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单层Ge2X4S2(X = P, As)是最近预测的一种二维层状材料, 它们不仅拥有高的光吸收系数, 同时还有较高的载流子迁移率, 这意味着它们在光电和热电领域可能有较好的应用前景. 本文通过第一性原理和玻尔兹曼输运理论系统地研究了这两种材料的热电性质. 结果表明, 单层Ge2P4S2和Ge2As4S2在室温下展现较低的晶格热导率, 沿armchair方向分别为3.93 W·m–1·K–1和3.19 W·m–1·K–1, zigzag方向分别为4.38 W·m–1·K–1和3.79 W·m–1·K–1, 这主要是由低的声子群速度、大的格林艾森参数以及小的声子弛豫时间造成的. 基于HSE06泛函计算出的能带结构表明单层Ge2As4S2是带隙值为1.21 eV的直接带隙半导体, 而单层Ge2P4S2是带隙值为1.13 eV的间接带隙半导体. 价带上明显的双重简并现象使得单层Ge2P4S2和Ge2As4S2具有较大的塞贝克系数, 其中300 K时沿armchair方向分别达到了1800 μV·K–1和2070 μV·K–1. 对于具有较小晶格热导率和较高功率因子的单层Ge2X4S2而言, 它们的热电性能是非常值得期待的. 本文计算表明, 当处于最佳的n型掺杂时, 单层Ge2P4S2和Ge2As4S2在500 K时都具有较高的热电优值, 在两个方向的最大值分别为3.06 (armchair方向)和3.51 (zigzag方向), 以及3.21 (armchair方向)和2.54 (zigzag方向), 这意味着它们在中温热电应用领域存在较大的应用价值.Monolayer Ge2X4S2 (X = P, As) are novel two-dimensional (2D) layered materials with suitable optical absorption properties in the visible range and high carrier mobility, so they possess broad application prospects in the photoelectric and thermoelectric fields. In this work, their thermoelectric properties are systematicly evaluated by using the first-principles and Boltzmann transport theory. For monolayer Ge2As4S2 and Ge2P4S2, their smaller phonon group velocities, low relaxation times and the large Grüneisen parameters result in ultra-low lattice thermal conductivities, which are 3.93 W·m–1·K–1 and 3.19 W·m–1·K–1 in the armchair direction, 4.38 W·m–1·K–1 and 3.79 W·m–1·K–1 in the zigzag directions at 300 K. Their electronic band structures reveal that the monolayer Ge2As4S2 is a semiconductor with a direct band gap of 1.21 eV, while the single-layer Ge2P4S2 owns an indirect band gap of 1.13 eV. Meanwhile, the twofold degeneracy of valence band provides a large p-type Seebeck coefficient that is 1800 μV·K–1 for Ge2P4S2 and 2070 μV·K–1 for Ge2As4S2 in the armchair direction. Obviously, monolayer Ge2X4S2 has smaller lattice thermal conductivity and higher power factor, thus it is worth exploring their thermoelectric properties. The results prove that monolayer Ge2As4S2 and Ge2P4S2 have outstanding thermoelectric performances at 500 K when they are treated by optimal n-type doping. The maximum ZT values of monolayer Ge2As4S2 and Ge2P4S2 are 3.06 (armchair direction) and 3.51 (zigzag direction), as well as 3.21 (armchair direction) and 2.54 (zigzag direction), indicating that monolayer Ge2X4S2 can be a potential candidate in the medium-temperature thermoelectric applications.
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图 1 (a) 单层Ge2X4S2的几何结构; (b) 对应的布里渊区; (c) 局域电荷密度. 其中紫色、红色以及蓝色球分别是Ge原子、P(As)原子和S原子
Fig. 1. (a) The structure of monolayer Ge2X4S2; (b) schematic diagram of the Brillouin zone; (c) electron localization function, where the purple, red and blue balls are Ge atoms, P (As) atoms and S atoms, respectively.
图 3 单层Ge2X4S2的晶格热导率与温度的关系(a), 累积晶格热导率与频率的关系(b), 各分支对累积晶格热导率的贡献(c), 累积晶格热导率与声子平均自由程的关系(d)
Fig. 3. (a) Lattice thermal conductively with respect to temperature, (b) cumulative lattice thermal conductivity as function of frequency, (c) the contribution of phonon acoustic mode (ZA, TA, LA) and optical modes (Opt) to the total lattice thermal conductivity, (d) cumulative lattice thermal conductivity as a function of mean free path (MFP) of monolayer Ge2X4S2.
图 4 单层Ge2X4S2的声子群速度(a), (b), 格林艾森参数(c), (d), 声子弛豫时间和三声子散射相空间(e), (f), 图 (e)和图(f)中的插图为三声子散射相空间
Fig. 4. (a), (b) The phonon group velocity, (c), (d) Grüneisen parameters, and (e), (f) phonon relaxation time of monolayer Ge2X4S2. The inset of Fgiure (e) and Figure (f) is three-phonon scattering phase space.
图 6 单层Ge2X4S2的电子输运参数(a), (b)塞贝克系数; (c), (d)电导率; (e), (f) 电子热导率; (g), (h) 功率因子
Fig. 6. Electronic transport parameters of monolayer Ge2X4S2 include the Seebeck coefficient ((a), (b)), electrical conductivity ((c), (d)), electronic thermal conductivity ((e), (f)), and power factor ((g), (h)) as function of the chemical potential.
表 1 弛豫后的结构参数
Table 1. The parameters of optimal structure.
晶格常数/Å 键长/Å Ge—S Ge—As (P) As—As (P—P) Ge2As4S2 7.10 2.48 2.60 2.48 Ge2P4S2 6.77 2.50 2.49 2.21 表 2 300 K下单层Ge2X4S2的弹性模量C 2D, 形变势常数El, 有效质量m*, 载流子迁移率μ和弛豫时间τ
Table 2. Calculated elastic modulus C 2D, DP constant El, effective mass m*, carrier mobility μ and scattering time τ for electron and hole in monolayer Ge2X4S2 at 300 K.
方向 类型 C 2D/(J·m–2) El /eV m* /me μ/(cm–2·V–1·s–1) τ/ps Ge2P4S2 armchair electron 55.46 2.04 0.22 3277.22 0.41 hole 55.46 3.32 0.65 131.23 0.05 zigzag electron 48.44 1.97 0.22 3676.88 0.46 hole 48.44 3.23 0.65 162.32 0.06 Ge2As4S2 armchair electron 48.94 2.30 0.19 3146.80 0.34 hole 48.94 6.56 0.24 220.71 0.03 zigzag electron 43.25 2.13 0.19 1943.61 0.21 hole 43.25 6.23 0.24 275.50 0.04 -
[1] Yang J, Xi L L, Qiu W J, Wu L H, Shi X, Chen L D, Yang J H, Zhang W Q, Uher C, Singh D J 2016 npj Comput. Mater. 2 1
Google Scholar
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Google Scholar
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Google Scholar
[4] Hicks L D, Harman T C, Dresselhaus M S 1993 Appl. Phys. Lett. 63 3230
Google Scholar
[5] Zhao L-D, Chang C, Tan G J, Kanatzidis M G 2016 Energy Environ. Sci. 9 044
[6] Liu Y C, Wang W X, Yang J, Li S 2018 Adv. Sustainable Syst. 2 1800046
Google Scholar
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Google Scholar
[8] Yan X, Poudel B, Ma Y, Liu W S, Joshi G, Wang H, Lan Y C, Wang D Z, Chen G, Ren Z F 2010 Nano Lett. 10 3373
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Google Scholar
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