The Role of Substrates in 2D Spin-Crossover Systems: Insights From Monte Carlo Simulations Within the Ising-Like Model

Spin-crossover (SCO) molecular solids are a class of coordination compounds exhibiting hysteretic thermal transitions between low-spin (LS) and high-spin (HS) states, making them capable of collective switching between these two states in response to external stimuli such as temperature, pressure, and electric fields. This bistable behavior directly paves the way for breakthrough technological applications in the field of molecular sensors, molecular switches, and actuators. For thermally induced spin transitions, the transition temperature ( T up ) on heating, at which the system switches from the LS to the HS, is strongly influenced by the ligands coordinating the metal center. In this study, we investigate the effect of the substrate on T up in SCO nanostructures, focusing on how subtle substrate-induced interactions can modulate the transition temperature. To this end, we model substrate effects through an extended Ising-like Hamiltonian, solved using Monte Carlo simulations. The results show that substrate interactions can be used to significantly fine-tune the thermal transition temperature, modify the width of the hysteresis, and induce either abrupt or gradual switching as needed. This groundbreaking control offers a radical new perspective for the design and optimization of next-generation SCO devices, enabling the creation, among other applications, of precision-engineered temperature sensors for complex systems.