Microsecond Valley Lifetime of Defect-Bound Excitons in Monolayer ${\mathrm{WSe}}_2$

In atomically thin two-dimensional semiconductors such as transition metal dichalcogenides (TMDs), controlling the density and type of defects promises to be an effective approach for engineering light-matter interactions. We demonstrate that electron-beam irradiation is a simple tool for selectively introducing defect-bound exciton states associated with chalcogen vacancies in TMDs. Our first-principles calculations and time-resolved spectroscopy measurements of monolayer ${\mathrm{WSe}}_2$ reveal that these defect-bound excitons exhibit exceptional optical properties including a recombination lifetime approaching 200 ns and a valley lifetime longer than $1\mu \mathrm{s}$. The ability to engineer the crystal lattice through electron irradiation provides a new approach for tailoring the optical response of TMDs for photonics, quantum optics, and valleytronics applications.

Figure 1

(a) Electron-beam irradiation of monolayer ${\mathrm{WSe}}_2$ creates defects associated with selenium vacancies in the crystal lattice. (b) Photoluminescence spectrum acquired for as-exfoliated (pristine, dashed line) ${\mathrm{WSe}}_2$ at 5 K. The two prominent peaks correspond to the neutral ($X^0$) and negatively charged ($X^{-}$) excitons. Following electron irradiation, a broad spectral band ($D$) appears at a lower energy due to emission associated with selenium vacancies. (c) Scaling of the maximum intensity of the neutral, charged, and defect exciton peaks with average power of the continuous-wave excitation. The $X^0$ and $X^{-}$ peaks scale linearly with exponent $k=1$. The defect peak $D$ scales sublinearly with exponent $k=0.7$.

Figure 2

DFT calculations of the electronic band structure, projected density of states, and linear absorption spectrum (${ϵ}_2$) for the 6-by-6 supercell Brillouin zone for (a) a single selenium vacancy (${\mathrm{Se}}_1$) and (b) a double selenium vacancy (${\mathrm{Se}}_2$). The $K$ point of the original Brillouin zone is folded back to the $\overline{K}$ point of the 6-by-6 supercell. The blue curves in the band diagram are attributed to the defects. (c) Schematic of the band-to-band transitions near the $K$ valley that predominantly contribute to the emission spectrum.

Figure 3

(a) Time-resolved photoluminescence dynamics for three different emission energies within the defect band (points). The curves are fit with a biexponential decay function (solid lines). (b) Lifetimes and amplitudes of the slow (squares) and fast (circles) decay components from the biexponential fits.

Figure 4

(a) A comparison of the cocircularly ($+/+$) and cross-circularly ($+/-$) polarized photoluminescence spectrum (solid lines) reveals a maximum valley polarization of $P_c=30\%$ (dashed line). (b) Normalized cocircularly (red circles) and cross-circularly (blue circles) polarized dynamics and the degree of circularly polarization $P_c$ (squares).