Surface effects on temperature-driven spin crossover in Fe(phen)₂(NCS)₂

Despite their importance in molecular spintronics, the surface effects on spin crossover (SCO) behaviors are still poorly understood. Here, we report the impact of substrates on thermal SCO in Fe(phen)₂(NCS)₂ (phen = 1,10-phenanthroline) deposited on metallic surfaces and monolayer two-dimensional materials. By first-principles calculations, we show that temperature-driven SCO is preserved on both hexagonal boron nitride and molybdenum disulfide (MoS₂), while low-spin ground states are locked on metal surfaces, including Cu(111), Ag(111), and Au(111). On the contrary, the molecule in contact with graphene exhibits a high-spin ground state. We demonstrate that the spin transition temperature T_c depends critically on surface environments, and we correlate this effect with the modification of electronic structures and molecular vibrations upon adsorption. In particular, a sulfur vacancy in MoS₂ considerably increases T_c. These findings open a way to nanoscale applications related to spin state bistability.