Title: Nanoscale Chemical Interrogation of Surfaces using Tip-Enhanced Near-field Optical Microscopy (TENOM)
Advisor: Michael Gordon
As our ability to engineer, design, and modify systems with nanoscale features continues to advance, characterization methods need to keep pace. Light-matter interactions, in particular optical spectroscopy, provide a wealth of information on the vibrational and electronic structure of matter that can be directly related to physical properties such as ordering, phase, chemistry, charge transport, etc. However, the fundamental wave-like nature of light prevents radiation from being focused to arbitrarily small length scales using traditional optics. This is known as the diffraction limit and is on the order of several hundred nanometers for optical wavelengths (~λ/2). One method of overcoming diffraction is to couple light to nanostructures (optical antennae) that support resonant oscillations of conduction electrons (plasmons). These charge oscillations generate intense optical fields that are spatially controlled by the antenna size, rather than the radiation wavelength. The set of related techniques utilizing this mechanism are collectively referred to as tip-enhanced near-field optical microscopy (TENOM).
This work details the design, construction, and experimental validation of a TENOM instrument for applications in near-field spectroscopy and super-resolution imaging. Chemical maps of patterned metal-free phthalocyanine films were obtained with simultaneous collection of fluorescence, Raman, and topographic data, all with lateral spatial resolutions below 50 nm (<λ/10). Additionally, finite-difference time-domain optical simulations were used to study the fundamental physics of plasmonic antennae with the goal of maximizing local electric field strengths. A quantitative comparison of several antenna designs was carried out, which has not been possible experimentally due to poor reproducibility in fabrication procedures and variability in methods of measuring optical near-fields. It is shown that designs with conical resonant cavities at their apex support intense localized surface plasmon resonances that may allow TENOM signals to be improved by 1-2 orders of magnitude, greatly extending the applicability of the technique as a general means of surface characterization.