Plasmonic metal-based nanoparticles have garnered significant attention due to a plethora of novel optical device designs which are able to make the most of enhanced light emissions and absorptions that may be controlled by the size, shape and composition of a nanostructure.
Greatly enhanced electromagnetic fields result from the interaction of light leading to resonance of the free electrons, which is particularly present in noble metal structures. It is, therefore, critical to identify the plasmon resonance modes and hot spot locations.
It must be noted, however, that this presents a significant experimental challenge as a result of weak light emission levels as well as the particle size being smaller than the diffraction limit.
The most complete analysis of cathodoluminescence (CL) emissions is offered by the Monarc™ Pro system, which empowers all users, from novice to expert, to capture the highest quality data.
Methods and Materials
The structures investigated in this research were gold-palladium (Au-Pd) chemically-synthesized nanostars with a nominal side length of approximately 120 nm. Nanostars were firstly suspended in fluid before being deposited on a silicon substrate and then dried to the surface.
A Monarc Pro (model 450.P.WAR) with an optional filter housing and a linear polarizer was used to perform CL measurements in a conventional FE-SEM.
Nanophotonic materials have historically been considered complex and difficult to analyze as a result of the low flux of emitted photons and additional complexity in co-aligning both the SEM and the optical collection system.
However, the polarization-filtered spectrum images of Figure B are able to be collected in just 120 seconds, thanks to the Monarc system’s optimized optical design and auto-alignment capability.
Gatan is the market-leading global manufacturer of software and instrumentation used to enhance and extend electron microscopes — ranging from specimen manipulation and preparation to imaging and analysis.
Figure 1. (a) In-lens secondary electron image of a typical Au-Pd nanostar, (b) colorized polarization-filtered CL spectrum image with opposing polarizations displayed in blue and red (acquired in 120 s) overlaid on the (simultaneously collected) in lens secondary electron image, and (c) the difference of averaged angle-resolved (AR) CL patterns for excitations at the tips of the isolated nanostar using the same colorization as in Figure b. The CL intensity of the isolated nanostar shows spatial variation consistent with the highest local density of optical states (LDOS) enhancement at the tips; filtering the CL intensity by polarization suggests that the most intense plasmonic modes generated by the electron beam are dipolar and resonant at opposing tips. The AR-CL patterns reveal that the emission patterns lie orthogonal to their respective dipole axes. Image Credit: Gatan Inc.
Summary
The local density of optical states in a metallic nanostar far below the optical diffraction limit was revealed by CL in an SEM.
The selectivity of excitation was demonstrated by polarization-filtered emission of an isolated nanostar, whilst angle-resolved CL displayed the emission direction from plasmonic modes activated at the nanostar tips and confirmed their status dipolar resonances with broad isotropy along the polar direction.
Credit(s)
Thanks are extended to Dr. Emilie Ringe of Rice University for generously providing the specimen.
This information has been sourced, reviewed and adapted from materials provided by Gatan Inc.
For more information on this source, please visit Gatan Inc.