Direct observation of patterned self-assembled monolayers and bilayers on silica-on-silicon surfaces

Self-assembled monolayers (SAMs) of organic molecules are widely employed in surface chemistry and biology, and serve as ultra-fine lithographic resists. Due to their small thickness of only a few nanometers, the analysis of patterned monolayer surfaces using conventional methods requires thorough point-by-point scanning using complicated equipment. In the work reported herein, patterned monolayers are simply and directly observed using a bright-field optical microscope. Features as narrow as 500 nm are properly recognized. The monolayers modify the spectral reflectivity pattern of a silica-on-silicon thin film, and introduce a contrast between bare and monolayer-coated regions of the substrate. The method can also distinguish between regions of single-layer and bi-layer coatings. The observations are supported by calculations, and by control experiments using atomic force microscopy, scanning Raman spectrometry and scanning reflection spectrometry. We show here that chemical reactions leading to the formation of a bi-layer of SAMs correspond to an optical contrast visible to the naked eye. This contrast, in turn, could provide a simple and effective differentiation between monolayers and adsorbed analytes, with possible applications in chemical and/or biological sensing. The method is also applicable to the study of graphene-on-SAM devices.