Date of Graduation
8-2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Physics (PhD)
Degree Level
Graduate
Department
Physics
Advisor/Mentor
Paul Thibado
Committee Member
Bothina H Manasreh
Second Committee Member
Jeff Dix
Keywords
critical point drying;electron-beam lithography;energy harvesting;graphene;graphene transfer;variable capacitor
Abstract
Freestanding graphene has constantly moving ripples. Due to its extreme flexibility, graphene responds to ambient vibrations and changes its curvature from concave to convex and vice versa. During a ripple inversion 10,000 atoms move together, suggesting the presence of kinetic energy which can be harvested. In this study we present circuitry and semiconductor studies for harvesting energy from graphene vibrations. The goal of the study is to develop a graphene energy harvesting chip which can serve as a battery replacement in low power electronics. In the first study we determined the best circuit for harvesting vibrational low power. To do this, we tested different full-wave rectifier topologies, which included a rectifier with 4 diodes and 1 storage capacitor, 2 diodes and 1 capacitor, 2 diodes and 2 capacitors, 2 diode-wired transistors and 1 capacitor, and 2 diode-wire transistors and 2 capacitors. The best circuit that we found used a rotatable variable capacitor (VC) as a power source, a DC bias for charging the VC, and a full-wave rectifier with 2 diode-wired transistors and 2 capacitors. Here, the rotating VC mimics the moving freestanding graphene. We show that when the VC is rotated, one plate of the VC moves relative to the other, generating a sinusoidal current signal, and allowing power to be harvested. The best circuit had a maximum output power of 10 nW and efficiency of 50 % at this power. We performed LTspice simulations to support these findings. We also wanted to know what circuit works best if the signal from graphene had a mix of frequencies rather than a single sine wave signal. In the second circuitry study we used a white gaussian noise signal from a power supply to determine the best circuit for harvesting noise power. We used an input signal with 10s of millivolts RMS, and a range of frequency up to 50 MHz to test our circuits. We tested a full-wave rectifier, a differential drive voltage multiplier, a Cockcroft Walton voltage multiplier, and a charge pump. The best circuit for harvesting noise power is the Schottky diode full-wave rectifier with 1-picofarad storage capacitors. This circuit yielded maximum output power in the order of 10s of nanowatts. These findings were also verified with LTspice simulations. Lastly, we fabricated an array of graphene variable capacitors (GVC) on Si/SiO2 substrate. First, we spin-coated PMMA onto the substrate at 1500 RPM. Next, we used electron-beam lithography to write a 2-um tip-region inside a 5-um well-region. This pattern was etched in buffered oxide etch (BOE) to form a cone-shaped tip at the center of the well, which connects to an elongated trench. Next, gold was deposited to form conducting traces and contact pads. We used atomic force microscope (AFM) to determine the tip-to-surface distance, which allows us to estimate the capacitance of the GVC. After, we use the scotch-tape method to transfer graphene above the tip region to form a capacitor. We measured the tip-graphene capacitance in time and found its range to be 100 attofarad.
Citation
Harerimana, F. (2023). Circuitry and Semiconductor Studies for Making a Graphene Energy Harvesting Device. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/4900