Browsing by Subject "Phonon"

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Characterization of thermal and hydrodynamic phonon transport using transient thermoreflectance
(2021-04-06) Jeong, Jihoon; Wang, Yaguo; Shi, Li; Liu, Yuanyue; Lin, Jung-fu; Bank, Seth R.
Thermal management becomes a significant challenge due to generating more power per unit volume in developing high-power density micro-/nano-sized electronics. The critical feature size of nano- or micro-electronics has already reached less than 10 nm. The thermal conductivity of the semiconducting, dielectric, and metallic materials used in fabricating these microelectronics can be reduced significantly at a small dimension because of increased boundary scattering of phonons and electrons, known as the heat carriers in solids. The size effects on the thermal transport property are considered one of the most fundamental problems for thermal management, which needs to be understood thoroughly for next-generation electronic devices' rational design. This work seeks to advance the understanding of how heat travels in solid materials. Utilizing precise methodologies for characterizing thermophysical properties is the most critical job. Transient thermoreflectance (TTR) and grating imaging techniques are developed to measure thermal conductivity in both lateral and horizontal directions. These techniques are employed to study materials from bulk to nanostructure, including thin-film and nanoparticles, to be characterized. A new regime of phonon transport between ballistic and diffusive transports, called hydrodynamic phonon transport, is observed with a novel method of heat pulse measurement. With thermal characterization using the TTR, four bulk materials, including glass, sapphire, GaAs, and Si are examined for interfacial thermal resistance and cross-plane thermal conductivity. Nanostructures are also tested, such as MoS2 thin-films over a wide range of thicknesses and digital alloy samples of GaAs/InAs. Cu nanoparticles' effective thermal properties are also measured, including heat capacity, density, and thermal conductivity to improve laser sintering technology in the additive manufacturing industry. Thermal characterization using the grating imaging technique, the in-plane thermal conductivity is measured in GaAs/AlAs superlattices and Cu nano-films on Si substrate, representing the bulk and layered structures, respectively. A new regime of phonon transport, hydrodynamic, is observed in graphite using optical heat pulse measurement. The measurement results reveal transient lattice cooling near the adiabatic center of a ring-shaped pump beam at temperatures between 80 and 120 K.
Synthesis and characterization of compound crystals with weak phonon couplings
(2022-09-22) Lee, Hwijong; Shi, Li, Ph. D.; Zhou, Jianshi; MacDonald, Allan H; Tutuc, Emanuel; Wang, Yaguo
Recent advances in experimental and computational techniques have advanced the understanding of phonon transport and phonon couplings to charge and spin degrees of freedom. As an illustrating example, the unusual phonon band features of Boron Arsenide (BAs) result in simultaneously high lattice thermal conductivity and high intrinsic carrier mobility, which make BAs an emerging III-V semiconductor for high-performance electronics devices. Meanwhile, magnon coupling with phonons allows for the thermal generation of spin waves, which can be converted into an electrical signal for readout or vice versa via the spin Hall effect in a normal metal in contact with the magnetic material. This work seeks to advance the understanding of the coupled transport phenomena in electronic and magnetic materials with unusual phonon-mediated behaviors and to address some of the fundamental questions on the interactions among energy, charge, and spin carriers in the semiconducting BAs, semimetal θ phase tantalum nitride (θ-TaN), and the magnetic insulator yttrium ion garnet (YIG). These questions are addressed through several experimental approaches based on surface electronic state measurements, steady-state bulk thermal and electrical properties measurements, frequency-dependent spin caloritronic measurements, and electron microscopy. Scanning tunneling spectroscopy measurements of cleaved BAs surfaces show a bulk bandgap of 2.1 eV as well as surface electronic states inside the bulk bandgap. These findings are relevant to the use of BAs as an active layer in future-generation electronic devices. With a similarly large phonon energy gap as semiconducting BAs, θ-TaN is grown via a high-pressure technique for transport measurements to investigate the theoretical prediction of a high thermal conductivity of this semimetal compound with a small electron density of states near the Fermi level. The results show both the effect of grain boundary scattering on suppressing the thermal conductivity and the potential of further increasing the thermal conductivity by increasing the grain size and reducing the defect concentrations. Besides these two investigations of electronic and phononic structures and transport, lock-in measurements are used to investigate the frequency dependence of the spin Seebeck effect (SSE) and detect a spin Peltier magnetoresistance (SPMR) at a heterostructure made of a platinum (Pt) thin film on YIG. The observed frequency dependence of the second harmonic SSE and first harmonic SPMR are analyzed with a model that accounts for both interface and bulk spin Seebeck effects to understand the elusive magnon transport properties.
Thermal transport in individual single-wall carbon nanotubes
(2007-05) Pettes, Michael Thompson, 1978-; Shi, Li, Ph. D.
This thesis presents an experimental study of phonon transport in individual suspended single-wall carbon nanotubes (SWCNTs). A microfabricated device consisting of two adjacent suspended membranes, each with a platinum resistance heater and thermometer, was used to directly measure the thermal conductance of three individual SWCNTs. These results show the effects of Umklapp phonon-phonon scattering remain weak and the thermal conductance remains roughly proportional to the ballistic conductance throughout the temperature range of 100 to 490 K. Additionally, through the use of transmission electron microscopy analysis we have for the first time directly obtained the diameter of a nanotube for which thermal measurements were obtained and determined the thermal conductivity of this SWCNT. The thermal conductivity of this 1.6 nm diameter, 4.72 μm long nanotube increases with temperature as ~T[superscript 1.5] throughout the temperature range indicating static scattering processes dominate transport in this regime. The measured thermal conductivity is greater than 1000 W/m·K above room temperature making it one of the best thermal conductors known.