Mapping the plasmonic modes of silver nanoparticle aggregates

Abstract

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).