Finite Size Effects on the Switching Dynamics of Spin‐Crossover Thin Films Photoexcited by a Femtosecond Laser Pulse

Abstract

Using ultrafast optical absorption spectroscopy, we have studied the room-temperature spinstate switching dynamics induced by a femtosecond laser pulse in high-quality thin films of the molecular spin-crossover complex [Fe(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl). These measurements reveal that the early, sub-picosecond, low-spin to high-spin photoswitching event, with linear response to the laser pulse energy, can be followed under certain conditions by a second switching process occurring on a tens of nanoseconds timescale, enabling nonlinear amplification. Such out-of-equilibrium dynamics is discussed in light of characteristic timescales associated with the different switching mechanisms, i.e. the electronic and structural rearrangements of photo-excited molecules, the propagation of strain waves at the material scale and the thermal activation above the molecular energy barrier. Importantly, size reduction effects are evidenced on the second switching step. Notably, this nonlinear process appears to be completely suppressed in the thinnest (50 nm) film due to the efficient heat transfer to the substrate, allowing the system to retrieve the thermal equilibrium state on the 100-ns timescale.These results provide a first milestone towards the assessment of the physical parameters that

Molecular Spin Crossover Materials: Review of the Lattice Dynamical Properties

“…In particular, the elastic switching step was shifted by two orders of magnitude toward shorter timescales (20 ns for crystals and 300 ps for nanocrystals) in agreement with the sample size reduction ratio, while the “thermal” step was suppressed due to a more efficient heat transfer with the external environment. Finite size effects have been also observed recently in high quality, thin SCO films revealing a complete suppression of both the thermal and elastic steps in the thinnest films (50 nm) due to the enhanced heat exchange as well as the large temporal decoupling of the lattice expansion from the intramolecular switching phenomenon …”

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

“…For our thermal-cycling experiment, the temperature of the wire was modulated between 50 and 88°C by applying 200-µs-long current pulses at a frequency of 200 Hz. Note that the heating duration (200 µs) was chosen long enough compared to the thermally activated molecular spin-state switching time (a few tens of nanoseconds above room temperature 38 ), ensuring that a full LS-HS-LS thermal switching cycle was completed for each current pulse. At selected times, the excitation frequency was lowered to 0.2 Hz in order to record simultaneously the temporal evolution of the optical reflectivity (λ = 452 nm) on the heated microwire.…”

“…Regarding the temporal resolution, our thermometer is intrinsically limited by the thermally induced switching dynamics of the SCO molecules. From recent femtosecond optical spectroscopy measurements 38 , we know that the thermally activated spin-state switching in the thin films of 1, governed by the LS ↔ HS intramolecular energy barrier, occurs within a few tens of nanoseconds. This time scale thus defines the intrinsic response time of the film, in terms of optical reflectivity change, to any variation of temperature.…”

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

“…Recently, we have shown that very high‐quality, nanocrystalline films of 1 can be obtained by vacuum thermal evaporation followed by solvent vapor annealing . These films exhibit a well‐reproducible thermal SCO (similar to the bulk material) as well as an ultrafast (≈10 −13 s) light‐induced spin state switching at room temperature . With decreasing film thickness, the transition is slightly shifted to higher temperatures, which we attributed to the stabilization of the LS phase by surface energy and stress .…”