Plasmonics is the study and applications of the interaction between electromagnetic wave (light) and the free electrons in metal. Silver (Ag) has been established as the hitherto best base material for plasmonics. The plasmonic behavior of Ag is largely dictated by its engineered structure at nano- and meso-scale.

The objectives of this thesis are (i) to develop electrochemical methods for the fabrication of arrayed, pattern-controlled and size-tunable hierarchical Ag mesostructures, (ii) to study the morphological transformation and growth mechanism with respect to different experimental parameters in electrochemical deposition, (iii) to study the optical property, especially, ultra-broadband plasmonic property associated with the morphological features and their applications in surfa...[ Read more ]

Plasmonics is the study and applications of the interaction between electromagnetic wave (light) and the free electrons in metal. Silver (Ag) has been established as the hitherto best base material for plasmonics. The plasmonic behavior of Ag is largely dictated by its engineered structure at nano- and meso-scale.

The objectives of this thesis are (i) to develop electrochemical methods for the fabrication of arrayed, pattern-controlled and size-tunable hierarchical Ag mesostructures, (ii) to study the morphological transformation and growth mechanism with respect to different experimental parameters in electrochemical deposition, (iii) to study the optical property, especially, ultra-broadband plasmonic property associated with the morphological features and their applications in surface-enhanced Raman (SERS) and hyper-Raman (SEHRS) scattering.

A method was developed for the pattern-controlled and size-tunable growth of arrayed hierarchical Ag mesostructures by combining the conventional photolithography with various electrochemical deposition techniques under potentiostatic, galvanostatic, and potentiodynamic conditions, respectively. Two morphological diagrams for Ag mesostructures, one for precursor concentration versus deposition potential and another for supporting electrolyte concentration versus deposition potential, were established for the first time for faceted, dendritic, fractal and DBM structures.

The linear optical properties of the as-fabricated hierarchical Ag mesostructures, including UV-Vis-NIR extinction spectra and SERS were studied systematically. The ultra-broadband extinction from ultraviolet to near-infrared region of the as-fabricated Ag mesostructures was observed. The SERS intensities from a monolayer of self-assembled 4-MBA on Ag mesostructures prepared with different deposition time and irradiation locations were examined and the enhancement factors estimated to be ~10⁷, ~10⁸, ~10⁹ for dendritic, DBM, and fractal Ag mesostructures at 632.8 nm, respectively, thus established Ag fractal mesostructure as the best candidate for the substrate in SERS for chemical and biological applications.

An optically “hot surface” composed of uniformly-structured Ag fractal mesostructure was developed for the first time for SERS and SERRS. A uniform SERS average enhancement factor of 6.2 × 10⁹ was obtained with the excitation at 632.8 nm using 4-MBA as the probe molecule. The Ag fractal “hot surface” can serve as a good substrate in the nonlinear SEHRS measurement from different kinds of probing molecules, including chemisorbed 4-MBA, and physisorbed R6G and CV, respectively. The SERHRS spectra of CV were successfully obtained under the very low average laser power (lower than 10 mW), which suggests that the as-fabricated Ag fractal “hot surface” can be applied in the ultrasensitive SEHRS measurement.

A method combining photolithography with electrochemical deposition technique was developed for the first time for the fabrication of arrays of single crystalline single Ag polyhedral particles and the single bi-particle “hot spot” with a uniform structure and tunable size. The effects of size, edges/vertexes, “hot spot”, surface roughness of Ag polyhedron on the single particle SERS were studied.