WS $_2$多型性电子性质的电镜观察
Illuminating the electronic properties of WS$_2$ polytypism with electron microscopy
摘要：
定制层状材料的特定堆叠序列（多型体）代表了识别和设计新物理特性的强大策略。虽然建立在过渡金属dichalcogenides（TMD）与2 H或3R结晶相的纳米结构已被常规研究，我们的知识的基础上混合2 H/3R多型是有限的。在这里，我们报告的混合2 H/3R的独立WS $_2$纳米结构的表征显示花状配置通过先进的透射电子显微镜。我们将它们丰富多样的形状-形态组合与相关的局部电子特性（如边缘、表面和体等离子体）相关联。结合机器学习的电子能量损失光谱显示，2 H/3R多型体显示出与$E_{\rm BG}=1.6^{+0.3}_{-0.2}~{\rm eV}$的间接带隙。此外，我们确定的EEL光谱中的能量增益峰的存在下，其特征在于由一个增益-损失比$I_G/I_L>1$。这种特性可以用来开发原子级薄的TMD纳米结构和基于它们构建的设备的新的冷却策略。我们的研究结果代表了一个垫脚石，以提高对混合结晶相的基础上的TMD纳米结构的理解。
英文摘要：
Tailoring the specific stacking sequences (polytypes) of layered materials represents a powerful strategy to identify and design novel physical properties. While nanostructures built upon transition-metal dichalcogenides (TMDs) with either the 2H or 3R crystalline phases have been routinely studied, our knowledge of those based on mixed 2H/3R polytypes is far more limited. Here we report on the characterization of mixed 2H/3R free-standing WS$_2$ nanostructures displaying a flower-like configuration by means of advanced transmission electron microscopy. We correlate their rich variety of shape-morphology combinations with relevant local electronic properties such as their edge, surface, and bulk plasmons. Electron energy-loss spectroscopy combined with machine learning reveals that the 2H/3R polytype displays an indirect band gap with $E_{\rm BG}=1.6^{+0.3}_{-0.2}~{\rm eV}$. Further, we identify the presence of energy-gain peaks in the EEL spectra characterized by a gain-to-loss ratio $I_G/I_L>1$. Such property could be exploited to develop novel cooling strategies for atomically thin TMD nanostructures and devices built upon them. Our results represent a stepping stone towards an improved understanding of TMDs nanostructures based on mixed crystalline phases.
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