Molecular-scale dynamics of light-induced spin cross-over in a two-dimensional layer

Abstract

Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensional spin cross-over layers.

The disentangling of hysteretic spin transition, polymorphism and metastability in bistable thin films formed by sublimation of bis(scorpionate) Fe(ii) molecules

“…In one case, some of us demonstrated the collective transformation of a 2D crystal of Fe[HB(3,5--(Me) 2 Pz) 3 ] 2 on a gold surface, based on the light--induced excited spin--state trapping (LIESST) process, 25 using scanning tunneling microscopy. 26 Memory effects in the conductance or transport properties are presently searched for with electronic devices based on SCO films, 24 nanostructures, single nanoparticles 27 or molecules 28 . The SCO properties of the pseudo--octahedral Fe[HB(3,5--(Me) 2 Pz) 3 ] 2 compound were first reported by Jesson et al, 29 but apart from the crystal structure that was solved at 289 K, 30 the structural and thermodynamical properties of the bulk material are unknown.…”

Direct Visualization of Local Spin Transition Behaviors in Thin Molecular Films by Bimodal AFM

“…Recently, much attention has been attracted to emergent physical properties and potential applications of nanoscale SCO materials in the form of nanoparticles, thin films, and nano‐heterostructures . Many aspects of the related physics focus on the study of the effect of surfaces/interfaces on the phase stability and phase transition dynamics . At reduced sizes, the increasing contribution of surfaces to the total free energy may give rise to structural and electronic changes in the material, which actually go beyond the surface layers.…”

“…The surface/interface energy can be therefore viewed as a key parameter, which governs the SCO properties at the nanoscale . Understanding surface/interface phenomena is not only important to elucidate nanoscale SCO behaviors but also opens up possibilities for engineering these nanomaterials . However, thin films and nanostructures usually have nonuniform properties due to defects, grain boundaries, strain distributions, and so forth.…”

“…Scanning probe microscopy (SPM) provides an attractive means for quantitative and noninvasive studies of local phase change behaviors in various materials under external stimuli (magnetic and electric fields, temperature, light irradiation, etc.). Solid–solid phase transitions in thin films can be visualized through different material properties (e.g., magnetic, optical, electrical, thermal, mechanical, and electromechanical) by SPM techniques such as piezoresponse force microscopy (PFM), Kelvin probe force microscopy (KPFM), scanning near‐field optical microscopy (SNOM), scanning tunneling microscopy (STM), scanning microwave microscopy (SMM), and magnetic force microscopy (MFM) …”

Enhanced Cooperativity in Supported Spin-Crossover Metal–Organic Frameworks

“…It is only recently that supported examples have emerged, starting from 3-dimensional (3D) materials with thin-films grown on gold. 11,16,17 0D systems followed based on single SCO molecules, [18][19][20][21][22][23][24] or molecular junctions. 25 However, 2D monolayer organized networks exhibiting SCO behavior have not been reported so far.…”