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Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
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, Structural Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 12, 6893-6901 p.Article in journal (Refereed) Published
Abstract [en]

The ability to understand the phonon behavior in small metal oxide nanostructures and their surfaces is of great importance for thermal and microelectronic applications in successively smaller devices. Here the development of phonons in successively larger ZnO wurtzite quantum dots (QDs) is investigated. Raman spectroscopic measurements for particles from 3 to 11 nm reveal that the E-2 Raman active optical phonon at 436 cm(-1) is the first mode to be developed with a systematic increase with particle size. We also find a broad phonon band at 260-340 attributed to surface vibrations. The E-1-LO mode at 585 cm(-1) is the next to be developed while still being strongly suppressed in the confined particles. Other modes found in bulk ZnO are not developed for particles below 11 nm. Results from density functional theory showed an excellent agreement with the experimental molecular vibrations in the zinc acetate precursor and phonon modes in bulk ZnO. To elucidate the vibration behavior and phonon development in the ZnO QDs under nonzero temperature conditions and incorporating surface reconstruction, we performed reactive force field calculations. We show that the experimentally developed phonon modes in the QDs are the ones expected from dynamic theory. In particular, we show that the surface phonon modes in the very outermost surface (5 angstrom) can explain the observed broad phonon band and give the precise relation between the intensity of the surface and bulk phonons as the particle size increases. Calculations with temperatures between 50K and 1000K also show distinction of temperature effects in the material and that the phonon peaks are not generally shifted when the system is heated and quantum confined but instead reveal a dependence on the symmetry of the phonon mode. 

Place, publisher, year, edition, pages
2012. Vol. 116, no 12, 6893-6901 p.
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-129300DOI: 10.1021/jp300985kISI: 000302051100015OAI: oai:DiVA.org:uu-129300DiVA: diva2:339920
Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2014-04-29Bibliographically approved
In thesis
1. ZnO Quantum Dots: Size Dependent Optical, Vibrational and Photoelectrochemical Properties
Open this publication in new window or tab >>ZnO Quantum Dots: Size Dependent Optical, Vibrational and Photoelectrochemical Properties
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis is devoted to quantum dots of ZnO in the size regime 2.5-10 nm.

The focus is directed towards their size dependent properties with special

emphasis on the optical, vibrational and photocatalytic properties. The particles

were synthesized by hydrolysis in alkaline zinc acetate solution.

Analysis and characterisation were performed on both growing particles in

solution as well as on thin particle films. The main characterisation techniques

were UV-vis, florescence, XRD, Raman, electrochemical- and photoelectrochemical

methods.

Based on a large set of XRD and UV-vis measurements an empirical relation

between the band gap and the particle size were developed. This enabled

size resolved measurements of the visible fluorescence on growing particles

in solution. We report evidence of mobile trap states responsible for the fluorescence

and discuss a possible surface dependent mechanism for this.

The quantum confinement of the phonon modes were investigated with

Raman spectroscopy and molecular dynamic simulations. We report on a

size depended suppression of the phonon modes and the contribution from

surface effects.

The absolute position of the conduction band edge was determined as a

function of particle size with different photoelectrochemical methods that we

describe in detail. We demonstrate that most of the size dependent shift in

the band gap occurs by a change in the position of the conduction band. We

also show that the parabolic band approximation is valid in a region of

slightly less than 0.1 eV from the conduction band edge, and this even under

an external electric field. An interesting electroabsorption phenomenon

where the absorption locally increases under an applied potential are described

and explained in terms of the quantum confined Stark effect and the

Franz-Keldysh effect.

The produced films of the particles show antireflective properties on conducting

glass substrates, which could be interesting from a technological

perspective. The films are demonstrated to show photocatalytic activity with

respect to degradation of organic dyes and for solar water splitting.

Place, publisher, year, edition, pages
T.J Jacobsson production, 2012. 195 p.
Keyword
ZnO, nanoparticels, quantum dots
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-193235 (URN)
Supervisors
Available from: 2013-01-30 Created: 2013-01-29 Last updated: 2013-01-30Bibliographically approved
2. 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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1134
Keyword
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
Identifiers
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)
Opponent
Supervisors
Available from: 2014-04-24 Created: 2014-03-27 Last updated: 2014-05-27Bibliographically approved

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Raymand, DavidJacobsson, JesperHermansson, KerstiEdvinsson, Tomas

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