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

TL;DR

This paper demonstrates a simple optical microscopy method to directly observe and distinguish patterned self-assembled monolayers and bilayers on silica-on-silicon surfaces, facilitating applications in sensing and device fabrication.

Contribution

The study introduces a straightforward optical contrast technique for visualizing and differentiating SAMs and bilayers without complex scanning equipment.

Findings

Optical contrast allows direct visualization of monolayers and bilayers.

Method distinguishes single-layer from bi-layer coatings.

Supports applications in sensing and device fabrication.

Abstract

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. 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. The results are useful for electro-optic devices, selective wafer-bonding protocols and lab-on-a-chip test systems. 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, enabling such detection to provide a simple, yet effective differentiation between monolayers and adsorbed analytes with possible applications for chemical and/or biological sensing.