Charge Mobility and Dynamics in Spin-Crossover Nanoparticles Studied by Time-Resolved Microwave Conductivity

We use the electrodeless time-resolved microwave conductivity (TRMC) technique to characterize spin-crossover (SCO) nanoparticles. We show that TRMC is a simple and accurate means for simultaneously assessing the magnetic state of SCO compounds and charge transport information on the nanometer length scale. In the low-spin state from liquid nitrogen temperature up to 360 K the TRMC measurements present two well-defined regimes in the mobility and in the half-life times, in which the former transition temperature T R occurs near 225 K. Below T R, we propose that an activationless regime taking place associated with short lifetimes of the charge carriers points at the presence of shallow-trap states. Above T R, these states are thermally released, yielding a thermally activated hopping regime where longer hops increase the mobility and, concomitantly, the barrier energy. The activation energy could originate not only from intricate contributions such as polaronic self-localizations but also from dynamic disorder due to phonons and/or thermal fluctuations of SCO moieties.