Self-assembled metal-dielectric metasurfaces produced by pulsed laser deposition for SERS detection

Abstract

Metasurfaces consisting of arrays of Au or Ag nanoparticles (NPs) on dielectric surfaces show unique optical properties related to the excitation of surface plasmon resonances (SPRs) that play a key role in the enhancement of fluorescence signals of active molecules, nanostructures, or surface Raman enhanced spectroscopy (SERS). In particular, SERS is becoming a key tool in areas such as molecular characterization, sensing, or detection of contaminants among others. However, a major drawback for the widespread use of SERS has been the lack of reliable practical techniques to produce solid substrates that could be used in sensors. This has frequently been related to the irreproducibility associated with the methods used to immobilize colloidal solutions of NPs into solid substrates. Therefore, a widespread use of SERS in these applications requires the development of reproducible SERS substrates having metal nanostructures with characteristic dimensions in the range of a few nanometers to tens of nanometers, and thus, different top-down and bottom-up approaches have been attempted. In this work we apply sequential pulsed laser deposition (PLD) to the synthesis of efficient SERS substrates consisting of Au NPs-Al2O3 metasurfaces. The as-deposited metasurfaces do not require any post-deposition treatment to show SERS activity, since PLD allows a precise control over the amount of deposited metal and an adequate range of interparticle spacing to enhance the SERS response. The characteristic Au NP size is close to 10 nm whereas the interparticle distance is in the 10-20 nm range. We compared the sensitivity in SERS of the produced metasurfaces with that of commercial ones produced by Langmuir-Schaefer technique and Klarite® substrates using the pesticide paraquat as a test molecule. The effect of the deposition time and the presence of Al2O3 on metasurface were tested by investigating the SERS efficiency when using the adenine molecule as a reference standard. The spectral differences observed between adenine deposited on a flat surface of Si and that of a PLD substrate and the SERS enhancement clearly indicates the existence of a SERS effect.