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Author ORCID Identifier

https://orcid.org/0000-0003-0931-739X

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

Year Degree Awarded

2019

Month Degree Awarded

May

First Advisor

Todd Emrick

Second Advisor

Bryan Coughlin

Third Advisor

Michael Barnes

Subject Categories

Materials Chemistry | Organic Chemistry | Physical Chemistry | Polymer Chemistry

Abstract

This dissertation describes the modification of 2D transition metal dichalcogenides (TMDCs). These materials exhibit unique electronic properties, ranging from metallic to insulating, and can transport either electrons (n-type) or holes (p-type). Polymers containing electronically-active moieties offer a path to control the electronic properties of a 2D material without altering the inherent structure of the semiconductor. This dissertation focuses on the synthesis of polymers bearing chalcogen-rich or zwitterionic moieties to alter the electronic and solution properties of 2D materials.

Chapter 2 describes polymers containing sulfur-rich tetrathiafulvalene (TTF) and their effects as electroactive coatings on the TMDC molybdenum disulfide (MoS2). These polymers were anticipated to not only promote adhesion to MoS2 through sulfur-sulfur interactions but also modify the work function of the semiconductor through the donation of electrons at the semiconductor/polymer interface. TTF polymers were synthesized by ring-opening metathesis copolymerization (ROMP) and reversible addition fragmentation chain transfer (RAFT) polymerization. TTF polymers stabilize suspensions of chemically exfoliated MoS2 nanosheets, contrary to a pyrene-substituted polymer of similar structure, demonstrating the importance of a sulfur-rich structure for interaction with MoS2. Kelvin probe force microscopy (KPFM) was used to examine the shift in work function after a thin polymer layer was applied to MoS2 which revealed a decrease in work function by 0.24 eV, expected for n-doping.

Chapter 3 examines the complementary case to TTF—doping with a sulfur rich electron acceptor bithiazolidinylidene (BT). Functional BT monomers were synthesized by the reaction of a primary amine with carbon disulfide and dimethylacetylene dicarboxylate. BT-containing polymers were then accessed by condensation of a BT-diol with hexamethylene diisocyanate to form polyurethanes. The polymers exhibited thermal stability and solubility in an array of solvents and, upon coating single layer MoS2 grown by chemical vapor deposition (CVD), the SPC decreased by 0.16 V, signifying an increase work function and confirming p-doping.

Chapter 4 progresses the previous two chapters through the analysis of the underlying substrate and its role in the efficacy of doping MoS2 with small molecules TTF and BT. CVD grown MoS2 on silicon oxide (SiO2) and aluminum oxide (Al2O3, sapphire) were coated with thin layers of TTF and BT and the change in work function was monitored by KPFM. MoS2 on Al2O3 showed a work function decrease of 1.24 eV when coated with TTF, displaying a remarkable increase in the efficacy of n-doping compared to using SiO2 as the underlying dielectric. Similarly, when coated with BT, MoS2 with Al2O3 as the dielectric displayed a 0.8 eV increase in the work function representing a four-fold increase in the magnitude of work function shift when compared to using SiO2 as the underlying substrate. We rationalize this increase in the efficacy of doping MoS2 by an increase in the static polarizability of the substrate when using Al2O3 causing a decrease in the effective measured dipole screening being probed at the dopant/semiconductor interface.

Chapter 5 concludes with the development of zwitterionic photoresists used to simultaneously pattern and dope 2D materials. We developed a novel photoresist composed of zwitterionic poly(sulfobetaine methacrylate) (PSBMA) copolymers with methyl methacrylate and implemented these photoresist in the fabrication of graphene transistors. Multiple copolymers were synthesized by conventional free-radical polymerization in trifluoroethanol with feed ratio matching experimental incorporations. These zwitterionic photoresists displayed resolutions approaching 100 nm, matching conventional methacrylic photoresists. Transistors were fabricated on CVD-grown single layer graphene deposited on p-type Si/SiO2 and the polymer was used to pattern over the device to afford multiple, unique devices on a single graphene flake. Polymer covered devices showed n-doping indicated by a shift in the charge neutrality point in the current-voltage curves. Furthermore, a single device with polymer covering half of the device exhibited p-n junction characteristics (high on currents at high gate biases) demonstrating the ease of fabrication of these devices using this class of polymer photoresists.

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