Single-source clusters enable tailoring of structures and nanostructures

Abstract

The performance of thin films in display devices, smart windows, self-cleaning coatings, and solar cells depends strongly on the crystal structure, microstructure, and chemical composition of the host materials. The sources of heterogeneity resulting from existing technologies can be identified by scrutinizing the film-formation processes. Populations of different intermediate chemical clusters produced in gas phase or solution are deposited on a substrate. These populations exhibit different reactivities, making control over nucleation, growth, coalescence, and mass-transfer processes very difficult. This is a big problem that materials scientists have tried to solve. Analytical techniques show films consisting of crystallites (‘grains’) of different sizes, shapes, and structures. Structural and chemical defects inside the crystallites and at their boundaries are quenched during aggregation and packing. This heterogeneity affects, among others, charge-carrier mobility and ion diffusion. To overcome this problem, a promising approach was developed by generating a single cluster, controlling its atomic packing tomatch that of the host material’s crystal structure, and then using it as a single source to grow films. The tendency of these clusters to self-assemble and better pack leads to ordered and relatively stable structures. This enables us to synthesize homogeneous films of different packing orders. From a fundamental point of view, this ‘single-cluster-source approach’ also allows a systematic study of the relationship between a material’s structure and its properties. We recently reported the first use of tritungstic groups as single-cluster source to process homogeneous films. We successfully used high-resolution transmission electron microscopy (HRTEM) to image the atomic arrangement in the resulting homogeneous electrochromic film. A new hexagonal phase was Figure 1. (top left) High-resolution transmission-electron-microscopy (HRTEM) image of a new tungsten trioxide hexagonal structure. (top right) The same image after Fourier filtering and a structural model for the new metastable hexagonal phase. (bottom left) A model for oxide clusters made of three tungsten atoms (tritungstic groups) and their condensation process.