| dc.description.abstract | Ever since their theoretical description in 2005, topological insulators have drawn scientific interest due to their high potential for application in spintronics and quantum computing. In 2007 the existence of this exotic phase of matter was demonstrated experimentally in a HgTe quantum well, fabricated through the use of lithography. Solvo-chemical methods however, are an attractive approach in the synthesis of 0-, 1- and 2-dimensional materials due to their cost-effectiveness. Furthermore solution-processing grants access to a series of facile structural transformations. For example, due to their well-defined geometry, 0-dimensional PbSe quantum dots can be used as building blocks for the formation of 2-dimensional quantum well superlattices through oriented attachment. Subsequently, cation exchange can be used to transform the PbSe quantum well into a selenide of choice. This provides a viable route towards the creation of 2-dimensional compositional analogues that structurally match the topological insulators proposed by theorists.
Therefore, the aim of the project was to investigate the single step cation exchange of PbSe-to-HgSe on both spherical PbSe nanocrystals (NCs) and PbSe quantum wells, since 2-dimensional HgSe superlattices have been proposed to behave as topological insulators. Invested parameters were time, temperature, precursor chemistry, addition of co-ligands, PbSe pre-treatment and cation ratio. The preservation of the anionic framework was not observed. However, HgSe NCs were synthesized through a digestive ripening type reaction. For both PbSe and HgSe, oleic acid was found to serve as solvation framework for the crystal-to-solution phase transitions.
By changing the ratio of Pb and Hg atoms present during the reaction, NCs with an average diameter of about 4 and 9 nm were formed. The 9 nm NCs were found to be HgSe and are formed at a specific (Hg:Pb) ratio. Herein, removal of ligands from the PbSe NCs surface is thought to be an important step needed to be performed prior to Hg-precursor addition. For this purpose thiols (in combination with pyridine) can be used. The 9 nm prism-shaped HgSe NCs are of significantly smaller size dispersion than reported in literature for so far. Furthermore, the synthesis of well-defined, equally shaped crystals is the first step towards their assembly into 2-dimensional superstructures. | |
