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基于密度泛函理论的第一性原理计算, 对单层TiOCl2的电子结构、输运性质和光学性质进行了理论研究. 对单层TiOCl2材料的声子谱、分子动力学和弹性常数的计算结果表明, 该材料在常温下能稳定存在, 并具有较好的动力学、热力学和机械稳定性. 电子结构分析表明, 单层TiOCl2是一种间接窄带隙半导体(能隙为1.92 eV). 在应力调控下, 单层TiOCl2材料的能带结构、输运性质和光学性质均发生明显变化. 沿a方向施加–4%的收缩应力后, 单层TiOCl2由间接带隙变为直接带隙, 带隙减小至1.66 eV. 同时TiOCl2还表现出明显的各向异性特征, 电子沿b方向传输(迁移率约为803 cm2·V–1·s–1), 空穴则沿a方向传输(迁移率约为2537 cm2·V–1·s–1). 此外, 施加收缩应力还会使单层TiOCl2材料的光吸收率、反射率和透射率的波峰(谷)发生红移, 进入可见光的紫色波段, 表明单层TiOCl2在未来光电器件领域有着潜在应用前景.Based on first-principles calculations, the electronic structure, the transport and optical properties of TiOCl2 monolayer are systematically investigated. The vibrational, thermodynamic, and mechanical properties of TiOCl2 monolayer are studied by phonon spectrum, molecular dynamics and elastic constants calculations. All these results indicate that the TiOCl2 monolayer possesses good structural stability at room temperature and excellent mechanical properties. The electronic structure analysis shows that the TiOCl2 is an indirect band gap (1.92 eV) semiconductor. Its band structure can be significantly affected by in-plane stress. Specifically, the TiOCl2 monolayer undergoes an indirect-to-direct band gap transition under –4% uniaxial stress along the a-axis and the gap size decreases to 1.66 eV. Moreover, the TiOCl2 monolayer exhibits obvious anisotropy characteristics, and its electron mobility is 803 cm2·V–1·s–1 along the b-axis, whereas the hole mobility reaches 2537 cm2·V–1·s–1 along the a-axis. The wave peaks (valleys) of the absorptivity, reflectivity and transmittance shift toward the violet part of the visible band by the stress. All these appealing properties make the TiOCl2 monolayer a promising candidate for applications in optoelectronic devices.
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Keywords:
- TiOCl2 monolayer /
- first-principles /
- electronic structure /
- optical properties
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图 1 单层TiOCl2的结构示意图 (a), (b) 侧视图; (c) 俯视图; (d) (0, –1, 2.30588) 面的电子局域函数(ELF)
Fig. 1. (a), (b) Side and (c) top views of TiOCl2 monolayer; (d) electron localization function (ELF) map of TiOCl2 monolayer along (0, –1, 2.30588) planes.
图 2 单层TiOCl2的2 × 2 × 1超晶胞结构v(a) 声子谱; (b) 分子动力学模拟
Fig. 2. (a) Phonon dispersion curves of TiOCl2 monolayer; (b) evolution of total energy and snapshots of TiOCl2 monolayer from AIMD simulations.
图 3 单层TiOCl2 (a)杨氏模量和(b))泊松比的极坐标图
Fig. 3. Polar diagrams for Young’s moduli (a) and Poisson’s ratio (b) of TiOCl2 monolayer.
图 4 单层TiOCl2的能带结构图(左)和分波态密度图(右)
Fig. 4. Electronic band structure (left) and density of states (right) of TiOCl2 monolayer.
图 5 不同应变与单层TiOCl2带隙的关系图
Fig. 5. Electronic band gap of TiOCl2 monolayer under various in-plane strains.
图 6 单层TiClO2能带结构与应力变化关系
Fig. 6. The band structure of TiOCl2 monolayer under various strains along a-axial.
图 7 单层TiOCl2的导带底(CBM), 价带顶(VBM)局域电荷密度图及轨道投影能带图
Fig. 7. The partial charge densities of the conduction band minimum (CBM), valence band maximum (VBM) and projected band structure of TiOCl2 monolayer.
图 8 ε = 0%和ε = –4%时, 单层TiOCl2的吸收率(a)、反射率(b)和透射率(c)
Fig. 8. (a) Absorption, (b) reflectance and (c) transmission of TiOCl2 monolayer under ε = 0% and ε = –4%.
表 1 沿a方向ε = 0%和ε = –4%时, 单层TiOCl2沿a, b方向的载流子有效质量及迁移率
Table 1. Calculated effective mass and carrier mobility for TiOCl2 monolayer under different uniaxial strain.
strain carrier ma/m0 mb/m0 μa
/(cm2·V–1·s–1)μb
/(cm2·V–1·s–1)ε = 0% electron 135.14 0.89 0.11 157.32 hole 1.33 2.14 347.72 113.97 ε = –4% electron 11.14 0.69 5.45 802.71 hole 0.36 1.98 2537.25 246.15 
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[1] Novoselov K S, Geim A K, Morozov S V, Jiang D, Zhang Y, Dubonos S V, Grigorieva I V, Firsov A A 2004 Science 306 666
Google Scholar
[2] Singh E, Singh P, Kim K S, Yeom G Y, Nalwa H S 2019 ACS Appl. Mater. Interfaces 11 11061
Google Scholar
[3] Gong C, Hu K, Wang X, Wangyang P, Yan C, Chu J, Liao M, Dai L, Zhai T, Wang C, Li L, Xiong J 2018 Adv. Funct. Mater. 28 1706559
Google Scholar
[4] Wang Q H, Kalantar-Zadeh K, Kis A, Coleman J N, Strano M S 2012 Nat. Nanotechnol. 7 699
Google Scholar
[5] An M, Dong S 2020 APL Mater. 8 110704
Google Scholar
[6] Iannaccone G, Bonaccorso F, Colombo L, Fiori G 2018 Nat. Nanotechnol. 13 183
Google Scholar
[7] Bhimanapati G R, Lin Z, Meunier V, Jung Y, Cha J, Das S, Xiao D, Son Y, Strano M S, Cooper V R, Liang L, Louie S G, Ringe E, Zhou W, Kim S S, Naik R R, Sumpter B G, Terrones H, Xia F, Wang Y, Zhu J, Akinwande D, Alem N, Schuller J A, Schaak R E, Terrones M, Robinson J A 2015 ACS Nano 9 11509
Google Scholar
[8] Xia F, Wang H, Hwang J C M, Neto A H C, Yang L 2019 Nat. Rev. Phys. 1 306
Google Scholar
[9] Mannix A J, Kiraly B, Hersam M C, Guisinger N P 2017 Nat. Rev. Chem. 1 0014
Google Scholar
[10] 徐依全, 王聪 2020 物理学报 69 184216
Google Scholar
Xu Y Q, Wang C 2020 Acta Phys. Sin. 69 184216
Google Scholar
[11] Xia F, Mueller T, Lin Y M, Valdes-Garcia A, Avouris P 2009 Nat. Nanotechnol. 4 839
Google Scholar
[12] Bonaccorso F, Sun Z, Hasan T, Ferrari A C 2010 Nat. Photonics 4 611
Google Scholar
[13] Bao W, Jing L, Jr J V, Lee Y, Liu G, Tran D, Standley B, Aykol M, Cronin S B, Smirnov D, Koshino M, McCann E, Bockrath M, Lau C N 2011 Nat. Phys. 7 948
Google Scholar
[14] Radisavljevic B, Radenovic A, Brivio J, Giacometti V, Kis A 2011 Nat. Nanotechnol. 6 147
Google Scholar
[15] Fuhrer M S, Hone J 2013 Nat. Nanotechnol. 8 146
Google Scholar
[16] Mak K F, Lee C, Hone J, Shan J, Heinz T F 2010 Phys. Rev. Lett. 105 136805
Google Scholar
[17] Li L, Yu Y, Ye G J, Ge Q, Ou X, Wu H, Feng D, Chen X H, Zhang Y 2014 Nat. Nanotechnol. 9 372
Google Scholar
[18] Liu H, Neal A T, Zhu Z, Luo Z, Xu X, Tománek D, Ye P D 2014 ACS Nano 8 4033
Google Scholar
[19] Wood J D, Wells S A, Jariwala D, Chen K S, Cho E, Sangwan V K, Liu X, Lauhon L J, Marks T J, Hersam M C 2014 Nano Lett. 14 6964
Google Scholar
[20] You H P, Ding N, Chen J, Dong S 2020 Phys. Chem. Chem. Phys. 22 24109
Google Scholar
[21] Kresse G, Hafner J 1993 Phys. Rev. B 47 558
Google Scholar
[22] Kresse G, Furthmuller J 1996 Phys. Rev. B 54 11169
Google Scholar
[23] Perdew J P, Burke K, Ernzerhof M 1996 Phys. Rev. Lett. 77 3865
Google Scholar
[24] Monkhorst H J, Pack J D 1976 Phys. Rev. B 13 5188
Google Scholar
[25] Gonze X, Lee C 1997 Phys. Rev. B 55 10355
Google Scholar
[26] Togo A, Tanaka I 2015 Scr. Mater. 108 1
Google Scholar
[27] Chandrasekaran A, Mishra A, Singh A K 2017 Nano Lett. 17 3290
Google Scholar
[28] Song Y Q, Yuan J H, Li L H, Xu M, Wang J F, Xue K H, Miao X S 2019 Nanoscale 11 1131
Google Scholar
[29] Xu L C, Du A, Kou L 2016 Phys. Chem. Chem. Phys. 18 27284
Google Scholar
[30] Cadelano E, Palla P L, Giordano S, Colombo L 2010 Phys. Rev. B 82 235414
Google Scholar
[31] Li Y, Yu C, Gan Y, Kong Y, Jiang P, Zou D F, Li P, Yu X F, Wu R, Zhao H, Gao C F, Li J 2019 Nanotechnology. 30 335703
Google Scholar
[32] Liu F, Ming P, Li J 2007 Phys. Rev. B 76 064120
Google Scholar
[33] Miao N, Xu B, Bristowe N C, Zhou J, Sun Z 2017 J. Am. Chem. Soc. 139 11125
Google Scholar
[34] Hur T B, Hwang Y H, Kim H K, Lee I J 2006 J. Appl. Phys. 99 64308
Google Scholar
[35] Bardeen J, Shockley W 1950 Phys. Rev. 80 72
Google Scholar
[36] Yu T, Zhao Z, Sun Y, Bergara A, Lin J, Zhang S, Xu H, Zhang L, Yang G, Liu Y 2019 J. Am. Chem. Soc. 141 1599
Google Scholar
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Google Scholar
[38] Cai Y, Zhang G, Zhang Y W 2014 J. Am. Chem. Soc. 136 6269
Google Scholar
[39] Matthes L, Pulci O, Bechstedt F 2016 Phys. Rev. B 94 205408
Google Scholar
[40] Matthes L, Pulci O, Bechstedt F 2014 New J. Phys. 16 105007
Google Scholar
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