nanostructured surfaces and self

7/28/2019 Nanostructured Surfaces and Self

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http://www.utwente.nl/tnw/pin/onderzoek/nanostructured_surfaces_and_self-assembly_of_nanocolloidal_films.doc/

Nanostructured surfaces and self-assembly of

nanocolloidal filmsNanostructured surfaces and self-

assembly of nanocolloidal films

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.

1. Unambiguous optical characterisation of nanocolloidal films

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.


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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 layergrowth 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.


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5. Microcontact printing to pattern nanocolloidal films

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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