Acoustic Vibrations of Single Suspended Gold Nanostructures

The acoustic vibrations for single gold nanowires and gold plates were studied using time-resolved ultrafast transient absorption. The objective of this work was to remove the contribution of the supporting substrate from the damping of the acoustic vibrations of the metal nano-objects. This was achieved by suspending the nano-objects across trenches created by photolithography and reactive ion etching. Transient absorption measurements for single suspended gold nanowires were initially completed in air and water environments. The acoustic vibrations for gold nanowires over the trench in air last typically for several nanoseconds, whereas gold nanowires in water are damped more quickly. Continuum mechanics models suggest that the acoustic impedance mismatch between air and water dominates the damping rate. Later transient absorption studies on single suspended gold nanowires were completed in glycerol and ethylene glycol environments. However, our continuum mechanical model suggests nearly complete damping in glycerol due to its high viscosity, but similar damping rates are seen between the two liquids. The continuum mechanics model thus incorrectly addresses high viscosity effects on the lifetimes of the acoustic vibrations, and more complicated viscoelastic interactions occur for the higher viscosity liquids.

Nearly all of the suspended gold nanowires in the transient absorption experiments exhibited several acoustic vibrations. This was attributed to the pentagonal cross-section of the nanowires. Two dimensional finite element simulations for gold pentagons show that similar frequencies exist for the vibrations of the apexes and the edges of the nanowires. Good agreement was found between the measured acoustic vibrations using transient absorption and the finite element calculations.

The suspension technique was extended to large single crystalline gold plates. Transient absorption measurements on suspended gold plates exhibit thickness vibrations, which can be modeled using a simple second order differential equation. The acoustic vibrations for suspended gold plates last for over 3 nanoseconds. Gold plates also exhibit surface plasmon polaritions when hit by light at the edge of the structure. Due to their two-dimensional structure, gold plates can be milled into specific shapes with a focused ion beam, offering many opportunities for plasmonics applications.