Strong excitonic effects and optical helicity in group-IV 2H-phase transition-metal dihalide monolayers for solar energy harvesting applications

Palheta J.M.T.; de O. Batista A.L.; Bastos C.M.O.; Guedes-Sobrinho D.; Santos A.S.; Rêgo C.R.C.; Flores E.M.; Dias A.C.; Piotrowski M.J. · Computational Condensed Matter (2026) · DOI: 10.1016/j.cocom.2026.e01341

Resumen

We present a comprehensive first-principles investigation of 2H-phase transition metal dihalide (TMDH) monolayers composed of group-IV transition metals (Ti, Zr, and Hf) combined with Cl, Br, and I. Structural, electronic, optical, and excitonic properties were systematically analyzed by combining density functional theory with a maximally localized Wannier function tight-binding framework for solving the Bethe–Salpeter equation. Dynamical, thermal, and mechanical stability were assessed through phonon dispersion calculations, ab initio molecular dynamics simulations, and elastic constant analysis, confirming that all nine investigated monolayers satisfy the adopted stability criteria. The optimized lattice parameters range from 3.27 ̊ A to 3.82 ̊ A, increasing systematically with the halide atomic number. All monolayers exhibit semiconducting behavior, with HSE06+SOC fundamental band gaps ranging from 0.07 eV to 1.54 eV, strongly influenced by chemical composition and relativistic effects. We found the optical response to be nearly isotropic in-plane, while pronounced excitonic effects emerge due to the reduced dielectric screening in the two-dimensional limit. Exciton binding energies reach values as high as 630 meV, and TiI2 is identified as an excitonic insulator, characterized by a negative exciton ground-state energy. Finally, the solar energy harvesting potential of these materials was evaluated through power conversion efficiency calculations within the Shockley–Queisser and spectroscopy-limited maximum efficiency formalisms. Several of these monolayers are predicted to have efficiencies larger than 30 % within the Shockley–Queisser limit, which identifies 2H TMDH monolayers as promising candidates for optoelectronic, photovoltaic, and valleytronic applications.

Instituciones: Universidad Nacional Jorge Basadre Grohmann