Semester

Fall

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Matthew B. Johnson

Committee Member

Alan D. Bristow

Committee Member

Mikel B. Holcomb

Committee Member

Edward M. Sabolsky

Abstract

CuAlO2 is among several ternary delafossites, which is a rare p-type semiconductor with potential applications as a transparent conductive oxide, photocatalyst, and spintronics when doped with transition metal ions. Reported in this thesis are results from our investigations of CuAl1-xFexO2 (x = 0 to1) with a focus on the x-dependence of structural, magnetic, vibrational, optical properties, and the role of defects and impurities. Samples are prepared by solid-state reactions.

We performed a complete study of magnetic properties to investigate the possibility of room temperature ferromagnetic alloys, which are used in transparent ferromagnet applications, suggested by a computational study. Analysis of magnetization (M) vs. temperature (T, from 2 to 300 K) data by Curie-Weiss law confirms Fe3+ as the electronic state of Fe; this analysis also yields a negative θ characteristic of an antiferromagnetic Fe3+-Fe3+ exchange coupling and magnitudes of x in good agreement with the nominal values. The isothermal M vs. H (up to H= 90 kOe) data analyzed by the modified Brillouin function support the results obtained from the M vs. T analysis. High-resolution M-H loop measurements at 300 K and 10 K show negligible coercivity (HC)at 10 K but HC ~ 100 Oe at 300K. The results suggest that the room temperature ferromagnetism can originate from hematite impurity, but not for CuAl1-xFexO2 alloys.

The understanding of the phonon dynamics of alloys is crucial because they have a fundamental indirect bandgap. I introduce a new approach, which is applicable to anisotropic, dilute alloys with allowance for a large variety of alloying elements. This approach has significant advantages over previously reported methods, especially for the lattice dynamics of such complex alloys. We use this approach to model the effects of Fe-doping on the vibrational modes in dilute alloys of CuAl1-xFexO2 (x = 0-0.10) delafossite powders. Raman and FTIR spectroscopies are performed to measure optical phonon frequencies. For the phonon calculations, an approach using a disordered supercell is not feasible because it is computationally expensive. Instead, we developed our weighted dynamical matrix (WDM) approach that uses a straightforward ordered supercell for force-constant calculations of the CuAlO2 and CuFeO2 parent endpoints and combines them using a WDM approach. Computationally, when Fe is substituted for Al (increasing x), an increase in the bond length is observed, leading to a redshift in the peak positions in all the phonon modes vs. x, in agreement with the experimentally observed trend.

CuAlO2, with an indirect bandgap ~3 eV, is a good candidate for photocatalysis applications because of its chemical stability and absorption in the solar region. However, efforts to improve its optical absorption continue. Here, I report the effect of alloying on the optical absorption of CuAl1-xFexO2(x = 0.0-1.0) by measuring the optical absorption of alloys from 1 to 6 eV and comparing these results to electronic band structures calculated using density functional theory (DFT) calculations. The calculations use DFT+U supercell methods, including spin. Experimentally, we observe a new absorption feature associated with Fe at about 1.8 eV for x = 0.01 shifting to 1.4 eV for x = 0.10. The energy of this feature and its redshift with x agrees with calculated spin-down Fe-3d states. This added feature will lead to more optical absorption.

The major conclusions from this research are that by alloying CuAlO2 with Fe, the optical absorption will improve. Also, we proposed a straightforward, computationally efficient WDM approach, which confirms the redshift associated with the optical phonon modes. With our magnetic measurements, we confirmed that the dilute alloys are not ferromagnetic at room temperature.

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