Liliya Nikolova

The recent surge of interest in nanostructured materials stems from the remarkable effects arising due to the size reduction. Interesting novel properties (optical, electronic, catalytic, magnetic, etc.) occur as the dimensions from a practically infinite solid crystal are reduced to a system composed of a small number of atoms. To optimize the properties of such novel systems however, more knowledge needs to be acquired on their structure and how it affects their properties.
The equilibrium states of the materials are well known and probed by large variety of techniques for characterisation, while the information on the intermediate states is not complete. To reveal the process of structural transformation and to understand the dynamics, a description of the states during the transition is need. At present, there is no well-developed general method for the atomic-level structural determination of short-lived transient states.
The transmission electron microscope (TEM) is a powerful and versatile tool for materials characterisation offering very high spatial resolution. However, it is not suitable for direct imaging of structural transitions because of the poor temporal resolution. In fact, the images capture could require few seconds. The goal of this research project is the modification of a TEM to Dynamic TEM (DTEM) to improve the temporal resolution and to reveal the dynamics of irreversible processes of structural transformation (as nucleation, interphase boundary motion, shock propagation, radiation damage, solid state chemical reactions, etc). Therefore, one of the key modifications of the TEM is the replacement of the conventional thermionic or field emission gun with a source of type photocathode. By this manner generation of packets of electrons with high density through pulsed laser excitement is obtained. Hence, high temporal resolution (nanosecond or better) is achievable while maintaining the high spatial resolution (few nanometers). The structural transition in the sample is initiated by a ‘pump’ laser pulse. The specimen images are taken at a variable time-delay after the excitation using the short burst of electrons that takes a ‘snapshot’ of the sample. The method permits to make a movie of the evolution of the sample’s structure.