Ultrafast optical and X-ray spectroscopy of strongly cooperative spin-crossover nanoparticles

In this thesis, we have investigated the light-induced switching events in nanoparticles of the strongly cooperative [Fe(Htrz)2(trz)](BF4) (Fe-Triz) spin-crossover (SCO) material. Here, SCO materials serve as a prototypical bistable material to study photo-induced phase transformations as a promising avenue towards material control by reversible switching. We have employed ultrafast optical and X-ray spectroscopies on a series of nanoparticle sizes to explore the mechanisms involved in light-induced switching at the nanoscale. After excitation into an MLCT band, we observe that Fe-Triz nanoparticles undergo a <200 fs spin transition to the high spin (HS) state. Following HS formation, we observe dynamics across several decades in time. In the first few picoseconds, we see a size-dependent oscillation which was identified to be an acoustic breathing mode. This phonon is excited by the nearly-simultaneous (<200 fs) formation of the larger volume HS states. Over the next 30 ps, a strong spectral shifting occurs in the transient signal. Using a combination of spectroscopic evidence—with theoretical support from time-dependent density functional theory and Monte Carlo mechanoelastic simulations—we determined that this spectral evolution is due to the build-up of HS states at distorted geometries within the nanoparticles. This results from the mechanical restoration of the lattice from the high elastic energy photoexcited distribution to the lower energy HS distribution with more HS states at the surface and distorted geometries. The mechanically relaxed lattice ultimately undergoes biexponential decay back to the low spin (LS) state with time constants of 300 ps and 3 ns. The two timescales of decay result from the different HS stability at the bulk and distorted geometries. When the laser-induced excitation fraction exceeds ~2%, we see another feature: an additional series of LS→HS switching during the lattice expansion. This elastic amplification was previously observed for relatively large crystals—and until now, had not been observed in nanoparticles. Fitting this elastic step to a sequential model allowed extraction of the time constant and magnitude of the elastic response. With the inclusion of laser-induced thermal effects, we were able to reproduce the magnitude of this amplification with mechanoelastic simulations. By measuring nanoparticles of different sizes, we showed that the elastic amplification time was consistent with the longitudinal expansion timescale. For our smallest particles, the elastic step was 5 ps, which is orders of magnitude faster than previously observed. We have demonstrated nanoscale miniaturization of cooperative, elastic interactions in SCO materials. Our ultrafast UV broadband tools have enabled us to uncover spectroscopic features of bulk and distorted HS species. Our measurements of a strongly cooperative SCO material far below the phase transition have illustrated the dynamic competition between thermal and pressure effects during the elastic step. Since the size reduction of Fe-Triz has not yet led to a loss of hysteresis, we encourage synthetic efforts to target even smaller particles than reported here, where surface effects will interface with the limits to cooperative behavior. As this size reduction comes with a speeding up of the material response, femtosecond spectroscopy and scattering methods will become the requisite approaches to study non-equilibrium dynamics.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Haddock, Tyler Nelson

Contributors dc:contributor

• van der Veen, Renske
• Vura-Weis, Joshua
• Girolami, Gregory S
• Gewirth, Andrew A

• ultrafast
• spectroscopy
• spin-crossover
• [Fe(Htrz)2(trz)](BF4)
• cooperative
• cooperativity
• nanoparticles

Statement dc:rights

• Copyright 2022 Tyler Haddock

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/115547

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F115547