UC Berkeley

This dissertation presents new advancements in the theory, design, and fabrication of nanoplasmonic structures for molecular detection.

After introducing nanoplasmonics, I ask a deceivingly simple question: are the various commonly used measures of localized surface plasmon resonance (LSPR) really equivalent? I then develop a theoretical framework that proves that important differences occur among the measures of LSPR. Specifically, the optimum wavelength for maximizing the near field can be significantly redshifted compared to the optimum wavelength for far field scattering, which can in turn be significantly redshifted compared to the optimum wavelength for absorption. I show how these differences are dependent upon critical parameters, such as the material, the particle size, and particle shape. This theory, based on the quasi-static approximation, is also verified rigorously using using Mie theory and finite element analysis. The implications on design for molecular detection are discussed.

I then focus on fabrication techniques, developing new tools which can control the geometry and interparticle distance of nanoplasmonic antenna initially fabricated with nanosphere lithography. I introduce active polymer fabrication and electroless metal deposition as two methods that can significantly decrease the nanogaps in nanoplasmonic systems and create enhancements in spectroscopic signal for molecular detection.

Next, I investigate the issue of nanoplasmonic geometry in great detail for four structures, with a focus on new systems in which the close proximity of two or more nanoplasmonic components causes their resonances to be linked together (i.e. plasmon-coupling). I demonstrate the coupling in these structures can cause high electric fields ideal for molecular detection. Four such structures are discussed: the nanocrescent, the crescent-shaped nanohole, the plasmon enhanced particle-cavity (PEP-C) system, and the core-satellite system. The nanocrescent and its counterpart, the crescent-shaped nanohole, utilize both sharp tips and intra-particle coupling to achieve high local field enhancement. The PEP-C system combines particle and cavity geometries to create a high density of hot-spots. The core-satellite system is based on the interaction of a central nanoparticle with multiple smaller particles.

Finally, I introduce a special class of bioinspired nanoplasmonic structures called nanocorals, designed for molecular sensing within or on the surface of living cells. Nanocorals are unique in that they combine cellular specific targeting with biomolecular sensing, yet decouple the two functional modes. Analogous to natural sea corals that use rough surfaces to maximize surface area for efficient capture of light and food particles, nanocorals utilize a highly roughened surface at the nanoscale to increase analyte adsorption capacity, and create a high density of surface-enhanced Raman spectroscopy (SERS) hot-spots.