Tailoring the Band gap and magnetic properties by Bismuth substitution in Neodymium Chromite

The intrinsic distortions present in r are earth ortho chromites (RCrO 3 ) observed from lanthanum to lutetium (in R - site) can influence the magnetic properties like Neel Transition and weak ferromagnetic coupling . A n on - magnetic cation with similar ionic radius would be a suitable candidate to engineer the inherent distortions of particular orthochromite. In this study Bismuth (Bi 3+ ) with a 6s 2 lone pair was chosen to substitute in Neodymium (Nd 3+ ) site of NdCrO 3 (NCO) to tailor the int rinsic structural distortions. Th e variation of optical absorption edge evidently suggests that Bi ( 6 s 2 ) substituted in the m agnetic rare earth Nd +3 influences the Cr - O overlap integral. Th e interaction of Bi cation with oxygen bonds influences the structural distortions through Cr - O polyhedra which are evident from Raman scattering studies . The observed structural and magnetic properties of similar ionic radius of Bi 3+ in Nd 3+ reveals that intrinsic structural distortions are interrelated to enhanced weak ferromagnetic component and change in Neel and spin reorientation temperatures in our compou nds . In addition a reduction in the optical band gap of NCO from 3.1 eV to 2. 6 eV w as observed

Nanoscale Electrical Investigation of Transparent Conductive Electrodes Based on Silver Nanowire Network

peer reviewedPresently, metallic nanowires (NWs) are the most promising materials to fabricate flexible transparent electrodes as an alternative to indium tin oxide. Here, the high performance of transparent conductive electrodes (TCEs) based on silver nanowires (AgNWs) percolation networks is reported. With optimized experimental conditions for the deposition, the AgNWs result in low sheet resistance of 10 Ω sq−1 combined with a high optical transmittance of 92.6% at λ = 550 nm. This leads to a valuable figure of merit as compared to other TCEs. In this study, the nanoscale electrical properties of the AgNWs are measured via conductive atomic force microscopy to characterize the percolation network. The electrical resistivity value calculated for a single AgNW is found to be about 12.35 µΩ cm, while a nanoscale conductivity map over an AgNW network bridging two electrodes has revealed high levels of current within the network over a distance of more than 1000 µm. The favorable determined conductivity results along with the high optical properties of the AgNWs network strongly suggest that thin-film electrodes based on AgNWs will be a potential approach for future flexible electronic devices

The development and validation of a scoring tool to predict the operative duration of elective laparoscopic cholecystectomy

Background: The ability to accurately predict operative duration has the potential to optimise theatre efficiency and utilisation, thus reducing costs and increasing staff and patient satisfaction. With laparoscopic cholecystectomy being one of the most commonly performed procedures worldwide, a tool to predict operative duration could be extremely beneficial to healthcare organisations. Methods: Data collected from the CholeS study on patients undergoing cholecystectomy in UK and Irish hospitals between 04/2014 and 05/2014 were used to study operative duration. A multivariable binary logistic regression model was produced in order to identify significant independent predictors of long (> 90 min) operations. The resulting model was converted to a risk score, which was subsequently validated on second cohort of patients using ROC curves. Results: After exclusions, data were available for 7227 patients in the derivation (CholeS) cohort. The median operative duration was 60 min (interquartile range 45–85), with 17.7% of operations lasting longer than 90 min. Ten factors were found to be significant independent predictors of operative durations > 90 min, including ASA, age, previous surgical admissions, BMI, gallbladder wall thickness and CBD diameter. A risk score was then produced from these factors, and applied to a cohort of 2405 patients from a tertiary centre for external validation. This returned an area under the ROC curve of 0.708 (SE = 0.013, p  90 min increasing more than eightfold from 5.1 to 41.8% in the extremes of the score. Conclusion: The scoring tool produced in this study was found to be significantly predictive of long operative durations on validation in an external cohort. As such, the tool may have the potential to enable organisations to better organise theatre lists and deliver greater efficiencies in care

PANC Study (Pancreatitis: A National Cohort Study): national cohort study examining the first 30 days from presentation of acute pancreatitis in the UK

Abstract Background Acute pancreatitis is a common, yet complex, emergency surgical presentation. Multiple guidelines exist and management can vary significantly. The aim of this first UK, multicentre, prospective cohort study was to assess the variation in management of acute pancreatitis to guide resource planning and optimize treatment. Methods All patients aged greater than or equal to 18 years presenting with acute pancreatitis, as per the Atlanta criteria, from March to April 2021 were eligible for inclusion and followed up for 30 days. Anonymized data were uploaded to a secure electronic database in line with local governance approvals. Results A total of 113 hospitals contributed data on 2580 patients, with an equal sex distribution and a mean age of 57 years. The aetiology was gallstones in 50.6 per cent, with idiopathic the next most common (22.4 per cent). In addition to the 7.6 per cent with a diagnosis of chronic pancreatitis, 20.1 per cent of patients had a previous episode of acute pancreatitis. One in 20 patients were classed as having severe pancreatitis, as per the Atlanta criteria. The overall mortality rate was 2.3 per cent at 30 days, but rose to one in three in the severe group. Predictors of death included male sex, increased age, and frailty; previous acute pancreatitis and gallstones as aetiologies were protective. Smoking status and body mass index did not affect death. Conclusion Most patients presenting with acute pancreatitis have a mild, self-limiting disease. Rates of patients with idiopathic pancreatitis are high. Recurrent attacks of pancreatitis are common, but are likely to have reduced risk of death on subsequent admissions. </jats:sec

Influence of Strain and Anisotropy on Structure of BiFeO3 Epilayers and their Utilization as Interface Driven Heterostructures for Multiferroic Device Applications

Bismuth ferrite (BFO) a room temperature multiferroic material has been studied extensively in recent years due to the interesting features it exhibits with a ferroelectric transition temperature (TC~1103 K) and a canted antiferromagnetic spin cycloid (period ~ 64nm) type ordering with a Neel temperature (TN~ 643 K). At room temperature BFO stabilizes in rhombohedral structure with R3c symmetry in bulk ceramics. It was demonstrated that the substrate strain and orientations can be used for controlling the structural domain variants, polarization directions, for which there have been sufficient experimental as well as theoretical evidences based on manipulation of the ferroelectric domain structures by using vicinal substrate or strain tuning. A minor change in c/a ratio was observed and the BFO layer was expected to possess a tetragonal symmetry with out-of-plane polarization components. The epitaxial BFO grown on other substrates including YAlO3, LaAlO3, LaSrAlTiO4, etc. with a high c/a ratio has shown significant change in the ferroelectric behavior of the material. More over the high epitaxial strain will cause the ferroelectric Curie temperature to come down further toward the Neel temperature of BFO, which is expected to improve the magneto-electric coupling properties. The strain driven structural transformations with varying thickness and temperature driven transformation of the BFO layer on these substrates were studied extensively. These transformations happen through different types of monoclinic distortions associated with the transition. These structural transformations were observed, using the different substrate strain, chemical pressure, and thickness of the epitaxial BFO film. And these structural phases coexist together in the film it can mimic the morphotropic phase boundary like behavior with high enhancement in the piezoelectric coefficients by the mechanisms explains by polarization extension and polarization rotation. First part of this work, we address the tuning the structure of BFO using a template assisted strain varying method, which is achieved previously by different means of changing the substrates, thickness of BFO and compositional change. We also able to mimic the ix morphotropic phase boundary like region in BFO by altering the epitaxial strain condition on the same substrate, with same thickness of the BFO film. In the second part the effects of structural distortions, interface roughness; anisotropy etc. on exchange bias is studied. Chapter 1 begins with the brief introduction to the perovskites materials, piezoelectric materials, materials with high d33 values in their morphotropic phase boundaries etc. Then classifications of multiferroic materials, followed by the literature reviews on the phase instability of epitaxial Bismuth ferrite and its properties are explained. The exchange bias phenomenon and the literature review on the bismuth ferrite based heterostructures were detailed. Chapter 2 deals with the experimental techniques and the working principles of each instrument are explained. The solid state synthesis route and coprecipitation route for target preparation are detailed. The working principles and the methodology of pulsed laser deposition, different scanning methods of high resolution X-ray diffraction and piezoresponse force microscopy techniques are then detailed. Chapter 3 deals with the optimization of the epitaxial bismuth ferrite on different substrates and different orientations are detailed. The growth parameters of the bismuth ferrite are first optimized by depositing the films in different conditions of temperature and partial pressure of oxygen. The optimized parameters were used for depositing the BFO films in different strained conditions and orientations. These films are characterized using HRXRD techniques to confirm the phase, orientations and strain in the films. Further the ferroelectric and piezoelectric characterization is performed using the PFM, to map the ferroelectric domain pattern, local piezoelectric coefficients etc. The films were showing good ferroelectric and piezoelectric characteristics. Chapter 3 deals with the tuning of the different phase of bismuth ferrite using a template layer to stabilize in rhombohedral, tetragonal and mixed phase of both these phase. A La0.77Sr0.33MnO3 (LSMO) buffer layer is used to vary the strain conditions of 20nm thick epitaxial BFO layer grown on LAO substrate. Lowering x the thickness (~2nm) of LSMO, a completely strained layer is able to transfer the strain to BFO layer and stabilize the tetragonal phase with an out-of-plane strain of ~ 16% altering the c/a ratio of BFO to ~1.2. Increasing the thickness of LSMO layer to 10nm results in a mixed phase rhombohedral (R) and tetragonal (T) domains with monoclinic distortion. Further increment of LSMO layer thickness to 20 nm stabilizes rhombohedral phase of BFO. The structural phase confirmation and the monoclinic distortion were confirmed using X-ray diffraction and reciprocal space mapping studies. In addition, the domain studies carried out with Piezoresponce force microscopy (PFM) reveals that the tetragonal phase with weak monoclinic distortion possessed 180 domains with dominant out-of-plane components of polarization. However, the mixed phase (R+T with monoclinic distortion) clearly revealed the various plausible polarization components in both out-of-plane and in-plane components. In the pure rhombohedral phase, the domain pattern transformed into fractal type domains which is commonly observed in BFO epilayers. Further, we have implemented a thermodynamically consistent model based on phase field approach to investigate the role of epitaxial strain on BFO layer to study the formation of domain patterns with various plausible polarization components. The piezoelectric d33 coefficient is found to be ~46 pm/V for the 20nm mixed phase BFO, which is relatively larger when compared to the single phase epitaxial films of given thickness (~20nm). Chapter 4 deals with the dependence of the structural distortion, interface roughness, and anisotropy in exchange bias phenomena between antiferromagnetic BFO and soft ferromagnetic layers. An exchange bias of 510 Oe was observed in the tetragonal phase of BFO with a ‘c/a’ ratio of 1.22, which is comparable with the exchange bias shown by the (111) oriented rhombohedral phase (360 Oe). Both the tetragonal (001) and rhombohedral (111) layers possess ferroelectric polarization normal to the sample surface and so the domain walls are mostly 180o oriented which is expected to have a minimum effect on the exchange bias. However, the weak strain induced structural variants in the (111) oriented rhombohedral BFO and the monoclinic distortion present in tetragonal BFO is expected to play a key xi role in defining the ferroelectric domain wall nature and thereby exhibiting exchange bias characteristics. The sample with low interface roughness exhibited a high exchange bias of ~222 Oe, than that of the sample with high interface roughness. . The Ferroelectric domain size of (001) oriented BFO is twice as large as in (011) oriented sample, and comparatively large domain size is observed in (111) oriented sample, confirming the presence of much higher domain wall density in the sample oriented in (011) direction. This confirms the higher exchange bias in (011) oriented sample. The coercivity enhancement has been observed in all the samples. Further, the optimizing the roughness in epitaxialy oriented film, controlling the ferroelectric domains with 71o ferroelectric domain wall, with a control of modulated spin cycloid vector will be the key for device based application as proposed for the reversible control of exchange bias using external electric field. Chapter 6 details the summary and conclusion of the thesis work and the future prospects of the work are also discussed

Template assisted strain tuning and phase stabilization in epitaxial BiFeO3 thin films

Strain engineering is a key to develop novel properties in functional materials. We report a strain mediated phase stabilization and epitaxial growth of bismuth ferrite(BiFeO3) thin films on LaAlO3 (LAO) substrates. The strain in the epitaxial layer is controlled by controlling the thickness of bottom electrode where the thickness of the BFO is kept constant. The thickness of La0.7Sr0.3MnO3(LSMO) template layer was optimized to grow completely strained tetragonal, tetragonal/rhombohedral mixed phase and fully relaxed rhombohedral phase of BFO layers. The results were confirmed with coupled-θ-2θ scan, and small area reciprocal space mapping. The piezoelectric d33 (˜ 45-48 pm/V) coefficient of the mixed phase was relatively larger than the strained tetragonal and relaxed rhombohedral phase for a given thickness

Tip Induced Surface Defect migration and Conductivity Studies in Tetragonal, Rhombohedral and Mixed-Phase epitaxial BiFeO3 Thin Films

BiFeO3 (BFO) is the most studied room temperature multiferroic compound. In this work we demonstrate a template assisted growth process through which the growth strain is controlled to achieve required phase of BFO. Growth of (∼20nm) fully strained tetragonal (T), rhombohedral (R) and mixed phase of T and R of Bismuth ferrite (BiFeO3) was achieved by varying the thickness of the template layer. The different phases were confirmed by using high resolution x-ray diffractions studies. The conductivity map of all the three phases were carried out using an atomic force microscope operating in conductive mode. Tip induced surface defect migration within a given grain was observed in pure phases and the conductivity map confirmed the same. The room temperature resistivity is found to be decreasing systematically from 1.1×106 Ωm , 935×105 Ωm and 1.16×104 Ωm respectively for tetragonal, mixed phase and rhombohedral phase BFO. In the case of mixed phase both the nano- scale and macroscopic leakage current studies show low conductivity, which could be due to the increased pinning sites that increases the energy barrier for the defect migration. The local nano-scale measurements and conductivity mapping corroborates well with the macroscopic studies

Realization of rhombohedral, mixed, and tetragonal like phases of BiFeO3 and ferroelectric domain engineering using a strain tuning layer on LaAlO3(001) substrate

BiFeO3 (BFO), a room temperature multiferroic, undergoes a series of structural transformations under varying strain conditions by utilizing appropriate substrates for a specific strain condition. In this study, epitaxial thin films of BFO were grown on La0.7Sr0.3MnO3 +/-delta (LSMO), a strain tuning layer on LaAlO3[LAO (001)] substrates, using pulsed laser ablation. LSMO layers of varying thicknesses from 2 nm to 20 nm were grown followed by a BFO layer of a fixed thickness (20 nm). A strained layer of similar to 2 nm thick LSMO stabilizes the tetragonal like phase of BFO. Increasing the thickness of the LSMO layer to 10 nm results in a mixed phase with rhombohedral (R) and tetragonal (T) domains, and a further increment of the LSMO layer thickness to 20 nm stabilizes the rhombohedral phase of BFO. The tetragonal phase with weak monoclinic distortion possessed 180 degrees domains with dominant out-of-plane polarization components. However, the mixed phase (R + T) possessed various plausible polarization components in both out-of-plane and in-plane directions. Further, a thermodynamically consistent model based on the phase field approach was implemented to investigate the role of strain on the formation of domain patterns with various polarization components and piezoelectric coefficients. The simulated domain structure exhibited a similar transformation on the dominant polarization components as observed in experiments across different phases of BFO. Our simulations show that the elastic constraint along the z-direction enhances the tetragonality of BFO. The piezoelectric (d(33)) coefficient was found to be similar to 46 pm/V for the 20 nm mixed phase BFO, which was nearly a 200% increment compared to the single phase BFO thin films on LAO. Published under license by AIP Publishing