Introduction
Nanostructured surfaces, films with morphological features in the nanometer range and ordered assemblies of nanometer-sized particles are an interesting class of nanomaterials with great technological potential. Innovative application fields for these new materials include high-density information storage media, biological sensor arrays, magnetic fluids, medical diagnostics and catalysts.
Compared to conventional surface science techniques, such as gas-phase synthesis and nanostructuring or deposition of nanoparticles under ultrahigh-vacuum condition, the soft-matter approach is a scientifically and economically interesting alternative. In this approach self-organisation in the bottom-up formation of nanostructured interfaces in liquid environment and self-assembled deposition of nanoparticles from colloidal suspension play a predominant role.
In this research theme, the emphasis is on identifying and gaining control over the relevant growth and/or ordering parameters during and after the formation of structured interfaces or the deposition of nanometer-sized entities. A prerequisite for investigating the formation process of nanostructured surfaces is the ability to accurately characterise the surfaces in situ during and ex situ after the manufacture of well-defined surfaces and interfaces.
The standard approach in the analysis of the spectroscopic ellipsometry experiments is to describe the dielectric properties of a heterogeneous layer, such as an adsorbed layer of colloids, with an effective medium approximation (EMA). However, with continuously decreasing feature sizes into the low nanometer range, the commonly used EMAs, such as those established by Bruggeman and Maxwell-Garnett do not give adequate results. The so-called thin film theory developed by Bedeaux and Vlieger gives an excellent quantitative description of the optical response of nanocolloidal layers. In this theory, the incorporation of image dipoles and laterally interacting entities is essential. A prerequisite for the description of the optical properties of the nanocolloidal film is the knowledge of the optical characteristics of a single entity. The limited size of the gold colloids results in a different polarizability than that of bulk gold.
 
2. Ionic strength dependent deposition of gold nanocrystals
Atomic force microscopy (AFM) and scanning electron microscopy (SEM) are used to characterise nanocolloidal gold films deposited on APTES-derivatised silicon/silicon oxide surfaces. Adsorption of citrate-stabilised gold colloids from suspension leads to a random spatial distribution of isolated particles, and can be adequately described by the random sequential adsorption (RSA) model. The minimum distance between neighbouring nanocrystals is related to the extent of their repulsive interaction in solution, which arises from surface charges on the particles. Consequently, it is tuneable via the ionic strength. Additionally, the deposition kinetics of nanocolloidal gold particles is investigated using single wavelength reflectometry in a stagnation point flow geometry.
 
3. Field-assisted nanocolloid self-assembly
Apart from methods using bonding or self-organisation on the molecular level (e.g. deposition on functionalised substrates or Langmuir-Blodgett techniques), the soft-matter approach for deposition of colloidal films includes established "physical" techniques (e.g. spin- and dip-coating or spray deposition), which offer only limited control over the deposition and ordering processes. Better control of layer growth can be achieved by electrophoretic deposition (EPD), an established technique for depositing charged colloidal particles onto electrodes. It is particularly useful for nanometer-sized particles for which gravitational forces are too small to direct the particles to a substrate. Recent work, however, has shown that the electric field is more than just a substitute for gravity. The formation of highly ordered colloidal layers was observed during EPD of micrometer-sized silica and polystyrene particles and nanometer-sized gold particles. Similarly, magnetophoretic deposition (MPD) can be used to direct particles onto a substrate, making use of the motion of magnetic particles in magnetic field gradients.
4. Structural and optical anisotropy in nanoporous anodic aluminium oxide
Porous aluminum oxide has stimulated considerable interest as a nanostructural template, primarily because of the self-organized, tunable formation of extremely well-aligned cylindrical pores. Apart from the application of aluminum oxide films as filtration membranes, they are frequently used to fabricate nanowires with large aspect ratios.
We use spectroscopic ellipsometry and other standard techniwues to characterize porous aluminum oxide obtained by anodization of aluminum films. Results on our film samples with a well-defined geometry show that anodization of aluminum is reproducible and results in a porous aluminum oxide network with randomly distributed, but perfectly aligned cylindrical pores perpendicular to the substrate.
The ellipsometry spectra are analyzed using an anisotropic optical model, partly based on the original work by Bruggeman. The model adequately describes the optical response of the anodized film in terms of three physically relevant parameters: the film thickness, the cylinder fraction, and the nanoporosity of the aluminum oxide matrix.
 
6. Quantum confinement effects in the dielectric function of PbSe
Monolayers of lead selenide (PbSe) of a few nanometers in height are made by electrodeposition on a Au(111) substrate. The optical properties of these nanocrystal films are investigated using spectroscopic ellipsometry. The experimental results indicate that the dielectric function of the PbSe nanocrystals is thickness-dependent. These results are compared to electronic band structure calculations of the imaginary part of the dielectric function. It is demonstrated that the size-dependent variation of the dielectric function is affected by quantum confinement at well-identifiable points in the Brillouin zone, different from the position of the band-gap transition.
7. Optical characterization of self-assembled poly(ferrocenylsilane) films
Spectroscopic ellipsometry experiments are employed to investigate films of different poly(ferrocenylsilane) films. In one study, organometallic multilayer films are deposited electrostatically onto a variety of substrate using layer-by-layer self-assembly of polyanions and polycations. To complement UV/Visible absorption spectroscopy, spectroscopic ellipsometry is used to measure the development of film thickness with the number of bilayers.
In another study, poly(ferrocenyldimethylsilanes), composed of alternating ferrocene and dimethylsilane units in their main chain and featuring a thiol end group, were self-assembled to redox-active monolayers on gold. Surface plasmon resonance spectroscopy and spectroscopic ellipsometry measurements, performed under electrochemical control, are employed to determine thickness changes upon oxidizing and reducing the surface-grafted polymers.
 
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