A Theoretical Study of Ultra-Thin Films with the Wurtzite and Zinc Blende Structures
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1035-L09-08
A Theoretical Study of Ultra-Thin Films with the Wurtzite and Zinc Blende Structures Frederik Claeyssens1, Colin L. Freeman2, John H. Harding2, and Neil L. Allan1 1 School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, United Kingdom 2 Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, United Kingdom ABSTRACT Results of periodic ab initio density functional theory calculations on thin films of (i) wurtzite ZnO (hexagonal) which terminate with the non-polar (101¯ 0) surface, and with the polar (0001) and (0001¯ ) surfaces (ii) zinc blende (cubic) ZnO which terminate with the non-polar (110) and with the polar (111) surfaces. Thin (less than18 layer) films of wurtzite ZnO which terminate with the polar (0001) and (0001¯ ) surfaces are found to be higher in energy than corresponding films in which these polar surfaces flatten out forming a new planar ‘graphitic’like structure in which the Zn and O atoms are coplanar and the dipole is removed. This is the lowest energy surface for ultra-thin films. For zinc-blende ZnO a graphitic-type solution, but with a different stacking of ZnO layers, is also comparable to energy to the non-polar (110) and polar (111) solutions. Consequences for crystal growth and the stabilization of thin films and nanostructures are discussed. INTRODUCTION Zinc oxide is an abundant, non-toxic, wide band gap semiconductor (~3.4 eV) with many useful properties including, among others, ultra-violet absorption and emission, piezoelectricity, high voltage-current nonlinearity and catalytic activity [1]. Such behaviour has prompted investigations of its use for applications in luminescence, solar cells, surface acoustic wave devices, waveguides, microsensors and photocatalysts. Ultra-violet lasing, at room temperature, from both films and nanotubes of ZnO on sapphire has also been reported [2]. Many properties depend crucially on crystal size, orientation and, in particular, morphology. A remarkable superhydrophobicity to super-hydrophilicity transition has been observed at the {0001} surface of aligned hexagonal ZnO nanorod films [3]. Thin films of ionic and semi-ionic solids such as zinc oxide provide an unrivalled opportunity for experimental characterization and fabrication of polar nanostructures with unusual behaviour. Of particular interest are films which terminate in ‘polar’ (Tasker Type III) surfaces [4], i.e. each repeat unit perpendicular to the surface has a non-zero dipole moment (Figure 1). An electrostatic instability results from the resulting macroscopic electric field. The experimentally observed prevalence of the growth of hexagonal ZnO by the (0001) (Zn cation outermost) and (0001¯ ) (O anion outermost) surfaces under a wide range of growth conditions is an excellent example of an apparent violation of Tasker’s rule [4]. This requires stabilisation of the surface by one of three mechanisms. In the first of these the surface can reconstruct via major changes in surface sto
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