Module-Analysis-Assisted Design of Deep Ultraviolet

Mar 5, 2019 - The diversity of potential structures and the competition of material functionalities make material design still a big challenge. Here, ...
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Module-Analysis Assisted Design of Deep Ultraviolet Fluorooxoborates with Extremely Large Gap and High Structural Stability Zhihua Yang, Bing-Hua Lei, Wenyao Zhang, and Shilie Pan Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b05175 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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Chemistry of Materials

Module-Analysis Assisted Design of Deep Ultraviolet Fluorooxoborates with Extremely Large Gap and High Structural Stability Zhihua Yang,*a Bing-Hua Lei,ab Wenyao Zhangab and Shilie Pan*a a CAS

Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, CAS; Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China. b Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China.

ABSTRACT: The diversity of potential structures and the competition of material functionalities make material design still a big challenge. Here we use down-to-top design by screening functional modules to realize targeted design of deep ultraviolet (DUV, < 200 nm) nonlinear optical (NLO) materials. By adjusting boron-oxygen-fluorine configurations, we analyse the functional modules that can balance the competition between the UV transmittance and the second harmonic generation (SHG) response, two key functionalities in DUV region. Among these units, the BO3F module is screened as the optimal module with both large energy gap and second-order microscopic susceptibility. As a demonstration, we introduce the BO3F module to Sr2Be2B2O7, an excellent NLO material suffering instability problem. The calculation results indicate that this introduction not only eliminates instability issue but the artificial compounds all have the shortest DUV cutoff edge in the commercial borate systems. Compared with KBe2BO3F2, the sole crystal used to produce DUV, their cutoff edges have about 20 nm blue shift.

Chemical bond associated microscopic structures with combination and spatial arrangement information determine functional properties in materials1, 2. Microscopic modules with optimizing spatial arrangement for functional properties are also called functional modules (FMs)1, 3. To predict and design the potential functional materials effectively before plentiful experimental trialand-error4-6, quantifying and screening the FMs are one of prerequisites7. Boron has versatile chemical properties because of its special position situated between metals and non-metals in the periodic table. In inorganic borate materials, a boron atom usually links with either three oxygen atoms to form a triangle BO3 with sp2 hybridization or four oxygen atoms to form a tetrahedron BO4 with sp3 hybridization. And the BO3 and BO4 modules may be further linked to form isolated rings, cages or infinite chains, layers and networks by sharing bridging oxygen atoms or edge8-13. For functional materials, performances are determined by FMs. Planar BO3 modules is easy to be polarized. When it polymerizes via parallel arrangement, i.e. the parallel BO3 FMs, the materials will possess a large

birefringence or second harmonic generation (SHG) effect. Borates become a class of excellent nonlinear optical (NLO) crystals, the key materials for solid state lasers to produce coherent light through cascaded frequency conversion, which are in urgent demands in the laser micromachining, material-processing, photolithography technique, optical measurements10, 14-21 and attract intensive interests of material scientists22-28. Especially, in ultraviolet (UV) and deep ultraviolet (DUV), KBe2BO3F2 (KBBF) and RbBe2BO3F2 (RBBF) are capable of directly generating DUV coherent light under sixth harmonic generation of Nd:YAG laser. However, they don't grow into large single crystals because of the layered habit, which hinders the further application. Recently, fluorooxoborates become the hotpot for exploring DUV NLO materials.29-35 Among these compounds, NH4B4O6F (ABF)32 has the best combined NLO properties. Its strong SHG response with 2.5 times that of KBBF, a DUV cutoff edge of 156 nm, an appropriate birefringence of 0.11 @ 1064 nm, a DUV phasematching wavelength of 158 nm, surpass that of KBBF.

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Figure 1. Energy levels and gaps of NLO material modules: BO3F, BO3, ang-B2O5, str-B2O5, B3O6 groups, and oxygen, boron and fluorine are marked as red, pink and blue, respectively. RY*, LP, BD and BD* stand for Rydberg, lone pair, bonding and antibonding orbital. Taking BD-BO for an example, it means the bonding orbital of boron and oxygen. Red and blue refer to the highest occupied molecular orbital (HOMO) as well as the lowest unoccupied molecular orbital (LUMO), and the energy gaps are listed in the last row

Actually, the appearance of fluorooxoborates can be followed from module analysis. To reach the DUV region, the polymerization or interaction of modules is crucial. Taking RbCaBO336, β-BaB2O4 (β-BBO)37, LiB3O5 (LBO)38 for example, the degree of polymerization of modules is isolated BO3 (RbCaBO3) < isolated B3O6 (BBO) < networked-B3O7 (LBO), their cutoff edges are 200, 189 nm and 155 nm, respectively. To shed a light on the change of polymerization for band gap, an ab-initio package, Gaussian39, is employed to calculate the energy levels for typical modules as seen in Figure 1. It is worth noticing that in order to reflect the electronic structure in crystal as closely as possible, the input files comes from the original crystal structure directly without any relaxation. Overall, the isolated BO3 unit has the smallest energy gap controlled by the lone pair (LP) 2p-orbitals of O and B. When two isolated BO3 units polymerize into B2O5, the LP B-2p orbital in conduction band will hybrid with a LP O-2p orbital, which will enhance the energy gap. In addition, to check the influence of different connection formations in Figure 1, it is found that the energy gap is not sensitive to the B-O-B angle (θ) on bridge oxygen. Energy gap of the B2O5 group with θ < π (Ang-B2O5) is only 0.3 eV less than that of B2O5 with θ = π (Str-B2O5). From isolated BO3 to the B3O6 modules, the energy gap increases as the degree of polymerization increases. Until all of them are polymerized, the cluster will have the largest energy gap, which is beneficial to large band gap for a crystal such as LBO. Considering the superiority of BO3 in SHG response

and optical anisotropy, BO3 is the perfect monomer to be polymerized, then the compound would be B2O3. However, B2O3 is not usually on a crystalline state. Therefore, we need introduce cations and anions to enrich structure diversity and explore high-performance materials from them.

Figure 2. Modules of the known alkali-metal fluorooxoborates: (a) B3O3F6 in Na3B3O3F6, (b) B3O5F3 in Li2B3O4F3, (c) B4O8F in NH4B4O6F and Rb4B4O6F, (d) B4O8F in CsB4O6F, CsKB8O12F2 and CsRbB8O12F2, (e) B5O10F2 in Li2Na0.9K0.1B5O8F2, (f) B6O11F in LiB6O9F, (g) B6O11F2 in Li2B6O9F2 and NaRbB6O9F2, (h) B6O11F2 in Na2B6O9F2, (i) B6O11F3 in K3B6O9F3; where red, green and blue atoms refer to oxygen, boron and fluorine, respectively.

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Chemistry of Materials For cations, alkali or alkaline-earth elements are the best choice for UV/DUV NLO materials because of few influences on bandgap. Similarly, in terms of transition region according to previous researches, fluorine is a suitable atom. Following this strategy, such compounds can be represented as Am(B2O3)nFm (A = alkali metals or ammonium). Reviewing the discovered materials, A = NH4, Rb, Cs, m=1, n=2, there are NH4B4O6F32, RbB4O6F30 and CsB4O6F31; A = mixed cations of Cs and Rb or K, m=2, n=4, there are CsKB8O12F2 and CsRbB8O12F230. Actually, these Am(B2O3)nFm-like compounds are fluorooxoborates, all

oxygen atoms are bridged and the fluorine atoms are connected with boron. Most of these fluorooxoborates possess excellent NLO properties in deep-UV NLO materials. In addition, when boron is substituted by Be atoms and oxygen became tridentate, for this family, they can be represented as Am[BelB(3-l)O3]nF[n(3-3l)-m]. When A= K, Rb, Cs, l=2, m=2, n=1, they are KBBF family, KBe2BO3F2, RbBe2BO3F2, CsBe2BO3F2. Therefore, it is no doubt that these fluorooxoborates have excellent NLO properties because they are derived from borate according to the criteria of DUV NLO materials.

Figure 3. Hyperpolarizability, HOMO-LUMO gaps, and polarizability anisotropy of designed six-membered rings including fluorine in rings. (a) Hyperpolarizability anisotropy of different six-membered rings; where Region I–IV refers to the [3 : 3T], [3: Δ+2T], [3 : 2Δ+T], and [3 : 3Δ] configuration, respectively. (b) Contour map related to the HOMO-LUMO gaps, hyperpolarizability of polarizability anisotropy. (c) Comparison of screened superior FBU in HOMO-LUMO gaps, hyperpolarizability and polarizability anisotropy. (d) Crystal structure of SBOF improved from SBBO.

To rationally design new fluorooxoborates, it is important to understand the environment rule of the known structure, explore the structure property relationship and find the FMs with special spatial arrangement. Previously, we proved by the first-principles calculations that the (BOxF4−x)(x+1)− (x = 1, 2, 3) groups ([BOF] for simplification) in fluorooxoborates40 benefit for extending a short cutoff edge down to DUV region without layering tendency41. By introducing [BOF] modules, we have obtained a series of new fluorooxoborates with good

performances. To date, there are 13 alkali metal fluorooxoborates and one ammonia-fluorooxoborates (see Table S1 synthesized mostly by the standard solid-state reaction in sealed silica tube42. In view of the B-O framework, it changes from two dimensional structures like in AB4O6F (A = NH4, Rb, Cs) to three dimensional structures like in A2B6O9F2 (A = Li, Na), as shown in Figures S1-S4 in the supplementary materials. Microscopically, in the known alkali metal fluorooxoborates, one or combination of BO3, BO4, BO3F, or BO2F exist while “BOF3” has not been found. Herein we label Δ as BO3 and T as tetrahedra: BO4 (T0), BO3F (T1), and BO2F2 (T2). Figure 2

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and S1-S4 give the structural characteristic and the corresponding B-O-F framework. The B3O3F6 module is composed of three BO2F2 by sharing O atoms, or 3: [(3:T2)] according to the classification proposed by Burns et al. and summarized by Touboul et al.15 And two units are involved like B4O8F-1 module with 4 : [(3 : 2Δ + T1) + (1: Δ)]; B4O8F-2 with 4 : [(3 : 3Δ) + (1 : T1)]; B6O11F with 6 : [(3 : 3Δ)+(3 : 2Δ + T1)]; B6O11F2-2 with 6 : 2[ (3 : 2Δ+T1)]; three units: B5O10F with 5 : [5 : (2Δ + T0 + 2T1)], B6O11F2-1 with 6 : [5 : (4Δ+T0) + (1 : T2)]; four kinds of basic units: B6O11F3 with 6 : [5 : (3Δ + T0 + T1) + (1 : T2)]. One can see that the investigated structures contain six-membered rings, which may be one of the energy minimizing optimizing systems.

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in Figure 3c, the [3 : 2Δ+T] and [3 : Δ+2T] series, especially [3 : 2Δ+T], are superior FMs for DUV region because introducing T1 and T2 can adjust the properties into a balance. Therefore, [BOF] is a good module which can be used to design NLO materials, especially for DUV. Table 1. The bandgaps from HSE06, cutoff edges from the calculated bandgap and SHG coefficients of designed artificial compounds compared with SBBO and KBBF (the parentheses are experimental values). Crystals

Symmetry

SrB2O3F2

P63cm

8.7

Eg (eV)

d15 = -0.16, d33 = 1.33

dij (pm/V)

SrBaB4O6F4

P63cm

8.4

d15 = -0.11, d33 = 1.68

BaB2O3F2

P63cm

8.3

d15= -0.04, d33 = 1.95

SBBOa

P 6 c2

5.7 (7.5)

d22 = 0.28 (1.62)

KBBF

R32

7.7 (8.2)

d11 = 0.49 (0.48)

a)The

large difference in bandgap as well as calculated optical properties and experimental values is caused by unstable structure of SBBO43.

Figure 4. Phonon spectra of SBBO(a) and artificial structures SBOF(b), SrBaB4O7F2 (c) as well as Ba2B4O7F2(d).

Spired by the regularity of the structures, we take sixmembered rings containing fluorine as the template to check the functionality of the [BOF] modules. Herein, we design the possible six-membered rings containing [BOF] modules in rings as compared with the B3O6 ring (Figure S5). Considering the terminal character of fluorine, BOF3 cannot be set to the rings. Therefore, only Δ, T0, T1, and T2 can be involved in a ring. In the [3 : 2Δ+T]-configuration, only two microstructures exist: [3 : 2Δ+T1], and [3 : 2Δ+T2]. While in the [3 : Δ+2T]-configuration, there are five kinds of rings, and nine rings for [3 : 3T]. Then, we investigate their electronic structure and polarizability characters using the DFT method implemented by Gaussian09 package at 6-31 G level. It should be noticed that during the calculation the modeling system (bond length, angles) adopts statistical average information of existed structures. Polarizability anisotropy (δ), HOMO- LUMO gaps, and hyperpolarizability (β) of designed rings are investigated, which can reflect the macroscopic birefringence, band gap and NLO properties of crystals. Figure 3a shows hyperpolarizability from I to IV region, where I-IV refer to [3 : 3T], [3 : Δ+2T], [3 : 2Δ+T], and [3 : 3Δ] configurations. For the hyperpolarizability, it is unrespective that FMs with the [BOF] units can have high hyperpolarizability, which can even be larger than that of [3 : 3Δ]. More importantly, FMs with [BOF] also expand large band gaps as shown in Figure 3b, which indicates that FMs can get the balance of DUV criterion for a crystal (band gap > 6.2 eV and SHG effect > 1 KH2PO4 (KDP, 0.39 pm/V). As shown

To verify the advantages of the BO3F modules, an excellent template is found in borates. As reported previously43, Sr2Be2B2O7 (SBBO) suffers from instability problem. The imaginary phonon modes appear in the vicinity of the high symmetry points A and Г q-point. As a result, the structure has not been solved (the structure convergence index > 0.06) and large size crystals with good quality have not been obtained. Figure 4a is the phonon spectra of SBBO we recalculate with linear response method44 in CASTEP45, a first-principles module based on plane wave pseudopotential method. The calculation detail can be found in the ESI. Clearly, as same as the previous research, there are imaginary phonon modes around A and Г q-point. In addition, the Jmol46 open source is employed to observe the vibration modes of imaginary phonon modes. The results imply that the vibration modes mainly originate from the BO3 groups along z-axis. Observing it carefully, the large distance (3.92 Å) between these two BO3 groups may cause the instability. Therefore, to improve the structural stability, two F atoms are inserted between these BO3 groups to expect a stable structure and the toxic beryllium atoms are substituted by boron atoms for valence balance. In this case, the artificial compound SrB2O3F2 (SBOF) is obtained (Figure 3d). In addition, the cation in SBOF can be substituted from magnesium to barium. Therefore, there are three new sturctures listed in Table 1. Comparing with SBBO, the new structures without beryllium are environmentally friend and the tetrahedral BO3F modules are superior-large-gap units owing to the larger energy gap between LUMO and HOMO contributed by the hybridization between B and F, which has a distinct advantage in obtaining short UV cutoff edge compared with those containing the BO3 groups. After geometric optimization, the phonon spectra of the artificial SrB2O3F2, SrBaB4O6F4 and BaB2O3F2 for comparison expectedly, the imaginary modes around A as well as Г q-point in these three compounds disappear and no imaginary frequency is observed, which indicate that they are stable and our strategy is valid.

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Chemistry of Materials The combining NLO properties of designed materials we calculated are listed in Table 1. Curiously, the band gap and optical properties of SBBO have large difference compared with the experimental results. We also believe that this is because of the unstable phase. As stated previously, the positions of oxygen atoms are statistically distributed and difficult to be determined by experiment since its structural convergence factor is large than 0.06. Therefore, in this work, we take KBBF as a reference. For band gap, artificial A2B4O6F4 (A=Sr or Ba) have an apparent enhancement, which indicates that A2B4O6F4 is beneficial to transmission of lower DUV than KBBF. It further proves that introducing BO3F modules can blue-shift the band gap. In addition, for artificial SrBaB4O6F4 and BaB2O3F2, although the bandgaps increase, the NLO properties have obvious enhancement as compared with those of SBBO. The calculation results imply that the NLO coefficients of artificial A2B4O6F4 (A=Sr or Ba) are about 2.7~4.0 times that of KBBF. Besides, it is difficult to explain because there is only BO3F tetrahedral groups but no BO3 πconjugation groups in the A2B4O6F4 (A=Sr or Ba) systems. In addition, the birefregences we calculated are shown in Figure S6, among these artificial compounds, BaB2O3F is phase-matchable. Macroscopically, what kind of electronic structure captures the superior to induce strong SHG? As we know that the macroscopic polarization under strong light irradiation, is related to the properties of Bloch wave function based on the modern theory of polarization47. And SHG at the low frequency region is characterized by the Berry connection and a symmetric metric tensor48-50. Consequently, strong orbital hybridizations can enhance the SHG response. The fluorooxoborates with the noncentrosymmetric, the general SHG coefficients are comparable to that of KDP, and artificial A2B4O6F4 (A=Sr or Ba) have relatively large NLO coefficients. The spanning of fluorine is checked because it reflects the bonding behaviors as shown in Figure S7. One can see that the spanning of fluorine in fluorooxoborates is wider than that in borate fluorides, which reflects the delocalization of fluorine orbitals. Besides, owing to the strong electronegativity of fluorine atom, it is hard to form covalent bond, while the interaction fluorine and boron is relatively strong as compared to that of metal cations and Fluorine. The bond population, which may be used to assess the covalent or ionic nature of a bond, is around 0.4~0.5 for B-F indicating a covalent bond nature. And owing to the difference of the covalence between B-F and B-O (~0.9), the [BOF] has apparent noncentrosymmetric distribution of electronic density, which makes that BOF has relatively strong hyperpolarizability. Therefore, BOF groups with strong delocalization of fluorine orbitals can result in good SHG performances. In summary, we study the functional modules, structure, electronic structure and properties of recent DUV fluorooxoborates. The results reveal that the [BOF] groups are excellent functional modules for designing or synthesizing DUV NLO materials. They can realize the balance between large SHG responses and wide band gaps.

Further, the introducing of the BO3F module to SBBO eliminates the instability issue. It is found that the new artificial SrB2O3F2 compound has the shortest UV cutoff edge in borate systems, namely, largest band gap. More importantly, under such large band gap, it still has a very large NLO coefficient about 3.4 KDP, the largest one with banddap >8.0 eV. Therefore, the [BOF] functional modules are proved to be efficient especially in DUV region.

ASSOCIATED CONTENT Supporting Information. Structures of fluorooxoborates. the spanning of p orbitals of fluorine. Structure information of thirteen alkali metal fluorooxoborates and one ammoniafluorooxoborates.

AUTHOR INFORMATION Corresponding Author *[email protected] (Z. H. Yang) *[email protected] (S. L. Pan).

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grant Nos. 11774414, 11474353), Tianshan Innovation Team Program (Grant No. 2018D14001), the National Key Research Project (Grant Nos. 2016YFB1102302, 2016YFB0402104), Shanghai Cooperation Organization Science and Technology Partnership Program (2017E01013).

ABBREVIATIONS UV, ultraviolet; DUV, deep-ultraviolet; NLO, nonlinear optical; SHG, second harmonic generation.

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