Exciton-exciton annihilation (EEA) in counterdoped monolayer transition metal dichalcogenides (TMDCs) can be suppressed by favorably changing the band structure with strain. The photoluminescence (PL) quantum yield (QY) monotonically approaches unity with strain at all generation rates. In contrast, here in bilayers (2L) of tungsten diselenide (WSe2) we observe a nonmonotonic change in EEA rate at high generation rates accompanied by a drastic enhancement in their PL QY at low generation rates. EEA is suppressed at both 0% and 1% strain, but activated at intermediate strains. We explain our observation through the indirect to direct transition in 2L WSe2 under uniaxial tensile strain. By strain and electrostatic counterdoping, we attain ∼50% PL QY at all generation rates in 2L WSe2, originally an indirect semiconductor. We demonstrate transient electroluminescence from 2L WSe2 with ∼1.5% internal quantum efficiency for a broad range of carrier densities by applying strain, which is ∼50 times higher than without strain. The present results elucidate the complete optoelectronic photophysics where indirect and direct excitons are simultaneously present and expedite exciton engineering in a TMDC multilayer beyond indirect-direct bandgap transition.
Bibliographical noteFunding Information:
This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the U.S. Department of Energy, under contract no. DE-AC02-05Ch11231. H.K. acknowledges support from Samsung Scholarship. N.H. acknowledges support from the Postdoctoral Fellowships for Research Abroad of Japan Society for the Promotion of Science.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Physics and Astronomy(all)