Correlative workflows to probe oxygen vacancies in functional oxides: from atomic site HAADF-STEM/EELS to synchrotron-based spectroscopies
Dr. Regina Ciancio
CNR-IOM Trieste, Italy
16:15 - 17:15 Tuesday 23 January 2024 TUG 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 complex oxides with atomic precision. Oxygen vacancies are known to play crucial roles in determining the physical properties of oxides and controlling and manipulating the defect structure provides a degree of freedom for harvesting and tailoring their functional properties.
The correlation between the electronic, functional and atomic scale properties in functional oxides requires a detailed analysis by advanced local probe techniques, allowing the accurate determination of the atomic positions, the chemical composition and the electronic state with atomic resolution. 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 materials locally, with atomic resolution, and often in a quantitative way by the smart combination of imaging and spectroscopy.
This lecture will review the tremendous impact of atomic resolution STEM-EELS, complemented by spectroscopic analysis, atomistic calculations and multislice simulation of atomic resolution STEM in oxide thin films to efficiently integrate the sample averaging information provided by synchrotron-based spectroscopies.
One relevant example is represented by anatase TiO2 thin films, where oxygen vacancies are observed to form arrays of ordered superstructure [1] which reflect into a Ti3+ / Ti4+ mixed population. 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 TinO2n-x Magneli phases, which are commonly claimed to occur in titanium dioxide [2]. Another relevant case is La0.7Sr0.3MnO3 thin films where the evidence of a preferential formation of oxygen vacancies at the interface with the substrate induce a structural shift of Mn ions from their equilibrium position [3]. In addition, a reduction of Mn valence state is also found at film/substrate interfacial region thus providing a possible explanation of the dead-layer in manganite thin films. One last example is provided by LaNiO3 thin films grown on SrTiO3, where Differential Phase Contrast (DPC) enlighten the presence of an ordered sequence of oxygen deficient layers along the growth direction of the films. Such an oxygen-defective phase has been tentatively related to tensile-strain induced mechanism rather than polar/no-polar effets at film/substrate interface [4].
In all cases, 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 tune the functional properties of the thin films using strain-driven thin film technology.
[1] Nano Lett. 2020, 20, 6444-6451
[2] ACS Appl. Mater. Interfaces 2017, 9, 23099-23106
[3] ACS Appl. Mater. Interfaces 2021, 13, 46, 55666-55675
[4] Unpublished (2023).
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