Work in the group centres on the chemistry and physics of solids and interfaces. It involves the synthesis of semiconductor nanostructures with controlled dimensions and the self-assembly of such building blocks into more complex architectures (quantum-dot molecules and solids). We are interested in the size-dependent optical and electrical properties of individual nanocrystals, ensembles of non-interacting nanocrystals and systems in which the building blocks are coupled electronically. Molecular simulations are used to study the self-assembly processes, as well as the molecular scale structure and growth of thin films and single crystals. The optical properties of lanthanide ions in host lattices are investigated in relation to luminescent materials for lighting. The study of interfacial chemistry and photoelectrochemistry is important for semiconductor nanostructures, porous semiconductors and single-crystalline materials. Consideration of possible applications and contacts with industry constitute an important aspect of work in the group.

Synthesis

A wide range of materials are made in the group. These include: phosphors, semiconductor nanocrystals (quantum dots (QDs)), QD-molecules and higher architectures (2-D arrays, 3-D solids) and porous semiconductors. Such materials are made by a variety of methods: solid-state reactions, wetchemical (colloidal) synthesis, electrodeposition, anodic etching, chemical vapour deposition and template synthesis.

Characterization
The group has extensive facilities for optical, (opto)electrical and electrochemical studies. Examples of work in progress include:

1. Optical investigations
   • Luminescence of lanthanide ions in solids.
   • Light absorption and emission in QDs, QD molecules, quantum wires and QD solids. In such studies the role of quantum-size effects is central.
2. Electronic properties
   • The electronic structure of QDs and QD systems (scanning tunneling spectroscopy).
   • Storage and transport of charge in QD solids (transistor techniques).
3. Photonic systems
   • Quantum-dot emission in photonic crystals.
   • Light scattering and random lasing in disordered porous media.
4. Interfacial chemistry
   • Influence of capping on optical and electronic properties of QD systems.
   • Mechanisms of porous etching of semiconductors.
   • Etching and photoelectrochemistry of crystalline semiconductors (Si, GaN, SiC).

Modelling

Molecular simulation is a powerful tool in understanding the structure and dynamics of nano-materials as well as their formation processes. These simulations describe the thermodynamics and the interplay on an atomic scale in systems of up to some 100000 atoms, depending on temperature, pressure and other macroscopic parameters. Analytical theories usually apply to either much smaller (one or a few atoms or molecules) or much larger (macroscopic) systems. Molecular simulation fills the gap between the two.

Examples of systems being studied at present are:

• The self assembly of nanopart.
• Structure formation in model biomembranes.
• Catalysis mechanisms of certain zeolites.

Applications

The materials and processes studied in the group are important for a wide range of (potential) applications including lighting (phosphors, GaN-based LEDs), solar cells, microelectromechanical systems and (nano)device technology.