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J. Appl. Phys. 113, 014304 (2013); (11 pages).
Kin Mun Wong, S. M. Alay-e-Abbas, A. Shaukat, Yaoguo Fang, Yong Lei.
Institut fUr Physik & IMN MacroNano® (ZIK), Technische Universität Ilmenau, Prof. Schmidt-Str. 26, Ilmenau 98693, Germany and
Department of Physics, GC University Faisalabad, Allama Iqbal Road, Faisalabad 38000, Pakistan and
Department of Physics, University of Sargodha, 40100 Sargodha, Pakistan.
Abstract
In this paper, all electron full-potential linearized augmented plane wave plus local orbitals method has been used to investigate the structural and electronic properties of polar (0001) and non-polar (10Ī0) surfaces of ZnO in terms of the defect formation energy (DFE), charge density and electronic band structure with the supercell-slab (SS) models. Our calculations support the size-dependent structural phase transformation of wurtzite lattice to graphite-like structure which is a result of the termination of hexagonal ZnO at the (0001) basal plane, when the stacking of ZnO primitive cell along the hexagonal principle c-axis is less than 16 atomic layers of Zn and O atoms. This structural phase transformation has been studied in terms of Coulomb energy, nature of the bond, energy due to macroscopic electric field in the [0001] direction and the surface to volume ratio for the smaller SS. We show that the size-dependent phase transformation is completely absent for surfaces with a non-basal plane termination, and the resulting structure is less stable. Similarly, elimination of this size-dependent graphite-like structural phase transformation also occurs on the creation of O-vacancy which is investigated in terms of Coulomb attraction at the surface. Furthermore, the DFE at the (10Ī0)/(Ī010) and (0001)/(000Ī) surfaces is correlated with the slab-like structures elongation in the hexagonal a- and c-axis. Electronic structure of the neutral O-vacancy at the (0001)/(000Ī) surfaces has been calculated and the effect of charge transfer between the two sides of the polar surfaces (0001)/(000Ī) on the mixing of conduction band through the 4s orbitals of the surface Zn atoms is elaborated. An insulating band structure profile for the non-polar (10Ī0)/(Ī010) surfaces and for the smaller polar (0001)/(000Ī) SS without O-vacancy is also discussed. The results in this paper will be useful for the tuning of the structural and electronic properties of the (0001) and (10Ī0) ZnO nanosheets by varying their size.
© 2013 American Institute of Physics.
Additional Information
For ZnO nanosheets (ultra-thin films) comprising of few Zn-O layers, it was found from density functional theory (DFT) calculations that the graphite-like structure was energetically more favourable as compared to the wurtzite crystal structure. The stability of this graphite-like structure of the ZnO nanosheets is only limited to the thickness of about few Zn-O layers (along the c-axis), beyond which they revert back to the wurtzite phase. Due to the special properties of graphene, these graphite-like ZnO nanosheets have attracted much interest, but the influence of oxygen vacancies (VO) on the important phase transition when the ZnO nanosheets revert back from the graphite-like phase to the wurtzite structure had been scarcely reported. In this article, this important result was highlighted by the DFT calculations using the full-potential linearized augmented plane-wave plus local orbitals method on the ZnO (0001) nanosheets where they are modelled by supercell slab by stacking the bulk unit cell of wurtzite ZnO in the c-axis of hexagonal lattice.
The transition from the bulk-like wurtzite structure to the graphite-like structure for the nanosheets of different sizes is due to a number of different factors. Importantly, the extreme surface Zn and O atoms of the smaller nanosheets have already lost one of the four bonds due to the surface termination as compared to bulk ZnO, a larger surface to volume ratio for these smaller nanosheets ensures that they are unable to compensate for these broken surface bonds together with the stronger Coulomb’s attraction from the interior atomic layers. This then results in movement of the surface atomic layers towards the interior of the nanosheet, flattening the atomic layers and hence a phase transformation from the wurtzite lattice to graphite-like lattice prevails for the thinner ZnO nanosheets. However, the creation of surface O-vacancy at the Zn-terminated (0001) surface of the ZnO (0001) nanosheets results in the removal of the stronger Coulomb’s attraction at the Zn-terminated (0001) surface. Therefore, reverting of the structural phase transformation from the graphite-like structure back to the wurtzite lattice occur even if the thickness of the ZnO nanosheet along the c-axis is less than or equal to 4 atomic graphite-like layers. The presence of oxygen vacancies results in eliminating the size-dependent graphite-like structural phase transformation for the defective ZnO nanosheets. The results in the article would have important implications for ZnO nanosheets in potential nanoscale applications.
