Substrates for surface-enhanced Raman spectroscopy

Surface enhanced Raman spectroscopy (SERS) utilizes surface plasmon resonance in metallic nanostructures to yield nearly a millionfold increase in the Raman signal of an analyte. The unprecedented sensitivity and specificity of SERS has great potential for applications in analytical chemistry and biological sensors. SERS is inherently a nanoscale phenomenon and recent advances in nanotechnology have generated an immense opportunity for the use of nanoparticles, nanowires and nanorods as substrates for SERS. This dissertation explores different aspects of utilizing bulk synthesized germanium oxide, zinc oxide nanowires, and metallic nanorods as substrates for SERS. It discusses the synthesis details and growth kinetics for the oxide nanowires and metallic nanorods. The germanium nanowire growth is carried out in a simple tube furnace chamber using two different temperature regimes - the traditionally high temperature synthesis and a novel low temperature synthesis. The high temperature synthesis (∼850°C) does not yield good control over length and diameter of the nanowires. The new low temperature synthesis technique overcomes these limitations by maintaining the source temperature at ∼650°C.;Zinc oxide nanowires are synthesized by thermal evaporation of zinc powder in an oxidizing environment. The as-synthesized bulk nanowires act as nanostructured template that is coated with a thin gold film to create a plasmon active surface for SERS application. In addition metallic nanorods of aluminum, copper, and silver were synthesized on glass slides in an e-beam equipped physical vapor deposition system using oblique angle deposition technique. Structural and chemical compositional characterization of the SERS substrates was done using a scanning electron microscope, transmission electron microscope and energy dispersive X-ray spectroscopy (EDS). The SERS performance is evaluated using model analytes of 4-methylbenzenthiol, 1,2-benzenedithiol and trans-1,2-bis(4-pyridyl)ethylene. The substrates yield strong and unambiguous Raman spectra from just a monolayer or few femto moles of analyte. The Raman enhancement factors are computed by comparing the intensity of Raman signal from SERS substrate to that from the bulk analyte. The metallic nanorods substrates show a metal dependent SERS enhancement with silver yielding an order of magnitude stronger enhancement compared to other metals. The oxide nanowires based substrates show an average Raman enhancement factor of ∼106 with good reproducibility of signal over the tested area. The dissertation puts in to perspective how noble metal coated germanium oxide and zinc oxide nanowires can be used as robust SERS platform for detection of trace levels of chemical species.