Yin-Lian Zhu

Observation of a periodic array of flux-closure quadrants in strained ferroelectric PbTiO3 films

Getting closure in ferroelectric films Ferroelectric materials have a spontaneous electric polarization that can be manipulated for applications. The polarization is usually not uniform throughout the material, and for nanosized ferroelectrics, polarization can be quite complex. Using scanning transmission electron microscopy, Tang et al. found that in thin films of the ferroelectric PbTiO3, the polarization vector rotated in space, forming a closed loop, the so-called flux closure. The flux closure structures formed an array, with the period dependent on the width of the thin film, and caused the buildup of considerable strain within the crystal lattice of the material Science, this issue p. 547 Scanning transmission electron microscopy is used to observe closed polarization loops in the ferroelectric PbTiO3. Nanoscale ferroelectrics are expected to exhibit various exotic domain configurations, such as the full flux-closure pattern that is well known in ferromagnetic materials. Here we observe not only the atomic morphology of the flux-closure quadrant but also a periodic array of flux closures in ferroelectric PbTiO3 films, mediated by tensile strain on a GdScO3 substrate. Using aberration-corrected scanning transmission electron microscopy, we directly visualize an alternating array of clockwise and counterclockwise flux closures, whose periodicity depends on the PbTiO3 film thickness. In the vicinity of the core, the strain is sufficient to rupture the lattice, with strain gradients up to 109 per meter. Engineering strain at the nanoscale may facilitate the development of nanoscale ferroelectric devices.

Giant linear strain gradient with extremely low elastic energy in a perovskite nanostructure array

Although elastic strains, particularly inhomogeneous strains, are able to tune, enhance or create novel properties of some nanoscale functional materials, potential devices dominated by inhomogeneous strains have not been achieved so far. Here we report a fabrication of inhomogeneous strains with a linear gradient as giant as 106 per metre, featuring an extremely lower elastic energy cost compared with a uniformly strained state. The present strain gradient, resulting from the disclinations in the BiFeO3 nanostructures array grown on LaAlO3 substrates via a high deposition flux, induces a polarization of several microcoulomb per square centimetre. It leads to a large built-in electric field of several megavoltage per metre, and gives rise to a large enhancement of solar absorption. Our results indicate that it is possible to build up large-scale strain-dominated nanostructures with exotic properties, which in turn could be useful in the development of novel devices for electromechanical and photoelectric applications.

Local Enhancement of Polarization at PbTiO3/BiFeO3 Interfaces Mediated by Charge Transfer

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 enhanced. Electron energy loss spectrum and X-ray photoelectron spectroscopy investigations demonstrate the oxygen vacancy accumulation as well as the transfer of Fe3+ to Fe2+ at the interface. On the basis of the polar catastrophe model, FeO2/PbO interface is determined. First-principles calculation manifests that the oxygen vacancy at the interface plays a predominate role in inducing the local polarization enhancement. We propose a charge transfer mechanism that leads to the remarkable polarization increment at the PbTiO3/BiFeO3 interface. This study may facilitate the development of nanoscale ferroelectric devices by tailing the coupling of charge and lattice in oxide heteroepitaxy.

Impact of interfacial effects on ferroelectric resistance switching of Au/BiFeO3/Nb:SrTiO3(100) Schottky junctions

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.

Microstructural evolution of (PbZrxTi1-xO3/PbZryTi1-yO3)n epitaxial multilayers (x/y^0.2/0.4, 0.4/0.6) - dependence on layer thickness

The microstructure of ferroelectric [PbZr x Ti1− x O3/PbZr y Ti1− y O3] n epitaxial multilayers (x/y = 0.2/0.4, 0.4/0.6) deposited on SrRuO3-coated SrTiO3 substrates by pulsed-laser deposition with different layer periodicity and layer thickness was characterized by means of transmission electron microscopy. Electron diffraction and contrast analysis revealed a very clear and well-separated layer sequence. The microstructures of PbZr0.2Ti0.8O3/PbZr0.4Ti0.6O3 and PbZr0.4Ti0.6O3/PbZr0.6Ti0.4O3 multilayers show a similar tendency in the dependence on the individual layer thickness. Whereas with thick individual layers, tetragonal a-domains are confined to specific layers of the two types of multilayers, below a certain critical thickness of the individual layers, a-domains extend over the whole film. This indicates a transition into a uniform tetragonal lattice and strain state of the whole multilayer. Increasing the layer periodicity further, the interfaces in PbZr0.4Ti0.6O3/PbZr0.6Ti0.4O3 multilayers become rough, and complex a-domain configurations appear.

Giant Polarization Sustainability in Ultrathin Ferroelectric Films Stabilized by Charge Transfer

Ferroelectricity is generally deteriorated or even vanishes when the ferroelectric films are downsized to unit cell scale. To maintain and enhance the polarization in nanoscale ferroelectrics are of scientific and technological importance. Here, giant polarization sustainability is reported in a series of ultrathin PbTiO3 films scaled down to three unit cells grown on NdGaO3 (110) substrates with La0.7 Sr0.3 MnO3 as bottom electrodes. Atomic mappings via aberration-corrected scanning transmission electron microscopy demonstrate the robust ferroelectricity for the sub-10 nm thick film. For the 1.2 nm thick film, the polarization reaches ≈50 µC cm-2 . The 2 nm thick film possesses a polarization as high as the bulk value. The films ranging from 10 to 35 nm display a giant elongation of out-of-plane lattice parameter, which corresponds to a polarization of 100 µC cm-2 , 20% larger than that of the bulk PbTiO3 . The giant enhancement of polarization in the present films is proposed to result from the charge transfer at the La0.7 Sr0.3 MnO3 /PbTiO3 interface, as supported by the anomalous decrease of Mn valence measured from X-ray photoelectron spectroscopy. These results reveal the significant role of charge transfer at interfaces in improving large polarizations in ultrathin ferroelectrics and are meaningful for the development of future electronic devices.

Controlled Growth and Atomic-Scale Mapping of Charged Heterointerfaces in PbTiO3/BiFeO3 Bilayers

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.

Robust ferromagnetism of single crystalline CoxZn1−xO (0.3 ≤ x ≤ 0.45) epitaxial films with high Co concentration

In contrast to conventional dilute magnetic semiconductors with concentrations of magnetic ions of just a few percent, here, we report the fabrication of epitaxial CoxZn1−xO single crystalline films with Co concentrations from xu2009=u20090.3 up to 0.45 by radio-frequency oxygen-plasma-assisted molecular beam epitaxy. The films retain their single crystalline wurtzite structure without any other crystallographic phase from precipitates, based on reflection high energy electron diffraction, X-ray diffraction, transmission electron microscopy, and Raman scattering. The results of X-ray diffraction, optical transmission spectroscopy, and in-situ X-ray photoelectron spectroscopy confirm the incorporation of Co2+ cations into the wurtzite lattice. The films exhibit robust ferromagnetism and the magneto-optical Kerr effect at room temperature. The saturation magnetization reaches 265u2009emu/cm3 at xu2009=u20090.45, which corresponds to the average magnetic moment of 1.5 μB per Co atom.

Polarization Rotation in Ultrathin Ferroelectrics Tailored by Interfacial Oxygen Octahedral Coupling

Multiple polar states and giant piezoelectric responses could be driven by polarization rotation in ferroelectric films, which have potential functionalities in modern material applications. Although theoretical calculations have predicted polarization rotation in pure PbTiO3 films without domain walls and strains, direct experiment has rarely confirmed such polar states under this condition. Here, we observed that interfacial oxygen octahedral coupling (OOC) can introduce an oxygen octahedral rotation, which induces polarization rotation in single domain PbTiO3 films with negligible strains. We have grown ultrathin PbTiO3 films (3.2 nm) on both SrTiO3 and Nb:SrTiO3 substrates and applied aberration-corrected scanning transmission electron microscopy (STEM) to study the interfacial OOC effect. Atomic mappings unit cell by unit cell demonstrate that polarization rotation occurs in PbTiO3 films on both substrates. The distortion of oxygen octahedra in PbTiO3 is proven by annular bright-field STEM. The critical thickness for this polarization rotation is about 4 nm (10 unit cells), above which polarization rotation disappears. First-principles calculations manifest that the interfacial OOC is responsible for the polarization rotation state. These results may shed light on further understanding the polarization behavior in ultrathin ferroelectrics and be helpful to develop relevant devices as polarization rotation is known to be closely related to superior electromechanical responses.

Microstructure of the potentially multiferroic Fe/BaTiO3 epitaxial interface

The Fe/BaTiO3 thin-film layered structure is a prototype of charge-mediated composite multiferroics, which is a promising but challenging route to achieve a sizable magnetoelectric effect. The real structure of the interface between the ferromagnetic Fe and ferroelectric BaTiO3 layers is crucial. In this paper, epitaxial Fe layers were successfully grown on top of BaTiO3 layers by carefully controlling the pulsed laser deposition and magnetron sputtering procedures. A detailed study of interfacial structure and defects at the Fe/BaTiO3 interface was carried out by transmission electron microscopy (TEM). Electron diffraction patterns and diffraction contrast images reveal a definite epitaxial relationship between Fe and BaTiO3 (001) films and a semi-coherent interface with nearly periodic interfacial dislocations. Based on high-resolution TEM images from both [010] and [110] direction observations, the interfacial dislocations were found to be partial with Burgers vectors and line directions of ⟨010⟩. By employing high-resolution Z-contrast imaging, the positions of individual atoms columns were resolved. The formation mechanism of interfacial dislocations was proposed in terms of geometrical models of the interface structure. On the basis of the remaining strain analysis in each layer, the effects of both BaTiO3 thickness and the SrTiO3 substrates on the density of the interface defects were discussed.