Date of Award

Summer 8-15-2016

Author's School

Graduate School of Arts and Sciences

Author's Department

Chemistry

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

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.

Language

English (en)

Chair and Committee

William E. Buhro

Committee Members

Richard A. Loomis, Liviu Mirica, Bryce Sadtler, Li Yang

Comments

Permanent URL: https://doi.org/doi:10.7936/K76W98G2

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