Hydroxylation-driven Type-I to Type-II band alignment transition in WS₂/m-plane ZnO heterostructures

The integration of two-dimensional transition-metal dichalcogenides (TMDs) with wide-bandgap semiconductors is attractive for next-generation optoelectronic devices. The WS₂/ZnO heterostructure is particularly promising, leveraging the high carrier mobility and tunable exciton properties of WS₂ as an emissive layer and the conductivity and scalability of ZnO as a substrate and electron transport layer. This heterostructure exhibits the essential Type-I band alignment (WS₂ as the smaller bandgap material) required for high-efficiency light-emitting diodes (LEDs) via strong spatial overlap and effective radiative recombination. However, the fabrication and transfer processes often expose the ZnO surface to water, which is known to induce significant surface hydroxylation, potentially reaching high coverage. This work employs first-principles calculations at the HSE06 level, using 0.375 exact exchange (α) to investigate the fundamental impact of water adsorption and hydroxylation on the optoelectronic properties of the WS₂/m-plane (101̄0) ZnO interface. Our analysis reveals that hydroxylation of the surface Zn and O atoms drives a profound electronic structure transition. The desired Type-I alignment is converted to a less efficient Type-II (staggered) alignment, with the conduction band offset shifting downward by approximately 1.3 eV, where electrons and holes become spatially separated across the junction, directly impeding LED recombination efficiency. Furthermore, localized partial hydroxylation on surface oxygen sites is shown to induce in-gap states, which contribute to exciton localization effects. Interfacial chemisorbed (non-dissociated) water, studied as separate models, maintains the band offsets but impedes charge transfer, as confirmed by Bader charge model analysis. This study provides a comprehensive, atomic-level view of how minor surface modifications, commonly unavoidable during fabrication, can fundamentally tune the quantum-well alignment. While this transition is detrimental for Type-I LED efficiency, the ability to control the band alignment from Type-I to Type-II through simple surface modification opens new avenues for realizing other high-performance devices, such as photodetectors or tunable light sources.