Resolving the optical anisotropy of low-symmetry 2D materials

Abstract

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.

In‐Plane Optical Anisotropy of Low‐Symmetry 2D GeSe

“…R zz represents the reflectance along zigzag direction while R ac denotes the reflectance along armchair orientation. In principle, the N (θ) is always zero for all angles toward the isotropic materials, which was also observed in the previous experiments . With regard to anisotropic materials, the value of N (θ) varied periodically with the angle of incident polarized light rotated from 0° to 180°, thus endowing ADRDM to directly evaluate the anisotropic level of low‐symmetry 2D layered materials and identify the crystal orientation.…”

“…To experimentally verify the in‐plane anisotropic optical reflection, we performed ADRDM measurements on the mechanically cleaved GeSe flakes. The ADRDM can directly visualize the optical anisotropy of samples in an in situ and nondestructive manner, therefore being used to rapidly identify the crystalline orientation . As shown in Figure a, the detection principle of ADRDM is to quantitatively measure the normalized reflectance difference (Δ R ) along two arbitrary orthogonal in‐plane directions ( m and n ) of sample at perpendicular incidence angle, which can be defined aswhere R m and R n are the reflectance intensities along m and n directions.…”

“…The ADRDM can directly visualize the optical anisotropy of samples in an in situ and nondestructive manner, therefore being used to rapidly identify the crystalline orientation . As shown in Figure a, the detection principle of ADRDM is to quantitatively measure the normalized reflectance difference (Δ R ) along two arbitrary orthogonal in‐plane directions ( m and n ) of sample at perpendicular incidence angle, which can be defined aswhere R m and R n are the reflectance intensities along m and n directions. The relationship between the dimensionless value N (θ) and the angle of incident light can be described aswhere θ is the rotated angle of linearly incident polarized light.…”

“…The N (θ) was subsequently recorded with the incident polarization rotating from 0° to 180°, which was maximized at ≈175° whereas minimized at ≈85°. In particular, ADRDM, serving as an imaging technique, can simultaneously collect N (θ) at all pixels within the whole field of view, thus providing an in situ visualization of the anisotropy contrast . As shown in Figure c, differing from the reflectance difference (RD) signals of the isotropic substrate that were always zero (green) and did not change with the rotating angles, the intensity of N (θ) on the GeSe flakes exhibited its minimum value at 75°–90° (dark blue) while maximum value at 165°–180° (dark red).…”

Weak Interlayer Interaction in 2D Anisotropic GeSe2

“…Before studying the interlayer interaction in GeSe 2 , we first investigated the in‐plane anisotropy of GeSe 2 by ADRDM, which was a nondestructive, surface‐sensitive, rapid and directly visualizing detection technique to characterize the optical anisotropy of the sample. The detection principle of ADRDM was to directly measure the normalized difference in reflectance (Δ R ) between two arbitrary orthogonal directions in the surface plane ( a and b ) when the sample is irradiated by incident polarized light, which can be defined aswhere R a and R b are the intensities of reflectance along a and b directions. The dimensionless value N(θ) alters as incident direction of linearly polarized light changes, which can be described aswhere θ represents the angle between incident polarized light and x axis of GeSe 2 .…”