Phonon softening and direct to indirect band gap crossover in strained single-layer MoSe2

Motivated by recent experimental observations of Tongay et al. [Nano Lett. 12, 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 (E2g and E1g in bulk) exhibit significant redshifts (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 band gap 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.