Be2CO3F2 Monolayer: A Flexible Ultraviolet Nonlinear Optical

May 23, 2019 - Multifunctional monolayer materials with attractive properties and novel applications are of present research interest. In this contrib...
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Be2CO3F2 Monolayer: A Flexible Ultraviolet Nonlinear Optical Material via Rational Design Yilimiranmu Rouzhahong, Bingbing Zhang, Ailijiang Abudurusuli, Shilie Pan,* and Zhihua Yang* CAS Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Key Laboratory of Electronic Information Materials and Devices, Xinjiang Technical Institute of Physics & Chemistry, 40-1 South Beijing Road, Urumqi 830011, China

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S Supporting Information *

ABSTRACT: Multifunctional monolayer materials with attractive properties and novel applications are of present research interest. In this contribution, we design a new monolayer Be2CO3F2 (BCF) by taking a KBe2BO3F2 unique crystal that is able to produce a high energetic laser by a direct second harmonic generation method, as a parent. The cohesive energy, positive phonon modes, and elastic constants reveal that the BCF monolayer is dynamically and mechanically stable, and the appropriate cleavage energy predicts the experimental realization possibility. The property investigations demonstrated that the monolayer BCF has the significantly superiority in flexibility over the representative flexible optoelectronic material MoS2 based on the calculation of Young’s modulus. Additionally, the monolayer BCF possesses both a large band gap (5.2 eV) and a second harmonic generation response. These results demonstrate that the monolayer BCF may provide better applications as a promising multifunctional material in the flexible nonlinear optical fields. We hope that this research will pave a new way for designing new generation multifunctional devices.

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INTRODUCTION

fore, the dimensionality reduction strategy is one way to extend their applications as NLO materials.16 Development of optoelectronic technology requires more and more integrated and miniaturized NLO equipment. 2D NLO material characteristics such as large controllable NLO coefficients,10 room temperature operations,17 fast response speed,18 integrated and miniaturized properties, etc. are conducive for achieving commercial applications. In addition, the monolayer materials have been considered as an important class of materials with superior multifunctionality.19 With sustained efforts of many scientists discovering 2D monolayer materials with remarkable properties, several chemical systems like group IV and group III−V element compounds and transition metal oxides have been considered as promising systems for searching 2D monolayer materials.20 With respect to monolayer materials, they reveal a good flexibility,21 large photoluminescence,17,22 valley polarizations,23 strong exciton effects,18,24 and strong SHG responses. As such, several applicable monolayers have been developed, including insulators (hexagonal boron nitride (h-BN)),25 semiconductors (MoS2, black phosphorus (BP), etc.),26 topological insulators (Bi2Se3),27 and photocatalysts (C3N4).28 More recently, revealed social demands on flexible optoelectronics have spurred requirements for monolayer materials providing multifunctional characteristic with constant performance under the strain condition. As a result, scientists have been devoting

Nonlinear optical (NLO) materials are a type of important materials that possess wide applications in future photoelectric devices.1,2 Therefore, related major efforts have been made to explore NLO materials with remarkable properties,3 such as borates,4 phosphates,5 carbonates,6 nitrates,7 and sulfurets, which have made great contributions to expand the laser wavelengths from the deep-ultraviolet to the infrared region. According to recent experimental and theoretical studies, 2D compounds may have larger NLO effects than 3D compounds due to reduced dimensionality;8 in addition, by using stress and electric fields, it is easy to control NLO properties of 2D materials as compared to 3D bulks.9,10 Besides this, the exploration of SHG for a monolayer material is also important owing to its potential applications in photonic and optoelectronic fields.10 For instance, MoS2 and GaTe have been considered as next generation laser generators with a large SHG response (SHG response for MoS2 monolayer is 8 × 104 pm2/V,11 and for GaTe monolayer is 6.72 × 104 pm2/ V12). However, it should be noticed that the transition metal dichalcogenide and GaTe possess narrow band gaps about 2.0 eV, and this small band gap is hard to use in high energetic light regions. As one learns from bulk crystals, the famous borate- or carbonate-based NLO crystals such as LiB3O5, βBaB2O4, KBe2BO3F2 (KBBF), ABCO3F (A = K, Rb, Cs; B = Ca, Sr, Ba), etc.13−15 have advantages in large band gaps and strong SHG responses, but they suffer some problems in phase matchability, crystal growth, and mechanical stabilities. There© XXXX American Chemical Society

Received: December 22, 2018

A

DOI: 10.1021/acs.inorgchem.8b03575 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. KBBF crystal structure (a) and monolayer [Be2BO3F2]− extending in the xy plane (b). Top (c) and side view (d) of the BCF monolayer.

massive efforts to realize the flexible electronics and optoelectronics. The MoS2 2D sheet is a well-known typical example for experimentally realized flexible electronics, while few-layer BC2N29 and monolayer Be5C230 are examples for theoretical prediction of flexible nanoelectronics and optoelectronics. In this paper, in order to search the multifunctional monolayer with combining flexibility and ultraviolet NLO performance, we concentrated our attention on the KBBF crystal. The KBBF crystal is a special deep ultraviolet (UV) NLO material with the capacity to generate a deep UV (177.3 nm) coherent laser by a direct SHG.31 The cleavage characteristic along the c-axis of KBBF makes it hard to grow thick crystals, which limits its applications in the industrial and commercial fields as a 3D crystal. Therefore, taking KBBF as a template can retain its merit of deep-UV nonlinear optics and expand its potential applications in other functionalities. In this perspective, we focused on the relevance of rational design of new multifunctional monolayer exhibiting more flexibility and a strong SHG response. With the aid of the first-principles calculation, a KBBF-type Be2CO3F2 (BCF) monolayer was designed. The stability of BCF was checked by cohesive energy, phonon dispersions, and mechanical stability criteria. In addition, the flexibility and SHG response were analyzed. The mechanical anisotropy of BCF was explored by the angular dependence of the Young’s modulus and Poisson ratios. Property investigations demonstrate that the BCF monolayer is a superior flexible material with ultraviolet bandgap and rather large SHG responses.

work, the properties of monolayer were theoretically calculated by using the plane-wave pseudopotential based on density functional theory with the CASTEP program.32 For structural optimization and prediction of the properties like mechanical, electronic, and optical properties, the Perdew−Burke− Erenzerh (PBE) functional33 with generalized gradient approximation (GGA) was employed. Therefore, the kinetic energy cutoff was 600 eV under the Norm conserving pseudopotentials. The system energy was iterated until a tolerance of 2 × 10−7 eV/atom was attained for the electronic relaxation. We set the layer in the xy plane, and adopted a 25 Å supercell length in the z direction to avoid the interaction between the layers. The Monkhorst−Pack k-point used in sampling was 5 × 5 × 1. Moreover, the SHG coefficients were calculated with the use of length-gauge formula under the zero frequency limit.34,35

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RESULTS AND DISCUSSION A. Design and Structure. In the KBBF structure, the honeycomb-type [Be2BO3F2]− layers are formed by [BO3]3− planar trigonal groups and [BeO3F]5− tetrahedral groups. In order to design the expected monolayer, potassium atoms were deleted to remove the original K−F ionic interactions. According to the anionic group theory, the planar [BO3]3−and [CO3]2− anionic groups are good nonlinear optical groups, and the similar planar triangle structures with the π-conjugated molecular orbitals are capable to generate the large second-order susceptibility.36 Therefore, in the design and modeling, with the aim of enhancing SHG effects, keeping the original layering framework and electric neutrality, the [BO3]3− planar triangle in the [Be2BO3F2]− layer is substituted by the [CO3]2− planar triangle. Be has unique combination properties: its tetrahedral coordination has great contribution to the stability for six-membered rings,37 and the tetrahedral

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CALCULATION DETAILS Properties of BCF monolayer were examined by employing the highly efficient first-principles calculations that are commonly utilized to investigate different properties of materials. In this B

DOI: 10.1021/acs.inorgchem.8b03575 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry group BeO3F has a larger HOMO−LUMO (the highest occupied molecular orbitals−the lowest unoccupied molecular orbitals) gap compared to that of the replaceable tetrahedral group AlO3F, which is beneficial for opening a large band gap.38 However, one should be very careful, as BeO is toxic if used as the raw material. Therefore, a new honeycomb monolayer Be2CO3F2 (BCF) with P321 symmetry (No. 150) was designed, and the optimized structure compared with KBBF is shown in Figure 1. It can be seen that the monolayer is honeycomb structure which is confirmed by repeating inplane Be−C−O six membered rings with out-plane F, which retains the layer character of KBBF. Such a structure is quite similar to some recent designed 2D materials, like halogen and hydrogen-saturated silicene monolayer,39 hydrogen-saturated graphene monolayer,40 and AlBeBO3F2 (ABBF) monolayer.41 In the Be−C−O−F ring of the BCF monolayer, each beryllium atom links with three neighboring oxygen atoms and one fluorine atom. Bond lengths for Be−O and Be−F are 1.72 and 1.36 Å, and the angle of O−Be−O is 106.1°. While each carbon atom connects with three neighboring oxygen atoms, the bond length for C−O is 1.29 Å, and the angle of O−C−O is 120°. B. Stability. Before investigating the functionality of the BCF monolayer, the structural stability was examined and confirmed by the following theoretical calculation methods: (1) Cohesive energy: We calculated the cohesive energy of the BCF monolayer by Ecoh = (2EBe + EC + 3EO + 2EF − EBCF)/8, where EBe, EC, EO, EF, and EBCF represent the total energy of corresponding elements (Be, C, O and F) and BCF unit cell. Calculated cohesive energy is 5.87 eV/atom, which is smaller than that of graphene (7.85 eV/atom), but larger than those of the typical 2D materials, i.e., 3.98 eV/atom of silicone42 and 3.48 eV/atom of BP. The proper cohesive energy means that BCF has exploitation possibilities. (2) Phonon dispersion: we evaluated the dynamical stability by phonon dispersion as shown in Figure S1. No imaginary phonon modes prove the good kinetic stability of the BCF monolayer. Besides, via the phonon dispersion calculation of the BCF bulk, we also found that the bulk has the dynamical stability. It is possible to synthesize the BCF bulk compound experimentally by tube sealing or hydrothermal methods. (3) Elastic constants: we checked the mechanical stability by estimating elastic constants via the GGA calculations. For the trigonal symmetry, the mechanical stability is decided by seven independent elastic constants, namely C11 = 43.9 N·m−1, C12 = 9.1 N·m−1, C13 = −7.9 N·m−1, C14 = −1.6 N·m−1, C15 = 0 N·m−1, C33 = 6.4 N· m−1, and C44 = 6.1 N·m−1. The elastic constants fulfill the mechanical stability criteria:43 C11 > |C12|, C11 > 0, C33 > 0, C33 > 0, [(C11 + C12)C33 − 2C132] > 0, [(C11 − C12)C44 − 2C142] > 0, which indicates that the BCF monolayer is stable under the mechanical distortion. Therefore, the BCF monolayer has a possibility to be synthesized in experimentally based on its energetically favorability, positive phonon modes, and elastic constants that satisfy the mechanically stability criteria. And to further evaluate exfoliation feasibility, we estimated the cleavage energy of BCF monolayer by calculating minimum energy that consumed to exfoliate one layer from stacking layers.44 As represented in Figure 2, the cleavage energy gradually increases in the 0−5 Å separation distance region, and it converges to an unchanged value 0.79 J·m−2 when the separation distance was larger than 5 Å. The unchanged cleavage energy 0.79 J·m−2 is larger than that of exfoliated monolayer like graphene (0.37 J·m−2)45 and BP

Figure 2. Cleavage energy for the formation of BCF monolayer. The cleavage energy is calculated by introducing five BCF layers, in which the top one is flexible while others are fixed.

(0.367 J.m−2),46 but it is smaller than those of experimentally synthesized GeP3 (1.14 J·m−2)47 and GaN2 (1.09 J·m−2).48 The cleavage energy value is appropriate; this suggests that it is quite feasible to exfoliate a BCF monolayer from the BCF bulk compound by the physical vapor deposition (PVD) method. C. Flexibility and Mechanical Properties . The flexibility of the 2D material renders it as a promising application in flexible devices, so the flexibility of materials is investigated based on the calculation of Young’s modulus.49,50 Young’s modulus is a strain capacity indicator, which is defined by the stress strain ratio, which reflects the possibility of more flexibility with smaller values of the Young’s modulus. Our calculated in-plane Young’s modulus is 43.9 GPa, and the perpendicular direction Young’s modulus is 5.9 GPa. Compared with the Young’s modulus of well-known 2D materials, the obtained in-plane Young’s modulus (43.9 GPa) is smaller than those of black phosphorene (BP) (70.3 GPa),51 silicene (62 GPa),52 and MoS2 (129 GPa),53 as shown in Figure 3. Therefore, according to the calculated Young’s modulus, the newly designed monolayer BCF demonstrates superiority as an applicable flexible device. The difference of the Young’s modulus in different directions reflects the anisotropic nature of BCF monolayer. Usually, it is rare in 2D materials.54−56 To systematically analyze anisotropy in the mechanical properties, the direction dependence of the Young’s modulus and Poisson ratios were calculated; the obtained results are shown in Figure 4. Figure 4a is the orientation-dependent Young modulus, it is clear to see the apparent anisotropy of Young’s modulus with the maximal value (43.9 GPa) along the in-plane direction (θ = 0°), and the minimum (5.9 GPa) along the out-of-plane direction (θ = 90°). The orientation dependent on Poisson’s ratio ν(θ) was also investigated to check a material tending to contract in the direction transverse to the direction of stretching, which can also reflect the mechanically anisotropy. Poisson’s ratio of common isotropic elastic materials is less than 0.5. However, in the BCF monolayer, as shown in Figure 4b, the Poisson ratios are larger than 0.5 in the region of [0°, 50.4°], and smaller than 0.5 from 50.4° to 90° (see the Supporting Information for calculation details). Poisson’s ratio has a maximum value of 0.71 at θ = 15° and values of 0.67 (@θ = 0°), 0.17 (@ θ = 90°) at other angles. The direction dependent Young’s modulus and C

DOI: 10.1021/acs.inorgchem.8b03575 Inorg. Chem. XXXX, XXX, XXX−XXX

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In this work, in order to check application of the BCF monolayer as a laser device, SHG responses of BCF were analyzed. BCF monolayer has only one nonvanishing independent NLO tensor component (d11) under the limitation of Kleinman symmetry. d11 was calculated by DFT under the zero frequency limit, d11(vac) is 0.22 pm/V with the vacuum layer 25 Å (convergence). The NLO coefficient of BCF is larger than that of the ABBF monolayer with d33(vac) = −0.137 pm/V under the same vacuum layer, 25 Å.39 By inspecting angular dependent SHG responses, one can determine crystallographic orientation, SHG anisotropy, and intensity.57,58 The incident electric field produces the second harmonic signal ê2ω along the parallel electric field and perpendicular to the incident electric field, respectively. In the parallel direction, the generated signal is ê2ω = [cos θ sin θ 0], while for perpendicular direction the produces signal is ê2ω = [−sin θ cos θ 0]. The intensity of the second harmonic signal is ISHG = |ê2ω·d⃡ êω2|2, where d⃡ is the second-order susceptibility tensor for the P321 symmetry group. Therefore, we obtain parallel (I∥) and perpendicular (I⊥) polarized signals:

Figure 3. Young’s modulus of the well-known 2D materials as compared with BCF monolayer. The smaller the Young’s modulus is, the more flexible the material is.

Poisson’s ratio indicate that the BCF monolayer is highly anisotropic. D. Electronic Structure. Ultraviolet laser devices with short wavelengths are necessary in real applications like photocatalysis, optical storage, ophthalmic-nan surgery, and sterilization. Large band gap monolayers are limited, so there are great demand to explore large bandgap monolayer materials. The obtained band structures are plotted in Figure S2a; it demonstrates that the BCF monolayer is a direct bandgap compound with 5.2 eV bandgap. Its maximum valence band (VB) and minimum conduction band (CB) are at the Γ point. And the 5.2 eV bandgap is also comparable with 2D hexagonal boron−nitride with a bandgap value of 6 eV. Besides this, we also used other pseudopotentials to calculate the band gap and found that there existed no differences. Computed partial density of states (PDOS) is shown in Figure S2b. From Figure S2b, the orbitals from the Be and hybridized orbitals from C−O groups dominate the bottom of the CBs. The VBs are mainly formed by 2p orbitals from the O and F atoms. E. Nonlinear Optical Properties. For a noncentrosymmetric material, its NLO propriety is particularly important in consideration of possible practical application in lasers, switches, frequency conversion, and electro-optic modulators.

I = |d11 cos3 θ − 3d11 cos θ sin 2 θ|2

(1)

I⊥ = |d11 sin 3 θ − 3d11 sinθ cos 2 θ|2

(2)

The here obtained corresponding polar plots are shown in Figure 4, where the red and blue lines are I⊥ and I∥, respectively. The angular-dependent SHG responses (Figure 5) reveal the highly polarized SHG response of the BCF monolayer. Its maximal value was located at 0°, 120°, and 240° in the parallel direction and at 90°, 210°, and 330° for the perpendicular direction.

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CONCLUSIONS In summary, with the great wish to explore multifunctional 2D materials, based on the typical NLO crystal KBBF, a novel BCF monolayer was designed and characterized theoretically. The DFT computations predicted that the BCF monolayer is a multifunctional material with flexibility and ultraviolet NLO properties. The moderate cleavage energy indicates the experimental viability of the BCF monolayer. Besides, proper cohesive energy, no imaginary frequency modes in phonon dispersion, and elastic constants indicate that the BCF monolayer is stable. Our calculation shows that the BCF monolayer is a direct band gap NLO material with a bandgap value of 5.2 eV and rather large SHG responses that are larger than those for ABBF monolayers. More interestingly, the BCF

Figure 4. Orientation dependence of Young’s modulus (a) and Poisson’s ratio (b) for the monolayer BCF. D

DOI: 10.1021/acs.inorgchem.8b03575 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Angular dependent SHG responses of the BCF monolayer. Blue (red) plot indicates SHG responses perpendicular I⊥ (parallel I∥) to the incident electric field. Green circle indicates the maximal value.

monolayer has unexpected flexibility with a rather small Young’s modulus. Therefore, the BCF monolayer is expected to not only possess promising future like the KBBF family crystal in UV region but also have wide application like the graphene family monolayers. We expect our theoretical research work will be beneficial to the experiment and hope to pave a new path for new NLO monolayer materials and laser devices.

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(3) Halasyamani, P. S.; Zhang, W. Viewpoint: Inorganic Materials for UV and Deep-UV Nonlinear-Optical Applications. Inorg. Chem. 2017, 56, 12077−12085. (4) Kong, F.; Huang, S. P.; Sun, Z. M.; Mao, J. G.; Cheng, W. D. Se2 (B2O7): A New Type of Second-Order NLO Material. J. Am. Chem. Soc. 2006, 128, 7750−7751. (5) Yu, P.; Wu, L. M.; Zhou, L. J.; Chen, L. Deep-Ultraviolet Nonlinear Optical Crystals: Ba3P3O10X (X = Cl, Br). J. Am. Chem. Soc. 2014, 136, 480−487. (6) Luo, M.; Ye, N.; Zou, G.; Lin, C.; Cheng, W. Na8Lu2 (CO3) 6F2 and Na3Lu (CO3) 2F2: Rare Earth Fluoride Carbonates as Deep-UV Nonlinear Optical Materials. Chem. Mater. 2013, 25, 3147−3153. (7) Song, Y.; Luo, M.; Lin, C.; Ye, N. Structural Modulation of Nitrate Group with Cations to Affect SHG Responses in RE (OH) 2NO3 (RE= La, Y, and Gd): New Polar Materials with Large NLO Effect after Adjusting ph Values of Reaction Systems. Chem. Mater. 2017, 29, 896−903. (8) Zhu, Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R. Graphene and Graphene Oxide: Synthesis, Properties, and Applications. Adv. Mater. 2010, 22, 3906−3924. (9) Varma, S. J.; Kumar, J.; Liu, Y.; Layne, K.; Wu, J.; Liang, C.; Nakanishi, Y.; Aliyan, A.; Yang, W.; Ajayan, P. M.; Thomas, J. 2D TiS2 Layers: A Superior Nonlinear Optical Limiting Material. Adv. Opt. Mater. 2017, 5, 1700713. (10) Wu, J.; Yang, Y.; Gao, H.; Qi, Y.; Zhang, J.; Qiao, Z.; Ren, W. Electric Field Effect of GaAs Monolayer from First Principles. AIP Adv. 2017, 7, No. 035218. (11) Malard, L. M.; Alencar, T. V.; Barboza, A. P. M.; Mak, K. F.; de Paula, A. M. Observation of Intense Second Harmonic Generation from MoS2 Atomic Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 87, 201401. (12) Susoma, J.; Karvonen, L.; Säynätjoki, A.; Mehravar, S.; Norwood, R. A.; Peyghambarian, N.; Kieu, K.; Lipsanen, H.; Riikonen, J. Second and Third Harmonic Generation in Few-Layer Gallium Telluride Characterized by Multiphoton Microscopy. Appl. Phys. Lett. 2016, 108, No. 073103. (13) Xia, Y.; Chen, C.; Tang, D.; Wu, B. New Nonlinear Optical Crystals for UV and VUV Harmonic Generation. Adv. Mater. 1995, 7, 79−81. (14) Chen, C.; Wu, Y.; Jiang, A.; Wu, B.; You, G.; Li, R.; Lin, S. New Nonlinear-Optical Crystal: LiB3O5. J. Opt. Soc. Am. B 1989, 6, 616− 621. (15) Mutailipu, M.; Zhang, M.; Zhang, B.; Wang, L.; Yang, Z.; Zhou, X.; Pan, S. SrB5O7F3 Functionalized with [B5O9F3]6− Chromophores: Accelerating the Rational Design of Deep-Ultraviolet Nonlinear Optical Materials. Angew. Chem., Int. Ed. 2018, 57, 6095−6099. (16) Autere, A.; Jussila, H.; Dai, Y.; Wang, Y.; Lipsanen, H.; Sun, Z. Nonlinear Optics with 2D Layered Materials. Adv. Mater. 2018, 30, 1705963. (17) Ye, Y.; Wong, Z. J.; Lu, X.; Ni, X.; Zhu, H.; Chen, X.; Zhang, X.; et al. Monolayer Excitonic Laser. Nat. Photonics 2015, 9, 733.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03575. Phonon dispersion, detailed information on bulk, shear modulus, calculation formula of angular dependent Young’s modulus and Poisson ratio, bandgap, and PDOS (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(Z.Y.) E-mail address: [email protected]. *(S.P.) E-mail address: [email protected]. ORCID

Shilie Pan: 0000-0003-4521-4507 Zhihua Yang: 0000-0001-9214-3612 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 11774414, 61366001, 61604126), the Western Light Foundation of CAS (Grant No. 2016-QNXZ-B-9), Shanghai Cooperation Organization Science and Technology Partnership Program (Grant No. 2017E01013), and the Tianshan Innovation Team Program (Grant No. 2018D14001).

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DOI: 10.1021/acs.inorgchem.8b03575 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry (58) Li, Y.; Rao, Y.; Mak, K. F.; You, Y.; Wang, S.; Dean, C. R.; Heinz, T. F. Probing Symmetry Properties of Few-Layer MoS2 and hBN by Optical Second-Harmonic Generation. Nano Lett. 2013, 13, 3329−3333.

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DOI: 10.1021/acs.inorgchem.8b03575 Inorg. Chem. XXXX, XXX, XXX−XXX