Why Are Polar Surfaces of ZnO Stable?

David Mora-Fonz,Matthew R Farrow,Tomas Lazauskas,Alexey A Sokol,C Richard A Catlow,Scott M Woodley

doi:10.1021/acs.chemmater.7b01487

Abstract

We probe and rationalize the complex surface chemistry of wurtzite ZnO by employing interatomic potential calculations coupled with a Monte Carlo procedure that sampled over 0.5 million local minima. We analyze the structure and stability of the (0001) and (0001) ZnO surfaces, rationalizing previous patterns found in STM images and explaining the (1 × 1) periodicity reported by LEED analysis. The full range of Zn/O surface occupancies was covered for a (5 × 5) supercell, keeping |mZn – mO|/N ≈ 0.24 where m and N are the numbers of occupied surface sites and total surface sites, respectively. Our calculations explain why the (5 × 5) reconstructions seen in some experiments and highlight the importance of completely canceling the inherent dipole of the unreconstructed polar surfaces. The experimentally observed rich reconstruction patterns can be traced from the lowest occupancy, showing the thermodynamically most stable configurations of both polar surfaces. Triangular and striped reconstructions are seen...

Highlights

Modeling or predicting surface structures often relies on intuition and input from experiment
The proposed approach is novel in the field of surface structure prediction and, in modeling realistic extended systems across the field of the chemistry of materials, where grand canonical Monte Carlo methods have been primarily confined to Boltzmann lattice simulations and related techniques
To explain the many contradicting experimental findings,[1,3−12,19−23] we examine in detail the most stable surface structures that will emerge upon crystal growth along the polar directions, which coincide with the c axis of the wurtzite crystal structure

Summary

Introduction

Modeling or predicting surface structures often relies on intuition and input from experiment. For complex surfaces, this approach is often insufficient to determine their real structure and fails to explain the effect of experimental conditions that are vital in understanding the surface chemistry of oxides To address this failure, we have implemented unbiased Monte Carlo methods for canonical and grand canonical ensembles and applied them to the intriguing case of the polar surfaces of wurtzite ZnO. We do attribute the stability of the polar surfaces to the high degree of disorder but calculate the relevant thermodynamic surface potential (grand, or Landau surface potential) and show that it can become comparable with or even lower than the corresponding surface energy of the competing nonpolar surfaces, which is a first proof of the observed surface stability These results are key both to fundamental and applied surface chemistry of ZnO, which in turn is one of the most important advanced functional materials. A recent study on CeO2 by Loṕ ez’ group[15] has demonstrated the importance of configurational entropy in stabilizing polar surfaces in general

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Conclusion