Epitaxial TiO_x Surface in Ferroelectric BaTiO₃: Native Structure and Dynamic Patterning at the Atomic Scale

Abstract

Surfaces and interfaces of ferroelectric oxides exhibit enhanced functionality, and therefore serve as a platform for novel nano and quantum technologies. Experimental and theoretical challenges associated with examining the subtle electro-chemo-mechanical balance at metal-oxide surfaces have hindered the understanding and control of their structure and behavior. Here, combined are advanced electron-microscopy and firstprinciples thermodynamics methods to reveal the atomic-scale chemical and crystallographic structure of the surface of the seminal ferroelectric BaTiO 3 . It is shown that the surface is composed of a native <2 nm thick TiO x rock-salt layer in epitaxial registry with the BaTiO 3 . Using electronbeam irradiation, artificial TiO x sites with sub-nanometer resolution are successfully patterned, by inducing Ba escape. Therefore, this work offers electro-chemo-mechanical insights into ferroelectric surface behavior in addition to a method for scalable high-resolution beam-induced chemical lithography for selectively driving surface phase transitions, and thereby functionalizing metal-oxide surfaces.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10. 1002/adfm.201902549. in oxidation states of the participating ions. Hence, functional-oxide surfaces and interfaces constitute a rich platform for novel phases that are attractive for high-performance miniaturized electronic devices [7,8] as well as for chemical catalyses. [9] Because ferroelectric oxides comprise regions with varying crystallographic and electric polarization orientations, there has been a growing interest in their outer surface and domain wall functionality, which is expressed as enhanced conductivity, [10][11][12] magnetism, [4] and even superconductivity. [13] The chemical origin of such a functional behavior is typically attributed to either oxygen vacancy dynamics [14][15][16][17] or cation segregation, [18] while other studies look at the effects of intrinsic symmetry breaking. [4] Despite the accumulated knowledge on domain walls, the structure and behavior of ferroelectric surfaces, which are responsible for domain stabilization and are attractive for, e.g., nano lithography [19][20][21][22][23] and catalysis, [9,[24][25][26][27] have remained elusive. Specifically, the longstanding challenge in understanding how the surface mediates between the absence of electric and mechanical fields in the vacuum and the polarization, and strain in the bulk is not merely experimental or theoretical, but even conceptual. [28] Hence, computational methods that were developed to explain the electro-chemical [29] and electro-mechanical [30] atomic-scale interactions in ferroelectrics have been adopted to describe experimental observations of the surface behavior. For example, Tsurumi et al. [31,32] combined dielectric measurements and density functional theory (DFT) calculations to demonstrate that nanoparticles (NPs) of the seminal nontoxic ferroelectric, BaTiO 3 , organize in a core-shell...

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“…The light‐yellow regions in Figure S1e correspond to Ba ( Z =56) columns and dark yellow contrasts correspond to Ti ( Z =22) or O ( Z =8) columns [18] . Interestingly, a surface reconstruction can be clearly seen in Figure 1 c,d, in which binary titanium oxide (TiO x ) phases with 3–4 unit‐cell thickness have formed instead of a ternary barium titanium (BaTiO 3 ) perovskite structure [19] . Owing to the newly formed TiO x phase, the peak [ A 1 (TO)] in Raman spectra splits into two peaks (Figure S4).…”

“…[18] Interestingly,asurface reconstruction can be clearly seen in Figure 1c,d, in which binary titanium oxide (TiO x )p hases with 3-4 unit-cell thickness have formed instead of at ernary barium titanium (BaTiO 3 )p erovskite structure. [19] Owing to the newly formed TiO x phase,t he peak [A 1 (TO)] in Raman spectra splits into two peaks (Figure S4). Thel ocal surface reconstruction may be attributed to the removal of Ba and partially Of rom the perovskite structure,w hich drives are combination of Ti and Od uring the synthesis process,t hus forming an epitaxial heterostructure of TiO x on BaTiO 3 .I n other words,p art of Ba-O plane in perovskite structure was firstly dissolved.…”

Engineering Individual Oxygen Vacancies: Domain-Wall Conductivity and Controllable Topological Solitons

“…(C) EELS spectra from domains and a domain wall near the Ti-L 23 peak, showing a reduced state of titanium ions at the domain walls ( Figure S5 ). 59 − 62 …”

“…To further confirm the charging at the vacant site, atomic-resolution EELS measurements were done across the domain wall. Figures 2 C and S5 reveal that the oxidation state of the titanium ion changes from Ti 4+ within the domains to a lower oxidation state at the domain wall, 59 − 62 where the oxygen vacancies are. Using the Kröger–Vink notation, the v O formation can be summarized: O O × → v O •• + 2 e ′ + 1 / 2 O 2 , so that the Ti ion transfers due to the vacancy from +4 to +2 to maintain charge neutrality, whereas it can also transfer to +3 to allow charging.…”

Giant Superelastic Piezoelectricity in Flexible Ferroelectric BaTiO3 Membranes

“…Such effects are examinable, because they are accompanied by a strong influence of the exposure time on the mechanical motion. 27 That is, the accumulated charge or power, not the flux or dose, are the driving excitations. Alternatively, if the effect is electromechanical,…”