Nanoscale thermal transport: size effects and transport regimes.
The blooming of the modern technologies is primarily attributed to the continuing miniaturization of electronic devices. As the characteristic lengths shrink into nanoscale, thermal management of these devices becomes the bottleneck for the further device performance improvements. The performance and the lifetime of the devices are highly dependent on the efficiency of heat dissipation. Poor heat dissipation efficiency leads to high operating temperatures, and thus performance degradation and reduced lifetimes. To achieve better heat dissipation efficiency in nanoscale devices, fundamental understanding of thermal transport at nanoscale is essential. One of the biggest challenges to understanding the phonon transport processes at nanoscale is the involvement of the size effects and various phonon transport regimes. In nanostructures or low dimensional solids, the dimensions of a system can be comparable to the characteristic lengths (phonon mean free paths and coherence length), and the phonon transport behavior is highly dependent on the transport regime.
The importance of the size effects and transport regimes is first shown in bulk crystals, by studying the thermal transport from ballistic to diffusive regime in two-dimensional bulk crystals. With varying sample sizes, the ordering of thermal conductivity among monolayer transition metal dichalcogenides MX2(M: Mo, W; X: S, Se) changes as phonon transport transits from the ballistic to diffusive regime, driven by the competition between the phonon frequency spectrum range and the scattering strength. This study highlights the necessity of considering the size effect and transport regime in analyzing thermal conductivity. It also expands our understanding of phonon scatterings by showing us the importance of the mid-frequency phonon branches bridging the phonon scattering spectrum.
In material systems involving interfaces, the size effects and transport regimes are equally significant as the material dimensions shrinks into nanoscale. For a material system with a bridged interface, our results show opposite trends of conductivity with the bridging thin film thickness in the harmonic limit and in the weak anharmonic regime, demonstrating that while in the harmonic limit the enhancement of conductance is limited due to fewer available channels, anharmonicity can strongly enhance the thermal conductance of the bridged structure due to added inelastic channels. In addition, we extend the findings to multilayers bridged interfaces -an exponential mass-graded system.
We also propose a method built in Non-Equilibrium Green's Function(NEGF) to illustrate the scattering mechanisms and transport regimes. Along with different scenarios of momentum and phase scattering processes being introduced to study phonon transport, models with the flexibility of adjusting the degree of phase and momentum separately are proposed and applied to superlattices to study the effects of period thickness and total sample length on the thermal conductance. These studies deepen our fundamental understanding of nanoscale thermal transport with long mean free paths, which is helpful and critical in the design of better heat dissipation devices and applications.