| This work focuses on the development and use of nonlinear optical techniques to study graphene surfaces in order to provide fundamental molecular level insight for guiding the advancement of graphene-based technologies. Specifically, using a combination of sum frequency generation (SFG) and second harmonic generation (SHG) this work examines the structure, molecular orientation, and the adsorption of liquid organic and aqueous phase species specific to the fluid/solid interface over various carbon substrates.;First, time-delayed SFG was optimized to resolve vibrational signatures of toluene adsorbed onto highly ordered pyrolytic graphite (HOPG) at the air/solid interface. Coupled with the polarization null angle method, the molecular tilt angle of the toluene methyl C3v axis from surface normal was calculated, thereby determining its interface-specific molecular orientation.;Second, the aqueous/single layer graphene interface was investigated with SHG. The chi(3) technique was applied to compare the effective surface charge density and magnesium ion adsorption at the aqueous/graphene/fused silica interface to the aqueous/bare fused silica interface. The presence of pristine or defected single layer graphene films was found to have a minimal influence on the electrical double layer. Additional SHG studies were carried out to evaluate how a single layer graphene sheet acts as a barrier or membrane. To this end, the acid-base chemistry at a silica surface was investigated with and without graphene present between the aqueous and solid phases. Changes in the SHG signal intensity as a function of bulk solution pH indicates that protonation and deprotonation of surface silanol groups occur in an unimpeded fashion despite the presence of a single layer of graphene. Along with SHG experiments, Raman spectroscopy, scanning electron microscopy, and scanning transmission electron microscopy are used to pinpoint a possible route for proton diffusion through the graphene sheet.;Finally, SFG was used to investigate the structure of the organic solvent/graphene/alpha-alumina interface. A binary mixture of 1-hexanol and cyclohexane-d12 was utilized to investigate organic solvent interactions at an oxide interface in the presence and absence of pristine and defected single layer graphene films. Similar to the aqueous/graphene systems, these results demonstrate that the underlying support substrate regulates the electrostatic field at the interface. |
