Multicolor Chemical Imaging by Sum Frequency Generation Imaging Microscopy of Monolayers on Metal Surfaces

Abstract

Sum frequency generation imaging microscopy (SFG-IM) is a unique surface-specific technique that can detect the spatial distributions of differing monolayer species based on chemical contrast. Here SFG images of microcontact-patterned multicomponent self-assembled monolayers derived by the adsorption of alkanethiols on gold were analyzed by factor analysis (FA) utilizing a library consisting of SFG alkanethiol spectra to determine the chemical identity and spatial distribution of the patterned monolayers across the images. By utilizing the spectral library as a target test for factor analysis, we correctly identified the monolayer species, and their spatial distributions were mapped. The chemical identity and spatial distribution of a random-pattern multicomponent alkanethiol sample were determined and mapped. Furthermore, utilizing the alkanethiol library, factor analysis was able to identify an unknown monolayer region, the vibrational spectra of which were not present in the target library. The results demonstrate the capability of FA combined with the spectral library to determine the chemical composition and spatial distribution of organic molecules on the surface of multicomponent complex chemical systems acquired by SFG-IM.

Resolution Limit in Infrared Chemical Imaging

“…Although we focus on use of a single IR wavelength for imaging throughout the optical train for simplicity, the formalism developed here can also be modified to suit these emerging modalities. The analytical framework can also be extended to other techniques, including second-order nonlinear spectroscopies such as sum frequency generation (SFG) − or second-harmonic generation (SHG) microscopy, , with an additional constraint on signal localization arising from the signal origin being from the interfaces…”

Leaving the Limits of Linearity for Light Microscopy

“…This group has also shown how factor analysis, which includes principal factor analysis and target factor analysis, can be applied to low-signal vSFG microscope images to reduce the total acquisition time without sacrificing image resolution. 96,97 Within the past few years, vSFG was also extended into three dimensions via holography as demonstrated by Ma et al 98 SFG microscopy that is resonant with electronic transitions, eSFG, is less developed than its vibrational counterpart. There have only been a few reports of eSFG microscopy that have explored chiral solutions 99 and 2D material systems.…”

“…Baldelli and co-workers have taken a less conventional approach by implementing compressive-sensing, which they utilized for monitoring molecular adsorption and film formation on metallic surfaces. , An example of vSFG images of self-assembled monolayers depicting the chemicals adsorbed on a copper surface can be seen in Figure . This group has also shown how factor analysis, which includes principal factor analysis and target factor analysis, can be applied to low-signal vSFG microscope images to reduce the total acquisition time without sacrificing image resolution. , Within the past few years, vSFG was also extended into three dimensions via holography as demonstrated by Ma et al…”

SAMs on Gold Derived from Adsorbates Having Phenyl and Cyclohexyl Tail Groups Mixed with Their Phase-Incompatible Fluorinated Analogues

“…The previous literature has extensively examined the effect of combining incompatible functional groups, such as the mixed surface of methyl and hydroxymethyl groups, methyl and amino groups, and methyl and oligo­(ethylene glycol) groups . However, the number of studies exploring the effect of bulky functional groups is limited, − especially for fluorinated groups. − To study the effect of bulky perfluorinated tail groups in mixed phase-incompatible SAMs, we investigated mixed SAMs generated from two classes of adsorbates on Au(111) bearing either a cyclohexyl or phenyl tail group of the form C 6 H 11 (CH 2 ) 11 SH ( HCyH11SH ) and C 6 H 5 (CH 2 ) 12 SH ( PhH12SH ), respectively. The adsorbates were mixed with their perfluorinated analogues, C 6 F 11 (CH 2 ) 11 SH ( FCyH11SH ) and C 6 F 5 CF 2 (CH 2 ) 11 SH ( FPhFH11SH ), respectively, to generate the mixed surfaces on gold; the molecular structures of the adsorbates are shown in Figure .…”