Haoxue Han

Functionalization mediates heat transport in graphene nanoflakes.

The high thermal conductivity of graphene and few-layer graphene undergoes severe degradations through contact with the substrate. Here we show experimentally that the thermal management of a micro heater is substantially improved by introducing alternative heat-escaping channels into a graphene-based film bonded to functionalized graphene oxide through amino-silane molecules. Using a resistance temperature probe for in situ monitoring we demonstrate that the hotspot temperature was lowered by ∼28 °C for a chip operating at 1,300 W cm−2. Thermal resistance probed by pulsed photothermal reflectance measurements demonstrated an improved thermal coupling due to functionalization on the graphene–graphene oxide interface. Three functionalization molecules manifest distinct interfacial thermal transport behaviour, corroborating our atomistic calculations in unveiling the role of molecular chain length and functional groups. Molecular dynamics simulations reveal that the functionalization constrains the cross-plane phonon scattering, which in turn enhances in-plane heat conduction of the bonded graphene film by recovering the long flexural phonon lifetime.

Phonon interference and thermal conductance reduction in atomic-scale metamaterials

We introduce and model a three-dimensional (3D) atomic-scale phononic metamaterial producing two-path phonon interference antiresonances to control the heat flux spectrum. We show that a crystal plane partially embedded with defect-atom arrays can completely reflect phonons at the frequency prescribed by masses and interaction forces. We emphasize the predominant role of the second phonon path and destructive interference in the origin of the total phonon reflection and thermal conductance reduction in comparison with the Fano-resonance concept. The random defect distribution in the plane and the anharmonicity of atom bonds do not deteriorate the antiresonance. The width of the antiresonance dip can provide a measure of the coherence length of the phonon wave packet. All our conclusions are confirmed both by analytical studies of the equivalent quasi-1D lattice models and by numerical molecular dynamics simulations of realistic 3D lattices.

Ultracompact Interference Phonon Nanocapacitor for Storage and Lasing of Coherent Terahertz Lattice Waves

We introduce a novel ultra-compact nanocapacitor of coherent phonons formed by high-finesse interference mirrors based on atomic-scale semiconductor metamaterials. Our molecular dynamics simulations show that the nanocapacitor stores THz monochromatic lattice waves, which can be used for phonon lasing the emission of coherent phonons. Either oneor two-color phonon lasing can be realized depending on the geometry of the nanodevice. The two color regimes of the capacitor originates from the distinct transmittance dependance on the phonon wave packet incident angle for the two phonon polarizations at their respective resonances. Phonon nanocapacitor can be charged by cooling the sample equilibrated at room temperature or by the pump-probe technique. The nanocapacitor can be discharged by applying tunable reversible strain, resulting in the emission of coherent THz acoustic beams.

2D heat dissipation materials for microelectronics cooling applications

The need for faster and smaller, as well as more reliable and efficient consumer electronic products has resulted in microelectronic components that produce progressively more heat. The resultant reliability issues from the increased heat flux are serious and hinder technological development. One solution for microelectronics cooling applications is 2D materials applied as heat spreaders and these include monolayer graphene, graphene based films, and monolayer hexagonal boron nitride and BN based films. In addition, thermal performances of the graphene heat spreader were also studied under different packaging structures, including wire bonding, cooling fins and flip chips. Finally, 2D hexagonal Boron nitride (h-BN) heat spreaders, fabricated by different methods, were characterized by different thermal characterization methods, such as resistance temperature detector (RTD) and Infrared (IR) methods. In conclusion, these new novel 2D materials developed show great potential for microelectronics cooling applications.

Phonon Interference and Energy Transport in Nonlinear Lattices with Resonance Defects

We introduce and model a three-dimensional atomic-scale phononic metamaterial producing two-path interference phonon antiresonances to control the heat flux spectrum. We show that a crystal plane partially filled with defect-atom arrays causes a total phonon reflection at the frequencies determined by masses and interaction forces. Such patterned atomic planes can be considered as high-finesse atomic-scale interference phonon metamirrors. We emphasize the predominant role of the second phonon path and destructive interference in the origin of the total reflection in comparison with the Fano-resonance concept. The random defect distribution in the plane and the anharmonicity of interatomic bonds do not deteriorate the interference antiresonances. The width of the interference antiresonance dip can provide a measure of the coherence length of the phonon wave packet. All our conclusions are confirmed both by analytical studies of the equivalent quasi-one-dimensional lattice models and by numerical molecular dynamics simulations of realistic lattices in three dimensions.