Role of Pr segregation in acceptor-state formation at ZnO grain boundaries

Grain boundary structural transformation induced by co-segregation of aliovalent dopants

Impurity doping is a conventional but one of the most effective ways to control the functional properties of materials. In insulating materials, the dopant solubility limit is considerably low in general, and the dopants often segregate to grain boundaries (GBs) in polycrystals, which significantly alter their entire properties. However, detailed mechanisms on how dopant atoms form structures at GBs and change their properties remain a matter of conjecture. Here, we show GB structural transformation in α-Al₂O₃ induced by co-segregation of Ca and Si aliovalent dopants using atomic-resolution scanning transmission electron microscopy combined with density functional theory calculations. To accommodate large-sized Ca ions at the GB core, the pristine GB atomic structure is transformed into a new GB structure with larger free volumes. Moreover, the Si and Ca dopants form a chemically ordered structure, and the charge compensation is achieved within the narrow GB core region rather than forming broader space charge layers. Our findings give an insight into GB engineering by utilizing aliovalent co-segregation.

The effect of aliovalent doping on grain boundary is not yet fully understood at the atomic level. Here, the authors report grain boundary structural transformation in α-Al₂O₃ is induced by co-segregation of multiple dopants using atomic-resolution electron microscopy and theoretical calculations.

Engineering of atomic-scale flexoelectricity at grain boundaries

Flexoelectricity is a type of ubiquitous and prominent electromechanical coupling, pertaining to the electrical polarization response to mechanical strain gradients that is not restricted by the symmetry of materials. However, large elastic deformation is usually difficult to achieve in most solids, and the strain gradient at minuscule is challenging to control. Here, we exploit the exotic structural inhomogeneity of grain boundary to achieve a huge strain gradient (~1.2 nm-1) within 3-4-unit cells, and thus obtain atomic-scale flexoelectric polarization of up to ~38 μC cm-2 at a 24° LaAlO3 grain boundary. Accompanied by the generation of the nanoscale flexoelectricity, the electronic structures of grain boundaries also become different. Hence, the flexoelectric effect at grain boundaries is essential to understand the electrical activities of oxide ceramics. We further demonstrate that for different materials, altering the misorientation angles of grain boundaries enables tunable strain gradients at the atomic scale. The engineering of grain boundaries thus provides a general and feasible pathway to achieve tunable flexoelectricity.

Twin Boundary Superstructures Assembled by Periodic Segregation of Solute Atoms

Twinning is a common deformation mechanism in metals, and twin boundary (TB) segregation of impurities/solutes plays an important role in the performances of alloys such as thermostability, mobility, and even strengthening. The occurrence of such segregation phenomena is generally believed as a one-layer coverage of solutes alternately distributed at extension/compression sites, in an orderly, continuous manner. However, in the Mn-free and Mn-containing Mg-Nd model systems, we reported unexpected three- and five-layered discontinuous segregation patterns of the coherent {101̅1} TBs, and not all the extension sites occupied by solutes larger in size than Mg, and even some larger sized solutes taking the compression sites. Nd/Mn solutes selectively segregate at substitutional sites and thus to generate two new types of ordered two-dimensional TB superstructures or complexions. These findings refresh the understanding of solute segregation in the perfect coherent TBs and provide a meaningful theoretical guidance for designing materials via targeted TB segregation.

Novel Ultrahigh-Performance ZnO-Based Varistor Ceramics

The nonlinear response of a material to an external stimulus is vital in fundamental science and technical applications. The power-law current-voltage relationship of a varistor is one such example. An excellent example of such behavior is the power-law current-voltage relationship exhibited by Bi₂O₃-doped ZnO varistor ceramics, which are the cornerstone of commercial varistor materials for overvoltage protection. Here, we report on a sustainable, ZnO-based varistor ceramic, without the volatile Bi₂O₃, that is based on Cr₂O₃ as the varistor former and oxides of Ca, Co, and Sb as the performance enhancers. The material has an ultrahigh α of up to 219, a low I_L of less than 0.2 μA/cm², and a high E_b of up to 925 V/mm, making it superior to state-of-the-art varistor ceramics. The results provide insights into the design of materials with specific characteristics by tailoring states at the grain boundaries. The discovery of this ZnO-Cr₂O₃-type varistor ceramic represents a major breakthrough in the field of varistors for overvoltage protection and could drastically affect the world market for overvoltage protection.

Nonsymmetrical Segregation of Solutes in Periodic Misfit Dislocations Separated Tilt Grain Boundaries

Interfacial segregation is ubiquitous in mulit-component polycrystalline materials and plays a decisive role in material properties. So far, the discovered solute segregation patterns at special high-symmetry interfaces are usually located at the boundary lines or are distributed symmetrically at the boundaries. Here, in a model Mg-Nd-Mn alloy, we confirm that elastic strain minimization facilitated nonsymmetrical segregation of solutes in four types of linear tilt grain boundaries (TGBs) to generate ordered interfacial superstructures. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy observations indicate that the solutes selectively segregate at substitutional sites at the linear TGBs separated by periodic misfit dislocations to form such two-dimensional planar structures. These findings are totally different from the classical McLean-type segregation which has assumed the monolayer or submonolayer coverage of a grain boundary and refresh understanding on strain-driven interface segregation behaviors.

The structure and photoluminescence of a ZnO phosphor synthesized by the sol gel method under praseodymium doping

This work outlines some interesting results regarding the effects of Pr³⁺ substitution on the structural and optical properties of (x = 0 and 0.02) samples.

One-Dimensional Atomic Segregation at Semiconductor-Metal Interfaces of Polymorphic Transition Metal Dichalcogenide Monolayers

Interface segregation is a powerful approach to tailor properties of bulk materials by interface engineering. Nevertheless, little is known about the chemical inhomogeneity at interfaces of polymorphic two-dimensional transition metal dichalcogenides (TMDs) and its influence on the properties of the 2D materials. Here we report one-dimensional monatomic segregation at coherent semiconductor-metal 1H/1T interfaces of Mo-doped WS₂ monolayers. The monatomic interface segregation takes place at an intact transition metal plane and is associated with the topological defects caused by reflection symmetry breaking at the 1T/1H interfaces and the weak difference in bonding strength between Mo-S and W-S. This finding enriches our understanding of the interaction between topological defects and impurities in 2D crystals and enlightens a potential approach to manipulate the properties of 2D TMDs by local chemical modification and interface engineering for applications in 2D TMD electronic devices.

Direct Observation of Oxygen Vacancy Distribution across Yttria-Stabilized Zirconia Grain Boundaries

Crystalline interfaces in materials often govern the macroscopic functional properties owing to their complex structure and chemical inhomogeneity. For ionic crystals, however, such understanding has been precluded by the debatable local anion distribution across crystal interfaces. In this study, using yttria-stabilized zirconia as a model material, the oxygen vacancy distribution across individual grain boundaries was directly quantified by atomic-resolution scanning transmission electron microscopy with ultrahigh-sensitive energy-dispersive X-ray spectroscopy. Combined with dynamical scattering calculations, we unambiguously show that the relative oxygen concentrations increase in four high-angle grain boundaries, indicating that the oxygen vacancies are actually depleted near the grain boundary cores. These results experimentally evidence that the long-range electric interaction is the dominant factor to determine the local point defect distribution at ionic crystal interfaces.

Probing dopant segregation in distinct cation sites at perovskite oxide polycrystal interfaces

Although theoretical studies and experimental investigations have demonstrated the presence of space-charge-induced dopant segregation, most work has been confined largely to the crystal-free surface and some special grain boundaries, and to the best of our knowledge there has been no systematic comparison to understand how the segregation varies at different types of interfaces in polycrystals. Here, through atomic-column resolved scanning transmission electron microscopy in real polycrystalline samples, we directly elucidate the space-charge segregation features at five distinct types of interfaces in an ABO₃ perovskite oxide doped with A- and B-site donors. A series of observations reveals that both the interfacial atomic structure and the subsequent segregation behaviour are invariant regardless of the interface type. The findings in this study thus suggest that the electrostatic potential variation by the interface excess charge and compensating space charge provides a crucial contribution to determining not only the distribution of dopants but also the interfacial structure in oxides.

Space-charges in polycrystalline materials can drive segregation of dopants, however an in-depth understanding of this process is still missing. Here, the authors show that in polycrystalline perovskites the space-charge segregation and interfacial structure are nearly identical irrespective of the interface type.

Subsurface Space-Charge Dopant Segregation to Compensate Surface Excess Charge in a Perovskite Oxide

Since the first prediction by Frenkel, many follow-up studies have been carried out to show the presence of subsurface space-charge layers having the opposite sign to that of the excess charge at the surface, producing overall neutrality in ionic crystals. However, no precise experimental evidence demonstrating how the aliovalent solutes segregate in the space-charge region beneath the surface has been provided over the past several decades. By utilizing atomic-scale imaging and chemical probing in a perovskite oxide, the origin of the surface excess charge at the topmost surface and the position of segregated dopants in the space-charge region is precisely determined. The impact of the space-charge contribution to the dopant distribution near the surface in oxide crystals is explored.

Atomically ordered solute segregation behaviour in an oxide grain boundary

Grain boundary segregation is a critical issue in materials science because it determines the properties of individual grain boundaries and thus governs the macroscopic properties of materials. Recent progress in electron microscopy has greatly improved our understanding of grain boundary segregation phenomena down to atomistic dimensions, but solute segregation is still extremely challenging to experimentally identify at the atomic scale. Here, we report direct observations of atomic-scale yttrium solute segregation behaviours in an yttria-stabilized-zirconia grain boundary using atomic-resolution energy-dispersive X-ray spectroscopy analysis. We found that yttrium solute atoms preferentially segregate to specific atomic sites at the core of the grain boundary, forming a unique chemically-ordered structure across the grain boundary.

Solute segregation is challenging to experimentally identify at the atomic scale. Here, the authors report the direct observation of atomic site-dependent solute segregation behaviour in an yttria-stabilized zirconia grain boundary by atomic-resolution energy-dispersive X-ray spectroscopy.

Atomic structures and oxygen dynamics of CeO2 grain boundaries

Material performance is significantly governed by grain boundaries (GBs), a typical crystal defects inside, which often exhibit unique properties due to the structural and chemical inhomogeneity. Here, it is reported direct atomic scale evidence that oxygen vacancies formed in the GBs can modify the local surface oxygen dynamics in CeO2, a key material for fuel cells. The atomic structures and oxygen vacancy concentrations in individual GBs are obtained by electron microscopy and theoretical calculations at atomic scale. Meanwhile, local GB oxygen reduction reactivity is measured by electrochemical strain microscopy. By combining these techniques, it is demonstrated that the GB electrochemical activities are affected by the oxygen vacancy concentrations, which is, on the other hand, determined by the local structural distortions at the GB core region. These results provide critical understanding of GB properties down to atomic scale, and new perspectives on the development strategies of high performance electrochemical devices for solid oxide fuel cells.

Paving the way to nanoionics: atomic origin of barriers for ionic transport through interfaces

The blocking of ion transport at interfaces strongly limits the performance of electrochemical nanodevices for energy applications. The barrier is believed to arise from space-charge regions generated by mobile ions by analogy to semiconductor junctions. Here we show that something different is at play by studying ion transport in a bicrystal of yttria (9% mol) stabilized zirconia (YSZ), an emblematic oxide ion conductor. Aberration-corrected scanning transmission electron microscopy (STEM) provides structure and composition at atomic resolution, with the sensitivity to directly reveal the oxygen ion profile. We find that Y segregates to the grain boundary at Zr sites, together with a depletion of oxygen that is confined to a small length scale of around 0.5 nm. Contrary to the main thesis of the space-charge model, there exists no evidence of a long-range O vacancy depletion layer. Combining ion transport measurements across a single grain boundary by nanoscale electrochemical strain microscopy (ESM), broadband dielectric spectroscopy measurements, and density functional calculations, we show that grain-boundary-induced electronic states act as acceptors, resulting in a negatively charged core. Besides the possible effect of the modified chemical bonding, this negative charge gives rise to an additional barrier for ion transport at the grain boundary.

Antisite defects in LiCoPO4 nanocrystals synthesized via a supercritical fluid process

Antisite defects in cathode materials are important to understand the chemistry of cathode materials. Herein, we investigated the antisite defects in LiCoPO₄ nanocrystals synthesized via a supercritical fluid process.

Fabrication of an electrically-resistive, varistor-polymer composite

This study focuses on the fabrication and electrical characterization of a polymer composite based on nano-sized varistor powder. The polymer composite was fabricated by the melt-blending method. The developed nano-composite was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FeSEM), and energy-dispersive X-ray spectroscopy (EDAX). The XRD pattern revealed the crystallinity of the composite. The XRD study also showed the presence of secondary phases due to the substitution of zinc by other cations, such as bismuth and manganese. The TEM picture of the sample revealed the distribution of the spherical, nano-sized, filler particles throughout the matrix, which were in the 10–50 nm range with an average of approximately 11 nm. The presence of a bismuth-rich phase and a ZnO matrix phase in the ZnO-based varistor powder was confirmed by FeSEM images and EDX spectra. From the current-voltage curves, the non-linear coefficient of the varistor polymer composite with 70 wt% of nano filler was 3.57, and its electrical resistivity after the onset point was 861 KΩ. The non-linear coefficient was 1.11 in the sample with 100 wt% polymer content. Thus, it was concluded that the composites established a better electrical non-linearity at higher filler amounts due to the nano-metric structure and closer particle linkages.

Grain boundary atomic structures and light-element visualization in ceramics: combination of Cs-corrected scanning transmission electron microscopy and first-principles calculations

Grain boundaries and interfaces of crystals have peculiar electronic structures, caused by the disorder in periodicity, providing the functional properties, which cannot be observed in a perfect crystal. In the vicinity of the grain boundaries and interfaces, dopants or impurities are often segregated, and they play a crucial role in deciding the properties of a material. Spherical aberration (Cs)-corrected scanning transmission electron microscopy (STEM), allowing the formation of sub-angstrom-sized electron probes, can directly observe grain boundary-segregated dopants. On the other hand, ceramic materials are composed of light elements, and these light elements also play an important role in the properties of ceramic materials. Recently, annular bright-field (ABF)-STEM imaging has been proposed, which is now known to be a very powerful technique in producing images showing both light- and heavy-element columns simultaneously. In this review, the atomic structure determination of ceramic grain boundaries and direct observation of grain boundary-segregated dopants and light elements in ceramics were shown to combine with the theoretical calculations. Examples are demonstrated for well-defined grain boundaries in rare earth-doped Al(2)O(3) and ZnO ceramics, CeO(2) and SrTiO(3) grain boundary, lithium battery materials and metal hydride, which were characterized by Cs-corrected high-angle annular dark-field and ABF-STEM. It is concluded that the combination of STEM characterization and first-principles calculation is very useful in interpreting the structural information and in understanding the origin of the properties in various ceramics.

Large neutral barrier at grain boundaries in chalcopyrite thin films

The electronic structure of grain boundaries in polycrystalline Cu(In,Ga)Se2 thin films and their role on solar cell device efficiency is currently under intense investigation. A neutral barrier of about 0.5 eV has been suggested as the reason for the benign behavior of grain boundaries in chalcopyrites. Previous experimental investigations have in fact shown a neutral barrier but only a few 10 meV high, which cannot be expected to have a significant influence on the solar cell efficiency. Here we show that a full investigation of the electrical behavior of charged and neutral grain boundaries shows the existence of an additional narrow neutral barrier, several 100 meV high, which is tunneled through by the majority carriers but is sufficiently high to explain the benign behavior of the grain boundaries.

Atomic-scale imaging of individual dopant atoms in a buried interface

Determining the atomic structure of internal interfaces in materials and devices is critical to understanding their functional properties. Interfacial doping is one promising technique for controlling interfacial properties at the atomic scale, but it is still a major challenge to directly characterize individual dopant atoms within buried crystalline interfaces. Here, we demonstrate atomic-scale plan-view observation of a buried crystalline interface (an yttrium-doped alumina high-angle grain boundary) using aberration-corrected Z-contrast scanning transmission electron microscopy. The focused electron beam transmitted through the off-axis crystals clearly highlights the individual yttrium atoms located on the monoatomic layer interface plane. Not only is their unique two-dimensional ordered positioning directly revealed with atomic precision, but local disordering at the single-atom level, which has never been detected by the conventional approaches, is also uncovered. The ability to directly probe individual atoms within buried interface structures adds new dimensions to the atomic-scale characterization of internal interfaces and other defect structures in many advanced materials and devices.

Aberration-corrected Z-contrast imaging of SrTiO3 dislocation cores

Many fundamental problems in materials science, physics, and particular nanotechnology rely on the direct determination and characterization of atomic arrangements and electronic environments of individual interfaces or defects. In this paper, we will show how aberration-corrected Z-contrast imaging in combination with electron energy-loss spectroscopy can be used to directly measure the local atomic and electronic structures of dislocation cores in low-angle SrTiO3 [001] tilt grain boundaries. In particular, we will study two types of dislocation cores in a 3 degrees tilt grain boundary, a pure edge dislocation, and a dissociated dislocation core. While it is energetically favorable for an edge dislocation to dissociate into two partial dislocations in such a low-angle grain boundary, we can find pure edge dislocations that show a higher O vacancy concentration than the dissociated cores. We suggest that the increased oxygen vacancy concentration might help stabilizing the pure edge dislocations in 3 degrees tilt grain boundaries of SrTiO3.

Atomic-scale visualization of antisite defects in LiFePO4

We visualize the antisite exchange defects in LiFePO4 crystals with an ordered olivine structure by using annular dark-field scanning transmission electron microscopy (STEM). A recognizable bright contrast is observed in some of the Li columns of STEM images in a sample annealed at a lower temperature, which directly demonstrates the disordered occupations by Fe atoms. Furthermore, such exchange defects appear to be locally aggregated rather than homogeneously dispersed in the lattice, although their overall concentration is fairly low. The present study emphasizes the significance of atomic-level observations for the defect distribution that cannot be predicted by macroscopic analytical methods.