Sum Frequency Generation Imaging Microscopy: Factor Analysis of Mixed Monolayers
Surfaces and interfaces are considered as a boundary between a material and its surrounding environment and influence interactions with that environment. In order to gain a fundamental understanding of the underlying processes, it is critically important to know the chemical and physical properties of the surfaces or interfaces. To investigate surface properties, while controlling the surface chemically and spatially, a model system of multi-component patterned self-assembled monolayers (SAMs) with different terminal functional groups were produced by microcontact printing. The microcontact patterned self-assembled alkanethiol monolayers on gold surfaces were analyzed by sum frequency generation imaging microscopy (SFG-IM) technique to generate SFG images. The SFG-IM provides identity of surface molecular species, information about surface chemical structure, and spatial distribution overview of chemicals on the surface, which make it a useful technique in chemical imaging. The SFG images were then analyzed by factor analysis utilizing a library consisting of SFG alkanethiol spectra to determine the chemical identity and spatial distribution of the patterned monolayers over the images. By utilizing the spectral library as a target test for factor analysis, the monolayers were correctly identified and their spatial distributions were mapped. By analyzing a random pattern sample, factor analysis was able to identify an unknown monolayer region, vibrational spectra of which was not present in the target library. Additionally, it was determined by target factor analysis that the amount of solution deposited backfill mixing into the microcontact stamped region and the absorption kinetics correlate with previous studies. The 12 percent backfill monolayer mixing into the stamped monolayer region, quantified by target factor analysis, results are representative of low concentration solution studies. These results demonstrated the capability of factor analysis combined with the alkanethiol library to determine the chemical composition and spatial distribution of alkanethiol monolayers on the surface of multi-component chemical system acquired by SFG-IM.