Surface-enhanced Raman scattering from silver nanorod arrays fabricated by oblique angle deposition
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Surface-enhanced Raman scattering (SERS) has been proven to be a promising and powerful analytical tool in environment monitoring, chemical and biological sensing, disease diagnosing and homeland security checking. This dissertation focuses on the studies on a new SERS substrate platform --- Ag nanorod arrays. We have fabricated Ag nanorod array substrates with different length at various deposition angles using oblique angle deposition (OAD) and the detailed structural characterizations have been performed for these samples. Semi-ordered Ag nanorod arrays have also been fabricated using template OAD method combining with electron beam lithography method. For Ag nanorod array substrates with a fixed stucture, SERS characterizations related to the excitation configuration have been systematically investigated. The SERS intensity strongly depends on the laser incident angle, polarization states and the reflectance from the underlayer of substrates. In order to understand these unique SERS properties, a modified Greenler’s model has been proposed. The theoretical calculations from this model can qualitatively explain these SERS properties. The SERS activity is also strongly dependent on the specific structures, such as length, diameter, and tilted angle of Ag nanorod and so on. For a fixed tilted angle, there exists an optimum length of Ag nanorod for SERS activity. At the same length, larger SERS intensity can be obtained from a larger tilted angle of Ag nanorods. With the increase of the diameter of Ag nanorods, the SERS intensity from template Ag nanorods decreases when the diameter of Ag nanorod is larger than 100 nm. To understand the SERS mechanism, the origin of SERS from Ag nanorod array has been investigated. Due to the anisotropic absorbance nature of Ag nanorod layer, our experiments indicate that most of SERS signal come from the molecules adsorbed on the side surface of Ag nanorods, not from the so called “hot spots” at the corner of between Ag nanorods and Ag film. We believe the Ag nanorod absorbance as a function of the thickness plays a critical role.