Strain Mapping In Single-Layer 2D Crystals Via Raman Activity

TL;DR

This study uses density functional theory to analyze how in-plane biaxial strain affects the Raman active phonon modes in single-layer 2D materials, providing insights for strain characterization.

Contribution

It offers a comprehensive theoretical analysis of strain effects on Raman spectra in novel 2D materials, including predictions of mode shifts and activity changes.

Findings

Phonon frequencies soften under tensile strain and harden under compressive strain.

Opposite trends in Raman activity responses for in-plane and out-of-plane modes enable strain monitoring.

Certain modes, like the A-peak in Si and Ge, disappear under critical tensile strain.

Abstract

By performing density functional theory-based ab-initio calculations, Raman active phonon modes of novel single-layer two-dimensional (2D) materials and the effect of in-plane biaxial strain on the peak frequencies and corresponding activities of the Raman active modes are calculated. Our findings confirm the Raman spectrum of the unstrained 2D crystals and provide expected variations in the Raman active modes of the crystals under in-plane biaxial strain. The results are summarized as follows; (i) frequencies of the phonon modes soften (harden) under applied tensile (compressive) strains, (ii) the response of the Raman activities to applied strain for the in-plane and out-of-plane vibrational modes have opposite trends, thus, the built-in strains in the materials can be monitored by tracking the relative activities of those modes, (iii) in particular, the A-peak in single-layer Si and Ge disappear under a critical tensile strain, (iv) especially in mono and di- atomic single-layers, the shift of the peak frequencies is stronger indication of the strain rather than the change in Raman activities, (v) Raman active modes of single-layer ReX 2 (X=S, Se) are almost irresponsive to the applied strain. Strain-induced modifications in the Raman spectrum of 2D materials in terms of the peak positions and the relative Raman activities of the modes could be a convenient tool for characterization.