Over the past few decades, the rapid development and discovery of novel nanomaterials has revolutionized society in all facets. One key area these nanomaterials are particularly useful and valued at is for thermal applications. They are already showing promise in improving thermal interface materials (TIM) for preventing overheating in computer chips, enhanced clean power generation for thermoelectrics (TE), and passive cooling radiative thin film structures for low-cost air-conditioning and refrigeration. It is therefore critical to not just invent these nanomaterials, but also characterize their thermal properties. This enables better understanding for which applications they would be best designed for, as well as to discover new phenomena and underlying mechanisms in this relatively...[ Read more ]

Over the past few decades, the rapid development and discovery of novel nanomaterials has revolutionized society in all facets. One key area these nanomaterials are particularly useful and valued at is for thermal applications. They are already showing promise in improving thermal interface materials (TIM) for preventing overheating in computer chips, enhanced clean power generation for thermoelectrics (TE), and passive cooling radiative thin film structures for low-cost air-conditioning and refrigeration. It is therefore critical to not just invent these nanomaterials, but also characterize their thermal properties. This enables better understanding for which applications they would be best designed for, as well as to discover new phenomena and underlying mechanisms in this relatively unexplored nanoscale regime.

In this work, we developed two thermal property measurement platforms (3ω, micro-bridge thermometry) based on microelectromechanical systems (MEMS) devices, to characterize nanomaterials that have great potential applications. The 3ω system was used to characterize the interfacial thermal resistance (ITR) of graphene/Al₂O₃ and graphene/metal (Al, Au, Ti) interfaces. It is of importance to study and improve the thermal stability of these interface combinations as they will arise in next-generation, high-speed electronic devices. We found that post-annealing selectively enhanced only graphene/Ti interfaces, up to 40%. This was attributed to the inherently strong binding between Ti (and other metals of the same class) and carbon, allowing strong covalent bonds to form between them when annealed. These chemical bonds result in better phonon coupling across the interface and enhance the interfacial thermal transport. We proposed a variation of the Diffuse Mismatch Model to better understand the interfacial transport by considering the varying interface conditions.

The micro-bridge thermometry system was utilized to characterize the in-plane thermal conductivity (κ) of self-assembled metal/organic hybrid superlattice microbelts. This novel nanomaterial is of a unique structure consisting of a metal monolayer (Hg) sandwiched between vertically aligned polymer chains in an ordered and repeating superlattice manner. The in-plane κ in the normal direction of the polymer chains was measured to be as low as bulk polymers (~0.3 W/mK at 300 K). Boltzmann Transport Equations simulation of an individual unit layer confirmed a comparable trend of κ with temperature. However, at temperatures between 200 and 240 K, a deviation to the simulation was observed which was attributed to phase-change phenomena and confirmed by thermal analysis.

The bridge device was also used to investigate Si nanomeshes. The significant reduction of κ by creating pores in the Si thin film, without greatly influencing its electrical properties, make it very attractive as a potential TE material. We found that the pore geometry and arrangement have a significant influence on the κ. The neck size was determined to be the most significant in limiting κ. This is distinct from previous experiments and models which considered surface-to-volume ratio and porosity as the most important factors. We also found that the staggered arrangement of pores can reduce κ by up to 25% at low temperature compared to ordered pores of similar geometry. To better understand these phenomena, we developed a framework to predict the κ in nanostructures using the Monte Carlo method and spectral phonon transport properties. We found that the dependence of thermal conductivity on pore arrangement could be attributed to enhanced phonon backscattering for the staggered arranged structures, especially at low temperatures where the phonon mean free path (MFP) is much longer. It is predicted that the enhancement of backscattering as well as wave effects in single-crystal SiGe thin films might be more prominent due to the much longer MFP of dominant phonons in SiGe, providing a new strategy to engineering the κ of Si-based materials for TE applications.