Self-Assembled Thin Films: Optical Characterization
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Many different materials with truly new physical and chemical properties, consisting of controllably deposited colloid particles, are being developed. Particles with a variety of intrinsic properties are used, their sizes varying over at least three orders of magnitude. For photonic band gap materials, particle sizes are in the (sub)micron range, whereas for magnetic applications, such as ultra-high-density storage devices, they are in the low-nanometer range. A prerequisite for studying colloidal systems is the ability to characterize them unambiguously under relevant conditions. Among the large number of methods available for characterizing colloids and their superstructures, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are by far the most popular. For both very small as well as relatively large particles these are the most employed ex situ techniques. The use of various scanning probe microscopies—atomic force microscopy (AFM), scanning tunneling microscopy (STM), and magnetic force microscopy (MFM)—among colloid scientists is increasing; with these techniques experiments are also typically performed ex situ. , In fact, only optical methods have been employed in situ. For larger colloids, imaging techniques such as conventional or confocal microscopy are used. For sizes much smaller than the wavelength of light, such as with gold nanoparticles, only nonimaging (lateral averaging) in situ techniques are available. These include primarily ultraviolet/visible (UV/vis) absorption spectroscopy and also optical reflection techniques such as reflectometry and ellipsometry. The major advantage of ellipsometry as compared to reflectometry is its sensitivity to very small perturbations at an interface. Not only the deposition process but also the drying processes of (nano)colloidal particles at a solid–liquid interface can be studied. However, both the in situ and ex situ capabilities of the aforementioned reflection techniques depend on an unambiguous interpretation of recorded optical spectra.
Much work is devoted to the optical characterization of adsorbed proteins. The optical contrast between these protein films and other layers is usually small because of the absence of light absorption in the visible and a refractive index of 1.35–1.4, i.e., close to water and glass. The approach developed by De Feijter et al. gives satisfactory results in such cases. However, for larger colloids and light absorbing particles, such as metal nanocrystals, this approach no longer suffices.
This paper is devoted to the optical characterization of Au nanocolloids (radius a = 6.6 nm) adsorbed from solution onto a naturally oxidized silicon substrate. An unambiguous optical characterization of such a colloidal system is not straightforward. We present an analysis of ellipsometry spectra in the coverage range up to 40%. A comparison of the optically determined coverage with the coverage determined with SEM serves as a benchmark for the quality of the description of the optical response.
The standard approach in the analysis of the optical spectra is to describe the optical properties of a heterogeneous layer, such as an adsorbed layer of colloids, with an effective medium approximation (EMA). In this paper, we will show that commonly used approaches as the Bruggeman and Maxwell-Garnett EMA do not give adequate results. However, the so-called thin film theory developed by Bedeaux and Vlieger gives an excellent description of the optical response. In this theory, the incorporation of image dipoles and laterally interacting entities is essential. We will show that an extension which modifies the strength of the image dipole is required. This is necessary for two reasons: 1) there is a large optical contrast between the ambient and the silicon substrate, and 2) the presence of the natural oxide layer also influences the image dipole contribution. This oxide layer has a limited thickness that is three times smaller than the radius of the colloids, and thus the silicon–silicon oxide interface affects the image dipole contribution. A prerequisite for the description of the optical properties of the colloid layer is the knowledge of the optical characteristics of a single entity. The limited size of the Au colloids results in a different polarizability than that of bulk gold. An experimental determination of the absorbance of the Au colloid suspension provides the required modification of the dielectric function of bulk gold in order to incorporate size effects.