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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2018)

The thermal conductivity (k) of a semiconducting nanostructure is dramatically reduced from the bulk value due to boundary and interfacial scattering of energy carriers (phonons). The theoretical understanding of such nanoscale thermal phenomena is based on measurements of relatively simple nanostructures, such as thin films or nanowires. However, qualitatively new heat transfer mechanisms may emerge in more complicated nanostructures such as etched silicon nanomeshes or arbitrarily anisotropic thin films. New theoretical tools are needed to predict k of these nanostructures, and new experimental nanoscale temperature mapping tools would resolve questions about the dominant nanoscale mechanisms. In addition, nanothermometry techniques could be used to improve the thermal performance of technologies utilizing complicated nanostructures, which range from data storage devices to light-emitting diodes and microelectronics.

In this dissertation, I develop experimental, computational, and analytical tools to answer fundamental questions about heat transfer in complicated nanostructures. I begin by demonstrating two nanothermometry techniques in the scanning transmission electron microscope (STEM) utilizing temperature-dependent thermal diffuse scattering. Temperature mapping of a Joule-heated silicon carbide device in the STEM shows the path forward towards ultrahigh spatial resolution temperature mapping of complicated nanostructures. Then, I describe how phonon ray tracing simulations quantify the boundary scattering reduction of k in complicated nanostructures. Comparing these simulation results with collaborator’s k measurements reveals that thermal phonons behave like incoherent particles rather than like coherent waves in silicon nanomeshes, which are membranes with periodically etched holes. Lastly, I derive solutions of the Boltzmann transport equation for phonon transport in arbitrarily aligned anisotropic thin films, and use these solutions to extend a well-known bulk Onsager relation for anisotropic heat conduction into the boundary scattering regime. In summary, further research using these experimental and theoretical techniques can answer long-standing fundamental thermal questions and can be leveraged in the design of energy-efficient lighting technologies and improved data storage devices.

    Cover page: Heat Conduction in Complicated Nanostructures: Experiments and Theory
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2017)
    Phonons can display both wave-like and particle-like behaviour during thermal transport. While thermal transport in silicon nanomeshes has been previously interpreted by phonon wave effects due to interference with periodic structures, as well as phonon particle effects including backscattering, the dominant mechanism responsible for thermal conductivity reductions below classical predictions still remains unclear. Here we isolate the wave-related coherence effects by comparing periodic and aperiodic nanomeshes, and quantify the backscattering effect by comparing variable-pitch nanomeshes. We measure identical (within 6% uncertainty) thermal conductivities for periodic and aperiodic nanomeshes of the same average pitch, and reduced thermal conductivities for nanomeshes with smaller pitches. Ray tracing simulations support the measurement results. We conclude phonon coherence is unimportant for thermal transport in silicon nanomeshes with periodicities of 100 nm and higher and temperatures above 14 K, and phonon backscattering, as manifested in the classical size effect, is responsible for the thermal conductivity reduction.
      Cover page: Investigation of phonon coherence and backscattering using silicon nanomeshes
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2019)
      Considerable advances in manipulating heat flow in solids have been made through the innovation of artificial thermal structures such as thermal diodes, camouflages, and cloaks. Such thermal devices can be readily constructed only at the macroscale by mechanically assembling different materials with distinct values of thermal conductivity. Here, we extend these concepts to the microscale by demonstrating a monolithic material structure on which nearly arbitrary microscale thermal metamaterial patterns can be written and programmed. It is based on a single, suspended silicon membrane whose thermal conductivity is locally, continuously, and reversibly engineered over a wide range (between 2 and 65 W/m·K) and with fine spatial resolution (10-100 nm) by focused ion irradiation. Our thermal cloak demonstration shows how ion-write microthermotics can be used as a lithography-free platform to create thermal metamaterials that control heat flow at the microscale.
        Cover page: Ion Write Microthermotics: Programing Thermal Metamaterials at the Microscale
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