van der Waals Epitaxial Ultrathin Two-Dimensional Nonlayered

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Van der Waals epitaxial ultrathin two dimensional nonlayered semiconductor for highly efficient flexible optoelectronic devices Qisheng Wang, Kai Xu, Zhenxing Wang, Feng Wang, Yun Huang, Muhammad Safdar, Xueying Zhan, Fengmei Wang, Zhongzhou Cheng, and Jun He Nano Lett., Just Accepted Manuscript • Publication Date (Web): 20 Jan 2015 Downloaded from http://pubs.acs.org on January 21, 2015

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Van der Waals epitaxial ultrathin two dimensional nonlayered semiconductor for highly efficient flexible optoelectronic devices Qisheng Wang, Kai Xu, Zhenxing Wang, Feng Wang, Yun Huang, Muhammad Safdar, Xueying Zhan, Fengmei Wang, Zhongzhou Cheng and Jun He National Center for Nanoscience and Technology, Beijing 100190, China ABSTRACT: In spite of great progress in synthesis and application of graphene-like materials, it remains a considerable challenge to prepare two dimensional (2D) nanostructures of nonlayered materials which may bring us surprising physical and chemical properties. Here, we propose a general strategy for the growth of 2D nonlayered materials by van der Waals epitaxy (vdWE) growth with two conditions: (1) the nonlayered materials satisfy 2D anisotropic growth and (2) the growth is implemented on the van der Waals substrates. Large-scale ultrathin 2D Pb1-xSnxSe nanoplates (~15-45 nm) have been produced on mica sheets by applying this strategy. Benefiting from the 2D geometry of Pb1-xSnxSe nanoplates and the flexibility of mica sheet, flexible photodetectors which exhibit fast, reversible, and stable photoresponse and broad spectra detection ranging from UV to infrared light (375, 473, 632, 800 and 980 nm) are in situ fabricated based on Pb1-xSnxSe nanoplates. We anticipate that more nonlayered materials will be developed into 2D nanostructures through vdWE, enabling the exploitation of novel electronic and optoelectronic devices. KEYWORDS: 2D nanostructures, nonlayered materials, van der Waals epitaxy, flexible optoelectronic devices

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Two dimensional (2D) materials such as graphene1, BN2, MoS23, Bi2Se34, 5 and In2Se36 have recently attracted considerable interest due to their fascinating properties such as high electron mobility7, superconductivity8, quantum Hall effects9 and quantum anomalous Hall effect10. And they show great potential in the applications of electronic and optoelectronic devices, including photodetectors11, transistors12, 13, pressure sensors14, spin-valleytronic devices15, p-n diodes16, 17 and light-emitting diodes18. Furthermore, benefiting from their 2D geometry, planar materials exhibit high compatibility with traditional microfabrication techniques and flexible substrates, which makes them the ideal building blocks for the fabrication of wearable flexible electronic and optoelectronic devices19,

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. Importantly, due to strong in-plane covalent bonding in

individual atomic layer and weak van der Waals interaction between two adjacent layers, it is easy to engineer 2D nanostructures of layered materials by chemical vapor phase deposition3 or mechanical exfoliation21. So far the preparation of 2D nanostructures is mainly restricted to the layered materials with planar crystal structure22. However, many materials with important functions have nonlayered crystal structure such as SnTe, Pb1-xSnxSe and Pn1-xSnxTe, a kind of topological crystalline


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insulators (TCIs) with narrow band gap and special applications in not only infrared detection but also spintronics devices23-26. In contrast to layered materials, these materials normally have isotropic crystal structure and incline to stack into three dimensional (3D) nanostructures due to the lack of driving force for the 2D anisotropic growth27, 28. Therefore, it is a big challenge to synthesize ultrathin 2D nanostructures of nonlayered materials, which impedes the exploration of their novel physical and chemical properties and their applications as nanoscale electronic and optoelectronic devices as being discussed in this work. Here, by comparing the growth of Pb1-xSnxSe nanoplates and SnTe nanopyramids in this work and Te nanoplates in our previous work29, we have proposed a general strategy for the growth of 2D nanostructures of nonlayered materials by van der Waals epitaxy (vdWE) with two conditions: (1) the nonlayered materials satisfy 2D anisotropic growth and (2) the growth is implemented on the van der Waals (vdW) substrates. By utilizing this strategy, we have obtained large-scale 2D ultrathin Pb1-xSnxSe nanoplates (~15-45 nm) on layered mica sheets. Profiting from the insulativity and flexibility of mica sheets, flexible photodetectors based on Pb1-xSnxSe nanoplates were in situ fabricated on mica sheets. The devices display broad spectra response ranging from UV to infrared light (375, 473, 632, 800 and 980 nm) and fast, stable and reversible photoresponse even after being bent for 100 times with a bending radius of 4 mm, indicating that 2D Pb1-xSnxSe nanoplates-based photodetectors on mica sheets have a great application potential in flexible and wearable optoelectronic devices. Meanwhile, 2D ultrathin Pb1-xSnxSe nanoplates with distinct {100} surfaces lay a foundation for the study of topological surface states on highsymmetry {100} surfaces30. Figure 1(a) presents the cubic crystal structure of Pb1-xSnxSe. It will be demonstrated later that Pb1-xSnxSe shows strong 2D anisotropic growth with 410 or 450 oC as substrate temperature.


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With the utilization of vdW substrate, ultrathin Pb1-xSnxSe nanoplates will be obtained as shown in the schematic diagram (Figure 1(b)). Here, layered fluorophlogopite mica [KMg3(AlSi3O10)F2] with pseudohexagonal Z2O5 (Z=Si, Al) layered structure was used as the vdW substrates. Fluorophlogopite mica was firstly mechanically cleaved into flexible transparent thin flakes with exposed (001) surface (inset of Figure 1(c)). The synthesis was performed by thermally evaporating mixture of Pb and SnSe powders. More experimental details are described in the Supporting Information. The optical microscopy (OM) image of Pb1-xSnxSe nanoplates grown on mica sheets is shown in Figure 1(c). The OM images reveal that Pb1-xSnxSe nanoplates are triangle or square with smooth surface and this is further confirmed by the scanning electron microscopy (SEM) images in Figure 1(d), (e), (f) and (g). Domain size of Pb1-xSnxSe nanoplates grown at source temperature of 550 oC and substrate temperature of 410 oC ranges from 1 to 8 µm (Figure S1(a)). Lateral dimension further rises to 3-11 µm when the temperatures of source and substrate increase to 600 and 460 oC respectively (Figure S1(b)). The growth time of the nanoplates displayed in Figure 1(c), (d) and (e) was set to 1 min. The domain size dramatically increases to ~20 and ~50 µm when the growth time rises to 5 and 30 mins respectively (Figure 1(f) and (g)). Here the source and substrate temperatures are kept unchanged (600 oC for the former and 460 o

C for the latter). The growth density of nanoplates on mica is 11000/mm2 at substrate

temperature of 410 oC and source temperature of 550 oC. It drops to 880/mm2 with the increase of substrate temperature from 410 to 460 oC and source temperature from 550 oC to 600 oC. The parameters that influence the growth density of nanoplates are discussed in detail in the Supporting Information. Figure 1(h) is the atomic force microscopy (AFM) image of one square nanoplate with source temperature of 550 oC and substrate temperature of 410 oC. Statistics of


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AFM images demonstrate that the thickness of nanoplates ranges from 15 to 45 nm (Figure 1(j)). When the source temperature reaches 600 oC and the substrate temperature reaches 460 oC, the height increases to ~50-90 nm (Figure 1(i), Figure S1(c) and Figure S1(d)). Mechanism of temperature-modulated domain size and thickness of Pb1-xSnxSe nanoplates is analyzed in the Supporting Information. In order to investigate the crystal structure, we have performed transmission electron microscopy (TEM) on Pb1-xSnxSe nanoplates. Both the triangle and square nanoplates exhibit smooth surface as displayed by the low-resolution TEM images in Figure 2(a) and (d) respectively. High-resolution TEM (HRTEM) images (Figure 2(b) and (e)) demonstrate both triangle and square Pb1-xSnxSe nanoplates are well-defined single crystalline. Here the HRTEM images in Figure 2(b) and (e) were obtained from the regions labeled by yellow squares in Figure 2(a) and (d), respectively. Each of the in situ selected area electron diffractions (SAED) (insets of Figure 2(b) and (e)) exhibits a single set of perfect cubic pattern which is in agreement with the cubic symmetry of Pb1-xSnxSe. Therefore, the top and bottom surfaces of the triangle nanoplate can be assigned to (002) and (00-2) facets which belong to topologically protected surfaces. The cubic pattern of SAED also indicates the triangle is an equilateral right triangle with hypotenuse constituted by (-110) surface. HRTEM obtained from the region near hypotenuse (Figure S2(b)) gives clear crystal fringes of (110) planes. One possible explanation is that Pb1-xSnxSe first crystallizes towards [110] direction and forms a rectangle Pb1-xSnxSe nanoribbon (Figure S3(a)). And then Pb1-xSnxSe nanoribbon grows up along lateral [-110] and [1-10] directions with final exposed equivalent {200} crystal surfaces (Figure S3(b)). In most cases, epitaxial growth occurred at the two equivalent lateral surfaces of Pb1-xSnxSe nanoribbon which are (-110) and (1-10) surfaces. Thus we can see in Figure 1(c) most nanoplates are square.


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The lack of bottom corner in square nanoplate (Figure 2(d)) could be ascribed to an incomplete growth of Pb1-xSnxSe nanoribbon along (-110) or (1-10) direction. This phenomenon can also be observed in some SEM images (Figure 1(d) and (e)). As a result, what causes the 2D anisotropic growth of Pb1-xSnxSe nanoplates is that {110} equivalent surfaces have higher growth activation in comparison with other facets. TEM-EDX elemental mapping shows Pb, Sn and Se distribute evenly in the entire nanoplate without detectable phase separation (Figure 2(c)). Quantitative analysis of TEM-EDX indicates the elemental ratio of Pb, Sn and Se is ~0.21:0.79:1.00. Raman spectra (Figure 2(f)) of a Pb1-xSnxSe nanoplate displays an obvious peak at 112 cm-1 which originates from longitudinal optical (LO) phonon mode31. Above results confirm Pb1-xSnxSe nanoplates obtained in our work are of high quality. We now discuss the mediation of substrate surface electronic properties on the vapor deposition of Pb1-xSnxSe nanoplates. Nonlayered materials normally have three dimensional (3D) bonded crystal structures and lack an intrinsic driving force for 2D anisotropic growth. Consequently, 3D nanoarchitectures are often obtained in CVD growth process22. However, for some nonlayered materials, their anisotropic growth of 2D structure can be activated by modulating growth parameters such as temperature, pressure and flow rate. As demonstrated in this work, Pb1-xSnxSe is of 2D anisotropic growth due to the high growth activation of {110} surfaces. Thus, after nucleation, Pb1-xSnxSe grows along chemical inert surface of mica sheets without restriction of large lattice mismatch32, 33, also known as van der Waals epitaxial growth. Figure 3(a) and (b) exhibit the SEM images of 2D Pb1-xSnxSe nanoplates obtained on mica. The source temperatures are set to 550 and 600 oC respectively and the substrate temperatures are set to 410 and 460 oC respectively. The advantages of vdWE lie in the following aspects: (1) overlayer is perfectly relaxed without excessive strain in the heterointerface; (2) strict


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requirement of lattice matching is circumvented (Figure 3(e)), enabling the growth of defect-free overlayer with different crystalline symmetry to that of substrate and (3) chemically inert mica surface facilitates the migration of adatoms, promoting the lateral growth of 2D Pb1-xSnxSe nanoplates. This growth mode reminds us of the deposition of thin film by Volmer-Weber mode in which the interaction between adatoms and substrate is much weaker than that between adjacent adatoms. Irregular Pb1-xSnxSe microcrystals with high density are observed on mica surface at higher growth temperature of 510 oC and source temperature of 650 oC (Figure S4(a)), which would be resulted from (1) overmuch supply of vapor source at high source temperature and (2) over high migration rate of adatoms at high substrate temperature, both of which cause the fast growth of Pb1-xSnxSe microcrystals along multiple crystal directions and the stacks of products on mica surface. However, 2D Pb1-xSnxSe nanoplates cannot be acquired on the surfaces of 3D bonded Si substrate even if we exerted the same growth conditions as that on mica as shown in Figure 3(c) and (d). Although Pb1-xSnxSe nanoplates are observed on Si surface at substrate temperature of 410 oC and source temperature of 550 oC (Figure 3(c)), their structures are vertical instead of planar, making them incompatible with traditional microfabrication technologies. The apparent morphological differences between the products on mica and that on Si originate from the variations of substrate electronic structure. Due to its 3D bonded crystal structure, Si surface accommodates large numbers of unsaturated dangling bonds. Therefore, large lattice mismatch between Si (100) surface and Pb1-xSnxSe (~10.4%) impedes the direct epitaxial growth of Pb1-xSnxSe nanoplates along the substrate surface. And strong interaction between substrate and adatoms, caused by dangling bonds on the surface of 3D bonded Si, increases the migration energy barriers of adatoms. The nearly vertical geometry of Pb1-xSnxSe nanoplates dramatically reduce interface area and thus easily relieve strain energy via


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lateral relaxation. As for the products at higher growth temperature on Si (substrate temperature 460 oC and source temperature 600 oC), they are cubic or irregular microcrystals (Figure 3(d)). The vertical size of cubic microcrystals is much larger than that of Pb1-xSnxSe nanoplates on mica. Migration barrier energy Em of adatoms on Si surface is much bigger than that on mica because of the stronger interaction between adatoms and Si substrate. Considering the migration coefficient D is related to migration barrier energy by ‫ ∝ ܦ‬eିா೘/௞்

(1)

where k is the Boltzmann constant and T is the substrate temperature. Consequently, migration coefficient on Si surface (DSi) is far lower than that (Dmica) on mica with the same growth temperature and time. Thus we can see the stack of cubic Pb1-xSnxSe microcrystals on Si surface but planar Pb1-xSnxSe nanostructure on mica surface. Some Pb1-xSnxSe microcrystals with vertical orientation are also obtained (Figure 3(d)), which can be explained by the fact that vertical architecture favors strain relaxation due to its small interface area (Figure 3(f)). Therefore, by carefully analyzing the vdWE growth of Pb1-xSnxSe nanoplates in this work and Te nanoplates in our previous work29, we can conclude two conditions for the growth of 2D nonlayered materials by vdWE: (1) the nonlayered materials satisfy 2D anisotropic growth and (2) the growth is implemented on the vdWE substrates. This can be further confirmed by the growth of SnTe nanostructures (Figure S5). Although SnTe displays perfect vdWE growth that regular nanopyramids array is formed on mica sheets at growth temperature of 510 oC and source temperature of 650 oC, SnTe nanostructures are not planar architecture owing to the 3D isotropic growth. Benefiting from the flexibility of mica sheets and 2D geometry of Pb1-xSnxSe nanoplates, flexible photodetectors based on Pb1-xSnxSe nanoplates were in situ fabricated on mica sheets by


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traditional device fabrication techniques as shown in the Supporting Information. Figure 4(a) is the photograph of flexible photodetector of which the inset signifies the optical microscopy (OM) image of electrodes array. Figure 4(b) exposes the electrical transport characteristic of single triangle Pb0.21Sn0.79Se nanoplate device (inset of Figure 4(b)). Linear plot of current (I) versus voltage (V) demonstrates the Ohmic contact of our device. The pronounced photoresponse is observed under the illumination of 375, 473 and 632 nm lasers. Light intensity of 375, 473 and 632 nm lasers was set to 302.9, 232 and 82.6 mW/cm2 respectively. The device shows highly stability at various wavelength (375, 473 and 632 nm) as illustrated by time-resolved photoresponse in Figure 4(c), (e) and (f). Photocurrent exhibits almost the same level with light on. In addition, with the decrease of light intensity from 232 to 5.1 mW/cm2, the photocurrent gradually drops under the illumination of 473 nm laser. Here, photocurrent (∆I) is defined as the differences between Ion and Ioff and 2 V voltage bias is applied. However, even if we reduce the laser power to 5.1 mW/cm2, the device remains highly responsive, reflecting the high sensitivity of two-terminal Pb0.21Sn0.79Se nanoplate device. The linear relation (inset of Figure 4(d)) between light intensity and photocurrent indicates more photons generate more electrons in Pb0.21Sn0.79Se nanoplate device. Photoresponsivity (Rλ), defined as the photocurrent generated per unit power of incident light on the effective area, is estimated to be 5.95 A/W at the low irradiation intensity of 5.1 mW/cm2 and voltage bias of 2 V. The efficient irradiation area is 32.93 µm2. The response time (τon) and recovery time (τoff) can be estimated from an enlarged single circle as shown in Figure 4(d). The sharp rise and drop of current with light on and off respectively confirm the device is ultrasensitive to light. The τon and τoff are calculated to be 0.90 and 0.74 s respectively. The actual response time could be faster than the measured values due to the limitation of measuring accuracy of our instruments. We further carried out photoresponse


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experiments of Pb0.21Sn0.79Se nanoplate on near-infrared light (800 and 980 nm). As shown in Figure 4(g-i), the device shows reversible and fast response on both 800 and 980 nm infrared light. The light power of 800 and 980 nm near-infrared light is set to 540 and 3.06 mW respectively. Theoretically, photoresponse range of Pb0.21Sn0.79Se nanoplate can extend to midinfrared light (~3 µm) due to its narrow bandgap (~0.43 eV). Limited by experimental conditions, we have not executed the corresponding experiments. However, we have investigated the absorption of Pb1-xSnxSe nanoplates array on mica sheet which displays a wide absorption spectra from 250 nm to 2.5 µm (Figure S7), indicating the application potential of Pb1-xSnxSe nanoplates as mid-infrared detector. In order to assess the durability of the Pb1-xSnxSe nanoplates-based flexible optoelectronic devices, the device was bent for 100 times with a bending radius of 4 mm as shown in Figure 5. Decrease of both dark current and photocurrent are caused by contact barrier due to the relaxation of the electrode contacts (Figure 5(c)). By improving the device fabrication techniques, the contact stability would be enhanced. Although the photocurrent decreases after bending, the current of device increases sharply once the light is on, suggesting the high durability and sensitivity of the Pb1-xSnxSe nanoplates-based flexible photodetector. In summary, we have conducted the growth of large-scale ultrathin 2D Pb1-xSnxSe nanoplates with a thickness of ~15-45 nm through vdWE techniques on mica sheets. Together with our previous work about vdWE growth of 2D Te nanoplates, we conclude that vdWE can be universally used to synthesize 2D nanostructures of nonlayered materials on condition that they have the property of 2D anisotropic growth. Developing 2D nanostructures of nonlayered materials will enrich the family of 2D materials and introduce novel electronic and optoelectronic device applications. As demonstrated in this work, 2D Pb1-xSnxSe nanoplates-


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based flexible photodetectors display fast, reversible and stable photoresponse with wide spectra response ranging from UV to infrared light. Further, owing to the similarity of surface chemical properties between mica and other layered materials, it is strongly expected to obtain vertical heterostructures of nonlayered/layered semiconductors by using vdWE such as Pb1-xSnxSe nanoplates/few-layers BN which are promising materials in the fabrication of high performance electronic and optoelectronic devices.


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Figure 1

Figure 1. Schematic illustrations, SEM and AFM characterizations of van der Waals epitaxial ultrathin 2D Pb1-xSnxSe nanoplates. Pb1-xSnxSe with cubic crystal structure (a) can be tailored into ultrathin 2D nanostructures (b) by vdWE at certain growth temperature under which Pb1-xSnxSe is of 2D anisotropic growth. (c) OM image of Pb1-xSnxSe nanoplates with source temperature 600 oC and substrate temperature 460 oC, inset is the photograph of a flexible mica sheet. (d) Magnified SEM images of triangle and square Pb1-xSnxSe nanoplates with source temperature 550 oC and substrate temperature 410 oC. (e) Magnified SEM images of triangle and square Pb1-xSnxSe nanoplates with source temperature 600 oC and substrate temperature 460 oC. The growth time for the products shown in Figure 1(c), (d) and (e) is 1 min. (f) and (g) The growth time increases to 5 and 30 mins respectively keeping source temperature 600 oC and substrate temperature 460 oC. (h) AFM image of a Pb1-xSnxSe nanoplate with substrate temperature 410 oC, showing thickness of 21 nm. (i) AFM image of a Pb1-xSnxSe nanoplate with substrate temperature 460 oC, showing thickness of 81 nm. (j) Histogram of Pb1-xSnxSe nanoplates thickness with substrate temperature 410 oC, smooth curve is the Gaussian fit of the thickness distribution.


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Figure 2

Figure 2. TEM and Raman characterizations of Pb1-xSnxSe nanoplates. (a) and (d) are low-resolution TEM images of triangle and square Pb1-xSnxSe nanoplates respectively, yellow square denotes regions where HRTEM images were taken. (b) and (e) are HRTEM images of triangle and square Pb1-xSnxSe nanoplates respectively with corresponding SAED as the insets. (c) TEMEDX mapping of triangle Pb1-xSnxSe nanoplate from (a). (f) Raman spectra of Pb1-xSnxSe nanoplate with peak of 112 cm-1.


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Figure 3

Figure 3. Comparison of growth on mica and Si at various temperatures. (a) and (b) SEM images of Pb1-xSnxSe micronanostructures on mica. (c) and (d) SEM images of Pb1-xSnxSe micro-nanostructures on Si. (a) and (c) The source and substrate temperatures are set to be 550 and 410 oC respectively. (b) and (d) The source and substrate temperatures are set to be 600 and 460 oC respectively. The growth duration of all experiments are set to be 1 min. (e) and (f) Schematic illustrations of Pb1-xSnxSe micro-nanostructures growth process on the surface of mica and Si respectively.


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Figure 4

Figure 4. Flexible optoelectronic devices based on Pb1-xSnxSe nanoplates on mica sheets. (a) Photograph of 2D Pb1-xSnxSe nanoplates-based device, inset is the OM image of electrodes pattern. (b) I-V curves in the dark and in the presence of 375, 473 and 632 nm laser of a single Pb1-xSnxSe nanoplate photodetector, inset is the SEM image of the device. (c) Time-dependent photoresponse of Pb1-xSnxSe nanoplate device at various light intensity with voltage bias of 2 V and laser wavelength of 473 nm. (d) A separated response and reset cycle, inset is the power dependence of photocurrent at 473 nm and 2 V bias. (e) and (f) Timedependent photoresponse of Pb1-xSnxSe nanoplate device at 375 and 632 nm respectively. (g) I-V curves in the dark and in the presence of 800 nm laser of another Pb1-xSnxSe nanoplate photodetector, inset is the SEM image of the device. Time-dependent photoresponse of Pb1-xSnxSe nanoplate device with laser wavelength of 800 nm (h) and 980 nm (i). The light power of 800 and 980 nm lasers is set to be 540 mW and 3.06 mV, respectively. The corresponding voltage bias is 2 and 1.5 V, respectively.


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Figure 5

Figure 5. Durability measurements of Pb1-xSnxSe nanoplates-based flexible photodetector on mica sheet. (a) Photograph of instrument used for bending. Time trace of photoresponse before (b) and after (c) bending the device for 100 times.


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ASSOCIATED CONTENT Supporting Information Experimental section, SEM, TEM and AFM images, statistic of Pb1-xSnxSe nanoplates domain size, photoresponse of square Pb1-xSnxSe nanoplate. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no completing financial interest. ACKNOWLEDGMENT This work at National Center for Nanoscience and Technology was supported by 973 Program of the Ministry of Science and Technology of China (No. 2012CB934103), the 100-Talents Program of the Chinese Academy of Sciences (No. Y1172911ZX), the National Natural Science Foundation of China (No. 21373065) and Beijing Natural Science Foundation (No. 2144059).


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