High density periodic metal nanopyramids for surface enhanced raman spectroscopy


Jin, Mingliang (2012) High density periodic metal nanopyramids for surface enhanced raman spectroscopy. thesis.

Abstract:The work presented in this thesis is focused on two areas. First, a new type of nanotextured noble-metal surface has been developed. The new nanotextured surface is expected to enhance Raman scattering, called surface enhanced Raman scattering (SERS), from molecules absorbed on the surface due to large electromagnetic fields created in nanoscale gaps on the nanotextured metal surfaces by a laser excitation source. By collecting and analyzing the enhanced Raman (inelastically) scattered photons, the molecular bond information can be identified using methods from conventional Raman spectroscopy. Raman spectroscopy is very powerful analytical method in chemistry, biology and other scientific areas, since it provides molecular vibrational information, which is considered as fingerprint the molecule and material.
However, Raman spectroscopy is less commonly used compared to infrared (IR)
spectroscopy, for example, due to its extremely weak signal (approximately 1 in 107 photons is inelastically scattered). Over the past few decades, there has been a dramatic increase in Raman spectroscopy based on SERS since it can provide huge Raman scattered intensity enhancements commonly reaching million-fold levels and even billion-fold increases. A good SERS substrate should provide both large Raman enhancements (~106-108) over large areas (~mm2) in order to be used as an analytical measurement technique. Four important aspects should be considered when developing a new SERS substrate: i. the first and most important is that the nanoscale gaps should be a made from a noble metal (e.g. Ag and Au), when using laser sources in the visible spectrum, with gap distances less than 5nm ii. the substrate should contain a high density of nanogaps (nanogap/cm2) with homogeneous spatial distribution, iii. the geometric alignment of the nanogaps to the excitation laser polarization should be well-controlled in order to maximize the generation of the local surface plasmon, and iv. The nanogap should be easily accessible for molecular diffusion into the nanogap region.
Based on the description above we have developed a general technique to manufacture high density nanopyramid (NPy) and nanogroove (NG) array templates from (100) silicon and subsequently coated with thin layers of polycrystalline gold. Small pitch NPy arrays form spontaneously using anisotropic wet etching silicon following lithographic patterning of an etching mask. A sharp nanocrevice gap is created between two adjacent pyramids and is used to couple laser excitation into a local surface plasmon. The size and density of these nanocrevices are determined by the nanolithography dimensions and etching time. We have studied the behavior of the gold NPy surfaces using a combination of the numerical modeling, reflection spectroscopy and Raman spectroscopy. Reflection spectroscopy provides information about the coupling of the optical laser excitation into a local surface plasmon resonance that includes resonant coupling energy, metal interband transition effects, dielectric interface dependent resonant energy, and polarization alignment effects. Raman spectroscopy allows us to probe the enhancement properties of the gold NPy surface and extensive studies of the enhancement behavior of the NPy surfaces using surface adsorbed rhodamine-6G in water and monolayers of chemisorbed benzenethiol have been performed.
Item Type:Thesis
Electrical Engineering, Mathematics and Computer Science (EEMCS)
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Link to this item:http://purl.utwente.nl/publications/79373
Official URL:http://dx.doi.org/10.3990/1.9789036533225
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