Making amorphous ZnO: Theoretical predictions of its structure and stability

ZnO is a transparent semiconductor with optoelectronic, thermoelectric, and sensor applications, where using amorphous thin films presents great advantages. However, growing amorphous (a) films of pure ZnO proved challenging due to their rapid crystallization. We investigated the ability of bulk ZnO to form glass structures using well-tested interatomic potentials and a melt and quench procedure within isothermal-isobaric (NPT) ensemble. The geometries of some of the resulting structures were further optimized using density functional theory (DFT) calculations with the PBE functional. We demonstrate that cooling rates in melt and quench procedure equal or exceeding 100 Kps−1 lead to formation of stable amorphous structures. However, ZnO samples tend to crystallize at lower cooling rates. This result does not depend on the size of the periodic cell used in the calculations for cells containing more than 324 atoms. Using simulation cells with up to 768 000 atoms, we demonstrate that the expected average glass density is about 5.04 gcm−3 and the coordination numbers of Zn and O atoms are around 3.9. We calculate radial distribution functions and characterize the structures of amorphous ZnO samples. Using both the activation-relaxation technique and simulated annealing, we show that the obtained amorphous structures have low propensity to crystallization

Morphology of Cu clusters supported on reconstructed polar ZnO (0001) and (0001̄) surfaces

Unbiased Monte Carlo procedures are applied to investigate the structure of Cu clusters of various sizes deposited over reconstructed polar ZnO surfaces. Four distinct reconstructed polar ZnO surfaces (two Zn terminated (0001) reconstructions and two O terminated (000[1 with combining macron]) reconstructions) were investigated, having previously been determined to be the most stable under typical conditions, as revealed by the grand canonical ensemble studies. Random sampling was performed considering ∼400 000 random initial structural configurations of Cu atoms over the ZnO surfaces, with each structure being optimised using interatomic potential techniques, and the most stable resultant structures being refined using a plane-wave DFT approach. The investigation reveals the key role of surface adatoms and vacancies arising from the reconstruction of the polar ZnO surface in determining the morphology of deposited Cu clusters. Strong Cu–Zn interactions play an essential role in Cu cluster growth, with reconstructed polar ZnO surfaces featuring sites with undercoordinated Zn surface atoms promoting highly localised three dimensional Cu cluster morphologies, whist reconstructions featuring undercoordinated O atoms tend to result in more planar Cu clusters, in order to maximise the favourable Cu–Zn interaction. This is the first study that evaluates the thermodynamically most stable Cu/ZnO structures using realistic reconstructed ZnO polar surfaces, and thus provides valuable insights into the factors affecting Cu cluster growth over ZnO surfaces, as well as model catalyst surfaces that can be utilised in future computational studies to explore catalytic activity for key processes such as CO2 and CO hydrogenation to methanol

Development of Interatomic Potentials for Supported Nanoparticles: The Cu/ZnO Case

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 (101̅0) 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 (101̅0) surface but are rarely seen on terraces

Inverse Perovskite Oxysilicides and Oxygermanides as Candidates for Nontoxic Infrared Semiconductor and Their Chemical Bonding Nature

We have synthesized inverse-perovskite-type oxysilicides and oxygermanides represented by R3SiO and R3GeO (R = Ca and Sr) and studied their characteristics in the search for nontoxic narrow band gap semiconductors. These compounds exhibit a sharp absorption edge around 0.9 eV and a luminescence peak in the same energy range. These results indicate that the obtained materials have a direct-band electronic structure, which was confirmed by hybrid DFT calculations. These materials, made from earth abundant and nontoxic elements and with a relatively light electron/hole effective mass, represent strong candidates for nontoxic optoelectronic devices in the infrared range

Real and virtual polymorphism of titanium selenide with robust interatomic potentials

The first successful pairwise potential for a layered material, TiSe2, has been parameterised to fit the experimental data, using a genetic algorithm as the optimisation tool for the parameters of the interatomic potential

Morphological Features and Band Bending at Nonpolar Surfaces of ZnO

We employ hybrid density functional calculations to analyze the structure and stability of the (101̅0) and (112̅0) ZnO surfaces, confirming the relative stability of the two surfaces. We then examine morphological features, including steps, dimer vacancies, and grooves, at the main nonpolar ZnO surface using density functional methods. Calculations explain why steps are common on the (101̅0) surface even at room temperature, as seen in experiment. The surface structure established has been used to obtain the definitive ionization potential and electron affinity of ZnO in good agreement with experiment. The band bending across the surface is analyzed by the decomposition of the density of states for each atomic layer. The upward surface band bending at the (101̅0) surface affects mostly the valence band by 0.32 eV, which results in the surface band gap closing by 0.31 eV; at the (112̅0) surface, the valence band remains flat and the conduction band bends up by 0.18 eV opening the surface band gap by 0.12 eV

Morphology of Cu clusters supported on reconstructed polar ZnO (0001) and (000[1]) surfaces†

Unbiased Monte Carlo procedures are applied to investigate the structure of Cu clusters of various sizes deposited over reconstructed polar ZnO surfaces. Four distinct reconstructed polar ZnO surfaces (two Zn terminated (0001) reconstructions and two O terminated (000[1 with combining macron]) reconstructions) were investigated, having previously been determined to be the most stable under typical conditions, as revealed by the grand canonical ensemble studies. Random sampling was performed considering ∼400 000 random initial structural configurations of Cu atoms over the ZnO surfaces, with each structure being optimised using interatomic potential techniques, and the most stable resultant structures being refined using a plane-wave DFT approach. The investigation reveals the key role of surface adatoms and vacancies arising from the reconstruction of the polar ZnO surface in determining the morphology of deposited Cu clusters. Strong Cu–Zn interactions play an essential role in Cu cluster growth, with reconstructed polar ZnO surfaces featuring sites with undercoordinated Zn surface atoms promoting highly localised three dimensional Cu cluster morphologies, whist reconstructions featuring undercoordinated O atoms tend to result in more planar Cu clusters, in order to maximise the favourable Cu–Zn interaction. This is the first study that evaluates the thermodynamically most stable Cu/ZnO structures using realistic reconstructed ZnO polar surfaces, and thus provides valuable insights into the factors affecting Cu cluster growth over ZnO surfaces, as well as model catalyst surfaces that can be utilised in future computational studies to explore catalytic activity for key processes such as CO2 and CO hydrogenation to methanol

Heterostructures of GaN with SiC and ZnO enhance carrier stability and separation in framework semiconductors

A computational approach, using the density functional theory, is employed to describe the enhanced electron‐hole stability and separation in a novel class of semiconducting composite materials, with the so‐called double bubble structural motif, which can be used for photocatalytic applications. We examine the double bubble containing SiC mixed with either GaN or ZnO, as well as related motifs that prove to have low formation energies. We find that a 24‐atom SiC sodalite cage inside a 96‐atom ZnO cage possesses electronic properties that make this material suitable for solar radiation absorption applications. Surprisingly stable, the inverse structure, with ZnO inside SiC, was found to show a large deformation of the double bubble and a strong localisation of the photo‐excited electron charge carriers, with the lowest band gap of ca. 2.15 eV of the composite materials considered. The nanoporous nature of these materials could indicate their suitability for thermoelectric applications. "This is the peer reviewed version of the following article: Heterostructures of GaN with SiC and ZnO enhance carrier stability and separation in framework semiconductors, which has been published in final form at https://doi.org/10.1002/pssa.201600440. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.

Defect formation in In2O3 and SnO2: a new atomistic approach based on accurate lattice energies

We present a consistent interatomic force field for indium sesquioxide (In2O3) and tin dioxide (SnO2) 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 SnO2 and In2O3, 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

Defect formation in In2O3 and SnO2: a new atomistic approach based on accurate lattice energies

We present a consistent interatomic force field for indium sesquioxide (In2O3) and tin dioxide (SnO2) 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 SnO2 and In2O3, 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