Self-Assembled Thin Films: Optical Characterization

Abstract

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.