Neutral and charged oxygen vacancies induce two-dimensional electron gas near SiO2/BaTiO3 interfaces

Oxide interfaces are known to exhibit properties that are not found in the constituent materials when they are considered separately. At the nanoscale, interfaces often define the properties of entire nanostructures. Thus, materials with new properties and functions can be fabricated by assembling complex heterostructures from "building blocks", such as epitaxial thin films and core-shell nanoparticles.

Core-shell nanoparticles of BaTiO₃ (BTO), in the form of 300-500 nm grains coated with an ~5 nm thick silica layer, have a dielectric constant over 10⁵ and This makes the BTO-SiO₂ nanoparticles promising for the applications that require high dielectric permittivity, such as solid-state energy storage and random access memory devices. We use ab initio simulations to construct an atomistic model of the interface between amorphous SiO₂ (a-SiO₂) film and the BTO (001) surface and to explore the properties of defects at this interface.

According to our ab initio molecular dynamics simulations, the Si-O-Si bonds in the vicinity of the TiO₂-terminated BTO (001) surface break and Si-O-Ti bonds are formed instead. In half of these bonds, the oxygen atom originates from the silica part of the system, and in the other half, it is displaced outward from the near-interface TiO₂ plane. These displacements result in the formation of a polar region consisting of a layer of positively charged oxygen vacancies V²⁺ in the outermost TiO₂ plane and a layer of negatively charged O^2- interstitials in a-SiO₂ near the interface.

The formation of V²⁺ vacancies is accompanied by the electron density redistribution, which facilitates the formation of neutral oxygen vacanciesV_O in the near-interface region. The electrons associated with V_O form a quasi-two-dimensional electrons gas compensating the interface polar region.

We propose that this electron-rich near-interface layer is responsible for the colossal dielectric response observed in the BTO-SiO₂ nanoparticles, consistent with some of the recent experimental observations of potentially ultrahigh energy storage materials.