AFM-based Nanolithography Beyond Standard Diamond Nanoindenters: A Paradigm for Low-Cost Nanoindentation-Based Fabrication of SERS-Active Surfaces

Abstract

Nanolithography techniques based on laser interference, electrons, or ion beams have been widely extended for fabricating versatile nanostructured systems with applications in modern Surface-Enhanced Raman Spectroscopy (SERS)-based molecular sensors. Techniques like Focused Ion Beam (FIB) milling have been used for high-resolution nanolithography, representing a direct-writing and chemicals-free alternative. In recent years, Depth-Sensing Nano Indentation (DSNI), a low-cost technique presenting both advantages, has been explored as an emergent nanolithography tool for fabricating SERS-active nanostructured surfaces. In this work, we report an original fundamental and significant research that validates the use of low-cost Atomic Force Microscopy (AFM) probes, in contrast to standard diamond AFM-nanoindentation ones, for fabricating functional SERS-active surfaces by AFM-assisted DSNI. Both kinds of probes were used for fabricating AFM-nanostructured surfaces on 50 [nm]-thick Au thin films, which consisted of 20 × 20 nanoindentation arrays with a period of 300 [nm]. Methylene Blue (MB) was used as a Raman marker for preliminary studies on SERS activity, observing the best Raman signal enhancement for this period in a range from 300 to 1500 [nm]. The nanostructured surfaces presented considerably higher SERS enhancement factors (EFSERS) than simple Au thin film surfaces ( 1 × 102) for both probes. Low-cost AFM probes produced more efficient SERS-active surfaces than standard AFM-nanoindentation ones, achieving a 1 × 104 compared to 4 × 103 and improved spectral resolution of the target molecule vibrational modes. These surfaces were also tested using other analytes like Rhodamine 6G (R6G) and Nile Blue A (NB-A), observing similar advantages concerning the SERS activity. Our results set an interesting paradigm on the applications of low-cost AFM probes for DSNI-based nanolithography and their potential for the fabrication of efficient SERS-active surfaces, representing a substantial advance towards the replacement of conventional high-cost diamond AFM-nanoindentation probes and maximization of efficiency-cost ratios for this emerging technique.