The effects of doping and chemical ordering on the optical, electronic and photochemical properties of alpha-(Fe,Cr,V)2O3 epitaxial films and interfaces
EMSL Project ID
48144
Abstract
We propose to modify hematite - an abundant, naturally occurring mineral with some potential as a highly efficient photovoltaic and photocatalytic material - by doping with other transition metals and forming high-symmetry superlattice structures. Our dopants will be V and Cr. Our goal is to determine the effect of long-range order within the cation sublattice of alpha-Fe2O3–Cr2O3–V2O3 epitaxial films on the associated functional properties. Mixed-cation alloy films across the compositional spectrum, and digital superlattices (DS) with varying periodicities, will be deposited epitaxially on alpha-Al2O3(0001) substrates in excess atomic oxygen by molecular beam epitaxy (MBE). We will utilize state-of-the-art MBE growth, along with materials characterization by means of XRD, APT, RBS/PIXE, STEM/EELS and AFM to prepare structurally and compositionally well-defined specimens. Relevant optical and electronic properties measurements will be carried out with an eye toward elucidating the effect multiple internal interfaces vis a vis a single material with a random distribution of cations. To facilitate interpretation of the experimental results as well as to guide the experiments, we will also conduct theoretical ab initio studies of these systems focusing on their thermodynamic stability, geometrical structure, electronic properties and optical absorption spectra. We will use state-of-the-art methods based on density functional theory (DFT) and a periodic model to obtain the ground state properties of each system, and then use an embedded cluster model and time-dependent DFT to calculate excitation energies and the character of the optical transitions. We will explore the effect of V doping in ?-Fe2O3, as well as the effect of chemical ordering in alpha-(Fe, Cr, V)2O3 on bandgap, resistivity, photoconductivity, e- - h+ lifetimes and surface photochemical activity. We hypothesize that chemical ordering will reduce carrier recombination times, leading to higher efficiencies in photovoltaic energy conversion and surface photochemical processes relative to random solid solutions of the same composition. If successfully demonstrated for single and multiple internal interfaces in DS structures, this effect will enable novel band-gap engineering to maximize photo-induced activity. This research could pave the way for a new class of materials that can effectively and efficiently harvest visible light to drive photochemical reactions important for green chemistry, and capture visible sunlight for photovoltaic conversion. The identified EMSL resources are needed to: (i) facilitate the synthesis and adequate characterization of materials that are inherently complex and difficult to make by virtue of their detailed atomistic structures, (ii) measure the optical, electronic and photochemical properties of these materials, and, (iii) accurately calculate from first principles the structural and electronic properties of these materials in order to understand them in detail, and guide future experimental efforts.
Project Details
Start Date
2013-10-14
End Date
2014-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
Related Publications
Chamberlin SE, TC Kaspar, ME Bowden, V Shutthanandan, B Kabius, S Heald, DJ Keavney, and SA Chambers. 2014. "Structural perturbations of epitaxial ?-(Fe1-xVx)2O3 thin films driven by excess oxygen near the surface." Journal of Applied Physics 116:10. doi:10.1063/1.4903839
Henderson MA. 2014. "Roles of Fe2+, Fe3+, and Cr3+ Surface Sites in the Oxidation of NO on the (Fe,Cr)3O4(1 1 1) Surface Termination of an ?-(Fe,Cr)2O3(0 0 0 1) Mixed Oxide." Journal of Catalysis 318:53-60. doi:10.1016/j.jcat.2014.07.015
Kaspar TC, SE Chamberlin, ME Bowden, RJ Colby, V Shutthanandan, S Manandhar, Y Wang, PV Sushko, and SA Chambers. 2014. "Impact of Lattice Mismatch and Stoichiometry on the Structure and Bandgap of (Fe,Cr)2O3 Epitaxial Thin Films." Journal of Physics: Condensed Matter 26(13):5005. doi:10.1088/0953-8984/26/13/135005