Controlling the Electronic Properties in La1/3Sr2/3FeO33-[delta] Complex Perovskite Oxides

For nearly 5 decades, the global semiconductor industry has followed Moore's law, which employs the iterative concept of transistor scaling in silicon-based technology. Though this approach has been massively successful at maintaining consistent increases in computational speed and power, silicon technology is quickly approaching its physical limitations with respect to continued scaling. In recent years, a growing effort has been adopted to pursue new materials and technologies as alternative platforms for information processing. Complex oxides are a potential candidate material system for next generation electronic devices due to their rich material properties such as metal-insulator transitions, high Tc superconductivity, and colossal magnetoresistance. In particular, there is growing interest to understand and control the unique electronic properties of complex oxides for applications in transistor-like devices. This dissertation is focused on understanding the growth, characterization and application of La1/3Sr2/3FeO3 (LSFO) thin films, which are known to undergo an abrupt charge ordering phase transition at 190 K in bulk materials. This phase transition is accompanied by an order of magnitude increase in resistivity going from a conductive to insulating state as well as the spontaneous ordering of charge and antiferromagnetic spin structure along the [111] direction. Isocompositional cation-ordered superlattices of LSFO were synthesized via oxygen-assisted molecular beam epitaxy and explored through synchrotron X-ray diffraction, electronic transport, and density functional theory modeling. By adjusting the cation ordering of LaFeO3 (LFO), an antiferromagnetic insulator, and SrFeO3 (SFO), a conductor with a helical magnetic ground state, three isocompositional systems of LSFO were investigated. The superlattices were found to exhibit a charge ordering phase transition similar to LSFO for two of the three structures, as measured by an abrupt discontinuity in the temperature-dependent resistivity. Carrier behavior within the superlattices was also explored by fitting the temperature dependent resistivity to common conduction models. The conduction mechanism fits show that the transport at high temperatures is dominated by weakly insulating behavior due to small polaron conduction and at low temperatures the resistivity can be fit to both a novel power law and 3-dimensional variable range hopping. Additionally, reversible changes of the structural and electronic transport properties of La1/3Sr2/3FeO3−δ/Gd-doped CeO2 (GDC) heterostructures arising from the manipulation of δ are presented. Thermally induced oxygen loss leads to a c-axis lattice expansion and an increase in resistivity in an LSFO film capped with GDC. In a three-terminal device where a gate bias is applied across the GDC layer to alter the LSFO oxygen stoichiometry, the ferrite channel is shown to undergo an order of magnitude change in resistance using gate voltages of less than 1 V applied at 500 K. The changes in resistance remain upon cooling to room temperature, in the absence of a gate bias, suggesting solid state ionic gating of perovskite oxides as a promising platform for applications in non-volatile, multistate devices. Along with the experiments of controlling δ in a device format, the kinetics of oxygen loss as a function of biaxial strain was investigated.Ph.D., Materials Science and Engineering -- Drexel University, 201

The ASOS Surgical Risk Calculator: development and validation of a tool for identifying African surgical patients at risk of severe postoperative complications

Background: The African Surgical Outcomes Study (ASOS) showed that surgical patients in Africa have a mortality twice the global average. Existing risk assessment tools are not valid for use in this population because the pattern of risk for poor outcomes differs from high-income countries. The objective of this study was to derive and validate a simple, preoperative risk stratification tool to identify African surgical patients at risk for in-hospital postoperative mortality and severe complications. Methods: ASOS was a 7-day prospective cohort study of adult patients undergoing surgery in Africa. The ASOS Surgical Risk Calculator was constructed with a multivariable logistic regression model for the outcome of in-hospital mortality and severe postoperative complications. The following preoperative risk factors were entered into the model; age, sex, smoking status, ASA physical status, preoperative chronic comorbid conditions, indication for surgery, urgency, severity, and type of surgery. Results: The model was derived from 8799 patients from 168 African hospitals. The composite outcome of severe postoperative complications and death occurred in 423/8799 (4.8%) patients. The ASOS Surgical Risk Calculator includes the following risk factors: age, ASA physical status, indication for surgery, urgency, severity, and type of surgery. The model showed good discrimination with an area under the receiver operating characteristic curve of 0.805 and good calibration with c-statistic corrected for optimism of 0.784. Conclusions: This simple preoperative risk calculator could be used to identify high-risk surgical patients in African hospitals and facilitate increased postoperative surveillance. © 2018 British Journal of Anaesthesia. Published by Elsevier Ltd. All rights reserved.Medical Research Council of South Africa gran

Maternal and neonatal outcomes after caesarean delivery in the African Surgical Outcomes Study: a 7-day prospective observational cohort study.

BACKGROUND: Maternal and neonatal mortality is high in Africa, but few large, prospective studies have been done to investigate the risk factors associated with these poor maternal and neonatal outcomes. METHODS: A 7-day, international, prospective, observational cohort study was done in patients having caesarean delivery in 183 hospitals across 22 countries in Africa. The inclusion criteria were all consecutive patients (aged ≥18 years) admitted to participating centres having elective and non-elective caesarean delivery during the 7-day study cohort period. To ensure a representative sample, each hospital had to provide data for 90% of the eligible patients during the recruitment week. The primary outcome was in-hospital maternal mortality and complications, which were assessed by local investigators. The study was registered on the South African National Health Research Database, number KZ_2015RP7_22, and on ClinicalTrials.gov, number NCT03044899. FINDINGS: Between February, 2016, and May, 2016, 3792 patients were recruited from hospitals across Africa. 3685 were included in the postoperative complications analysis (107 missing data) and 3684 were included in the maternal mortality analysis (108 missing data). These hospitals had a combined number of specialist surgeons, obstetricians, and anaesthetists totalling 0·7 per 100 000 population (IQR 0·2-2·0). Maternal mortality was 20 (0·5%) of 3684 patients (95% CI 0·3-0·8). Complications occurred in 633 (17·4%) of 3636 mothers (16·2-18·6), which were predominantly severe intraoperative and postoperative bleeding (136 [3·8%] of 3612 mothers). Maternal mortality was independently associated with a preoperative presentation of placenta praevia, placental abruption, ruptured uterus, antepartum haemorrhage (odds ratio 4·47 [95% CI 1·46-13·65]), and perioperative severe obstetric haemorrhage (5·87 [1·99-17·34]) or anaesthesia complications (11·47 (1·20-109·20]). Neonatal mortality was 153 (4·4%) of 3506 infants (95% CI 3·7-5·0). INTERPRETATION: Maternal mortality after caesarean delivery in Africa is 50 times higher than that of high-income countries and is driven by peripartum haemorrhage and anaesthesia complications. Neonatal mortality is double the global average. Early identification and appropriate management of mothers at risk of peripartum haemorrhage might improve maternal and neonatal outcomes in Africa. FUNDING: Medical Research Council of South Africa.Medical Research Council of South Africa