The structure of free silver clusters, produced in an inert-gas-aggregation source and flowing in a molecular beam has been studied by electron diffraction. Large clusters (up to 11 nm in diameter) of both icosahedral and fcc structure are observed. Cluster structure is investigated as a function of evaporation temperature and molecular weight of the inert gas in the source. An increase in evaporation temperature leads to increasing formation of fcc particles in the beam, without a significant change in particle size. The existence of large icosahedra is ascribed to the formation of metastable structures, grown under conditions dominated by kinetic factors. Furthermore, these large icosahedra show pronounced relaxation, indicating that they are subject to significant internal strain. This strain appears to be accommodated by deformation of the lattice, rather than by the inclusion of defects. The diffraction findings are supported by transmission-electron-microscopy measurements made on samples taken from the beam.

Unsupported nanometer-sized copper particles in a molecular beam of inert carrier gas have been studied using electron diffraction. A distinct separation in the size distributions of fcc and icosahedral particles is observed: Icosahedra occur at sizes well below a diameter limit of 3.8 nm predicted for copper by molecular dynamics simulations, while fcc particles occur close to this limit or above. Molecular dynamics simulations have also provided information about particle temperature via thermal expansion and atomic dynamics.

The structure of small silver particles has been investigated in detail by use of electron diffraction. Particles are produced by the inert-gas aggregation technique and transferred to a diffraction apparatus by a molecular beam of inert carrier gas. There, their structure can be investigated free of interaction with a substrate or contaminants. The small particles are strongly size dispersed and a technique has been developed to analyze the results. Experimental diffraction measurements are compared with linear combinations of calculated diffraction patterns, from several types of structure, covering a range of diffracting domain sizes. It is possible, in this way, to identify a mixture of structures and estimate the size of the corresponding domains. The analysis clearly shows both multiply twinned (icosahedral and decahedral) and fcc particles, in the size range between 20 and 40 Å in diameter, present in the beam. The relative proportions of these depend on the conditions of particle production.

An electron diffraction study on unsupported nanometer-sized copper clusters has been combined with molecular-dynamics calculations to investigate size-related structural effects. The experimental conditions allow slow cluster growth, close to thermodynamic equilibrium, in an inert-gas-aggregation source. A distinct structural change is observed, which is correlated with cluster size: there is a predominance of icosahedra below 3.8 nm in diameter, and a clear separation of size distributions for icosahedra and fcc particles, which are larger. These results confirm the predictions of an earlier molecular-dynamics study. Further molecular-dynamics simulations have provided information on atomic dynamics and thermal expansion of interatomic distances and are compared with experimental data.

The special features of diffraction from particles in the nanometer size range are discussed and the problem of structure determination is considered. A direct method of structure analysis, known as Debye Function Analysis, is presented and evaluated. The method is capable of obtaining information about both the size and structure of domains in a sample, and can identify noncrystalline structures. Numerical simulations of observations are used to investigate the limitations of the technique, and also provide a general method for quantifying uncertainties in parameters estimated by Debye Function Analysis.