Surface Plasmon Mapping of Dumbbell-Shaped Gold Nanorods: The Effect of Silver Coating

Abstract

We report on the identification of surface plasmons in individual gold dumbbell-shaped nanoparticles (AuDBs), as well as AuDBs coated with silver. We use spatially resolved electron energy-loss spectroscopy in a scanning electron microscope, which allows us to map plasmon-energy and intensity spatial distributions. Two dominant plasmon resonances are experimentally resolved in both AuDBs and silver-coated AuDBs. The intensity of these features is peaked either at the tips or at the sides of the nanoparticles. We present boundary element method simulations in good agreement with the experiment, allowing us to elucidate the nature of such modes. While the lower-energy, tip-focused plasmon is of longitudinal character for all dumbbells under consideration, the second side-bound plasmon has a more involved symmetry, starting as a longitudinal quadrupole in homogeneous AuDBs and picking up transversal components when silver coating is added. The longitudinal dipolar mode energy is found to blue-shift upon coating with silver. We find that the substrate produces sizable shifts in the plasmons of silver-coated AuDBs. Our analysis portraits a complex plasmonic scenario in metal nanoparticles coated with silver, including a transition from the original homogeneous gold dumbbell plasmons to the modes of homogeneous silver rods. We believe that these findings can have potential application to plasmon engineering.

Excitation dependent Fano-like interference effects in plasmonic silver nanorods

“…Nanorod modes observed in EELS [17][18][19][20][21][22][23][24][25], CL [26,27], near field microscopy [28][29][30][31][32], and other mea-* smc204@cam.ac.uk † pam33@cam.ac.uk; www-hrem.msm.cam.ac.uk surements [33] have frequently been described as FabryPérot-like resonances [21,22,26,30], standing-wave patterns of surface charge and the corresponding near fields. These are characterized by intensity at antinodes in the standing-wave pattern, recorded as peak intensities in maps and line profiles along the plasmonic rod.…”

Plasmon Mapping in Au@Ag Nanocube Assemblies

“…6,7 Due to the possibility of combining a good spatial resolution (∼Å) and energy resolution (∼0.1 eV), such electron energy loss spectroscopy (EELS) experiments performed in a scanning transmission electron microscope (STEM) recently became very popular when mapping the different surface plasmon modes in nanostructures. The technique was used to map the plasmon resonant modes in different structures such as Au and Ag nanorods, 8−10 Ag triangles, 11 Au dumbbells, 12 bow-tie antennas, 13 nanocubes, 14 and many others. As an alternative to EELS, energy filtered TEM (EFTEM) imaging, using an energy selecting slit, can be used for the mapping of plasmonic resonances as well.…”

Relativistic force between fast electrons and planar targets

“…The retarding force along an electronʼs trajectory in an inhomogeneous medium has attracted the most attention in recent decades due to its interest for electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy (STEM) [4,5]. Nowadays this technique has become a very accurate tool currently used to characterize nanostructures and map plasmons in space with high resolution [6,7].…”