Sum Frequency Generation Imaging of Alkanethiol-covered Copper and Alloys: Spatial Distribution Analysis of Monolayer Order
Santos, Greggy M. 1983-
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The utilization of the sum frequency generation imaging microscope (SFGIM) to study heterogeneous samples of alkanethiol-covered reactive metals such as Cu and brass was demonstrated in this dissertation. This nonlinear optical microscope provides a spatial distribution overview of the surface as a description of conformation and orientation of these molecules at the interface. In the first study of this dissertation, the length scale dependence of the spatial distribution analysis of the SFGIM technique has been analyzed to provide a comprehensive evaluation of its ability of revealing hidden domains of monolayer defects from averaged SFG signals. By constructing maps from regions-of-interest (ROIs) of different length scales, measuring from 5 to 1000 μm, two-dimensional localized domains of defects are identified to contain populations of less-ordered monolayers. By generating the tilt angle distributions of the scaled ROIs, SFGIM determines the experimental mean orientation, θ, and distribution width, σ, as dictated by the SFGIM resolution limit. The capability of SFGIM in monitoring surface reactions was demonstrated by defining changes in the monolayer order as an effect of oxide growth on alkanethiol-covered copper. The spontaneous atmospheric oxidation of Cu was observed as a change in the nonresonant phase response of SFG spectral lineshapes obtained from the SFGIM setup (1064 nm). With the growth of the oxidized film, the changes in the methyl tilt orientation are spatially correlated to the shifts in the nonresonant phase. Regions with the most concentration of reduced surface Cu registered as surface domains with lower gauche defect signals and well-oriented molecules. The remarkable advantage of SFGIM against averaged SFG spectroscopy was demonstrated in its ability to spatially correlate the local structural behavior of interfacial molecules in response to the local physical properties of the surface. The heterogeneous local spatial kinetics was evaluated to have a spatial effect on gauche defect formations. Several experiments were performed to further test the imaging microscope for its capability in characterizing monolayer order on various metals such as gold, silver, copper, and brass in order to exploit its full potential in analyzing surface reactions, particularly atmospheric oxidation.