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High-index ferroelectric thin films show excellent dielectricity, piezoelectricity and switching behaviors. Understanding the misfit strain relaxation behavior may prove beneficial to gaining insights into the high-quality growth of high-index ferroelectric films. In this study, ferroelectric PbTiO3 thin films were deposited on the (101)-oriented (La, Sr)(Al, Ta)O3 substrate by pulsed laser deposition and were investigated using (scanning) transmission electron microscopy. Two types of misfit dislocations with line directions of and [010] were found at the interface. The dislocation exhibited Burgers vectors of a [011] or [01 $\overline{1}$ 1], while the [010] dislocation featured Burgers vectors of a[ $\overline{1}$ 01]. The former might be generated by gliding, and the latter by climbing. We propose that the misfit strain relaxation in this film system basically results from the formation of dislocations and the residual misfit strain is relaxed via the formation of 90° ac domains.

Understanding the interactions between deformation twins and plate-like phases in magnesium alloys is one of the key issues to tailor the microstructure of magnesium alloys for better mechanical pr...

Strongly correlated oxides exhibit multiple degrees of freedoms, which can potentially mediate exotic phases with exciting physical properties, such as the polar vortex recently found in ferroelectric oxide films. A polar vortex is stabilized by competition between charge, lattice, and/or orbital degrees of freedom, which displays vortex–ferroelectric phase transitions and emergent chirality, making it a potential candidate for designing information storage and processing devices. Here, by a combination of controlled film growth and aberration-corrected scanning transmission electron microscopy, we obtain nanoscale vortex arrays in [110]-oriented BiFeO3 films. These vortex arrays are stabilized in ultrathin BiFeO3 layers sandwiched by two coherently grown orthorhombic scandate layers, exhibiting a ferroelectric morphotropic phase boundary constituted by a mixed-phase structure of polar orthorhombic BiFeO3 and rhombohedral BiFeO3. Clear polarization switching and piezoelectric signals were observed in thes...

Oxygen vacancy configurations and concentration are coupled with the magnetic, electronic, and transport properties of perovskite oxides, and manipulating the physical properties by tuning the vacancy structures of thin films is crucial for applications in many functional devices. In this study, we report a direct atomic resolution observation of the preferred orientation of vacancy ordering structure in the epitaxial LaCoO3- x (LCO) thin films under various strains from large compressive to large tensile strain utilizing scanning transmission electron microscopy (STEM). Under compressive strains, the oxygen vacancy ordering prefers to be along the planes parallel to the heterointerface. Changing the strains from compressive to tensile, the oxygen vacancy planes turn to be perpendicular to the heterointerface. Aberration-corrected STEM images, electron diffractions, and X-ray diffraction combined with X-ray photoelectron spectroscopy demonstrate that the vacancy concentration increases with increasing misfit strains and vacancy distribution is more ordered and homogeneous. The temperature-dependent magnetization curves show the Curie temperature increases, displaying a positive correlation with the misfit strains. With change in the strain from compressive to tensile, anisotropy fields vary and show large values under tensile strains. It is proposed that oxygen vacancy concentration and preferred ordering planes are responsible for the enhanced magnetic properties of LCO films. Our results have realized a controllable preparation of oxygen vacancy ordering structures via strains and thus provide an effective method to regulate and optimize the physical properties such as magnetic properties by strain engineering.

Tuning multiple strain and polar states of ferroelectrics by using strain engineering is an essential approach for designing multifunctional electric devices such as multiple state memories. However, integrating multiple strain states is still a challenge, and in addition, revealing such strains and the resultant polar behaviors on the atomic level remains difficult. In this work we prepare PbZr0.52Ti0.48O3/PbTiO3 (PZT/PTO) superlattices on SrRuO3-buffered SrTiO3(001) substrates. Aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) reveals that the superlattice is coherent in both c (out-of-plane polar direction) and a (in-plane polar direction) domains. We find that the strain states of both PZT and PTO in c and a domains are variant, leading to four special strain states. For example, the tetragonality for PTO in c and a domains is 1.061 and 1.045, respectively. In contrast, PZT in c domains displays a tetragonality as giant as 1.107, which corresponds to 110 µC cm−2 spontaneous polarization, much larger than the bulk PZT; while PZT in a domains exhibits 1.010 tetragonality with about 70 µC cm−2 polarization. This study reveals a practical way to integrate multiple strain states and enhanced polarizations in ferroelectric films, which could be used as multifunctional electric elements.

High-index ferroelectric films as (101)-orientated ones exhibit enhanced dielectric responses, piezoelectric responses, and exotic ferroelectric switching behaviors, which are potential candidates for applications in memories and capacitors. However, possible domain patterns and domain wall structures in (101)-oriented ferroelectric thin films are still elusive, which results in difficulties in understanding the origin and further modulating their special properties. In this work, a series of PbTiO3 (PTO) thin films with 35, 50, 60, and 70 nm in thickness were grown on (101)-oriented (LaAlO3)0.29(SrTa1/2Al1/2O3)0.71 (LSAT(101)) substrates by pulsed laser deposition and investigated by both piezoresponse force microscopy (PFM) and (scanning) transmission electron microscopy ((S)TEM). PFM measurements reveal that periodic stripe domains are dominant in 50 nm thick PTO films. Besides stripe domains, a/c domains appear in films with thickness more than 60 nm. A thickness-dependent evolution of piezoresponse a...

Nanoindentation-induced phase transformation between silicon carbide (SiC) polymorphs has been investigated by a combined study of transmission electron microscopy and first principles calculations. In situ nanoindentation of 4H-SiC single crystals revealed the occurrence of a phase transformation from 4H to 3C in the vicinity of a crack. First principles calculations pointed out an extremely low energy barrier for this phase transformation, which could be attributed to the very smooth transition in the atomic and electronic structures across the 3C/4H interface. The phase transformation from 4H to 3C may retard the propagation of cracks and reduce the band gap of SiC.

Secondary phases precipitated in diffusion affected zones (DAZs) of a transient liquid phase (TLP) bonded Ni-based superalloy have a large impact on materials properties. Here we report an atomic-scale analysis of the crystal structures and elemental distributions within DAZ precipitates in a TLP-bonded single-crystal superalloy using a range of electron microscopy techniques. The predominant precipitate phases are found to be M3B2- and M5B3-type borides, where M is a mixture of transition metal elements. Atomically-resolved energy-dispersive X-ray spectroscopy in an aberration-corrected scanning transmission electron microscope (STEM) enabled the distributions of metal atoms within the precipitates to be determined. The observation that different metals occupy preferred sites within the crystals indicates that these phases are not uniform solid solutions, but instead are chemically ordered. Both borides contain two distinct metal lattice sites such that the phases can be described as L2SB2 and L4SB3, where L and S are elements with larger (e.g., W, Mo) and smaller (e.g., Cr, Co, Ni) atomic numbers, respectively. Ordering of W and Cr is found to be particularly strong. A systematic search for stable phases within the chemical space of the W-Cr-B ternary system using the crystal structure prediction (CSP) program USPEX provided further insights into the nature of the observed chemical ordering. This combination of STEM and CSP techniques provides a direct and robust way of determining chemical features and local structures of multicomponent phases with atomic resolution even when particle sizes are too small for analysis using conventional methods.

The dissolution behavior of γ/Laves eutectic within a typical Ni-based superalloy (Inconel 718) was investigated by in-situ and conventional transmission electron microscopy (TEM) techniques. The results show that the Laves phase dissolved firstly at 700°C, but a new Ni7Nb2 phase possessing similar composition with δ-Ni3Nb phase precipitated near Laves phase did not dissolve at temperatures up to 950°C. The new Ni7Nb2 phase with lattice parameters: a=0.473 nm, b=0.794 nm, c=1.21 nm and β=91.3°, shows a C2/m space group symmetry. The crystal structure of the new Ni7Nb2 phase can be understood by “triple layers” as the Ni7Zr2-type structure does, stacking perpendicular to the (001) planes. Instead of stacking with one single shift vector s corresponding to the Ni7Zr2-type structure, the stacking sequences of the new Ni7Nb2 can be characterized by alternate occurrence of +s and -s. Moreover, the structural change of γ/Laves eutectic constituent before and after heat treatment was investigated. The Ni7Nb2 could remain after heat treatment. Since the experimental Ni7Nb2 structure was shown to be unstable in the Ni-Nb binary system by calculation, it is suggested that Zr, Si and Co promote the formation of Ni7Nb2 phase in superalloys according to the segregation behaviors of elements. Moreover, as Ni7Zr2 is considered to be detrimental to the mechanical properties, Ni7Nb2 phase should also have negative influences and should be eliminated through possible approaches. The finding of the new phase should shed light on the microstructural controlling and heat treatment process formulating of superalloys.

Atomistic scale interface structures could tailor the dislocation behaviors at interface, as well as interface strengthening. In this work, we design a disordered interface by introducing an ultrathin amorphous layer in the Cu/Nb multilayers. Compared to the sharp interface, the disordered interface can smear the impinging dislocation with relaxing the in-plane and out-of-plane Burgers vectors, thus inducing a remarkable hardening in the Cu/Nb multilayers. The findings not only prove the hardening effect induced by dislocation interface smearing, but also shed light on developing advanced materials with ultrahigh hardness via disordered interface.

The quasi-one-dimensional (1D) metallic conductivity of the perovskite-related SrnNbnO3n+2 compounds is of continuing fundamental physical interest as well as being important for developing advanced electronic devices. The SrnNbnO3n+2 compounds can be derived by introducing additional oxygen into the SrNbO3 perovskite. However, the physical origin for the transition of electrical properties from the three-dimensional (3D) isotropic conductivity in SrNbO3 to the quasi-1D metallic conductivity in SrnNbnO3n+2 requires more in-depth clarification. Here we combine advanced transmission electron microscopy with atomistic first-principles calculations to unambiguously determine the atomic and electronic structures of the SrnNbnO3n+2 compounds and reveal the underlying mechanism for their quasi-1D metallic conductivity. We demonstrate that the local electrical conductivity in the SrnNbnO3n+2 compounds directly depends on the configuration of the NbO6 octahedra in local regions. These findings will shed light on t...

Functional oxide interfaces have received a great deal of attention owing to their intriguing physical properties induced by the interplay of lattice, orbital, charge, and spin degrees of freedom. Atomic-scale precision growth of the oxide interface opens new corridors to manipulate the correlated features in nanoelectronics devices. Here, we demonstrate that both head-to-head positively charged and tail-to-tail negatively charged BiFeO3/PbTiO3 (BFO/PTO) heterointerfaces were successfully fabricated by designing the BFO/PTO film system deliberately. Aberration-corrected scanning transmission electron microscopic mapping reveals a head-to-head polarization configuration present at the BFO/PTO interface, when the film was deposited directly on a SrTiO3 (001) substrate. The interfacial atomic structure is reconstructed, and the interfacial width is determined to be 5-6 unit cells. The polarization on both sides of the interface is remarkably enhanced. Atomic-scale structural and chemical element analyses exhibit that the reconstructed interface is rich in oxygen, which effectively compensates for the positive bound charges at the head-to-head polarized BFO/PTO interface. In contrast to the head-to-head polarization configuration, the tail-to-tail BFO/PTO interface exhibits a perfect coherency, when SrRuO3 was introduced as a buffer layer on the substrates prior to the film growth. The width of this tail-to-tail interface is estimated to be 3-4 unit cells, and oxygen vacancies are supposed to screen the negative polarization bound charge. The formation mechanism of these distinct interfaces was discussed from the perspective of charge redistribution.

Vertical heterostructures of two-dimensional (2D) crystals have led to the observations of numerous exciting physical phenomena and presented the possibilities for technological applications, which strongly depend on the quality, interface, relative alignment, and interaction of the neighboring 2D crystals. The heterostructures or hybrids of graphene and superconductors offer a very interesting platform to study mesoscopic superconductivity and the interplay of the quantum Hall effect with superconductivity. However, so far the heterostructures of graphene and 2D superconductors are fabricated by stacking, and consequently suffer from random relative alignment, weak interfacial interaction, and unavoidable interface contaminants. Here we report the direct growth of high-quality graphene/2D superconductor (nonlayered ultrathin α-Mo2C crystal) vertical heterostructures with uniformly well-aligned lattice orientation and strong interface coupling by chemical vapor deposition. In the heterostructure, both gra...

Structural distortions at the nanoscale are delicately linked with many exotic properties for ferroic thin films. Based on advanced aberration corrected scanning transmission electron microscopy, we observe BiFeO3 thin films with variant tensile strain states and demonstrate at an atomic scale the interplay of intrinsic spontaneous structural distortions with external constraints. Structural parameters (the rhombohedral distortion and domain wall shear distortion) under zero (BiFeO3/GdScO3) and 1.5% (BiFeO3/PrScO3) lateral strain states are quantitatively analyzed which are suppressed within a few unit cells near the film/substrate interfaces. In particular, an interfacial layer with asymmetrical lattice distortions (enhanced and reduced out-of-plane lattice spacing) on the two sides of 109° domain wall is resolved. These structural distortions near the film/substrate interface in ferroic thin films reveal intense tanglement of intrinsic distortions of BiFeO3 with external boundary conditions, which could provide new insights for the development of nanoscale ferroelectric devices.

Ferroelectrics hold promise for sensors, transducers, and telecommunications. With the demand of electronic devices scaling down, they take the form of nanoscale films. However, the polarizations in ultrathin ferroelectric films are usually reduced dramatically due to the depolarization field caused by incomplete charge screening at interfaces, hampering the integrations of ferroelectrics into electric devices. Here, we design and fabricate a ferroelectric/multiferroic PbTiO3/BiFeO3 system, which exhibits discontinuities in both chemical valence and ferroelectric polarization across the interface. Aberration-corrected scanning transmission electron microscopic study reveals an 8% elongation of out-of-plane lattice spacing associated with 104%, 107%, and 39% increments of δTi, δO1, and δO2 in the PbTiO3 layer near the head-to-tail polarized interface, suggesting an over ∼70% enhancement of polarization compared with that of bulk PbTiO3. Besides that in PbTiO3, polarization in the BiFeO3 is also remarkably ...

The pronounced mechanical property of pearlitic steels highly correlates with the ferrite (bcc-Fe)/cementite (Fe3C) boundaries inside. Unraveling the interface structure at an atomic level is essential for interpreting the material’s property. In the present study, using aberration-corrected scanning/transmission electron microscopy combined with density functional theory calculations, we reveal the atomic configuration as well as the electronic structure of the Fe/Fe3C interfaces with the Isaichev orientation in pearlite. The interface with terminating layer Fe–C–Fe in cementite has the lowest energy due to the formation of interfacial Fe–C bonds. Terrace steps which are frequently observed at the interfaces would not break the lattice match between the two phases.

By atomic-scale high-angle annular dark-field scanning transmission electron microscopy, the long-period stacking ordered (LPSO) structures in a near-equilibrium Mg97Zn1Y2 (at.%) alloy have been characterised. In addition to 18R and 14H, new polytypes of LPSO structures are analysed and determined as 60R, 78R, 26H, 96R, 38H, 40H, 108H and 246R. All of these LPSO structures feature AB′C′A building blocks with two Mg layers and three Mg layers sandwiched between them. The Bravais lattices and space groups of new polytypes of LPSO structures were easily determined via the newly introduced method. A structural relationship between the LPSOs is proposed.

Direct evidence of purely interfacial effects on resistance switching is demonstrated in Au/BiFeO3/Nd:SrTiO3(001) (Au/BFO/NSTO) Schottky junctions by reducing the thickness of ferroelectric interlayer BFO. The Au/BFO/NSTO junction shows large current rectification and hysteretic resistive switching behavior without any electroforming process. The conduction mechanism is dominated by interface-limited Fowler–Nordheim (FN) tunneling through a potential barrier formed at the BFO/NSTO interface. Measurements of polarization switching dynamics and capacitance–voltage characteristics provide direct evidence that the resistance switching in the Au/BFO/NSTO junction is ferroelectric and interfacially limited. The observed resistance switching behavior can be attributed to the ferroelectric polarization modulation of the barrier and depletion width of the p–n junction formed at the BFO/NSTO interface.

Electron beam irradiation is well known to induce damage in materials. The structural transformation involved in the damage is usually believed to be an irreversible solid state chemical reaction. Here we use in situ transmission electron microscopy (TEM) combined electron-energy loss spectroscopy (EELS) technique in an aberration-corrected TEM to track the structural transformation in spinel Mn3O4 induced by electron beam irradiation. It is clarified that spinel Mn3O4 is transformed to rocksalt structured MnO by irradiation and the reversed recovering transition from rocksalt MnO to spinel Mn3O4 can occur by aging in the gentle electron beam circumstance. The mechanisms including the role of O desorption/adsorption and the displacement of Mn and O involved in the reversible transformation processes are discussed. The work presents an implication that electron beam can modify the structure at atomic dimension yielding diverse assemblies of surfaces, interfaces and colorful properties.

Lateral heterostructures of 2D materials have several interesting properties and potential applications, and they are usually fabricated by chemical vapor deposition. However, it still remains a great challenge to fabricate 2D lateral heterostructures with well-controlled patterns and sharp interfaces. Herein, we found that the 2D α-Mo2C crystal, a recently emerging 2D superconductor, experiences a phase transition from the α phase to β phase on electron beam irradiation in a transmission electron microscope because of the migration of carbon atoms among the molybdenum octahedrons. Combined with first-principles calculations, the carbon atom migration paths and the corresponding energy barriers were discussed. Utilizing this unique phase transition property of 2D α-Mo2C crystal, we demonstrated the precise in situ construction of the lateral heterostructure of 2D superconducting α/β Mo2C with a well-controlled pattern and sharp interface using advanced aberration-corrected scanning transmission electron microscopy.

Ferroelectricity and electrical conductivity are two fundamentally incompatible properties that are difficult to simultaneously achieve in a material. Here, we combine these two contradictory properties by embedding conducting SrNbO3 micro/nanopillars into a ferroelectric SrNbO3.5 (i.e., Sr2Nb2O7) thin film. The high-Tc ferroelectric SrNbO3.5 thin film is epitaxially grown on a LaAlO3 substrate by pulsed laser deposition. The conducting SrNbO3 micro/nanopillars are introduced into the film via an electron-irradiation-induced SrNbO3.5-to-SrNbO3 phase transformation triggered by a focused electron beam. The sizes and distribution of the SrNbO3 micro/nanopillars can be accurately controlled through artificial manipulation of the electron-irradiation-induced SrNbO3.5-to-SrNbO3 phase transformation. The ferroelectric SrNbO3/SrNbO3.5 thin film with an in-plane polarization exhibits an electrical conductivity in the out-of-plane direction. Such conducting ferroelectric thin films may lead to the discovery of plentiful physical phenomena and have great potential for pyroelectric, photoelectric, and multiferroic applications.

In advanced fission and fusion reactors, structural materials suffer from high dose irradiation by energetic particles and are subject to severe microstructure damage. He atoms, as a byproduct of the (n, α) transmutation reaction, could accumulate to form deleterious cavities, which accelerate radiation-induced embrittlement, swelling and surface deterioration, ultimately degrade the service lifetime of reactor materials. Extensive studies have been performed to explore the strategies that can mitigate He ion irradiation damage. Recently, nanostructured materials have received broad attention because they contain abundant interfaces that are efficient sinks for radiation-induced defects. In this review, we summarize and analyze the current understandings on interface effects on He ion irradiation in nanostructured materials. Some key challenges and research directions are highlighted for studying the interface effects on radiation damage in nanostructured materials.

Controlling domain width, orientation, and patterns in oxide ferroelectrics are not only important for fundamental research but also for potential electronic application. Here, a series of PbTiO

The grain boundaries (GBs) of graphene and molybdenum disulfide have been extensively demonstrated to have a strong influence on electronic, thermal, optical, and mechanical properties. 2D transition-metal carbides (TMCs), known as MXenes, are a rapidly growing new family of 2D materials with many fascinating properties and promising applications. However, the GB structure of 2D TMCs and the influence of GB on their properties remain unknown. Here, we used aberration-corrected scanning transmission electron microscopy combined with electrical measurements to study the GB characteristic of highly crystalline 2D Mo

Fully lamellar Ti-6Al-4V alloys comprise body-centered cubic (BCC) β lamellae in large-sized, hexagonal close-packed (HCP) α colonies and exhibit outstanding toughness. Although α/β interfaces are considered to play a key role in plastic deformation connected to the toughness, the interface effects have not been revealed so far. In this work, we studied underlying deformation mechanisms of interface-related deformation modes at an atomic scale. After the cyclic loading, {1‾102} deformation twins were observed in the vicinity of fatigue crack surfaces. Moreover, the α/β interface structures before and after cyclic loading deformation were characterized via transmission electron microscopy (TEM). The initial α/β interfaces can be described by the terrace ledge kink model, consisting of (01‾10)α||(‾121)β terrace plane and (‾1100)α||(‾101)β ledge plane. TEM investigations reveal that deformation twins nucleate at the α/β interface and the corresponding nucleation is ascribed to the dissociation of basal type dislocations. More importantly, these twins can continuously propagate through multiple β phase lamella. The continuous propagation of twinning is accomplished through double kinking mechanism. In this manner, twinning in α phases and sequential kinking in β phases can effectively release the stress intensification at the crack tip and dissipate plastic work/energy, correspondingly enhancing fracture toughness of fully lamellar Ti-6Al-4V.

The continuous rotation of electric dipoles, which is inspired by unusual spin textures in magnetic materials, has been envisioned by theoretical modelings in last two decades. Although in electrically polar systems it was thought to be difficult to introduce continuous rotation of electric dipoles since similar Dzyaloshinskii–Moriya interaction like that of ferromagnets is still under study, external strains and interface depolarization fields have been then identified to be critical for rotating electric dipoles in nano-scale oxide films/superlattices. In this Perspective, we will briefly summarize the experimental finding of the newly identified topological polar structures and corresponding properties, such as polar flux-closure, vortex, skyrmion lattice, and meron. The critical importance of microscopy technologies, especially the advanced aberration-corrected transmission electron microscopy with ultra-high spatial resolutions, will be emphasized. Moreover, physical aspects to be addressed in the future, such as the strain maintenance and relaxation mechanisms of polar systems/superlattices, atomic maps of three-dimensional topological polar structures, and flexoelectricity-related properties, will be highlighted and envisioned.

Flexoelectricity has been shown to be an effective strategy to modulate the polarization configurations, domain structures, and physical properties in nanoscale ferroelectric thin films. However, the relations between the domain switching processes and flexoelectric effects remain elusive, which is essential for the design of nanoscale ferroelectric electric devices. In this work, strain-gradient and normal PbTiO3 films are fabricated and investigated to resolve this elusive relationship. By using large-scale and local piezoelectric force microscopy characterization, the ferroelectric domain switching in strain-gradient PbTiO3 films is found to be hard and hindered under applied electric fields compared with the normal ones. Successive atomic-scale scanning transmission electron microscopy imaging analysis manifests that the domains in the strain-gradient PbTiO3 films are stabilized by an additional effective strain gradient-induced flexoelectric field, which was introduced by negative pressure originated from vertically distributed Pb-rich anti-phase domains. This study proposes an effective method to stabilize the ferroelectric polarization in nanoscale ferroelectric films, thus facilitate improving the reliability of ferroelectric electronic devices.

Large compressive strains may introduce giant tetragonality and morphotropic phase boundaries in perovskite BiFeO3 films, where the coexisting tetragonal and rhombohedral phases (T like and R like phases) are identified to exhibit large piezoelectric response. Here, we have further achieved mechanical bending deformations in R like BiFeO3 through its neighboring T like BiFeO3 phases, where a strain gradient of ∼106/m was identified. Aberration-corrected scanning transmission electron microscopy revealed not only the strain distributions but also the atomic scale Fe polar displacement in the gradient R like BiFeO3. In spite of the giant strain gradient, the polarization direction in each R like BiFeO3 unit cell was found mainly along its diagonal direction, suggesting that potential flexoelectric coupling induced polarization in BiFeO3 is smaller than its spontaneous polarizations, while a large built-in electric field can be obtained via the large strain gradient. Our results indicate that the common phase coexistences in oxide materials could be further manipulated to introduce elastic strain gradients and tune the properties for oxide films.

Hybrid multiferroic systems composed of complex oxides have drawn sustained attention for decades due to their intriguing physical effects and potential technological exploitations. Designing innovative nanostructures of multiferroic nanocomposites to overcome inherent shortcomings for conventional layered (e.g. clamping effect) and vertically aligned (e.g. leaky) films are instrumental to realize genuine multiferroic behaviors. Here the self-assembled architecture with three-dimensional (3D) framework of heterostructures is fabricated, wherein well-ordered multiferroic e-Fe2O3 (e-FO) nanoboats are embedded in a ferroelectric perovskite PbTiO3 (PTO) matrix. The biphasic system combines strong interfacial magnetoelectric couplings of vertically aligned heterostructure and addressed leakage issue via preferential epitaxy of a high-resistance transition layer of PTO. Additionally, the two phases maintain full lattice coherence along vertical interfaces allowing for efficient interfacial strain coupling. Ferroelectricity and piezoelectric switching of the 3D nanostructured PTO:e-FO film have been corroborated macroscopically by polarization hysteresis (Ps ~ 45 μC cm−2) and locally by piezoresponse force microscopy, and strong magnetoelectric coupling has been manifested as a sizable modification of piezoelectric switching characteristics via applying a DC magnetic field, all conducting at room temperature. The novel 3D multiferroic-ferroelectric heterostructure offers great potential for nanoengineering of multiferroic composites, thus opens an avenue towards superior microelectronics and spintronics.

Giant piezoelectricity has been thought to be originated from polarization rotation, electric-field-induced phase transformation or nano-domain reorientation in diverse complicated morphotropic phase boundaries (MPB) based piezoelectric ceramics. However, the deterministic contribution to the peak piezoelectricity is still elusive. Here, a full stage of piezoelectric response is presented in pure PbTiO3 films epitaxially constrained as the monoclinic phase on SrTiO3(111), where the as-prepared films feature complicated monoclinic nano-domains with {201} domain boundaries. By applying external voltage, the piezoelectric response undergoes three distinct stages which correspond, respectively, to the three existing mechanisms in terms of polarization rotation, nano-domain reorientation, and electric-field-induced phase transformation. A direct correlation between the electric-poling process and the piezoelectric response is established, confirming that the phase transformation from the monoclinic to tetragonal plays a dominant role for the superior piezoelectric properties of the oxide. This study clarifies the role of monoclinic phase by integrating the three individual mechanisms of piezoresponse into a single-phase perovskite oxide, which may facilitate the development of high-performance piezoelectric materials for electronic device applications.

Effects of crystal tilt on the determination of the relative positions of different ion columns in compounds such as ferroelectric PbTiO3 are of critical importance, because the displacements of Ti and O relative to Pb correlate directly to the spontaneous polarization and ferroelectric properties. Here a study about the effects of small-angle crystal tilt on the relative image spots positions of different ions in PbTiO3 was carried out under high angle angular dark-field (HAADF) and bright-field imaging for aberration corrected Scanning Transmission Electron Microscope. The results indicate that crystal tilt affects the relative positions of Pb, Ti, and O greatly, and the effects are proved to depend highly on crystal tilt angle and PTO thickness. HAADF image simulations on PbTiO3, SrTiO3, and SrRuO3 indicate that the difference in atomic number is a main contributor to the relative image spot position change of different ion columns when crystal tilts.

Solute atoms segregation to the interfaces, such as grain boundary or twin boundary, often plays a critical role in modulating the properties of a metallic alloy. Deformation induced segregation to the interfaces has been a subject of significant research, since this is one of the key issues to fully understand the deformation mechanism and microstructure evolution in service of engineering materials. By means of the high-resolution aberration-corrected scanning transmission electron microscopy (STEM), we report the investigations of segregations to symmetrical boundaries, kink boundary (KB) and twin boundary (TB), in the Mg-Zn-Y alloys containing long period stacking ordered (LPSO) phases subjected to a compression at room temperature. We found that Zn atoms preferentially segregate to the deformation-induced symmetrical KBs in the LPSO structures and sandwiched Mg layers, while only a small amount of Y atoms concentrate at KB in LPSO structure. These enriched atoms may be in a random distribution, form nanoscale clusters or in a periodic pattern. Furthermore, solute atoms would rather decorate the segment of coherent TBs than enrich the overlapped TBs. Based on the direct atomic observations, the segregation mechanisms to the featured microstructures are proposed.

Although the strong coupling of polarization to spontaneous strain in ferroelectrics would impart a flux-closure with severe disclination strains, recent studies have successfully stabilized such a domain via a nano-scaled multi-layer growth. Nonetheless, the detailed distributions of polarizations in three-dimensions (3D) and how the strains inside a flux closure affect the structures of domain walls are still less understood. Here we report a 3D polarization texture of a 4-fold flux closure domain identified in tensile strained ferroelectric PbTiO3/SrTiO3 multilayer films. Ferroelectric displacement analysis based on aberration-corrected scanning transmission electron microscopic imaging reveals highly inhomogeneous strains with strain gradient above 107/m. These giant disclination strains significantly broaden the 90° domain walls, while the flexoelectric coupling at 180° domain wall is less affected. The present observations are helpful for understanding the basics of topological dipole textures and indicate novel applications of ferroelectrics through engineering strains.

The properties of two-dimensional (2D) materials such as graphene and monolayer transition metal dichalcogenides are strongly influenced by domain boundaries. Ultrathin transition metal carbides are a class of newly emerging 2D materials that are superconducting and have many potential applications such as in electrochemical energy storage, catalysis, and thermoelectric energy conversion. However, little is known about their domain structure and the influence of domain boundaries on their properties. Here we use atomic-resolution scanning transmission electron microscopy combined with large-scale diffraction-filtered imaging to study the microstructure of chemical vapor deposited high-quality 2D α-Mo2C superconducting crystals of different regular shapes including triangles, rectangles, hexagons, octagons, nonagons, and dodecagons. The Mo atom sublattice in all these crystals has a uniform hexagonal closely packed arrangement without any boundaries. However, except for rectangular and octagonal crystals, the C atom sublattices are composed of three or six domains with rotational-symmetry and well-defined line-shaped domain boundaries because of the presence of three equivalent off-center directions of interstitial carbon atoms in Mo octahedra. We found that there is very small lattice shear strain across the domain boundary. In contrast to the single sharp transition observed in single-domain crystals, transport studies across domain boundaries show a broad resistive superconducting transition with two distinct transition processes due to the formation of localized phase slip events within the boundaries, indicating a significant influence of the boundary on 2D superconductivity. These findings provide new understandings on not only the microstructure of 2D transition metal carbides but also the intrinsic influence of domain boundaries on 2D superconductivity.

KTaO3 is an incipient ferroelectric with its properties influenced by defects and the purity. In this paper, we investigated the microstructures of KTaO3 by X-ray diffraction (XRD) and Cs-corrected scanning transmission electron microscopy (STEM). XRD and electron diffraction indicate that a second phase of K6Ta10.8O30 precipitates from the matrix. Four orientation relationships were identified: (100)m // ( 0)p, [001]m // [0 0 1]p; (1 0 0)m // ( 0)p, [001]m // [0 0 1]p; (1 0 0)m // (0 0 1)p, [0 0 1]m // [3 1 0]p and (100)m // ( 3 0)p, [0 0 1]m // [3 1 0]p. High resolution imaging shows that the interfaces between the second phase and KaTaO3 are distinct and coherent. Atomically resolved analysis identified two kinds of novel ordered defects at the interfaces with the termination planes either KO plane of KTaO3 or TaO2 plane of KTaO3.

Revealing strains on the unit-cell level is essential for understanding the particular performance of materials. Large-scale strain variations with a unit-cell resolution are important for studying ferroelectric materials since the spontaneous polarizations of such materials are strongly coupled with strains. Aberration-corrected high-angle-annular-dark-field scanning transmission electron microscopy (AC-HAADF-STEM) is not so sensitive to the sample thickness and therefore thickness gradients. Consequently it is extremely useful for large-scale strain determination, which can be readily extracted by geometrical phase analysis (GPA). Such a combination has various advantages: it is straightforward, accurate on the unit-cell scale, relatively insensitive to crystal orientation and therefore helpful for large-scale. We take a tetragonal ferroelectric PbTiO3 film as an example in which large-scale strains are determined. Furthermore, based on the specific relationship between lattice rotation and spontaneous polarization (Ps) at 180° domain-walls, the Ps directions are identified, which makes the investigation of ferroelectric domain structures accurate and straightforward. This method is proposed to be suitable for investigating strain-related phenomena in other ferroelectric materials.

It is known that high-index perovskite ferroelectric thin films exhibit giant permittivity, piezoelectric response, and a particular switching behavior. However, the fine domain configuration in high-index ferroelectric films is not yet clarified, which triggers difficulties to further modulate their electric properties. In this work, we focus on the tetragonal PbTiO3 thin films with thicknesses of 20, 40, 55, and 70 nm deposited on a [101]-oriented KTaO3 substrate. By using piezoresponse force microscopy and state-of-the-art scanning transmission electron microscopy, the stripe a/c domains with alternately wide c and narrow a domains are observed in all these PbTiO3 films. The periodic stripe a/c domains with {101} domain walls extend along the in-plane [ 11 1 ¯ ] or [ 1 1 ¯ 1 ¯ ] direction, which almost completely relaxes the misfit strain between the PbTiO3 films and the KTaO3 substrate. The domain width decreases as the film thickness is reduced following the square root dependence. This results in an increase of ferroelastic a/c domain walls and promotes the enhancement of the piezoresponse amplitude for the thinner PbTiO3 films. In addition, the piezoresponse amplitude of a 20 nm PbTiO3 film is comparable to that of a 40 nm PbTiO3 film, which indicates that the piezoelectric response of ferroelectric films may saturate at a certain film thickness and scarcely increase even after the film thickness reduces further. These results clarify the domain configurations of [101]-oriented PbTiO3 thin films and provide useful information for understanding the relationship between microstructures and piezoelectric properties in ferroelectric films.

Topological structures in ferroelectric materials play a crucial role in the potential applications of high-density memories and are currently the subject of intensive interest. Interfaces with local symmetry breaking have garnered wide attention in designing the topological domains in ferroelectric films by regulating the different boundary conditions. Here, we present multiple topological polar nanodomains near the heterointerfaces in the trilayered systems of BiFeO3/GdScO3/BiFeO3 grown on [110]-oriented TbScO3 substrates. The formation and stabilization of these topological polar states depend on the electrical and mechanical boundary conditions of the BiFeO3 layers. Aberration-corrected transmission electron microscopy observation reveals that the topological polar nanodomains, including nano-scale vortices and flux-closures at the termination of 109° domain walls and the semi-vortices at the end of 180° domain walls, are stabilized in the BiFeO3 layers confined by two orthorhombic structures. Furthermore, the formation of flux-closures near the BiFeO3/GdScO3 interface is influenced by the domain structures in the adjacent BiFeO3 layers, which is preferred by the 180° domain patterns rather than the 109° domain patterns. This work provides further understanding into the influences of boundary conditions on topological polar configurations and would offer guidance for designing novel topological states that enable the development of high-density memory devices.

Topological lattices such as vortices are of importance in both scientific research and application potential. Here, we observed periodic vortex-antivortex pairs in PbTiO3/SrTiO3 multilayered films deposited on orthorhombic (110)-oriented GdScO3 by pulsed laser deposition. Domain patterns of the PbTiO3 layers containing topological structures were analyzed in detail by using advanced transmission electron microscopy. It was found that by increasing the PbTiO3 thickness, a/c domains with extra c domains will form in the triangle a domains in pure flux-closure structures. Atomically resolved high-angle annular dark field-scanning TEM imaging demonstrates that this specific domain structure results in vortex-antivortex pairs at the junctions of extra c domains and the original flux-closure structure, forming a periodic vortex-antivortex array throughout the PbTiO3 layer. These results suggest a pathway for designing new topological structures in ferroelectric films.

Boron segregation and boride formation at grain boundaries are generally accepted in Ni-based superalloys. However, the addition of boron was found to result in the formation of a high density of nanosized M5B3 (where M is a mixture of Cr, Mo) particles in γ/γ′ eutectics of a Ni-based superalloy during high-temperature creep in this work. M5B3 phase precipitated at γ/γ′ eutectics keeps a well-defined orientation relationship with the eutectics, which is [001]γ/γ′//[001]M5B3, (020)γ/γ′//(130)M5B3. And these nanosized borides could block dislocation slip, result in stress concentration surrounding γ/γ′ eutectics and facilitate crack initiation at γ/γ′ eutectic/matrix interface during the creep at high temperatures.

The present study reports an atomic level quasi in-situ TEM observation of the redistribution of alloying element Cu in the corrosion process of a B4C-reinforced Al-Mg-Si-Cu composite. Cu atoms were found to combine with Mg and form a thin layer at the interfaces between the aluminum matrix and nano-reinforcements (i.e. Mg(Al)B2 and Al3BC). Dealloying of the Cu/Mg-rich layer was revealed. The following decomposition of Mg(Al)B2 resulted in narrow corrosion trenches forming around Mg(Al)B2 and then the localized pitting of the matrix. In addition, the liberated Cu ions were then redeposited and formed noble Cu nano-particles upon cathodic Mg(Al)B2 and B4C particles.

We investigated the structure, symmetry, and distribution of stacking faults and growth twins in 18R and 14H long-period stacking ordered (LPSO) structures in a near-equilibrium Mg97Zn1Y2 (at.%) alloy by transmission electron microscopy. The stacking faults and growth twins here result from the insertion of the n-Mg components in the LPSO structure, and some of them possess inversion centers or mirror symmetry. The stacking faults and growth twins in 18R LPSO depend on whether the sum of inserted Mg layers of all n-Mg components in defects is even number or odd number, respectively, while only stacking faults appear in 14H LPSO. The gliding of Shockley partial dislocation pairs induces the transformation of stacking faults and motion of twin boundary. Statistically, the periodical arrangement of stacking faults in 18R or 14H LPSO structures results in the formation of superstructures, such as 62L, 86L, 152L, and 235L.

International audience; In ferroelectric thin films, the domain structure defines ferroelectric switching pathways and thus influences device performance. In epitaxial bismuth ferrite (BiFeO3) films, fractal-like domains have been observed, but direct evidence of their origins has remained unclear. Here, we show that the nature of the ferroelectric domain structure-i.e. striped vs. fractal-like-in epitaxial BiFeO3 is defined by the strain profile across the film-substrate interface. In samples with fractal-like domains, X-ray diffraction analysis reveals strong strain gradients, while geometric phase analysis using atomic resolution scanning transmission electron microscopy reveals that within a few nanometers of the film-substrate interface, the out of plane strain shows an anomalous dip while the in-plane strain is constant. Electron energy-loss near edge structure at the oxygen K edge shows that in the vicinity of the interface, the oxygen coordination is locally modified; this combined with the anomalous strain behavior thus drives the formation of fractal-like domains. Conversely, if uniform strain is maintained across the interface, characteristic striped domains are formed. Interestingly, conversion from the fractal-like arrangement to striped domains is found possible by an ex-situ thermal treatment step. Critically, the antiferromagnetic state of the BiFeO3 is influenced by the domain structure, whereby the fractal-like domains disrupt the long-range spin cycloid. Finally, as a demonstration of the applicability of this concept, we show that a carefully engineered lower electrode with large strain gradient can be used to induce fractal domains.

Understanding cavity-interface interaction is crucial in designing high-strength, radiation-tolerant nanocomposites. Here, bubbles near the Cu–Nb interface in Nb could be absorbed by Cu voids wetting the interface due to the high system energy difference produced by the huge pressure difference between the bubbles and voids and high mobility of bubbles, and no bubble-denuded zone forms owing to fast Brownian motion of bubbles, which keeps bubbles distributed homogeneously. However, owing to the low system energy difference generated by the low internal pressure difference and low mobility, voids near the Cu–Nb interface in Nb would simply coalesce with Cu voids wetting the interface, leading to the void-interface wetting to crossing transition and the formation of void-denuded zones due to the negligible migration of internal Nb voids.

Strongly correlated oxides exhibit multiple degrees of freedoms, which can potentially mediate exotic phases with exciting physical properties, such as the polar vortex recently found in ferroelectric oxide films. A polar vortex is stabilized by competition between charge, lattice, and/or orbital degrees of freedom, which displays vortex-ferroelectric phase transitions and emergent chirality, making it a potential candidate for designing information storage and processing devices. Here, by a combination of controlled film growth and aberration-corrected scanning transmission electron microscopy, we obtain nanoscale vortex arrays in [110]-oriented BiFeO