Significant progress has been made in the past two decades about the micro/nanoscale heat conduction. Many computational methods have been developed to accommodate the needs to investigate new physical phenomena at micro/nanoscale and support the applications like microelectronics and thermoelectric materials. In this review, we first provide an introduction of state-of-the-art computational methods for micro/nanoscale conduction research. Then the physical origin of size effects in thermal transport is presented. The relationship between the different methods and their classification are discussed. In the subsequent sections, four commonly used simulation methods, including first-principles Boltzmann transport equation, molecular dynamics, non-equilibrium Green's function, and numerical solution of phonon Boltzmann transport equation will be reviewed in details. The hybrid method and coupling scheme for multiscale heat transfer simulation are also briefly discussed.

Electron-phonon (e-p) interaction and transport are important for laser-matter interactions, hot-electron relaxation, and metal-nonmetal interfacial thermal transport. A widely used approach is the two-temperature model (TTM), where e-p coupling is treated with a gray approach with a lumped coupling factor G ep and the assumption that all phonons are in local thermal equilibrium. However, in many applications, different phonon branches can be driven into strong nonequilibrium due to selective e-p coupling, and a TTM analysis can lead to misleading or wrong results. Here, we extend the original TTM into a general multitemperature model (MTM), by using phonon branch-resolved e-p coupling factors and assigning a separate temperature for each phonon branch. The steady-state thermal transport and transient hot electron relaxation processes in constant and pulse laser-irradiated single-layer graphene (SLG) are investigated using our MTM respectively. Results show that different phonon branches are in strong nonequilibrium, with the largest temperature rise being more than six times larger than the smallest one. A comparison with TTM reveals that under steady state, MTM predicts 50% and 80% higher temperature rises for electrons and phonons respectively, due to the "hot phonon bottleneck" effect. Further analysis shows that MTM will increase the predicted thermal conductivity of SLG by 67% and its hot electron relaxation time by 60 times. We expect that our MTM will prove advantageous over TTM and gain use among experimentalists and engineers to predict or explain a wide ranges of processes involving laser-matter interactions.

Graphene has attracted enormous attention due to its extraordinary physical properties, which have potential for increasing the thermal conductivity of nanocomposites or nanofluids, and the thermal resistance between graphene and the surrounding matrices arises as an important issue. In this paper, the thermal transport at the graphene-water interface is investigated by molecular dynamics simulations. The interfacial thermal resistance decreases with the graphene layer number. Interlayer functionalization by oxygen atoms is applied to tune the interfacial thermal resistance. A peak thermal resistance reduction of nearly 50% is generated with the oxygen ratio of only 0.5% for two-layer graphene. Based on the analyses of vibrational density of states, it is found that lower thermal resistance is consistent with more vibrational density of states overlaps at the interface. Our results are instructive for improving the interfacial thermal transport in graphene-based nanocomposites and nanofluids.

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