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AFM and STM Study of ZnO Nanoplates
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics. (Materialfysik)
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics. (Fysik)
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics. (Materialvetenskap)ORCID iD: 0000-0003-4165-1515
(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 [en]
ZnO, Nanostructures, Microwave assisted growth, AFM, STM
National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kau:diva-27155OAI: oai:DiVA.org:kau-27155DiVA: diva2:619265
Available from: 2013-05-02 Created: 2013-05-02 Last updated: 2015-04-15Bibliographically approved
In thesis
1. Growth and Characterization of ZnO Nanocrystals
Open this publication in new window or tab >>Growth and Characterization of ZnO Nanocrystals
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
ZnO, Nanocrystals, Surface physics, XPS, SEM, AES, AFM, STM
National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
urn:nbn:se:kau:diva-27156 (URN)978-91-7063-503-8 (ISBN)
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

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