Direct Observation of Plasmon Band Formation and Delocalization in Quasi-Infinite Nanoparticle Chains.

Chains of metallic nanoparticles sustain strongly confined surface plasmons with relatively low dielectric losses. To exploit these properties in applications, such as waveguides, the fabrication of long chains of low disorder and a thorough understanding of the plasmon-mode properties, such as dispersion relations, are indispensable. Here, we use a wrinkled template for directed self-assembly to assemble chains of gold nanoparticles. With this up-scalable method, chain lengths from two particles (140 nm) to 20 particles (1500 nm) and beyond can be fabricated. Electron energy-loss spectroscopy supported by boundary element simulations, finite-difference time-domain, and a simplified dipole coupling model reveal the evolution of a band of plasmonic waveguide modes from degenerated single-particle modes in detail. In striking difference from plasmonic rod-like structures, the plasmon band is confined in excitation energy, which allows light manipulations below the diffraction limit. The non-degenerated surface plasmon modes show suppressed radiative losses for efficient energy propagation over a distance of 1500 nm.

Cross-section transmission electron microscopy characterization of the near-surface structure of medical Nitinol superelastic tubing.

The application of Nitinol in a wide variety of medical implants is progressively increasing because of its unique mechanical properties, durability and biocompatibility. However, as Nitinol consists of about 50 at.% of toxic Ni, certain applications are still hindered by the concern of free Ni release in the surrounding tissue. The latter is controlled by the structure of near-surface layers and can be strongly affected by various surface treatments. A proper application of advanced cross-section sample preparation techniques allows us to characterize the Nitinol near-surface structure down to the nanoscale by means of transmission electron microscopy (TEM). Elemental maps of the Ti, O and Ni distribution, concentration profiles, quantification of composition as well as atomic resolution images at the surface of a Nitinol tubing are presented and the results obtained with different sample preparation and analytical characterization techniques are compared. In addition to a strong decrease of Ni towards the surface of the oxide layer and a Ti depleted layer underneath the oxide, also a possible transformation from TiO to TiO(2) is documented.