Strain tuning of excitons in monolayer ${\mathrm{WSe}}_2$

We investigate excitonic absorption and emission in monolayer (1L) ${\mathrm{WSe}}_2$ under tensile strain. We observe a redshift of 100 meV in the $A$ exciton energy and a decrease of 25 meV in its estimated binding energy under 2.1% strain. Surprisingly, the linewidth of the $A$ exciton decreases by almost a factor of two under strain, from 42 to 24 meV at room temperature. We explain this effect as the result of suppression of phonon-mediated exciton scattering channels. We show that such a suppression results from the relative shift under strain of a secondary valley in the conduction band that is nearly degenerate with the $K$ valley where the $A$ exciton is formed.

Figure 1

(a) Absorption spectra of 2.1% strained and unstrained 1L ${\mathrm{WSe}}_2$. (b) Strain-dependent PL spectra of 1L ${\mathrm{WSe}}_2$ (offset). Strain dependence of the strength of (c) absorption and (d) PL due to the $A_{1s}$ exciton; (e) $E_{2s-1s}$, and $A$ exciton binding energy; (f) $E_{B-A}$. Hollow circles are data points and solid lines are corresponding linear fits in (c)–(f).

Figure 2

(a) Experimentally measured strain dependence of the total linewidths of the $A$ and $B$ excitons in 1L ${\mathrm{WSe}}_2$ and ${\mathrm{WS}}_2$. Strain dependence of the extracted homogeneous linewidths and calculated contributions to the $A$ exciton linewidth in 1L (b) ${\mathrm{WSe}}_2$ and (c) ${\mathrm{WS}}_2$ (see SM Sec. S8 [28]). The heights of the colored areas between the solid lines represent respectively the contributions from the radiative decay ${\gamma }_{\mathrm{rad}}$ (yellow), intravalley exciton-phonon scattering ${\gamma }_{KK-KK}$ (orange), and intervalley exciton-phonon scattering ${\gamma }_{KK-KQ}$ (turquoise). (d) Schematic representation of the line broadening mechanisms and the strain dependence of the relative energy levels of $KQ$ and $KK$ states.

Figure 3

(a) Schematic illustration of the absorption spectra of strained (top, offset, in blue) and unstrained (bottom, in red) 1L ${\mathrm{WSe}}_2$ due to the $A$ exciton. Vertical dashed lines denote the energies of the $1s$ state (left) and quasiparticle band gap (right). For clarity, only the $2s$ state among the excited excitonic states is shown. (b) Calculated strain-dependent positions of the $A$ exciton $1s, 2s$ states, quasiparticle band gap, and $KQ$ state.