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Ground State Structure and Physical Properties of Silicon Monoxide Sheet T ⁎ Yanyan Chen, Yupeng Shen, Yiheng Shen, Xiaoyin Li, Yaguang Guo, Qian Wang

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Ground State Structure and Physical Properties of Silicon Monoxide Sheet T ⁎ Yanyan Chen, Yupeng Shen, Yiheng Shen, Xiaoyin Li, Yaguang Guo, Qian Wang Applied Surface Science 527 (2020) 146759 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Full Length Article Ground state structure and physical properties of silicon monoxide sheet T ⁎ Yanyan Chen, Yupeng Shen, Yiheng Shen, Xiaoyin Li, Yaguang Guo, Qian Wang Center for Applied Physics and Technology, Department of Materials Science and Engineering, HEDPS, BKL-MEMD, College of Engineering, Peking University, Beijing 100871, China ARTICLE INFO ABSTRACT Keywords: Two-dimensional (2D) silicon oxides have received considerable attention in recent years because of their ex- 2D silicon monoxide (SiO) cellent compatibility with the well-developed Si-based semiconductor technology. Though the ground state Global structure search structure of 2D silicon dioxide (SiO2) was identified many years ago, the corresponding study of 2D silicon Physical properties monoxide (SiO) has not yet been reported. Herein, using global structure search method combined with density Ground state structure functional theory and Boltzmann transport theory, we find the ground state structure of 2D SiO, named Orth-SiO, and systematically study its physical properties. Orth-SiO is found to be thermally, dynamically and mechani- cally stable, and possesses a direct band gap of 1.52 eV, which is much smaller as compared to that of α-2D silica (7.31 eV). Moreover, it exhibits significant anisotropies in mechanical properties and optical adsorption due to its exceptional atomic configuration. The lattice thermal conductivity of Orth-SiO is 80.45 and 33.79 W/mK along the x and y directions at room temperature, respectively, much higher than that of many Si-based ma- − − terials due to its weak anharmonicity. In addition, the carrier mobility is 8.5 × 103 cm2·V 1·s 1, much greater than that of many other oxides. These results add new features to silicon oxides family. 1. Introduction while the high pressure SiO phases are metallic. However, when going from 3D to 2D, to the best of our knowledge, Silicon monoxide (SiO), which possesses high interstellar abun- the ground state structure of 2D SiO has not yet been identified. To dance, is the most common oxide of silicon in the universe [1]. The date, only three possible structures of 2D SiO, namely ep-sSiO, a-sSiO widely used method of synthesizing SiO is heating silicon dioxide with and z-sSiO have been reported [14]. This motivates us to explore the silicon [2], although many other methods for preparing high-purity SiO ground state structure of 2D SiO and study its properties by using the have also been proposed [3,4]. Due to its mature preparation techni- particle swarm optimization (PSO) algorithm combined with first ques, wide band gap and low refractive index, the commercial SiO is principles calculations. Parallel to the expansion from 3D SiO2 to 3D widely used as the protective film of optical fibers and dielectric ma- SiO, our study expands the low-dimensional silicon oxide family from terial of capacitors [5–7]. However, most of the commercial SiO are in 2D SiO2 to 2D SiO, providing new understandings on how the geometry amorphous phase where the conventional diffraction methods, such as and properties of silicon oxides change with the dimensionality and X-ray diffraction, X-ray Raman scattering, small-angle X-ray scattering, oxygen concentration, which are helpful for experimentalists to tune cannot provide detailed information on their structures [8]. Therefore, the structure and properties by changing oxygen concentration. there is no assured conclusion about the structure of SiO [9,10]. Moreover, the amorphous SiO is metastable and will decompose to Si 2. Computational methods and SiO2 under certain conditions [6,11,12]. So, it is highly desirable to identify the possible crystalline structures of 3D and 2D SiO [10,13,14]. The ground state of 2D SiO sheet is predicted by using the particle Mankefors et al. predicted the probable structures of 3D SiO by element swarm optimization (PSO) algorithm implemented in the Crystal substitution in known crystal structures, including rock salt, zinc ble- structure AnaLYsis by Particle Swarm Optimization (CALYPSO) nede, wurtzite and SnO [13]. Subsequently, AlKaabi et al. explored the package [15,16]. The population size and the number of generations ground state structures of 3D SiO at 1 atm and high pressures up to 200 are set to thirty and fifty, respectively, to ensure the convergence. GPa through the evolutionary and random structure searches [10], and Considering the fact that silicon atom prefers to form a tetrahedral found that the ground state structure of 3D SiO is a semiconductor configuration and the experimental thickness of 2D hexagonal silica is ⁎ Corresponding author. E-mail address: [email protected] (Q. Wang). https://doi.org/10.1016/j.apsusc.2020.146759 Received 11 March 2020; Received in revised form 3 May 2020; Accepted 21 May 2020 Available online 25 May 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved. Y. Chen, et al. Applied Surface Science 527 (2020) 146759 4.34 Å [17,18], the buffering thickness is set as 4.5 Å to accommodate Table 1 the buckling of structure. The vacuum space of 16 Å is included to avoid Relative energy ΔE (in eV/atom) with respect to the lowest energy structure, the interaction from its periodic images. lattice parameters (in Å), Si-Si and Si-O bond lengths (in Å) of Orth-SiO, z-sSiO, The geometric optimization and calculations of electronic structure a-sSiO and ep-sSiO. are performed using density functional theory (DFT) as included in the 2D SiO allotropes ΔEa b dSi-Si dSi-O Vienna ab initio Simulation Package (VASP) [19]. The interactions be- tween ion cores and valence electrons are described within the pro- Orth-SiO 0.000 5.50 6.91 2.41 1.67 jector augmented wave (PAW) method [20,21] with the kinetic energy z-sSiO 0.047 4.04 2.81 2.41 1.68 ff (4.04) (2.71) (2.41) (1.68) cuto of 500 eV. The electron exchange-correlation interaction is a-sSiO 0.061 7.06 2.80 2.40, 2.43 1.68 treated by the Perdew-Burke-Ernzerhof (PBE) functional [22] within (7.08) (2.81) (2.40,2.43) (1.68) the generalized gradient approximation (GGA) [23]. To obtain the ac- ep-sSiO 0.212 7.42 4.02 2.40 1.70 curate electronic band structure, we recalculate the band structure with (7.40) (4.06) (2.41) (1.70) the hybrid functional of Heyd-Scuseria-Ernzerhof (HSE06) [24,25]. The *The numbers in parentheses are from Ref. [14]. first Brillouin zone is represented by using the Monkhorst–Pack scheme − [26] with a grid density of 2π × 0.02 Å 1. The convergence criteria for − atoms. total energy and atomic force components are set to 10 8 eV and − It is worth mentioning that the previously proposed z-sSiO, a-sSiO 10 6 eV/Å, respectively. The thermal stability is examined by using ab and ep-sSiO [14] are also obtained as the metastable SiO phases during initio molecular dynamics (AIMD) simulation. the structure search process. This verifies the accuracy and effectiveness The calculation of lattice thermal conductivity is based on the semi- of the structure search method. We list their relative energies with re- classical phonon Boltzmann transport equation (BTE) [27] as im- spect to the lowest energy structure (Orth-SiO), the lattice parameters, plemented in ShengBTE code [28], where the required harmonic and the Si-O and Si-Si bond lengths in Table 1 for comparison. On can second-order interatomic force constants (IFCs) and the anharmonic see that the average Si-Si bond length of Orth-SiO is comparable to that third-order IFCs are calculated using the Phonopy package [29].To of other three SiO allotropes, while the average Si-O bond length is balance both the calculation accuracy and computational cost, the 7th slightly shorter, leading to the lower energy of Orth-SiO. We also note nearest neighbors are considered. The phonon dispersion is obtained that the Si-Si bond length of 2.41 Å in Orth-SiO is larger than that of with the finite displacement method [30] and the specific heat capacity low-buckled silicene (2.28 Å) [31] and diamond-Si (2.37 Å) [10], is further calculated using the second-order IFCs. showing a single bond character. While the Si-O bond length of 1.67 Å is larger than the value of 1.63 Å in the ground state structure of 2D α 3. Results and discussion SiO2 ( -2D silica) [32]. We then examine the dynamic stability of Orth-SiO by calculating 3.1. Geometric structure and stability its phonon spectra. Fig. 2(a) displays the phonon dispersion along the high-symmetry paths of the first Brillouin zone. One can see that all the By performing the global structure search for 2D SiO, we obtain the frequencies of vibrational modes are positive, confirming that Orth-SiO energetically most stable configuration, namely, the ground state is dynamically stable. In addition, we notice that the highest frequency structure of 2D SiO, as displayed in Fig. 1. This structure belongs to of Orth-SiO at the Γ point reaches 28 THz, which is higher than that of Cmma space group (No. 67) and has an orthogonal unit cell with eight z-sSiO, a-sSiO and ep-sSiO (about 25 THz) [14], implying the stronger Si atoms and eight O atoms, so we name it Orth-SiO. The optimized bonding and better energetics [33]. lattice constants are a = 5.50 Å and b = 6.91 Å, and the side view The thermal stability of Orth-SiO is studied by performing AIMD shows that Orth-SiO possesses a finite thickness of 3.60 Å.
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