First-Principles Study of Nonlinear Optical Properties of Pristine and Substrate-Supported SnS, SnSe, and SnTe...

2D monolayers of tin monochalcogenides (SnX, with X = S, Se, and Te) offer environmental stability, making them promising for various technological applications. Although proposed as ferroelectrics, their nonlinear optical (NLO) activity has not been fully explored. Here, we use density functional theory to quantify their NLO response by calculating the (hyper)polarizability. These monolayers are semiconducting and form a stable Type II heterostructure with TiO₂(001), which is advantageous because it effectively suppresses electron-hole recombination in a device. The interfacial electronic states, created by the interaction of the (Sn) lone pair electrons with the substrate's electronic states, lead to increased polarizability (α) and first dipole hyperpolarizability (β) of the supported monolayers. Additionally, a significant enhancement in both β (-ω; ω, 0); β (0; 0, 0), and β (-2ω; ω, ω) is predicted for the supported SnSe monolayer. This may be due to a resonance effect associated with an interfacial charge-transfer transition, which appears to be closely aligned with the incident photon energy at λ = 1064 nm. This overall amplification of the NLO response indicates that these supported chalcogenide monolayers are promising candidates for next-generation nanoscale photonic technologies.