Strain-gradient mediated local conduction in strained bismuth ferrite films

It has been recently shown that the strain gradient is able to separate the light-excited electron-hole pairs in semiconductors, but how it affects the photoelectric properties of the photo-active materials remains an open question. Here, we demonstrate the critical role of the strain gradient in mediating local photoelectric properties in the strained BiFeO3 thin films by systematically characterizing the local conduction with nanometre lateral resolution in both dark and illuminated conditions. Due to the giant strain gradient manifested at the morphotropic phase boundaries, the associated flexo-photovoltaic effect induces on one side an enhanced photoconduction in the R-phase, and on the other side a negative photoconductivity in the morphotropic [Formula: see text]-phase. This work offers insight and implication of the strain gradient on the electronic properties in both optoelectronic and photovoltaic devices

Structure and ferroelectricity at the atomic level in perovskite oxides.

Ferroelectricity is a phenomenon that has been studied for nearly 100 years, forming one arm of the large field of ferroics that includes ferromagnetism and ferroelasticity. Ferroelectric materials have shown promise in a wide range of devices such as non-volatile memory devices, micro mechanical actuators and infrared sensors. Further potential can be realised by combining the ferroelectric properties with other ferroic properties to form more complex devices. This thesis aims to leverage the power of transmission electron microscopy to examine ferroelectricity down to the atomic scale. First, the polarisation in ferroelectric tunnel junction devices, consisting of an ultrathin ferroelectric between two electrodes, is studied in context of the depolarisation field created by bound charges in the ferroelectric. This drives the formation of vortex-type structures in the polarisation, however, the free charges in the electrodes act to screen the depolarisation field. This thesis investigates whether curling polarization structures are innate to ferroelectricity or induced by the absence of electrodes. Here it is shown that in unpoled (La0:7Sr0:3)MnO3/PbTiO3/Co ferroelectric tunnel junctions, the polarization in active PbTiO3 layers 3.6 nm thick forms Kittel-like domains, while for 2.4 nm of PbTiO3 there is a complex flux-closure curling behaviour resembling an incommensurate phase. Reducing the thickness to 1.2 nm, there is an almost complete loss of switchable polarization associated with an internal gradient. Additionally, the polarisation for thicker PbTiO3 films extends into the (La0:7Sr0:3)MnO3 whilst the 1.2 nm PbTiO3 film is dominated by the substrate. Next, the relaxor-like ferroelectric Pb2ScTaO6 has been examined, a highly dynamic system that is typified by the presence of polar nano regions that fluctuate rapidly in time. The properties of Pb2ScTaO6 are strongly dependent on the presence of ordering of the Ta and Sc ions into a chequerboard pattern. Here, highly ordered (> 85 %) Pb2ScTaO6 is examined in terms of its local ordering, where small, local fluctuations in the order are accompanied by disorder at anti phase boundaries. Furthermore, evidence for a phase transition at 220 K to the ferroelectric state is found from electron diffraction and dielectric data. The polar fluctuations are also examined, directly using dark field imaging and from scattering phenomena in atomic resolution scanning transmission electron microscopy. It is found that the polar fluctuations do not change in frequency or size as a function of temperature though electron beam effects cannot be excluded. Finally, the application of geometric phase analysis, used to measure strain, is considered when applied to atomic resolution, Z-contrast images. It is demonstrated that, in such image, an additional phase can be present. If the structure changes from one area to another in the image (e.g. across an interface), the change in this additional phase will appear as a strain in conventional geometric phase analysis, even if there is no lattice strain.The origin of the artefact is formalised and strategies to avoid this pitfall are outlined

Quantitative high-dynamic-range electron diffraction of polar nanodomains in Pb2 ScTaO6

Highly B‐site ordered Pb2ScTaO6 crystals are studied as a function of temperature via dielectric spectroscopy and in situ high‐dynamic‐range electron diffraction. The degree of ordering is examined on the local and macroscopic scale and is determined to be 76%. Novel analysis of the electron diffraction patterns provides structural information with two types of antiferroelectric displacements determined to be present in the polar structure. It is then found that a low‐temperature transition occurs on cooling at ≈210 K that is not present on heating. This phenomenon is discussed in terms of the freezing of dynamic polar nanodomains where a high density of domain walls creates a metastable state

Interlacing in atomic resolution scanning transmission electron microscopy

Fast frame-rates are desirable in scanning transmission electron microscopy for a number of reasons: controlling electron beam dose, capturing in-situ events or reducing the appearance of scan distortions. Whilst several strategies exist for increasing frame-rates, many impact image quality or require investment in advanced scan hardware. Here we present an interlaced imaging approach to achieve minimal loss of image quality with faster frame-rates that can be implemented on many existing scan controllers. We further demonstrate that our interlacing approach provides the best possible strain precision for a given electron dose compared with other contemporary approaches

Structural and photoelectric properties of tensile strained BiFeO3

An in-depth structural study of a 23-nm-thick BiFeO3 film grown on orthorhombic NdScO3(110)O substrates demonstrates the presence of a mixed phases. Atomic resolution scanning transmission electron microscopy measurements reveal an out-of-plane stripe domain structure typical of rhombohedral BiFeO3 films but with a polarization component along pseudocubic ⟨100⟩PC or canted from the ⟨111⟩PC towards the in-plane direction. Photovoltaic measurements display an anomalous modulation of the open circuit voltage as temperature is decreased that is attributed to a structural change associated with a transition to a single structural phase

Polarization curling and flux closures in multiferroic tunnel junctions

Formation of domain walls in ferroelectrics is not energetically favourable in low-dimensional systems. Instead, vortex-type structures are formed that are driven by depolarization fields occurring in such systems. Consequently, polarization vortices have only been experimentally found in systems in which these fields are deliberately maximized, that is, in films between insulating layers. As such configurations are devoid of screening charges provided by metal electrodes, commonly used in electronic devices, it is wise to investigate if curling polarization structures are innate to ferroelectricity or induced by the absence of electrodes. Here we show that in unpoled Co/PbTiO3/(La,Sr)MnO3 ferroelectric tunnel junctions, the polarization in active PbTiO3 layers 9 unit cells thick forms Kittel-like domains, while at 6 unit cells there is a complex flux-closure curling behaviour resembling an incommensurate phase. Reducing the thickness to 3 unit cells, there is an almost complete loss of switchable polarization associated with an internal gradient

How Fast is Your Detector? The Effect of Temporal Response on Image Quality

With increasing interest in high-speed imaging should come an increased interest in the response times of our scanning transmission electron microscope (STEM) detectors. Previous works have previously highlighted and contrasted performance of various detectors for quantitative compositional or structural studies, but here we shift the focus to detector temporal response, and the effect this has on captured images. The rise and decay times of eight detectors' single electron response are reported, as well as measurements of their flatness, roundness, smoothness, and ellipticity. We develop and apply a methodology for incorporating the temporal detector response into simulations, showing that a loss of resolution is apparent in both the images and their Fourier transforms. We conclude that the solid-state detector outperforms the photomultiplier-tube (PMT) based detectors in all areas bar a slightly less elliptical central hole and is likely the best detector to use for the majority of applications. However, using tools introduced here we encourage users to effectively evaluate what detector is most suitable for their experimental needs

Bi-ferroic memristive properties of multiferroic tunnel junctions

The giant tunnelling electroresistance (TER) and memristive behaviours of ferroelectric tunnel junctions make them promising candidates for future information storage technology. Using conducting ferromagnetic layers as electrodes results in multiferroic tunnel junctions (MFTJs) which show spin dependent transport. The tunnelling magnetoresistance (TMR) of such structures can be reversibly controlled by electric pulsing owing to ferroelectric polarisation-dependent spin polarisation at the ferroelectric/ferromagnetic interface. Here, we show multilevel electric control of both TMR and TER of MFTJs, which indicates the bi-ferroic or magneto-electric memristive properties. This effect is realised by manipulating the ferroelectric domain configuration via non-volatile partial ferroelectric switching obtained by applying low voltage pulses to the junction. Through electrically modulating the ratio between up- and down-polarised ferroelectric domains, a broad range of TMR (between ∼3% and ∼30%) and TER (∼1000%) values can be achieved. The multilevel control of TMR and TER using the electric pulse tunable ferroelectric domain configuration suggests a viable way to obtain multiple state memory