Strain engineering in bilayer WSe2 over a large strain range

Abstract

Both Raman and photoluminescence spectroscopies have been employed to investigate the variation of lattice vibrational and electronic properties of bilayer WSe2 for a large strain range. Upon application of uniaxial tensile strain, the initially degenerate in-plane E2g Raman mode evolves into two discrete modes as a consequence of inversion symmetry breaking. The decrease of direct bandgap energy accompanied by an increase of indirect bandgap energy leads to an indirect-to-direct bandgap crossover, which expectedly occurs at 2% strain. However, under higher strain, a phenomenon of strain relaxation appears in the WSe2 crystal. Such relaxation gives rise to a cease of further redshift of direct bandgap and a significant increase of direct excitonic linewidth. Our findings can provide a deeper understanding in the effects of strain engineering in WSe2 and are believed to be applicable to other layered transition-metal dichalcogenides.