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. 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.


Introduction
Self-assembled monolayers (SAMs) are ordered, single-molecular layers of organic materials which may form spontaneously on various surfaces, in solution or in the gas phase [1,2]. These thin films have proven to be powerful tools for controlling surface chemistry and are the basis of applications ranging from sensors [3][4][5] to controlling surface free energy and adhesion. The patterning of SAMs is of interest for diverse potential applications such as ultra-thin lithographic resists [6], positioning and attachment of different particles [7] and electronic molecular devices [8].
Many methods are routinely used for characterizing the thickness, composition and order of SAMs, such as ellipsometry [9], atomic force microscopy (AFM) [10], Fourier-transform infra-red spectroscopy (FTIR) [11], x-ray photo-electron spectroscopy (XPS) [12], and scanning tunneling microscopy (STM) [13]. Contact angle goniometry measurements are widely used in the estimation of surface free energy of the monolayers [14]. Some of these methods are inherently statistical, and are based on an averaging over large uniform surfaces, whereas others are extremely localized and require a thorough, nanoscale point-by-point sampling. In addition, these observation methods require sophisticated and expensive equipment. Some of them involve meticulous sample preparation, and/or could be destructive to the sample being examined. These characterization methods do not provide a rapid, convenient analysis of patterned SAM-coated surfaces.
Spectral analysis of reflections from one or a few molecular layers can resolve changes in optical path length on a sub-nanometer scale [15][16][17][18][19]. Gauglitz and coworkers employed spectral interferometry to monitor the swelling of thin polymer films exposed to different analytes, as well as antigen-antibody reactions [15]. A review of chemical and biological applications of spectral interferometry point-sensors is provided in [16]. Reflective interferometry was extended to the spatially resolved analysis of SAM patterns on a substrate, in a significant series of works by Rothberg and associates [17][18][19]. The deposition of molecular films could be observed with sub-Angstrom-level resolution based on relatively large contrast [17][18][19]. These experiments required polarization control, careful collimation and angular alignment of the interrogating beam, and narrow-band optical filtering. The lateral resolution of the measurements was limited to a length scale on the order of tens of microns. The direct imaging of sub-micron SAM patterns using a simple, readily available setup has not yet been reported. Similar methodology has been applied to the characterization of exfoliated graphene films [20], in which small regions of single-atomic layers must be identified. Over the last few years, several groups reported the direct observation of single-layer graphene using standard, bright-field microscopy, when the films are deposited on silica-on-silicon substrates [21][22][23]. The thickness of the silica layer is designed to provide wavelength-varying reflectivity within the visible range. The presence of a graphene monolayer slightly modifies the spectral reflectivity pattern. Although small, these spectral variations can be recognized by the naked eye [20,24]. The technique can also distinguish between regions containing different numbers of graphene using visibl Daaboul et substrate [26 In this w silica-on-sil microscope filtering. Th SAM-formi compared w dependent presence of are only a fe

Theory
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Conclusi
The