How SERS responses of probe molecules depend on topographies of the substrates? A vis-a-vis exploration

The present paper reflects on the results of sytematic studies that link SERS responses of probe molecules with the topographical features of three different SERS active substrates prepared through integration of Langmuir-Blodgett and self-assembly technique. The topographies of the substrates are mapped directly from the AFM images. Relevant topographical parameters of three efficient SERS active substrates are elaborately investigated in terms of lateral correlation length, the interface width, Hurst and Lyapunov exponents. The lateral correlation lengths have been estimated from auto-correlation function, while the interface width and respective values of Hurst exponents are explored from the height-height correlation function. The comparative studies on all the three substrates suggest that the lateral correlation length in the range 136-286 nm favour SERS activities. Estimations of Hurst exponents >0.5 for all the above referred three substrates further signify prevalence of long range correlations driven by super diffusion of the nanocolloids on the respective LB film substrates. Moreover, all the substrates exhibit fractal nature of the plasmonic aggregates that may favour the suitable architecture responsible for SERS enhancements. The chaotic behaviours of the as prepared substrates have also been estimated from Lyapunov exponents and 2D phase space trajectories. The paper thus essentially provides a one to one mapping between SERS spectroscopy at the molecular scale with the corresponding microscopy of the substrates wherein the probe molecules are adsorbed. These information may find relevance in the pragmatic fabrication of future SERS active substrates for sensing and diagnostic applications.