Unveiling Spin Transition at Single Particle Level in Levitating Spin Crossover Nanoparticles

TL;DR

This study demonstrates real-time, substrate-free control and monitoring of spin crossover transitions in levitating nanoparticles, enabling potential applications in ultralow-power optical devices and nanoscale sensors.

Contribution

It introduces a contact-free platform combining a quadrupole Paul trap with optical microscopy to manipulate and observe spin transitions in individual levitating nanoparticles.

Findings

Reversible opto-volumetric changes up to 6% in single nanoparticles.

Precise switching thresholds for spin states at the single-particle level.

Mechanical pressure modulates the spin transition similarly to optical control.

Abstract

The ability to control and understand the phase transitions of individual nanoscale building blocks is key to advancing the next generation of low-power reconfigurable nanophotonic devices. To address this critical challenge, molecular nanoparticles (NPs) exhibiting a spin crossover (SCO) phenomenon are trapped by coupling a quadrupole Paul trap with a multi-spectral polarization-resolved scattering microscope. This contact-free platform simultaneously confines, optically excites, and monitors the spin transition in Fe(II)-triazole NPs in a pressure-tunable environment, eliminating substrate artifacts. Thus, we show light-driven manipulation of the spin transition in levitating NPs free from substrate-induced effects. Using the robust spin bistability near room temperature of our SCO system, we quantify reversible opto-volumetric changes of up to 6%, revealing precise switching thresholds at the single-particle level. Independent pressure modulation produces a comparable size increase, confirming mechanical control over the same bistable transition. These results constitute full real-time control and readout of spin states in levitating SCO NPs, charting a route toward their integration into ultralow-power optical switches, data-storage elements, and nanoscale sensors.