State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, No. 92 Weijin Road, CN-300072 Tianjin, China.
Nanoscale. 2018 May 3;10(17):8329-8337. doi: 10.1039/c7nr09173g.
Optical anisotropy is one of the most fundamental physical characteristics of emerging low-symmetry two-dimensional (2D) materials. It provides abundant structural information and is crucial for creating diverse nanoscale devices. Here, we have proposed an azimuth-resolved microscopic approach to directly resolve the normalized optical difference along two orthogonal directions at normal incidence. The differential principle ensures that the approach is only sensitive to anisotropic samples and immune to isotropic materials. We studied the optical anisotropy of bare and encapsulated black phosphorus (BP) and unveiled the interference effect on optical anisotropy, which is critical for practical applications in optical and optoelectronic devices. A multi-phase model based on the scattering matrix method was developed to account for the interference effect and then the crystallographic directions were unambiguously determined. Our result also suggests that the optical anisotropy is a probe to measure the thickness with monolayer resolution. Furthermore, the optical anisotropy of rhenium disulfide (ReS2), another class of anisotropic 2D materials, with a 1T distorted crystal structure, was investigated, which demonstrates that our approach is suitable for other anisotropic 2D materials. This technique is ideal for optical anisotropy characterization and will inspire future efforts in BP and related anisotropic 2D nanomaterials for engineering new conceptual nanodevices.
各向异性是新兴低对称二维（2D）材料最基本的物理特性之一。它提供了丰富的结构信息，对于创建各种纳米尺度器件至关重要。在这里，我们提出了一种方位分辨的微观方法，可以直接解析正常入射时两个正交方向上归一化的光差。差分原理确保了该方法仅对各向异性样品敏感，对各向同性材料不敏感。我们研究了裸和封装黑磷（BP）的各向异性，并揭示了对光学各向异性的干涉效应，这对于在光学和光电设备中的实际应用至关重要。我们开发了一个基于散射矩阵方法的多相模型来解释干涉效应，然后明确确定了晶体方向。我们的结果还表明，光学各向异性是测量具有单层分辨率的厚度的探针。此外，我们还研究了具有 1T 扭曲晶体结构的另一种各向异性 2D 材料-二硫化铼（ReS2）的各向异性，这表明我们的方法适用于其他各向异性 2D 材料。该技术非常适合光学各向异性表征，并将激发未来在 BP 和相关各向异性 2D 纳米材料方面的努力，以实现新型概念纳米器件。
