Evaluation of substrates for surface-enhanced Raman scattering




Zhong, Muyang

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Surface-enhanced Raman scattering (SERS) has long been the interest of researchers in chemistry, physics and engineering, especially since the discovery that SERS can probe into the system down to the single molecule (SM) level. Despite the large number of publications regarding the fabrication of SERS substrates, it has been a challenge in the field to quantify the SERS signal and universally compare substrates. Traditionally, enhancement factor (EF) is used as an indicator of substrate quality, but the EF calculation is hugely dependent on the estimation of the surface coverage and other factors that are determined largely subjectively. Therefore, this thesis aims at discussing other parameters that can also be used to evaluate different substrates. Six different SERS substrates of Ag or Au nanoparticles of different sizes were fabricated by nanosphere lithography (NSL) and characterized by electron microscopy and UV-vis spectroscopy. SERS substrates were mapped for different concentrations of a probe molecule. Through subsequent baseline correction and principle component analysis (PCA), the "intensity" of individual spectrum was obtained and the shapes of intensity histograms of each substrate were acquired. Instead of calculating EF, five criteria (six quantification methods in total) were employed to comprehensively evaluate the six substrates. These were density of hot spots (characterized by the number of zero-intensity events), enhancement (represented by mean intensity), spatial variation (calculated by RSD of intensity), repeatability (realized by cross correlation) and histogram shape (quantified by skewness and kurtosis). These new methods provide insights to the understanding of the properties of SERS substrates in terms of hot spots. Different substrates may exhibit better performance in terms of one criterion but worse in terms of others. Those variations in performance can be explained by their surface morphology. These more elaborated methods are believed to provide a more comprehensive approach to evaluate and compare substrates than the traditional EF values. The thesis also paves the way for future study on SM-SERS and fabricating better SERS substrates.



surface-enhanced Raman scattering, SERS substrates