Interplay between Epitaxial Growth and Spin-Crossover Properties of Molecular Ultrathin Films on Metallic Surface

For their incorporation in molecular spintronic devices, it is mandatory to understand how the properties of spin-crossover molecules are modified when they are in direct contact with metallic substrates and how the growth of molecular films is governed by the substrate. In this context, we investigate in detail the structure of [Fe^II(HB(3,5-(CH₃)₂Pz)₃)₂] (Pz = pyrazolyl) ultrathin films adsorbed on Cu(110) using grazing incidence X-ray diffraction measurements, along with their spin-crossover properties measured by X-ray absorption spectroscopy. For submonolayer coverage, the molecules self-assemble into two equivalent domains that are in a perfect epitaxial relationship with the Cu(110) substrate at room temperature. In parallel, the molecules are fully locked in a high spin (HS) state at low temperature. Density functional theory calculations show that there is a complex interplay between the epitaxial strain effect and the binding to the substrate that governs the orientation growth and the spin-crossover properties of the molecular films. For thicker thicknesses, a layer-by-layer growth of the (100) planes of the molecular bulk crystal with the release of the epitaxial constraint is observed, while the spin-crossover properties of the films are partially recovered with the opening of a hysteresis. The results obtained are very different from those observed on thin films adsorbed on Cu(111), indicating that not only the nature of the substrate but also its symmetry influences the spin-crossover properties of molecules.