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Photoemission study of ZnO nanocrystals: Thermal annealing in UHV and induced band bending
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. (Materialfysik)
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. (Materialvetenskap)ORCID iD: 0000-0003-4165-1515
Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. (Fysik)
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. Vol. 612, 10-15 p.
Keyword [en]
ZnO, Nanocrystals, XPS, Annealing, Band bending
National Category
Nano Technology
Research subject
Physics
Identifiers
URN: urn:nbn:se:kau:diva-27016DOI: 10.1016/j.susc.2013.02.001ISI: 000317809100005OAI: oai:DiVA.org:kau-27016DiVA: diva2:617224
Available from: 2013-04-22 Created: 2013-04-22 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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