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 in ultrahigh vacuum (UHV) conditions. The metal FON surface is a materials general, cost-effective, and highly SERS-active surface. The SERS activity and thermal stability were investigated by adsorbing benzene, pyridine, and C-60 onto the AgFON surface. We chose these adsorbates for the following reasons: (1) vibrational spectroscopy and temperature-programmed desorption (TPD) behavior of benzene adsorbed onto metal surfaces has been widely investigated 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 can be performed. TPD demonstrates that the AgFON surface has two classes of adsorption sites: (1) those that mimic the behavior of single crystal surfaces and (2) defect sites with higher adsorbate binding energies. Room temperature annealing does not irreversibly destroy the SERS enhancement capability of this surface, thereby permitting for repeated use in UHV experiments. The AgFON surface morphology and localized surface plasmon resonance frequencies, as monitored by UV-vis extinction, change as the AgFON surface temperatures increases from 300 to 548 K, and the SERS activity corresponds with these changes. Because the AgFON surface is thermally stable at room temperature and retains high SERS-activity following temperature annealing to 573 K, it is unlikely that adatoms or adatom clusters play a significant role as adsorption sites supporting the chemical enhancement mechanism. Rather, one can conclude that the electromagnetic enhancement mechanism is the most likely origin of the SER spectra from benzene, pyridine, and C-60 adsorbed on AgFON surfaces.