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.
We report on both the theoretical and experimental design of a black phosphorus (BP)-based reflective linear polarizer on Si/SiO substrate in visible range using the Fabry-Perot cavities method. Thanks to the optical anisotropy of BP, polarization wavelength regulation and a high extinction ratio are achievable via optimizing the thickness of BP. Using azimuth-dependent reflectance difference microscopy, we directly measured a huge optical anisotropy of 1.58, corresponding to an extinction ratio of ∼9 dB, from a 96 nm BP on a silicon substrate capped by 260 nm thermally oxidized silicon at a wavelength of 690 nm for the first time, to the best of our knowledge. Our results not only provide a new route to designing nanoscale polarizers based on anisotropic two-dimensional (2D) materials, promoting the application of 2D materials in integrated optoelectronics and system-on-chip, but also suggest a modulation technique for optical anisotropy by integrating the BP film with cavity structures.
Understanding and engineering the interface between metal and two-dimensional materials are of great importance to the research and development of nanoelectronics. In many cases the interface of metal and 2D materials can dominate the transport behavior of the devices. In this study, we focus on the metal contacts of MoTe (molybdenum ditelluride) FETs (field effect transistors) and demonstrate how to use post-annealing treatment to modulate their transport behaviors in a controlled manner. We have also carried out low temperature and transmission electron microscopy studies to understand the mechanisms behind the prominent effect of the annealing process. Changes in transport properties are presumably due to anti-site defects formed at the metal-MoTe interface under elevated temperature. The study provides more insights into MoTe field effect devices and suggests guidelines for future optimizations.
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