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 (0001̅ ) 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 |m Zn − m O |/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, inter alia, on both polar surfaces, and hexagonal patterns also appear on the O terminated surface. Our results explain the main experimental structures observed on these complex surfaces. Moreover, grand canonical simulations of ZnO polar surfaces reveal that disorder is favored and, thus, configurational entropic factors is the the cause of their stability.

We have developed and implemented a new global optimization technique based on a Lamarckian genetic algorithm with the focus on structure diversity. The key process in the efficient search on a given complex energy landscape proves to be the removal of duplicates that is achieved using a topological analysis of candidate structures. The careful geometrical prescreening of newly formed structures and the introduction of new mutation move classes improve the rate of success further. The power of the developed technique, implemented in the Knowledge Led Master Code, or KLMC, is demonstrated by its ability to locate and explore a challenging double funnel landscape of a Lennard-Jones 38 atom system (LJ). We apply the redeveloped KLMC to investigate three chemically different systems: ionic semiconductor (ZnO), metallic Ni and covalently bonded C. All four systems have been systematically explored on the energy landscape defined using interatomic potentials. The new developments allowed us to successfully locate the double funnels of LJ, find new local and global minima for ZnO clusters, extensively explore the Ni and C (the buckminsterfullerene, or buckyball) potential energy surfaces.

We present a consistent interatomic force field for indium sesquioxide (In 2 O 3 ) and tin dioxide (SnO 2 ) that has been derived to reproduce lattice energies and, consequently, the oxygen vacancy formation energies in the respective binary compounds. The new model predicts the dominance of Frenkel-type disorder in SnO 2 and In 2 O 3 , in good agreement with ab initio defect calculations. The model is extended to include free electron and hole polarons, which compete with charged point defects to maintain charge neutrality in a defective crystal. The stability of electrons and instability of holes with respect to point defect formation rationalises the efficacy of n-type doping in tin doped indium oxide (ITO), a widely employed transparent conducting oxide in optoelectronic applications. We investigate the clustering of Sn substitutional and oxygen interstitial sites in ITO, finding that the dopants substitute preferentially on the cation crystallographic d site in the bixbyite unit cell, in agreement with experiment.The force field described here provides a useful avenue for the investigation of the defect properties of extended transparent conducting oxide systems, including solid solutions.

We present a potential model that has been parametrized to reproduce accurately metal−metal oxide interactions of Cu clusters supported on ZnO. Copper deposited on the nonpolar (101̅ 0) ZnO surface is investigated using the new pairwise Cu−ZnO interatomic potentials including repulsive Born−Mayer Cu−O and attractive Morse Cu−Zn potentials. Parameters of these interactions have been determined by fitting to periodic supercell DFT data using different surface terminations and Cu cluster sizes. Results of interatomic potential-based simulations show a good agreement both structurally and energetically with DFT data, and thus provide an efficient filter of configurations during a search for low DFT energy structures. Upon examining the low energy configurations of Cu clusters on ZnO nonpolar surfaces for a range of cluster sizes, we discovered why Cu islands are commonly observed on step edges on the (101̅ 0) surface but are rarely seen on terraces.