Electron diffraction studies of unsupported clusters
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The motivation to study the structure of clusters is the possible observation of unusual structures for small cluster sizes. The presence of such structural size effects is generally associated with the optimization of surface energy in clusters. For metals that have face centred cubic (FCC) structure in the bulk, optimization of the surface energy typically results in icosahedral or decahedral structures being preferred for nanometre sized clusters. The inert gas aggregation (IGA) technique has been used to produce a beam of clusters (diameter ≤10 nm) for structural studies using electron diffraction. Studying the clusters while in a molecular beam, as opposed to on a substrate, means that the clusters are unsupported and thus free of any perturbing effects due to a substrate. The use of a beam also means each cluster is subjected to only a brief exposure to the electron beam, minimizing effects due to the electron beam. Attempts to obtain diffraction patterns from Zn clusters were unsuccessful, however using Pb it was possible to obtain diffraction patterns from clusters using a wide range of parameters in the IGA source. The experimental diffraction patterns result from the range of different sized and structured clusters produced by the source. The analysis reflects the distribution of cluster sizes and structures by combining diffraction patterns from model clusters with a range of sizes and structures to produce a best fit to the experimental pattern. In general, two sets of model clusters are used: the first set contains models with up to ~6500 atoms, created using bulk and symmetry properties for clusters with FCC, decahedral and icosahedral structure. The second set contains the same sizes and structures as the first, however each model has been relaxed using molecular dynamics (MD) simulations. In the analysis of several experimental diffraction patterns, models with twinned FCC, liquid, anti-Mackay icosahedral and shaved icosahedral structures are also considered. Domain size estimates are obtained using the fit results; cluster size estimates are made from samples collected from the beam and observed in a TEM. Size estimates are also made using the Scherrer formula and the Fourier inversion method . Analysis of diffraction patterns from Pb clusters shows that changing the type of inert gas produces the greatest variation in the size and structure of the clusters. The small clusters produced using He are found to be based on icosahedral structures. The clusters produced using Ar are larger than those produced using He and the diffraction patterns are difficult to interpret. The patterns bear a strong resemblance to those from decahedra, but diffraction patterns from decahedra are similar to those from twinned FCC structures, and from a combination of shaved icosahedra and FCC structures. FCC structure is not observed, which is both interesting and surprising.