Self-assembled monolayers (SAMs) can be applied in a variety of technical areas. Infrared characterization of these materials
is a challenge because of the low concentrations on the surfaces. This article reviews the sampling methods for characterization
and presents data comparing the different approaches.
Self-assembled monolayers (SAMs) have been used to produce ultrathin, ordered films in a number of technical areas. Initial
work on SAMs was published in 1946 (1), but the field has grown tremendously over the past 25 years. SAMs are used in a variety
of applications. A quick search on Google Scholar shows applications in material science, biology and biochemistry, sensors,
electrochemistry, and nanotechnology.
SAMs are typically assembled on metal or metal oxide surfaces. Other substrates such as glass are also used depending on the
application. Characterization of these ultrathin films can take many forms. Scanning probe microscopy techniques provide morphological
information. Electrochemical characterization is also used. Infrared (IR) and Raman spectroscopies are used routinely to characterize
these supramolecular systems.
Methods of Infrared Characterization
Three methods are available to probe the chemistries of these materials in the infrared, each with their own advantages and
disadvantages. In some cases, the techniques provide similar results and the choice can be made based on simplicity and cost.
In other cases, the choice is not as simple.
Infrared Reflection Absorption Spectroscopy (IRRAS)
Very thin coatings on metal surfaces can be examined by reflection of the IR radiation from the substrate to the detector
in a specular reflectance mode.
Specular reflectance is an external reflection method in which light is reflected from a smooth solid surface where the angle
of incidence on the sample is exactly equal to the angle of reflection. Specular reflection spectroscopy comprises two distinct
types of examinations. The first consists of a metal (a highly reflecting substrate) on which a material has been deposited
(for example, films or contaminants). This would be the type of examination applicable to SAMs. The second type of reflection
examination involves the reflection from the front surface of a sample (similar to a reflection from a mirror surface).
In the situation of a coating on a reflective surface, the IR beam that is reflected from the metal surface interacts with
the coating (sample). As the metal is generally noninteracting, the resulting spectrum is actually an absorption spectrum
of the coating material (that is, a reflection-absorption spectrum).
In the infrared reflection absorption spectroscopy (IRRAS) experiment, as the angles of incidence and therefore reflection
change from near normal to near grazing, the pathlength increases providing more sample to interact with the IR radiation.
Films that are thinner than the wavelength of light do not give rise to a spectrum when the light is perpendicular to the
surface since the standing wave has a node at the surface. There is no interaction with the sample. Parallel (p or 90°) polarized
radiation on the other hand was shown by Greenler (2) to have a significant enhancement in the intensity for an absorbing
species at close to grazing angle. This effect has been exploited in studying SAMs.
Most sampling accessories have been designed so that the sample is placed face down on a horizontal supporting stage. A high
polished substrate (for example, shiny aluminum or a silver or gold mirror) is used for the collection of the reference background
for ratioing. The amount of light reflected from a sample is usually quite low, only a few percent, so longer scan times are
typical to obtain good signal-to-noise ratios. Experiments such as this have been performed on monolayers with thicknesses
of approximately 1 nm.
Figure 1 shows a schematic of a commonly available specular reflectance accessory (Pike 80Spec). With a sample in this accessory,
a polarizer is placed in the beam before the sample. Typically, a mercury-cadmium-telluride (MCT) detector and thousands of
scans are necessary to obtain good spectra.
Figure 1: The optical layout of a fixed-angle specular reflectance accessory (Pike 80Spec).
A number of accessories are available to collect this type of data. Some have fixed angles, as with the one shown in Figure
1, and some offer more flexibility and allow multiple angles of incidence.
This technique has three major disadvantages. The first is that one needs to have a clean sample to use as a background. Some
of the substrates used (gold, in particular) will react with air and you never have a "pure, clean" surface. An even worse
problem is that because the sample itself is so thin, the absorbances are very low. Minor changes in water vapor may show
up in the spectrum and may actually be larger than the analytical bands of interest. The third problem is one of dynamic range.
In the IRRAS experiment, the sample signal is very low compared to the total energy reaching the detector, especially at the
centerburst. Placing a polarizer in the beam will help this situation, but the dynamic range problem can still exist.