Spin crossover molecules have two states (a magnetic one and a non-magnetic one) that can be modified by external stimuli. The possibility of switching from one state to another with stimuli is very interesting for applications in molecular spintronics: scientists keep searching for new supports, always more miniaturized, for the information bits. But the preservation of these properties is questioned in low dimensionality. An international collaboration between researchers from French (MPQ, Paris ; SIXS beamline, SOLEIL ; ICMMO, Orsay ; SPEC, Saclay), Indian (JNCASR, Jakkur) and Romanian (Faculty of Physics, Isasi) institutions have shown that when spin crossover molecules are deposited on a gold surface, the substrate influences the crystalline organization of the molecules and the strain induced in the molecular network forces the coexistence of both magnetic and non-magnetic molecular states at low temperature.
The spin-crossover molecules are composed of a central metallic ion surrounded by organic ligands that induce a degeneration lift on the metal d orbitals. Depending on the competition between the electron pairing energy and the ligand field energy, the orbitals can be filled in two ways leading to two different spin states that correspond to a magnetic state and a non-magnetic state. Thus, by adapting the ligands, it is possible to obtain bistable molecules that can switch from a non-magnetic state to a magnetic state by external stimuli such as a temperature or light. Nonetheless, if the properties of these molecules are well controlled in bulk, they are strongly modified when the molecules are in the form of nanoparticles or form a two-dimensional network.
The present work focuses on how the substrate, via epitaxial constraints, can influence the molecular state within a monolayer. For this, grazing incidence X-ray diffraction measurements realized on SixS beamline have been coupled to ab-initio calculations and numerical simulations. The spin cross-over molecules considered in this study are formed by an FeII ion surrounded by organic ligands, as shown in the figure.
The left figure shows a reciprocal space map measured on a sub-layer of spin-crossover molecules adsorbed on a gold substrate (Au (111)). From this type of measurement, it has been possible to identify the diffraction peaks related to the spin cross-over molecules and to show that they organize in an epitaxial network on the Au (111) substrate thus forming large well-oriented domains. Moreover, the orientation of the adsorbed molecules on the surface was determined precisely by comparing the relative intensities of the different diffraction peaks with ab-initio calculations.
Figure: Left, reciprocal space map allowing the determination of the lattice parameters of the spin crossover molecular monolayer on the gold surface (Au (111)). In the center of the map, a molecule is represented (C, H, B, N and Fe atoms are in brown, white, green, blue and light brown, respectively and the gold surface is in yellow). Right, fraction of molecules in a magnetic state (high spin, HS) obtained by mechano-elastic simulation as a function of the ratio between the substrate (ksub) and the molecules (kmol) spring constants. Due to the strain in the molecular layer, the coexistence of both molecular states can be obtained at low temperature.
In order to understand the role of the substrate on the thermal transition of the molecules from a magnetic state at high temperature to a non-magnetic state at low temperature, a simple mechano-elastic model has been developed modelling the molecule-molecule and molecule-substrate interactions by springs (their constants are called kmol and ksub respectively). It has thus been possible to show that it exists a range of parameters for which at low temperature some of the molecules are stabilized in the magnetic state (so-called high spin, HS). The figure on the right shows the evolution of the proportion of molecules in the magnetic state (HS fraction) as a function of the ratio of the spring constants (ksub / kmol). It is thus possible to see that for small ratio (here less than 0.05) the properties of the molecules are unaffected by the surface, that is to say that all molecules are in a non-magnetic state as expected for bulk at low temperature. Beyond this value, the magnetic state is stabilized due to the epitaxial strain and it is possible to observe the coexistence of the two states even at low temperature.
The coexistence of both spin states has been evidenced, in a previous study, at low temperature by scanning tunneling microscopy measurements performed in the Laboratory Matériaux et Phénomènes Quantiques (Université Paris/CNRS) and by x-ray absorption spectroscopy (XAS) measurements performed on DEIMOS beamline. The XAS measurements realized on a 1.5 monolayer thick molecular layer on Au(111) have indeed shown an uncomplete thermal transition and thus the coexistence of both molecular states.
The results obtained on the influence of the strain, which are part of the FET-Open European project COSMICS, enable a better understanding of the SCO molecule/metal interfaces and path the way to the control of their properties through epitaxial constrain.