Ferroelectric domain walls exhibit a number of new functionalities that are not present in their host material. One of these functional characteristics is electrical conductivity that may lead to ...future device applications. Although progress has been made, the intrinsic conductivity of BiFeO3 domain walls is still elusive. Here, the intrinsic conductivity of 71° and 109° domain walls is reported by probing the local conductance over a cross section of the BiFeO3/TbScO3 (001) heterostructure. Through a combination of conductive atomic force microscopy, high‐resolution electron energy loss spectroscopy, and phase‐field simulations, it is found that the 71° domain wall has an inherently charged nature, while the 109° domain wall is close to neutral. Hence, the intrinsic conductivity of the 71° domain walls is an order of magnitude larger than that of the 109° domain walls associated with bound‐charge‐induced bandgap lowering. Furthermore, the interaction of adjacent 71° domain walls and domain wall curvature leads to a variation of the charge distribution inside the walls, and causes a discontinuity of potential in the 110p direction, which results in an alternative conductivity of the neighboring 71° domain walls, and a low conductivity of the 71° domain walls when measurement is taken from the film top surface.
The distinct intrinsic conductivity of the 71° and 109° domain walls in BiFeO3
is revealed by using a combination of local probe techniques. It is found that the intrinsic conductivity of the 71° domain walls is an order of magnitude larger than that of the 109° domain walls. Atomic resolution electron energy loss spectroscopy (EELS) analysis and phase‐field simulation demonstrate clear bandgap lowering and the weakly charged nature of the 71° domain walls, while a small bandgap change is observed at the 109° walls.
Using epitaxy and the misfit strain imposed by an underlying substrate, it is possible to elastically strain oxide thin films to percent levels—far beyond where they would crack in bulk. Under such ...strains, the properties of oxides can be dramatically altered. In this article, we review the use of elastic strain to enhance ferroics, materials containing domains that can be moved through the application of an electric field (ferroelectric), a magnetic field (ferromagnetic), or stress (ferroelastic). We describe examples of transmuting oxides that are neither ferroelectric nor ferromagnetic in their unstrained state into ferroelectrics, ferromagnets, or materials that are both at the same time (multiferroics). Elastic strain can also be used to enhance the properties of known ferroic oxides or to create new tunable microwave dielectrics with performance that rivals that of existing materials. Results show that for thin films of ferroic oxides, elastic strain is a viable alternative to the traditional method of chemical substitution to lower the energy of a desired ground state relative to that of competing ground states to create materials with superior properties.
Crystal defects affect the thermal and heat-transport properties of materials by scattering phonons and modifying phonon spectra
. To appreciate how imperfections in solids influence thermal ...conductivity and diffusivity, it is thus essential to understand phonon-defect interactions. Sophisticated theories are available to explore such interactions, but experimental validation is limited because most phonon-detecting spectroscopic methods do not reach the high spatial resolution needed to resolve local vibrational spectra near individual defects. Here we demonstrate that space- and angle-resolved vibrational spectroscopy in a transmission electron microscope makes it possible to map the vibrational spectra of individual crystal defects. We detect a red shift of several millielectronvolts in the energy of acoustic vibration modes near a single stacking fault in cubic silicon carbide, together with substantial changes in their intensity, and find that these changes are confined to within a few nanometres of the stacking fault. These observations illustrate that the capabilities of a state-of-the-art transmission electron microscope open the door to the direct mapping of phonon propagation around defects, which is expected to provide useful guidance for engineering the thermal properties of materials.
The distribution of charge density in materials dictates their chemical bonding, electronic transport, and optical and mechanical properties. Indirectly measuring the charge density of bulk materials ...is possible through X-ray or electron diffraction techniques by fitting their structure factors
, but only if the sample is perfectly homogeneous within the area illuminated by the beam. Meanwhile, scanning tunnelling microscopy and atomic force microscopy enable us to see chemical bonds, but only on the surface
. It remains a challenge to resolve charge density in nanostructures and functional materials with imperfect crystalline structures-such as those with defects, interfaces or boundaries at which new physics emerges. Here we describe the development of a real-space imaging technique that can directly map the local charge density of crystalline materials with sub-ångström resolution, using scanning transmission electron microscopy alongside an angle-resolved pixellated fast-electron detector. Using this technique, we image the interfacial charge distribution and ferroelectric polarization in a SrTiO
/BiFeO
heterojunction in four dimensions, and discover charge accumulation at the interface that is induced by the penetration of the polarization field of BiFeO
. We validate this finding through side-by-side comparison with density functional theory calculations. Our charge-density imaging method advances electron microscopy from detecting atoms to imaging electron distributions, providing a new way of studying local bonding in crystalline solids.
•CuFeO2 crystals up to 50 mm in length and up to 10 mm in diameter were grown by OFZ.•Crystals will be used for MBE growth of other functional delafossites.•The melting of CuFeO2 is not simply ...incongruent, the gas phase is also involved.
CuFeO2 single crystals up to 50 mm in length and up to 10 mm in diameter were grown by the optical floating-zone method. Stoichiometric polycrystalline rods with a diameter of 6–12 mm were used as feed materials to produce crystals of sufficient size to be used as substrates for the growth of thin films of delafossites. For stable growth along the c-axis, low growth rates of 0.4 mm/h are necessary. Due to the incongruent melting behavior of CuFeO2, a stable melt zone requires adjustment of the lamp power during growth. The melting of CuFeO2 is not simply incongruent because the thermodynamic equilibrium includes more than two solid phases and the melt; the gas phase is also involved. The crystals were characterized by X-ray diffraction and X-ray fluorescence measurements.
Elementary particles such as electrons or photons are frequent subjects of wave-nature-driven investigations, unlike collective excitations such as phonons. The demonstration of wave-particle ...crossover, in terms of macroscopic properties, is crucial to the understanding and application of the wave behaviour of matter. We present an unambiguous demonstration of the theoretically predicted crossover from diffuse (particle-like) to specular (wave-like) phonon scattering in epitaxial oxide superlattices, manifested by a minimum in lattice thermal conductivity as a function of interface density. We do so by synthesizing superlattices of electrically insulating perovskite oxides and systematically varying the interface density, with unit-cell precision, using two different epitaxial-growth techniques. These observations open up opportunities for studies on the wave nature of phonons, particularly phonon interference effects, using oxide superlattices as model systems, with extensive applications in thermoelectrics and thermal management.
We describe a hybrid pixel array detector (electron microscope pixel array detector, or EMPAD) adapted for use in electron microscope applications, especially as a universal detector for scanning ...transmission electron microscopy. The 128×128 pixel detector consists of a 500 µm thick silicon diode array bump-bonded pixel-by-pixel to an application-specific integrated circuit. The in-pixel circuitry provides a 1,000,000:1 dynamic range within a single frame, allowing the direct electron beam to be imaged while still maintaining single electron sensitivity. A 1.1 kHz framing rate enables rapid data collection and minimizes sample drift distortions while scanning. By capturing the entire unsaturated diffraction pattern in scanning mode, one can simultaneously capture bright field, dark field, and phase contrast information, as well as being able to analyze the full scattering distribution, allowing true center of mass imaging. The scattering is recorded on an absolute scale, so that information such as local sample thickness can be directly determined. This paper describes the detector architecture, data acquisition system, and preliminary results from experiments with 80–200 keV electron beams.
DC and pulse voltage-induced metal–insulator transition (MIT) in epitaxial VO2 two terminal devices were measured at various stage temperatures. The power needed to switch the device to the ON-state ...decrease linearly with increasing stage temperature, which can be explained by the Joule heating effect. During transient voltage induced MIT measurement, the incubation time varied across 6 orders of magnitude. Both DC I–V characteristic and incubation times calculated from the electrothermal simulations show good agreement with measured values, indicating Joule heating effect is the cause of MIT with no evidence of electronic effects. The width of the metallic filament in the ON-state of the device was extracted and simulated within the thermal model.
Controlled switching of resistivity in ferroelectric thin films is demonstrated by writing and erasing stable, nanoscale, strongly charged domain walls using an in situ transmission electron ...microscopy technique. The resistance can be read nondestructively and presents the largest off/on ratio (≈105) ever reported in room‐temperature ferroelectric devices, opening new avenues for engineering ferroelectric thin‐film devices.
Multiferroic BiFeO3 (BFO) films with spontaneously formed periodic stripe domains can generate above-gap open circuit voltages under visible light illumination; nevertheless the underlying mechanism ...behind this intriguing optoelectronic response has not been understood to date. Here, we make contact-free measurements of light-induced currents in epitaxial BFO films via detecting terahertz radiation emanated by these currents, enabling a direct probe of the intrinsic charge separation mechanisms along with quantitative measurements of the current amplitudes and their directions. In the periodic stripe samples, we find that the net photocurrent is dominated by the charge separation across the domain walls, whereas in the monodomain samples the photovoltaic response arises from a bulk shift current associated with the non-centrosymmetry of the crystal. The peak current amplitude driven by the charge separation at the domain walls is found to be 2 orders of magnitude higher than the bulk shift current response, indicating the prominent role of domain walls acting as nanoscale junctions to efficiently separate photogenerated charges in the stripe domain BFO films. These findings show that domain-wall-engineered BFO thin films offer exciting prospects for ferroelectric-based optoelectronics, as well as bias-free strong terahertz emitters.
PDF
