Subwavelength metal nanogap surface for surface-enhanced Raman spectroscopy

Loan Le Thi Ngoc

Research output: ThesisPhD Thesis - Research UT, graduation UT

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Abstract

In this dissertation, a new top-down nanofabrication echnology is presented to realize large area metal nanowire rrays with tunable sub-20 nm separation nanogaps without the use of chemical etching or milling of the metal layer. Gold and silver nanowire arrays are presented with high-density on the order of 105 cm−1, variable diameters down to 50 nm, variable nanogaps down to 5 nm, and very large nanogap length density on the order of 1 km cm−2. A key point of this structure is that the coupledmode localized surface plasmon resonance wavelength in the nanogaps between adjacent nanowire array dimers can be precisely tuned to match the wavelengths of common laser sources (532 nm, 632.8 nm, and 785 nm), with a spatially averaged surface-enhanced Raman scattering (SERS) analytical enhancement factor in the range of 107 to 108. The SERS substrates provide large, uniform, and reproducible SERS electromagnetic field enhancements over a large area, which is suitable for analytical applications, and have been used for several types of measurements in the thesis. First, silver nanowire array surfaces have been used as a Raman surface spectroscopy tool to study the surface conformation characteristics of amine terminated alkanethiol self-assembled monolayers (SAMs). The surface conformation has implications for amine group availability, which is important when SAMs are used as linking layers for the binding of secondary molecules, which is particularly important for biosensing and medical diagnostics applications. Second, Au-nanowires array surfaces were used to study a broad background continuum accompanying SERS spectra, and have been found that plasmon-modulated photoluminescence is the main contributor to the SERS background. Finally, an integrated SERS spectroelectrochemical (SEC) analysis system has been developed that combines a small volume microfluidic sample chamber with a compact three-electrode configuration for in situ surface-enhanced Raman spectroelectrochemistry. The SEC system includes a gold nanowire array surface that serves dual roles as the electrochemical working electrode and a SERS-active surface, a microfabricated platinum counter electrode, and an external Ag/AgCl reference electrode. In situ SERS spectroelectrochemistry measurements of the talloporphyrin hemin are demonstrated showing shifts of the oxidation marker band of the iron core with the gold nanowire working electrode under potential control.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • University of Twente
Supervisors/Advisors
  • van den Berg, Albert, Supervisor
  • Carlen, Edwin T., Advisor
Thesis sponsors
Award date22 May 2015
Place of PublicationEnschede
Publisher
Print ISBNs978-90-365-3861-9
DOIs
Publication statusPublished - 22 May 2015

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