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Be3BN3 monolayer with ultrawide band gap and promising stability for deep ultraviolet applications
Computational Materials Science 177 (2020) 109552
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Be3BN3 monolayer with ultrawide band gap and promising stability for deep ultraviolet applications ⁎
Haijun Zhanga, Chengcheng Xub, Kun Lia, Yiru Dia, Mingchao Wanga, , Xiaomeng Zhoua, a b
T
⁎
College of Economics and Management, College of Science, Civil Aviation University of China, Tianjin 300300, PR China School of Physics and Materials Science, Anhui University, Hefei 230601, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene-like material Deep-ultraviolet absorption Experimental feasibility Sustainable electronic gap First-principles calculations
In this study, the Be3BN3 two-dimensional (2D) system with strong ionicity and ultrawide band gap was theoretically obtained. The computational results suggest that the Be3BN3 sheet is thermodynamically stable and may be experimentally realized through dehydrogenation and polymerization of the B@Be3N3H6 cluster, which is an isoelectronic molecule to benzene and has promising stability and strong aromaticity. This newly designed 2D material exhibits ultrawide band gap of 5.25 eV and deep ultraviolet (DUV) absorption (λ < 222 nm), implying its possible applications for photodetectors in the visible-blind or solar-blind regions. Remarkably, the sustainable DUV response (λ < 305 nm) under external strain, electric field and high temperature indicates the probable utilization of the Be3BN3 monolayer in severe environment. This newly designed graphene-like material could further enrich the variety of ultrawide-gap nanomaterials and propose an outstanding candidate for DUV applications.
1. Introduction Since the discovery of mechanically exfoliated graphene in 2004 [1], which is an exemplary model of ultrathin two-dimensional (2D) materials with novel geometries and unexpected characteristics [2], tremendous studies have been performed on exploring graphene-like nanostructures to further enrich family members of 2D ultrathin materials [3,4]. The graphene-like materials, including hexagonal boron nitride (h-BN) [5,6], transition metal dichalcogendes (TMDs) [7,8], graphitic carbon nitride (g-C3N4) [9,10], layered metal oxides [11,12], MXenes [13–15], metal-organic frameworks (MOFs) [16,17], monoelemental graphene analogues [18–22], boron carbide/carbonitride [23–25] and so on, exhibit unique structural characteristics and versatile physicochemical properties for a wide spectra of applications. The various practical applications of ultrathin 2D nanomaterials are predominantly determined by their specific mechanical, electronic, optical, or magnetic properties. Take the electronic band gap as an example, both the gapless graphene and metallic MXene have ultrahigh electrical mobility and can be used as high-performance supercapacitor electrodes [26,27], while the 2D HgTe/CdTe superlattice with narrow band gap (48 to 101 meV) is a stable alternative for application in farinfrared optoelectronic devices [28]. The few-layer MoS2 nanosheets with moderate band gaps (1.35 to 1.82 eV) are promising phototransistors for the detection of green and red light [8]. The band gap ⁎
around 2.70 eV results in the visible-light absorption and appropriate redox potential of the graphitic carbon nitride, which makes the g-C3N4 nanosheet a high-efficiency photocatalysts [10]. Additionally, the wide gap semiconductor films and one-dimensional nanostructures (Eg > 3.0 eV), including III-Nitrides, SiC, diamond, and metal oxides, can be utilized as ultraviolet (UV) photodetectors in the visible-blind or solar-blind regions [29]. Despite the great achievements in exploration of graphene-like materials, the exceptional rarity of 2D wide-gap semiconductors is a potential barrier to the utilization of these materials [5,30–33]. Especially, the materials with deep UV (350–190 nm) responsive are rarely obtained but play a key role in applications of environment security, information technology, medical treatment, flame and radiation detection, inter-satellite communications, et al. [28,34]. Moreover, the 2D wide-band-gap materials with the absorption ranging from 400 to 10 nm can also be utilized in the transparent field-effect transistors [35], high-temperature die attach materials [36], deep UV (DUV) optoelectronic devices [37–38], et al. In view of the significance and scarcity of the nanomaterials with UV or DUV response, it is indispensable to seek for other 2D wide-gap materials to further enrich their diversity and broaden their various applications. Considering the wide band gap resulting from the strong ionicity within the h-BN nanosheet [39–41], introduction of another boron-like element into 2D B-N system may be a good strategy to reassemble the
Corresponding authors. E-mail addresses: [email protected] (M. Wang), [email protected] (X. Zhou).
https://doi.org/10.1016/j.commatsci.2020.109552 Received 23 October 2019; Received in revised form 16 January 2020; Accepted 20 January 2020 0927-0256/ © 2020 Elsevier B.V. All rights reserved.
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2D framework and theoretically design new graphene-like materials with novel geometry and wide band gaps. Xie et al found that [42], due to the different electronegativities of B and N atoms, the asymmetrical shapes of π bonding states in VBM and π* antibonding states in CBM result in a significant iconicity between B and N atoms and the opening of a large energy gap at Fermi level. By computing the band structures of atomic chains of group III-V elements, Zaluev and D’yachkov found that going from the BN chain to the AlP, GaAs, and InSb chains is accompanied by a decrease in the chemical bond ionicity, leading to a gradual decrease of the π-π* gaps. Therefore, the strong chemical bond ionicity should play a significant role in determining the insulating characteristics of h-BN nanosheet. The ultrawide band gap may be obtained by composing the 2D framework with the elements of boron and nitrogen, as well as beryllium element similar to boron. Herein, the beryllium element with electron deficiency similar to B element was introduced into the 2D B-N system to compose novel chemical bonds of Be-N, B-N and Be-B interactions in this study. Accordingly, a brand-new Be3BN3 monolayer with wide band gap of 5.25 eV was theoretically obtained. Different from the doping strategy for deep π-hole in Be-doped h-BN [43], this Be3BN3 monolayer is not composed of the hexagonal boron nitride framework and Be dopant, but is constructed by extending quasi-planar Be3BN3 cluster into 2D framework. This newly predicted Be3BN3 monolayer has not only absorption edge at solar-blind region (λ < 222 nm) but also pronounced thermal stability and sustainable band gap at the temperature up to 1000 K, indicating its superiority in deep ultraviolet application. 2. Computational details The first-principles calculations in this work were performed by employing the projector-augmented plane wave (PAW) [44] method as implemented in the Vienna ab inito simulation package (VASP) [45] code. The generalized gradient approximation in the form of PBE [46] was used to describe the electron exchange–correlation functional. The energy of ionic relaxation is converged at 10-6 eV, and the force convergence was set at 10-3 eV/Å. The cutoff energy of 600 eV was adopted in all computations and 11 × 11 × 1 Monkhorst-Pack k-points mesh was utilized in the geometry relaxation and self-consistent calculations. Considering the underestimation of band gaps by PBE functional, hybrid functional proposed by Heyd-Scuseria-Ernzerhof (HSE06) [47] was employed to compute the band structures and dielectric constants of Be3BN3 monolayers. In all computations of geometry optimization and chemical potential of elements, spin polarization was considered. The details of computing the phonon dispersion, ab initio molecular dynamics (AIMD) simulations, optical properties, and geometric and electronic properties of B@Be3N3H6 cluster were represented in the Supporting information.
Fig. 1. Geometric structure (upper) and phonon dispersion (lower) of theoretically predicted Be3BN3 monolayer.
investigated 2D framework, boron atoms were introduced into hexagonal BeN monolayer to construct a thermodynamic stable 2D network consists of Be, B and N elements, which is labeled as Be3BN3 monolayer (Fig. 1a). As presented in Fig. 1b, the computational results of phonon dispersion suggest the kinetic stability of this novel 2D crystal constructed by motifs of B-centered Be3N3 rings (B@Be3N3). As the result of different electron distribution between the Be3BN3 and hBN monolayers, the outer Be3N3 rings of B@Be3N3 motifs within the former are not regular hexagon. There is not any shared edges or atoms/points between all B-centered Be3N3 rings, which is due to the interval distribution of B@Be3N3 clusters within the Be3BN3 monolayer. There is not any appreciable imaginary frequency in the phonon spectrum of Be3BN3 monolayer and the highest frequency of it can reach up to 1395 cm−1 (41.84 THz), indicating the kinetic stability of Be3BN3 monolayer and robust Be-B-N connection in this 2D crystal. Moreover, the thermal stability was also assessed by performing the AIMD simulations at the temperatures of 500 K, 1000 K and 1400 K. The snapshots of Be3BN3 monolayer at the end of 10 ps simulations (Fig. 2) suggest that the 2D framework with hexagonal B@Be3N3 motifs are well maintained at the temperature of 1000 K, which indicate their promising thermal stability and possible utilizations at relatively high temperatures.
3. Results and discussion 3.1. Geometric structure and thermodynamic stability of Be3BN3 monolayer In view of the similarity of Be and B elements, we attempted to replace the boron atoms within BN nanosheet by Be atoms to compose the BeN nanosheets (Fig. S1a) and investigate the stability and physicochemical characteristics of the latter. We found that the BeN monolayer with honeycomb structure is kinetically instable, as suggested by the appreciable imaginary frequency in the phonon spectrum (Fig. S1b). On the purpose of obtaining the stable framework containing Be, B and N elements, the stability of modified hexagonal BN (h-BN) monolayer with interval-distribution Be atoms at the center of B3N3 rings, namely the BeB3N3 monolayer, was also assessed (Fig. S2a). There is also obvious negative frequency in the phonon spectrum of BeB3N3 sheet, indicating the dynamical instability of these modified hBN. In order to achieve in both the stabilization and ionicity of
3.2. Stabilization mechanism and experimental feasibility The pronounced thermal and dynamic stability of Be3BN3 monolayer may result from the strong ionicity of and novel electronic configuration of this Be-B-N network. To evaluate the bonding strength of this newly designed 2D materials, the cohesive energy was calculated according to the formula of Ecoh = (3EBe + EB + 3EN − EBe3BN3)/7, in which the EBe, EB, EN and EBe3BN3 are total energies of a single Be atom, a single B atom, a single N, and one unit cell of Be3BN3 monolayer, 2
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Fig. 2. Snapshots of the Be3BN3 monolayers at the end of 10 ps AIMD simulations at the temperatures of (a) 500 K, (b) 1000 K and (c) 1400 K. Both the top (upper) and side (lower) views are demonstrated.
respectively. The computed cohesive energy of Be3BN3 sheet (5.06 eV/ atom) is obviously higher than that of hypothetical BeN monolayer in Fig. S1a (3.57 eV/atom) and is slightly higher than that of modified hBN monolayer with Be atoms in Fig. S2a (4.96 eV/atom), which implies the strongest connection between Be, B and N atoms in the Be3BN3 sheet. Also, the cohesive energy of Be3BN3 sheet is comparable to other graphene-like materials, such as the iron borides (5.56–5.79 eV/atom), [48] Cu2Si (3.46 eV/atom), [49] and beryllium carbide (4.86 eV/atom), [50] indicating the robust chemical bonds in Be3BN3 monolayer. For a deep insight into the bonding nature and electronic configuration, the electron localization functions (ELF) [51] were calculated to analyze the stabilization mechanism of Be3BN3 monolayer. The ELF is a useful tool for description of ground-state electron distributions and classification of the chemical bonds. The ELF values of 1.0 designate the completely localized electrons, while the values approach to zero denote extremely low density of electrons. The isosurface of ELF for Be3BN3 monolayer was plotted with the iso-value of 0.35 au (Fig. 3a) and The ELF results demonstrate that there are little electrons locate at the Be atoms and most of ground-state electrons are distributed around N atoms, which is also suggested by distribution of red color region within the ELF map (Fig. 3b). The localized electrons around N atoms suggest that the Be and B atoms are donors and N atoms are acceptors of electrons during the formation of chemical bonds, indicating the greatly ionic nature of Be-N (B-N) bonds and strong connection between Be (or B) and N atoms. Moreover, the strong ionicity within the Be3BN3 sheet may lead to its ultrawide band gap, and thus, deep UV absorption. Different from the doping strategy for deep π-hole in Be-doped h-BN [43], this Be3BN3 monolayer is composed of the hexagonal beryllium nitride framework by partially inserting boron atoms into the center of Be3N3 rings. Our design of periodic 2D Be3BN3 monolayer is initially inspired by our finding of the B@Be3N3H6 cluster. Without considering the electrons of hydrogen, the number of outer electrons in B@Be3N3H6 cluster is 24 (Fig. S3), which is equal to those of benzene (C6H6) and borazine (B3N3H6). Therefore, the B@Be3N3H6 is isoelectronic molecule to benzene and borazine, which suggest that the electronic configuration of B@Be3N3H6 cluster (Be3BN3 monolayer) is similar to that of benzene/borazine (graphene/h-BN). To further investigate the electronic configuration of B@Be3N3H6 cluster, its geometric and electronic properties are calculated by using the GAUSSIAN 09 code. The optimized geometry of B@Be3N3H6
Fig. 3. Isosurface of ELF (upper) plotted with the value of 0.35 au and ELF map (lower) sliced perpendicular to (0 0 1) direction for Be3BN3 sheet.
possesses a quasi-planar structure with C3v symmetry, as shown in Fig. S4. The B-N, Be-N bond distances are 1.46 and 1.72 Å, respectively. The smallest harmonic vibration frequencies, HOMO-LUMO energy gap and
3
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Fig. 4. The valence molecular orbitals of B@Be3N3H6 cluster.
vertical ionization potential of B@Be3N3H6 are 145.33 cm−1, 5.99 eV and 8.05 eV, respectively. These results are comparable to those of benzene [52], indicating the high stability of B@Be3N3H6 cluster. The nucleus-independent chemical shifts (NICS) values at a distance of 1 Å above and below the B atom (NICS(1)) are −2.53 and −3.99 PPM, which suggests the aromatic characteristics of Be3BN3H6. The valence molecular orbitals (MOs) of B@Be3N3H6 (Fig. 4) demonstrate that the HOMO-5 and the degenerate HOMO-3 are delocalized π MOs. Thus, B@ Be3N3H6 have six π electrons and exhibits aromatic character, which is in agreement with the NICS results. The B@Be3N3H6 cluster has electronic configuration similar to benzene (or borazine), which is in accordance with the isoelectronic characteristics of B@Be3N3H6, benzene and borazine (Fig. S3). For a rough estimate of the formation difficulty of Be3BN3 monolayer, we calculated the formation energies of Be3BN3, h-BN and graphene monolayers (Table S1). The formation energy of Be3BN3 monolayer is comparable to those of h-BN and graphene, indicating that the newlydesigned Be3BN3 monolayer may be experimentally realized via dehydrogenation and polymerization of B@Be3N3H6 cluster, which is similar to the fabrication of graphene [53], h-BN [54] and other nanosheets [55,56] through dehydrogenation and polymerization of corresponding molecules or clusters.
Fig. 5. Band structure and PDOS of the Be3BN3 monolayer.
implying that B atoms have little effect on the band edges (Fig. S5c). Remarkably, the VBM is mainly contributed by p electrons of N atoms, while the CBM consist of s orbitals of N atoms (Fig. S5d). The conduction band level adjacent to CBM is mainly composed of Be-p and B-p state (Fig. S5b-c). The dielectric constants of the newly predicted Be3BN3 sheet were also calculated to investigate the optical absorption properties. According to equation (S1), the positive value of imaginary part ε2(ω) demonstrate the effective light absorption at this frequency (energy). The imaginary parts of dielectric constants calculated by HSE06 method demonstrate that the Be3BN3 monolayer has optical absorption edge locating at ~5.6 eV (Fig. 6), which is corresponding to the wavelength of 222 nm. The optical absorption characteristics suggest the deep UV applications of this Be3BN3 sheet, such as the deep ultraviolet (350–190 nm) detectors utilized in environment security, information technology, medical treatment, astronomical observation and inter-satellite communications [33]. Also, the newly predicted 2D materials may be utilized as visibleblind and solar-blind ultraviolet detectors for flame and oil spill detection, which are useful for solving the technical challenge of operating these detectors with a large background radiation of sunlight. DUV detection in the wavelength range of 210–300 nm is useful for flame, oil spill and missile detection, and traditional approaches include wide band gap semiconductor UV photodetectors and vacuum-based photoemission detectors [58].
3.3. Electronic structure and deep-ultraviolet response As a newly designed 2D material with promising stability, strong ionicity and experimental feasibility, it is worthwhile to further investigate the electronic structure and optical absorption for exploring its potential applications. Especially, the strong ionicity of Be3BN3 sheet implies its possible characteristics of ultrawide band gap and extremely deep UV absorption. By employing the HSE06 functionals, which was proven a promising method for analyzing the electronic and optical properties of wide-gap semiconductors or insulators, the band structure and dielectric constants of Be3BN3 monolayer were computed. As represented in Fig. 5, the Be3BN3 monolayer has a ultrawide band gap of 5.25 eV, which are comparable to those of other 2D materials including h-BeO (5.45 eV) [29], h-BN (5.56 eV) [57], h-BeS (4.25 eV) [31], et al. Both the valence band maximum (VBM) and conduction band minimum (CBM) located at the Γ point of k space suggest the direct band gap of Be3BN3 sheet. For Be atoms in the Be3BN3 monolayer, their p electrons give a little contribution to the VBM and the s orbitals of them mainly contribute to the deep energy level in the conduction and valance bands (Fig. S5b). Both the s and p electrons of B atoms have energy level higher (or lower) then the CBM (or VBM) level,
3.4. Sustainability of ultrawide band gap Despite of the outstanding optical absorption and possible DUV 4
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simulation at the temperatures of 500, 1000 and 1400 K, respectively, were calculated and represented at Fig. S6b. To avoid the computational burden of band structure calculations for excessively large superlattice including 112 atoms, the band structures of d-Be3BN3 were computed through the PBE functional, which usually underestimate the band gaps of semiconductors. To correct the underestimation band gaps calculated by PBE functionals, the band structure of Be3BN3 unitcell at 0 K by using both the HSE06 and PBE functionals were calculated to provide a benchmark for the correction PBE gaps. We found that the band gaps of Be3BN3 sheet computed by PBE and HSE06 functionals are 4.13 eV (Fig. S7) and 5.25 eV (Fig. 5), respectively, which suggest that the scissors value of +1.12 eV should be added to correct the PBE band gaps. Accordingly, the band gaps of d-Be3BN3 in the Fig. S6b are all corrected by scissors value of +1.12 eV. The width of band gap can be well maintained at the temperature of 500 K (5.11 eV) and will be reduced to 4.11 eV at 1000 K (or 4.07 eV at 1400 K). For Be3BN3 monolayer, the edge of absorption spectrum could be well sustained at ~243 nm under the temperature of 500 K, and there would be redshift of optical absorption to 302 and 305 nm at the temperature of 1000 and 1400 K, respectively. Remarkably, these absorption edges of 243, 302 and 305 nm can still reach the requirement of DUV detectors (350–190 nm) [33].
Fig. 6. Imaginary parts of dielectric constants for Be3BN3 monolayer computed by HSE06 functionals.
Table 1 The band gaps of Be3BN3 monolayer under different strength of strain. Strength of Strain
5% 4% 3% 2% 1% 0% −1% −2% −3% −4% −5%
Band Gaps under Biaxial Strain (eV)
5.12 5.14 514 5.18 5.22 5.25 5.39 5.32 5.33 5.34 5.35
4. Conclusion
Band Gaps under Uniaxial Strain (eV) x-direction
y-direction
5.15 5.18 5.21 5.23 5.24 5.25 5.27 5.28 5.29 5.30 5.30
5.11 5.15 5.18 5.21 5.23 5.25 5.27 5.29 5.30 5.31 5.32
The Be3BN3 monolayer constructed by B@Be3N3 cluster, which is isoelectronic molecule to dehydrogenated benzene/borazine with promising stability and aromaticity, was theoretically investigated in this study. Through first-principles computations, we found that the Be3BN3 sheet has not only pronounced thermodynamic stability, but also the ultrawide band gap (5.25 eV) and deep ultraviolet absorption (< 222 nm), which implies the DUV applications of this graphene-like material with novel geometric structure and unique chemical composition. Similar to the h-BN sheet, the insulating characteristics of Be3BN3 monolayer may result from the strong ionicity of it. Remarkably, the ultrawide band gap of Be3BN3 is sustainable under the external stress of −5% ~ 5%, electric fields up to 0.3 V/Å, and extremely high temperature of 1000 K, indicating its possible DUV applications in hostile environments. Noteworthy, this newly designed Be3BN3 monolayer may be fabricated by dehydrogenation and polymerization of B@Be3N3H6 cluster, which is similar to synthesis of graphene, h-BN and other nanosheets through polymerization of corresponding molecules. This Be3BN3 sheet with promising stability, DUV absorption and experimental feasibility could further enrich the family of 2D ultrawide band gap materials and provide a candidate for the DUV applications under severe environment.
applications of these newly predicted graphene-like material, the DUV photodetectors should still need to have the capability of operating at high temperatures and in hostile environments [33]. To evaluate the sustainability of DUV absorptions under severe environment, the effects of strains, external electric fields and extremely high temperatures on the band gap of Be3BN3 sheet were investigated. Under the tensile strain, all the band gaps (5.11–5.24 eV) of Be3BN3 monolayer are smaller than that of fully relaxed structure, while the compression strains lead to slightly larger band gaps (5.27–5.39 eV), as listed in Table 1. Accordingly, the external strain has little influence on the band gap, and thus, deep UV absorption of the Be3BN3 sheet, indicating its probable DUV utilization under external stress. Moreover, we also evaluate the effect of external electric field (Efield) on the electronic properties of Be3BN3 monolayer. As shown in Fig. S6a, the increase of external Efield, which are added along the z direction perpendicular to the Be3BN3 sheet, leads to slight decrease of its band gap. When the added Efield increase to 0.3 V/Å, the band gap is decreased to the value of ~5.00 eV, resulting in the optical absorption edge of ~248 nm. This wavelength is still located at the range of optical-absorption requirements of deep ultraviolet detectors (350–190 nm) [33]. Noteworthy, the DUV detectors or other DUV devices may be utilized at the extremely high temperature, and therefore, the pronounced thermal stability and sustainable DUV response under high temperature is an indispensable characteristic for the practical application of this newly designed Be3BN3 monolayer. Consequently, the band structures of distorted Be3BN3 (d-Be3BN3) sheets obtained after the AIMD
CRediT authorship contribution statement Haijun Zhang: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Writing - original draft, Writing review & editing. Chengcheng Xu: Conceptualization, Data curation, Formal analysis, Validation, Visualization. Kun Li: Formal analysis, Investigation, Software, Validation, Visualization, Writing - original draft. Yiru Di: Data curation, Formal analysis, Software, Validation, Visualization. Mingchao Wang: . : Software, Supervision, Validation, Writing - review & editing. Xiaomeng Zhou: Formal analysis, Supervision, Validation, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 5
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Acknowledgements
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This work was supported in China by NSFC (No. 21403001, 51776219 and 51802343), Tianjin Education Commission (2018KJ247) and start-up funds of Blue-Sky Young Scholar and Blue-Sky Distinguished Scholar in Civil Aviation University of China. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.commatsci.2020.109552. References [1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomically thin carbon films, Science 306 (2004) 666–669. [2] A.K. Geim, K.S. Novoselov, The rising of graphene, Nat. Mater. 6 (2007) 183–191. [3] C. Tan, X. Cao, X. Wu, Q. He, J. Yang, X. Zhang, J. Chen, W. Zhao, S. Han, G.H. Nam, M. Sindoro, H. Zhang, Recent advances in ultrathin two-dimensional nanomaterials, Chem. Rev. 117 (2017) 6225–6331. [4] Q. Tang, Z. Zhou, Graphene-analogous low-dimensional materials, Prog. Mater. Sci. 58 (2013) 1244–1315. [5] Y. Lin, T.V. 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Yang, Metal-organic framework nanosheets as building blocks for molecular sieving membranes, Science 346 (2014) 1356–1359. [17] T. Rodenas, I. Luz, G. Prieto, B. Seoane, H. Miro, A. Gorma, F. Kapteijn, F.X. Llabrés, J. Gascon, Metal-organic framework nanosheets in polymer composite materials for gas separation, Nat. Mater. 14 (2015) 48–55. [18] H. Liu, Y. Du, Y. Deng, P. Ye, Semiconducting black phosphorus: synthesis, transport properties and electronic applications, Chem. Soc. Rev. 44 (2015) 2732–2743. [19] P. Vogt, P.D. Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M.C. Asensio, A. Resta, B. Ealet, G.L. Lay, Silicene: compelling experimental evidence for graphene like two-dimensional silicon, Phys. Rev. Lett. 108 (2012) 155501. [20] G. Yang, Z. Xu, Z. Liu, S. Jin, H. Zhang, Z. Ding, Strain- and fluorination-induced quantum spin hall insulators in blue phosphorene: a first-principles study, J. Phys. Chem. C 121 (2017) 12945–12952. [21] S. Zhang, Z. Yan, Y. Li, Z. Chen, H. Zeng, Atomically thin arsenene and antimonene: semimetal-semiconductor and indirect-direct band-gap transitions, Angew. Chem. Int. Ed. 54 (2015) 3112–3115. [22] P. Ares, F. Aguilar-Galindo, D. Rodríguez-San-Migue, D.A. Aldave, S. Díaz-Tendero, M. Alcamí, F. Martín, J. Gómez-Herrero, F. Zamora, Mechanical isolation of highly stable antimonene under ambient conditions, Adv. Mater. 28 (2016) 6332–6336. [23] H. Zhang, Y. Liao, G. Yang, X. Zhou, Theoretical studies on the electronic and optical properties of honeycomb BC3 monolayer: a promising candidate for metal-free photocatalysts, ACS Omega 3 (2018) 10517–10525. [24] H. Zhang, X. Li, X. Meng, S. Zhou, G. Yang, X. Zhou, Isoelectronic analogues of graphene: the BCN monolayers with visible-light absorption and high carrier mobility, J. Phys.: Condens. Matter 31 (2019) 125301. [25] H. Chang, K. Tu, X. Zhang, J. Zhao, X. Zhou, H. Zhang, B4C3 monolayer with impressive electronic, optical, and mechanical properties: a potential metal-free photocatalyst for CO2 reduction under visible light, J. Phys. Chem. C (2019), https://doi.org/10.1021/acs.jpcc.9b06744. [26] G. Zhao, T. Wen, C. Chen, X. Wang, Synthesis of graphene-based nanomaterials and their application in energy-related and environmental-related areas, RSC Adv. 2 (2012) 9286–9303.
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Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci
Be3BN3 monolayer with ultrawide band gap and promising stability for deep ultraviolet applications ⁎
Haijun Zhanga, Chengcheng Xub, Kun Lia, Yiru Dia, Mingchao Wanga, , Xiaomeng Zhoua, a b
T
⁎
College of Economics and Management, College of Science, Civil Aviation University of China, Tianjin 300300, PR China School of Physics and Materials Science, Anhui University, Hefei 230601, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene-like material Deep-ultraviolet absorption Experimental feasibility Sustainable electronic gap First-principles calculations
In this study, the Be3BN3 two-dimensional (2D) system with strong ionicity and ultrawide band gap was theoretically obtained. The computational results suggest that the Be3BN3 sheet is thermodynamically stable and may be experimentally realized through dehydrogenation and polymerization of the B@Be3N3H6 cluster, which is an isoelectronic molecule to benzene and has promising stability and strong aromaticity. This newly designed 2D material exhibits ultrawide band gap of 5.25 eV and deep ultraviolet (DUV) absorption (λ < 222 nm), implying its possible applications for photodetectors in the visible-blind or solar-blind regions. Remarkably, the sustainable DUV response (λ < 305 nm) under external strain, electric field and high temperature indicates the probable utilization of the Be3BN3 monolayer in severe environment. This newly designed graphene-like material could further enrich the variety of ultrawide-gap nanomaterials and propose an outstanding candidate for DUV applications.
1. Introduction Since the discovery of mechanically exfoliated graphene in 2004 [1], which is an exemplary model of ultrathin two-dimensional (2D) materials with novel geometries and unexpected characteristics [2], tremendous studies have been performed on exploring graphene-like nanostructures to further enrich family members of 2D ultrathin materials [3,4]. The graphene-like materials, including hexagonal boron nitride (h-BN) [5,6], transition metal dichalcogendes (TMDs) [7,8], graphitic carbon nitride (g-C3N4) [9,10], layered metal oxides [11,12], MXenes [13–15], metal-organic frameworks (MOFs) [16,17], monoelemental graphene analogues [18–22], boron carbide/carbonitride [23–25] and so on, exhibit unique structural characteristics and versatile physicochemical properties for a wide spectra of applications. The various practical applications of ultrathin 2D nanomaterials are predominantly determined by their specific mechanical, electronic, optical, or magnetic properties. Take the electronic band gap as an example, both the gapless graphene and metallic MXene have ultrahigh electrical mobility and can be used as high-performance supercapacitor electrodes [26,27], while the 2D HgTe/CdTe superlattice with narrow band gap (48 to 101 meV) is a stable alternative for application in farinfrared optoelectronic devices [28]. The few-layer MoS2 nanosheets with moderate band gaps (1.35 to 1.82 eV) are promising phototransistors for the detection of green and red light [8]. The band gap ⁎
around 2.70 eV results in the visible-light absorption and appropriate redox potential of the graphitic carbon nitride, which makes the g-C3N4 nanosheet a high-efficiency photocatalysts [10]. Additionally, the wide gap semiconductor films and one-dimensional nanostructures (Eg > 3.0 eV), including III-Nitrides, SiC, diamond, and metal oxides, can be utilized as ultraviolet (UV) photodetectors in the visible-blind or solar-blind regions [29]. Despite the great achievements in exploration of graphene-like materials, the exceptional rarity of 2D wide-gap semiconductors is a potential barrier to the utilization of these materials [5,30–33]. Especially, the materials with deep UV (350–190 nm) responsive are rarely obtained but play a key role in applications of environment security, information technology, medical treatment, flame and radiation detection, inter-satellite communications, et al. [28,34]. Moreover, the 2D wide-band-gap materials with the absorption ranging from 400 to 10 nm can also be utilized in the transparent field-effect transistors [35], high-temperature die attach materials [36], deep UV (DUV) optoelectronic devices [37–38], et al. In view of the significance and scarcity of the nanomaterials with UV or DUV response, it is indispensable to seek for other 2D wide-gap materials to further enrich their diversity and broaden their various applications. Considering the wide band gap resulting from the strong ionicity within the h-BN nanosheet [39–41], introduction of another boron-like element into 2D B-N system may be a good strategy to reassemble the
Corresponding authors. E-mail addresses: [email protected] (M. Wang), [email protected] (X. Zhou).
https://doi.org/10.1016/j.commatsci.2020.109552 Received 23 October 2019; Received in revised form 16 January 2020; Accepted 20 January 2020 0927-0256/ © 2020 Elsevier B.V. All rights reserved.
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2D framework and theoretically design new graphene-like materials with novel geometry and wide band gaps. Xie et al found that [42], due to the different electronegativities of B and N atoms, the asymmetrical shapes of π bonding states in VBM and π* antibonding states in CBM result in a significant iconicity between B and N atoms and the opening of a large energy gap at Fermi level. By computing the band structures of atomic chains of group III-V elements, Zaluev and D’yachkov found that going from the BN chain to the AlP, GaAs, and InSb chains is accompanied by a decrease in the chemical bond ionicity, leading to a gradual decrease of the π-π* gaps. Therefore, the strong chemical bond ionicity should play a significant role in determining the insulating characteristics of h-BN nanosheet. The ultrawide band gap may be obtained by composing the 2D framework with the elements of boron and nitrogen, as well as beryllium element similar to boron. Herein, the beryllium element with electron deficiency similar to B element was introduced into the 2D B-N system to compose novel chemical bonds of Be-N, B-N and Be-B interactions in this study. Accordingly, a brand-new Be3BN3 monolayer with wide band gap of 5.25 eV was theoretically obtained. Different from the doping strategy for deep π-hole in Be-doped h-BN [43], this Be3BN3 monolayer is not composed of the hexagonal boron nitride framework and Be dopant, but is constructed by extending quasi-planar Be3BN3 cluster into 2D framework. This newly predicted Be3BN3 monolayer has not only absorption edge at solar-blind region (λ < 222 nm) but also pronounced thermal stability and sustainable band gap at the temperature up to 1000 K, indicating its superiority in deep ultraviolet application. 2. Computational details The first-principles calculations in this work were performed by employing the projector-augmented plane wave (PAW) [44] method as implemented in the Vienna ab inito simulation package (VASP) [45] code. The generalized gradient approximation in the form of PBE [46] was used to describe the electron exchange–correlation functional. The energy of ionic relaxation is converged at 10-6 eV, and the force convergence was set at 10-3 eV/Å. The cutoff energy of 600 eV was adopted in all computations and 11 × 11 × 1 Monkhorst-Pack k-points mesh was utilized in the geometry relaxation and self-consistent calculations. Considering the underestimation of band gaps by PBE functional, hybrid functional proposed by Heyd-Scuseria-Ernzerhof (HSE06) [47] was employed to compute the band structures and dielectric constants of Be3BN3 monolayers. In all computations of geometry optimization and chemical potential of elements, spin polarization was considered. The details of computing the phonon dispersion, ab initio molecular dynamics (AIMD) simulations, optical properties, and geometric and electronic properties of B@Be3N3H6 cluster were represented in the Supporting information.
Fig. 1. Geometric structure (upper) and phonon dispersion (lower) of theoretically predicted Be3BN3 monolayer.
investigated 2D framework, boron atoms were introduced into hexagonal BeN monolayer to construct a thermodynamic stable 2D network consists of Be, B and N elements, which is labeled as Be3BN3 monolayer (Fig. 1a). As presented in Fig. 1b, the computational results of phonon dispersion suggest the kinetic stability of this novel 2D crystal constructed by motifs of B-centered Be3N3 rings (B@Be3N3). As the result of different electron distribution between the Be3BN3 and hBN monolayers, the outer Be3N3 rings of B@Be3N3 motifs within the former are not regular hexagon. There is not any shared edges or atoms/points between all B-centered Be3N3 rings, which is due to the interval distribution of B@Be3N3 clusters within the Be3BN3 monolayer. There is not any appreciable imaginary frequency in the phonon spectrum of Be3BN3 monolayer and the highest frequency of it can reach up to 1395 cm−1 (41.84 THz), indicating the kinetic stability of Be3BN3 monolayer and robust Be-B-N connection in this 2D crystal. Moreover, the thermal stability was also assessed by performing the AIMD simulations at the temperatures of 500 K, 1000 K and 1400 K. The snapshots of Be3BN3 monolayer at the end of 10 ps simulations (Fig. 2) suggest that the 2D framework with hexagonal B@Be3N3 motifs are well maintained at the temperature of 1000 K, which indicate their promising thermal stability and possible utilizations at relatively high temperatures.
3. Results and discussion 3.1. Geometric structure and thermodynamic stability of Be3BN3 monolayer In view of the similarity of Be and B elements, we attempted to replace the boron atoms within BN nanosheet by Be atoms to compose the BeN nanosheets (Fig. S1a) and investigate the stability and physicochemical characteristics of the latter. We found that the BeN monolayer with honeycomb structure is kinetically instable, as suggested by the appreciable imaginary frequency in the phonon spectrum (Fig. S1b). On the purpose of obtaining the stable framework containing Be, B and N elements, the stability of modified hexagonal BN (h-BN) monolayer with interval-distribution Be atoms at the center of B3N3 rings, namely the BeB3N3 monolayer, was also assessed (Fig. S2a). There is also obvious negative frequency in the phonon spectrum of BeB3N3 sheet, indicating the dynamical instability of these modified hBN. In order to achieve in both the stabilization and ionicity of
3.2. Stabilization mechanism and experimental feasibility The pronounced thermal and dynamic stability of Be3BN3 monolayer may result from the strong ionicity of and novel electronic configuration of this Be-B-N network. To evaluate the bonding strength of this newly designed 2D materials, the cohesive energy was calculated according to the formula of Ecoh = (3EBe + EB + 3EN − EBe3BN3)/7, in which the EBe, EB, EN and EBe3BN3 are total energies of a single Be atom, a single B atom, a single N, and one unit cell of Be3BN3 monolayer, 2
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Fig. 2. Snapshots of the Be3BN3 monolayers at the end of 10 ps AIMD simulations at the temperatures of (a) 500 K, (b) 1000 K and (c) 1400 K. Both the top (upper) and side (lower) views are demonstrated.
respectively. The computed cohesive energy of Be3BN3 sheet (5.06 eV/ atom) is obviously higher than that of hypothetical BeN monolayer in Fig. S1a (3.57 eV/atom) and is slightly higher than that of modified hBN monolayer with Be atoms in Fig. S2a (4.96 eV/atom), which implies the strongest connection between Be, B and N atoms in the Be3BN3 sheet. Also, the cohesive energy of Be3BN3 sheet is comparable to other graphene-like materials, such as the iron borides (5.56–5.79 eV/atom), [48] Cu2Si (3.46 eV/atom), [49] and beryllium carbide (4.86 eV/atom), [50] indicating the robust chemical bonds in Be3BN3 monolayer. For a deep insight into the bonding nature and electronic configuration, the electron localization functions (ELF) [51] were calculated to analyze the stabilization mechanism of Be3BN3 monolayer. The ELF is a useful tool for description of ground-state electron distributions and classification of the chemical bonds. The ELF values of 1.0 designate the completely localized electrons, while the values approach to zero denote extremely low density of electrons. The isosurface of ELF for Be3BN3 monolayer was plotted with the iso-value of 0.35 au (Fig. 3a) and The ELF results demonstrate that there are little electrons locate at the Be atoms and most of ground-state electrons are distributed around N atoms, which is also suggested by distribution of red color region within the ELF map (Fig. 3b). The localized electrons around N atoms suggest that the Be and B atoms are donors and N atoms are acceptors of electrons during the formation of chemical bonds, indicating the greatly ionic nature of Be-N (B-N) bonds and strong connection between Be (or B) and N atoms. Moreover, the strong ionicity within the Be3BN3 sheet may lead to its ultrawide band gap, and thus, deep UV absorption. Different from the doping strategy for deep π-hole in Be-doped h-BN [43], this Be3BN3 monolayer is composed of the hexagonal beryllium nitride framework by partially inserting boron atoms into the center of Be3N3 rings. Our design of periodic 2D Be3BN3 monolayer is initially inspired by our finding of the B@Be3N3H6 cluster. Without considering the electrons of hydrogen, the number of outer electrons in B@Be3N3H6 cluster is 24 (Fig. S3), which is equal to those of benzene (C6H6) and borazine (B3N3H6). Therefore, the B@Be3N3H6 is isoelectronic molecule to benzene and borazine, which suggest that the electronic configuration of B@Be3N3H6 cluster (Be3BN3 monolayer) is similar to that of benzene/borazine (graphene/h-BN). To further investigate the electronic configuration of B@Be3N3H6 cluster, its geometric and electronic properties are calculated by using the GAUSSIAN 09 code. The optimized geometry of B@Be3N3H6
Fig. 3. Isosurface of ELF (upper) plotted with the value of 0.35 au and ELF map (lower) sliced perpendicular to (0 0 1) direction for Be3BN3 sheet.
possesses a quasi-planar structure with C3v symmetry, as shown in Fig. S4. The B-N, Be-N bond distances are 1.46 and 1.72 Å, respectively. The smallest harmonic vibration frequencies, HOMO-LUMO energy gap and
3
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Fig. 4. The valence molecular orbitals of B@Be3N3H6 cluster.
vertical ionization potential of B@Be3N3H6 are 145.33 cm−1, 5.99 eV and 8.05 eV, respectively. These results are comparable to those of benzene [52], indicating the high stability of B@Be3N3H6 cluster. The nucleus-independent chemical shifts (NICS) values at a distance of 1 Å above and below the B atom (NICS(1)) are −2.53 and −3.99 PPM, which suggests the aromatic characteristics of Be3BN3H6. The valence molecular orbitals (MOs) of B@Be3N3H6 (Fig. 4) demonstrate that the HOMO-5 and the degenerate HOMO-3 are delocalized π MOs. Thus, B@ Be3N3H6 have six π electrons and exhibits aromatic character, which is in agreement with the NICS results. The B@Be3N3H6 cluster has electronic configuration similar to benzene (or borazine), which is in accordance with the isoelectronic characteristics of B@Be3N3H6, benzene and borazine (Fig. S3). For a rough estimate of the formation difficulty of Be3BN3 monolayer, we calculated the formation energies of Be3BN3, h-BN and graphene monolayers (Table S1). The formation energy of Be3BN3 monolayer is comparable to those of h-BN and graphene, indicating that the newlydesigned Be3BN3 monolayer may be experimentally realized via dehydrogenation and polymerization of B@Be3N3H6 cluster, which is similar to the fabrication of graphene [53], h-BN [54] and other nanosheets [55,56] through dehydrogenation and polymerization of corresponding molecules or clusters.
Fig. 5. Band structure and PDOS of the Be3BN3 monolayer.
implying that B atoms have little effect on the band edges (Fig. S5c). Remarkably, the VBM is mainly contributed by p electrons of N atoms, while the CBM consist of s orbitals of N atoms (Fig. S5d). The conduction band level adjacent to CBM is mainly composed of Be-p and B-p state (Fig. S5b-c). The dielectric constants of the newly predicted Be3BN3 sheet were also calculated to investigate the optical absorption properties. According to equation (S1), the positive value of imaginary part ε2(ω) demonstrate the effective light absorption at this frequency (energy). The imaginary parts of dielectric constants calculated by HSE06 method demonstrate that the Be3BN3 monolayer has optical absorption edge locating at ~5.6 eV (Fig. 6), which is corresponding to the wavelength of 222 nm. The optical absorption characteristics suggest the deep UV applications of this Be3BN3 sheet, such as the deep ultraviolet (350–190 nm) detectors utilized in environment security, information technology, medical treatment, astronomical observation and inter-satellite communications [33]. Also, the newly predicted 2D materials may be utilized as visibleblind and solar-blind ultraviolet detectors for flame and oil spill detection, which are useful for solving the technical challenge of operating these detectors with a large background radiation of sunlight. DUV detection in the wavelength range of 210–300 nm is useful for flame, oil spill and missile detection, and traditional approaches include wide band gap semiconductor UV photodetectors and vacuum-based photoemission detectors [58].
3.3. Electronic structure and deep-ultraviolet response As a newly designed 2D material with promising stability, strong ionicity and experimental feasibility, it is worthwhile to further investigate the electronic structure and optical absorption for exploring its potential applications. Especially, the strong ionicity of Be3BN3 sheet implies its possible characteristics of ultrawide band gap and extremely deep UV absorption. By employing the HSE06 functionals, which was proven a promising method for analyzing the electronic and optical properties of wide-gap semiconductors or insulators, the band structure and dielectric constants of Be3BN3 monolayer were computed. As represented in Fig. 5, the Be3BN3 monolayer has a ultrawide band gap of 5.25 eV, which are comparable to those of other 2D materials including h-BeO (5.45 eV) [29], h-BN (5.56 eV) [57], h-BeS (4.25 eV) [31], et al. Both the valence band maximum (VBM) and conduction band minimum (CBM) located at the Γ point of k space suggest the direct band gap of Be3BN3 sheet. For Be atoms in the Be3BN3 monolayer, their p electrons give a little contribution to the VBM and the s orbitals of them mainly contribute to the deep energy level in the conduction and valance bands (Fig. S5b). Both the s and p electrons of B atoms have energy level higher (or lower) then the CBM (or VBM) level,
3.4. Sustainability of ultrawide band gap Despite of the outstanding optical absorption and possible DUV 4
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simulation at the temperatures of 500, 1000 and 1400 K, respectively, were calculated and represented at Fig. S6b. To avoid the computational burden of band structure calculations for excessively large superlattice including 112 atoms, the band structures of d-Be3BN3 were computed through the PBE functional, which usually underestimate the band gaps of semiconductors. To correct the underestimation band gaps calculated by PBE functionals, the band structure of Be3BN3 unitcell at 0 K by using both the HSE06 and PBE functionals were calculated to provide a benchmark for the correction PBE gaps. We found that the band gaps of Be3BN3 sheet computed by PBE and HSE06 functionals are 4.13 eV (Fig. S7) and 5.25 eV (Fig. 5), respectively, which suggest that the scissors value of +1.12 eV should be added to correct the PBE band gaps. Accordingly, the band gaps of d-Be3BN3 in the Fig. S6b are all corrected by scissors value of +1.12 eV. The width of band gap can be well maintained at the temperature of 500 K (5.11 eV) and will be reduced to 4.11 eV at 1000 K (or 4.07 eV at 1400 K). For Be3BN3 monolayer, the edge of absorption spectrum could be well sustained at ~243 nm under the temperature of 500 K, and there would be redshift of optical absorption to 302 and 305 nm at the temperature of 1000 and 1400 K, respectively. Remarkably, these absorption edges of 243, 302 and 305 nm can still reach the requirement of DUV detectors (350–190 nm) [33].
Fig. 6. Imaginary parts of dielectric constants for Be3BN3 monolayer computed by HSE06 functionals.
Table 1 The band gaps of Be3BN3 monolayer under different strength of strain. Strength of Strain
5% 4% 3% 2% 1% 0% −1% −2% −3% −4% −5%
Band Gaps under Biaxial Strain (eV)
5.12 5.14 514 5.18 5.22 5.25 5.39 5.32 5.33 5.34 5.35
4. Conclusion
Band Gaps under Uniaxial Strain (eV) x-direction
y-direction
5.15 5.18 5.21 5.23 5.24 5.25 5.27 5.28 5.29 5.30 5.30
5.11 5.15 5.18 5.21 5.23 5.25 5.27 5.29 5.30 5.31 5.32
The Be3BN3 monolayer constructed by B@Be3N3 cluster, which is isoelectronic molecule to dehydrogenated benzene/borazine with promising stability and aromaticity, was theoretically investigated in this study. Through first-principles computations, we found that the Be3BN3 sheet has not only pronounced thermodynamic stability, but also the ultrawide band gap (5.25 eV) and deep ultraviolet absorption (< 222 nm), which implies the DUV applications of this graphene-like material with novel geometric structure and unique chemical composition. Similar to the h-BN sheet, the insulating characteristics of Be3BN3 monolayer may result from the strong ionicity of it. Remarkably, the ultrawide band gap of Be3BN3 is sustainable under the external stress of −5% ~ 5%, electric fields up to 0.3 V/Å, and extremely high temperature of 1000 K, indicating its possible DUV applications in hostile environments. Noteworthy, this newly designed Be3BN3 monolayer may be fabricated by dehydrogenation and polymerization of B@Be3N3H6 cluster, which is similar to synthesis of graphene, h-BN and other nanosheets through polymerization of corresponding molecules. This Be3BN3 sheet with promising stability, DUV absorption and experimental feasibility could further enrich the family of 2D ultrawide band gap materials and provide a candidate for the DUV applications under severe environment.
applications of these newly predicted graphene-like material, the DUV photodetectors should still need to have the capability of operating at high temperatures and in hostile environments [33]. To evaluate the sustainability of DUV absorptions under severe environment, the effects of strains, external electric fields and extremely high temperatures on the band gap of Be3BN3 sheet were investigated. Under the tensile strain, all the band gaps (5.11–5.24 eV) of Be3BN3 monolayer are smaller than that of fully relaxed structure, while the compression strains lead to slightly larger band gaps (5.27–5.39 eV), as listed in Table 1. Accordingly, the external strain has little influence on the band gap, and thus, deep UV absorption of the Be3BN3 sheet, indicating its probable DUV utilization under external stress. Moreover, we also evaluate the effect of external electric field (Efield) on the electronic properties of Be3BN3 monolayer. As shown in Fig. S6a, the increase of external Efield, which are added along the z direction perpendicular to the Be3BN3 sheet, leads to slight decrease of its band gap. When the added Efield increase to 0.3 V/Å, the band gap is decreased to the value of ~5.00 eV, resulting in the optical absorption edge of ~248 nm. This wavelength is still located at the range of optical-absorption requirements of deep ultraviolet detectors (350–190 nm) [33]. Noteworthy, the DUV detectors or other DUV devices may be utilized at the extremely high temperature, and therefore, the pronounced thermal stability and sustainable DUV response under high temperature is an indispensable characteristic for the practical application of this newly designed Be3BN3 monolayer. Consequently, the band structures of distorted Be3BN3 (d-Be3BN3) sheets obtained after the AIMD
CRediT authorship contribution statement Haijun Zhang: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Writing - original draft, Writing review & editing. Chengcheng Xu: Conceptualization, Data curation, Formal analysis, Validation, Visualization. Kun Li: Formal analysis, Investigation, Software, Validation, Visualization, Writing - original draft. Yiru Di: Data curation, Formal analysis, Software, Validation, Visualization. Mingchao Wang: . : Software, Supervision, Validation, Writing - review & editing. Xiaomeng Zhou: Formal analysis, Supervision, Validation, Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 5
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Acknowledgements
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