Work function dependent photogenerated carrier dynamics of defective transition metal dichalcogenide heterostructures

Vacancies are commonly introduced in the preparation of transition metal dichalcogenide (TMD) heterostructures, severely affecting photogenerated carrier dynamics. Herein, we systematically explore the carrier dynamics of TMD heterostructures (metal: Mo, W; chalcogen: S, Se, Te) with the most common chalcogen vacancies by nonadiabatic molecular dynamics simulations. For TMD monolayers, a chalcogen vacancy induces defect states around the conduction band minimum forming a recombination center, while in heterostructures, locations of defect states in different sublayers exhibit clear work function dependence. Specifically, the defect states of a sublayer with a high work function are pushed down and act as photogenerated carrier recombination centers, while those of a sublayer with a low work function are pushed up and defect tolerant. This is because the delocalized charge distribution weakens the overlap of photogenerated electrons and holes. This indicates that repairing the defects of sublayers with a high work function can eliminate the adverse effect of vacancies. As a further demonstration, oxygen passivation is employed to repair the defects of sublayers with a high work function, which fully recovers the 10 ns scale of the photogenerated carrier lifetime. In this paper, we provide an in-depth understanding of defective carrier dynamics and guidelines for high-performance TMD heterostructure photoelectric devices.

Figure 1

Stacking configurations of (a) ${\mathrm{MoS}}_2$/${\mathrm{WS}}_2$ heterostructure and (d) S-S heterostructure. Band structures of (b) Perf and (e) S-S heterostructures. I–VI in (e) refer to different electron trapping states. Mechanism of e-h transfer and recombination of (c) Perf and (f) S-S heterostructures. $①$ and $②$ in (c) refer to generation of photogenerated carriers. $③\text{−}⑤$ in (c) refer to transfer and recombination of electrons and holes. $⑥\text{−}⑩$ in (f) refer to electron transfer process among conduction band minimum (CBM), defect states, and valence band maximum (VBM).

Figure 2

(a) e-h recombination within 1000 ps of different processes for S-S heterostructure. (b) Nonadiabatic coupling (NAC) of S-S heterostructure. (c) Charge densities of S-S heterostructure at valence band maximum (VBM) and trap I (named as S-S@VBM and S-S@trap), ${\mathrm{WS}}_2$ monolayer at VBM and S vacancy (named as ${\mathrm{WS}}_2$@VBM and ${\mathrm{WS}}_2$@trap). The isosurface value is $2.0\times {10}^{-3}\mathrm{eV}/{\AA }^3$. (d) Fourier transform (FT) spectrum of the autocorrelation function for Perf and S-S heterostructures in conduction band minimum (CBM), VBM, and defect states. (e) e-h recombination of different processes for S-S-io, S-S-oi, S-S-h, and $\mathrm{S}\text{−}{\mathrm{S}}_{2\mathrm{v}}$. Insets are corresponding atomic structures with different locations, concentrations, and types of S vacancies, and gray balls represent vacancies.

Figure 3

e-h recombination different processes of (a) O-Mo, (b) O-W, and (c) O-Mo-W. Inset is corresponding atomic structure and mechanism of e-h recombination for ${\mathrm{O}}_2$ repairing heterostructures. $⑤\text{−}⑩$ of inset refer to electron transfer process among conduction band minimum (CBM), defect states, and valence band maximum (VBM). Time evolution of energy levels for (d) O-Mo, (e) O-W, and (f) O-Mo-W.

Figure 4

Band structures of (a) ${\mathrm{MoSe}}_2$/${\mathrm{WSe}}_2$, (b) ${\mathrm{MoTe}}_2$/${\mathrm{WTe}}_2$, (c) ${\mathrm{MoS}}_2$/${\mathrm{WSe}}_2$, (d) ${\mathrm{MoSe}}_2$/${\mathrm{WS}}_2$, (e) ${\mathrm{MoS}}_2$/${\mathrm{MoSe}}_2$, and (f) ${\mathrm{WS}}_2$/${\mathrm{WSe}}_2$ heterostructures with S-family vacancies in different sublayers.