Electrostatic Functionalization and Passivation of Water-Exfoliated

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Electrostatic Functionalization and Passivation of WaterExfoliated Few-Layer Black Phosphorus by Poly Dimethyldiallyl Ammonium Chloride and Its Ultrafast Laser Application Qingliang Feng, Hongyan Liu, Meijie Zhu, Jing Shang, Dan Liu, Xiaoqi Cui, Diqin Shen, Liangzhi Kou, Dong Mao, Jianbang Zheng, Chun Li, Jin Zhang, Hua Xu, and Jianlin Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00556 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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ACS Applied Materials & Interfaces

Electrostatic Functionalization and Passivation of Water-Exfoliated Few-Layer Black Phosphorus by Poly Dimethyldiallyl Ammonium Chloride and Its Ultrafast Laser Application Qingliang Feng,*† Hongyan Liu,† Meijie Zhu,† Jing Shang,§ Dan Liu,ǁ Xiaoqi Cui,† Diqin Shen,† Liangzhi Kou,⊥ Dong Mao,*†Jianbang Zheng,† Chun Li,§ Jin Zhang,ǁ Hua Xu,*‡ Jianlin Zhao†

† MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions, Shaanxi Key Laboratory of Optical Information Technology, School of Science, Northwestern Polytechnical University, Xi’an 710072, China Email: [email protected]; [email protected] ‡ Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, Shaanxi Key Laboratory for Advanced Energy Devices, Shaanxi Engineering Lab for Advanced Energy Technology, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710119, China Email: [email protected] §Department of Engineering Mechanics, Northwestern Polytechnical University, Xi’an 710072, China

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ǁ Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ⊥School of Chemistry, Physics and Mechanical Engineering Faculty, Queensland University of Technology, Garden Point Campus, QLD 4001, Brisbane, Australia KEYWORDS: black phosphorus, electrostatic functionalization, PDDA, stability, ultrafast lasers

ABSTRACT: Few-layer black phosphorus (BP), which exhibits excellent optical and electronic properties, has great potential application in nanodevices. However, BP inevitably suffers from the rapid degradation in ambient air due to the high reactivity of P atoms with oxygen and water, which greatly hinders its wide applications. Herein, we demonstrate the electrostatic functionalization as an effective way to simultaneously enhance the stability and dispersity of aqueous phase exfoliated few-layer BP. The poly dimethyldiallyl ammonium chloride (PDDA) is selected to spontaneously and uniformly adsorb on the surface of few-layer BP via electrostatic interaction. The positive charge-center of N atom of PDDA, which passivates the lone-pair electrons of P, plays a critical role in stabilizing the BP. Meanwhile, the PDDA could serve as hydrophilic ligands to improve the dispersity of exfoliated BP in water. The thinner PDDA-BP nanosheets can stabilize in both air and water even after 15 days exposure. Finally, the uniform PDDA-BP-polymer film was used as saturable absorber to realize passive mode-locking operations in a fiber laser, delivering a train of ultrafast pulses with the duration of 1.2 ps at 1557.8 nm. This work provides a new way to obtain highly stable few-layer BP, which shows great promise in ultrafast optics application.

1. INTRODUCTION


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Few-layered BP has shown great potential application in many fields such as sensing,1 lithium ion battery,2 photodetectors,3 ultrafast optics,4-6 and optoelectronics,7,8 due to the outstanding physical and chemical properties.9 The most distinguished advantage of few-layer BP lies in its tunable direct bandgap (1.1 eV),10, 11 anisotropy,12, 13 high carrier mobility,8, 14 and large on/off ratio.13 Modulating the properties of BP to widen its application has been an important subject in this field. Several approaches have been developed to tune the basic properties of few-layer BP, especially the chemical stability for its large-scale synthesis, to broaden investigation in fundamental research and applications in optoelectronics. The primary challenge of the synthesis and application of BP is still the oxidation-induced instability of few-layer BP nanosheets in ambient condition. Recently, different approaches have been developed to exfoliate BP bulk crystal down to fewlayer BP, such as mechanical exfoliation and liquid-phase exfoliation.15 And correspondent passivation techniques also have been developed to suppress chemical degradation of few-layer BP nanosheets. For the mechanical exfoliated few-layer BP nanosheets, coating with AlOx layer via atomic layer deposition,16 photochemical etching,17,18 metal-ion-modified,19 encapsulation with graphene or hexagonal boron nitride (h-BN) as well as aryl diazonium modified20 have been used to keep BP nanosheets away from oxidation. For the liquid-phase exfoliation, several organic compounds have been used as the solvents or stabilizer to exfoliate few-layer BP nanosheets in a sealed system. For example, 1-methyl-2-pyrrolidone (NMP),21 and 1-cyclohexyl2-pyrrolidone (CHP)22 could forms a tightly packed solvation shell around BP nanosheets to preventing it from chemical degradation. However, few-layered BP nanosheets prepared with above mentioned technology still suffers low dispersity and chemical unstable in water. Thus, it


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is urgent issues to develop no additional contamination methods for effective exfoliation of fewlayer BP in water. Aqueous solution is a frequently used system to exfoliate few-layer graphene, few-layer MoS2, and carbon nanotubes with excellent dispersity, what’s more important is that this system is no additional contamination, low-cost, safety, and easy to scale up. Compared with other 2D materials, the few-layer BP is more chemically unstable due to the P atoms can easily react with oxygen and water, resulting in it rapidly degradation both in air and water. Therefore, it is essential to explore effective passivation techniques to scalable produce highly stable few-layer BP aqueous dispersion. Besides, the dispersity of few-layer BP in aqueous solution is a another fundamental issue accounting for fabrication of uniform BP thin film, which is essential for further applications in optoelectronic devices. From the structural aspect, in the honeycomb structure of BP, every phosphorus atom possesses five valent electrons (3s23p3), where three electrons are distributed in three 3p orbitals and one lone pair electrons are distributed in one 3s orbital. The electrons in 3p orbital are covalently bonded to three adjacent phosphorus atoms, while the electrons in 3s orbital are reserved. These lone-pair electrons of 3s orbital feature high reactivity with oxygen to form POx groups, leading to rapid degradation of BP in ambient conditions. Thus, how to passivate the lone pair electrons of 3s orbital, which prevent the reaction between phosphorus atom and oxygen, should be the key for enhancing the stability of BP. In this work, we propose the introduction of PDDA to produce high-stable and high-dispersive few-layer BP nanosheets aqueous in a large scale. Herein, the positive and hydrophilic PDDA molecules are uniformly adsorbed on the BP nanosheets by electrostatic interactions between


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positive-charged N atoms of PDDA and lone-pair electrons in P 3s orbitals of BP, whereby stabilizing the lone-pair electrons. In addition, the alkyl group of PDDA chains can act as the physical isolation layer, further blocking the permeation of O2 and H2O molecules. The combined electrostatic stabilization and physical isolation effects guarantee a stable environment of BP. In addition, the innate hydrophilicity of PDDA due to positive-charged polymer backbones could greatly enhance the dispersity of BP nanosheets in water. High-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM), micro-zone Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were employed to investigate the morphology, chemical composition and stability of the water-exfoliated BP nanosheets against the exposure time to ambient condition. Additionally, the high concentration PDDA-BP nanosheets dispersion was used to fabricate a uniform flexible film with polyvinyl alcohol (PVA) base materials. The PDDA-BP-PVA uniform films exhibit saturable absorption property with the modulation depth and saturable intensity of 1.1 % and 221 MW/cm2, respectively. Picosecond mode-locked pulses are generated in an erbium-doped fiber laser using the PDDABP-PVA film, further confirming the ultrafast saturable absorption property of as-prepared BP nanosheets. 2. Experimental Section 2.1. Sample preparation. PDDA (20 µl, Sigma-Aldrich, 20 wt % in H2O) was dropped into ultra-water (20 ml) and sonicated for 5 mins at 5% power via a TU-650Y ultrasonicator. Then, the BP bulk crystal (10 mg, MK NANO, >99.999%) was mixed with as-prepared water solution, and sonicated for 120 mins at 80% power to form a mixture aqueous solution with different layers of PDDA functionalized BP nanosheets. The mixture aqueous dispersion was centrifuged at a low speed of 6000 rpm for 10 mins to remove thick and unexfoliated bulk BP using a Cence-


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H1850 centrifuge. The monolayer and bilayer PDDA functionalized BP nanosheets can obtain with higher speed of 15 000 rpm for 10 mins (PDDA-BP-1), and tri-layer to seven layer mixed samples can obtain with lower speed of 11 000 rpm for 10 mins (PDDA-BP-2). 2.2. Characterization. The aqueous solution of sample (10 µl) was dripped onto transmission electron microscope (TEM) microgrids, and the TEM, HR-TEM and SAED patterns were carried out on a FEI-Themis Z operated at 200 kV. And the aqueous solution of sample (20 µl) was dripping onto SiO2/Si (300 nm) substrate for following measurements. Scan Asyst mode AFM was done on a Bruker Dimension ICON multimode microscope. Raman and XPS spectra were measured on a Renishaw micro-Raman spectroscope (514 nm excitation) and Kratos Axis Ultra DLD (Al Kαhυ=1486.6 eV) under ambient conditions. The absorption spectra were acquired on a UV3600PLUS UV–vis–NIR spectrophotometer. The zeta potentials were done on a Zeta sizer Nano ZS. 2.3. Mode-locked fiber laser. The laser resonator is composed of a WDM, an EDF of 1.5 m, an output coupler (OC), a polarization in sensitive isolator (ISO), a polarization controller (PC) and a BP–PVA SA. The SMF length is about 23 m. The EDF with an absorption coefficient of 3 dBm-1 is used as the gain medium, which is pumped by a 980 nm laser diode through a 980/1550 WDM. The 10% OC is employed to output the laser emission. A PC is used to tune the polarization of the laser. An ISO is used to force the unidirectional operation of the laser in cavity, and the PDDA-BP–PVA film is served as a mode locker. The optical spectrum and the output pulse train were simultaneously monitor on a Yokogawa AQ6370 optical spectrum analyzer and a Tektronix DPO 3054 digital phosphor oscilloscope


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combined with a high-speeds PIN photodetector. The corresponding radio frequency was measured by an Agilent E4440A PSP series spectrum analyzer. 3. Results and Discussions 3.1. Exfoliation in Water and Basic Characterization. The few-layer BP was prepared by water exfoliation with PDDA functionalized from bulk BP crystal. As shown in Figure 1a, PDDA (20 µl) was dropped into ultra-water (20 ml) and sonicated for 5 mins to form homogeneous solution. Then, the BP bulk crystal (10 mg) was mixed with as-prepared water solution, and sonicated for 120 mins to form a mixture aqueous solution with different layers of PDDA functionalized BP nanosheets. Firstly, the mixture aqueous dispersion was centrifuged at a low speed of 6000 rpm to remove thick and unexfoliated bulk BP. By modulating the rotation speed of centrifugation, BP nanosheets aqueous dispersion with different layer number distribution can be obtained. Monolayer and bilayer PDDA functionalized BP nanosheets (PDDA-BP-1) can be obtained at a higher speed centrifugation of 15000 rpm, and tri-layer to seven layer mixed samples (PDDA-BP-2) can be obtained at a lower speed centrifugation of 11000 rpm. The functionalization and passivation mechanism of few-layer BP nanosheets is shown in Figure 1c, one Gouy-Chapman-Stern layer structure was formed on the surface of BP after the PDDA functionalization. Firstly, one layer of the positive and hydrophilic PDDA was adsorbed on the surface of few-layer BP nanosheets, forming the stern layer via charge-charge interaction. It can passivate the lone-pair electrons of P atoms, and the alkyl group can also keep BP nanosheets away from oxidization as the physical layer isolating the O2 and H2O molecules. And then, the outside Gouy layer (diffusion layer), mainly consisting of PDDA chains and part of


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H2O molecules, could stabilize the stern layer and enhance the dispersity of BP nanosheets in water. Ab initio density functional theory (DFT) calculations are performed to investigate the energy difference of the phosphorene absorbed PDDA chains. The results show that the binding energy is -42.85 meV/Å2 (-1.02 kcal/Å2), indicating that the PDDA chains is physically absorbed on single layer phosphorene due to the van der Waals interaction between N and P atoms. As a result, as-exfoliated few-layer PDDA-BP nanosheets show excellent dispersity in water as shown in Figure 1a and S1, and also exhibit typical Tyndall effect of nanomaterials. Figure S1 shows the photograph of PDDA-BP-1 and PDDA-BP-2 solutions, where the color difference indicates that the yield of PDDA-BP-2 nanosheets is higher than the PDDA-BP-1 nanosheets. Figure S2 shows the data of pure BP nanosheets without PDDA functionalized in water. The result indicating that the pure BP in water is too unstable to stay in water for 6 days.


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Figure 1. (a) Schematic of the fabrication process of exfoliated few-layer BP in water by PDDA, Tyndall effect of BP colloid solution via centrifugation at 15 000 rpm (top) and 11 000 rpm (down) with bulk BP crystal (10 mg). (b) Schematic of adsorption process for PDDA-BP nanosheets in water; (c) Schematic of stabilization mechanism for PDDA-BP nanosheets in water. The morphology and crystal structure of as-synthesized PDDA-BP nanosheets were investigated using HR-TEM and selected area electron diffraction (SAED) patterns by dipping the aqueous solution onto the TEM micro grids. Figure 2a and b shows the atomic model of monolayer and folded-bilayer BP nanosheets.23 The low magnification TEM images of PDDABP-1 nanosheets show a uniform morphology and size distribution (Figure 2c). And the normal TEM images of PDDA-BP-2 nanosheets are shown in Figure S3. As shown in Figure 2d-f, the HR-TEM images provide (111) lines with spacing of 0.25 nm, (001) lines with spacing of 0.44 nm, (021) lines with spacing of 0.34 nm, (041) lines with spacing of 0.22 nm, and (110) lines with spacing of 0.32 nm. These results demonstrate that the obtained PDDA-BP-1 nanosheets have high quality and maintain its original in-plane crystal structure. It needs to be noted that the BP quantum dots (QDs) are also observed in the aqueous solution with centrifugation rate at 15000 rpm. And the HR-TEM images of BP QDs were shown in Figure 2 (g, h), the lattice parameters of BP QDs perfectly match with that of bulk materials.24 Furthermore, SAED pattern shows only one set of orthorhombic spots (Figure 2i), indicating the naturally AA stacking mode of as-synthesized PDDA-BP-1 nanosheets. The layer number distribution of the as-exfoliated PDDA-BP nanosheets could be modulated by tuning the centrifugation speed, which was confirmed via atomic force microscope (AFM) measurement. As shown in Figure 3a and b, AFM images of PDDA-BP nanosheets obtained at


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both centrifugation speeds of 15 000 rpm (Figure 3a and Figure S4) and 11 000 rpm (Figure 3b) exhibit uniform and flatten surface, suggesting the homogeneous adsorption of PDDA molecules on BP nanosheets. The statistic thickness distribution of PDDA-BP nanosheets is shown in Figure 3c, with the average height of around 2.1 nm (PDDA-BP-1, corresponding to bilayer) and 4.5 nm (PDDA-BP-2, corresponding to five layer), respectively.1, 12


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Figure 2. Crystal Structural characterization of water-exfoliated PDDA-BP nanosheets with 15000 rpm centrifugation. (a) Atomic structure of monolayer BP crystal. a: 0.438 nm; b: 0.33 nm; (b) Schematic showing of the monolayer (left) and folded bi-layer (right) BP crystal; (c) Normal TEM images of PDDA-BP-1 nanosheets. Scale bar: 100 nm; (d-f) HR-TEM images of PDDABP-1 nanosheets with different lattice space. Scale bar: 1 nm; (g, h) HR-TEM images of BP QDs in aqueous solution with centrifugation at 15 000 rpm. Scale bar: 2 nm; (i) SAED pattern of PDDA-BP-1 nanosheets. Scale bar: 1/5 nm. Raman spectra were utilized to confirm the structure information of the water-exfoliated PDDA-BP nanosheets. Three primary Raman peaks at 362.0 cm-1, 438.1 cm-1 and 465.9 cm-1 were observed (Figure 3d), which are assign to , B2g and vibration modes, respectively. All these peaks are corresponding well to that of bilayer BP, demonstrating that the well-preserved crystal structure of BP.11 The optical bandgap of PDDA-BP-1 and PDDA-BP-2 were confirmed by measuring their absorbance spectra. As shown in Figure 3e-f, two obvious peaks located at 960 nm and 1155 nm were observed, which are consist with the monolayer and bilayer BP nanosheets as previous reported.25 Inherently, the peak position of PDDA-BP show a slightly blue shift compared to the mechanical exfoliated and organic solvent exfoliated bilayer BP, which should be attribute to the chemical doping of bilayer BP by positive electron of PDDA chains. Absorption spectra of pure PDDA water solution and PDDA-BP-2 nanosheets in ethyl alcohol are shown in Figure S5, respectively. It can be find that there is no any adsorption peak appears in Figure S5a. Chemical functionalize process and surface charge properties of asexfoliated PDDA-BP nanosheets were further confirmed by XPS spectra and Zeta potential. As shown in Figure 3g, the zeta potential of both PDDA-BP-1 and PDDA-BP-2 are positive and around at 37.7 mV, which further indicate the successful adsorption of PDDA molecules on the


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surface of BP sheets. The XPS full spectra of PDDA-BP nanosheets are shown in Figure S6. The XPS spectra of Cl elements are shown in Figure S7. In Figure 3h, one obvious peak of N 1s located at high binding energy around 401.2 eV is observed, which is assigned to N elements in the PDDA chains. And another peak was the adsorbed N atoms in the surface of materials. As shown in Figure 3i, a couple peaks of P element were located at 129.7 eV (P 2p3/2) and 130.5 eV (P 2p1/2), which corresponding well to that of bulk BP. Besides, a much weaker peak of oxidized BP (i.e., POx) groups was also observed at ∼133.0 eV in the freshly as-exfoliated PDDA-BP-1 nanosheets, indicating a much weak but inevitable oxidization of BP during exfoliation process.16 Fortunately, this weak oxidization of BP would not develop further due to the protective function of PDDA as demonstrate below.


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Figure 3. (a, b) AFM images of water-exfoliated BP nanosheets with centrifugation speeds of 15 000 rpm (a) and 11 000 rpm (b). Scale bar: 5 µm. (c) Thickness distribution of PDDA-BP-1 and PDDA-BP-2 nanosheets; (d) Raman spectra of PDDA-BP-1 nanosheets (red) and PDDA-BP-2 nanosheets (black). Spectra are normalized to the silicon peak at 521 cm-1. (e, f) Absorption spectra of water-exfoliated BP with PDDA-BP-2 (e) and PDDA-BP-1 (f). Inset: a wide spectrum with the range between 300 and 1300 nm. (g) The Zeta potential analysis of the water-exfoliated PDDA-BP nanosheets. (h, i) XPS spectra of N element and P element for PDDA-BP-1 nanosheets.


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3.2. Stabilization of PDDA-BP nanosheets. The stability of BP nanosheets is a key fact that greatly affects on its properties and application. The schematic of interface adsorption for exfoliated few-layer BP nanosheets under air condition was revealed in Figure S8. Only a layer of PDDA chains was electrostatic adsorbed on the surface of few-layer BP nanosheets after sample was dried in vacuum, which is denoted as stern layer. In air condition, the O2 molecules or other polarity molecules will adsorbed on the surface of PDDA to neutralize surface positive charge, forming a Gouy layer and stabilize the PDDA layers finally. As a result, similar to the water, this method could also protect and passivate BP nanosheets from oxidation in air. To further investigate the chemical stability and oxidization mechanism of PDDA-BP nanosheets, AFM, Raman spectra and XPS were used to monitor the morphology, crystal structure and chemical composition evolution versus exposure time. To observe the morphology and basal plane evolution of PDDA-BP nanosheets, one special sample with porous structure was selected. From the AFM images of PDDA-BP-1 (Figure 4a, PDDA-BP-2 in Figure S9.), three typical model of morphology evolution were observed. The first model is marked with red circle, where the edge of PDDA-BP could stable and no further oxidation in such long time. The second model is marked with black circle, where the oxidation was began at the edge and expand slowly under air condition. And the last one was marked with white circle, where the adjacent edges are connected and form a large exposed edge during its weak oxidation process. And the weak oxidation mainly occurs at the edge of the exfoliated PDDA-BP nanosheets. In general, the results demonstrate that the exfoliated BP nanosheets were mainly chemical stable in 15 days after the PDDA chains functionalized and passivated (Figure S10).


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Figure 4. Stability investigation of exfoliated PDDA-BP nanosheets. (a) AFM images of asprepared bilayer PDDA-BP-1 nanosheets at the special region after 1, 7, 10 and 15 days of exposure under ambient conditions, respectively (scale bar, 3 µm). (b) Raman spectra evolution (λ=532 nm) of bilayer PDDA-BP nanosheets against the exposure time in air. Raman spectra are normalized to the silicon peak at 521 cm-1; (c) XPS spectra of P element in bilayer PDDA-BP on a same sample against the exposure time in air; (d) The intensity ratio of the / from Raman spectra of PDDA-BP-1 after 15 days exposure in ambient conditions; (e) The intensity ratio of the P /2 / as a function of exposure time from XPS. Inset: Photographs of a freshly prepared PDDA-BP-1 dispersion (left) and after 15 days exposure in air condition (right) with bulk BP crystal (20 mg) ultrasonicated in ultra-water (20 ml) in ambient condition.


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Recent studies have shown that the variation of Raman spectra can reflex the oxidation process of few-layer BP exposed in air. As shown in Figure 4b, the Raman spectra of assynthesized PDDA-BP samples at the same region show a negligible variations, especially the values of / , which is sensitive to the oxidation of BP,26 are always higher than 0.4 during 15 days exposure in air (Figure 4d). In addition, the roughness evolution of PDDA-BP-1 and PDDA-BP-2 against the exposure time also shows no obvious variation during such long time exposure (Figure S11). The high stability of PDDA-BP was also further confirmed by tracking XPS spectra at the same nanosheets over time (Figure 4c). The two primary peaks at 129.74 and 130.31 eV assigned to 2p3/2 and 2p2/1 of P maintains well during such long time exposure. Meanwhile, the oxidized peak at ~133.0 eV shows inappreciable variation, and the value of peak intensity of POx/P3/2 also show no obvious increase (Figure 4e), indicating the oxidation of BP was greatly retarded owing to the timely passivation by adsorbing PDDA molecular. The left one of inset photograph in Figure 4e is a freshly prepared PDDA-BP-1 dispersion and the right one is after 15 days. There is almost no visually difference between them, the aqueous solution still retains high stability and excellent dispersity without any obvious sedimentation and aggregation under 15 days exposure in ambient environment. 3.3. Application to Ultrafast Fiber Laser. For further studying the optoelectronic properties of water-exfoliated PDDA-BP nanosheets, the high concentration PDDA-BP-2 nanosheets dispersion was incorporated into a host polymer to form a uniform flexible film using wetchemistry techniques. Because of the bandgap dependents on layer numbers for BP, the lockmode based on PDDA-BP-2 nanosheets was investigated at 1550 nm. Due to the excellent dispersity of PDDA-BP-2 nanosheets in water, poly vinyl alcohol (PVA) was used as the host polymer for its hydrophilic nature to fabricate uniform PDDA-BP-PVA film (insert, Figure 5a).


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And then, the nonlinear optical response of the PDDA-BP-PVA film was investigated by the balanced twin-detector measurement system. As shown in Figure 5a, the thin PDDA-BP-PVA film exhibits clear saturable absorption property. And then the experiment date was fitting with the formula:

T(I)=1- αs / (1+I/IS) - αns

(1)

Where T (I) is transmission, αs and αns are the saturable (i.e., modulation depth) and nonsaturable absorption, respectively. I is the input intensity and Is is the saturable intensity. The modulation depth and saturation intensity were found to be about 1.1 % and 221 MWcm-2, respectively. And then an erbium-doped fiber laser was built to test the performance of the PDDA-BP-PVA saturable absorber. The schematic of the fiber laser is shown in Figure 5a, and its parameters were shown in Experimental Section. Self-started mode locking is achieved in the fiber laser at the pump power of 35 mW by using the PDDA-BP-PVA saturable absorber. Figure 5b shows the typical output spectrum of the mode-locked pulses, which is centered at 1557.8 nm with a 3 dB bandwidth of 2.61 nm. Obvious Kelly spectral sidebands appear on the spectrum, indicating the fiber laser operates at soliton mode-locking regime. Figure 5c was a second harmonic generation autocorrelation trace of the mode-locked pulses. It has a full width at half maximum (FWHM) of 1.93 ps using a sech2 fit, and the pulse duration is calculated to be 1.20 ps. The insert of Figure 5c is the output pulse train, with a pulse-pulse interval of 158.48 ns, matching well with the cavity round-trip time. The radio frequency (RF) spectrum in Figure 5d shows that the fundamental repetition rate is 6.317 MHz, with a signal-to-noise ratio (SNR) of 65 dB, indicating good mode-locking stability. Lasers mode-locked based on kinds of two-dimensional materials are summarized in Table


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1.Comparing all experimental results, few-layer PDDA-BP exfoliated in water is ideal saturable absorbtion material for fiber mode-locked laser.

Figure 5. (a) Measured saturable absorption data and its corresponding fitting curve of BP-PVA firm;(b)The experimental setup of the mode-locked fiber laser based on BP saturable absorber (SA). LD: laser diode (980 nm). WDM: wavelength division multiplexer. EDF: erbium-doped fiber (1.5 m, EDFC-980-HP). OC: output coupler. ISO: polarization insensitive isolator. SMF: Single-mode fiber (20 m, G652D). PC: polarization controller. BP-SA: PDDA-BP-PVA film based saturable absorber. (c) A typical output spectrum; (d) Autocorrelation trace of output pulses. (e) Output pulse train; (f) RF spectrum. Insert: wideband RF spectrum of 600 MHz. 4. Conclusions In summary, we have developed a PDDA-mediated exfoliation strategy to produce stable fewlayer BP nanosheets in water-phase at a large scale. It is the positive charge at the PDDA skeleton that act as a barrier passivate the lone-pair charge at the P atoms of BP, and thus


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enhance the chemical stability of BP. Both bilayer and few-layer PDDA-BP water dispersion with uniform size and thickness were obtained through different speed of centrifugation. The time dependent Raman and XPS spectra evolution indicate a high chemical stability of the PDDA-BP nanosheets aqueous even after 15 days exposure under air. Furthermore, the hydrophilic side-chain of PDDA promotes the superior dispersity of PDDA-BP nanosheets in water phase. Time resolution AFM measurement indicates just a much weak oxidation occurs at the edge of PDDA-BP nanosheets after 15 days exposure in air, while the base plane still maintains well. Thus, the as-exfoliated few-layer PDDA-BP exhibited superior dispersity and high chemical stability in water phase. Finally, the uniform PDDA-BP-PVA film was used as a saturable absorber to realize passive mode-locking operations in a fiber laser, generating a train of ultrafast pulses with the duration of 1.2 ps at 1557.8 nm. This work provides an effective and compatible passivation route for large scale preparation of few-layer BP nanosheets with excellent chemical stability and dispersity in water, which paves the way for BP films based ultrafast optoelectronic device application.


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Table 1. Mode-locked lasers based on kinds of two-dimensional materials.

Character of SA Materials

Preparation methods

Type of SA

WSe2/ MoSe2

WTe2

Liquid exfoliation

Liquid exfoliation

/Mw

Repetition rate/MHz

1571.45

63.7

2.92

0.946

5.9

27

Tm-doped fiber

1910

300

5.8（FWHM ）

0.739

36.8

28

12.5 MW· cm-2

Er-doped fiber

1558.14

\

1.25 （ FWHM）

2.18

15.59

29

4.5 mW

Er-doped fiber

15321570

30

3.39

0.94

4.96

30

Er-doped

1557.6

2.1

1.25

5.31

2.2

1.18

5.03

4.14

0.77

13.98

18.55% (25 layers）

10.74 MW·cm-2

8.1% (15 layers ）

6.55 MW·cm-2

4.1%

Optically deposited BP on D-shaped fiber

Optical deposited BP on microfiber

Er-doped fiber

\

3.31%

10.9%

PVA-WSe2 0.5% \

\

PVA-MoSe2 0.4%

WTe2-PVA deposited side-

2.85%

Ref.

Pulse duration /ps

waveleng th/nm

PVA composite

Typical Laser specifications

Threshold of pump power

3dB bandwidth/nm

Saturation intensity

Mechanical exfoliation

Liquid exfoliation

Laser type Modulation depth

Transferred BP on fiber end

BP

Operatio n

64.6 W

fiber

1562.6

Er-doped fiber

1556.2

25

31

32


20

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polished fiber

3-7 layers BP

Water exfoliation

BP-PVA composite

221 1.1% MW·cm-2

Er-doped fiber

1557

35

2.6

1.2

This article

6.317


21


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ASSOCIATED CONTENT Supporting Information. TEM, HR-TEM images and SAED pattern of PDDA-BP-2 nanosheets, full-band XPS spectra, AFM images of PDDA-BP-1 nanosheets, AFM images of PDDA-BP-2, photographs of PDDABP-1 aqueous solution and roughness of PDDA-BP nanosheets against the exposure time in ambient condition (PDF) The authors declare no competing financial interest. AUTHOR INFORMATION Corresponding Author *Email: [email protected] *[email protected] *Email: [email protected] Author Contributions † Q. L. Feng and H. Y. Liu contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by the National Natural Science Foundation of China (11634010, 51502167, 91622117), the Fundamental Research Funds for the Central Universities in


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Northwestern Polytechnical University (3102016QD071), and the Fundamental Research Funds for the Central Universities (GK201502003), the Funded Projects for the Academic Leaders and Academic Backbones, Shaanxi Normal University (16QNGG011). ACKNOWLEDGMENT The authors acknowledge the insightful suggestions from Dr. N.N. Mao and Porf. Xi Ling at Boston University. ABBREVIATIONS BP, black phosphorus; PDDA-BP-1, Monolayer and bilayer PDDA functionalized BP nanosheets obtained with higher speed of 15000 rpm for 10 mins; PDDA-BP-2, tri-layer to seven layer mixed samples obtained with lower speed of 11 000 rpm for 10 mins. REFERENCES (1) Hanlon, D.; Backes, C.; Doherty, E.; Cucinotta, C. S.; Berner, N. C.; Boland, C.; Lee, K.; Harvey,A.; Lynch, P.; Gholamvand, Z.; Zhang, S.; Wang, K.; Moynihan, G.; Pokle, A.; Ramasse, Q. M.; McEvoy, N.; Blau, W. J.; Wang, J.; Abellan, G.; Hauke, F.; Hirsch, A.; Sanvito, S.; O'Regan, D. D.; Duesberg, G. S.; Nicolosi, V.; Coleman, J. N. Liquid Exfoliation of Solvent-Stabilized Few-Layer Black Phosphorus for Applications beyond Electronics. Nat. Commun. 2015, 6, 8563. (2) Chen, L.; Zhou, G. M.; Liu, Z. B.; Ma, X. M.; Chen, J.; Zhang, Z. Y.; Ma, X. L.; Li, F.; Cheng, H. M.; Ren, W. C. Scalable Clean Exfoliation of High-Quality Few-Layer Black Phosphorus for a Flexible Lithium Ion Battery. Adv. Mater. 2016, 28, 510-517. (3) Youngblood, N.; Chen, C.; Koester, S. J.; Li, M. Waveguide-Integrated Black Phosphorus Photodetector with High Responsivity and Low Dark Current. Nat. Photonics 2015, 9, 247–252.


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The table of contents Electrostatic functionalized BP nanosheets shows high stability even after 15 days exposure in both air and water. The PDDA-BP-polymer film was used as saturable absorber to realize passive mode-locking operations in a fiber laser. This work paves a new and environment friendly way to obtain highly stable few-layer BP nanosheets. Keywords: black phosphorus, electrostatic functionalization, PDDA, stability, ultrafast lasers TOC Figure


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Electrostatic Functionalization and Passivation of Water-Exfoliated

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