Multiscale simulations of thermal transport in graphene-based materials and across metal-semiconductor interfaces

Yan Wang, Purdue University

Abstract

The rapid advance in modern electronics and photonics is pushing device design to the micro- and nano-scale, and the resulting high power density imposes immense challenges to thermal management. When device size shrinks to the same order of or even below the wavelength or mean-free-path of heat carriers, the transport of heat carriers and the interaction between them will differ from those in the macroscopic regime. This imposes challenges on designing micro/nano-devices with required thermal performance, while, at the same time, also opens the door for designing novel materials and structures with promising thermal properties.^ This research explores structures with unique heat transfer properties for thermal management applications. It also seeks to build a more accurate and comprehensive understanding of electron and phonon transport, and the coupling between them, in order to guide the design of strategies to enhance heat dissipation in solid-state devices. Graphene is a unique material with 2D lattice structure and single-atomic-layer thickness, and we explore several mechanisms that can affect thermal transport in it. The thermal conductivity (κ) of zigzag-edged graphene nanoribbons (GNRs) is found to be higher than that of armchair-edge ones in our molecular dynamics (MD) simulations, and phonon localization at edges is attributed to underlie such edge-chirality dependence. Thermal rectification (TR) is a phenomenon in which heat flows more easily in one direction than in the opposite direction, which is particularly useful for thermal management. Using MD simulations, we find significant TR in asymmetrically defected GNRs and pristine GNRs with asymmetric geometry. However, TR in these two structures arises from different mechanisms. In the former case, GNRs are pristine on one side while defective on the other, and TR is caused by the different temperature dependence of the thermal conductivity of the two sides. In the latter case, TR can be enabled by phonon lateral confinement when the width of the GNR is smaller than the phonon mean free path. These findings will provide useful guidance to the fabrication of thermal rectifiers from pristine materials including but not limited to graphene. ^ A two-temperature non-equilibrium MD simulation technique is developed to atomistically model electron-phonon coupled thermal transport across metal/semiconductor interfaces. On the metal side, the lattice part of thermal transport is modeled with MD while the electronic part is simultaneously modeled with the Fourier’s law using the finite difference method. On the semiconductor side, electrons are neglected and only phonons are considered. Our method naturally accounts for the effect of defects, interface, temperature, etc., on thermal properties of phonons and also includes the coupling between electron and phonon. We use this technique to compute the thermal boundary resistance (TBR) of Si/Cu and CNT/Cu interfaces. In a region within a “cooling lengt” distance to the interface, electron and phonon are revealed to be in thermal non-equilibrium, which considerably impedes heat transfer across the interface. The TBR of CNT/Cu interfaces predicted using our method is in better agreement with experimental results than conventional MD methods.^ A two-temperature Boltzmann transport equation method is also built, which considers electron and phonon on both sides of the interface. It was reported that an interlayer with intermediate phonon spectra between two dielectric materials could reduce the phononic interfacial thermal resistance. In this work, we show that an appropriate choice of interlayer materials with relatively strong electron-phonon coupling could significantly enhance interfacial thermal transport across metal-dielectric interfaces. Our Boltzmann transport simulations demonstrate that such enhancement is achieved by the elimination of electron-phonon nonequilibrium near the original metal-dielectric interface. Moreover, we reveal that interlayer can substantially accelerate hot electron cooling in thin films with weak electron-phonon coupling, for example, Cu, Ag, and Au, supported on a dielectric substrate. At the same time, lattice heating in the thin film is largely reduced.^ A Monte-Carlo simulation approach is proposed to solve electron-phonon coupled thermal transport problems in metal-semiconductor heterojunctions. This approach enables us to conduct a spectral electron-phonon simulation considering the selection rules for three-phonon scatterings. We demonstrate the approach using a Au-Si bilayer system under ultrafast laser radiation. Nonequilibrium between electrons and different phonon modes are observed. This approach enables first-principles-based simulation of heat transfer across metal-nonmetal interfaces, which will be useful for designing thermoelectric devices and for thermal management of electronic devices.^ The results from this study offer new understandings of nanoscale thermal transport involving multiple types of heat carriers, and the approaches developed have strong predictive capability, which will aid the thermal design of novel micro- or nano-devices. This research also provides new perspectives of atomic- and nano-scale engineering of materials and structures to enhance efficiency of thermal management.^

Degree

Ph.D.

Advisors

Xiulin Ruan, Purdue University.

Subject Area

Mechanical engineering|Condensed matter physics|Energy

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