Tailoring Linear and Nonlinear Optical Properties of 2D Sc2C MXenes via Surface Termination Modulation

Abstract

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 (Sc2CTx, 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, 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) 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.