Synthesis of Two-Dimensional Copper Indium Disulfide Nanocrystals and Study of Exciton Splitting in Thin Nanosheets

This dissertation presents the preparation of two-dimensional (2D) CuInS2 nanocrystals by the ion-exchange method and the exciton splitting observed in thin CuInS2 nanosheets. After monitoring the one-pot synthesis of CuInS2 by energy-dispersive X-ray spectroscopy (EDS), an ion-exchange method is developed as an improvement for the synthesis of CuInS2 nanocrystals, to control the morphology and size. CuInS2 nanocrystals are prepared by ion exchange with template Cu2-xS nanoplatelets and InX3 [X = chloride, iodide, acetate (OAc), or acetylacetonate (acac)]. The morphologies of the resultant nanocrystals depend on the InX3 precursor and the reaction temperature. Exchange with InCl3 at 150 °C produces CuInS2 nanoplatelets having central holes and thickness variations, whereas the exchange at 200 °C produces intact CuInS2 nanoplatelets in which the initial morphology is preserved. Exchange with InI3 at 150 °C produces CuInS2 nanoplatelets in which the central hollowing is more extreme, whereas exchange with In(OAc)3 or In(acac)3 at 150 °C produces intact CuInS2 nanoplatelets. The results establish that the ion exchange occurs through the thin nanoplatelet edge facets. The hollowing and hole formation are due to a nanoscale Kirkendall Effect operating in the reaction-limited regime for displacement of X– at the edges, to allow insertion of In3+ into the template nanoplatelets. Additionally, nanosheets of Cu2-xS having discrete thicknesses of 5, 6, and 8 crystallographic monolayers (MLs) are prepared by adaptation of literature methods. These are converted to CuInS2 nanosheets having the same, discrete, monolayer thicknesses by cation-exchange reactions. The 6- and 8-monolayer CuInS2 nanosheets exhibit broad, nearly featureless extinction (absorption) spectra that are generally characteristic of CuInS2 nanocrystals. However, the thinnest, 5-ML CuInS2 nanosheets exhibit structured extinction spectra having a well-resolved doublet feature with peaks at 2.74 and 2.54 eV. An exciton-splitting model assigns the transitions to excitons derived from inequivalent A, B, and C holes from the bulk band structure of CuInS2. This splitting is analogous to the heavy-hole, light-hole splitting observed in the absorption spectra of III-V quantum wells, and pseudo-2D II-VI nanocrystals. The model semi-quantitatively accounts for the magnitude of the observed splitting, which is largely due to quantum confinement and the differing A, B, and C hole effective masses. The model also accounts for the lack of resolved splitting in CuInS2 nanocrystals having larger dimensions