Electron diffraction studies of unsupported bismuth clusters
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This study uses electron diffraction to investigate the structure of clusters, which are small particles containing between 3 and 105 atoms. Because of their size clusters represent the link between molecules and bulk material. Of particular interest is the appearance of structures that are forbidden in the bulk as the particle size decreases. One of the main reasons why structural changes occur is the increased proportion of surface atoms in small clusters which may lead to a rearrangement of the atoms in a cluster due to the need to minimise surface as well as bulk energies. Electron diffraction applied to cluster beams allows the study of the structure of small particles, free of contamination and without interaction with a substrate. The exposure time of the clusters in the beam is very short limiting the possibility of altering the clusters by the electron beam. The high intensity cluster beam necessary for this diffraction experiment is generated by an inert-gas aggregation source. The two main parts of this thesis work are the development of a new detector system to record electron diffraction patterns and the study of the structure of unsupported Bi clusters. Furthermore, two new analysis tools based on existing techniques have been developed and implemented. The new detector system uses a pair of linear diode arrays to measure the radial intensity profile of the Debye-Scherrer ring diffraction pattern. The scattered electrons are detected through secondary charge created on impact in the semiconductor material of the sensor pixels. The detector operates differently compared to its predecessor enhancing the quality of the diffraction patterns. The diffraction patterns from Bi particles can be categorised into patterns from crystalline clusters and patterns with liquid/amorphous features. In the case of the crystalline particles, the diffraction patterns indicate a rhombohedral structure. It has been found that the size estimates determined from the diffraction patterns are smaller than the estimates calculated from TEM images of deposited clusters. This suggests that the particles consist of domains that are separated by lattice defects. The mean sizes of the crystalline particles range from 109 to 231 Å. In the case of liquid/amorphous particles, the patterns have been compared to patterns of liquid drops calculated from structure data of liquid Bi. Although the features in the patterns are similar, the peak positions and relative intensities are different suggesting a new, probably amorphous structure.