In-plane uniaxial strain in black phosphorus enables the identification of crystalline orientation

TL;DR

This paper introduces a universal, non-polarization-dependent method using in-plane uniaxial strain and Raman spectroscopy to accurately identify the crystalline orientation of black phosphorus, applicable to other 2D anisotropic materials.

Contribution

The authors develop a strain-based Raman technique that does not depend on optical polarization or sample thickness, enabling reliable identification of BP's crystalline axes.

Findings

Raman shifts vary with strain angle, revealing crystalline orientation.

Only 20-40% of substrate strain effectively transfers to BP flakes.

The method is extendable to other 2D anisotropic materials.

Abstract

The identification of the crystalline axis of anisotropic black phosphorus (BP) is key for the study of its physical properties and for its optical and electronic applications. Herein, we show that by applying an in-plane uniaxial strain and measuring the changes of Raman shifts, the crystalline axis of BP can be reliably determined. The strain effects on Raman shifts are angle-dependent, which can be expressed as a combination of Raman responses under zigzag and armchair strains. Different from previous polarized optical spectroscopic means, the proposed method does not rely on the laser polarization, the excitation wavelength, the sample thickness and the material of the stretchable substrate. Besides, the effective strain applied on BP from the stretched substrate is estimated, showing that only 20% to 40% of the strain can be effectively transferred to BP flakes from polyethylene terephthalate substrates. Our method provides not only an effective and universal approach to identify the crystalline orientation of layered BP, but also a model to extract additional information in strain-related studies, and can be extended to other 2D anisotropic materials.