Date of Graduation

5-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics (PhD)

Degree Level

Graduate

Department

Physics

Advisor/Mentor

Thibado, Paul M.

Committee Member

Wang, Yong

Second Committee Member

Kumar, Pradeep

Keywords

AFM; critical point dryer; Graphene; Materials science; photolithography; variable capacitor; vibrational energy

Abstract

Since it was first isolated and characterized in 2004, graphene has shown the potential for a technological revolution. This is due to its amazing physical properties such as high electrical conductivity, high thermal conductivity, and extreme flexibility. Freestanding graphene membranes naturally possesses an intrinsic rippled structure, and these ripples are in constant random motion even room temperatures. Occasionally, the ripples undergo spontaneous buckling (change of curvature from concave to convex and vice versa) and the potential energy associated with this is a double well potential. This movement of graphene is a potential source of vibrational energy.

In this dissertation, we want to exploit this movement of freestanding graphene to design and create an array of freestanding graphene-based variable capacitors on 100 mm silicon wafer substrates. Our intent is to develop a device that can be highly duplicated and potentially incorporated into an integrated circuit to power low power electronics.

This work is based on a two-mask photolithography process. The first photolithographic mask creates long trenches terminated by square wells which have a cone-shaped tip feature etched at its center. These trenches, wells, and tip features are created by isotropic wet etching of the underlying sacrificial SiO2 layer with hydrofluoric acid for 5 minutes at room temperature. The second photolithographic mask lays out metal traces from the tip to its bonding pad along the trench, and a second bonding pad opposite the square well. Creation of these conductive pathways and contact pads is done by deposition of Cr and Au. Finally, I perform large area graphene transfer to the tip regions and use critical point dryer to dry the substrate. This ensures that graphene is left freestanding over the tip feature.

This graphene-tip feature junction forms a variable capacitor where graphene is the movable plate, and the etched tip feature is the fixed plate. Capacitance of up to 60aF is measured from these device structures. In a broad picture, this graphene-tip variable capacitor can be incorporated in a low power energy harvesting circuit as the power source component. It can be used to power low power electronics such as remote sensors. Harnessing this energy associated with graphene vibrations could be source of clean renewable energy and an alternative to batteries.

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