Phonon Softening and Direct to Indirect Bandgap Crossover in Strained Single Layer MoSe2

TL;DR

This study investigates how biaxial strain influences the electronic, vibrational, and optical properties of single layer MoSe2, revealing a transition from direct to indirect bandgap and significant phonon and Raman shifts.

Contribution

It demonstrates the effects of elastic biaxial strain on MoSe2's properties, including a direct to indirect bandgap crossover and phonon dispersion changes, providing insights for strain engineering.

Findings

Raman peaks exhibit significant red shifts with strain

Bandgap transitions from direct to indirect at 3% strain

Flexural phonon dispersion changes from quadratic to linear

Abstract

Motivated by recent experimental observations of Tongay et al. [Tongay et al., Nano Letters, 12(11), 5576 (2012)] we show how the electronic properties and Raman characteristics of single layer MoSe2 are affected by elastic biaxial strain. We found that with increasing strain: (1) the E' and E" Raman peaks (E1g and E2g in bulk) exhibit significant red shifts (up to 30 cm-1), (2) the position of the A1' peak remains at 180 cm-1 (A1g in bulk) and does not change considerably with further strain, (3) the dispersion of low energy flexural phonons crosses over from quadratic to linear and (4) the electronic band structure undergoes a direct to indirect bandgap crossover under 3% biaxial tensile strain. Thus the application of strain appears to be a promising approach for a rapid and reversible tuning of the electronic, vibrational and optical properties of single layer MoSe2 and similar MX2 dichalcogenides.