Design of nanostructured organic films
This project is aimed at the development of new methodologies for fabrication and characterisation of highly ordered films of known and controllable structure. Generally the films of interest are organic films containing functional molecules such as porphyrins and other highly conjugated materials. The control of the structure of thin films at a molecular level is essential for light harvesting applications in such devices as solar cells. Porphyrins and related conjugated systems such as phthalocyanines 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. Our expertise in characterisation methods (X-ray reflectivity, synchrotron grazing incidence diffraction and X-ray photoelectron spectroscopy) is heavily used 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
In 2008 this work proceeded in two major directions. The first involved the continuation of methods that form two dimensional arrays at the air/water interface, using the fact that this is an environment that can assist self assembly of hydrophobic materials that interact with an aqueous solution. In this case, ions that form square coordinate complexes (particularly palladium) were used in soluble form to direct assembly.
The other direction is a strategy to directly assemble films on a solid substrate from solution using layer by layer deposition. This removes the need to transfer structures from one interface to another, with the risk of alteration of the ordered structure in a way that may be hard to control and predict.
The use of multilayer films that show excellent electrical rectification has also been explored. A combination of a donor layer and an acceptor layer gives some of the highest rectification ratios ever measured for organic materials.
A novel method for producing ordered films has been further explored. It relies on electrostatic interactions between charged porphyrins and polyelectrolytes to exert control over the aggregation behaviour. While earlier work focused on a specific system, we have now extended this to other combinations of charge and structure, in order to optimise the desired multilayer structures.
Some research highlights include:
The aggregation behaviour in multilayer polyelectrolyte/porphyrin films is, as a result of recent work, now quite well understood. It has been shown that the aggregation varies depending on whether the porphyrin is at the outermost layer or not, and now we have applied other deposition techniques and other combinations to further elucidate the mechanism of self assembly.
A mechanism has been proposed to describe the pH dependence of spectroscopic properties of these composite films, in which protonation of the nitrogen atoms in the porphyrin ring significantly affects film morphology.
Future plans and directions
The emphasis for this project will be on systems that are self-assembled directly onto solid substrates. In general this will involve layer by layer deposition, although other methods are also being tested.
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.
