Design of nanostructured organic films
This project is aimed at the development of a new methodology for fabrication and characterisation of highly ordered films of known and controllable structure. The control of the structure of thin films at a molecular level is essential for light harvesting applications in devices such as solar cells. Although many techniques for producing layered films have been demonstrated, practical methods for controlling the structure in the plane of the surface have not been explored. Organic materials such as porphyrins are ideal for the formation of functional films as thin as a single molecular layer. In this project we are applying novel methods to construct, by way of an organic linker layer directly onto a silicon substrate, highly ordered films of porphyrin derivatives. We will use our expertise in characterisation methods (X-ray reflectivity, synchrotron grazing incidence diffraction and X-ray photoelectron spectroscopy) to demonstrate that the desired assembly has been achieved. Films produced in this way have applications in photonics, molecular electronics, molecular recognition and catalysis.
Major achievements and highlights
In 2007, one focus has been on the transfer of the ordered films that were previously formed at the air/water interface onto solid substrates. Such transfer is necessary in order to fabricate useful films of these materials. Some surprising behaviour has been observed, however, it is now possible to produce stable multilayer films of known composition.
A novel method for producing ordered films, developed in 2006, has been further explored. It relies on electrostatic interactions between charged porphyrins and polyelectrolytes to exert control over the aggregation behaviour.
Measurements of spectroscopic properties have been combined with X-ray reflectometry to show that the outermost layer forms different aggregate structures to the underlying layers, leading to optical properties that alternate depending on whether the outer layer is composed of porphyrin or polyelectrolyte.
Research highlights
For the first time it has been possible to definitively determine the aggregation behaviour in mutilayer polyelectrolyte/porphyrin films. It was shown that the aggregation varies depending on whether the porphyrin is at the outermost layer or not.
A mechanism has been proposed to describe the pH dependence of the spectroscopic properties of these composite films.
A new type of multilayer structure has been designed which consists of a bipyridinium layer followed by a porphyrin layer. These structures show promising rectifying behaviour, and this could be useful for molecular diode structures.
Future plans and directions
We have hypothesised that new ways to order functional molecules could lead to substantially better results than have currently been achieved. In particular, square planar four-coordinate metals such as Pt and Pd are to be investigated, using their tendency to coordinate with the lone pair of electrons, for example, on a nitrogen atom on the molecule.
The electrical properties of a number of new structures consisting of donor-acceptor systems incorporating porphyrins will be investigated. In particular, structures with very high rectification ratios will be sought.
Collaborations
Collaborator |
Organisation |
Dr Stephen Holt |
ISIS Facility, UK |
Dr Gary Foran |
ANSTO |
Prof Geoffrey Ashwell |
Bangor University, UK |
