Author ORCID Identifier
Semester
Fall
Date of Graduation
2025
Document Type
Dissertation
Supplemental dataset
Degree Type
PhD
College
Statler College of Engineering and Mineral Resources
Department
Mechanical and Aerospace Engineering
Committee Chair
Konstantinos Sierros
Committee Member
Alan Bristow
Committee Member
Xingbo Liu
Committee Member
Edward Sabolsky
Committee Member
Charter Stinespring
Abstract
Exfoliated 2D nanomaterials have led to significant advancements in physics, chemistry, and materials science over the last few decades. This is because of their sensitivity to changes in surface chemistry and defects as well as quantum confinement effects originating from their nanoscale dimensions. In contrast to bottom-up growth techniques for synthesizing films of 2D materials, such as graphene, transitional metal dichalcogenides and MXenes, liquid phase exfoliation (LPE) has been widely explored as a lower cost, top-down approach for extracting 2D nanoflakes from their bulk counterparts. A significant drawback in films and coatings using LPE nanoflakes is that the properties and uniformity often are inferior to those of grown films. This divergence can be understood as a reduction in the network carrier mobility resulting from both increased resistance in transport between flakes and degraded material properties of the constituent flakes. Therefore, improvements in films and coatings utilizing liquid exfoliated 2D nanoflakes may result from focusing on the characterization of the junction and material level carrier transport while employing strategies to engineer these components for tuned applications and performance.
As the focus in this work, the inter-flake (junction) resistances and intra-flake (material) resistances are studied using exfoliated graphene flakes as a model system, where macroscopic network and microscopic material level studies are performed on percolated graphene flake networks. For engineering the macroscopic transport, it was confirmed that film structuring and morphology are crucial aspects for determining the inter-junction transport. In particular, electromechanical and impedance spectroscopic measurements showing reduction in the network junction resistance by mechanical straining films on stretchable substrates. Such resistance reductions are due to external mechanical forces which densify the film and therefore reduce void space/porosity. Material properties are probed through a combination of Raman spectroscopy, X-ray photoelectron spectroscopy, and ultrafast optical techniques such as Terahertz time-domain spectroscopy and Terahertz transient absorption spectroscopy. It is found that the size of the exfoliated nanoflake plays a vital role in tuning the electrical conductivity characteristics of the flakes through modification of the intrinsic carrier concentration and carrier scattering time. Modifying the flake surface chemistry, through UV exposure in the presence of a photocatalyst, provided a tuning method for the oxygen-to-carbon ratio, impacting surface wetting behavior, AC conductivity characteristics, and photocarrier generation. While the work performed separately characterizes the transport at these disparate size scales, future work may focus on employing hybrid processing and cross-scale characterization strategies targeted at further reducing interflake and intra-flake carrier scattering for targeted device applications and performance. This would provide further benefits in establishing global correlations between film and flake morphology, chemistry, and carrier dynamics in exfoliated nanoflake systems.
Recommended Citation
Loh, Harrison Alexander, "Characterization and Engineering of Inter- and Intra-Electrical Transport in Percolated Graphene Networks" (2025). Graduate Theses, Dissertations, and Problem Reports. 13071.
https://researchrepository.wvu.edu/etd/13071
Included in
Condensed Matter Physics Commons, Nanoscience and Nanotechnology Commons, Optics Commons, Polymer and Organic Materials Commons