Relative stability of nanosized wurtzite and graphitic ZnO from density functional theory

Influence of Dispersion Interactions on the Polymorphic Stability of Crystalline Oxides

The accurate determination of relative phase stabilities using DFT methods is a significant challenge when some of these can vary by only a few kJ/mol. Here, we demonstrate that for a selection of oxides (TiO₂, MnO₂, and ZnO) the inclusion of dispersion interactions, accomplished using the DFT-D3 correction scheme, allows for the correct ordering and an improved calculation of the energy differences between polymorphic phases. The energetic correction provided is of the same order of magnitude as the energy difference between phases. D3-corrected hybrid functionals systematically yield results closest to experiment. We propose that the inclusion of dispersion interactions makes a significant contribution to the relative energetics of polymorphic phases, especially those with different densities, and should therefore be included for calculations of relative energies using DFT methods.

Size dependent structural and polymorphic transitions in ZnO: from nanocluster to bulk

We report on an extensive survey of (ZnO)_N nanostructures ranging from bottom-up generated nanoclusters to top-down nanoparticles cuts from bulk polymorphs. The obtained results enable us to follow the energetic preferences of structure and polymorphism in (ZnO)_N systems with N varying between 10-1026. This size range encompasses small nanoclusters with 10s of atoms and nanoparticles with 100s of atoms, which we also compare with appropriate bulk limits. In all cases the nanostructures and bulk systems are optimized using accurate all-electron, relativistic density functional theory based calculations with numeric atom centered orbital basis sets. Specifically, sets of five families of (ZnO)_N species are considered: single-layered and multi-layered nanocages, and bulk cut nanoparticles from the sodalite (SOD), body centered tetragonal (BCT), and wurtzite (WZ) ZnO polymorphs. Using suitable fits to interpolate and extrapolate these data allows us to assess the size-dependent energetic stabilities of each family. With increasing size our results indicate a progressive change in energetic stability from single-layered to multi-layered cage-like nanoclusters. For nanoparticles of around 2.6 nm diameter we identify a transitional region where multi-layered cages, SOD, and BCT nanostructures are very similar in energetic stability. This transition size also marks the size regime at which bottom-up nanoclusters give way to top-down bulk-cut nanoparticles. Eventually, a final crossover is found where the most stable WZ-ZnO polymorph begins to energetically dominate at N ∼ 2200. This size corresponds to an approximate nanoparticle diameter of 4.7 nm, in line with experiments reporting the observation of wurtzite crystallinity in isolated ligand-free ZnO nanoparticles of 4-5 nm size or larger.

Evidence for multi-polymorphic islands during epitaxial growth of ZnO on Ag(1 1 1)

A range of models of free standing and Ag(1 1 1)-supported stoichiometric ZnO films with coverages between 2-3 monolayers are studied using density functional calculations. Following experimental observations we focus on stoichiometric hexagonal and triangular ad-layer islands grown on top of two complete ZnO monolayers. The adlayer islands display distinct edge and corner reconstructions and are found to induce a structural transition extending from the island core to the layered phase below. Based on our results we propose a general model of ad-layer triangular island structure based on seven regions exhibiting four distinct polymorphs.

Pressure-induced phase transition in hydrothermally grown ZnO nanoflowers investigated by Raman and photoluminescence spectroscopy

This paper reports the pressure-dependent photoluminescence and Raman spectral investigation of hydrothermally synthesized ZnO nanoflowers at room temperature. Intrinsic near-band-edge UV emission from ZnO nanoflowers is monotonously blue-shifted under pressures up to 13.8 GPa with a pressure coefficient of 26 meV GPa(-1), and this pressure value is nearly 5 GPa above the transition pressure from the wurtzite to the rock salt phase for bulk ZnO. The Raman band corresponds to the wurtzite phase, the [Formula: see text] and [Formula: see text] modes were observed up to about 11 GPa from the spectra. The apparent discrepancy in the transition pressures as determined from photoluminescence and Raman studies suggests that it is a consequence of the gradual phase transition, in which the smallest nanoparticles are expected to remain in the wurtzite phase up to 13-15 GPa.