Atomic Site HAADF-STEM and EELS of Complex Oxides Thin Films—The Role of Oxygen Vacancies

Complex oxides are fascinating systems which host a vast array of unique phenomena, such as high temperature (and unconventional) superconductivity, ‘colossal’ magnetoresistance, all forms of magnetism and ferroelectricity, as well as (quantum) phase transitions and couplings between these states. The recent years have witnessed considerable achievements in the ability to grow thin film heterostructures of these materials with atomic precision. With this level of control, the electrostatic boundary conditions at oxide surfaces and interfaces can be used to form new electronic phases or novel low-dimensional states at the interfaces inaccessible in bulk oxides. Oxygen vacancies play crucial roles in determining the physical properties of metal oxides and controlling and manipulating the defect structure provides a degree of freedom for harvesting and tailoring the functional properties of oxides. This is the case of anatase titanium dioxide TiO₂, where the formation of oxygen vacancies has been proved to effectively tune the amount of Ti3+ ions [1-2] and to extend the photoresponse of TiO₂ from the UV towards the visible-light region [3].<br/>The correlation between the functional and atomic scale properties requires a detailed analysis by local probe techniques with advanced capabilities, allowing the accurate determination of the atomic positions, the chemical composition and the electronic state with atomic resolution. In this regard, aberration-corrected STEM and the possibility to couple STEM imaging (in Z-contrast or Annular Bright Field) and EELS spectroscopy enables to determine the chemistry, crystal and electronic structure of new materials locally, with atomic resolution, and often in a quantitative way by the smart combination of imaging and spectroscopy.<br/>This lecture will review the themendous impact of having access to atomic resolution HAADF-STEM, complemented by spectroscopic analysis, atomistic calculations and multislice simulation of atomic resolution STEM in oxide thin films. The case of anatase TiO₂ thin films will be described, where oxygen vacancies are observed to form arrays of ordered superstructures with varying oxygen content which reflect into a Ti³⁺/Ti⁴⁺ mixed population [4]. The combination of atomically resolved STEM-EELS along with extensive atomistic and multislice simulations lead to the validation of a new model for the defect structures in anatase that excludes the typical shear-plane structures, like the Ti_nO_2n-x Magnéli phases, which are commonly claimed to occur in titanium dioxide. The relevant case of La_0.7Sr_0.3MnO₃ thin films will also be discussed where the evidence of a structural shift of Mn ions correlated with the reduction of Mn valence state is found to be correlated with the preferential formation of oxygen vacancies at the interface with the substrate [5].<br/>In both the cases, the identification of secondary structures as well as the determination of the intrinsic structure of oxygen defects is a crucial step to improve the functionalities of such material systems and to open up new options to engineer devices with targeted properties.<br/><br/>References<br/>[1] Role of Oxygen Deposition Pressure in the Formation of Ti Defect States in TiO2(001) Anatase Thin Films, <i>ACS Appl. Mater. Interfaces</i> 2017, 9, 23099−23106<br/>[2] Distinct behavior of localized and delocalized carriers in anatase TiO2 (001) during reaction with O2, <i>Phys. Rev. Mater</i>. 4, 025801 (2020)<br/>[3] Tuning the Optical Absorption of Anatase Thin Films Across the Visible-To-Near-Infrared Spectral Region, <i>Phys. Rev. Appl</i>. 2020, 13, 044011<br/>[4] Unveiling Oxygen Vacancy Superstructures in Reduced Anatase Thin Films, <i>Nano Lett.</i> 2020, 20, 6444−6451<br/>[5] Evidence of Mn-Ion Structural Displacements Correlated with Oxygen Vacancies in La_0.7Sr_0.3MnO₃ Interfacial Dead Layers, <i>ACS Appl. Mater. Interfaces</i> 2021, 13, 46, 55666–55675