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Author ORCID Identifier

https://orcid.org/0000-0002-9952-9187

AccessType

Open Access Dissertation

Document Type

dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Chemistry

Year Degree Awarded

2023

Month Degree Awarded

February

First Advisor

Michael D. Barnes

Second Advisor

Kevin Kittilstved

Third Advisor

Ricardo B. Metz

Fourth Advisor

Ashwin Ramasubramaniam

Subject Categories

Materials Chemistry | Physical Chemistry

Abstract

My thesis focuses on understanding the changes in electronic properties of two-dimensional materials produced by surface interactions not limited to charge exchange. Recent work from our group demonstrated that both small molecules and polymers can function as effective charge dopants for monolayered 2D materials such as MoS2 and graphene, changing the Fermi energy by either donating or accepting electron density to/from the 2D material. Additionally, the underlying support material was found to play a significant role, where higher dielectric constant supports result in larger magnitude of Fermi energy shift of the 2D material because less of the dopant interaction is lost polarizing the support. This work here was motivated by the desire to understand in greater detail how electronic properties of 2D materials can be tailored by exploiting different types of surface interactions, and how the properties can be characterized in a manner which is both specific to the 2D layer, and insightful with respect to the impact of different environmental factors. To this end, two questions were asked: 1) How significant is polarization of the 2D layer in determining electronic properties, compared to charge exchange? 2) What is the role of the overlayer and the underlayer (support) in electronic properties vs. the determined properties? The questions were addressed using two experimental platforms. In the first, we employed a zwitterionic polymer to test the response of graphene to permanent surface dipoles. The interaction was probed using Kelvin probe force microscopy to detect changes in the Fermi energy, electrostatic force microscopy to detect changes in surface charges and polarizability and was examined from the graphene side and the polymer side by employing a unique inverted sample construction along with a normal orientation sample. This allowed us to probe the source of changes in the Fermi energy, disentangling dipole effects from those of charge exchange, and to observe the magnitude of electric charge screening present in the graphene layer. In the second platform, we used a polymer with perfluorinated linear alkyl side chains to examine the role of fluorine and surface dipole formation in the p-doping of MoS2, using a second unfluorinated polymer as a control. In the system, the photoluminescence signatures of three body excited states (trions, i.e., charged excitons) were used to unambiguously determine changes in the MoS2 carrier density which was compared with Kelvin probe force microscopy measurements of changes in the Fermi energy. Our goal was to elucidate the role dipoles and surface polarization play in the electronic properties of 2D materials and provide tools to approach the doping and characterization of these materials. We probed phosphorylcholine containing zwitterionic polymer coatings of graphene, and fluorinated polymer coatings of MoS2 using a combination of Kelvin probe force microscopy, electrostatic force microscopy, and photoluminescence. Our key findings were that polarization, both due to the presence of surface dipoles or due to dipole formation in response to fluorinated groups, strongly influences the Fermi energy and can have a larger impact than the exchange of charge. We found that charge exchange accounted for only about 10% of the 261 meV shift in the Fermi energy of graphene due to phosphorylcholine polymers. Additionally, we found that graphene has an astounding electrical ‘opacity’. When viewing a surface interaction through graphene, charge screening within graphene reduces the observed magnitude of the interaction by a factor of ~26. We found that perfluoropolymers are capable of both destabilizing trions and shifting the Fermi energy in MoS2, by ~40x more than the calculated reduction in charge would predict. These findings underline the significance of surface interactions in driving the electronic properties of 2D materials, particularly those involving dipoles, and demonstrate methods of exploiting them to tailor the Fermi energy and photoluminescence properties of two prototypical materials in graphene and MoS2. Further, the way multiple scanning probe techniques can be incorporated into chemical analysis is demonstrated, along with the combination of photoluminescence with scanning probe measurements.

DOI

https://doi.org/10.7275/32802674

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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