Tailoring linear and nonlinear optical properties of 2D Sc₂C MXenes via surface termination modulation

Surface termination engineering is a powerful strategy to tailor the electronic and optical properties of two-dimensional MXenes for optoelectronic applications. In this work, we present a systematic theoretical investigation into the stability and second-order nonlinear optical (NLO) responses of a series of Sc-based MXenes (Sc₂CT_x, where T = O, F, OH). By screening twelve termination configurations, we identify six dynamically stable phases-including mixed-termination Sc-MXenes-exhibiting wide band gap tunability from 0.56 to 1.82 eV. Remarkably, Sc₂C(OH)₂ exhibits a giant static second-harmonic generation (SHG) coefficient of 191.4 pm V^-1, significantly outperforming benchmark NLO crystals like AgGaSe₂. Using band and atomic-resolved susceptibility decomposition, we reveal that the Sc-3d orbitially controlled covalency, modulated by the electronegativity of terminations, dictates the SHG response. Notably, we identify Sc₄C₂F(OH)₃ as a standout candidate that balances a substantial SHG response (d₁₂ = 185.0 pm V^-1) with optical transparency in the mid-infrared region. Our findings not only establish Sc-based MXenes as high-efficiency platforms for frequency conversion but also provide a general route for tailoring NLO performance via surface engineering in 2D materials.