Experimental and theoretical characterization of structure in thin disordered films

The electron microscope provides an ideal environment for the structural analysis of small volumes of amorphous and polycrystalline materials by enabling the collection of scattering information as a function of energy loss and momentum transfer. The scattered intensity at zero energy loss can be readily processed to a reduced density function, providing information on nearest-neighbour distances and bond angles. A method for collecting and processing the scattered intensity, which allows for the collection of an energy-loss spectrum for a range of momentum transfers, is discussed. A detailed structural determination from a reduced density function alone is difficult and it is shown that a more detailed structural model can be obtained by combining the experimental reduced density function with model structures obtained from molecular dynamics based on first-principles quantum mechanics. This method is applied to tetrahedral amorphous carbon, as an example of a monatomic network, and to aluminium nitride, as a prototype for a binary amorphous alloy.