These recent years, study of surface plasmon resonances in transmission electron microscope has undergone an increasing interest. Indeed, low-loss EELS has proven its remarkable efficiency to probe plasmonic resonances at the nanometer scale while recent developments in cathodoluminescence (CL) spectroscopy brought a new insight into the coupling of plasmons and far field [1]. Several types of particles have already been numerically and experimentally studied ranging from nano-disk to nano-prism which plasmonic behaviors are now fairly understood [2,3]. However, despite this systematic effort to characterize the widest variety of plasmonic nano-particles, some structures still remain challenging e.g. the nano-cube. In the latest case, the difficulties encountered are plural. First, this structure being highly symmetric, plasmonic modes exhibit a significant number of natural degeneracies leading to superposition or hybridization of modes, which dramatically harden their understanding. Second, cube’s plasmonic modes turned out to be very sensitive to the geometry of the underlying nano-structure (e.g. edge rounding [4]) and to be strongly affected by the presence of substrate [5]. Third, particularly because of the degeneracies mentioned above, the coupling between two nanocubes brings unexpected difficulties, which are enhanced when the inter-particle gap goes below 1nm. Although this coupling has been widely tackled recently, no definitive theory has been given and this question remains controversial [6,7].

In the present work, we experimentally and numerically investigated plasmonic silver nanocubes and their coupling using STEM-EEL and STEM-CL spectroscopies (see figures 1 and 2) aiming at giving a clear and complete understanding of these modes. The cube samples have been prepared by chemical growth, which provides them a high degree of crystallinity. This property dramatically enhance the CL signal and thus enable us to obtain remarkably relevant CL-maps (see figure 2). For studying the coupling, in order to get rid of the quantum charge transfer problem arising at very small gap regime (<0.5nm), we restricted ourselves to inter-particle separations larger than 5nm. Complementary BEM simulations [8] have been carried out to understand on a deeper level the observed plasmonic modes by computing the corresponding charge distributions. We observed a good agreement between computations and experiments, which strengthen our conclusion. In addition to be consistent with the earlier studies, our work bring an overall understanding of the nanocubes coupling.

References

[1] Kociak, Stephan, Chemical Society Reviews 43, 3865-3883, 2014.

[2] Schmidt and al, Nano Lett. 12 (11), 5780–5783, 2012.

[3] Nelayah and al, Nano Lett. 10 (3), 902–907, 2010.

[4] Grillet and al, ACS Nano 5 (12), 9450–9462, 2011.

[5] Mazzuco and al, Nano Lett. 12 (3), 1288–1294, 2012.

[6] Tan and al, Science 243 (6178), 1496-1499, 2014.

[7] Knebl and al, Phys. Rev. B. 93 (8), 081405(R), 2016.

[8] Hohenester, Trügler, Comput. Phys. Commun 183, 370, 2012.

Figures:

Figure 1 : a) STEM-CL image of a nanocube dimer. b-c-d) (In red) CL spectra taken at position P1, P2 and P3 represented on a. Scale bar 100nm.

Figure 2 : a) Raw Energy filtered CL-maps. b) Raw Energy filtered CL-maps. The white contour represents the dimer showed on figure 1.a.

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