Date of Award

2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Electrical & Systems Engineering

First Advisor

Marija Drndic

Abstract

Nanopore-based devices provide the ability to detect, analyze and manipulate molecules by monitoring changes in ionic current and sieving molecules dissolved in an electrolyte. While devices with single nanopores can be used as molecular sensors and analyzers, including as a possible high-throughput DNA sequencer, devices with multiple nanopores (nanoporous devices) can be used to filter out ions from solutions, with possible use in water desalination. Sensitivity and molecular flux can be enhanced by using two-dimensional (2D) materials, like graphene and transition metal dichalcogenides (TMDs), as the nanopore membrane. However, these devices face challenges yet to be solved, including (a) fast DNA translocation velocity through 2D nanopores that limits temporal resolution required to achieve DNA sequencing, and (b) sensitive fabrication techniques that prevents large-scale commercialization of such devices. Additionally, TMD nanoporous membranes have been predicted to possess higher permeability of water molecules than their graphene counterparts, but no related experiments have been presented. In this dissertation, we explore not only ways to tackle the stated limitations, but also perform ion selectivity measurements through ion-irradiated TMD nanoporous devices.

First, we investigate ionic flow and associated leakage currents in voltage-gated graphene nanopores predicted to help slow down DNA translocation velocity. We extract important parameters that can help reduce leakage currents while enhancing the signal strength and gating control.

Next, we report DNA detection with high sensitivity through monolayer tungsten disulfide (WS2) nanopores fabricated via electron-beam drilling and observe laser irradiation induced expansion of the pore, which we are able to control with nanometer precision. Follow-up experiments are performed, wherein we characterize this technique by irradiating intact suspended WS2 membranes to fabricate nanoporous membranes and measure dependence of the induced defect sizes and density on laser power density. This process can be fine-tuned in future studies to enable facile creation of both nanopores and nanoporous devices based on TMDs.

Additionally, we study and calibrate sub-nm defect formation in suspended molybdenum disulfide membranes using ion-beam irradiation. Ionic current characterization of the devices exhibits selective ionic transport, thus laying experimental foundation for future studies on TMD-based nanoporous devices for water desalination.

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