2D MoS2 Nanopores: Wafer-scale Fabrication and Monolayer Stability for Long-term Single-Molecule Sensing

Biologically inspired solid-state nanopores are artificial openings or apertures in thin membranes similar to natural protein ion channels in a lipid bilayer of cell membranes. In solid-state nanopores, a thin insulating membrane with single or multiple pores separates two conductive salt solutions. When an electric field is applied across this membrane, electrically charged species such as ions pass through these nanopore(s), generating a nanopore ion current. In essence, nanopores are single-molecule sensors and valuable tools for studying biophysics. For instance, an intrinsically charged biomolecule such as DNA can be electrophoretically threaded through the nanopore, transiently blocking the ionic current characteristic of the molecule passing through the pore.With progress in two-dimensional (2D) materials, the marriage of nanopores with 2D materials - "2D nanopores" have emerged as a new class of ultra-thin membrane solid-state nanopores. Molybdenum disulfide (MoS2) is a 2D material with an atomic thickness (0.7 nm) that approaches the inter-base distance of two DNA bases and is a lucrative 2D material for the DNA sequencing application. However, there are inherent challenges and bottlenecks with using MoS2 due to sensitive fabrication and inherent challenges of the 2D materials leading to low device yield. In this thesis, I will demonstrate ways to improve high-throughput production and the development of more reliable and durable nanopore devices. In the second chapter, I will introduce MoS2 material as a 2D nanopore system and elaborate fabrication of nanopore substrates. I will discuss various problems and issues related to substrate fabrication, transfer, nanopore-creation, and nanopore measurements. Finally, I list a step-by-step protocol and troubleshooting guide for early-stage 2D nanopore researchers. In the third chapter, I specifically focus on "chip-scale" transfer strategies for MoS2 grown using chemical vapor deposition (CVD). I introduce two transfer approaches - direct-transfer and stamp-assisted- with just water as a medium. I will demonstrate these transfer approaches and discuss their advantages and limitations. Furthermore, I discuss hydrocarbon contamination with 2D materials and their implications in nanofluidics. In the fourth chapter, I demonstrate a scalable transfer from chip-scale to a larger "wafer-scale" for batch fabrication of nanopore substrates for single-molecule DNA sensing. With PDMS-based polymer, I will demonstrate 3-inch monolayer MoS2 transfer on nanopore substrates with 128 nanopore devices with high transfer efficiency (>70%). Moreover, the technique is etchant-free, and growth substrates are recyclable after transfer. In the fifth chapter, I study nanopore instability and address those issues. I address the delamination issue by chemically modifying Si-substrates with an organosilicon that increases the adherence of monolayer MoS2 layers. This surface pre-treatment helped reinforce 2D layer attachment to the substrate, increasing the nanofluidic devices' durability. Further, we show that the nanopore enlargement due to dissolved oxygen in an aqueous solution can be considerably reduced in a low dissolved oxygen concentration in solution. These strategies improved MoS2 nanopore stability and enabled long-term DNA sensing. This will pave way toward more durable 2D nanopore sensors.