Psi-k - Ab initio (from electronic structure) calculation of complex processes in materials

Working Group 8 - Electronics at the Molecular Level

Geert Brocks, University of Twente, the Netherlands

The need

This field will, on the one hand, become extremely important and, on the other hand, it is somewhat underdeveloped in Europe, and therefore requires particular fostering. It is a highly active field in both academic and industrial laboratories in USA. While we do not wish to ape everything that the Americans do, several groups in Europe are starting or planning to work in this area because of its intrinsic scientific challenge, as well as its probable future technological significance. No spokesperson to join the Steering Committee has yet emerged precisely because so few people have been working in this field in Europe, but the need for one will continue to be kept in mind.

Scientific background

In the past decades, electronic computers have grown increasingly powerful as their basic elements, transistors and wires, became smaller and smaller. Present-day silicon technology allows integrated circuits on the micron scale. Even if fabrication techniques can be refined to further reduce the size, the laws of quantum mechanics prevent transistors of traditional design to function usefully once their smallest features shrink to less than 0.1 microns. If the miniaturization of electronic devices is to continue, present-day device designs have to be replaced by entirely new concepts which make use of the quantum mechanical effects that dominate the scale smaller than 0.1 microns.

The basic idea of molecular electronics is to use single molecules or chains of molecules as the basic elements (transistors, wires, etc.) of electronic devices. In this way the size of computers can be reduced dramatically, allowing for an enormous increase in their speed and memory. Comparing traditional electronic circuits with molecular circuits, the time cycle order of magnitude will go from nanoseconds to femtoseconds.

In the past few years, several experiments have provided data on the current-voltage characteristics of single atoms or molecules captured between STM tips. The leads can be combined with a huge variety of molecules allowing one to custom-build a virtually infinite number of distinct possible molecular electronic devices. Reliable theoretical tools are needed to guide experimental choices.

Research horizons

The principal objective of this research is the development and application of first-principles theoretical methods to calculate reliably the transport through single molecules. The goal is to understand how structural and external factors (electromagnetic fields, temperature) influence the molecular current-voltage characteristics.

Understanding the electronic properties in the vicinity of the contact region of the molecule and the substrate is a key point of the molecule-substrate interface for electronic devices. The rearrangements of the electrons of the molecule and substrate upon reaching contact determines the strength of the molecule-substrate bond and the transport properties. The charge transfer between the molecule and the substrates alignes the electronic structure of both constituents and determines such the possible resonance tunneling phenomena and the contact resistance. If the contact resistance between substrate leads and molecule becomes too large, the systems cannot be used as fast devices with low power dissipation.

The calculations are based on modern ab-initio techniques such as stationary and time-dependent density-functional theory. The investigations include

MEMBERS OF THE WORKING GROUP