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The early stages of polar ZnO growth on Ag(111)

The early stages of polar ZnO growth on Ag(111). Charlotte Phillips University of Cambridge Supervisor: Dr. P. Bristowe. Overview: . Low-emissivity optical coatings Purpose and outline of the current work Constraints of the work Preliminary findings on O adsorption Future work.

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The early stages of polar ZnO growth on Ag(111)

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  1. The early stages of polar ZnO growth on Ag(111) Charlotte Phillips University of Cambridge Supervisor: Dr. P. Bristowe

  2. Overview: • Low-emissivity optical coatings • Purpose and outline of the current work • Constraints of the work • Preliminary findings on O adsorption • Future work Charlotte Phillips - Supervisor : Dr. P. Bristowe

  3. Heat-loss through windows increases the amount of energy needed to regulate building temperature [1] Heat is lost through conduction, convection and radiation Conduction and convection are minimised by filling the cavity with argon, so radiation must still be addressed => Allow only certain wavelengths of light through the window, keeping heat on one side of the glass using low-emissivity coatings Emissivity is a measure of the heat radiated by a material to the heat radiated by a blackbody at the same temperature [2] Low-emissivity optical coatings: Charlotte Phillips - Supervisor : Dr. P. Bristowe

  4. A typical low-E stack: TiO2 SnO2 Anti-scratch AR layers Barrier layer Low-E material Growth layer ZnO Weak interface Ag Very weak interface ZnO TiO2 Float Charlotte Phillips - Supervisor : Dr. P. Bristowe

  5. Purpose of the current work: • Determine the early growth mechanisms of ZnO on Ag(111) • Graphitic sheets or Wurtzite ZnO structure? • 1C/3C interface with Ag(111)? • Determine the mechanisms that make the upper ZnO/Ag interface stronger than the lower? • How does the strength of the O-Ag bonds alter as subsequent layers of Zn and O are added? • Does sub-surface O deposition in the Ag surface affect interfacial strength? Glass Charlotte Phillips - Supervisor : Dr. P. Bristowe

  6. Build up two double layers of polar ZnO on Ag(111), one layer at a time, in order to simulate the deposition and relaxation of ZnO on Ag Steps: Allow the Ag(111) surface to relax Determine the optimal configuration of a layer of O on Ag(111) Add the subsequent layers of Zn and O in both Wurtzite and graphitic structures, allowing relaxation at each stage, in order to determine the most favourable configuration of the first few layers of ZnO on Ag Determine the lowest energy structures using the total energy/atom for each model post relaxation Work outline: Wurtzite structure Graphitic structure - Harding et al, J. Mater. Chem. 15,139 (2005) Charlotte Phillips - Supervisor : Dr. P. Bristowe

  7. Constraints on the calculations: In order to describe the growth of ZnO on Ag using DFT (density functional theory) certain assumptions and approximations are made: • Thermal effects are ignored • The Ag(111) surface is assumed to be perfectly flat with no defects • Only the exact amount of Zn and O necessary to create ZnO is added to the surface (¾ ML) • Initially, only surface adatoms will be considered Charlotte Phillips - Supervisor : Dr. P. Bristowe

  8. Magnetron sputtering produces a (0001)ZnO surface which promotes the growth of (111)Ag The high-strain (1x1) coherent interface has not been observed Instead, a (2x√3) coincidence with the Ag slab rotated with respect to the ZnO slab is observed → referred to as the R30 (2x√3) interface (Arbab) → the subsequent interface making up the ZnO/Ag/ZnO stack would be a (0001)ZnO/(111)Ag R30 (2x√3) interface Observed structure of ZnO/Ag interfaces: The (1x1) (2xsupercell) coherent (left) and (2x√3) R30 (right) ZnO/Ag interfaces Charlotte Phillips - Supervisor : Dr. P. Bristowe

  9. Quantum mechanical modelling: • Density Functional Theory used • Approximations: • XC functional – GGA(PBE) • Plane waves, kinetic energy cutoff – 460eV • Ultra-soft pseudopotentials • k-point mesh – (3x5x1) • Fixed supercells • CASTEP Charlotte Phillips - Supervisor : Dr. P. Bristowe

  10. Setting up a surface calculation: • Interlayer-relaxed calculations – fixed atomic positions, interlayer distance varied • Geometry optimisation – relaxation of atomic positions • Supercell approximation: dimensions a = 11.1684Å, b = 5.5842Å, c = 30Å • Vacuum size = 15Å • Ag compressed by 2.6%, in ZnO R30(2x√3) unit cell Charlotte Phillips - Supervisor : Dr. P. Bristowe

  11. Surface calculations: • Relax the Ag(111) surface • Determine the optimal vertical distance between a layer of O and the Ag(111) surface • Allow ¾ML of O to relax on the surface • 3 different sets of initial O positions: • All fcc (like Scheffler et al), fcc-hollow Wurzite ZnO sites (off fcc), on-top Wurtzite ZnO sites All fcc; fcc-hollow Wurtzite; on-top Wurtzite. Charlotte Phillips - Supervisor : Dr. P. Bristowe

  12. Preliminary findings on O adsorption: Charlotte Phillips - Supervisor : Dr. P. Bristowe

  13. Preliminary findings on O adsorption: • Full relaxation of first three layers of a Ag(111) slab • No significant changes in atomic structure observed • Interlayer-relaxed d(O-Ag) for a layer of O on Ag(111): • All fcc : 1.4Å • Fcc-hollow (Wurtzite): 1.8Å • On-top (Wurtzite): 1.8Å • Minimum energy d(O-Ag) for each structure the same for the Wurtzite positions, slightly lower for the all fcc positions (0.007%) • For the volume relaxed calculations the all fcc sites are energetically favoured • However, when full relaxation of the interface is allowed, the Wurzite positions are favoured Charlotte Phillips - Supervisor : Dr. P. Bristowe

  14. O on Ag(111) on Wurtzite fcc-hollow sites: Preliminary findings on O adsorption: geometry optimisation: Fcc sites Charlotte Phillips - Supervisor : Dr. P. Bristowe

  15. O/Ag(111) all on fcc sites, d(O-Ag) = 1.2, 1.8: Preliminary findings on O adsorption: geometry optimisation: d(O-Ag) = 1.2Å d(O-Ag) = 1.8Å No change Slight relaxation Charlotte Phillips - Supervisor : Dr. P. Bristowe

  16. Preliminary findings on O adsorption – geometry optimisation: • All fcc sites: • No significant relaxation is observed for initial d = 1.2Å • Relaxed d(O-Ag) ~ 1.4Å • Compared with d(O-Ag) = 2.2Å for a ZnO/Ag(111) interface • Fcc hollow Wurtzite sites (off-fcc): • Relaxation is observed, with atoms adopting the fcc positions • Average relaxed d(O-Ag) ~ 1.8Å • Compared with d(O-Ag) = 2.2Å for a ZnO/Ag(111) interface Charlotte Phillips - Supervisor : Dr. P. Bristowe

  17. Future work: • Building up the layers of Zn and O on the Ag surface • Using both graphitic and Wurtzite starting structures • Adding subsurface O atoms and determining the adsorption energies of on-surface O Charlotte Phillips - Supervisor : Dr. P. Bristowe

  18. Acknowledgements: • Dr. P. D. Bristowe and Dr. Z. Lin • Dr. Paul Warren, John Ridleagh and Monica Hughes, Pilkington Glass Plc. • HPCx and EPSRC Charlotte Phillips - Supervisor : Dr. P. Bristowe

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