Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide

Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena.publishedVersio

Modeling of electrochemical and photoelectrochemical impedance functions for films

The aim of this study was to develop mathematical models for electrochemical and photoelectrochemical impedance transfer functions for mixed conducting films. These models include electrochemical impedance spectroscopy (EIS), photoelectrochemical impedance spectroscopy (EIS), intensity-modulated photovoltage spectroscopy (IMVS), and intensity-modulated photocurrent spectroscopy (IMPS) for mixed conducting photoelectrochemical thin films, and intensity-modulated photocurrent spectroscopy (IMPS) for mixed conducting microporous thin films. In addition a short study of the step size dependency of the convergence for numerical modeling approach using Newman's BAND(J) subroutine to solve a second order diffusion equation has beed performed.It was found that the derived photoelectrochemical impedance (EIS) transfer function and the intensity-modulated photocurrent spectroscopy (IMPS) transfer function were dependent on the degree of mixed conductivity, while the intensity-modulated photovoltage spectroscopy (IMPS) transfer function was independent with degree of mixed conductivity. This was explained by the steady-state concentration profiles for the extreme cases of t+=1 and t+=t-=0.5. The concentration at solution interface increases with increasing light intensity when a sufficiently large light adsorption coefficient is assumed. For the pure electrical conducting system (t+=1), the concentration close to the support is constant. So when the current oscillates, only the concentration close to the solution interface oscillates. For the mixed conducting system (t+=0.5), concentration at both the electrode interfaces oscillates. Thus, the electrode kinetics at both interfaces affect the impedance measurements for an applied current density. Under open circuit conditions for IMVS measurements, only the photogenerated charge carriers close to the solution will contribute to the impedance, since the applied current is zero and the transfer function is independent of the transport numbers.Newman's BAND(J) subroutine has been proven to be valid to solve partial differential equations with complex numbers, needed to calculate impedance spectra, in previous work. It was found in this study that the convergence of the BAND(J) subroutine does not follow the expected convergence toward the numerical solution with decreasing step size. One possible source for this unexpected trend was proposed to be that the error related to this numerical approach is dependent on higher derivatives of the solution. It was advised that a Richardson's iteration method study for higher order derivatives should be performed for this routine to find the appropriate error dependency.The electrochemical impedance transfer function for the mixed conducting thin film electrode showed a reflective-like behavior, as expected from previous work. A dome in the low frequency region of the impedance plane plot was observed. The dome occurs at a frequency equal to the effective rate constant k. Thus, this dome corresponds to the charge limiting process of recombination across the bulk electrode. By decreasing the rate constant of recombination, the impedance increases, in accordance with previous work.The photoelectrochemical impedance transfer function for the mixed conducting thin film electrode showed two domes in the impedance plane plot with different imaginary parts in the low frequency region of the impedance spectrum corresponding to two distinct charge transfer limiting processes in opposite directions. It was found that the current associated with charge transfer limiting process oscillating in the same direction as the potential occurred at a frequency equal to the chosen effective rate constant k. Thus, this dome corresponds to the charge limiting process of recombination in the bulk of the electrode material. The current oscillating in the opposite direction of the potential was assumed to be associated with a back charge transfer process between the electrode and the electrolyte, and should be described by the electrode kinetics at the solution interface. In the high frequency region, the system is diffusion limited, and for high light intensities the system shows a reflective-like behavior.The intensity-modulated photovoltage spectroscopy (IMVS) impedance transfer function for the mixed conducting thin film electrode showed one dome in the impedance plane plot. It was found that the dome occurs at a frequency equal to the chosen value for the effective rate constant k. Thus, the dome corresponds to the charge limiting process of recombination across the bulk electrode. For sufficiently low k-values, the system is charge transfer limiting in the low frequency region, and diffusion limiting in the high frequency region.The intensity-modulated photocurrent spectroscopy (IMPS) impedance transfer function for the mixed conducting thin film electrode showed two distinct domes in the impedance plane plot in the same quadrant for large rate constants and applied steady state current densities, corresponding to two different charge limiting processes. It was found that one of the domes occurred at a frequency equal to the chosen value of the effective rate constant k. Thus, this dome corresponds to the charge limiting process of recombination across the electrode. The other dome is assumed to correspond to back charge transfer with the electrolyte, and the rate of the charge transfer should be described by the electrode kinetics at the solution interface. By decreasing the applied current density and increasing the light intensity, a shift in the quadrant of the impedance plane plot was observed. The proposed explanation to this is the competition between recombination and charge generation to keep the potential constant. When the rate of generation exceeds the recombination, the charge transfer at the solution interface changes direction in order to maintain a constant potential. This was investigated in more detail for steady-state conditions, where it was found that for high rate constants the current increases with increasing light intensity to keep a constant potential, in accordance with Fick's first law of diffusion. For low rate constants the current decreases with increasing potential, thus the charge transfer between the electrode and electrolyte changes sign and the net current density is in opposite direction than the light intensity. For intermediate rate constants, a change from increasing to decreasing current with increasing light intensity is observed. This indicates that the recombination process accommodates for the increased current by increasing the light intensity in order to keep the potential constant for low light intensities. At a threshold light intensity, the current density is changing from increasing to decreasing with increasing light intensity, indicating that the recombination cannot accommodate for the increased current by increasing the light intensity to keep the potential constant, and the potential is kept constant by changing the direction of the charge transfer between electrode and electrolyte.The intensity-modulated photocurrent spectroscopy (IMPS) transfer function for the microporous mixed conducting thin film electrode showed a reflective-like behavior, where a charge transfer limiting process in the lower frequency limit and a diffusion controlled process in the high frequency region where observed. The charge limiting process was found to be the recombination process described by the effective rate constant k. The model, however, seemed to break for low rate constants, and a further study of this model was suggested.A limitation of the derived model was that the steady-state concentration under zero light and zero illumination is zero. In previous work, an additive term in the steady-state concentration expression corresponding to the equilibrium concentration was proposed. By including an additive term directly does not fulfill the steady-state diffusion equation as written. One possible solution was proposed, in which the equilibrium concentration is introduced in the diffusion equation, and the diffusion equation is then solved with appropriate boundary conditions. This was suggested to be studied in more detail in further work

First-principles study of topologically protected vortices and ferroelectric domain walls in hexagonal YGaO3

Ferroelectric behavior on the meso- and macroscopic scale depends on the formation and dynamics of domains and controlling the domain patterns is imperative to device performance. While density functional theory (DFT) calculations have successfully described the basic properties of ferroelectric domain walls, the necessarily small cell sizes used for the calculations hampers DFT studies of complex domain patterns. Here, we simulate large-scale complex ferroelectric domain patterns in ferroelectric YGaO 3 using multisite local orbitals as implemented in the DFT code conquest. The multisite local orbital basis set gives similar bulk structural and electronic properties, and atomic domain wall structures and energetics as those obtained with conventional plane-wave DFT. With this basis set model, 3600-atom cells are used to simulate topologically protected vortices. The local atomic structure at the vortex cores is subtly different from the domain walls, with a lower electronic band gap suggesting enhanced local conductance at these cores

Oxygen vacancies in the bulk and at neutral domain walls in hexagonal YMnO3

We use density functional calculations to investigate the accommodation and migration of oxygen vacancies in bulk hexagonal YMnO3, and to study interactions between neutral ferroelectric domain walls and oxygen vacancies. Our calculations show that oxygen vacancies in bulk YMnO3 are more stable in the Mn-O layers than in the Y-O layers. Migration barriers of the planar oxygen vacancies are high compared to oxygen vacancies in perovskites, and to previously reported values for oxygen interstitials in h-YMnO3. The calculated polarization decreases linearly with vacancy concentration, while the out-of-plane lattice parameter expands in agreement with previous experiments. In contrast to ferroelectric perovskites, oxygen vacancies are found to be more stable in bulk than at domain walls. The tendency of oxygen vacancies to segregate away from neutral domain walls is explained by unfavorable Y-O bond lengths caused by the local strain field at the domain walls

Hydrothermal synthesis of hexagonal YMnO3 and YbMnO3 below 250 °C

The hydrothermal synthesis of hexagonal YMnO3 and YbMnO3 are reported using high KOH mineraliser concentrations (>10 M) and low temperatures (<240 °C). The relation between reaction parameters and resulting phase purity were mapped by ex situ and in situ X-ray diffraction. Excess Y2O3 resulted in two-phase product with hexagonal YMnO3 with different lattice parameters. An unusual microstructure was observed in which particles have a hexagonal shape with a highly crystalline edge and either a hollow or polycrystalline interior. An Ostwald ripening mechanism was proposed to explain this phenomenon. Solid-state reactions and density functional theory calculations were performed to determine plausible defect chemistry which can lead to the observed phases with different lattice parameters

Domain wall mobility and roughening in doped ferroelectric hexagonal manganites

The macroscopic performance of ferroelectric and piezoelectric devices depends strongly on domain wall dynamics. It is clear that structural defects, such as vacancies, interstitials, and dopants codetermine the dynamics, but the microscopic understanding of the wall-defect interactions is still at an early stage. Hexagonal manganites are among of the most intensively studied systems with respect to static domain wall properties and thus are ideal model materials for studying domain wall mobility in the presence of defects. Here we study the mobility of domain walls in the hexagonal manganites and how it is affected by cation dopants using density functional theory calculations. The results are correlated with scanning probe microscopy measurements on single crystals, to confirm an increasing domain wall roughness for the dopants we predict to pin the walls. The pinning originates from elastic strain fields around the walls interacting with the local crystal perturbations surrounding a dopant. The pinning strength is correlated with the local change in order parameter amplitude caused by the dopant. As a computationally friendly alternative to large supercell calculations, we demonstrate that domain wall pinning can be predicted from the dopants’ effect on the free-energy landscape of polarization switching. This approach allows to directly probe the effect of defects on domain wall mobility using a fraction of the computational cost, opening the door to detailed modeling and understanding of the critical pinning process of domain walls

Intrinsic and extrinsic conduction contributions at nominally neutral domain walls in hexagonal manganites

Conductive and electrostatic atomic force microscopy (cAFM and EFM) are used to investigate the electric conduction at nominally neutral domain walls in hexagonal manganites. The EFM measurements reveal a propensity of mobile charge carriers to accumulate at the nominally neutral domain walls in ErMnO3, which is corroborated by cAFM scans showing locally enhanced direct current conductance. Our findings are explained based on the established segregation enthalpy profiles for oxygen vacancies and interstitials, providing a microscopic model for previous, seemingly disconnected observations ranging from insulating to conducting behavior. In addition, we observe variations in conductance between different nominally neutral walls that we attribute to deviations from the ideal charge-neutral structure within the bulk, leading to a superposition of extrinsic and intrinsic contributions. Our study clarifies the complex transport properties at nominally neutral domain walls in hexagonal manganites and establishes the possibility to tune their electronic response based on oxidation conditions, opening the door for domain-wall-based sensor technology

Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide

Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena

Observation of Electric-Field-Induced Structural Dislocations in a Ferroelectric Oxide

Dislocations are 1D topological defects with emergent electronic properties. Their low dimensionality and unique properties make them excellent candidates for innovative device concepts, ranging from dislocation-based neuromorphic memory to light emission from diodes. To date, dislocations are created in materials during synthesis via strain fields or flash sintering or retrospectively via deformation, for example, (nano)-indentation, limiting the technological possibilities. In this work, we demonstrate the creation of dislocations in the ferroelectric semiconductor Er(Mn,Ti)O3 with nanoscale spatial precision using electric fields. By combining high-resolution imaging techniques and density functional theory calculations, direct images of the dislocations are collected, and their impact on the local electric transport behavior is studied. Our approach enables local property control via dislocations without the need for external macroscopic strain fields, expanding the application opportunities into the realm of electric-field-driven phenomena

Electronic bulk and domain wall properties in B-site doped hexagonal ErMnO3

Acceptor and donor doping is a standard for tailoring semiconductors. More recently, doping was adapted to optimize the behavior at ferroelectric domain walls. In contrast to more than a century of research on semiconductors, the impact of chemical substitutions on the local electronic response at domain walls is largely unexplored. Here, the hexagonal manganite ErMnO3 is donor doped with Ti4+. Density functional theory calculations show that Ti4+ goes to the B site, replacing Mn3+. Scanning probe microscopy measurements confirm the robustness of the ferroelectric domain template. The electronic transport at both macroscopic and nanoscopic length scales is characterized. The measurements demonstrate the intrinsic nature of emergent domain wall currents and point towards Poole-Frenkel conductance as the dominant transport mechanism. Aside from the new insight into the electronic properties of hexagonal manganites, B-site doping adds an additional degree of freedom for tuning the domain wall functionality