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Geometry design based plasmon hybridization represents an efficient means for pursuing desired spectral response and near-field enhancement in hybrid structures. Here, we propose to merge individual metal nanostructures into a superstructure to simultaneously realize plasmon mode inheritance and hybridization and electric near-field nanofocusing. As a model demonstration, we combine a metal nanoring supporting two localized surface plasmon resonances (LSPRs) and a metal nanocone sustaining one LSPR and one propagating surface plasmon polaritons (SPPs) mode into a hollow nanocone superstructure, and show numerically that such superstructure exhibits multiple plasmon resonance bands in the visible and near-infrared range and efficient nanofocusing of electric near-fields to the nanocone open apex. We further fabricate the designed superstructure with an ion-track membrane template method that allows for flexible control over relevant structural parameters by varying track etching time. Dark-field scattering measurements on single hollow nanocones confirm the presence of multiple plasmon resonances; surface-enhanced Raman spectroscopy further corroborates that the nanofocusing of electric fields plays a critical role in signal enhancement. We believe that such multi-resonant plasmonic superstructures with efficient near-field nanofocusing capability can find great potential in fundamental nonlinear optics and hot-carrier science studies and practical applications in surface-enhanced spectroscopies and broadband solar light harvesting.

Chemical stability of metal nanowires is a great concern for many practical applications. Metal nanowires deposited in the pores of various templates have been considered oxidization-resistant because the templates provide natural protection. Here we present the first ever attempt to investigate the oxidation behavior of copper nanowires supported in a template matrix using a surface-plasmon-based optical spectroscopy method combined with high-resolution transmission electron microscopy (TEM). Although the nanowires are protected by being embedded in the polymer template matrix, unexpected oxidation is clearly observed in TEM imaging, which unambiguously reveals the possible underlying oxidation mechanism responsible for the oxidation passivation. By measuring the optical properties with localized surface plasmon resonance spectroscopy, the oxidation behavior of the copper nanowires is studied as a function of storage time in both air and vacuum and thus uncovers the oxidation dynamics of copper. Finally, we demonstrate that the oxidation can be entirely prevented by sealing the open ends of the wires, which is confirmed by monitoring the electrical resistance of single nanowires. Our results have significant importance in understanding the oxidation behavior of metal nanostructures in general and also provide useful guidelines to estimate their electrical functionality in optoelectronic devices.

The environmentally friendly preparation of nanomaterials with controlled structural features represents a development trend of nanoscience and nanotechnology. In this work, using a cyanide-free bath, gold nanowires with controlled crystallinity and preferred crystallographic orientation have been prepared by electrochemical deposition in home-made polycarbonate ion track-etched templates. Single-crystal and polycrystal gold nanowires with preferred orientations along the [111] and [100] directions have been obtained by selecting fabrication parameters. The influence mechanisms of nanopore diameter, applied voltage, and deposition temperature on structural properties are proposed. In addition, single-crystal nanowires with [100] preferred orientation show enhanced field emission, which may be attributed to their single-crystal structure and the lower work function of loosely packed crystal planes.