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

Abstract

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 MoSe 2 are affected by elastic biaxial strain. We found that with increasing strain: (1) the E and E Raman peaks (E 2g and E 1g in bulk) exhibit significant redshifts (up to ∼30 cm −1 ), (2) the position of the A 1 peak remains at ∼180 cm −1 (A 1g 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 MoSe 2 and similar MX 2 dichalcogenides.

Origin of Indirect Optical Transitions in Few-Layer MoS2, WS2, and WSe2

“…Increase in the in-plane lattice constant by ∆a leads to reduction of direct and indirect transition energies at a different rate (Figure 2a and d) in agreement with the previous studies 16,18,19,24,30,31 . The changes are more pronounced for transitions involving the K point.…”

“…The indirect-to-direct crossover in these MX 2 compounds (M=Mo,W and X=S,Se) results from local shift of valence band hills and conduction band valleys in the Brillouin zone 4,7,9,10,[16][17][18][19] . In single layers, the conduction band minimum (CBM) and valence band minimum (VBM) coinciding at the K point, making them direct gap semiconductors.…”

Stable ultra-thin CdTe crystal: a robust direct gap semiconductor

“…55 Previously we showed that electronic band structure of single-layer MoSe 2 undergoes a direct to indirect band gap crossover under tensile strain. 56 Moreover, strain induced phase transition (from semiconducting 2H phase to metallic 1T' phase) is observed in MoTe 2 . 57 Therefore, in contrast to typical ultra-thin materials, monolayer CdTe exhibits robust and moderate band gap that covers the broad range of the solar spectrum, which are essential for its utilization in future electronics.…”

Strain engineering in semiconducting two-dimensional crystals

“…For TMDs, a number of DFT calculations have studied the dependence of the band structure and phonon modes with external strain [51,53,121,[123][124][125][126][127][128][129][130], and the consequences on different physical properties have been discussed in Refs. [131][132][133].…”