Enhanced Raman Scattering On Nanostructures From Nanowire SERS Sensor To Double-tip TERS Spectroscopy

Plasmons are quasiparticles resulting from the quantization of collective oscillations of the free charges in a conductive medium, and surface plasmons are plasmons which are confined to the surface of the conductive medium. The surface-plasmon polaritons (SPP) are quasipartcles formed when the suface plasmons are coupled with photon field. If the surface of a metal exhibits tiny structures or the size of a metal partcle is much smaller than the wavelength of the excitation light, the excited surface plasmons will be bounded to the tiny structures of the metal or the metal particle, leading to the formation of localized surface plasmons (LSPs). The LSPs can be excited by light and, in a resonant condition, a vast locally enhanced electromagnetic field can be created in the vicinity of the nanometer sized metal structures. Localized surface plasmon resonance (LSPR) on nanometer sized metal structures has many usages, of which two important ones are surface-enhanced Raman scattering (SERS) and tip-enhanced Raman scattering (TERS). The mechanisms of SERS can be ascribed to the electromagnetic enhancement part and the chemical enhancement part. For the former one, the locally enhanced electromagnetic field coherently amplifies both the excitation and the emission of absorbed molecules, resulting in the enhancement of the Raman signal. TERS has an advantage over the SERS in the sense that it not only provides the Raman signal enhancement with presence of the sharp tip, but also provides high spatial resolution topographic and near-field optical informations simultaneously.The thesis is organized as follows. Chapter 1 presents the introduction to the concepts of plasmons and SPPs with examples. Chapter 2 discusses SERS related issues and gives details about the polarization dependence of SERS on gold nanoparticle-decorated nanowire arrays. Chapter 3 describes TERS related issues with an emphasis on the technology developments. Finally, in Chapter 4, an STM study of gold surfaces and gold surface reconstructions due to particle depositions and molecule adsorptions is presented.