Vacancies and adatoms unlock reactivity in 2H- TiBr$_2$ monolayers

Defect engineering and functionalization enable the precise modulation of a material's local reactivity, electronic doping, and optical response. Motivated by its promising optical and excitonic characteristics, we show that the electronic properties of the 2H-phase TiBr$_2$ monolayer can be effectively tuned via vacancy engineering and adatom functionalization, thereby unlocking potential applications in catalysis, sensing, and optoelectronic devices. Herein, using density functional theory (DFT), we investigate pristine (TiBr$^{pri}_2$) and its modified monolayers by including vacancies Br-type (TiBr$^{vac}_2$), as well as those functionalized by common atomic adsorbates (X = H, C, N, and O, denoted as X/TiBr$_2$). For the pristine surface, all adatoms exhibit weak physisorption, with adsorption energies | E$_{ads}$ |< 1e⁢V. In contrast, the introduction of Br vacancies dramatically enhances the surface reactivity, resulting in strong chemisorption at the vacancy sites (for all adatoms) with large adsorption energies and substantial electronic modifications. The present work provides a comprehensive energetic decomposition framework (adsorption, interaction, and distortion energies) applied to transition-metal dihalide monolayers. ... mehrWe expect that similar defect- and adatom-induced reactivity may emerge in structurally related 2D transition-metal halides. In addition, our findings demonstrate that the electronic and adsorptive properties of TiBr$_2$ monolayers can be rationally engineered through defect and adatom manipulation.