Rational Design of Low-Dimensional Hybrid Organic/Inorganic Interfaces for Enhanced Second-Harmonic Generation

Abstract

Hybrid interfaces formed by push-pull organic molecules physisorbed on two-dimensional (2D) semiconductors provide a structurally tunable platform for engineering second-harmonic generation (SHG). However, predicting and optimizing their macroscopic response remains a formidable challenge due to the phase-sensitive interference between the nonlinear responses of the adlayer and substrate. Here, we develop a physics-informed computational screening framework that decomposes the effective second-order susceptibility into its constituent substrate, molecular, and electric-field-induced SHG channels. Validated by fully atomistic first-principles calculations across representative interface structures, this model maps the complete orientation-resolved tensor landscape of chemically modulated carbon-conjugated polar molecules on 2D substrates with distinct symmetry. Crucially, our findings dismantle the conventional reliance on static polar descriptors, demonstrating that the ground-state permanent dipole magnitude is a fundamentally unreliable proxy for macroscopic nonlinear response. Instead, we show that the ultimate criterion for SHG maximization is encoded in the phase-resolved projection of the full anisotropic molecular hyperpolarizability tensor. On non-centrosymmetric substrates, the coherent interference between the physisorbed adlayer and the underlying 2D matrix induces a characteristic Fano-like asymmetry and ranking reversals that are entirely absent on centrosymmetric platforms. By establishing that functionalization topology, dynamic spatial orientation, and substrate point-group symmetry constitute a single, non-separable parameter space, this work provides a systematic blueprint for the rational assembly, predictive discovery, and non-invasive characterization of next-generation low-dimensional hybrid materials for nonlinear optoelectronics.