Predetermined and selective placement of nanoparticles onto large-area substrates with nanometre-scale precision is essential to harness the unique properties of nanoparticle assemblies, in particular for functional optical and electro-optical nanodevices. Unfortunately, such high spatial organization is currently beyond the reach of top-down nanofabrication techniques alone. Here, we demonstrate that topographic features comprising lithographed funnelled traps and auxiliary sidewalls on a solid substrate can deterministically direct the capillary assembly of Au nanorods to attain simultaneous control of position, orientation and interparticle distance at the nanometre level. We report up to 100% assembly yield over centimetre-scale substrates. We achieve this by optimizing the three sequential stages of capillary nanoparticle assembly: insertion of nanorods into the traps, resilience against the receding suspension front and drying of the residual solvent. Finally, using electron energy-loss spectroscopy we characterize the spectral response and near-field properties of spatially programmable Au nanorod dimers, highlighting the opportunities for precise tunability of the plasmonic modes in larger assemblies.

The relationship between composition and plasmonic properties in noble metal nanoalloys is still largely unexplored. Yet, nanoalloys of noble metals, such as gold, with transition elements, such as iron, have unique properties and a number of potential applications, ranging from nanomedicine to magneto-plasmonics and plasmon-enhanced catalysis. Here, we investigate the localized surface plasmon resonance at the level of the single Au−Fe nanoparticle by applying a strategy that combines experimental measurements using near field electron energy loss spectroscopy with theoretical studies via a full wave numerical analysis and density functional theory calculations of electronic structure. We show that, as the iron fraction increases, the plasmon resonance is blue-shifted and significantly damped, as a consequence of the changes in the electronic band structure of the alloy. This allows the identification of three relevant phenomena to be considered in the design and realization of any plasmonic nanoalloy, specifically: the appearance of new states around the Fermi level; the change in the free electron density of the metal; and the blue shift of interband transitions. Overall, this study provides new opportunities for the control of the optical response in Au−Fe and other plasmonic nanoalloys, which are useful for the realization of magnetoplasmonic devices for molecular sensing, thermo-plasmonics, bioimaging, photocatalysis, and the amplification of spectroscopic signals by local field enhancement.

The status of microscopic X-ray fluorescence analysis with tube excitation and synchrotron radiation is reviewed in terms of the lateral resolution, minimum detection limits and elemental sensitivity that can be achieved. As illustrations, the utilization of two typical state-of-the-art instruments for the analysis of geological material is described; one of the instruments is based on tube excitation, the other is installed at a synchrotron X-ray source. The analytical implications of the use of X-ray microprobes installed at a third generation storage ring, and in particular at the European Synchrotron Radiation Facility (ESRF), are discussed.

Multiresonant plasmonic nanoantennas have recently gained a lot of attention due to their ability to enhance nonlinear optical processes at the nanoscale. The first nanostructure designed for this purpose was an aluminum antenna composed of three arms, designed to be resonant at both the fundamental and the second harmonic frequencies. It was demonstrated that second harmonic generation induced by its resonances at both the fundamental and second harmonic wavelengths is higher than the one from a simple dipolar nanoantenna supporting a resonance at the fundamental wavelength only. However, the underlying mechanisms leading to this strong nonlinear signal are still unclear. In this study, both advanced simulations and experiments are combined to investigate in details the role of the mode coupling in the enhancement of second harmonic generation. By varying the length of the nanoantenna arms, it is clearly demonstrated that second harmonic generation is enhanced when the coupling between the quadrupole and the dipole modes at the second harmonic wavelength is significant. Indeed, using a numerical analysis based on the spatial selection of the second harmonic sources, it is shown that the second harmonic quadrupolar mode, which is directly excited by the fundamental pump, can resonantly transfer its energy to the second harmonic dipolar mode supported by another part of the nanoantenna due to near-field coupling. The study of the second harmonic generation mechanisms of double resonant plasmonic systems is important for the design of efficient second harmonic meta-devices such as coherent extreme-ultraviolet sources, ultrasensitive index, and chiral plasmonic sensors.

While plasmonic antennas composed of building blocks made of the same material have been thoroughly studied, recent investigations have highlighted the unique opportunities enabled by making compositionally asymmetric plasmonic systems. So far, mainly heterostructures composed of nanospheres and nanodiscs have been investigated, revealing opportunities for the design of Fano resonant nanostructures, directional scattering, sensing and catalytic applications. In this article, an improved fabrication method is reported that enables precise tuning of the heterodimer geometry, with interparticle distances made down to a few nanometers between Au−Ag and Au−Al nanoparticles. A wide range of mode energy detuning and coupling conditions are observed by near field hyperspectral imaging performed with electron energy loss spectroscopy, supported by full wave analysis numerical simulations. These results provide direct insights into the mode hybridization of plasmonic heterodimers, pointing out the influence of each dimer constituent in the overall electromagnetic response. By relating the coupling of nondipolar modes and plasmon−interband interaction with the dimer geometry, this work facilitates the development of plasmonic heterostructures with tailored responses, beyond the possibilities offered by homodimers. KEYWORDS: optical nanoantennas, plasmonic heterostructures, bimetallic antennas, electron energy loss spectroscopy, electron beam lithography, eigenmodes C ollective oscillations of the conduction electrons in metal nanostructures, known as localized surface plasmon resonances, 1 have been intensively studied and designed by manipulating both the nanostructures size and geometry, as well as their dielectric environment.2 These investigations, carried out over the entire visible light spectrum, including the near-UV and near-IR, have demonstrated the control of both nanoscale optical fields and far field radiation, 1 leading to the concept of optical nanoantennas.2,3 The electromagnetic properties of these structures are governed by their eigenmodes, ranging from dipoles to high order multipoles. 4,5 When the nanoparticles are arranged in pairs or multimers, these modes couple to each other and hybridize, producing further optical properties. 6−8 In the simplest and most common geometry, involving the coupling of two selfsimilar spherical nanoparticles separated by a nanogap, a wellknown dipole−dipole interaction along the dimer axis is produced. 6 This coupling generates an intense and confined near field in the gap region, which can enable large nanoscale fluorescence enhancement 9 and surface enhanced Raman scattering down to the single molecule level. 10,11 Thanks to a complete description of the plasmonic coupling for spherical self-similar nanoparticle dimers, Nordlander and coauthors 7 have shown that the gap dependent hybridization in such nanostructures stems from the interaction of the uncoupled eigenmodes. Specifically, the uncoupled modes hybridize with bonding and antibonding interactions a...