Mapping grain boundary heterogeneity at the nanoscale in a positive temperature coefficient of resistivity ceramic

Despite being of wide commercial use in devices, the orders of magnitude increase in resistance that can be seen in some semiconducting BaTiO3-based ceramics, on heating through the Curie temperature (TC), is far from well understood. Current understanding of the behavior hinges on the role of grain boundary resistance that can be modified by polarization discontinuities which develop in the ferroelectric state. However, direct nanoscale resistance mapping to verify this model has rarely been attempted, and the potential approach to engineer polarization states at the grain boundaries, that could lead to optimized positive temperature coefficient (PTC) behavior, is strongly underdeveloped. Here we present direct visualization and nanoscale mapping in a commercially optimized BaTiO3-PbTiO3-CaTiO3 PTC ceramic using Kelvin probe force microscopy, which shows that, even in the low resistance ferroelectric state, the potential drop at grain boundaries is significantly greater than in grain interiors. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy reveal new evidence of Pb-rich grain boundaries symptomatic of a higher net polarization normal to the grain boundaries compared to the purer grain interiors. These results validate the critical link between optimized PTC performance and higher local polarization at grain boundaries in this specific ceramic system and suggest a novel route towards engineering devices where an interface layer of higher spontaneous polarization could lead to enhanced PTC functionality

Deterministic dual control of phase competition in strained BiFeO3 : a multiparametric structural lithography approach

UK Research and Innovation, MR/T043172/1, Raymond G. P. McQuaid; Department for Employment and Learning, Northern Ireland, USI-082, Amit Kumar; Engineering and Physical Sciences Research Council, EP/S037179/1, Amit Kumar; EP/L015323/01, Nathan Black.The realization of a mixed-phase microstructure in strained BiFeO3 (BFO) thin films has led to numerous novel effects derived from the coexistence of the tetragonal-like monoclinic phase (T phase) and rhombohedral-like monoclinic phase (R phase). Strong strain and polarization differences between the phases should result in a high level of transformation plasticity, which enables the continuous alteration of the relative proportion of R and T states in response to external forces. Although the potential for utilizing such plasticity to control mixed-phase populations under external stimuli is evident, direct experimental evidence backed by equilibrium predictions has not yet been fully demonstrated. Here we demonstrate deterministic control of mixed-phase populations in an epitaxially strained BFO thin film through the application of localized stresses and electric fields in a reversible manner. The results illustrate and rationalize deterministic control of mixed phases in strained BFO films, which could be crucial in tuning their functional properties. The findings also highlight a new multiparametric technique in the scanning probe lithography toolbox based on tip-assisted electric and strain field manipulation of functional properties that might find application beyond the ferroelectric domain and structural phase lithography.Publisher PDFPeer reviewe

Exploring domain continuity across BaTiO3 grain boundaries: Theory meets experiment.

Polycrystalline ferroelectrics constitute the basis of many advanced technologies, including sensors and actuators. Their intricate domain patterns, and switching, drive the macroscopic electrical and mechanical properties of the material, where the domain switching behaviour is largely influenced by the grain-grain interaction of the domain walls. Domain wall continuity across grain boundaries is speculated to affect the domain wall – grain boundary interaction, although the true impact of this phenomenon on the ferroelectric properties, and the conditions under which continuity occurs, are not yet well understood. Whilst there are some theoretical reports, the link to experimental evidence is limited, greatly hindering the applicability and fundamental understanding of current polycrystalline based devices. In this work, we close this gap by studying several grain junctions in free-standing BaTiO3 thin films using microscopy techniques and rationalising the domain configurations with reference to martensite theory. A pleasing agreement of minimal strain and polarisation mismatch for a pair of domain variants were found in cases where domain wall continuity across grain boundaries was observed, confirming that domain continuity is related to the compatibility conditions at the grain boundary. Following this experimental validation, the mismatches for various combinations of Euler angles in bi-grain junctions were theoretically explored, offering valuable insights into specific cases where domain continuity can be expected. These results offer an advancement in the understanding of grain-grain-domain interactions and provides a template for the prediction and control of domain wall continuity in polycrystalline ferroelectrics, appealing to those working in polycrystal design and domain engineering

Towards 3D characterisation of site-controlled InGaAs pyramidal QDs at the nanoscale

In this work, we report an extensive investigation via transmission electron microscopy (TEM) techniques of InGaAs/GaAs pyramidal quantum dots (PQDs), a unique site-controlled family of quantum emitters that have proven to be excellent sources of single and entangled photons. The most striking features of this system, originating from their peculiar fabrication process, include their inherently 3-dimensional nature and their interconnection to a series of nanostructures that are formed alongside them, such as quantum wells and quantum wires. We present structural and chemical data from cross-sectional and plan view samples of both single and stacked PQDs structures. Our findings identify (i) the shape of the dot, being hexagonal and not triangular as previously assumed, (ii) the chemical distribution at the facets and QD area, displaying clear Indium diffusion, and (iii) a near absence of Aluminium (from the AlAs marker) at the bottom of the growth profile. Our results shed light on previously unreported structural and chemical features of PQDs, which is of extreme relevance for further development of this family of quantum emitters

Effects of multiple local environments on electron energy loss spectra of epitaxial perovskite interfaces

The role of local chemical environments in the electron energy loss spectra of complex multiferroic oxides was studied using computational and experimental techniques. The evolution of the O K-edge across an interface between bismuth ferrite (BFO) and lanthanum strontium manganate (LSMO) was considered through spectral averaging over crystallographically equivalent positions to capture the periodicity of the local O environments. Computational techniques were used to investigate the contribution of individual atomic environments to the overall spectrum, and the role of doping and strain was considered. Chemical variation, even at the low level, was found to have a major impact on the spectral features, whereas strain only induced a small chemical shift to the edge onset energy. Through a combination of these methods, it was possible to explain experimentally observed effects such as spectral flattening near the interface as the combination of spectral responses from multiple local atomic environments