Image Contrast in Sum Frequency Generation Microscopy Based on Monolayer Order and Coverage

Sum frequency generation imaging microscopy (SFGIM) was used to study a binary system of self-assembled monolayers (SAMs) on evaporated gold. A two-step process that consists of microcontact printing (μCP) and the solution backfilling method is used to generate a sample surface with two distinct SAMs domains. The high contrast chemical identification and the homogeneity map of the SAMs for two different head groups are revealed by processing the three-dimensional images (xy-surface and infrared wavenumbers) obtained from SFGIM. For conformation order analysis, the amplitude ratios of the methylene and methyl symmetric resonance obtained by a nonlinear curve fit of the spatially and spectrally resolved SFG images are used to reconstruct spatial image maps. The conformation order map shows the distribution of gauche defects in SAMs with improved contrast across monodentate and bidentate alkanethiol monolayer regions. The results show that variation of the head group attached on the surface dominantly influences the conformation disorder and surface coverage of the molecules, while the terminal methyl group in both monodentate and bidentate SAMs shows a similar orientation angle.