The optical properties of the noble metal nanoparticles (NPs) are dominated by localized surface plasmon resonances (LSPR) [1]. A spherical NP suspended in vacuum would present a LSPR mode that can be modelled as a dipole, hence called dipolar mode. When NPs are close enough to each other, they couple splitting the plasmonic modes of the same order and creating two new modes, the bonding dipolar plasmonic mode (BDP), and the antibonding dipolar plasmonic mode (ADP) [2]. The BDP is a low-energy mode while the ADP resonates at a value slightly higher than the dipolar mode of a sphere. The exact energy value for both modes depends on the inter-particle distance, being smaller as they are closer to each other [3]. It also depends on the aspect ratio of the group with lower energy values as the aspect ratio gets larger [4]. The third conditioning factor is the geometric shape of the cluster. In the same way that triangular NPs have plasmonic modes at lower energies than a sphere [5], a triangular or rhomboidal shaped group of NPs shows plasmonic modes at smaller energies than a spherical one [6].

In this work, silver NPs were created and were forced to cluster. Samples were taken at different stages of the aggregation process. They were analyzed at a large scale by UV-Vis spectroscopy (UV-Vis) and at nanometre scale by energy-filtering transmission electron microscopy (EFTEM). The individual, dipolar mode was clearly identified for isolated NPs corresponding to the early stages of the clustering process. As bigger clusters are created, the collective modes become more apparent.

This work was supported by the Spanish MINECO (projects TEC2014-53727-C2-1-R, 2-R and CONSOLIDER INGENIO 2010 CSD2009-00013), Generalitat Valenciana (PROMETEOII/2014/059) and Junta de Andalucía (PAI research group TEP-946).  The research leading to these results has received funding from the European Union Seventh Framework Program [FP/2007/2013] under Grant Agreement No. 312483 (ESTEEM2) and H2020 Program (PROMIS ITN European network).

References

1.             Maier, S.A., Plasmonics: Fundamentals and Applications. 1st ed. 2007: Springer: New York.

2.             Halas, N.J., et al., Plasmons in Strongly Coupled Metallic Nanostructures. Chemical Reviews, 2011. 111(6): p. 3913-3961.

3.             Duan, H.G., et al., Nanoplasmonics: Classical down to the Nanometer Scale. Nano Letters, 2012. 12(3): p. 1683-1689.

4.             Barrow, S.J., et al., Surface Plasmon Resonances in Strongly Coupled Gold Nanosphere Chains from Monomer to Hexamer. Nano Letters, 2011. 11(10): p. 4180-4187.

5.             Koh, A.L., et al., High-Resolution Mapping of Electron-Beam-Excited Plasmon Modes in Lithographically Defined Gold Nanostructures. Nano Letters, 2011. 11(3): p. 1323-1330.

6.             Diaz-Egea, C., et al., High spatial resolution mapping of surface plasmon resonance modes in single and aggregated gold nanoparticles assembled on DNA strands. Nanoscale Research Letters, 2013. 8(1): p. 337.

Figures:

Figure 1 Schematic representation of the way plasmonic modes are summed into the collective response of clusters

Figure 2 (a) Pseudo-spherical silver nanoparticle shown in a bright field TEM image (a1) and two EFTEM images (a2) and (a3) acquired at 2.3 eV and 3.4 eV energy loss. (b) Cluster of silver nanoparticles. Bright field TEM image (b1) and EFTEM images (b2, b3, b4 and b5) acquired at the indicated energy losses.

« Back to The 16th European Microscopy Congress 2016

EMC Abstracts - https://emc-proceedings.com/abstract/mapping-the-plasmonic-modes-of-silver-nanoparticle-aggregates/