Interface nanochemistry and band alignment control in emerging metal chalcogenide nanostructures.

Emerging metal chalcogenide semiconductors, including kesterites, tin and antimony chalcogenides, elemental selenium, and related oxychalcogenides, combine strong absorption with adaptable processing but remain limited by buried junctions and electrical contacts. In these systems, the operational band profile can depend on interdiffusion, secondary phases, charged defects, interface dipoles, Fermi-level pinning, and the thermal or chemical history of the junction. This Topical Review uses the term reactive heterophase region (RHR) as an explicit synthesis of established interface-layer and transition-layer descriptions, extended to reactive chalcogenides by coupling composition and phase, defect charge, electrostatic potential, and process or operating history. RHR is not proposed as a universal new phase: the framework includes the abrupt and weakly coupled limits and is invoked only when evidence or a physically justified process model supports a finite reactive interval. We first define natural, near-interface, and operational band offsets and consolidate the roles of pinning, dipoles, trap-assisted tunnelling, and ultrathin de-pinning layers. We then apply a concept-to-evidence-to-design-rule sequence to absorber/buffer junctions, back contacts, passivating contacts, van der Waals interfaces, chalcogenide-silicon tandems, and photodetectors. A claim-driven metrology section replaces a technique catalogue with an evidence architecture linking chemistry, ensemble and local energetics, device function, and stability. It also systematically distinguishes reactive-interface effects from alternative causes of discrepant offsets, including surface preparation, charging and referencing, valence-band fitting, facet dependence, lateral heterogeneity, sample damage, and model dependence. Case studies on Cu2ZnSn(S,Se)4(CZTSSe), SnS, Sb2Se3/Sb2(S,Se)3, and selenium illustrate transferable and material-specific limits. The review closes with compact reporting and reliability standards intended to improve causal attribution, reproducibility, and cross-study comparison.