Surface Enhanced Raman Spectroscopy: silver/gold nanoparticles deposited on graphene monolayers

Surface Enhanced Raman Spectroscopy (SERS) seems to be one of the most powerful analytical tools for monitoring of environmental contaminants at low concentrations. Increasing the intensity of Raman signal has been regularly... [ view full abstract ]

Surface Enhanced Raman Spectroscopy (SERS) seems to be one of the most powerful analytical tools for monitoring of environmental contaminants at low concentrations. Increasing the intensity of Raman signal has been regularly observed to be as high as 10¹⁴ for optimized systems, reaching up to single molecule detection level. 

We present results of SERS study using two excitation wavelengths observed primarily for analytes adsorbed onto silver and gold nanoparticles deposited on monolayered graphene.

The metals provide localized surface plasmon resonance, considerably increasing local electromagnetic field and combination with graphene based materials are expected better SERS performances due to concentration effect through a π-π stacking.

Graphene monolayer was prepared by the conventional CVD technique on a Cu substrate (CH₄/H₂, 800 °C). The Cu/graphene sheet was then accommodated in a glass reactor and the surface was covered by Ag or Au nanoparticles. The laser ablation (ArF excimer laser, 193 nm, 100 mJ/pulse) of elemental metal targets was used to cover graphene surface by nanoparticles with diameters up to 25 nm. The deposition process was optimized (deposition time, pressure of a background gas) to obtain the best SERS performance. Deposition in vacuum was found to afford completely graphitized samples due to excess energy of the metal atoms/clusters, reaching the graphene surface. The deposition of metal nanoparticles was therefore conducted at pressures 2 - 15 Pa of helium. At 10 Pa the metal nanoparticles were deposited onto the graphene surface without its graphitization.

The prepared SERS substrates were characterized by scanning and transmission electron microscopies, electron and x-ray diffractions and by spectroscopy techniques (UV-Vis, FTIR, Raman and XPS). SERS studies were performed using excitation wavelengths 473 nm (Ag based substrates) and 532 nm (Au based substrates). Raman spectra of the model compounds (Rhodamine 6G, Fluoresceine, methylene blue, aminothiophenole) will be presented.