First-principles study of electric-field-tunable exciton potentials in twisted ZnS bilayers

The electric field-regulated excitonic behavior of twisted hexagonal wurtzite homojunctional ZnS and heterojunctional ZnS/ZnSe was systematically studied using first-principles density functional theory implemented in the Quantum Espresso package. For the ZnS homojunction, lattice twisting intrinsically induces a 17.5% bandgap redshift (3.83 eV → 3.16 eV at θ = 30°) under zero-field conditions through interlayer orbital hybridization. Upon applying an external electric field, the moiré excitonic potential (modeled via Morse parameters) exhibits a characteristic non-monotonic response. In particular, the confinement potential depth initially increases due to enhanced interlayer dipole coupling, reaching a maximum of 75 meV at a critical field (E_crit) of 0.1 V/nm. Beyond the threshold, field-induced charge separation dominates, reducing exciton stability. In contrast, the ZnS/ZnSe heterojunction shows reduced bandgap tunability (11% reduction) and, crucially, necessitates a significantly stronger critical field (E_crit = 0.3 V/nm) to achieve its peak Morse potential (60 meV). The disparity in critical field requirements, where the heterojunction underperforms at the lower field (0.1 V/nm) optimal for the homojunction, stems from the interfacial type-II band alignment, which creates additional potential barriers. Our findings establish quantitative relationships among twist angle, electric field strength, and exciton confinement dynamics, providing critical insights into designing 2D optoelectronic devices with programmable exciton transport.