Density functional theory calculations for solid-state oxide compounds in heterointerface, bulk, and nanoparticle structures

Abstract

Oxide compounds are among the most abundant and diverse stoichiometries found naturally and laboratory synthesized either as a solid, liquid or in gas form. In addition, many elements are able to form more than one oxide stoichiometry. The abundant diversity of oxide compounds in conjunction with nanostructured designs, such as thin films heterojunctions and nanoparticles, makes these materials capable to produce a wide variety of properties ranging from insulators to superconductors, ferromagnetism, enhanced catalytic activity, enhanced mechanical properties, superparamagnetism, a wide range of absorption and emission optical spectra, and many other remarkable phenomena. For these reasons, oxide compounds constitute one of the most important fields of research in materials science with applications in almost all levels of our current technology and have the potential to realize novel and emergent technologies and devices.<br /> <br /> Given the electronic structure of oxygen, which makes it easy to take two electrons from other atoms, the bonds between ions in an oxide crystal have a stronger polar behavior than in conventional non-oxide compounds. This makes the valence electrons of such oxides to have stronger interactions between each other, hence these materials are said to be strongly correlated. In addition, numerous solid-state oxide materials are typically formed using elements with partially filled <em>d</em> and <em>f</em> electronic orbitals which are further characterized by having a strong electron localization. The strong polar behavior of oxide crystals in conjunction with the strong localized nature of the <em>d</em>-block and <em>f</em>-block elements, are essentially responsible for the complex electron correlation in these oxide materials, consequently responsible for the wide range of emergent phenomena.<br /> <br /> Ironically, the key factor of "strongly correlated electrons" that makes oxide solid-state materials have such a wide variety of emergent properties is also the most challenging to study. There is a necessity for improvement and optimization of the current computational methods and capabilities to achieve the maturity and confidence needed to model the properties of these materials with the accuracy and details needed to use it as a guide, or better yet as a recipe, in the synthesis and fabrication of functional/multifunctional novel devices. For strongly correlated materials the development of suitable methods which are both accurate and computational efficient is still a challenge. In virtue of this, the focus of this work is on the analysis of solid-state oxide compounds in the form of heterointerfaces, bulk and nanoparticles using first principle calculations based on the Density Functional Theory (DFT). Three different systems were studied, which covers a bulk, a heterointerface, and a nanoparticle solid-state oxide compound.<br /> <br /> As the heterointerface system, the emergence of 2D electrical conductivity at the interface of strontium titanate and lanthanum aluminate (STO/LAO) at the (001), (110), and (111) crystallographic planes were studied. In this case regular DFT formalism based in the General Gradient Approximation by Perdew-Burke-Ernzerhof (GGA-PBE) functional was used due to its accuracy in the calculation of the mechanical properties of STO/LAO. The calculations showed that whereas at the (001) interface the conductivity appears at a critical thickness of 4 monolayers of LAO, the (110) and (111) planes have no clear critical thickness, these two interfaces were always conductive. Nevertheless, the number of conductive electrons per unit cell increases with the thickness of the LAO layer in the (110) and (111) interfaces. The main mechanism responsible for the conductivity in the (110) and (111) planes was attributed to the large structure reconstruction that locally changes the energy levels at the interface causing charge transfer and accumulation at the layers close to the interface.<br /> <br /> As the bulk system, the optical properties of zinc oxide (ZnO) and aluminum doped zinc oxide (Al:ZnO) were studied at different Al concentrations. To accurately describe the optical properties of ZnO and Al:ZnO the Hubbard correction was implemented to the DFT formalism. Calculation of the absorption spectrum showed that increasing the aluminum concentration shifts the absorption edge to shorter wavelengths, consistent with the increase in the measured bandgap due to the Moss-Burstein effect. The inclusion of the aluminum impurity narrows the absorption spectrum and decreases the overall peak at ultraviolet wavelengths, showing a higher transparency at wavelengths of 50 nm and shorter.<br /> <br /> As the nanoparticles system, the morphology of uranium dioxide (UO₂) nanoparticles at different surface oxidation conditions and surfactant ligands concentration was studied. In this case the implementation of the Hubbard correction to the DFT formalism proved to be of critical importance do to the highly localized nature of the 5<em>f</em>-electron of uranium. The nanoparticles morphology was studied by the calculation of the surface energies of different UO₂ crystallographic planes ((001), (110), and (111)) as a function of the relative oxygen chemical potential under the influence of different ligands, and different concentrations. The calculations shown that, for materials with low oxidation potential, such as uranium dioxide, the oxygen environment and capping ligand concentration are competing factors in determining the nanoparticle morphology.