Magneto-Optics of Plasmas, Bipolarons in Organic Semiconductors, and Electro-Optics of 2D Metals
Open Access
- Author:
- Schrecengost, Jonathon
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 06, 2023
- Committee Members:
- John Mauro, Program Head/Chair
Jon-Paul Maria, Major Field Member
Noel Giebink, Chair & Dissertation Advisor
Joshua Robinson, Major Field Member
Sahin Ozdemir, Outside Unit & Field Member - Keywords:
- Faraday Rotation
Magneto-Optic
Transparent Conducting Oxides
Plasmonics
Organic Electronics
Bipolarons
Magnetoresistance
OLED
2D Polar Metals
Confinement Heteroepitaxy
Electric-Double-Layer Gating - Abstract:
- Chapter 1 investigates Faraday rotation in transparent conducting oxide (TCO) thin films. In this work I built and programmed a magneto-optic (MO) polarimeter to measure world record breaking Faraday rotations in cadmium oxide thin films doped with In or Gd ions. I derive from first principles, the theory to understand and explain these results, as well as extrapolate beyond our measurement capabilities. Both theory and experiment demonstrate that near the epsilon-near-zero (ENZ) frequency, applied magnetic fields induce large differences between the refractive indices of right and left-hand circularly polarized light, leading to large Faraday rotations. Doped CdO is particularly of interest because its ENZ frequency is tunable across the near-infrared (NIR) to mid-infrared (MIR) by varying the dopant concentration, and its high optical mobility leads to substantially higher MO figures of merit than other plasmonic materials. With extremely large Verdet constants (~3∙10^5-10^6 deg T-1 m-1) tunable across the NIR to MIR, doped CdO thin films show great promise in MO applications such as Faraday rotators, optical isolators, non-reciprocal linear↔circular polarizers, tunable bandpass filters, and integrated Si photonic chip applications. Chapter 2 focuses on bipolarons in organic electronics and their suspected role in organic magnetoresistance (OMAR). The first subchapter focuses on work we published of an improved theoretical model of organic semiconductor/electrode interfaces which supports the causality of significant (>1%) interfacial bipolaron density resulting from image charge stabilization. This work was intended to be combined with work in the second subchapter, which investigates the role of bipolarons in organic magnetoresistance (OMAR). However, I show that previously reported experimental evidence of bipolaron mediated OMAR were thermal artifacts unrelated to applied magnetic fields. These artifacts were a result of their experimental setup, which brought a permanent magnet near the organic device; improving the dissipation of Joule heating, lowering the device operating temperature, and reducing electrical conductivity. The results were replicated with the same effect using a non-magnetic metal, and demonstrated on a more stable commercial organic light-emitting diodes (OLED). The OLED acted as ideal devices for thermal modeling (COMSOL Multiphysics software), which predicted a 20 °C temperature reduction in the organic layer. This work extends beyond OMAR metrology, and generally applies to a variety of sensitive electronic measurements where the thermal environment may change, such as varying the microscope objective while characterizing field-effect-transistors (FETs). In Chapter 3 the optical absorption of two-dimensional polar metals (2D PMets) synthesized via confinement heteroepitaxy (CHet) is investigated. 2D PMets consist of 1-3 atomic layers of metal intercalated between a SiC substrate and graphene that are theorized to have an out-of-plane permanent electric dipole moment resulting from the non-centrosymtmetric interface. 2D PMets, such as bilayer Ga, exhibit unique optical absorption attributed to a quantum-confined interband transition (~1.9 eV). In this work, 2D Ga FETs were side-gated using electric-double-layer (EDL) gating with polyethylene oxide (PEO):CsClO4. AC EDL gating (2 Hz) modulates the FET’s channel current, indicating high AC electric fields (Erms ~ 1.36 V/nm) which Stark shifted the interband transition, resulting in measurable changes in reflectivity (ΔR/R ~ 8∙10^-4). A combination of transfer matrix modeling coupled with experimental data reveal the Stark shift (~1.5 meV) to be linear, corresponding to a ~0.05 Debye change in the permanent dipole moment between the ground and excited state of 2D Ga. These results for the first time experimentally validate the presence of permanent dipole moments in PMets, as well as demonstrate a powerful new technique to safely deploy large time varying electric fields to 2D materials in the form of AC EDL-gating.