Silver nanorod array substrates fabricated by oblique angle deposition: Morphological, optical, and SERS characterizations

Silver nanorod arrays with different lengths fabricated by oblique angle deposition at various vapor deposition angles have been studied systematically on their morphologies, optical reflections, and surface-enhanced Raman scattering (SERS) responses. The tilting angle β of Ag nanorods increases with the increase of the deposition angle θ, while the diameter D and density n of Ag nanorods grow as power laws of the length L, D ∼ L ^pand n ∼ L^-γ, where the exponents are p ∼ 0.30-0.34 and γ ∼ 0.23-0.40 for different deposition angles, respectively. The optical reflectance from these substrates depends not only on the length of the Ag nanorods but also on the deposition angle. It is found that the SERS enhancement factor decreases nearly monotonically with the increase of the reflectance at SERS excitation wavelength, and the highest SERS enhancement factor can reach close to 10⁹. We have developed a phenomenological model based on the assumption that the absorbance (reflection) of the nanorod array is directly linked to the local electric field, and it predicts a similar trend to that of the experimental observation. The empirical results can help us to design better Ag nanorod array SERS substrates, and can also be used as a quality control measurement method for SERS substrate production.