Electronic coupling in self-assembled nanocrystal arrays, a scanning tunneling microscopy study

Abstract

Colloidal semiconductor nanocrystals (NCs) are one the most actively studied components of modern nanoscience. The high degree of control over their size and shape makes it possible to accurately tune their opto-electronic properties through quantum confinement. Colloidal nanocrystals can serve as building blocks for more complex architectures. Formation of binary superlattices is a particularly important development in this field as it opens up new avenues towards nanostructured metamaterials with engineered electronic and optical properties. The properties of these metamaterials depend on the nature of the NC building blocks and the degree of electronic coupling between them. In this research, the effects of electronic coupling between NC building blocks in an array have been studied, by measuring the density of states on a local (nanometer) scale by scanning tunneling microscopy. STM is based on measurement of the tunnel current between an atomically sharp tip and a sample and can yield topographic information at sub-nanometer resolution. Scanning tunneling spectroscopy (STS) is used to measure the electronic structure of single nanocrystals or nanocrystal assemblies. The strength of these techniques lies in the combination of high resolution topographic imaging and local electronic spectroscopy. To study the electronic properties and electronic coupling between PbSe NCs in large ordered structures, a new technique has been developed to stabilize monomers, dimers, trimers and larger aggregates of PbSe in an inert matrix of CdSe NCs. The experiments showed that the coupling strength is influenced by the number of neighboring PbSe NCs whereas the CdSe NCs do not show electronic coupling effects at all. In large ordered assemblies of PbSe NCs, electronic coupling is also observed, but varies in strength. Despite the high structural quality, the electronic structure varies from site to site in the array. In thicker layers built from CdSe NCs, it was shown that the potential distribution and therefore the electric field influences the electron transport through the NC solid. The results of this study are important to understand how the tunnel contacts and electronic coupling can affect the current transport in NC solids. At the end of the study, the first results on the formation of binary structures by self-assembly are described. Experiments demonstrated that the size-ratio as well as the concentration ratio of PbSe and CdSe NCs influence the formation of ordered binary structures. It was demonstrated that with a size ratio of 0.57 mainly AB2 and AB13 structures were formed. The results in this thesis contribute to the understanding of electronic coupling and electron transport which is an essential issue in future opto-electronic devices based on nanocrystal solids. Local variations in the electronic structure should not be disregarded in the design of NC materials with tailored properties.