Undergraduate Honors Theses

Thesis Defended

Spring 2012

Document Type

Thesis

Department

Physics

First Advisor

Prof. Markus Raschke

Abstract

Understanding chemical properties and their physical mechanisms at the level of molecules is of fundamental scienti c interest and is important for the development and optimization of applications in a broad range of elds such as catalysis, photovoltaics, and biology. Room-temperature tip-enhanced Raman spectroscopy (TERS) is used for its ability toprobe molecular vibrational modes on the nanoscale. The organic dye molecule, malachite green (MG) and others, are deposited onto an evaporated gold substrate. The molecular dyes are then resonantly photoexcited to their S1 excited states by a 633 nm continuous wave Helium-Neon laser with the help of an enhancement from a nanoantenna situatedwithin nanometers of the molecules. The Raman response of the dye molecules is enhanced and spectrally resolved at room-temperature. Future e orts will be to compare these measurements at variable and liquid-nitrogen temperatures by combining a room-temperature TERS system with a characterized low-temperature vacuum chamber. Future low-temperature measurements should encompass changes in linewidth of the Raman spectrum, which depends on the lifetime of the vibrational excitation. Vanishing Raman peaks and shifts in peaks, due to vibrational freezing and mode constriction. More interesting at low-temperatures is the e ect on single to few-molecule's randomly switching between bright and dark states. These e ects give vital information about the e ect temperatures have on relaxation and decay pathways in molecules.

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