The spin crossover (SCO) compound is a typical example of a bistable molecular material. The SCO is an entropy-driven mechanism involving the switching between a diamagnetic low-spin (LS) state and a paramagnetic high-spin (HS), as a response to the application of an external perturbation such as a variation of temperature, pressure, light, magnetic or an electric field.
We present in this contribution the analysis of the shape effect in SCO nanoparticles. Indeed, the ratio surface/volume in a nanoparticle strongly depends on the shape of the nano-object. Here, we consider a nanoparticle containing a fixed number (let’s say 36) of SCO molecules and we analyse all the possibilities of shape constructions for square- and rectangular-shaped lattice configurations. We associate to each of them the parameter t, which represents the ratio between surface and volume numbers of molecules. Thus, for a square 6×6, we obtain t=0.56, while for rectangular-shaped SCO nanoparticles of size, 9×4, 12×3 abnd 18×2, the repective t values are 0.61, 0.72 and 1. For a typical parameter in the framework of the Ising-like model we show that the increase of t value drives the appearance of a first-order transition accompanied with a hysteresis loop, the width of which increases with t.
Concerning the application of these SCO, we present the possibility of a new design concept for dual spin crossover based sensors for concomitant detection of both temperature and pressure. It is conjectured from numerical results obtained by mean field approximation applied to an Ising-like model that using two different spin crossover compounds containing switching molecules with weak elastic interactions it is possible to simultaneously mesure pressure P and temperature T.
Keywords: Phase transition, Spin crossover, sensors, nanoparticles