turn-on voltage, the EL emissions of our devices can be qualitatively understood through a bipolar EL mechanism. As shown in Figure 4c, when the ITO electrode is applied with negative voltage, the formed Schottky barrier is in forward bias. In such a case, when the applied bias is above the turn-on voltage, the current mainly includes electrons that flow from ITO to CsPbBr3; namely, large numbers of electrons would be injected into CsPbBr3. Meanwhile, when the EL devices worked under high current conditions, the holes that drifted by the external electric field to the drain contact (negative bias here) would be back-scattered and accumulated near the contact, pulling the band down and further promoting

of laser position. By fitting of the curves with single exponential function, a carrier diffusion length of ∼3.5 μm can be extracted, which is comparable to the previously reported values in MAPbI3 perovskite nanostructures.29 Importantly, compared with the n-type semiconductor−metal devices, the photocurrent distributions here are opposite,55 suggesting the formation of p-type Schottky barriers and local downward band bending at the CsPbBr3/ITO interface.56 Such contact barrier is further confirmed by the corresponding transistor characteristics of the device, which will be discussed later. Therefore, the photocurrent behavior can be explained by a back-to-back p-type Schottky diode model (Figure 3d). The current of the device is limited by the reversely biased Schottky diode (positively biased contact), and the dark current coming from electron diffusion Jn and holes tunneling Jp is small at low bias. When laser illumination takes place at areas near the positively biased contact, the photogenerated electrons and holes can be effectively separated by the strong local electric field, producing large photocurrent across the interface. When illumination is away from the positively biased contact, the generated minority carriers (electrons) are redistributed and decrease exponentially under the electric field, leading to an obvious decrease of the photocurrent. Kelvin probe force microscopy (KPFM) was further used to confirm the surface potential and energy band alignments at the contacts. Figure 4a represents the atomic force microscopic (AFM) image of the nanoplate and its corresponding position scanned along line AB, showing a height of ∼130 nm with a smooth surface. Figure 4b gives the surface potential image of the nanoplate and its corresponding potential variation scanned along the line. The surface potential also distributes uniformly on the nanoplate, but it changes abruptly at the edges. This surface potential measurement demonstrates that the Fermi level of the CsPbBr3 nanoplate is about 35 meV lower than that 9872

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Figure 5. (a) Output characteristics Ids−Vds of CsPbBr3 nanoplate FET with different gate voltages. Inset is the schematic of the FET. (b) Semilog plot of the gate transfer characteristic Ids−Vg measured at Vds = 3.0 V and swipe rate 1.1 V/s. Inset shows the same plot in linear scale.

likely to be affected by the defects at the dielectric−perovskite interface, resulting in the decrease of the mobility.64 The swipe rate influences on the charge carrier mobility of the devices was also investigated. Figure S8 shows the transfer characteristics at a swipe rate of 0.8, 1.1, 1.7, and 3.3 V/s. With increasing the swipe rate, the hole mobility shows a decrease from 0.85 to 0.3 cm2 V−1 s−1. In order to underline the reproducibility of the devices, we fabricated tens of nanoplate transistors, which shows that the hole mobility μp is mostly located between 0.2 and 1.0 cm2 V−1 s−1 (Figure S9). Moreover, the electrical characteristics of a representative transistor after several swipes show that on/off current ratio, threshold voltage, and mobility are almost unchanged (Figure S10), indicating the high stability of the achieved devices. The above results demonstrate the formation of high-performance ITO/p-type CsPbBr3 Schottky diodes, which agrees well with the scanning photocurrent and surface electrostatic potential measurements and further supports the working mechanism of the CsPbBr3 nanoplate EL devices.

the injected current of minority carriers (electrons). As a result, the injected electrons and accumulated holes then recombine to generate luminescence in the CsPbBr3 region near the contact. In this case, the EL intensity is proportional to the drain current, which agrees with the plot of EL intensity versus current (Figure 2e). The bipolar EL mechanism has been reported to be possible in such unipolar devices and appears to explain our EL emissions well, which always occur near the negatively biased electrodes. In addition, through investigating transistor characteristics of the CsPbBr3 nanoplate devices, we clearly confirm the formation of back-to-back ITO/p-type CsPbBr3 Schottky diodes which are responsible for the EL discussed above. The schematic of the device structure (Figure 5a) shows that the CsPbBr3 device is first coated with 1 μm thick poly(methyl methacrylate) film as dielectric layer, and then 50 nm thick Au film was deposited as the top-gate. The output and transfer characteristics were measured under the dark condition. Figure 5a represents the Ids−Vds curves with gate voltages varying from −20 to 25 V, and the exponential shape of these output curves obviously reveals the existence of Schottky barriers in our devices.57 The corresponding transfer characteristic of the FET is measured at drain voltage of 3 V and swipe rate of 1.1 V/s. The results also show a p-type conductivity of the device with an on/off ratio of 104 (Figure 5b). This p-type character of the as-grown samples may come from the negatively charged Pb (VPb′) according to previous work.58,59 Further investigations indicate that the devices have a gate dielectric leakage current of only around 0.03 nA at an applied voltage of −20 V, less than 0.01% of the drain current, and the hysteresis of transfer characteristics (shift in threshold voltage depending on the direction of the swipe gate−source voltage) ΔVth is around 1.2 V, smaller than that of the previously reported halide perovskite microplate transistors (see Figure S6).60−62 The small hysteresis and low leakage current both demonstrate the good quality of the achieved perovskite nanoplates transistors. Field-effect hole mobility can be calculated with the following equation,63 μ = L2gm/(CVds) = Lgm/(WCiVds), where W represents the width of the nanoplate, L is the channel length, and Ci is the gate capacitance per unit area (Ci = ε·ε0·S/d = 3 nF/cm2). The transconductance gm = dI·ds/dVg can be extracted from the curve to be 2.3 nS, and the hole mobility is estimated to be 0.26 cm2 V−1 s−1 at the gate voltage of 16 V. It is noted that the devices constructed with thicker nanoplates always have mobility higher than that of thinner ones (Figure S7), which may be attributed to thinner nanoplates being more

CONCLUSIONS In summary, we have successfully fabricated ITO/CsPbBr3 nanoplate/ITO EL devices by a direct vapor-phase growth of single CsPbBr3 nanoplates on two ITO electrodes. The working mechanism of the EL devices was investigated through scanning photocurrent and surface electrostatic potential measurements, which revealed the EL originates from the radiative recombination of injected electrons and the accumulated holes at the ITO/p-type CsPbBr3 Schottky junction. Studies of field-effect transistors based on such CsPbBr3 nanoplates revealed the p-type doping of CsPbBr3 nanoplates with a hole mobility of 0.26 cm2 V−1 s−1. The EL of the devices exhibits a very narrow line width of ∼22 nm and an EQE of ∼0.2%, which are all superior to those of the previously reported semiconductor−metal Schottky junction type nanocale EL devices. Our results not only provide deeper understanding of the charge transport properties in inorganic lead halide perovskites but also show a simple and practicable method to fabricate nanoscale optoelectronic devices (such as light-emitting diodes) based on halide perovskite nanostructures. EXPERIMENTAL SECTION ITO Electrode Fabrication. An ITO-coated glass substrate with patterned electrodes was first prepared by a conventional lithograph and etching method. First, an ITO glass substrates with a ∼200 nm 9873

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ACS Nano thick ITO layer was ultrasonically cleaned in the acetone and deionized water. After that, a photoresist pad was patterned to cover ITO glass by a UV lithography and development process. The ITO glass was immersed vertically in the etching solution, without stirring, during the etching process. Later, the photoresist was removed by acetone. Finally, patterned electrodes (200 nm) were made on top of the glass. Synthesis of CsPbBr3 Nanoplates. The CsPbBr3 perovskite nanoplates were synthesized by vapor-phase approach using a homebuilt CVD system. Several pieces of patterned ITO electrodes glasses (10 mm × 10 mm) were placed downstream of the quartz tube mounted in a furnace (OTF-1200X). An alumina boat loaded with mixed powders of PbBr2 and CsBr was put inside the heating center of a quartz tube. Considering the different vapor pressures of these materials, the molar ratio of PbBr2 and CsBr was set to 1:2. Prior to heating, the quartz tube was pumped down, which was followed by a 60 sccm flow of high-purity Ar (99.999%) and maintained at a pressure at 300 Torr. Then the furnace was heated to a setting temperature at 570−600 °C in 30 min and maintained at this temperature for 15 min. After the growth, the tube was naturally cooled to room temperature. Measurements of the Properties. Photoluminescence measurements were performed with a confocal μ-PL system (WITec, alpha300). The 488 nm CW laser was used as the excitation source. The laser was introduced to the confocal system and focused onto the samples with a 100× objective. The PL signals were collected through the same objective and detected by a CCD spectrometer (300 g/mm grating). EL measurement of the CsPbBr3 devices was performed in ambient conditions with the same confocal μ-PL system and CCD spectrometer. Keithley 2400 source-measure unit was used as the power source. The spatially resolved scanning photocurrent measurements were performed in ambient conditions with the same confocal μ-PL system. Briefly, a 488 nm laser spot with beam diameter of 1−2 μm scanned over the device, and the resulting photocurrent was recorded as a function of the laser position. The KPFM measurements were performed with a Bruker Dimension ICON AFM with Pt/Ircoated tips (ACCESS EFM).

Innovative Research Team in Higher Educational Institutions of Hunan Province, Joint Research Fund for Overseas Chinese, Hong Kong and Macau Scholars of the National Natural Science Foundation of China (No. 61528403), and The Foundation for Innovative Research Groups of NSFC (Grant 21521063).

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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/acsnano.7b03660. XRD pattern and EDS spectrum of the as-grown CsPbBr3 nanoplates; typical I−V characteristics of the CsPbBr3 nanoplates, PL quantum yield estimation of the CsPbBr3 nanoplates, EQE estimation of the EL devices (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Xiao Wang: 0000-0002-2973-8215 Yongping Fu: 0000-0003-3362-2474 Song Jin: 0000-0001-8693-7010 Anlian Pan: 0000-0003-3335-3067 Author Contributions ∥

X.H., H.Z., and Z.J. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to the National Natural Science Foundation of China (Nos. 51525202, 61574054, 51672076, and 61505051), the Aid Program for Science and Technology 9874

DOI: 10.1021/acsnano.7b03660 ACS Nano 2017, 11, 9869−9876

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DOI: 10.1021/acsnano.7b03660 ACS Nano 2017, 11, 9869−9876