Vibrational Imaging at the Nanoscale: Surpassing the Diffraction Limit Using Tip-Enhanced Raman Spectroscopy

A deep understanding of the chemical composition of surfaces, interfaces or nanoscale structure with a high spatial resolution is an important goal in nanoscience and nanotechnology. Structural information can be collected using a variety of high spatial resolution techniques such as atomic force microscopy (AFM), scanning tunneling microscopy (STM), scanning electron microscopy (SEM), or transmission electron microscopy (TEM). Nevertheless, these methods do not offer molecular information such as vibrational spectroscopy techniques that allow one to collect molecular or lattice vibrations yielding to a precise picture of the molecular interactions in bulk materials as well as in surfaces and interfaces. Unfortunately optical spectroscopy techniques are limited in terms of spatial resolution and sensitivity due to the poor signal/noise ratio of the localized measurement. Surface- and tip-enhanced Raman spectroscopy (SERS and TERS) are advanced spectroscopic techniques, which are becoming widely used and show a great potential for the structural characterisation of biological systems. Surface-enhanced spectroscopy (SERS) was developed to improve the sensitivity of the chemical measurements by using rough silver or gold surfaces. The challenge of the simultaneous improvement of the spatial resolution and sensitivity was addressed by combining high resolution optical microscopy with the high sensitivity of surface-enhanced spectroscopy and was termed tip-enhanced Raman spectroscopy (TERS). In this thesis, gap-mode TERS is developed for the study of a variety of materials. TERS is used in conjunction with gold nanoplates to serve as an ultraflat substrate that can possibly be functionalized. TERS investigation of monolayers adsorbed onto gold nanoplates such as alkoxy substituted azobenzene thiol and 4-nitrothiophenol is conducted. The monolayer is probed with a silver coated AFM tip in order to obtain the largest electromagnetic field enhancement and the effect of the excitation (linearly or radially polarized) is conducted. TERS is also used to probe graphene flakes and differentiate the edges of a few-layer graphene flakes with a spatial resolution better than 20 nm. Last, TERS was used to investigate single DNA molecules deposited onto gold nanoplates. The DNA, cDNA and pure plasmid were investigated with TERS probing the distribution of nucleobases at a specific location with a spatial resolution which was, in the best conditions below 10 nm.

Name thesis:degree_name

Ph D

Discipline thesis:degree_discipline

Chemistry

Grantor dc:publisher

The University of Western Ontario

Year dc:date.issued

2015

Author dc:creator

• Pashaee, Farshid

Advisor dc:contributor.advisor

• Francois Lagugné-Labarthet

• Tip-enhanced Raman spectroscopy
• atomic force microscopy
• localized surface plasmon resonances
• near-field measurements
• spatial resolution
• radial and linear polarization

Language dc:language.iso

en_ca

Handle dc:identifier.uri

https://hdl.handle.net/20.500.14721/33455

OAI identifier oai:identifier

oai:uwo.scholaris.ca:20.500.14721/33455