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二维WTe2由于特殊的晶格对称性而衍生出量子自旋霍尔效应、非线性霍尔效应等奇异性质. 确定其晶体结构的细节, 是理解这些性质的重要出发点. 本文利用温度、偏振依赖的拉曼光谱与光学二次谐波产生, 详细研究了一至三层WTe2的晶格对称性. 实验发现, 单层样品具有可观的二次谐波产生信号, 表明其晶格中心对称性破缺, 且偏振依赖符合
$ {C}_{s} $ 点群, 与此前普遍认为的中心对称的$ 1{T}' $ 结构不同. 双层样品具有更为显著的二次谐波产生信号, 且信号强度的温度依赖与铁电相变一致, 表明层间堆叠产生了更强的中心对称性破缺, 提供了该体系中存在层间滑移铁电的证据. 三层样品的二次谐波产生信号约为单层样品的五倍, 但比双层样品弱一个数量级, 表明层间堆叠导致其具有较弱的中心对称性破缺. 仅单层与双层样品中出现了多个显著的二阶拉曼散射峰, 其温度依赖反映出电子结构对层间耦合高度敏感. 这些结果将有助于完善对原子级厚度WTe2物理性质的理解.Two-dimensional WTe2 possesses a special crystal symmetry, leading to novel properties such as quantum spin Hall effect and nonlinear Hall effect. Determining the details of its crystal structure is essential for understanding these interesting properties. Here, we report an optical study on the crystal symmetry of monolayer, bilayer, and trilayer WTe2, using temperature and polarization dependent Raman spectroscopy and optical second harmonic generation (SHG). We find that monolayer WTe2 is noncentrosymmetric as indicated by its sizable SHG, in contrast to the commonly believed centrosymmetric 1T' structure. The polarization dependence of the SHG is consistent with the Cs point group. Bilayer WTe2 exhibits SHG signal more than one order of magnitude higher than in the monolayer and trilayer samples, with its temperature dependence reflecting the ferroelectric phase transition, evidencing strong inversion symmetry breaking induced by layer stacking and interlayer-sliding ferroelectricity. We also observe prominent second-order resonant Raman scattering peaks only in monolayer and bilayer WTe2, but not in thicker samples, and their temperature dependence indicates an electronic structure highly sensitive to interlayer coupling. These results will be useful for further exploring the properties of atomically thin WTe2.-
Keywords:
- tungsten ditelluride /
- Raman scattering /
- optical second harmonic generation /
- ferroelectricity
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图 1 (a) 单层(左图)和三层(右图)WTe2的晶体结构. 单层中的黑点表示
$ {C}_{2 a} $ 螺旋轴. 右图对比了将三层WTe2进行$ {C}_{2 a} $ 操作之后的结构(阴影部分), 在第二层中无法与原结构重合, 因此双层与三层WTe2均不具有中心对称性; (b) 不同厚度WTe2的300 K拉曼光谱. 50 cm–1以下和以上的光谱分别在垂直($ \perp $ )和平行(//)偏振条件下获得. P0—P9为拉曼声子; (c), (d) P0, P1和P8峰位的厚度依赖, 由拟合图(b)中数据获得; (c)中实线为根据$\sqrt{1-\mathrm{cos}(\mathrm{\pi }/N)}$ 拟合的结果, N为层数Fig. 1. (a) Crystal structures of monolayer (left) and trilayer (right) WTe2. The black dots in the monolayer structure indicate
$ {C}_{2 a} $ screw rotation axes. The trilayer structure does not overlap with its counterpart after$ {C}_{2 a} $ operation (shaded) in the second layer, meaning that bilayer and trilayer WTe2 do not possess inversion symmetry. (b) Raman spectra of WTe2 with thickness ranging from 1 to 4 layers, collected at 300 K. The spectral ranges below and above 50 cm–1 are measured in the cross ($ \perp $ ) and parallel (//) polarization configurations, respectively. P0–P9 denote the observed Raman modes. (c), (d) Thickness dependence of the peak frequencies for P0, P1, and P8, obtained by fitting analysis of the data in Figure (b). The solid line in Figure (c) is a fit to$\sqrt{1-\mathrm{cos}(\mathrm{\pi }/N)}$ , where N is the layer number.
图 2 (a)—(c) 1—3层 (1L—3L) WTe2在平行偏振条件下的温度依赖拉曼散射强度图; (d)—(f) 分别为图(a)—(c)中300—460 cm–1范围内在若干温度点的拉曼谱. 星号(*)标注的峰来自于蓝宝石衬底
Fig. 2. (a)–(c) Temperature dependent Raman scattering intensity maps for monolayer, bilayer, and trilayer (1L–3L) WTe2, measured in the parallel polarization configuration. (d)–(f) The corresponding spectra between 300 and 460 cm–1 at selected temperatures. The peaks marked by the asterisks are from the sapphire substrate.
图 3 (a), (b) 双层 WTe2样品在 4 K 的偏振角度依赖拉曼散射强度图, 分别在平行与垂直偏振条件下测得; (c) 各个拉曼峰的峰强随偏振角度的依赖. 实心与空心符号为实验数据, 实线为根据文中模型拟合的结果
Fig. 3. (a), (b) Polarization angle dependent Raman scattering intensity maps for bilayer WTe2, measured in the parallel and cross polarization configurations, respectively, at 4 K. (c) Polarization angle dependent intensity for each Raman mode. The filled and empty symbols are experimental data, and the solid lines are fits as described in the text.
图 4 (a) 机械剥离获得的WTe2样品的光学照片. 数字和虚线分别代表样品的层数和不同厚度区域之间的分界线. 比例尺: 5 μm; (b), (c) 对图(a)中方框区域进行SHG扫描的成像图, 均在垂直偏振条件下获得, 入射光偏振方向分别对应于图(e)中的
${30}^{\circ}$ 和${90}^{\circ}$ ; (d), (e) 单层WTe2的偏振角度依赖SHG, 分别在平行($//$ )和垂直($ \perp $ )偏振条件下获得. 实线为根据正文中模型拟合的结果. 所有数据在4 K获得; (f) 上图为单层WTe2在文献中普遍认为所具有的中心对称的$1{T}{'}$ 结构, 同图1(a). 下图为单层WTe2中心对称性破缺的一种可能结构[9], 称作${1{T}}_{{d}}$ Fig. 4. (a) Optical image of mechanically exfoliated WTe2, with layer numbers marked for the thin regions. The dashed lines represent the boundaries between regions of different thickness. Scale bar: 5 μm. (b)–(c) SHG intensity maps for the region marked by the square in (a), obtained in the cross polarization configuration, with the incident polarization angle set to
${30}^{\circ}$ and${90}^{\circ}$ in (e), respectively. (d)–(e) Polarization angle dependent SHG for monolayer WTe2, measured in the parallel ($//$ ) and cross ($ \perp $ ) polarization configurations, respectively. The solid lines are fits as discussed in the text. All data were obtained at 4 K. (f) The upper part is a schematic of the commonly adopted$1{T}{'}$ structure for monolayer WTe2 (same as in Fig. 1(a)), which is centrosymmetric. The lower part shows a possible structure with broken inversion symmetry[9], referred to as$ {1 T}_{d} $ .
图 5 (a)—(d) 不同厚度WTe2在4 K的偏振角度依赖SHG. 实心与空心符号对应的数据分别在平行(
$//$ )与垂直($ \perp $ )偏振条件下测得, 实线为根据正文中的模型拟合的结果; (e) 由图(a)—(d)中垂直偏振下的数据在0°—360°范围内积分得到的SHG强度的厚度依赖(双层2H-MoS2的原始数据未给出)Fig. 5. (a)–(d) Polarization angle dependent SHG at 4 K for WTe2 samples of various thickness. The filled and empty symbols represent data collected in the parallel (
$//$ ) and cross ($ \perp $ ) polarization configurations, respectively. The solid lines are fits as described in the text. (e) Thickness dependence of the SHG, obtained by integrating the cross polarization data in Figure (a)–(d) from 0° to${360}^{\circ}$ . The raw data for bilayer 2H-MoS2 are not shown.
图 6 实心符号为1—3层样品的SHG在垂直偏振下的温度依赖, 对应于左侧y轴. 空心符号为图2(b)中双层样品P11拉曼峰积分强度的温度依赖, 对应于右侧y轴
Fig. 6. Left axis: temperature dependent SHG intensity for monolayer, bilayer, and trilayer WTe2 in the cross polarization configuration, shown as filled circles. Right axis: temperature dependent integrated intensity for the P11 peak in the Raman spectra in Fig. 2(b) for bilayer WTe2, shown as empty squares.
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[1] Wang Q H, Kalantar-Zadeh K, Kis A, Coleman J N, Strano M S 2012 Nat. Nanotechnol. 7 699
Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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