  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.
