​Creating order out of chaos: magnetic alignment makes plasmonic nanostructures shine

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Plasmonic nanostructures are materials featured with localized surface plasmon resonance (LSPR). With an anisotropic shape, plasmonic nanorods display two LSPR modes due to the oscillation of conduction electrons along their short (transverse mode) and long-axis (longitudinal mode) under light excitation. In principle, aligning the anisotropic nanostructures along or perpendicular to the polarization of linearly polarized light can selectively excite the individual plasmon mode, offering opportunities in fine-tuning of the LSPR strength and providing an additional degree of freedoms for designing many advanced devices. However, in conventional systems they are randomly orientated in colloidal dispersions or solid substrates, therefore limiting their potential in practical applications. It has remained a critical challenge to control the collective orientation of anisotropic nanostructures along specific directions with the desired precision, flexibility, and scalability.

In our present work, we develop a universal nanoscale magnetic assembly approach to precisely control the spatial orientation of anisotropic plasmonic nanostructures (Figure 1). We take advantage of the templating methods for the confined growth of hybrid magnetic-plasmonic nanorods, which have coupled magnetic-plasmonic anisotropy, excellent parallel alignment, and defined structural ratio. Simply applying a magnet can effectively align the hybrid rods along any specific directions as needed. We have achieved active and selective tuning of their transverse and longitudinal plasmon modes for mechanochromic devices (Figure 2).


Figure 1. Schematic illustration of the orientation-dependent plasmonic excitation of nanorods for mechanochromic devices.

The design of mechanochromic responses is not straightforward. “How can we use our materials to detect the complex mechanical movements and deformations”, asked Dr. Yin during a walk-and-talk discussion. It was a hot summer and I did not have an answer to the question at that time. But it kept inspiring me. Later, we realized that the mechanical perturbation normally induces deformations in elastomers, which can be recognized by spectral techniques if the plasmonic rods are homogeneously aligned. Based on this simple principle, we have designed plasmonic films with observable mechanochromic responses to linear and angular mechanical perturbations, including pressing, stretching, rotating, bending, and twisting.


Figure 2. Left: A TEM image of the hybrid magnetic-plasmonic nanorods synthesized by a templating method. Right: Orientation-dependent plasmonic excitation of Au nanorods.

How to accurately describe the orientation-dependent plasmonic excitation in the deformed plasmonic films is another challenge that we met during our manuscript preparation. The chemistry department at UCR has been offering Quantum Mechanics for graduate students majoring in physical chemistry. It was about four years ago, and I learned about operators, expectation values, and how to describe the linear algebra and linear operators on vector spaces by bra-ket notation. It is simple, historical, and mathematically succinct in a way that provides an elegant tool to define the plasmonic excitation under linear polarization. I was not expecting any practical use of it while taking the class, which, however, all came back to me when I tried to describe the linear excitation of the hybrid nanorods under complex mechanical perturbations. In combination with the finite-element analysis, we provided both analytical and numerical methods to calculate the linear plasmonic excitation of anisotropic plasmonic nanostructures.   

                

Plasmonic films showing asymmetric and programmable mechanochromic responses to bending.

This is not going to be the end of the story. We are currently further exploring the unique properties of the orientation-dependent plasmonic excitation and trying to expand the nanoscale magnetic assembly to tune other physical properties of plasmonic nanostructures. It is an exciting research field, which I am passionate about as it has both fundamental importance and broad application prospects.

If you are interested in the story, please read our paper “Coupling magnetic and plasmonic anisotropy in hybrid nanorods for mechanochromic responses” published in Nature Communications. 

Li, Z., Jin, J., Yang, F. et al. Coupling magnetic and plasmonic anisotropy in hybrid nanorods for mechanochromic responses. Nat Commun 11, 2883 (2020). https://doi.org/10.1038/s41467...

Zhiwei Li

Postdoctoral Researcher, University of California, Riverside

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