| Plasmonic materials are emerging as key platforms for applications that rely on the manipulation of light at small length scales. Materials that possess sub-wavelength metallic features support either localized or propagating surface plasmons that can induce huge local electromagnetic fields at the metal surface, facilitating a host of extraordinary optical phenomena. For many of the breakthrough photonic, spectroscopic, and optoelectronic applications of plasmonics, the bottom-up fabrication of these materials from low-dimensional structures has yet to be explored. Because colloidal metal nanostructures can be readily synthesized with controlled shapes and sizes, and because these structures also generate plasmon-mediated evanescent fields near their surfaces when irradiated with light, Ag nanocrystals and nanowires are ideal building blocks for rationally designed plasmonic materials.;This dissertation addresses three major challenges: (1) the synthesis of Ag polyhedral nanocrystals and nanowires, (2) the bottom-up organization of these nanostructures into one-, two-, and three-dimensional assemblies, and (3) the application of these assemblies as spectroscopic sensing platforms. Faceted Ag colloids were synthesized in high yield and with remarkable monodispersity using the polyol process, where Ag+ is reduced in the presence of a polymer capping agent that serves to regulate nucleation and crystallographic growth direction. The resulting nanocrystals and nanowires are bound exclusively by {100} and {111} crystal planes, where nanowires possess pentagonal cross-sections and nanocrystals possess octahedral symmetry. Because allowed plasmon modes are explicitly dictated by geometric considerations, each shape exhibits a unique scattering spectrum in the optical wavelengths. These shaped colloidal building blocks were assembled into ordered groupings and superlattices to achieve controlled electromagnetic coupling between individual nanostructures. Of particular note is the use of Langmuir-Blodgett assembly for the construction of two-dimensional nanocrystal superlattices with continuously variable interparticle spacing and density. For the first time, we demonstrate the complete bottom-up fabrication of a macroscopic material with a tunable plasmonic response in the visible wavelengths.;Lastly, we show that these nanoscale materials behave as exceptional substrates for surface-enhanced Raman spectroscopy (SERS). Assemblies of Ag nanowires and nanocrystals facilitate intense electromagnetic field enhancement due to charge localization near the sharp corners, edges, and junctions of the nanocrystals. We not only demonstrate that these assemblies can achieve high chemical sensitivity and specificity, but exhibit their utility as portable field sensors for toxins and explosives. For the first time, we demonstrate that SERS can be employed for the facile detection of low-level arsenic concentrations in ground water. In addition, we show the feasibility of integrating these Ag nanocrystals into microfluidic, multiplexed "lab-on-a-chip" devices, where SERS can be used for the in situ sensing of low-volume analytes. |
