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Switching dynamics of ferroelectric Langmuir -Blodgett copolymer films
Ferroelectric switching dynamics in ferroelectric copolymer films of poly(vinylidene fluoride-trifluoroethylene) can vary over nine orders of magnitude; 100 seconds for the slowest ultrathin (1-50 nm) Langmuir-Blodgett films to 100 ns for the fastest polymorphous spin-coat films (∼50 μm thick). These ultra-thin films share many of the same ferroelectric properties of bulk films such as polarization, phase transition temperatures, crystalline structure, and high electrical resistance (>10 MΩ). The slow nature of switching in ultrathin films is believed to be caused by the intrinsic nature of the switching. The polarization is no longer switching by nucleation and domain wall growth enabled by defects and nanostructures in the polymorphous samples. We investigate this hypothesis by the introduction of defects in the form of nucleation sites and/or grain boundaries by electron irradiation, production of individual ferroelectric nano-crystals, and the introduction of domain wall boundaries through Direct Laser Interference Patterning (DLIP). Electron-irradiation was performed for a large range of doses from 16 to 110 Mrad, on ultra thin films 36 nm thick. It was thought that the defects introduced by electron irradiation could act as nucleation sites, promoting faster switching. However, the primary effect of electron irradiation was the decrease in crystallinity and therefore the fraction of ferroelectric material. Even for lower doses the increase in switching speed was negligible in comparison to the loss of ferroelectricity. The introduction of false domain walls through laser annealing was used to produce more complex and controlled shapes than given by the nanomesas. We investigated patterning by continuous-wave direct write, and by pulsed laser irradiation DLIP. We have demonstrated the ability to pattern films reversibly into films of ferroelectric regions surrounded by paraelectric phase, as well as irreversibly ferroelectric regions surrounded by melted copolymer. The investigation of switching dynamics of individual ferroelectric crystals, through the production of ferroelectric 'nanomesas', were examined. Ferroelectric nanomesas are created through heat treatment and self organization, and have an average height of 7 to 10 nm and diameters of 80 to 100 nm depending on the formation conditions. Nanomesas are highly crystalline and in the ferroelectric phase and switch faster than 50 μs. These studies have demonstrated the ability to demonstrate switching speeds in ultrathin films over seven orders of magnitude.
Othon, Christina M, "Switching dynamics of ferroelectric Langmuir -Blodgett copolymer films" (2005). ETD collection for University of Nebraska - Lincoln. AAI3194123.