Gold Nanobipyramid-Directed Growth of Length-Variable Silver Nanorods with Multipolar Plasmon Resonances

We report on a method for the preparation of uniform and length-variable Ag nanorods through anisotropic Ag overgrowth on high-purity Au nanobipyramids. The rod diameters can be roughly tailored from similar to 20 nm to similar to 50 nm by judicious selection of differently sized Au nanobipyramids. The rod lengths can be tuned from similar to 150 nm to similar to 550 nm by varying the Ag precursor amount during the overgrowth process and/or by anisotropic shortening through mild oxidation. The controllable aspect ratios, high purity, and high dimensional uniformity of these Ag nanorods enable the observation of Fabry-Perot-like multipolar plasmon resonance modes in the colloidal suspensions at the ensemble level, which has so far been demonstrated only on Au nanorods prepared electrochemically with anodic aluminum oxide templates. Depending on the mode order and geometry of the Ag nanorods, the multipolar plasmon wavelengths can be readily tailored over a wide spectral range from the visible to near-infrared region. We have further elucidated the relationships between the multipolar plasmon wavelengths and the geometric dimensions of the Ag nanorods at both the ensemble and single-particle levels. Our results indicate that the Au nanobipyramid-directed, dimensionally controllable Ag nanorods will be an attractive and promising candidate for developing multipolar plasmon-based devices and applications.