First-principles study of bilayers ZnX and CdX (X=S,Se,Te) direct band-gap semiconductors and their van der Waals heterostructures

We conducted comprehensive first-principles investigations of the structural, electronic, and optical properties of hexagonal $\mathrm{Zn}X$ and $\mathrm{Cd}X$ ($X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) and their van der Waals heterostructures. Our results indicate that all materials are thermally and dynamically stable, in contrast to earlier works. Electronic structure calculations with a hybrid functional revealed that the bilayers $\mathrm{Zn}X$ and $\mathrm{Cd}X$ are characterized by a direct band gap (at the $\mathrm{\ensuremath{\Gamma}}$ point), primarily lies within the visible spectrum of the sunlight (with an exception for ZnS). Moreover, we found the band edges (VBM/CBM of the bilayers) lying below/above the oxidation/reduction potentials (${E}^{{\mathrm{O}}_{2}/{\mathrm{H}}_{2}\mathrm{O}}/{E}^{{\mathrm{H}}^{+}/{\mathrm{H}}_{2}}$) depending on the environment's pH. The effects of mechanical strain on the electronic properties of the bilayers have been thoroughly investigated, revealing an impressive tunability of the band gap, energy position of the band edges, and the ratio of the electron and hole effective masses. The calculated optical absorption spectra showed that the bilayers $\mathrm{Zn}X$ and $\mathrm{Cd}X$, with the exception of ZnS, absorb in the visible region. Besides that, we found exciton binding energies between 0.30 and 0.96 eV for ZnTe and CdS bilayers, confirming that the reduced screening effect in 2D systems leads to higher values of exciton binding energies. Furthermore, our results indicated that the ZnTe/CdS heterostructure exhibits a band gap within the visible sunlight spectra. The band edges are located in the bilayer ZnTe resulting in a type-I band offset. However, upon compressive strain, we verified the emergence of the type-II band alignment, as a result, the first absorption peak is redshifted and the exciton exciton wave function spreads out in both materials. Overall, our findings provide valuable insights into the potential of these materials for various technological applications in the fields of the photonics, photocatalysis, and optoelectronics.