Tiffany C. Kaspar

Pacific Northwest National Laboratory

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Scott A. Chambers

Pacific Northwest National Laboratory

Timothy C. Droubay

Pacific Northwest National Laboratory

Mark Bowden

Pacific Northwest National Laboratory

Steven R. Spurgeon

National Renewable Energy Laboratory

V. Shutthanandan

The University of Texas at El Paso

Peter V. Sushko

Pacific Northwest National Laboratory

Steve M. Heald

Argonne National Laboratory

Daniel K. Schreiber

Pacific Northwest National Laboratory

R. Comès

Auburn University

Daniel R. Gamelin

University of Washington

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Pacific Northwest National Laboratory

169

Battelle

Environmental Molecular Sciences Laboratory

Richland College

Government of the United States of America

University of Washington

John Wiley & Sons (United States)

Argonne National Laboratory

Physical Sciences (United States)

Lawrence Berkeley National Laboratory

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Role of structural defects in mediating disordering processes at irradiated epitaxial Fe3O4/Cr2O3 interfaces

Physical Review Materials (2023)

Tiffany C. Kaspar Steven R. Spurgeon Kayla Yano Bethany E. Matthews Mark Bowden

Disordering processes in oxide materials are complicated by the presence of interfaces, which can serve as either point defect sinks or accumulation sites; the response depends on factors such as interfacial structure, chemistry, and termination. We have characterized the disordering of epitaxial ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}(111)/{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}(0001)$ thin film heterostructures after 400 keV ${\mathrm{Ar}}^{2+}$ radiation at room temperature. The density of misfit dislocations in both the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ overlayer and ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ buffer layer is varied by changing the thickness of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ to be pseudomorphically strained to the ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ (0001) substrate (5 nm thick) or partially relaxed (20 nm thick), as confirmed by Bragg filtering analysis of scanning transmission electron microscopy images. In both cases, irradiation leads to damage accumulation on the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ side of the heterointerface, as shown by Rutherford backscattering spectrometry measurements in the channeling geometry. However, the interface with more misfit dislocations exhibits disordering at a faster rate than the less-defective interface. Likewise, layer-resolved electron energy loss spectroscopy reveals interfacial reduction of Fe after irradiation at the more defective interface. Intermixing of Cr across the interface is observed by atom probe tomography, which is likely facilitated by the generation of Cr interstitials in ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ under irradiation.

Overlayer

10.1103/physrevmaterials.7.093604

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Citations (1)

Built‐In Potential in Fe₂O₃‐Cr₂O₃ Superlattices for Improved Photoexcited Carrier Separation

Advanced Materials (2015)

Tiffany C. Kaspar Daniel K. Schreiber Steven R. Spurgeon Martin E. McBriarty Gerard M. Carroll

Hematite (α-Fe2 O3) is engineered to improve photoexcited electron-hole pair separation by synthesizing Fe2O3-Cr2O3 superlattices (SLs) with precise atomic control. The different surface terminations exhibited by Fe2O3 and Cr2O3 determine the hetero-junction interface structure and result in controllable, noncommutative band offset values. This controllable band alignment is harnessed to generate a built-in potential as large as 0.8 eV in Fe2 O3-Cr2O3 SLs.

Band offset

10.1002/adma.201504545

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Citations (27)

Mid-gap electronic states inZn1−xMnxO

Physical Review B (2010)

Claire A. Johnson Kevin R. Kittilstved Tiffany C. Kaspar Timothy C. Droubay Scott A. Chambers

Electronic absorption, magnetic circular dichroism (MCD), photoconductivity, and valence-band x-ray photoelectron (XPS) spectroscopic measurements were performed on epitaxial ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ films to investigate the origin of the mid-gap band that appears upon introduction of ${\text{Mn}}^{2+}$ into the ZnO lattice. Absorption and MCD spectroscopies reveal ${\text{Mn}}^{2+}$-related intensity at energies below the first excitonic transition of ZnO, tailing well into the visible energy region, with an onset at $\ensuremath{\sim}2.2\text{ }\text{eV}$. Photoconductivity measurements show that excitation into this visible band generates mobile charge carriers, consistent with assignment as a ${\text{Mn}}^{2+/3+}$ photoionization transition. XPS measurements reveal the presence of occupied ${\text{Mn}}^{2+}$ levels just above the valence-band edge, supporting this assignment. MCD measurements additionally show a change in sign and large increase in magnitude of the excitonic Zeeman splitting in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ relative to ZnO, suggesting that $sp\text{\ensuremath{-}}d$ exchange in ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ is not as qualitatively different from those in other II-VI diluted magnetic semiconductors as has been suggested. The singular electronic structure feature of ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ is the presence of this ${\text{Mn}}^{2+/3+}$ ionization level within the gap, and the influence of this level on other physical properties of ${\text{Zn}}_{1\ensuremath{-}x}{\text{Mn}}_{x}\text{O}$ is discussed.

10.1103/physrevb.82.115202

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Citations (41)

Data for EMSL Project 49141 from March 2022

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information) (2022)

Edgar C. Buck Tiffany C. Kaspar Sébastien Kerisit Joseph V. Ryan Carolyn I. Pearce

10.25582/data.2022-03.2449181/2356732

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Surface structure of α-Cr2O3(0001) after activated oxygen exposure

Surface Science (2013)

Tiffany C. Kaspar Sara E. Chamberlin Scott A. Chambers

Oxidation state

10.1016/j.susc.2013.09.005

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Citations (26)

Infrared optical absorption in low-spin Fe²⁺-doped SrTiO₃

Journal of Physics Condensed Matter (2016)

R. Comès Tiffany C. Kaspar Steve M. Heald Mark Bowden Scott A. Chambers

Band gap engineering in SrTiO3 and related titanate perovskites has long been explored due to the intriguing properties of the materials for photocatalysis and photovoltaic applications. A popular approach in the materials chemistry community is to substitutionally dope aliovalent transition metal ions onto the B site in the lattice to alter the valence band. However, in such a scheme there is limited control over the dopant valence, and compensating defects often form. Here we demonstrate a novel technique to controllably synthesize Fe(2+)- and Fe(3+)-doped SrTiO3 thin films without formation of compensating defects by co-doping with La(3+) ions on the A site. We stabilize Fe(2+)-doped films by doping with two La ions for every Fe dopant, and find that the Fe ions exhibit a low-spin electronic configuration, producing optical transitions in the near infrared regime and degenerate doping. The novel electronic states observed here offer a new avenue for band gap engineering in perovskites for photocatalytic and photovoltaic applications.

10.1088/0953-8984/28/3/035901

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Citations (11)

Non-destructive characterization of corroded glass surfaces by spectroscopic ellipsometry

Journal of Non-Crystalline Solids (2017)

Tiffany C. Kaspar Joelle T. Reiser Joseph V. Ryan Nathalie A. Wall

Characterization

Ellipsometry

Surface layer

10.1016/j.jnoncrysol.2017.10.054

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Citations (23)

Correlative Imaging of Phase Separation in Fe2TiO4 Thin Films Prepared by Conventional Ga and Xe Plasma FIB Processing

Microscopy and Microanalysis (2020)

Steven R. Spurgeon Bethany E. Matthews Ron Kelley Tiffany C. Kaspar Scott A. Chambers

Journal Article Correlative Imaging of Phase Separation in Fe2TiO4 Thin Films Prepared by Conventional Ga and Xe Plasma FIB Processing Get access Steven Spurgeon, Steven Spurgeon Pacific Northwest National Laboratory, Richland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Bethany Matthews, Bethany Matthews Pacific Northwest National Laboratory, Richland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Ron Kelley, Ron Kelley Thermo Fisher Scientific, Hillsboro, Oregon, United States Search for other works by this author on: Oxford Academic Google Scholar Tiffany Kaspar, Tiffany Kaspar Pacific Northwest National Laboratory, Richland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Scott Chambers Scott Chambers Pacific Northwest National Laboratory, Richland, Washington, United States Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 26, Issue S2, 1 August 2020, Pages 186–187, https://doi.org/10.1017/S1431927620013720 Published: 01 August 2020

National laboratory

10.1017/s1431927620013720

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Surface termination control of charge transfer and band alignment across a semiconductor–crystalline-oxide heterojunction

Physical Review Materials (2023)

Matthew Chrysler Judith Gabel T.-L. Lee Zihua Zhu Tiffany C. Kaspar

Charge redistribution across heterojunctions has long been utilized to induce functional response in materials systems. Here we examine how the composition of the terminating surface affects charge transfer across a heterojunction consisting of Si and the crystalline complex oxide $\mathrm{SrTi}{\mathrm{O}}_{3}$. Itinerant electrons in Si migrate across the interface toward the surface of $\mathrm{SrTi}{\mathrm{O}}_{3}$ due to surface depletion. The electron transfer in turn creates an electric field across the interface that modifies the interfacial dipole associated with bonding between $\mathrm{SrTi}{\mathrm{O}}_{3}$ and Si. The modification in the dipole leads to a change in band alignment, in which the conduction band of $\mathrm{SrTi}{\mathrm{O}}_{3}$ moves from being above the valence band of Si in energy, to below it. By capping the $\mathrm{SrTi}{\mathrm{O}}_{3}$ surface with ultrathin ($\ensuremath{\le}1$ nm) layers of BaO, SrO, or $\mathrm{Ti}{\mathrm{O}}_{2}$, charge transfer across the interface can be weakened or inhibited. Ab initio modeling implicates the adsorption of oxygen associated with exposure to ambient conditions as driving the surface depletion in $\mathrm{SrTi}{\mathrm{O}}_{3}$. The electronic coupling between the surface and buried interface expands the functionality of semiconductor--crystalline-oxide heterojunctions.

10.1103/physrevmaterials.7.084604

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Citations (3)

Electronic, magnetic, and optical properties of Semiconducting Spinel Fe 2 CrO 4

APS (2017)

Timothy C. Droubay Tiffany C. Kaspar Iffat Nayyar D. J. Keavney Peter V. Sushko

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