Hysteresis and change of transition temperature in thin films of Fe{[Me₂Pyrz]₃BH}₂, a new sublimable spin-crossover molecule

Thin films of the spin-crossover (SCO) molecule Fe{[Me₂Pyrz]₃BH}₂ (Fe-pyrz) were sublimed on Si/SiO2 and quartz substrates, and their properties investigated by X-ray absorption and photo-emission spectroscopies, optical absorption, atomic force microscopy, and superconducting quantum interference device. Contrary to the previously studied Fe(phen)₂(NCS)₂, the films are not smooth but granular. The thin films qualitatively retain the typical SCO properties of the powder sample (SCO, thermal hysteresis, soft X-ray induced excited spin-state trapping, and light induced excited spin-state trapping) but present intriguing variations even in micrometer-thick films: the transition temperature decreases when the thickness is decreased, and the hysteresis is affected. We explain this behavior in the light of recent studies focusing on the role of surface energy in the thermodynamics of the spin transition in nano-structures. In the high-spin state at room temperature, the films have a large optical gap (similar to 5 eV), decreasing at thickness below 50 nm, possibly due to film morphology. (C) 2015 AIP Publishing LLC.