Photo‐Thermal Switching of Individual Plasmonically Activated Spin Crossover Nanoparticle Imaged by Ultrafast Transmission Electron Microscopy

Abstract

Spin Crossover (SCO) is a promising switching phenomenon when implemented in electronic devices as molecules, thin films or nanoparticles. Among the properties modulated along this phenomenon, optically induced mechanical changes are of tremendous importance as they can work as fast light-induced mechanical switches or allow to investigate and control microstructural strains and fatigability. The development of characterization techniques probing nanoscopic behaviour with high spatio-temporal resolution allows to trigger and visualize such mechanical changes of individual nanoscopic objects. Here we use Ultrafast Transmission Electron Microscopy (UTEM) to precisely probe the length changes of individual switchable nanoparticles induced thermally by nanosecond laser pulses. This allows us to reveal the mechanisms of spin switching, leading to the macroscopic expansion of SCO materials. This study was conducted on individual pure SCO nanoparticles and SCO nanoparticles encapsulating gold nanorods that serve for plasmonic heating under laser pulses. Length changes are compared with time-resolved optical measurements performed on an assembly of these particles.

Dynamics of Spin Crossover Molecular Complexes

“…Furthermore, increased heating also speeds up the switching time, down the 10 ns time range for the highest fluence values, on nanoparticles [ 99 ]. This behavior is confirmed by time-resolved electron microscopy, showing a sub-µm particle expansion under illumination, that fully takes place below 20 ns for the highest fluence used [ 104 , 105 ]. Generally, one should be cautious about the thermal heating processes, as SCO materials may have limited thermal conductivity [ 106 ].…”

“…A broad understanding of the limits to fidelity and reliability is, however, not extensive given the limited number of studies in the literature (see a pertinent review in [ 113 ]). Even though there are indications that laser pulses of high fluence allow for only a limited number of operations [ 105 ], recent reports indicate that SCO materials should be capable of sustaining a significant number of thermal switching events (up to 10 7 ) [ 114 ]. Further improvements can also be gained by nanomaterials design.…”

“…One example is an experimental evidence that the required laser power for photo-switching the spin state can be reduced by combining SCO particles, coated with silica, with gold nanoparticles [ 115 ]. Exciting the plasmon resonance of the gold nanoparticles, leads to the Au nanoparticles acting as local nano heaters [ 115 , 116 ], with indications of lower fluence needed for switching [ 117 ] and faster thermal switching related to a more efficient heating process [ 105 ]. With the help of better material design, in the perspective of using smaller SCO particles, one can consider that the optical device applicability remains an open issue.…”

Analytics at the nanometer and nanosecond scales by short electron pulses in an electron microscope

“…Here, photon-induced electronic transitions between different states of excitation in molecules lead to structural transformations visible by imaging or diffraction. While the electronic transitions occur in femtoseconds and cannot be detected by UTEM, reversible structural changes in the pico-to nanosecond regime can be imaged [45,46].…”

Time-resolved Analytical Electron Microscopy with Single Nanosecond Electron Pulses

“…Furthermore, the integration of EELS provides the analytical information so that the composition of the specimen can be determined as a function of time with nanosecond resolution, and the kinetics of fast reactions can be studied at the nanoscale. In this presentation, additional examples for irreversible and reversible transformations [6] will be shown [7].…”