Surface-enhanced Raman scattering detected temperature programmed desorption: Optical properties, nanostructure, and stability of silver film over SiO2 nanosphere surfaces

In this work, silver film over silica nanosphere (AgFON) surfaces are shown to be thermally stable, SERS-active substrates that are suitable for use i n ultrahigh vacuum (UHV) conditions. The metal FON surface is a materials g eneral, cost-effective, and highly SERS-active surface. The SERS activity a nd thermal stability were investigated by adsorbing benzene, pyridine, and C-60 onto the AgFON surface. We chose these adsorbates for the following re asons: (1) vibrational spectroscopy and temperature-programmed desorption ( TPD) behavior of benzene adsorbed onto metal surfaces has been widely inves tigated and is a simple system to study, respectively; (2) characteristics of pyridine adsorption on the AgFON surface can be compared to a large body of previous studies; and (3) high-temperature studies of C-60 adsorption c an be performed. TPD demonstrates that the AgFON surface has two classes of adsorption sites: (1) those that mimic the behavior of single crystal surf aces and (2) defect sites with higher adsorbate binding energies. Room temp erature annealing does not irreversibly destroy the SERS enhancement capabi lity of this surface, thereby permitting for repeated use in UHV experiment s. The AgFON surface morphology and localized surface plasmon resonance fre quencies, as monitored by UV-vis extinction, change as the AgFON surface te mperatures increases from 300 to 548 K, and the SERS activity corresponds w ith these changes. Because the AgFON surface is thermally stable at room te mperature and retains high SERS-activity following temperature annealing to 573 K, it is unlikely that adatoms or adatom clusters play a significant r ole as adsorption sites supporting the chemical enhancement mechanism. Rath er, one can conclude that the electromagnetic enhancement mechanism is the most likely origin of the SER spectra from benzene, pyridine, and C-60 adso rbed on AgFON surfaces.