From Single to Multiple Ag-Layer Modification of Au Nanocavity Substrates: A Tunable Probe of the Chemical Surface-Enhanced Raman Scattering Mechanism

Abstract

We present experimental and computational results that enlighten the mechanisms underlying the chemical contribution to surface-enhanced Raman scattering (SERS). Gold void metallic arrays electrochemically covered either by a Ag monolayer or 10-100 Ag layers were modified with a self-assembled monolayer of 4-mercaptopyridine as a molecular Raman probe displaying a rich and unexpected Raman response. A resonant increase of the Raman intensity in the red part of the spectrum is observed that cannot be related to plasmon excitations of the cavity-array. Notably, we find an additional 10-20 time increase of the SERS amplification upon deposition of a single Ag layer on the Au substrate, which is, however, almost quenched upon deposition of 10 atomic layers. Further deposition of 100 atomic Ag layers results in a new increase of the SERS signal, consistent with the improved plasmonic efficiency of Ag bulk-like structures. The SERS response as a function of the Ag layer thickness is analyzed in terms of ab initio calculations and a microscopic model for the SERS chemical mechanism based on a resonant charge transfer process between the molecular HOMO state and the Fermi level in the metal surface. We find that a rearrangement of the electronic charge density related to the presence of the Ag monolayer in the Au/Ag/molecule complex causes an increase in the distance between the HOMO center of charge and the metallic image plane that is responsible for the variation of Raman enhancement between the studied substrates. Our results provide a general platform for studying the chemical contribution to SERS, and for enhancing the Raman efficiency of tailored Au-SERS templates through electrochemical modification with Ag films.

Continuous flow synthesis of ultrasmall gold nanoparticles in a microreactor using trisodium citrate and their SERS performance

“…The enhancement of the signals is mainly caused by two effects: electromagnetic (EM) enhancement and a chemical contribution (Osawa and Ikeda, 1991). Compared with EM enhancement, the charge transfer-based chemical contribution provides a much smaller enhancement and the actual mechanism is still in dispute (Otto, 2005;Otto et al, 1980;Tognalli et al, 2011). The electromagnetic enhancement utilizes the LSPR of nanostructures to form incident electric field which could drive the conduction electrons.…”

Synergetic Light-Harvesting and Near-Field Enhancement in Multiscale Patterned Gold Substrates

“…The sample is mounted on a micrometer stage, allowing progressive withdrawing from the solution during the deposition by steps of 500 µm, yielding discrete levels of truncation parameters for a given sample. 56 The polystyrene spheres are eventually removed via sonication in a sequence of solvents (dimethylformamide, acetone, ethanol and water) for 2 minutes per step.…”

ZnO/Ag porous nanosheets used as substrate for surface-enhanced Raman scattering to detect organic pollutant

“…[8][9][10] For many years, noble metals like Au, Ag, Cu nanostructures were used as traditional SERS substrates, due to the electromagnetic enhancement caused by surface plasmon resonance mechanism of the metal. [11][12][13][14][15][16] Recently, it was found that many semiconductors, such as TiO 2 , Fe 3 O 4 , ZnO, also possess SERS activity. [17][18][19] Particularly, ZnO, as an important semiconductor material with high chemical stability, has a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV.…”