Optical properties of noble metal nanoparticle arrays

Optical properties of noble nanoparticle arrays are studied by surface enhanced spectroscopies. The experiments and results are reported in this thesis.;Arrays of linear, size-controllable, one-dimensional (1D) silver nanoparticle rows have been prepared that demonstrate exceptional surface-enhanced Raman scattering (SERS) from incident light polarized along the axis of the arrays. These 1D rows were prepared by physical vapor deposition (PVD) of silver (∼ 0.25 angstrom/s) at an equivalent uniform film thickness of 2-7 nm onto highly oriented pyrolytic graphite (HOPG) surfaces at 400 °C. At these elevated temperatures, atomically deposited silver has a sufficiently high surface mobility to reorder into nanoparticles that associate to the graphite step edges. Both the average 15-60 nm nanoparticle sizes and 7-20 nm gaps between adjacent particles within the rows were controlled by the overall silver evaporation thickness. The rows were typically hundreds of microns long with 10-1000 nm spacing between adjacent 1D rows that correspond to the space between the graphite step edge spacing. Similar 1D gold and silver/gold hybrid nanoparticle rows have been prepared using the same method. The polarization dependence and the enhancement of the Raman scattered light was characterized using a monolayer of adsorbed thiophenol on the silver nanoparticle surfaces as Raman probe molecules. To understand this polarization selectivity, electromagnetic calculations modeled the response of the interacting nanoparticles with an incident light field, and the influence of particle size and gap spacing in the one dimensional arrays. Modeled Raman enhancement factors show consistency with the experimental results.;Following the PVD growth of Ag or Au nanoparticle arrays on HOPG surfaces, we electrochemically deposit cadmium around these nanoparticle arrays to form hybrid nanowires as thin as 50-60 nm in width. Further annealing of these hybrid nanowires in H2S at temperatures of 300-320 °C enables the formation of CdS nanowires around the Ag or Au nanoparticle cores. Using this combined PVD/electrochemistry/chemical modification approach we have been able to generate ordered 2-D arrays of hybrid semiconductor nanowires that are as small as 100 nm in diameter and 100s of microns in length. Under light illumination, the surface plasmon supported by the Ag or Au nanoparticle cores enhances the photoluminescence of the outer CdS nanowires.