6 research outputs found
Tailoring Electronic Properties in Semiconducting Perovskite Materials Through Octahedral Control
Perovskite oxides, which take the chemical formula ABO3, are a very versatile and interesting materials family, exhibiting properties that include ferroelectricity, ferromagnetism, mixed ionic/electronic conductivity, metal-insulator behavior and multiferroicity. Key to these functionalities is the network of BO6 corner-connected octahedra, which are known to distort and rotate, directly altering electronic and ferroic properties. By controlling the BO6 octahedral distortions and rotations through cationic substitutions, the use of strain engineering, or through the formation of superlattice structures, the functional properties of perovskites can be tuned. Motivating the use of structure-driven design in oxide heterostructures is the prediction of hybrid improper ferroelectricity in A'BO3/ABO3 superlattices. Two key design rules to realizing hybrid improper ferroelectricity are the growth of high quality superlattice structures with odd periodicities of the A / A' layers, and the control of the octahedral rotation pattern. My work explores the rotational response in perovskite oxides to strain and interface effects in thin films of RFeO3 (R = La, Eu). I demonstrate a synchrotron x-ray diffraction technique to identify the rotation pattern that is present in the films. I then establish substrate imprinting as a key tool for controlling the rotation patterns in heterostructures, providing a means to realize the necessary structural variants of the predicted hybrid improper ferroelectricity in superlattices. In addition, by pairing measured diffraction data with a structure factor calculation, I demonstrate how one can extract both A-site and oxygen atomic positions in single crystal perovskite oxide films. Finally, I show results from (LaFeO3)n/(EuFeO3)n superlattices (n = 1-5), synthesized to test the motivating predictions of hybrid improper ferroelectricity in oxide superlattices.Ph.D., Materials Science and Engineering -- Drexel University, 201
Distinguishing Thermal and Electronic Effects in Ultrafast Optical Spectroscopy Using Oxide Heterostructures
Measuring
time-resolved photoexcited properties in semiconductors
is critical to the design and improvement of light-harvesting devices.
Although ultrafast pump–probe spectroscopy offers a promising
route to understand carrier recombination mechanisms and quantify
lifetimes, thermal contributions to the transient optical response
can be significant and need to be properly accounted for to isolate
carrier-induced contributions. We demonstrate the use of broadband
ultrafast optical spectroscopy on type I heterostructures as a means
to isolate transient effects that are solely thermal in nature. Specifically,
we use transient absorption and reflectance spectroscopy to measure
the time-resolved optoelectronic changes in photoexcited epitaxial
bilayers of LaFeO<sub>3</sub>/LaMnO<sub>3</sub> and monolithic thin
films of these materials. Experiments and complementary numerical
modeling reveal that thermal effects dominate the transient absorption
and reflectance spectra above the band gap. Fitting the dynamics with
a thermal diffusion model yields thermal conductivities of 6.4 W m<sup>–1</sup> K<sup>–1</sup> for LaFeO<sub>3</sub> and 2.2
W m<sup>–1</sup> K<sup>–1</sup> for LaMnO<sub>3</sub>. In LaFeO<sub>3</sub>, an additional photoinduced absorption feature
below the band gap at ∼1.9 eV is assigned primarily to photoexcited
carriers and persists for over 3 ns. This work provides a direct demonstration
of how thermal and electronic contributions can be separated in transient
optical spectroscopies, enabling new insights into dynamical optical
properties of semiconductors
Static and Dynamic Optical Properties of La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>FeO<sub>3−δ</sub>: The Effects of A‑Site and Oxygen Stoichiometry
Perovskite
oxides are a promising material class for photovoltaic
and photocatalytic applications due to their visible band gaps, nanosecond
recombination lifetimes, and great chemical diversity. However, there
is limited understanding of the link between composition and static
and dynamic optical properties, despite the critical role these properties
play in the design of light-harvesting devices. To clarify these relationships,
we systemically studied the optoelectronic properties in La<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>FeO<sub>3−δ</sub> epitaxial films, uncovering the effects of A-site cation substitution
and oxygen stoichiometry. Variable-angle spectroscopic ellipsometry
was used to measure static optical properties, revealing a linear
increase in absorption coefficient at 1.25 eV and a red-shifting of
the optical absorption edge with increasing Sr fraction. The absorption
spectra can be similarly tuned through the introduction of oxygen
vacancies, indicating the critical role that nominal Fe valence plays
in optical absorption. Dynamic optoelectronic properties were studied
with ultrafast transient reflectance spectroscopy, revealing similar
nanosecond photoexcited carrier lifetimes for oxygen deficient and
stoichiometric films with the same nominal Fe valence. These results
demonstrate that while the static optical absorption is strongly dependent
on nominal Fe valence tuned through cation or anion stoichiometry,
oxygen vacancies do not appear to play a significantly detrimental
role in the recombination kinetics
Energy level alignment and cation charge states at the <tex>LaFeO_{3}/LaMnO_{3}$</tex> (001) heterointerface
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Risk of COVID-19 after natural infection or vaccinationResearch in context
Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health
Risk of COVID-19 after natural infection or vaccinationResearch in context
Summary: Background: While vaccines have established utility against COVID-19, phase 3 efficacy studies have generally not comprehensively evaluated protection provided by previous infection or hybrid immunity (previous infection plus vaccination). Individual patient data from US government-supported harmonized vaccine trials provide an unprecedented sample population to address this issue. We characterized the protective efficacy of previous SARS-CoV-2 infection and hybrid immunity against COVID-19 early in the pandemic over three-to six-month follow-up and compared with vaccine-associated protection. Methods: In this post-hoc cross-protocol analysis of the Moderna, AstraZeneca, Janssen, and Novavax COVID-19 vaccine clinical trials, we allocated participants into four groups based on previous-infection status at enrolment and treatment: no previous infection/placebo; previous infection/placebo; no previous infection/vaccine; and previous infection/vaccine. The main outcome was RT-PCR-confirmed COVID-19 >7–15 days (per original protocols) after final study injection. We calculated crude and adjusted efficacy measures. Findings: Previous infection/placebo participants had a 92% decreased risk of future COVID-19 compared to no previous infection/placebo participants (overall hazard ratio [HR] ratio: 0.08; 95% CI: 0.05–0.13). Among single-dose Janssen participants, hybrid immunity conferred greater protection than vaccine alone (HR: 0.03; 95% CI: 0.01–0.10). Too few infections were observed to draw statistical inferences comparing hybrid immunity to vaccine alone for other trials. Vaccination, previous infection, and hybrid immunity all provided near-complete protection against severe disease. Interpretation: Previous infection, any hybrid immunity, and two-dose vaccination all provided substantial protection against symptomatic and severe COVID-19 through the early Delta period. Thus, as a surrogate for natural infection, vaccination remains the safest approach to protection. Funding: National Institutes of Health