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Quantum Confined Stark Effects in ZnO Quantum Dots Investigated with Photoelectrochemical Methods
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 22, 12061-12072 p.Article in journal (Refereed) Published
Abstract [en]

The optical absorption behavior of ZnO quantum dots has been investigated as a function of particle size in the quantum confined regime, between 4 and 9 nm in diameter, by using photoelectrochemical methods. Thin films of quantum dots, with 18 different sizes, were prepared on conducting substrates where the Fermi level could be controlled potentiostatically simultaneously as absorption measurements were performed. While raising the Fermi level into the conduction band, the dominant effect is a decrease in absorption as a consequence of increased electron population in the conduction band. This is a potentiostatic analogue to the Burstein-Moss shift for degenerate semiconductors. For applied potentials in an interval of 0.2 eV below the conduction band edge, the absorption does, however, increases instead of decreases. This absorption increase was found to be caused by a transition into states located within the band gap, which are introduced as a consequence of the applied potential. The magnitude of this effect is for the smallest particles (4 nm) approximately 9% compared to the magnitude of the Burstein-Moss bleaching. The effect decreases with increased particle size and essentially disappears for particles approaching 9 nm. The phenomenon is analyzed in terms of the Stark effect where the consequence of the applied potential is a buildup of an electric field within the particles, breaking the symmetry and splitting the energy levels in the conduction band. The gradual disappearance of the effect for the growing particles gives the extent of the quantum confinement effects of this phenomenon. The size-dependent absorption probability is analyzed and gives important information concerning the nature of both the perturbed states above the conduction band edge and the formation of the subband edge states.

Place, publisher, year, edition, pages
2014. Vol. 118, no 22, 12061-12072 p.
National Category
Inorganic Chemistry
URN: urn:nbn:se:uu:diva-221257DOI: 10.1021/jp503098qISI: 000337013400064OAI: oai:DiVA.org:uu-221257DiVA: diva2:708214
Available from: 2014-03-27 Created: 2014-03-27 Last updated: 2014-07-16Bibliographically approved
In thesis
1. Highly Efficient CIGS Based Devices for Solar Hydrogen Production and Size Dependent Properties of ZnO Quantum Dots
Open this publication in new window or tab >>Highly Efficient CIGS Based Devices for Solar Hydrogen Production and Size Dependent Properties of ZnO Quantum Dots
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Materials and device concepts for renewable solar hydrogen production, and size dependent properties of ZnO quantum dots are the two main themes of this thesis.

ZnO particles with diameters less than 10 nm, which are small enough for electronic quantum confinement, were synthesized by hydrolysis in alkaline zinc acetate solutions. Properties investigated include: the band gap - particle size relation, phonon quantum confinement, visible and UV-fluorescence as well as photocatalytic performance. In order to determine the absolute energetic position of the band edges and the position of trap levels involved in the visible fluorescence, methods based on combining linear sweep voltammetry and optical measurements were developed.

The large band gap of ZnO prevents absorption of visible light, and in order to construct devices capable of utilizing a larger part of the solar spectrum, other materials were also investigated, like hematite , Fe2O3, and CIGS, CuIn1-xGaxSe2.

The optical properties of hematite were investigated as a function of film thickness on films deposited by ALD. For films thinner than 20 nm, a blue shift was observed for both the absorption maximum, the indirect band gap as well as for the direct transitions. The probability for the indirect transition decreased substantially for thinner films due to a suppressed photon/phonon coupling. These effects decrease the visible absorption for films thin enough for effective charge transport in photocatalytic applications.

CIGS was demonstrated to be a highly interesting material for solar hydrogen production. CIGS based photocathodes demonstrated high photocurrents for the hydrogen evolution half reaction. The electrode stability was problematic, but was solved by introducing a modular approach based on spatial separation of the basic functionalities in the device. To construct devices capable of driving the full reaction, the possibility to use cells interconnected in series as an alternative to tandem devices were investigated. A stable, monolithic device based on three CIGS cells interconnected in series, reaching beyond 10 % STH-efficiency, was finally demonstrated. With experimental support from the CIGS-devices, the entire process of solar hydrogen production was reviewed with respect to the underlying physical processes, with special focus on the similarities and differences between various device concepts.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 155 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1134
ZnO, Nanoparticles, Quanum Dots, Size Dependent Properties, Hematite, CIGS, Solar Water Splitting, Hydrogen Production, PEC, Photoelectrochemical cells, PV-electrolysis
National Category
Inorganic Chemistry Physical Chemistry Materials Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
urn:nbn:se:uu:diva-221260 (URN)978-91-554-8918-2 (ISBN)
Public defence
2014-05-23, Häggsalen, Ångström Laboratory, Lägehydsvägen 1, Uppsala, 09:15 (English)
Available from: 2014-04-24 Created: 2014-03-27 Last updated: 2014-05-27Bibliographically approved

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