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

Abstract

Thin films of the molecular spin‐crossover complex [Fe(HB(1,2,4‐triazol‐1‐yl)3)2] undergo spin transition above room temperature, which can be exploited in sensors, actuators, and information processing devices. Variable temperature viscoelastic mapping of the films by atomic force microscopy reveals a pronounced decrease of the elastic modulus when going from the low spin (5.2 ± 0.4 GPa) to the high spin (3.6 ± 0.2 GPa) state, which is also accompanied by increasing energy dissipation. This technique allows imaging, with high spatial resolution, of the formation of high spin puddles around film defects, which is ascribed to local strain relaxation. On the other hand, no clustering process due to cooperative phenomena was observed. This experimental approach sets the stage for the investigation of spin transition at the nanoscale, including phase nucleation and evolution as well as local strain effects.

Unprecedented switching endurance affords for high-resolution surface temperature mapping using a spin-crossover film

“…Besides, as the SCO phenomenon is also accompanied by a concomitant change of the mechanical, dielectric, and magnetic properties of the material, other approaches can be implemented to probe the thermal spin transition, instead of optical readout. Among them, quantitative AFM mechanical (Young's modulus) measurements 48,49 can be used to probe the temperature changes in the SCO-thin-film thermometer, providing higher spatial resolution, though at the detriment of the temporal resolution and ease-of-use.…”

Nanomechanical mapping of soft materials with the atomic force microscope: methods, theory and applications

“…242 Local spin transitions in thin molecular films have been determined by following the temperature dependence of the Young's modulus from the low to the high spin states. 243 In another application, it was reported that the surface elasticity of binary self-assembled monolayers increased with ligand ordering. 244 The local elastic and inelastic properties of polymer films, polymer-based materials 214,245 and flexible crystals 246 have been extensively studied by bimodal AFM.…”

Sequential Activation of Molecular and Macroscopic Spin‐State Switching within the Hysteretic Region Following Pulsed Light Excitation

“…The study of the dynamical properties of the spin-state switching process within the hysteretic region of SCO nano-objects [13,50] remains the ultimate experimental challenge. In this context, the use of scanning probe microscopy [51][52][53] and transmission electron microscopy [54,55] methods opens up interesting perspectives. By analogy to superparamagnetism, it is possible that, below a certain critical size, the nanoobject behaves as a single entity in which all SCO molecules switch coherently, without activation of the heterogeneous nucleation and growth process.…”