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Surface ferroelectric phase transitions
Abstract
I studied the surface ferroelectric phase transition, distinct from the bulk ferroelectric transition, of the crystalline ferroelectric copolymer films of poly vinylidene fluoride (70%) with trifluoroethylene (30%) P(VDF-TrFE) which were prepared by the Langmuir-Blodgett deposition method. The goal of this dissertation was to understand the origin of the surface ferroelectric transition. I undertook photoemission and inverse photoemission measurements to obtain both valence band and conduction band spectra. These measurements allowed me to probe the electronic structure and the changes in electronic structure associated with the surface ferroelectric phase transition. X-ray diffraction, neutron reflection, scanning tunneling microscopy and low energy electron diffraction were employed to measure the bulk and surface crystal structure. From these measurements, the bulk crystal structure change was observed across the bulk ferroelectric transition and the surface crystal structure was distinct from the bulk. I employed angle-resolved inverse photoemission spectroscopy to map the conduction band structure along several surface high symmetry directions. These measurements demonstrated a strong correlation between the changes in structure and the changes in the conduction band structure across the surface ferroelectric phase transition. The work function changes were measured from the cutoff edge of the secondary electron in photoemission. The work function measurements indicated a change of the surface polarization across the surface ferroelectric phase transition. I measured the angle-resolved core-level spectra to determine the depth profile. This allowed me to probe the physical rotation of the polar group (CH$\sb2$-CF$\sb2$). The electronic structure of the doped polymer P(VDF-TrFE) was measured, as a function of sodium doping and temperature, using photoemission, inverse photoemission and core-level photoemission spectroscopy to probe the electron and phonon correlation. This allowed me to identify a dynamic Jahn-Teller distortion. I identified the changes in metallicity and electronic structure across the surface ferroelectric phase transition of the ultrathin crystalline poly(vinylidene fluoride-trifluoroethylene) copolymers, for the first time. The copolymer P(VDF-TrFE) films are n-type semiconductors and have a band structure. The first conduction band structure mapping and the conduction band structure change of the copolymer P(VDF-TrFE) films across the surface ferroelectric phase transition were obtained. The results indicated the existence of a coupled ferroelectric/crystal structure/electronic band structure/metallicity phase transition. In this transition, the Brillouin zone size of the surface ferroelectric phase roughly doubles the one of the surface paraelectric phase, there is a transition from an all-trans to alternating gauche-trans conformation in crystal structure, and there is a semiconductor to semimetal phase transition across the surface ferroelectric transition. Strong electron-phonon correlations are implicated.
Subject Area
Condensation|Polymers|Molecules|Electromagnetism
Recommended Citation
Choi, Jaewu, "Surface ferroelectric phase transitions" (1998). ETD collection for University of Nebraska-Lincoln. AAI9908466.
https://digitalcommons.unl.edu/dissertations/AAI9908466