Laser annealing of nanocrystalline gold nanowires

Gold nanowire mesh electrode for electromechanical device

Ionic polymer-metal composite (IPMC) actuators were prepared with Nafion film as the ionic polymer and gold nanowire (Au-NW) mesh film as the metal electrodes by hot-pressing, which shortened preparation time within 1 h. As a reference, IPMC actuator consisting of Nafion film and gold foil (Au-foil) was also prepared. Au-NW mesh film can be an electrode with thinner (about 150 nm) and lower surface resistivity (about 0.5 Ω sq-1) than the conventional electrode prepared by electroless plating. Larger contact area of the Au-NW mesh electrode than the Au-foil electrode resulted in better actuation performance (60% larger peak-to-peak displacement in actuation). It was confirmed that the transformation behavior of Au-NWs differed depending on the external stimuli condition. Namely Au-NWs transformed to Au nanoparticles in the case of the heat stimulus only. Meanwhile, Au-NWs transformed to plates in the case of the heat and pressure stimuli. While higher temperature improved the adhesion of Au-NW mesh electrode to the Nafion surface, it induced the transformation of nanowire to plates. The IPMC actuator that the Au-NW mesh electrodes were hot-pressed at 90 ºC exhibited the highest capacitance and the largest peak-to-peak displacement in actuation. This research expanded the application field of gold nanowires to the electromechanical devices.

Water-Immersion Laser-Scanning Annealing for Improving Polycrystalline Au Films

A water-immersion laser-scanning annealing (WILSA) method was developed for the heat treatment of a deposited polycrystalline Au film on a glass. The material characterization using X-ray diffraction, field-emission scanning electron microscopy, and electron backscatter diffraction shows improved crystallinity with a more uniform crystallographic orientation of (111) and the grain growth of the annealed Au film. Additionally, the optical constants of the Au film before and after annealing were characterized by spectroscopic ellipsometry in the visible to near-infrared (NIR) regime, and the corresponding optical densities (ODs) were measured by transmittance spectroscopy. Our results show that the extinction coefficient and the OD of the annealed film are significantly reduced, particularly in the NIR regime. This is because the grain growth caused by the annealing reduces the density of grain boundaries, leading to the decrease of the loss of free electrons' scattering at grain boundaries. Hence, the damping effect of the surface plasmon is reduced. Additionally, the integrity of the WILSA-treated thin film is kept intact without pinholes, usually produced by the conventional thermal annealing. Based on the improved optical property of the WILSA-treated Au film, two performances of an insulator-metal-insulator (IMI) layered structure of biosensors are theoretically analyzed. Numerical results show that the propagation length of a long-range surface plasmon polariton along an IMI structure with an annealed Au film is significantly increased, compared to an unannealed film, particular in the NIR region. For the other application of using an IMI sensor to detect the shift of the surface-plasmon-resonance dip in the total internal reflection spectrum for the measurement of a change of the medium's refractive index, the sensitivity is also profoundly improved by the WILSA method. It is worth mentioning that the optimal heating conditions (laser wavelength, fluence, exposure time, and scanning step) depend on the thickness of the Au film. Our study provides a postprocess of WILSA to improve the optical properties of a deposited polycrystalline Au film for raising the sensitivity of the related biosensors.

Single-Crystalline Gold Nanowires Synthesized from Light-Driven Oriented Attachment and Plasmon-Mediated Self-Assembly of Gold Nanorods or Nanoparticles

Through the light-driven geometrically oriented attachment (OA) and self-assembly of Au nanorods (NRs) or nanoparticles (NPs), single-crystalline Au nanowires (NWs) were synthesized by the irradiation of a linearly-polarized (LP) laser. The process was conducted in a droplet of Au colloid on a glass irradiated by LP near-infrared (e.g. 1064 nm and 785 nm) laser beam of low power at room temperature and atmospheric pressure, without any additive. The FE-SEM images show that the cross sections of NWs are various: tetragonal, pentagonal or hexagonal. The EDS spectrum verifies the composition is Au, and the pattern of X-ray diffraction identifies the crystallinity of NWs with the facets of {111}, {200}, {220} and {311}. We proposed a hypothesis for the mechanism that the primary building units are aligned and coalesced by the plasmon-mediated optical torque and force to form the secondary building units. Subsequently, the secondary building units undergo the next self-assembly, and so forth the tertiary ones. The LP light guides the translational and rotational motions of these building units to perform geometrically OA in the side-by-side, end-to-end and T-shaped manners. Consequently, micron-sized ordered mesocrystals are produced. Additionally, the concomitant plasmonic heating causes the annealing for recrystallizing the mesocrystals in water.