CELNAPAG · Characterising the Energy Landscape of Nano-Polymer-Aggregates:Application to Synthetic and Bio-polymers

Final Activity Report Summary - CELNAPAG (Characterising the Energy Landscape of Nano-Polymer-Aggregates:Application to Synthetic and Bio-polymers)

During the fellowship the fellow worked in the field of energy landscapes applied to oxide materials. Oxides are important materials in chemistry and technology. Part of the research involved the study of the material silicon-dioxide (SiO2), also known as silica. This material is ubiquitous in our planet's geology, e.g. in the form of quartz and sand. Quartz is the highest density and most stable form of silica. Silica can occur in many other forms as well, either naturally or synthetically produced in the laboratory. Some of these materials have a much more open structure than the dense quartz phase. These materials can have (sub-) nanometer sized pores and cavities in them and are generally known as zeolites. Silica has many technological applications. It is important in optics, since it is the major component of glass. The zeolites, with their well-defined pore sizes are used as a sort of molecular sieve to separate components on the basis of their size. Furthermore silica is often used as a support for catalysts. Silica is also used as an insulator in microelectronics, e.g. in the form of the so called 'gate dielectric', which is the insulating layer between the gate of a transistor and the silicon surface. As the microelectronics technology progresses the components of a silicon chip become smaller and smaller. We are now rapidly approaching the regime where the width of the gate dielectric will be smaller than a nanometer. This was one of our motivations for looking at a nanoscale silica model system: silica nanoclusters in vacuum. In collaboration with Stefan Bromley from the University of Barcelona the fellow performed a 'global optimisation' study of silica clusters. The objective of this study was to try to find the most stable geometric arrangement of atoms in a silica cluster of a certain size. In this study we looked at clusters in the size range of up to 27 SiO2 units using a combination of methods. A quick but inaccurate method, i.e. a 'force field', was used to generate candidates for the most stable cluster geometry. Promising candidates were chosen for a more accurate, but more time-consuming, quantum chemical method ('Density Functional Theory'). It was found that these cluster geometries were very different from quartz or any of the other bulk forms of silica. Quartz-like structures were not expected to appear in clusters until sizes in the hundreds or thousands of SiO2 units. All the geometries found in the abovementioned study had defects in the form of dangling or double bonded oxygen atoms. These clusters were expected to be very reactive. During a second study the fellow looked at 'fully coordinated' clusters, i.e. clusters without defects. The fellow developed and improved a method for specifically generating these fully coordinated cluster geometries. These clusters were conjectured to have special properties making them useful as building blocks for new materials, with new properties, differing from the already existing zeolites and silica materials. In all the above studies clusters, i.e. finite systems, were studied. A new project that the fellow started was the study of bulk, i.e. infinite, oxide systems. An infinite system was modelled by taking a finite cell and periodically replicating it to form an infinite system. This led to mathematical complications that made the project especially challenging. The benchmark system the fellow was looking at was a mixture of calcium-oxide and magnesium-oxide. The fellow was in the process of applying the 'energy landscape' methods developed in the host group to this system. These methods included global optimisation, i.e. finding the most stable form, and the generation of transition states, i.e. structures that mediated minimum energy pathways between two minima. The project was still ongoing, but had already resulted in databases of various rearrangement mechanisms of atoms in the solid.

Data: CORDIS, © European Union