Event Details

Exploring structure with atomic resolution using electron imaging and diffraction

Abstract: One of the great advantages of microscopy is that its images can show structures that could not be predicted due to the limited imagination of the user. Since the realm of electron microscopy continues to expand - routine observation of materials at a truly atomic scale is now possible - the surprises that it produces become more interesting and relevant at a fundamental level. Of course, it is still possible to usemicroscopy to address specific and well defined questions, in which case precision, accuracy, and reproducibility are essential. This talk will give examples from both types of study.

Semiconductor nanowires have been predicted to have superior properties to bulk materials because defects with long-range strain fields such as dislocations should be unstable in very small crystals.Nevertheless, observation of the end of GaAsP nanowires grown by Ga-droplet vapour liquid-solid show high densities of line defects at the end where the droplet is converted to solid material. Atomic resolution imaging shows a variety of defects, which are either locked into sessile configurations or,surprisingly, have a null Burgers vector and no long-range strain field.Electron diffraction has several advantages for the study of materials, in particular its ability to access nm-sized volumes and higher sensitivity to lighter elements than X-rays or neutrons. Nevertheless, it is incapable of producing reliable intensities in simple scattering experiments due to multiple scattering effects and is thus rarely used.

This problem can be overcome by collecting large amounts of data with the use of computer control. The resulting diffraction patterns have a wealth of detail and we find that they can yield information about atom coordinates to sub pico-metre precision as well as thermal vibrations and bonding. Thus, highly accurate measurement of crystal structure and bonding maybe readily achievable using any modern TEM.