| Recent advances in wet-chemical synthesis have made it possible to control precisely the composition, size, shape, and surface chemistry of colloidal semiconductor quantum dots and magnetic or noble-metal nanocrystals. The size-dependent opto-electronic and magnetic properties of these nanometre-scale building blocks can be exploited in nanocrystal assemblies where new quantum phenomena are expected to emerge due to electronic or magnetic coupling. The proposed research will investigate meta-materials with engineered electronic structure where the properties are determined through choice of the building blocks and control of the electronic and magnetic interactions. These materials promise novel optical, electronic and magnetic properties, such as enhanced exciton lifetimes, charge- and spin-ordering, and zero-threshold lasing. Optical and electrical spectroscopies have been used to explore the sample-averaged properties of nanostructures; however, these techniques have inherently limited spatial resolution. There is clearly a need for the use of local probes, in particular scanning tunnelling microscopy (STM) and spectroscopy (STS) that can measure topography and electronic structure with sub-nanometre spatial resolution. In this proposal, low-temperature STM and STS will be the key experimental techniques used to explore the electronic properties of coupled nanoparticle systems and to relate them to the local geometry. The proposed research aims to study the effects of quantum mechanical coupling in nanocrystal assemblies ranging from only a few nanoparticles (quantum dot molecules) to superlattices with long-range order (artificial solids). I will use STS to probe the local (spin) density of states and also to map the wave-functions of the coupled states in real space. Detailed study of interactions between nanocrystals at a single nanoparticle level will bridge the existing knowledge gap between design and function which is highly desirable for the rational design of novel materials based on colloidal nanocrystals. |
