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 Charlotte Phillips - Supervisor : Dr. P. Bristowe

Low-emissivity optical coatings: 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] Charlotte Phillips - Supervisor : Dr. P. Bristowe

A typical low-E stack: Anti-scratch AR layers Barrier layer Low-E material Growth layer Float TiO 2 ZnO Ag ZnO SnO 2 TiO 2 Charlotte Phillips - Supervisor : Dr. P. Bristowe Very weak interface Weak interface

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? Charlotte Phillips - Supervisor : Dr. P. Bristowe Glass

Work outline: 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: 1. Allow the Ag(111) surface to relax 2. Determine the optimal configuration of a layer of O on Ag(111) 3. 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 4. Determine the lowest energy structures using the total energy/atom for each model post relaxation Charlotte Phillips - Supervisor : Dr. P. Bristowe - Harding et al, J. Mater. Chem. 15,139 (2005) Graphitic structureWurtzite structure

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: 1. Thermal effects are ignored 2. The Ag(111) surface is assumed to be perfectly flat with no defects 3. Only the exact amount of Zn and O necessary to create ZnO is added to the surface (¾ ML) 4. Initially, only surface adatoms will be considered Charlotte Phillips - Supervisor : Dr. P. Bristowe

Observed structure of ZnO/Ag interfaces: 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 Charlotte Phillips - Supervisor : Dr. P. Bristowe The (1x1) (2xsupercell) coherent (left) and (2x√3) R30 (right) ZnO/Ag interfaces

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

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

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 Charlotte Phillips - Supervisor : Dr. P. Bristowe All fcc; fcc-hollow Wurtzite; on-top Wurtzite.

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

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

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

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

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

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

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