Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61974068, 11704198)
Received Date:22 December 2020
Accepted Date:06 February 2021
Available Online:16 June 2021
Published Online:20 June 2021
Abstract:Since the discovery and synthesis of graphene, two-dimensional graphether and silicether materials have been predicted as novel semiconductors. A novel two-dimensional silicether/graphether heterostructure is designed by combining silicether and graphether, which has unique optical and electronic properties due to the properties of a single material synthesized by heterostructures. The electronic and optical properties of silicether/graphether heterostructure are studied by the first-principles calculations based on density functional theory. The binding energy and layer spacing for each of all considered 16 stacking patterns of the heterostructures are calculated. The results show that different stacking patterns have a small effect on the binding energy of the heterostructure. When the layer spacing is 2.21 ?, the stacking pattern in which the concave oxygen atoms of graphether are on the top of the concave oxygen atoms of silicether is the most stable. In addition, it has an indirect band gap of 0.63 eV, which is smaller than that of the silicether and graphether, respectively. By changing the external electric field and the biaxial strain strength, the band gap of the silicether/graphether heterostructure shows tunability. The compressive strain can increase the band gap of silicether/graphether heterostructure, while the band gap decreases with the tensile strain increasing. Especially, when the compressive strain is greater than –6%, the heterostructure undergoes an indirect-to-direct band gap transition, which is beneficial to its applications in optical devices. When the external electric field is applied, the band gap of the heterostructure changes linearly with the strength of the electric field, and the indirect band gap characteristic is maintained. The absorption coefficient of silicether/graphether heterostructure shows a strong peak in the ultraviolet light region. The maximum absorption coefficient can reach up to 1.7 × 105 cm–1 around 110 nm. Compared with that of monolayer graphether and silicether, the optical absorption of the heterostructure is significantly enhanced within the range from more than 80 nm to less than 170 nm. The results show that silicether/graphether heterostructure has an outstanding optical absorption in the ultraviolet region. Moreover, the silicether/graphether heterostructure also shows considerable absorption coefficient (1 × 104—4 × 104 cm–1) in the visible region, which makes it a potential material in photovoltaic applications. This work may provide a novel material with a promising prospect of potential applications in nanodevices. Keywords:first-principle/ silicether/graphether heterostructure/ electronic properties/ optical properties
其中, Etotal为硅醚/石墨醚异质结构的总能; Egraphether与Esilicether分别为独立的石墨醚和硅醚的总能. 其他函数表示为Eb/S, 其中S为硅醚/石墨醚异质结构的耦合面积. 根据Eb的定义, Eb的绝对值越大, 说明这两个单层之间的界面在能量上更稳定, 在实验中更容易获得[50]. 图1为计算的异质结构的所有16种堆砌方式的结合能和层间距, 计算结果表明, 这些构型之间Eb的差异很小, 说明堆砌方式对异质结构的总能没有显著影响. 尽管如此, 为了研究方便, 选择能量最低的方式, 即方式X (图2)作为本文的研究重点. 当层间距离为2.21 ?时, 异质结构的结合能(–9.39 meV/?2)最低, 说明该堆砌模式下的结构最稳定. 而当层间距离大于(小于)该值时, 异质结构的结合能逐渐增大, 稳定性随着层间距离的增大(减小)而降低. 图 1 16种堆砌方式在不同层间距下的结合能 Figure1. Binding energy of the sixteen stacking patterns under different interlayer distances.
图 2 (a) 异质结构堆砌方式X的俯视图; (b) 堆砌方式X的侧视图; 红色、黄色和灰色的球分别代表氧原子、硅原子和碳原子 Figure2. (a) Top view of stacking pattern X; (b) side view of the pattern X. O, Si and C atoms are presented by red, yellow and grey balls, respectively.
23.2.电子结构 -->
3.2.电子结构
在确定了异质结构的构型和稳定性后, 计算了硅醚/石墨醚的能带结构以了解其性质. 为了比较, 图3(a)和图3(b)分别显示了单个的石墨醚和硅醚的带隙. 从图3(a)和图3(b)可以看出, 计算得到的石墨醚是一种带隙为0.85 eV的直接带隙半导体, 导带底和价带顶位于Γ点. 而硅醚的间接带隙为1.46 eV, 导带底和价带顶位于Γ和Y点之间. 图3(c)表明异质结构的间接带隙为0.63 eV, 导带底位于Γ点, 而价带顶位于S和X点之间. 此外, 异质结构的带隙比石墨醚和硅醚的带隙要小. 图 3 能带结构图 (a)石墨醚; (b)硅醚; (c)硅醚/石墨醚异质结构, 其中点A, B和C分别为态A, B和C在能带结构中的位置 Figure3. Band structure: (a) Graphether; (b) silicether; (c) silicether/graphether heterostrure. Points A, B and C in panel (c) are the positions of states A, B and C in the energy band structure respectively.
计算了硅醚/石墨醚异质结构的总态密度(TDOS)和部分态密度(PDOS), 如图4所示. 从图4(a)可以看出, 硅醚/石墨醚异质结构的导带和价带是由石墨醚和硅醚层贡献的, 在图4(b)和图4(c)的PDOS结果中存在轨道杂化. 对于导带最小值(CBM), 能带由C-2p, Si-3p和Si-3s轨道态组成. 对于价带最大值(VBM), 能带主要由O-2p和Si-3p轨道态贡献, Si-3s和C-2p电子也较少分布在该区域. 图 4 硅醚/石墨醚异质结构的TDOS (a)和PDOS (b), (c) Figure4. Total density (a) and partial density (b), (c) of the state of the graphether/silicether heterostructure.
其中a为应变条件下的晶格常数, a0为本征晶格常数, ε的正值和负值分别表示对晶格常数的拉伸和压缩[55]. 改变晶格常数会改变体系中原子间的距离, 从而会改变原子间的相互作用, 最终导致结构的带隙发生变化. 如图5(a)所示, 在压缩应变下, 带隙值首先随着应变的增加而增加, 直到应变为–6%时, 带隙值增大为1.02 eV, 总体来说, 在压缩应变下, 硅醚/石墨醚异质结构的带隙是增加的. 然而在拉伸应变下, 随着应变的增加, 带隙急剧减小. 从图6可以看出, 在压缩应变下, CBM始终位于Γ点, 而VBM在–2%至–4%的应变范围保持在S-X路径, 并且在–6%至–8%的应变内转移到Γ点, 同时异质结构存在间接带隙到直接带隙的转变. 在拉伸应变下, CBM也是始终位于Γ点, VBM在4%—8%的应变范围内转移到X-Γ路径上, 带隙接近0 eV. 图 5 (a) 双轴应变下硅醚/石墨醚异质结构的带隙变化; (b) 双轴应变下态B和态C的能量; 应变为-6%时异质结构中(c)硅醚和(d)石墨醚的PDOS图; (e) 不同垂直电场强度下带隙变化 Figure5. (a) Band gap variation of graphether/silicether heterostructure under biaxial strain; (b) energy of states B and C under biaxial strain; the partial density of the state of (c) silicether and (d) graphether in the heterostructure at -6% strain; (e) the band gap variation of silicether/graphether heterostructure under perpendicular electric field.
图 6 双轴应变下的能带结构图 Figure6. Band structure of silicether/graphether heterostructure under biaxial strain.