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Controlling the Growth of Single Nanowires in a Nanowire Forest for Near Infrared Photodetection Zhiqiang Tang, Yuxiang Han, Mei Sun, Xing Li, Gongtao Wu, Song Gao, Qing Chen, Lian-Mao Peng, and Xianlong Wei ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00733 • Publication Date (Web): 15 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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ACS Applied Nano Materials

Controlling the Growth of Single Nanowires in a Nanowire Forest for Near Infrared Photodetection Zhiqiang Tang, Yuxiang Han, Mei Sun, Xing Li, Gongtao Wu, Song Gao, Qing Chen, Lianmao Peng and Xianlong Wei*

Key Laboratory for the Physics and Chemistry of Nanodevices and Department of Electronics, Peking University, Beijing 100871, China

ABSTRACT

Controlling one-dimension nanowire growth at single nanowire level is vital for building up multi-functional nanowire-based devices and getting insights into nanowire growth mechanisms. In this letter, we report the first control of single nanowire growth in a nanowire forest by touching a growing tungsten oxide nanowire with a nanoprobe inside an environmental scanning electron microscope. Compared with naturally growing nanowires, the touched nanowire exhibits an accelerated radial growth rate and decelerated axial growth rate, with the ratio of axial growth rate to radial growth rate decreasing by a magnitude of up to two orders. It is flexible to alternately accelerate and decelerate the growth rate of single nanowires via nanoprobe touching and detaching. The acceleration of the radial growth rate is attributed to the touching-induced local cooling of the touched nanowires and the consequent

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dominant vapor source deposition on the sidewall due to the reduced effective diffusion length of adatoms and the reduced equilibrium source vapor pressure. Our results represent the first controllable nanowire growth at single nanowire level and provide insights into the diffusion kinetics of nanowire growth. The as-grown nanowires exhibit potential applications in near-infrared photodetectors.

KEYWORDS Controllable nanowire growth, tungsten oxide nanowire, in-situ electron microscopy, environmental scanning electron microscope, photodetector

TEXT

The controllable growth of nanowires has attracted widespread attention for uncovering the growth mechanisms as well as acquiring tailored building blocks to construct functional devices. By controlling the parameters of growing processes (i.e. temperature, pressure, time, sources, substrates, etc.), a rich variety of nanowires with specific morphologies,[1-3] compositions[4-6] and crystal structures[7-9] could be controllably grown, usually in the form of arrays or forests that consists of large number of single nanowires. In these cases, all nanowires in an array or forest share nearly the same morphologies, compositions, crystal structures, and physical properties. However, there are rising demands for the nanowire arrays where nanowires exhibit distinct structures and properties, e.g. semiconductor nanowire arrays with spatially varying bandgap for developing wideband absorption solar cells[10,11] and photodetectors,[12,13] nanowire forests consisting of nanowires with distinct 2


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electrical properties for building multi-functional devices,[14,15] etc. It is therefore highly desirable to control the distinct growth of nanowires in a forest, ideally at single nanowire level. On the other hand, to get insights into the mechanisms for the controllable nanowire growth, it is important to well determine the nanowire growing conditions (i.e. temperature, pressure, sources, etc.) and correlate them to the compositions and structures of the grown nanowires. However, for the conventional controllable nanowire growth where the growing conditions are controlled globally, the local growth conditions are usually unknown due to the fluctuations of temperature, gas pressure and reactant concentrations. In this context, it is of significant importance to develop the methods for controlling the local growing conditions of nanowires to enable the controllable growth of single nanowires in a nanowire forest; however, it still proves challenging. Environmental electron microscopes provide a special venue for growing nanostructures with the growth dynamics being visualized in real time and function as a powerful tool to explore the growth dynamics and mechanisms.[16,17] Using in situ environmental electron microscope, the controllable growth of several kinds of nanowires has been reported, including the diameter and growth direction modulation of Si nanowires by electric field,[18] the growth kinetics control of GaP nanowires,[19] crystal phase switching in GaAs nanowires,[20] etc., and the compositions and structures of the in-situ grown nanowires were immediately determined by electron microscopy. The capabilities of visualizing growth dynamics in real time and determining the compositions and structures of the grown nanowires immediately motivate us to develop a method for controlling the local growing conditions of nanowires to achieve the control of single nanowire growth inside an electron 3



microscope, since the responses of the nanowire growth dynamics and the compositions and structures of nanowires to the modulation of local growth conditions can be immediately obtained. Tungsten oxide, with structural flexibility and versatile applications, have been widely reported for the photocatalysis,[21] electrochemistry[22] and phototherapy.[23] Lots of efforts have been made for the growth of tungsten oxide nanowires,[24,25,31] but controlling tungsten oxide nanowires growth at single level in an electron microscope has not been reported, which is crucial for studying the mechanism of non-catalytic metallic oxide nanowires growth. Herein, we report the modulation of local growth conditions of single tungsten oxide nanowires and the controllable growth of single tungsten oxide nanowires in a nanowire forest by using a nanoprobe inside an environmental scanning electron microscope (ESEM). Through mechanically touching a growing nanowire in a forest using a nanoprobe, the radial growth rate of the touched nanowire is accelerated and the axial growth rate is decelerated, with the ratio of axial growth rate to radial growth rate decreasing by a large magnitude of up to two orders. The growth rate can be controllably accelerated and decelerated via alternate nanoprobe touching and detaching. The acceleration of the radial growth rate is attributed to be the local temperature decrease of the nanowire induced by nanoprobe touching, which causes the dominant vapor source deposition on the sidewall. Our results represent the first controllable nanowire growth at single nanowire level and provide insights into the diffusion kinetics of nanowire growth.

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Figure 1. (a) SEM image showing the experimental setup for the in situ growth of tungsten oxide nanowires on a W filament heated by laser and the control of single nanowire growth using a probe. The laser is guided with an optical fiber. (b)-(d) SEM images of the framed area in (a) showing the growth of a single nanowire at 0 s (b), 314 s (c) and 1048 s (d) after it was touched by the W probe at its top surface. The probe was detached from the nanowires in (d). (e) Schematic illustration of the controlled growth of single nanowires by probe touching. 5



Tungsten oxide nanowires in the form of nanowire forest are synthesized on the surface of a W filament under laser irradiation in O2 atmosphere in an ESEM (FEI Quanta 600FEG) equipped with a multifunctional nanoprobe system.[26] Experiments are carried out in the low vacuum mode with 50 Pa pure O2 in the ESEM chamber. The 980-nm-wavelength laser with a power of 4.0 W is introduced to the chamber through an optical fiber tip to heat the W filament locally (Figure 1(a)). After heating for a few minutes, dense nanowire forest can be observed on the W filament and were determined to be monoclinic W18O49. A symmetric varying profile of morphology and density distribution is observed with respect to the center spot of laser irradiation. As the temperature near the center point is too high for nanowire growth and the sites far away from the center point shows too low temperature for nanowire growth, dense and high quality nanowires are only observed in a narrow zone about 100 µm from the center spot of laser irradiation. The real-time growth process was recorded as Video S1 in the Supporting Information. The composition/structure characterization of the grown tungsten oxide nanowire was shown in supplementary Figure S1. More experimental details and composition/structure characterization of the nanowires can also be found in our previously published paper [26]. To control the growth of a single nanowire in the forest, a W probe is manipulated to touch the top surface of a growing nanowire (Figure 1(a) and 1(b)). It is much intriguing that as the growing procedure keeps going, the touched nanowire that showed similar diameter to its neighbors at the beginning (Figure 1(b)) grows faster in radial direction leading to a ‘fatter’ nanowire as compared with the neighboring naturally growing nanowires (Figure 1(c)). The touched nanowire is easily identifiable in the forest due to its 6


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thick diameter after growth (Figure 1(d)). The results indicate that the growth of a single nanowire in a forest can be controlled via a W probe, as schematically shown in Figure 1(e). The top surface of a growing nanowire is believed to provide the driving force for noncatalytic nanowire growth as it has a lower chemical potential than those of side surfaces.[27] In Figure 1, the W probe contacts the tungsten oxide nanowire at its top surface, which may modified the top surface and destroy the preferred deposition of source molecules there. To exclude the possibility and get more insights into the interesting phenomenon, we also performed experiments where a W probe was manipulated to touch the sidewall of a growing nanowire. Same phenomenon was observed as shown in Figure 2. To describe the phenomenon more quantitatively, the diameter and length of the target nanowire (touched by W probe) and three reference nanowires (R1, R2 and R3) that grew naturally in neighbor were tracked during the growth. As the spatial resolution of SEM is limited at low vacuum mode, we tried our best to take high-resolution SEM photos by maximizing electron beam dwell time and photo pixel. To measure the length of nanowires as precise as possible, SEM images were amplified and a reference point with clear feature was defined for each nanowire (Figure 2(a)-2(f)). The length of the nanowire was measured relative to the reference point. It can be seen from Figure 2(a)–2(f) that the target nanowire shows considerable diameter broadening and slight length elongation, while the reference nanowires show the opposite results. The extraordinary growth behavior of the target nanowire is quite different from the knowledge that the radial growth could be neglected in nanowire growth dynamics.[25] The target nanowire exhibited a diameter expansion of ~105 nm, corresponding to an average radial growth rate of 0.15 nm s-1. In contrast, the reference nanowires showed the expansion 7



of 22 nm (R1), 38 nm (R2) and 8 nm (R3), respectively, corresponding to the average radial growth rate of 0.04, 0.06 and 0.01 nm s-1. The touched target therefore exhibited a radial growth rate several times larger than those of the naturally growing nanowires.

Figure 2. (a)-(f) SEM images showing the growth of a single nanowire at 108 s (a), 155 s (b), 290 s (c), 406 s (d), 526 s (e), and 674 s (f) when it was touched by the W probe at its sidewall. The touched nanowire is marked by arrow in red and three reference nanowires that grew naturally in neighbor are marked in blue, pink, and green. The colored triangles represent the reference locations for measuring the length of the four nanowires. (g)-(h) The plots of the nanowire diameter (g) and length (h) as a function of growth time. The touched nanowire shows enhanced radial growth rate compared with other nanowires. 8


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On the other hand, the length elongation of the target and reference nanowires was measured to be 0.5 µm (target), 1.5 µm (R1), 1.9 µm (R2) and 2.9 µm (R3), with average axial growth rate of 0.74, 2.22, 2.82 and 4.30 nm s-1, respectively. The anisotropic growth rate ratios (υ, defined as the ratio of axial growth rate to radial growth rate) of the three reference nanowires are therefore 55, 47, 430, in the same order as the previously reported ratio of 100.[28] For the target nanowire, the value of υ is 4.9, indicating the similar magnitude of the growth rate in axial and radial directions. Compared with R3, the target nanowire shows a maximal reduction of the anisotropic growth rate ratio up to about two orders of magnitude. The comparison of growth rates between target nanowire and the reference nanowires suggests that the accelerated growth in radial direction and decelerated growth in axial direction were induced by the W probe touching. It is also interesting to found that the nanowires with a high radial growth rate own a low axial growth rate, and vice versa. The negative relationship between the axial and radial growth rate indicates the competition of source molecules deposition on the sidewall and on the top surface. Although synthesis of nanowire is complicated and could be affected by various factors, the probe touching owns a unique and unambiguous effect on the growth rate of nanowires. To evaluate the reliability and flexibility of controlling nanowire growth by a W probe, the radial growth of a single nanowire was alternately accelerated and decelerated via alternate probe touching and detaching. The diameter of three nanowires, including a target nanowire and two reference ones, were tracked during alternate probe touching and detaching (Figure 3). Depending on whether the target nanowire was touched by the probe or not, the 9



growth of the nanowire in Figure 3 can be divided into three stages. In stage I when a tungsten oxide nanowire was touched by the probe (the insets of Figure 3), the diameter of the target nanowire increased quickly with time with a radial growth rate of 0.18 nm s-1, while the reference nanowires exhibited minor increase in diameter. In stage II, the probe was detached from the target nanowire and the radial growth rate of the target nanowire slowed down substantially to 0.07 nm s-1. In stage III, the probe was manipulated to touch the target nanowire again, and the radial growth rate of the target nanowire increased again to 0.16 nm s-1. In contrast with the controllable acceleration and deceleration of the target nanowire growth by the W probe, the two reference nanowires exhibited nearly constant radial growth rate during the whole growth process, with an average growth rate of 0.03 and 0.01 nm s-1, respectively. The controllable acceleration and deceleration of nanowire growth indicate the good reliability and flexibility of controlling single nanowire growth by a nanoprobe.

Figure 3. The plot of the nanowire diameter at different growth time when the nanowire (marked in red) was alternately touched by the W probe (stages I and III) and detached from 10


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the latter (stage II), indicating the flexibility of the controllable growth using the probe. The diameter of two naturally growing reference nanowires (marked in blue and green) were also plotted for comparison. The insets are SEM images at 69 s (stage I), 279 s (stage II) and 404 s (stage III).

We have totally carried out the controllable growth of fifteen nanowires by nanoprobe touching and they all showed significant enhanced radial growth similar to that in Figures 1-3 (another typical experiment results can be seen in supplementary Figure S6). There comes an interesting question why the touched nanowires show much faster radial growth than the naturally growing nanowires? To figure out the reason of this extraordinary phenomenon, it is necessary to explore the growth mechanism. In our experiments, nanowires not only grew on the surface of W filament, but also be found on the surface of optical fiber (supplementary Figure S2), which is used to guide the laser beam for heating as shown in Figure 1(a). Considering that no catalysts were used and no droplets were found on the top surface of nanowires, the growth procedure is thought to be in a physical vapor deposition manner in which source vapors evaporated from the directly irradiated region deposit on the nearby substrate with a lower temperature.[26] The growth of tungsten oxide nanowire therefore follows a vapor-solid (VS) mechanism, agreeing with the previous literatures about noncatalytic tungsten oxide growth.[24,25,28-31] In VS growth mechanism, naturally grown nanowires exhibit initial growth stage and saturated growth stage. Nanowires grow fast in axial direction and show negligible radial growth at the initial stage as a result of continuous supply of source molecules from substrate to the top surface and the preferred deposition of 11



source molecules on the top surface because of its lower chemical potential. Once the length is larger than the diffusion length of source molecules, the diffusion of source molecules to the top surface is limited and growth rate along axial direction slows down, resulting in radial broadening.[32-34] The radial broadening due to limited diffusion of source molecules to the top surface is thought to be responsible for the broadening of the reference nanowires in Figure 2, where the diameters of the reference nanowires started to increase with time after the moment of 300 s. The reason for the slight change of diameter of R3 in Figure 2 is that R3 was in its initial growth stage, while R1 and R2 were close to their saturation stage. We have carried out experiments in both initial (Figure 3) and saturated (Figure 2) stage and it can be seen that the effect of accelerated radial growth will be more obvious in the initial stage. To make the dynamics visualized more clearly as well as exclude the effect of natural broadening in the saturation stage, we mainly perform touching on the nanowire in the initial stage. However, it cannot explain the fast radial growth of the touched nanowire, which exhibited similar length with the reference nanowires but a radial growth rate up to ten times larger than those of the reference nanowires. In order to get more insights into the mechanism for the accelerated radial growth of the touched nanowires, both the target nanowire and the reference nanowires were picked up from the W filament and transferred to a transmission electron microscope (TEM, FEI Tecnai F20) for structure determination.[35-37] The nanowires are transferred from the W filament to the TEM following the three steps as shown in Figure S3. First, one probe is touched on the target nanowire and bonded tightly to the nanowire through electron beam induced deposition of amorphous carbon (Figure S3a). Next, the nanowire is uprooted from the W filament by 12


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retracting back the probe (Figure S3b). Last, the nanowire is placed on the carbon film of a normal TEM copper grid and the probe is detached from the nanowire (Figure S3c). The nanowires can now be characterized in TEM in a routine way by mounting the copper grid into a TEM holder. High-resolution TEM images indicate that both the target nanowire and the reference nanowire exhibited the same crystal structures and growth directions.[26] The typical high-resolution TEM images of the target nanowire and the reference nanowire are shown in Figure 4. The probe touching therefore induced no crystal phase change of the nanowire. However, in contrast with the smooth side surface of the reference nanowires, there are steps on the sidewall of the target nanowires (Figure 4(a) and supplementary Figure S4 for low magnitude TEM images). High-resolution TEM images indicate that the step shows a facet of (113) plane (Figure 4(a)). The steps on the sidewall of the target nanowire indicate that epitaxy growth took place there as a result of nanoprobe touching. This behavior of sidewall epitaxy growth is in accordance with tungsten oxide nanotubes growth in environmental transmission electron microscope, where epitaxy growth of steps on the sidewall was caused by the change of oxygen pressure.[38] In our experiments, the target and reference nanowires are expected to share the same gas pressure due to the close distance of a few microns between them. The growth of the steps on the sidewall here is attributed to the modulation of local growth conditions by the W probe. However, it is still unclear which growth parameter was modulated by the probe.

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Figure 4. High resolution transmission electron microscopy (HRTEM) images of a nanowire touched by W probe during growth (a) and a naturally growing nanowire (b). In contrast with smooth sidewall of naturally growing nanowires, steps were observed on the sidewall of the probe touched nanowire. The insets are the fast Fourier transform patterns of the HRTEM images, which show that the two nanowires both exhibit monoclinic structure and grow along [010] directions (the determination of the growth directions could be found in reference [26]). The step in (a) indicates sidewall epitaxy growth, and the step facet is indexed to be (113).

Both the W probe and filament were grounded and no bias voltage was applied between them, so the possible effect of electric field can be excluded. Considering the probes used in Figure 1-3 are made of W that may provide another material source for the tungsten oxide nanowire growth, we also performed experiments by using Au and Pt probes and observed the same phenomenon as that of W probe (supplementary Figure S5). The bending of nanowires was sometime induced by the probe touching (Figure 2), so mechanical strain is another possible reason for the observed phenomenon. However, a freestanding nanowire that 14


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was adhered to the W probe and thus free of strain was observed to exhibit similar phenomenon (supplementary Figure S6). The effect of mechanical strain is therefore thought to be negligible. It is worth noting that the temperature of the probe and nanowires is quite different and the temperature change of the nanowire is expected to take place when it is touched by a probe. As shown in Figure 1(a), the W probe and the optical fiber locate at the opposite sides of the W filament that is much thicker than the W probe and the optical fiber, so the W probe cannot be directly irradiated by the laser in the vacuum and is thought to stay at room temperature during nanowire growth. Considering the much larger thermal capacity of the W probe than that of a tungsten oxide nanowire, a growing nanowire will be immediately cooled down once it is touched by the W probe. Unfortunately, we cannot directly measure the temperature of a nanowire in the ESEM. To consolidate the local cooling effect on the nanowire caused by probe touching, we simulated the temperature distribution of a nanowire before and after it is touched by a probe (see supplementary Figure S7 for details). The dimensions of tungsten filament and probe can be thought to be infinite compared with that of the nanowire, so the temperature of tungsten filament and probe are set to fix at 973 K (the temperature for tungsten oxide nanowire growth[24,30]) and 298 K (room temperature) in simulation. Before probe touching (Figure S7c), the nanowire exhibits uniform temperature distribution at 973 K along the axis. After probe touching (Figure S7d), the nanowire exhibits an approximately linear temperature distribution from ~973 K (bottom end) to 298 K (top end). The results unambiguously suggest an overall cooling of the nanowire induced by probe touching. 15



Combining with the comparative experiments, TEM characterization and simulated temperature change of a touched nanowire, we therefore attribute the accelerated radial growth and decelerated axial growth of the touched nanowire to the cooling of the nanowire by the W probe. In the VS growth mechanism without catalysts, the dominant axial growth is attributed to the preferred deposition of source molecules on the top surface due to its lower chemical potential than that of the sidewalls.[39] The source molecules depositing on the top surface are supplied mainly by the diffusion from the substrate and the sidewall. While a growing nanowire is cooled down by the W probe, the decrease of temperature induce two effects: the reduced effective diffusion length of adatoms and the reduced equilibrium vapor pressure. Due to the reduced effective diffusion length, nucleation on the sidewall occurs and vapor molecules are captured by the nucleation islands. Furthermore, the decrease of equilibrium vapor pressure induced by local cooling may result in supersaturation of vapor,[28] which leads to rapid deposition of vapor molecules on the exposed facets of the touched nanowire. The vapor molecules from gaseous environment therefore uniformly deposit on both the top surface and the sidewall, and the radial growth is significant accelerated. The mechanism can well explain the experiment result that the anisotropic growth rate ratios decrease from more than 47 to 4.9 when a nanowire was touched by the W probe in Figure 2. The mechanism for accelerated radial growth by probe touching is schematically shown in Figure 5. The effect of temperature on the nanowire diameter was previously reported in literatures,[28,40,41] where the diameter of nanowire was found to be inversely proportional to the substrate temperature due the temperature dependent critical nucleus diameter. Here we show that the diameter of nanowires can be modulated in the 16


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growth stage through local temperature modulation by using a probe. These results suggest that touching induced temperature decrease of nanowire is the main reason for the phenomenon of accelerated radial growth rate.

Figure 5. Schematic diagram showing the transition from the preferred source molecules deposition on the top surface through surface diffusion for naturally growing nanowires to the uniform deposition on all the surface for the nanowire cooled by probe touching.

In conclusion, we report the controllable growth of single nanowires in a nanowire forest for the first time by modulating the local growth conditions using a probe in an ESEM. By touching a growing tungsten oxide nanowire using a probe, the radial growth rate of the touched nanowire was accelerated and its axial growth rate was decelerated, with the ratio of axial growth rate to radial growth rate decreasing by a large magnitude of up to two orders. It is flexible to accelerate and decelerate the growth rate in a well-controlled manner via alternate probe touching and detaching. The accelerated radial growth and decelerated axial 17



growth of the touched nanowires is attributed to the local cooling induced by probe touching, which causes the reduced effective diffusion length of adatoms and the reduced equilibrium vapor pressure. The decrease of temperature prevents source molecules diffusing from sidewall to the top and leads to a high degree of vapor supersaturation around the nanowire, which consequently results in uniform deposition of vapor source molecules on exposed surface. The controlled growth of single nanowire by probe touching indicates our effective controlling of the diffusion pathways for vapor molecules in a local environment. Our synthesized tungsten oxide nanowires are demonstrated to exhibit pronounced optoelectronic response to near-infrared irradiation of 980 nm with a responsivity of 12.84 A/W and have potential applications in photodetectors (supplementary Figure S8). Our technique of manipulating single growing nanowires under real time observation of electron microscope opens up a flexible way of controlling nanowire growth at single nanowire level. The control of single nanowire growth in a nanowire forest implies the possibility of tailoring the compositions, structures and properties of single nanowires one by one in a nanowire forest through controllable growth at single nanowire level.

ASSOCIATED CONTENT Supporting Information. Structure and composition characterization of tungsten oxide nanowires, nanowires grown on the optical fiber, transferring process of nanowires from W filament to TEM, TEM images of probe touched nanowires, SEM images of controlling nanowire growth using Pt and Au tip, growth procedure of a nanowire free of mechanical strain, simulated nanowire temperature change after probe touching, photodetectors based on 18


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a single nanowire, real-time video showing the in situ growth of tungsten oxide nanowires. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author * [email protected]

Notes The authors declare no competing financial interest

ACKNOWLEDGEMENTS

We thank Prof. L.W. Yu in Nanjing University for valuable discussion. This work was supported by the National Key Research and Development Program of China grants 2017YFA0205003 and 2016YFA0200802, and the National Science Foundation of China grant 61621061.

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