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

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

• Charlotte Phillips
• University of Cambridge
• Supervisor Dr. P. Bristowe

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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
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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
• gt 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
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A typical low-E stack

• Anti-scratch
• AR layers
• Barrier layer
• Low-E material
• Growth layer

TiO2
SnO2
ZnO
Weak interface
Ag
Very weak interface
ZnO
TiO2
Float
Charlotte Phillips - Supervisor Dr. P. Bristowe
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Purpose of the current work
Glass

• 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
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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
• 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

Graphitic structure
Wurtzite structure
- Harding et al, J. Mater. Chem. 15,139 (2005)
Charlotte Phillips - Supervisor Dr. P. Bristowe
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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
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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 (2xv3) coincidence with the Ag slab
  rotated with respect to the ZnO slab is observed
  ? referred to as the R30 (2xv3) interface (Arbab)
• ? the subsequent interface making up the
  ZnO/Ag/ZnO stack would be a (0001)ZnO/(111)Ag R30
  (2xv3) interface

The (1x1) (2xsupercell) coherent (left) and
(2xv3) R30 (right) ZnO/Ag interfaces
Charlotte Phillips - Supervisor Dr. P. Bristowe
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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
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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(2xv3) unit cell

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

• O on Ag(111) on Wurtzite fcc-hollow sites

Fcc sites
Charlotte Phillips - Supervisor Dr. P. Bristowe
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Preliminary findings on O adsorption geometry
optimisation

• O/Ag(111) all on fcc sites, d(O-Ag) 1.2, 1.8

d(O-Ag) 1.2Å d(O-Ag)
1.8Å
No change
Slight relaxation
Charlotte Phillips - Supervisor Dr. P. Bristowe
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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
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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
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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