Thermal Characterization of Thickness-Dependent Thin Graphite

Abstract

As emerging two-dimensional (2D) materials continue to attract significant attention for their remarkable physical properties, accurately determining their intrinsic heat conduction behavior has become a central challenge. Graphite, in particular, serves as a representative platform for studying phonon transport in reduced dimensions, yet its thickness-dependent thermal conductivity remains insufficiently understood. This gap underscores the need for systematic and high-fidelity experimental approaches. Throughout my Ph.D. research, I have addressed this challenge through three interconnected aims: developing residue-free fabrication techniques for 2D materials, establishing a localized transducer frequency-domain thermoreflectance (LT-FDTR) method for highly sensitive in-plane thermal characterization, and experimentally investigating the thickness-dependent thermal conductivity of suspended graphite films. In the first part of my dissertation, I developed a fabrication technique designed to enable residue-free processing of 2D materials for reliable intrinsic thermal property measurements. Conventional polymer-assisted transfer and lithography processes often leave behind contaminants or induce mechanical damage, both of which obscure the intrinsic thermal behavior. To overcome these limitations, I implemented a purely vdW-based fabrication scheme that eliminates the use of polymers and liquid solvents throughout the process. Using molybdenum disulfide (MoS2) as the primary material, which is chosen for its high vdW adhesion energy, I demonstrated that the method yields clean and high-quality individual flakes. Comprehensive optical, chemical, and electrical characterizations confirmed the absence of residues, oxidation, or strain, with devices fabricated using this technique exhibiting field-effect mobilities of up to 60 cm2/V-s and on/off ratios of ~108. In addition, the residue-free stamp system further enabled a versatile suite of manipulation capabilities, including pick-up, release, exfoliation, wiping-out, flipping, stretching, and folding processes. Analytical modeling clarified the underlying mechanisms, showing that pick-up is governed by vdW adhesion energies, while release relies on interfacial superlubricity. Furthermore, the technique was subsequently extended to h-BN and graphite, demonstrating its broader applicability, including direct relevance for graphite sample fabrication in this dissertation. The second part of this dissertation focuses on advancing the measurement methodology for suspended thin films to enhance sensitivity to in-plane thermal conductivity within the framework of frequency-domain thermoreflectance (FDTR). In conventional FDTR, the use of fully deposited metal transducers often suppresses lateral heat spreading and consequently limits sensitivity to in-plane thermal transport. To overcome this limitation, I developed a modified LT-FDTR approach that employs a small and localized metal transducer, thereby promoting lateral heat flow in the underlying sample and significantly improving sensitivity to in-plane conductivity. The method was validated using suspended silicon nitride (SiNx) films, and sensitivity and correlation analyses confirmed reliable decoupling between in-plane thermal conductivity and other thermal parameters. Furthermore, the LT-FDTR platform demonstrated broad applicability across materials with diverse thermal properties, including Al-coated SiNx, polyimide, and graphite films. For graphite in particular, the localized transducer patterning process can introduce surface contamination. To mitigate this issue, I implemented a vdW stencil lithography technique using freestanding SiNx membranes, enabling contamination-free transducer definition and reliable thermal measurements on suspended graphite films. Finally, the third part of the dissertation examines the thickness-dependent thermal conductivity of suspended graphite films by integrating residue-free fabrication with the LT-FDTR measurement technique. Clean and thin graphite samples were prepared using vdW-based transfer combined with stencil patterning, while thicker films were fabricated through a complementary repeated exfoliation method, enabling sample preparation across a broad thickness range of approximately 500 nm to 82 μm. Using the LT-FDTR approach, graphite films between 517 nm and 1.7 μm were characterized over temperatures from 80 to 300 K, revealing a clear decrease in thermal conductivity with decreasing thickness. In addition, the measurements identified a temperature-dependent peak near 140 K. Although hydrodynamic phonon transport could not be experimentally confirmed within this thickness and temperature window, the results nonetheless reconcile discrepancies in previously reported thickness- dependent thermal conductivities and provide a reliable experimental foundation for understanding phonon transport in graphite at reduced dimensions. Furthermore, through preliminary assessment using the steady-state thermoreflectance (SSTR) method, the feasibility of characterizing much thicker graphite films was demonstrated, suggesting that future work can extend this approach to deliver a high-quality dataset spanning an even wider thickness regime.