Organisers

• Peeters Francois (University of Antwerp (CGB))
• Pawel Hawrylak (Institute for Microstructural Sciences, National Research Council)
• Peter Kratzer (Univerisity Duisburg-Essen, Fachbereich Physik)

Supports

   CECAM

   SANDiE

   COST - MolSimu

Description

Scientific background:

There is currently significant interest in understanding lectronic and
optical properties of self-assembled semiconductor quantum dots, carbon, and in particular graphene, based nanostructures, and hybdrid nanostructures, such semiconductor quantum dots with magnetic ions at the atomistic level. The common theoretical challenge rests with a multitude of length scales involved and the resulting size of the system, involving often millions of atoms.
This is well illustrated by self-assembled semiconductor quantum dots, which form spontaneously when a few monolayers of a semiconductor are deposited on a lattice-mismatched substrate. These nanostructures are then solidified by the deposition of further material. By varying the semiconductors involved, the growth conditions, strain engineering, substrate lateral patterning or vertical stacking, a rich variety of novel materials with unique and tailored electronic and optical properties can be produced. Another new development is the incorporation of controlled number of magnetic ions into semiconductor nanostructures, combining semiconducting and magnetic properties at the atomic level. Recent advances in fabrication of controlled number of single carbon layers open up a possibility of building lateral carbon based nanostructures in a way similar to semiconductor quantum dots.

These materials allow the study of fundamentally new phenomena at the nanoscale as well the development of new electronic and optoelectronic devices.

One of the main purposes of the proposed meeting will be to bring together, in a dedicated workshop, scientists working in the different areas relevant in the understanding of semiconductor, carbon and magnetic nanostructures.

Scientific Objectives

Research topics:

Different areas of physics have to be integrated in order to
understand the physical properties of semiconductor, carbon, and magnetic nanostructures:

1. Semiconductor self-assembled quantum dots:

1.1. Self-organized growth:
Novel computational models for atomistic growth processes relevant for self-assembled nanostructures will be addressed: e.g. methods to bridge the gap between atomistic growth processes and mesoscopic growth phenomena; molecular dynamic simulations of structural relaxations and dynamic processes (i.e. inhomogeneous growth at surfaces). Some of the key issues to be addressed is the inter-relation between strain relaxation, intermixing and defect formation.

1.2. Strain calculations:
Strain drives the growth of self-assembled nanostructures and is responsible for the self-organization of e.g. quantum dots, i.e. its vertical alignment. It also strongly affects the conduction and valence bands. Different approaches (continuum approaches, atomistic theories: valence force field, Stillinger-Weber and Tersoff potentials, ...) to calculate the strain will be confronted with each other. In which cases does one need to go beyond the more simple continuum approaches? E.g. due to the relatively slow algebraic decay of the strain in the matrix of a self-assembled quantum dot one may question if atomistic theories with multi-million atoms are sufficient.

1.3. Multi-scale calculation of electronic structure:
The electronic structure calculation involves nanometers and up to a million of atoms while strain calculations involve microns and billions of atoms. Treatment of million atom nanostructures necessities phenomenological tight binding or pseudo-potential approaches, and the coupling of strain and electronic degrees of freedom is not obvious. The atomistic calculations will be assessed and compared with continuum approaches based on k.p-theory and effective mass theories. The phenomenological tight binding models will be also confronted with methods derived from ab-initio calculations, such as tb-DFT.

2. Many-particle effects:
Exchange and correlation effects play a crucial role in understanding the electronic and optical properties of interacting many-electron systems like self-assembled quantum dots. At present many-particle effects are treated in an ìad hocî manner by e.g. incorporating one-electron tight-binding states into an effective quasi-particle multi-exciton Hamiltonians. This is to be contrasted with self-consistent calculation of excitations in ab-initio GW-BSE approaches.
We will explore the possibility of a more microscopic approach to million atom nanostructures. The many-body problem is of course an unending challenge. We will explore the quantum dots as `laboratories for correlated electron systems’. In this context different speakers will address different approaches e.g. Hartree-Fock-CI, DFT-CI, tight-binding-CI, quantum Monte Carlo, etc and their applicability to and potential benchmarking in quantum dots.

2. Semiconductor quantum dots with magnetic ions:

Semiconductor quantum dots with magnetic ions present all the challenges of semiconductor quantum dots, and add magnetic ions. At present only effective mass description of carriers and phenomenological description of exchange interaction has been explored. We will asses the state of knowledge and explore the potential of atomistic multi-scale approaches to the problem of magnetic ions in quantum dots.

3. Graphene based nanostructures:

The physics of single carbon layers with tunable charge density has been extensively explored in intercalated graphite twenty years ago. However, the dopants (intercalants) controlling carrier density were in close proximity to the carbon layers. Recent progress allows for `modulation doping’ of graphene layers. Their 2D character implies ready transfer of lateral gating/patterning techniques create 0D nanostructures. We will explore the state of knowledge on graphene ribbons and dots in order to bridge the semiconductor and graphene communities.

4. Computational aspects: top down and bottom up.

The initial understanding of nanostructures started with top-down approach of effective mass and is continuously moving toward the atomic scale. At the same time the ab-initio approach, with the development of O(N)-methods and tb-DFT is continuously moving to larger systems. By bringing in experts in the large scale ab-initio methods we will explore the possibility of transferring some of these methods to million atom nanostructure calculations.


Motivation and objectives:

There are different international conferences in which self-assembled semiconductor nanostructures are discussed. E.g. in May 11-16, 2008 at QD2008 (International Conference on the Physics and Chemistry of Quantum dots) will be organized at Korea. In such international meetings the accent is predominantly on the experimental progress in this area. In such meetings there is no possibility for a confrontation of the theoretical methods and techniques which are used in this field. Therefore, there is a real lack of a discussion forum
on the computational approaches used in the field of semiconductor nanostructures. The proposed workshop will fill this gap as well expand into new areas : graphene, magnetic dots, and multi-scale modeling. Participants are limited to young and experienced researchers which are actively involved in the modeling of semiconductor and related nanostructures. The experimental state of the art will be known from e.g. QD2008.

References

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