Spin crossover molecule: A First-Principles Study*

Selection of supporting substrate for spin crossover (SCO) molecules is important for electronic application. Applying density functional theory calculations to the SCO molecule Fe[qsal(2,4-diterbutyl)]₂ adsorbed on Au(111), graphene, and hexagonal boron nitride (h-BN), we showed a contrast in the influence of the substrate on electronic structure and magnetic properties of the molecule. We find that while the nature of the adsorption of the molecule on the three substrates is similar, mainly via van der Waals interaction, the charge transfer from the highest occupied molecular orbital of the molecule to the substrates is not the same. It donates about 0.82 electron to the Au surface and as much as 0.56 electron to graphene, but hardly any to h-BN. As a result, electronic structure of SCO is affected by the Au surface and graphene with an introduction of an partially occupied state at near the Fermi level. The contrasting effects of the substrates are traced to differences in the orbital alignment between the molecular orbital of the SCO and the substrates. Extension of calculations to overlayers of the SCO molecules on the substrates points to interesting differences in the underlying physics.