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Understanding quantum confinement and ligand removal in solution-based ZnO thin films from highly stable nanocrystal ink

Abstract

We report a synthesis procedure for ZnO nanocrystals that is well-suited for producing solution-processed thin films. We obtained dodecane-thiol capped wurtzite ZnO nanocrystals with an average diameter of 4 nm that are monodisperse, highly soluble, and shelf-stable for many months. Compared to previous ZnO ink recipes, we demonstrate improved particle solubility and excellent ink stability, resulting in ZnO nanocrystal inks that are optimized for printed electronics applications. The ZnO nanocrystal solution exhibits an absorption peak at 341 nm (3.63 eV), which represents a blue-shift of approximately 0.3 eV from the bulk ZnO bandgap (~3.3 eV). This blue shift is consistent with previously reported models for an increased bandgap due to quantum confinement. We used variable-angle spectrocscopic ellipsometry (VASE) to determine the optical properties of solution-processed thin films of ZnO nanocrystals, which provides valuable insight into the changes in film composition and morphology that occur during thermal annealing treatments. The ZnO nanocrystals maintain their quantum confinement when deposited into a thin film, and the degree of quantum confinement is gradually reduced as the thermal annealing temperature increases. Using infrared absorption measurements (FTIR) and x-ray photoelectron spectroscopy (XPS), we show that the dodecanethiol ligands are removed from the annealed ZnO films, resulting in a high-purity semiconductor film with very low carbon contamination. Furthermore, we show that annealing at a suitable temperature results in complete ligand removal with only a slight increase in grain size. These results demonstrate the distinct advantages of colloidal nanocrystals for printed electronics applications: the composition and morphology of the solution-processed film can be carefully tuned by optimizing the size and surface coating of the nanocrystals in the ink.

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Publication details

The article was received on 02 Apr 2018, accepted on 03 Aug 2018 and first published on 03 Aug 2018


Article type: Paper
DOI: 10.1039/C8TC01536H
Citation: J. Mater. Chem. C, 2018, Accepted Manuscript
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    Understanding quantum confinement and ligand removal in solution-based ZnO thin films from highly stable nanocrystal ink

    Y. Sun, P. D. Donaldson, J. García-Barriocanal and S. L. Swisher, J. Mater. Chem. C, 2018, Accepted Manuscript , DOI: 10.1039/C8TC01536H

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