Growth Mechanism and Enhanced Yield of Black Phosphorus

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Growth Mechanism and Enhanced Yield of Black Phosphorus Micro-ribbons Ming Zhao, Yewu Wang, Haolei Qian, Xinyue Niu, Wei Wang, Liao Guan, and Jian Sha Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01709 • Publication Date (Web): 31 Dec 2015 Downloaded from http://pubs.acs.org on January 1, 2016

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Crystal Growth & Design

Growth Mechanism and Enhanced Yield of Black Phosphorus Micro-ribbons Ming Zhao, Haolei Qian, Xinyue Niu, Wei Wang, Liao Guan, Jian Sha, and Yewu Wang *

Department of Physics & State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, P. R. China

ABSTRACT Black phosphorus, a promising two-dimensional material to be widely used in many areas of electronics and optoelectronics, can be prepared traditionally by high pressure or fast low-pressure transport routes. However, there lacks a general understanding of the growth mechanism and it often suffers from poor yield and high cost. In this paper, we developed a facile method to synthesize large-scale black phosphorus micro-ribbons, significantly decreasing its cost and increasing its yield, and then it can commercially produce black phosphorus. The growth process of black phosphorus micro-ribbons has been investigated systematically and its growth mechanism has been speculated, which opens up the possible opportunity to directly grow black phosphorus micro-belts even few-layered nanobelts by adjusting the growth conditions. In addition, ribbon-like few-layered black phosphorus with large area can be easily exfoliated from the grown micro-ribbons because of their smooth and large area of the cleavage plane. The ribbon-like few-layered phosphorus is beneficial to investigate the anisotropic properties of black phosphorus.

* To

whom correspondence should be addressed. E-mail: [email protected].

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1. INTRODUCTION The successful exfoliation of graphite to graphene has opened a new research field studying on the properties of two-dimensional materials.1 Recently, many graphene like atomic-layer systems, including transition metal dichalcogenides (TMDs),2 hexagonal boron nitride (h-BN),3 silicene,4 has been reported and proved to be candidates for future electronic and optoelectronic applications.5-7 Since early 2014, black phosphorus (B-P) has been rediscovered as a promising new two-dimensional material (the schematic structure is shown in Figure 1) due to its widely tunable and direct bandgap by layer thickness from 0.3 eV in bulk to ~ 2.0 eV in a monolayer,8-11 high carrier mobility,12-14 and strong intrinsic in-plane anisotropic electrical, optical and phonon properties.12, 13, 15-17 Usually, the mechanical exfoliation and ultrasonic methods are the two major approaches to prepare few-layered B-P from bulk phosphorus.18-22 Therefore, the quality of the starting material B-P plays a crucial role in all cases. In 1914, Bridgeman discovered a method to convert white phosphorus to B-P under a pressure as high as 1.2GP with the temperature 200℃.23 Other synthetic methods such as mercury catalyzing,24 bismuth-flux,25 and chemical transport reaction26 have been reported in succession. Recently, Nilges et al. reported the access of B-P under relatively low pressure via chemical vapor transport,27 which was applied in commercial production of bulk B-P. However, this method still suffers from low yield, high cost (the use of tin (IV) iodide) and hash growth conditions (even involve toxic chemicals). In some recent cases, the growth temperature difference between the

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source materials and the growth site of B-P should keep at 45℃~50℃, otherwise, white phosphorus and violet phosphorus will be the main product containing a small amount of B-P.27, 28 The harsh growth conditions and the low yield result in the very high price of bulk B-P in the market. In addition, the size of the prepared few-layered B-P using the mechanical exfoliation or ultrasonic method from the bought bulk phosphorus is usually below 10 μm and its yield is also very low,12, 18, 22, 29, 30 which limits its scientific research progress. Herein, we developed a facile method to commercially produce B-P micro-ribbons. The source material was sealed in ampoule tube under atmospheric pressure and moreover the temperature difference between the source materials and the growth site of B-P is not necessary. Thus the muffle furnace with much larger reaction space was used to replace the tube furnace as the reactor, significantly increasing the yield of B-P. Additionally, tin (IV) iodide was replaced by iodine, cutting the preparing process of tin (IV) iodide and then resulting in the cost decrease of B-P. The growth process of B-P micro-ribbons has been investigated systematically and its growth mechanism has been speculated. Moreover, it has been demonstrated that the large size few-layered B-P can be more easily exfoliated from B-P micro-ribbons. The exfoliated ribbon-like few-layered phosphorus is also beneficial to investigate the anisotropic properties of B-P. The reliable large-scale and cheap method introduced here should play a significant role in the scientific research and applications of electronic and optoelectronic devices composed of few-layered B-P.

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2. EXPERIMENTAL SECTION 2.1 Synthesis of B-P Red phosphorus (Aladdin, 99.999% metal basis), Tin (Aldrich, powder, 99.99% trace metal basis) and iodine (Aldrich, 99.995% trace metal basis) are used as source materials for the synthesis of B-P. They were carefully transferred to a silica glass ampoule in a glove box. The silica glass ampoule was then directly sealed under atmospheric pressure after it was moved out from glove box. The sealed silica glass ampoules were finally loaded in muffle furnace for B-P micro-ribbon growth. The number of silica glass ampoule in furnace only depends on the space of the furnace. A typical growth process was set as follows: the temperature of furnace was slowly increased from room temperature to 863K and then kept at this temperature for 2 h. Subsequently, the temperature was decreased to 758K and kept at this temperature for 2 h. Then, the temperature was decreased to 393K slowly. Finally, the silica glass ampoules were moved out and cooled down to room temperature naturally. Long B-P micro-ribbons were densely distributed in the silica glass ampoule. The grown micro-ribbons were cleaned in ethyl alcohol by ultrasonic. Two experiments were designed to reveal the bottom-up growth behavior of B-P micro-ribbons. In the first experiment, the initial source materials were loaded on the top surface of carbon paper and then the carbon paper was horizontally transported into the ampoule tube. In the other experiment, a piece of semi-cylinder quartz with same diameter of ampoule tube was loaded into the ampoule tube, therefore, the semi-cylinder quartz and ampoule were contacted very closely.

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Eleven experiments were designed with the same growth conditions to figure out the growth mechanism of B-P micro-ribbons, while the growths were stopped at different stages and a series of photos have been taken at these stages. 2.2 Characterization of B-P A fraction of B-P micro-ribbon was carefully transferred on a glass slide for x-ray diffraction (XRD) measurement. It was carried out at room temperature on a PAN analytical x-ray diffractometer equipped with a monochromatic CuKα1 radiation. Raman spectroscopy measurement was carried out using an ANDOR (iXon Ultra) spectrometer equipped with a 532 nm laser and a power of 2 mW. The entrance slit width was fixed 100 μm and the data was collected with 5 s of integration time. The field emission scanning electron microscope (FE-SEM, HITACHI S-4800) and transmission electron microscopy (TEM, JEM-2010(HR)) were used to characterize the morphology and microstructure of B-P micro-ribbons. Energy dispersive x-ray analysis (EDAX, Oxford) was performed to examine the purity of the grown micro-ribbon with an Octane Plus detector. A strip of B-P was carefully grinded for 10 minutes in mortar with ethyl alcohol. Then, a few drops of the grinded solution were dropped on copper grids for TEM characterization. Optical microscopy (OLYMPUS BX51M) and atomic force microscopy (AFM, Di NanoScope 3D, company of Veeco) were used to observe the prepared few-layered B-P samples on SiO2 (300nm)/Si wafers. Few-layered B-P was prepared by mechanic exfoliation from the B-P micro-ribbon. To fabricate field effect transistors (FET) based on few-layered B-P, the

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exfoliated B-P thin flakes were transferred to a cleaned Si wafer with a 300nm-thick thermally grown SiO2 layer. Subsequently, copper TEM grids were used as the mask and Ti/Au (200nm/100nm) were deposited on B-P flakes by magnetron sputtering method as the drain and source electrodes. The electronic properties of FETs were measured in a scanning electron microscope chamber (~1e-3 Pa) with two tungsten probes. FET based on B-P few layer was fixed on the sample holder via which the gate bias was applied. Keithley 6487 and 2400 ammeters were used as the voltage sources to measured the Ids-Vds and Ids-Vg curves. 31

3. RESULTS AND DISCUSSION 3.1 Morphology and Structure of the Grown B-P Micro-ribbons The conversion ratio ( w% 

mass of black P 100% ) of B-P is 97%, which is mass of initial red P

much higher than that in reported references.27 Figure 2a shows the grown micro-ribbons in silica ampoule. Long micro-ribbons densely distribute in silica ampoule adhering to its sidewall. Figure 2b is the enlarged image of these grown micro-ribbons in the right part of silica ampoule in Figure 2a. The grown micro-ribbons were collected, cleaned and then dried as shown in Figure 2c. The crystalline structure of the long micro-ribbons was examined by XRD and the result is shown in Figure 3a. All the diffraction peaks can be well indexed as B-P and show a preferential orientation of (0k0). No peaks from other crystalline impurity such as tin, iodine, tin-iodide or tin-phosphide is observed under the instrumental resolution, confirming the product composed of predominantly B-P. Figure 3b shows the Raman spectroscopy of the grown B-P micro-ribbons. The peaks at 362.4cm-1,

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440.2 cm-1 and 466.8 cm-1 are due to vibrations of the crystalline lattice of B-P and they match well with the Raman shifts attributed to the A1g (out-of-plane mode), B2g (in-plane mode) and Ag2 (in-plane mode) phonon modes observed in bulk B-P.10, 18, 22, 30

SEM images of B-P micro-ribbons are shown in Figure 4. Figure 4a is a low magnification SEM image of B-P, which clearly indicates its micro-ribbon like morphology. The obvious layered structure is observed in the enlarged SEM image of a B-P micro-ribbon as shown in Figure 4b, and the thickness of the micro-ribbon is ~ 30μm. Figure 4c shows a micro-ribbon with the thickness of only 3.96μm, in which the thickness of layer is about several hundred nanometers. It is therefore possible to fabricate B-P nanobelts directly by adjusting the growth conditions. According to the XRD result, these layers are all (0k0), revealing a highly oriented growth of B-P. EDX characterizations shown in Figure 4d demonstrated that the micro-ribbons are only composed of P, which further confirms the purity of as-prepared B-P micro-ribbons. The detailed morphology and crystalline structure of the grown B-P micro-ribbon were studied by TEM. Figure 5a shows a low magnification TEM where the flat flake is clearly observed. The HRTEM image shown in Figure 5b indicates the B-P flake is highly crystalline with a lattice spacing of 0.218 nm, which corresponds to the (002) planes in the B-P crystal lattice. The inset in Figure 5b shows the corresponding SAED pattern. Together with the HRTEM image, the SAED pattern suggests that the B-P growth is along the [001] direction. 3.2 The Bottom-up Growth Behavior of B-P Micro-ribbons In order to observe the nucleation and growth sites of B-P, two experiments were

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designed. One is shown in Figure 6a, in which the initial source materials were loaded on the top surface of carbon paper and then the carbon paper was horizontally transported into the ampoule tube. The left schematic shows the sectional view of the ampoule tube. When the growth reaction finished, it is found that B-P micro-ribbons grow at the bottom of ampoule tube with long strips firmly clinging to the wall. However, no B-P was found on the top surface of carbon paper. The other is shown in Figure 6b. A piece of semi-cylinder quartz with same diameter of ampoule tube was loaded into the ampoule tube, therefore, the semi-cylinder quartz and ampoule were contacted very closely as shown in the schematic in Figure 6b. After growth, it was also found that B-P still grew at the bottom of ampoule tube with long strips as shown in the rear view of the semi-cylinder quartz. The Raman spectrum is shown in Figure 6c, which proves its B-P identity with three characteristic peaks A1g, B2g and A2g located at 362.9cm-1, 439.5cm-1, 467.4cm-1, respectively. However, B-P film or micro-ribbons were also not found on the top surface of semi-cylinder quartz according to the Raman analysis (Figure 6c). The above two experiments clearly reveal an interesting bottom-up growth behavior of B-P micro-ribbons. It also indicates the nucleation and growth sites are at the bottom of the silica tube, which will be investigated in detail in next section. 3.3 The Speculated Growth Mechanism of B-P Micro-ribbons The growth process of B-P micro-ribbons was investigated in detail to figure out the growth mechanism. Eleven experiments were designed with the same growth conditions (refer to Experimental Section), while the growths were stopped at different stages and the photos of ampoule tubes at these stages were taken

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immediately and shown in Figure 7. A possible growth mechanism was proposed according to the detailed investigations. The initial source materials including red phosphorus, I2, and Sn were loaded into the ampoule tubes and then sealed at room temperature as shown in Figure 7a. Two hours later, when temperature was increased to 523K, light yellow gas appeared in ampoule tube, while red phosphorus still at the tube bottom (Figure 7b). Increasing the temperature to 768K, red phosphorus began to sublimate, making the tube partly full of red gas as shown in Figure 7c. The whole ampoule tube is growing dark red as shown in Figure 7d when the temperature was increased to 863K, which indicates the complete sublimation of red phosphorus. During the cooling stage, when the temperature was decreased from 863K to 798K, brunet solid state products appeared at the bottom of the ampoule tube and continued to increase as shown in Figure 7e to Figure 7g. When the temperature was decreased to 773K, strip-like B-P appeared at the bottom of ampoule tube (Figure 7h). Further decreasing the temperature from 773K, the color of the ampoule tube gradually changed from red to light red, and finally transparent at 523K, and the corresponding photos are shown in Figure 7i-7k. Long B-P micro-ribbons clinging to the side wall from the bottom of ampoule tube were clearly observed when the temperature is below 523K. According to the aforementioned investigations, the possible growth process is shown in Figure 8a ~ 8e. Figure 8a is the initial state, in which the source materials including red phosphorus, iodine, and tin distributed at the bottom of ampoule tube. The growth of B-P micro-ribbons includes four stages. At the first stage, when the

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temperature was increased from 300K to 673K, iodine (I2) and tin (Sn) evaporated because their melt points are 386K and 505K, respectively. The whole silica tube appeared light yellow color. At the same time, tin reacted with iodine to form tin iodide as follows:

Sn  I 2

SnI 2

(1)

The schematic is shown in Figure 8b. Although the reaction is reversible, tin iodide would not be decomposed due to the excessive amount of Sn. When the temperature was further increased from 673K to 863K, the second growth stage, red phosphorus began to sublimate, and then the ampoule tube was fully filled with red phosphorus gas and appeared dark red. Red phosphorus gas reacted with tin iodide and tin gas to form P-Sn-I compound at this stage as shown in Figure 8c. When the temperature was decreased from 863K to 798K, the third growth stage as shown in Figure 8d, the gas in the silica tube began to condense and deposit at the tube bottom. It is well known that tin iodide (SnI2) plays a key role in the synthesis of B-P introduced by Schäfer.32 It works as mineralizer to improve the crystallization of stable or metastable phase via the phosphorus gas.33 The P-Sn-I compound precipitates along the bottom of tube, working as nucleation sites for the growth of B-P micro-ribbons. Figure 8e shows the fourth growth stage during from 798K to 523K. B-P micro-ribbons crystallized and grew with the help of mineralizer. This process lasted to 523K until the red phosphorus gas converted to B-P micro-ribbons completely, turning the silica tube clear and transparent. Figure 8f shows the rear of the grown B-P micro-ribbons. Many strips spread out from the core, which is indicated in the blue box with lots of red

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lines extending along B-P micro-ribbons. Figure 8g, a typical SEM image of the growth site of B-P, clearly shows B-P micro-ribbons cross each other at the initial growth sites. The enlarged image of the growth site (the cross center) is shown in Figure 8h, which obviously reveals that the micro-ribbons grew out from the growth site (the cross center). Figure 8i is the EDX pattern of the growth site in Figure 8h. The product at the growth site contains P/Sn/I three elements, which is agreed with our assumption that it works as nucleation sites for B-P’s growth. Moreover the P-Sn-I compound is the major byproducts in all grown products. Therefore, the P-Sn-I compound works as not only active nucleation sites but also the catalyst for the B-P growth. 3.4 FETs based on few-layered B-P Mechanical exfoliation of bulk B-P is a traditional method to fabricate monolayer or few-layered phosphorus flakes. However, usually, the small area and low yield of the fabricated monolayer or few-layered flakes restrict their applications in devices. Figure 9a is an enlarged optical image of B-P micro-ribbons. It shows a large area (460μm×700μm) of shining cleavage plane, indicating the smooth surface of a cleaved micro-ribbon. Mechanical exfoliation from this flat and very thin B-P micro-ribbon would be possible to increase significantly the area and yield of the fabricated monolayer or few-layered phosphorus. Figure 9b is the few-layered B-P fabricated by mechanical exfoliation method from the grown B-P ribbons. Figure 9c shows AFM image of the rectangular region in Figure 9b. The height in the dark blue area is estimated to be 11.3 nm. Raman spectrum (shown in Figure 9d) has been performed to verify its B-P identity with three typical modes located at 361.9 cm-1,

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439.8 cm-1 and 467.6 cm-1, respectively. Figures 9e shows other exfoliated few-layered B-P with thickness ~ 10 nm. The morphology of the few-layered phosphorus is similar with that of the grown B-P micro-ribbons, which is beneficial to investigate their anisotropic properties of optical and electronic. The schematic of the mechanic exfoliation process of phosphorus micro-ribbons is shown in Figure 9f. The micro-ribbon layers are arranged and piled up along [040] direction, which is vertical to their cleavage planes. This makes B-P micro-ribbons easier to be exfoliated along cleavage planes. Figure 10a shows optical image of field effect transistor based on the exfoliated few-layered phosphorus. The drain and source electrodes are Ti/Au metals. The thickness of the few-layered phosphorus is about 10.04 nm according to AFM measurement as shown in Figure 10b. Figure 10c is the Raman spectrum of the few-layered phosphorus, which clearly shows the three typical modes A1g, B2g and A2g located at 359.8 cm-1, 436.5 cm-1, 464.4 cm-1, respectively. Figures 10d and 10e are two typical electrical characteristic curves of the FET. The linear relation of Ids and Vds indicates good ohmic contact between electrodes and few-layered B-P. Using the following equation:

mFE =

where

dI DS L dVG CoxWVDS

(2)

dI DS is the slope of linear area in Figure 10e, L is channel length, W is width dVG

of few-layered B-P and Cox is the capacitance per unit area of gate oxide given by

Cox =

 0 r d

(  0 is permittivity of vacuum, εr = 3.9 for SiO2, and d = 300 nm for the

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thickness of SiO2). The mobility of hole can be calculated to be 200 cm2 V-1 s-1 for Vds=0.1V at room temperature, which is lower than the results in references.14 The degradation of BP in air atmospheres is thought to be the main reason. 34, 35 Many water clusters is clearly observed on the surface of BP as shown in Figure 10(b). The ribbon form of B-P can also be suitable to investigate its anisotropic properties, such as the mobility of carriers along armchair and zigzag directions, the quantum oscillations36 for large size flakes, and in-plane thermal conductivity for ribbons with clear axes.37

4. CONCLUSION In conclusion, a facile method has been developed to synthesize crystalline B-P micro-ribbons in large-scale. The growth process of B-P micro-ribbons has been investigated systematically and its growth mechanism has been speculated. The significant decreasing of the cost and increasing of the yield made this method to have commercial applications for B-P production. More importantly, it opens up the possible opportunity to directly grow B-P micro-belts even few-layered nanobelts by adjusting the growth conditions. In addition, it is easy to fabricate ribbon-like few-layered B-P with large area by mechanical exfoliation method because of the smooth and large area of the cleavage plane of the grown B-P ribbons. The ribbon-like few-layered phosphorus is beneficial to investigate the anisotropic properties of B-P.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (No.

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51272232), Program for New Century Excellent Talents in University, and the Fundamental Research Funds for the Central Universities.

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Captions: Figure 1 (a) Schematic layered structure of B-P, (b) Top view and side view of B-P with [001] direction along the armchair edge Figure 2 (a) the photo of the grown B-P micro-ribbons in ampoule tube, (b) the magnified photo of B-P micro-ribbons, and (c) the photo of the cleaned B-P micro-ribbons. Figure 3 (a) XRD pattern of the grown B-P micro-ribbons, (b) Raman spectrum of the grown B-P micro-ribbons. Figure 4 (a) the low-magnification SEM image of an individual B-P ribbon, (b) the high-magnification SEM image of the B-P ribbon, (c) the high-magnification SEM image of a thin ribbon, and (d) EDAX pattern of the grown B-P. Figure 5 (a) low-, (b) high-resolution TEM images of B-P ribbon, and (c) SAED pattern of the sample shown in (b). Figure 6 (a) a piece of carbon paper with initial source materials horizontally loaded in the ampoule tube, (b) a semi-cylinder quartz with initial source materials loaded in the ampoule tube. Sectional schematics (left) and photos (right) in figures 6(a) and 6(b) indicates the initial and the final stages of B-P growth, respectively. (c) Raman spectra of surface and rear parts of the grown sample in figure 6(b). Figure 7 (a) ~ (k), the photos of the ampoule tubes at different growth stages. Figure 8 (a) ~ (e), schematic of the micro-ribbon growth stages, (f) SEM image of the rear part of the grown product, and (g) SEM image of the surface part of the grown product , (i) EDAX spectrum of the core area shown in (h).

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Figure 9 (a) optical images of a B-P ribbon, showing a very smooth shining cleavage plane, (b) the few-layered B-P flake exfoliated from the grown ribbon, (c) AFM images of the rectangular region in (b), indicating an average height of about 11.3 nm and 3.6 nm in the dark and light blue area, (d) Raman spectrum of the sample in (b), (e) other ribbon-like few-layered phosphorus, and (f) schematic of the exfoliation process of B-P ribbons. Figure 10 (a) Optical image of the FET based the few-layered B-P, (b) AFM image of few-layered phosphorus in (a), (c) Raman spectrum of the exfoliated B-P, (d) Typical Ids-Vds characteristic curves of the FET shown in (a), and (e) Ids-Vg curves of the B-P FET measured with Vds = 0.1V..

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Figure 1

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Figure 2

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Figure 4

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Figure 8

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For Table of Contents use only Growth Mechanism and Enhanced Yield of Black Phosphorus Micro-ribbons Ming Zhao, Haolei Qian, Xinyue Niu, Wei Wang, Liao Guan, Jian Sha, and Yewu Wang

We developed a facile method to synthesize large-scale black phosphorus micro-ribbons, significantly decreasing its cost and increasing its yield.

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Growth Mechanism and Enhanced Yield of Black Phosphorus

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