Deformation-induced charge redistribution in ceria thin film at room temperature

Abstract

Tuning electronic properties through strain engineering of metal oxides is an important step toward understanding electrochemical and catalytic reactions in energy storage and conversion devices. Traditionally, strain engineering studies focused on movement of oxygen ions at high temperatures (500 degree Celcius and above), complicating electrical properties by introducing mixed electronic and ionic conductivity. In this study, we demonstrate room temperature charge redistribution in a CeO2 thin film as a result of phase transformation in a localized region by mechanical deformation. Mechanical indentation of the CeO2 thin film at room temperature results in irreversible deformation. Conductive-tip atomic force microscopy (C-AFM) analysis shows increased current passing through the locally deformed area of the CeO2 thin film. Electron energy loss spectroscopy (EELS) analysis equipped with transmission electron microscopy (TEM) suggests that the increase in current contrast in the deformed region arises from an increased concentration of Ce 3+ ions. We herein discuss the fundamental reason behind the increased amount of Ce 3+ ions in the deformed area, based on the atomic scale computational works performed by molecular dynamics (MD) simulations and first-principles density functional theory (DFT) calculations. Plastic deformation induces a phase transformation of cubic fluorite CeO2 into a newly formed T-CeO2 structure. This phase transformation occurs mainly by oxygen ions moving closer to cerium ions to release mechanical energy absorbed in the CeO2 thin film, followed by charge redistribution from the initial CeO2 to the newly created T-CeO2 structure.

Plastic deformations of ceria nanocubes under compression: An atomistic simulations study

“…Sayle et al simulated nanorod deformed along [110] where the relaxation process is driven by a reversible fluorite-to-rutile transition. Another tetragonal phase transition was identified by Park et al as a result of the compression of a thin film [32]. This tetragonal phase was found to be consistent with STEM imaging after compression experiments.…”

“…[31], Figure 4 of Ref. [32]); (ii) all the structures (both initial and final) can be described by a slight and continuous variation of the Ce atoms while the main difference between them lies in the arrangement of the oxygen sublattice.…”

“…Several elementary processes responsible for plastic dissipation in ceria nanocrystals were identified including amorphization under compression in cuboidal, polyhedral and coreshell particles [28,30], surface dislocation nucleation in compressed ⟨110⟩-oriented nanorods [29], and fluorite-to-rutile phase transformation in ⟨110⟩oriented nanorods under tension [31]. Park et al reported another phase transformation mechanism from the fluorite structure to a tetragonal phase occurring during the nanoindentation of CeO 2 thin films [32].…”

Introducing Spontaneously Phase‐Separated Heterogeneous Interfaces Enables Low Power Consumption and High Reliability for Phase Change Memory

“…The grain growth and atomic migration of the phase change part are suppressed by the nearby amorphous interfaces. [ 17,25,26 ] Through the element distribution characterization in Figure S4 of the Supporting Information, Sb atoms not only form Sb–Te grains in the distribution region of Te element, but also disperse around them, indicating that the precipitated phase is made up of Sb and O atoms. Combined with the results of XPS characterization, conclusion can be made that the embedded precipitated phase, consisting of Sb oxide with low thermal conductivity and low electrical conductivity, is formed via doping O atoms into Sb 2 Te 3 .…”