Effect of point defects on electronic and excitonic properties in Janus-MoSSe monolayer

Defects in transition-metal dichalcogenides (TMDs) monolayers are ubiquitous and have great effects on the electronic and optical properties. As a consequence, an in-depth understanding of how defects influence the performance of materials is crucial for the design and manipulation of electronic and optoelectronic devices. In this work, we investigate the effect of four common point defects, i.e., $V_{\text{S}}$, $V_{\text{Se}}$, $V_{{\text{MoS}}_3}$, and $V_{{\text{MoSe}}_3}$ on the electronic and excitonic properties in the Janus-MoSSe monolayer, by employing the first-principles method combined with the $GW$-Bethe-Salpeter equation. Results show that the presence of defects indeed alters the electronic and optical properties of the Janus-MoSSe monolayer, but in different ways. For monochalcogen vacancies, the first bright exciton ($X_1$) is more localized and has a smaller exciton radius as compared with that in the pristine Janus-MoSSe monolayer. In addition, the exciton binding energies become much larger, with the value up to 1.58 eV. As a result, excitons are easy to combine with defects, becoming centers of luminescence. While for the systems with ${\mathrm{MoS}}_3$ and ${\mathrm{MoSe}}_3$ vacancy clusters, the distribution of $X_1$ has changed significantly even though the exciton binding energies are comparable with that of the pristine system. Further analysis indicates that this can be ascribed to the competition mechanism between the existence of defects and Coulomb screening effect. Based on these findings, we obtained a thorough understanding of how point defects influence the nature of the Janus-TMD monolayer, which provides useful theoretical guidance for further experimental research.

Figure 1

Top views of optimized Janus-MoSSe monolayer with (a) $V_{\text{S}}$, (b) $V_{\text{Se}}$, (c) $V_{{\text{MoS}}_3}$, and (d) $V_{{\text{MoSe}}_3}$ defects.

Figure 2

The electronic band structures of (a) pristine Janus-MoSSe monolayer, and systems with (b) $V_{\text{S}}$, (c) $V_{\text{Se}}$, (d) $V_{{\text{MoS}}_3}$, and (e) $V_{{\text{MoSe}}_3}$ defects at the PBE level (black line) and the corresponding $G_0{W}_0$ level (red line). The Fermi level is set to zero.

Figure 3

The projected density of states (PDOS) of (a) pristine Janus-MoSSe monolayer and systems with (b) $V_{\text{S}}$, (c) $V_{\text{Se}}$, (d) $V_{{\text{MoS}}_3}$, and (e) $V_{{\text{MoSe}}_3}$ defects at the PBE level. The Fermi level is set to zero.

Figure 4

The calculated dielectric function ${\varepsilon }_2$ for (a) pristine Janus-MoSSe monolayer and systems with (b) $V_{\text{S}}$, (c) $V_{\text{Se}}$, (d) $V_{{\text{MoS}}_3}$ and (e) $V_{{\text{MoSe}}_3}$ defects. The GW band gaps for the first bright exciton are marked with vertical dash lines. Green bars represent the optical oscillator strengths of the constituting excitonic and band transitions. The insets show the energy region around the dark excitonic peaks.

Figure 5

Top view of the spatial distribution of $X_1$ for (a) pristine Janus-MoSSe monolayer and systems with (b) $V_{\text{S}}$, (c) $V_{\text{Se}}$, (d) $V_{{\text{MoS}}_3}$, and (e) $V_{{\text{MoSe}}_3}$ defects. The fixed hole position is indicated by red arrow. The isosurface value is set to 0.2 e/${\text{Å}}^3$.