Richard Palmer1

1, Swansea University, Swansea, , United Kingdom

If we are to translate the exquisite physics of atomic clusters into the synthesis of a new class of functional nanomaterials, massive scale-up of the rate of cluster depositon is required. The prize is a set of applications ranging from theranostics and water treatment to catalysis and memristors. Compared with colloidal synthesis of nanoparticles, the cluster beam approach is green - it involves no solvents and no effluents; particles can be size-selected as desired; and challenging combinations of metals (nanoalloys) can readily be produced. Yet to date the cluster approach has been held back by extremely low rates of particle production, largely confined to the sub-microgram per hour range. As one example, industrial catalysis R&D typically requires a gram of catalyst, or 10 mg of clusters at 1% loading on a suitable catalyst support. The cluster beam community is now rising to this scale-up challenge. I will discuss the development of a new kind of super-intense nanoparticle beam source, the “Matrix Assembly Cluster Source” (MACS). The MACS is based on ion beam sputtering of a solid rare gas matrix into which metal atoms are pre-loaded by evaporation. A scale-up of five orders of magnitude in cluster beam intensity has been achieved. I will address the mechanism of cluster formation in the matrix, highlighting the roles of multiple ion impact events in stimulating the ripening of the clusters, as well as the size-dependence of the cluster escape from the matrix to form the beam. However, the generation of intense cluster beams is not sufficient for the production of functional cluster-based materials. There is also a processing challenge to address. I will discuss a number of routes by which size-controlled clusters may be presented in a form matching the desired functional application, with specific reference to catalysis and theranostics. These examples of "formulation engineering on the nanoscale" include direct deposition of metal cluster beams onto micron-scale oxide powder support particles; bottom-up synthesis of novel cluster-decorated carbon powders; and functionalisation of deposited clusters to achieve biological compatibility. In all cases the synthetic work is supported by aberration-corrected Scanning Transmission Electron Microscopy (STEM) imaging. FInally I will discuss the validation challenge that we need to meet if we want to show that cluster-based functional materials are competitive with, or ideally superior to, advanced materials created by more traditional routes. I will focus on the hydrogenation (in both gas and liquid phases) of organic molecules over cluster catalysts as a basis for applications in the fine chemicals and pharmaceuticals sectors. Comparison with reference materials synthesised by standard methods begins to highlight the types of reaction in which cluster materials show a notable competitive edge. It may be that these three challenges will define the future of the field of cluster science.