Cold Injection Synthesis of Nanoheterostructures based on Cluster Decomposition

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Nanocrystals have attracted considerable interest, because they promise large potential for miniaturisation of functional materials and have physical properties that depend on their size. They have been synthesised by wet-chemical means with more and more complex architectures. Complexity is either achieved by making particles with anisotropic shape (e.g. nanorods) or in complex heterostructures of different materials (e.g. particles with multiple shells), or both. The deposition of a new material onto a nanostructure requires the reaction of usually two precursor molecules that react at the surface of a seed particle in a heterogeneous nucleation reaction. Therefore precursor reactivities need to be matched. In addition, lattice mismatch and effects of surface ligands need to be taken into account. Therefore, the fabrication of nanoparticles usually occurs at high temperatures. If complex morphologies are exposed to heat they quickly degrade. Anisotropic shapes melt into spherical particles, while interfaces in heterostructures form a material gradient. High temperature thus poses a limit to the complexity of nanostructures that are accessible by wet-chemical methods. Mild reaction conditions, low temperature, and the absence of precursor decomposition reactions would evade these restrictions and allow the synthesis of a much wider range of nanomaterials. In this project, we aim to establish a new synthetic route to form nanocrystalline heterostructures based on a "cold injection" approach. Here, chemical triggers rather than physical parameters initiate the growth reaction of a nanocrystal.We intend to employ ultrasmall semiconductor clusters as source of material for seeded growth. These clusters can be produced ex-situ and act as a reservoir of the target material. Due to their molecular structure they can be decomposed under well-defined conditions. By separating the formation of the semiconductor compound from the growth reaction the general approach will be applicable to a broad spectrum of seed materials. The use of a chemical trigger to initiate the reaction allows working under very mild conditions and thermodynamic control, which is crucial for thermally unstable morphologies. Fabrication of defined heterostructures will thus be possible at much reduced energy costs. We will apply the principle to two semiconductor model systems, the formation of "hard" potential steps in core/shell particles and regio-selective growth of anisotropic particles in only one direction. Both systems are of high interest for both a fundamental understanding of nanoheterostructures and future application of colloidal nanocrystals in complex materials and electronic devices. However, the proposed reaction will be highly beneficial beyond the scope of this project, e.g. for modification of nanocrystals with impurity doping, in which dopant diffusion must be suppressed, or for nanocrystals that have been assembled into a larger superstructure.