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Growth and Characterization of ZnO Nanocrystals
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The understanding of surfaces of materials is of crucial importance to all of us. Considering nanocrystals (NCs), that have a large surface to bulk ratio, the surfaces become even more important. Therefore, it is important to understand the fundamental surface properties in order to use NCs efficiently in applications. In the work reported in this thesis ZnO NCs were studied.

At MAX-lab in Lund, synchrotron radiation based Spectroscopic Photoemission and Low Energy Electron Microscopy (SPELEEM) and X-ray Photoelectron Spectroscopy (XPS) were used. At Karlstad University characterization was done using Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), Scanning Tunnelling Microscopy (STM), Auger Electron Spectroscopy (AES), and XPS.

The fundamental properties of ZnO surfaces were studied using distributions of ZnO NCs on SiO2/Si surfaces. The conditions for distribution of ZnO NCs were determined to be beneficial when using ethanol as the solvent for ultrasonically treated dispersions. Annealing at 650 °C in UHV cleaned the surfaces of the ZnO NCs enough for sharp LEEM imaging and chemical characterization while no sign of de-composition was found. A flat energy band structure for the ZnO/SiO2/Si system was proposed after 650 °C. Increasing the annealing temperature to 700 °C causes a de-composition of the ZnO that induce a downward band bending on the surfaces of ZnO NCs.

Flat ZnO NCs with predominantly polar surfaces were grown using a rapid microwave assisted process. Tuning the chemistry in the growth solution the growth was restricted to only plate-shaped crystals, i.e. a very uniform growth. The surfaces of the NCs were characterized using AFM, revealing a triangular reconstruction of the ZnO(0001) surface not seen without surface treatment at ambient conditions before. Following cycles of sputtering and annealing in UHV, we observe by STM a surface reconstruction interpreted as 2x2 with 1/4 missing Zn atoms.

Abstract [en]

Baksidestext

The understanding of the surfaces of materials is of crucial importance to all of us. Considering nanocrystals (NCs), that have a large surface to bulk ratio, the surfaces become even more important. In the work in this thesis ZnO NCs were studied.

The fundamental properties of ZnO surfaces were studied using distributions of ZnO NCs on SiO2/Si surfaces. Annealing at 650 °C in UHV cleaned the surfaces of the ZnO NCs enough for sharp LEEM imaging and chemical characterization while no sign of de-composition was found. A flat energy band structure for the ZnO/SiO2/Si system was proposed after 650 °C. Increasing the annealing temperature to 700 °C causes a de-composition of the ZnO that induce a downward band bending on the surfaces of ZnO NCs.

Flat ZnO NCs with predominantly polar surfaces were grown using a microwave assisted process. Tuning the chemistry in the growth solution the growth was restricted to only plate-shaped crystals, i.e. a very uniform growth. The surfaces of the NCs were characterized using AFM, revealing a triangular reconstruction of the ZnO(0001) surface not seen without surface treatment at ambient conditions before. Following cycles of sputtering and annealing in UHV, we observe by STM a surface reconstruction interpreted as 2x2 with 1/4 missing Zn atoms.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2013. , 56 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2013:26
Keyword [en]
ZnO, Nanocrystals, Surface physics, XPS, SEM, AES, AFM, STM
National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kau:diva-27156ISBN: 978-91-7063-503-8 (print)OAI: oai:DiVA.org:kau-27156DiVA: diva2:619268
Public defence
2013-06-14, 21A 342, Karlstad University, Karlstad, 13:15 (English)
Opponent
Supervisors
Available from: 2013-05-23 Created: 2013-05-02 Last updated: 2013-05-23Bibliographically approved
List of papers
1. ZnO nanocrystals on SiO2/Si surfaces thermally cleaned in ultrahigh vacuum and characterized using spectroscopic photoemission and low energy electron microscopy
Open this publication in new window or tab >>ZnO nanocrystals on SiO2/Si surfaces thermally cleaned in ultrahigh vacuum and characterized using spectroscopic photoemission and low energy electron microscopy
2010 (English)In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 28, no 3, 438-442 p.Article in journal (Refereed) Published
Abstract [en]

Thermal cleaning in ultrahigh vacuum of ZnO nanocrystals distributed on SiO2/Si surfaces has been studied using spectroscopic photoemission and low energy electron microscopy (SPELEEM). This study thus concern weakly bound ZnO nanocrystals covering only 5%–10% of the substrate. Chemical properties, crystallinity, and distribution of nanocrystals are used to correlate images acquired with the different techniques showing excellent correspondence. The nanocrystals are shown to be clean enough after thermal cleaning at 650 °C to be imaged by LEEM and x-ray PEEM as well as chemically analyzed by site selective x-ray photoelectron spectroscopy (μ-XPS). μ-XPS shows a sharp Zn 3d peak and resolve differences in O 1s states in oxides. The strong LEEM reflections together with the obtained chemical information indicates that the ZnO nanocrystals were thermally cleaned, but do not indicate any decomposition of the nanocrystals. μ-XPS was also used to determine the thickness of SiO2 on Si. This article is the first to our knowledge where the versatile technique SPELEEM has been used to characterize ZnO nanocrystals.

National Category
Physical Sciences
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-9765 (URN)10.1116/1.3372804 (DOI)000277241900010 ()
Available from: 2012-02-08 Created: 2012-02-08 Last updated: 2017-12-07Bibliographically approved
2. Photoemission study of ZnO nanocrystals: Thermal annealing in UHV and induced band bending
Open this publication in new window or tab >>Photoemission study of ZnO nanocrystals: Thermal annealing in UHV and induced band bending
2013 (English)In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 612, 10-15 p.Article in journal (Refereed) Published
Abstract [en]

ZnO nanocrystals distributed by spin-coating on SiO2/Si surfaces were annealed in UHV and studied in situ by synchrotron radiation based X-ray Photoelectron Spectroscopy. Changes in chemical composition and electronic structure of ZnO nanocrystal surfaces were found with increasing annealing temperatures. Annealing at 650 °C reduces the surface contaminant levels without any observed de-composition of ZnO. After annealing at 700 °C an initial de-composition of ZnO together with further reduction of contaminants was observed. As a result, 650 °C is found to be the optimal annealing temperature for thermal cleaning of ZnO nanocrystals. Chemical changes and induced point defect formation cause changes in the band structure of the ZnO/SiO2/Si system. An upward band bending of 0.7 eV on the surfaces of the ZnO nanocrystals was found after annealing at 300 °C. The bands on the surfaces of ZnO nanocrystals gradually bend downwards with increasing annealing temperatures. A downward band bending of 1.4 eV is the result after annealing at 750 °C for 1 h. This large downward band bending is explained as due to the change in balance of oxygen vacancies and zinc vacancies on the surfaces of ZnO nanocrystals.

Place, publisher, year, edition, pages
Amsterdam: Elsevier, 2013
Keyword
ZnO, Nanocrystals, XPS, Annealing, Band bending
National Category
Nano Technology
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-27016 (URN)10.1016/j.susc.2013.02.001 (DOI)000317809100005 ()
Available from: 2013-04-22 Created: 2013-04-22 Last updated: 2017-12-06Bibliographically approved
3. Preparation of ZnO nanocrystals for individual surface analysis
Open this publication in new window or tab >>Preparation of ZnO nanocrystals for individual surface analysis
2013 (English)Manuscript (preprint) (Other academic)
Keyword
ZnO, Nanocrystals, AFM
National Category
Nano Technology
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-27094 (URN)
Available from: 2013-05-02 Created: 2013-04-26 Last updated: 2014-10-14Bibliographically approved
4. Microwave assisted rapid growth of flat ZnO(0001) platelets
Open this publication in new window or tab >>Microwave assisted rapid growth of flat ZnO(0001) platelets
2013 (English)Manuscript (preprint) (Other academic)
Keyword
ZnO, Nanostructures, Microwave assisted growth
National Category
Condensed Matter Physics Nano Technology
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-27153 (URN)
Projects
Bio2Mat
Funder
Vinnova
Available from: 2013-05-02 Created: 2013-05-02 Last updated: 2014-02-10Bibliographically approved
5. AFM and STM Study of ZnO Nanoplates
Open this publication in new window or tab >>AFM and STM Study of ZnO Nanoplates
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The surface morphology and electronic structure of hexagonal ZnO nanoplates have been studied by Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM). It was found that these nanoplates are terminated by their polar (0001) surfaces. The AFM investigation was performed in the ambient conditions with the nanocrystals “as grown”. Surprisingly, the AFM images of the top surfaces revealed an interesting triangular reconstruction, which was earlier observed only after cycles of sputtering and annealing of the ZnO(0001) surface in Ultra High Vacuum (UHV) systems. The surface atomic and electronic structures of these nanoplates have been further studied by STM and Scanning Tunneling Spectroscopy (STS) in UHV. The STM images also showed a triangular structure with single atomic steps. In addition, a 2x2 surface reconstruction has been observed with high resolution STM. This reconstruction agrees well with the recently proposed model that involves the removal of 1/4 of the topmost Zn atoms on the ZnO(0001) surface.

Keyword
ZnO, Nanostructures, Microwave assisted growth, AFM, STM
National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-27155 (URN)
Available from: 2013-05-02 Created: 2013-05-02 Last updated: 2015-04-15Bibliographically approved

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