Coupled second-harmonic generation, surface plasmon resonance and AC impedance studies of full and partial monolayers in (Au,Ag)-alkanethiolate electrolyte systems

Optical second-harmonic generation (SHG) has been coupled with surface plasmon resonance (SPR) and AC impedance measurements to probe Ag and Au electrolyte interfaces in the presence of full and partial alkanethiolate self-assembled monolayers (SAMs) at potentials within the double layer charging region. Both SHG and AC impedance measurements of the Ag/alkanethiolate/electrolyte systems have beer performed for alkanethiols with different chain lengths (C_nH_2n+1SH, n = 8, 12, 16). The capacitance of the alkanethiolate SAMs is found to vary inversely with alkanethiol chain length, as expected, permitting direct determination of the dielectric constants for all films. The potential dependence of the second harmonic (SH) signal from metal/SAM/electrolyte systems is found to be much weaker than that from bare Ag or Au, a result explained in terms of a series capacitor model for the SAM-double layer combination. The coverage dependent SH signal intensity from Au/alkanethiolate/electrolyte systems has been measured. Ozonolysis is used to obtain partially covered hexedecanethiolate monolayer on Au surfaces, with the coverage being controlled by the O₃ exposure time and monitored by shifts in the surface plasmon resonance (SPR) position. A linear relationship between the film coverage and SH intensity has been observed with SH response increasing with decreasing coverage. The SH behavior for partial monolayers is dominated by the change in the DC electric field caused by SAM formation.