Date of Award
Doctor of Philosophy (PhD)
School of Materials Science and Engineering
Stephen H. Foulger, Committee Chair
There is a strong interest in organic materials for electrical devices due to several advantages that organic systems have over their inorganic counterparts including ease of processability and lower toxicity. Many of these organic materials can be utilized in the creation of thin-film devices that can be formed in high-throughput processes and with a very small profile. One such device that has emerged in recent years is the memristor which can be used in new computational concept or as a synaptic model. This work studies the alternating current (AC) and direct current (DC) electrical response of a number n-alkyl methacrylate polymers with a charge transporting pendant carbazole ring. The electrical properties of the polymers were studied as a function of n-alkyl length with n ranging from 2 to 11. The DC current (I)-voltage (V) response of the polymers was characterized by an erratic and bistable response, while their AC I-V response was a pinched hysteresis loop when measured between 1-100 Hz. For polymers with n < 9, their pinched hysteresis loop is characterized by "jump transitions" indicative of bistability, while polymers with n ≥ 9 had a pinched hysteresis loop that is smooth in appearance. Dielectric spectroscopy on the polymers indicates that as the n-alkyl length is increased, the rotation flexibility of the carbazole moiety is enhanced. The n-alkyl methacrylate polymers with a pendant carbazole ring spaced n ≥ 9 exhibited a lower activation energy and temperature for the onset of ring motion and resulted in polymer-based memristors that exhibit electrical characteristics, such as incrementally adjustable conductivity, that are potential candidates for mimicking synaptic plasticity. Further characterization was done on similar methacrylate systems with oxygen-substituted side chains and the addition of bulky phenyl groups to the carbazole moieties. From this work, the most promising candidate for synaptic modeling behavior was taken and further examined. It was shown that this polymer could be pulsed through a multitude of conductivity states and demonstrated behavior consistent with the Hebbian Learning Rule upon the application of pre- and post-synaptic pulses. The system was further characterized for the effects of different spike rates and voltages before being utilized in a flexible device. Other thin-film devices as well as novel processing methods were also demonstrated in this work including a biologically based reserve battery and a printed diode utilizing pentacene. The battery utilized standard alkaline chemistry where the zinc and manganese oxide electrodes are formed using stencil printing. Fish eggs are used to sequester the electrolyte out of the system until the application of force to the device. This application of force bursts the fish eggs and allows the battery to function by introducing the electrolyte into the system. A printed diode is also demonstrated through the use of a miniemulsion process that allows for the dispersion of the material into aqueous solution. This pentacene emulsion in water can then be used as the basis for the formation of diodes in a variety of fabrication processes.
McFarlane, Tucker, "n-Alkyl Methacrylate Polymeric Memristors for Synaptic Response Modeling: Organic and Biologically Relevant Thin Films" (2018). All Dissertations. 2263.