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3D Confocal Raman mapping of field enhancement
inside Supercluster metamaterials
Alberto Lauri¹, Leonora Velleman², Xiaofei Xiao¹, Emiliano Cortes¹, Joshua B. Edel², Vincenzo
Giannini¹, Aliaksandra Rakovich¹, Stefan A. Maier¹.
¹ The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, UK.
² Department of Chemistry, Imperial College London, South Kensington Campus, London, SW7 2AZ, United
Kingdom
Keywords: Self-Assembly, Raman spectroscopy, metamaterials, nanoparticles, Supercluster, surfaceenhanced.
Abstract
Assembly of nanoscale building blocks into hierarchical superstructure by self-assembly is one of the most
pursued topics in nanoparticles chemistry. The possibilities obtainable when individual components arrange
themselves into an ordered structure are limitless and of great interest, as some biological self-assembled
structures such as DNA, opals and proteins have shown. One of the most interesting types of coupling
available between different nanoparticles is a plasmonic one. Indeed, if we think of the electromagnetic
field surrounding a metallic nanoparticle excited near its plasmonic resonance in the same way as the wave
function of a molecule or an atom, the idea of nanoparticles assembled into a lattice structure comes
immediately as analogy. Self-assembled supercluster metamaterials are one of the possibilities when
metallic nanoparticles are used as building blocks. The strong plasmonic interaction allows the creation of
a strong field at the nanoparticles’ gap inside the supercluster, while the structural hierarchy introduces a
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wide range of modes spanning from the visible all the way to the microwave regime as shown by Turek et
al. (ACS Photonics 2016, 3, 35-42).
As Surface-Enhanced Raman Scattering (SERS) allows ultrasensitive detection of molecules located
several nanometers away from the surface of a metallic nanoparticle by exploiting the strong field
enhancement of a plasmonic mode, the strong field and the wide range of modes of the supercluster
metamaterials can provide an efficient platform for such measurements. Moreover the three-dimensionality
of this structure allows the mapping of Raman signal in all space dimensions.
Exploiting the properties of the self-assembled metamaterials supercluster, we report the experimental
measurement of near and mid- IR plasmonic collective modes inside colloidal gold nanoparticle
superclusters by monitoring the Raman scattering of 4-Mercaptobenzoic acid with a confocal microscope.
The strongly enhanced Raman signal allows measurement of the plasmonic mode with a lateral resolution
lower than 500 nm and a vertical one of 500 nm. As the supercluster structure possesses tunable optical
modes, different plasmonic responses are mapped according to the cluster size and the excitation
wavelength. Furthermore, the tunable optical properties of the supercluster, and its strong field
enhancement suggest their use for 3D surface-enhanced mapping.
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