Three-Dimensional Tomographic Mapping of Surface Plasmons of a Chiral Gold Nanoparticle Using STEM-EELS

Chiral metal nanoparticles are reported to have interesting chiroptical properties, which attracts researchers in the bio-sensing and nano-optics fields. The gold 432-hellicoids, the particles of interest in this work, were synthesized with chiral peptides and showed Circular Dichroism (CD) [1]. The distinguished three-dimensional chiral shape of the nanoparticle is thought to attribute the optical chirality.<br/>However, the origin of the chiroptical property of the 432-hellicoids has not been fully understood yet. To understand the physical ground of it, analyzing the possible induced plasmon modes in single-particle-level through simulations and measurements is the most first step.<br/>Electron Energy Loss Spectroscopy (EELS) in Transmission Electron Microscope (TEM) is a powerful characterization tool for this work. Scanning Transmission Electron Microscopy Electron Energy Loss Spectroscopy (STEM-EELS) has superior spatial resolution compared to any other characterization tools and good energy resolution. It enables sub-particle-level plasmon mapping, and the three-dimensional imaging is also possible with those maps at several different sample tilt-angles.<br/>In this work, Cs-corrected and Monochromated TEM (Themis Z, Thermofisher) was used to map the surface plasmons of a single 432-hellicoid nanoparticle. The Spectrum-Images (SI) with the energy range in 0 eV to 3 eV and 5nm-sized pixel were obtained for each sample tilt-angle. The samples were tilted along the axis of the holder (alpha-tilt) from -60 degree to +60 degree in the increment of 10 degree. The 13 SIs were post-processed to get information on plasmon energy and the spatial distribution, then used to reconstruct three-dimensional mapping of plasmon modes. The widely known MNPBEM, the plasmon simulation tool that numerically solves Maxwell equation, was used to interpret the experimental data [2].<br/>This work enables deep understanding of the plasmons in a single chiral nanoparticle. It also can be a step forward to great advancement in establishing design strategy its potential future application like chirality-dependent bio-sensing.<br/>[1] Lee, Hye-Eun, et al. "Amino-acid-and peptide-directed synthesis of chiral plasmonic gold nanoparticles." <i>Nature</i> 556.7701 (2018): 360-365.<br/>[2] Hohenester, Ulrich. "Simulating electron energy loss spectroscopy with the MNPBEM toolbox." <i>Computer Physics Communications</i> 185.3 (2014): 1177-1187.