# Plasmonically enhanced Fe(ii) coordination complexes allow SERS readout of spin state switching below the optical diffraction limit

## TL;DR

This paper shows how plasmonic nanovoids can be used to monitor spin state changes in iron-based materials at the nanoscale using SERS.

## Contribution

The first demonstration of using SERS to detect spin state switching in sub-micron SCO nano-objects within plasmonic hotspots.

## Key findings

- SERS enabled monitoring of spin state transitions in <1 µm SCO nano-objects within plasmonic nanovoids.
- Thermal hysteresis was retained in nanoscale clusters, indicating cooperative spin crossover behavior.

## Abstract

Creating and monitoring spin crossover (SCO) materials at the nanoscale is challenging since the spin transition phenomena are perturbed and methods for monitoring them are limited. Optical approaches for monitoring nanoscale SCO are attractive but limited by weak signal levels. Here, we demonstrate for the first time that surface enhanced Raman spectroscopy (SERS) allows enhanced readout of spin state transitions of even <1 µm SCO nano-objects confined within plasmonic nanovoids. Pressing dry crystalline [Fe(Htrz)2(trz)](BF4) (1) into the nanogaps between sheets of metal nanoparticles gave strong SERS signals but was unsuccessful since the surface perturbed the spin transition behaviour. However, when 1 was placed in the plasmonic hotspots between the Au cores in clusters of Au@SCO core–shell nanoparticles, SCO was retained and could be monitored using SERS. Importantly, the clusters showed thermal hysteresis loops which, although narrower than that of bulk 1 (9 K vs. 40 K), demonstrated that cooperative behaviour was retained in the nanovoids.

SERS has been used to observe temperature-driven spin crossover of an FeII coordination polymer confined in the nanovoids of <1 µm clusters of Au@SCO core–shell nanoparticles. Importantly, hysteresis is retained within the ∼nm hotspots.