**4. Heterostructures**

In electronics, especially for nanoelectronics, interfacial thermal resistance is a key factor that affects heat dissipation in devices and researchers have shown that the interfacial thermal transport can be largely enhanced using graphene-based nanocomposites as thermal interface materials. Graphene and its bilayer structure are the two-dimensional crystalline forms of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. Their realistic applications in emerging nanoelec‐ tronics usually call for thermal transport manipulation in a controllable and precise manner. Equilibrium molecular dynamics simulations were performed by Zhang et al. to investigate the effect of interlayer *sp*<sup>3</sup> bonding density on the thermal conductivity of bilayer graphene, especially the results of the randomly and regularly bonded bilayer graphene structures were compared in detail [70]. The thermal conductivity of randomly bonded bilayer graphene decreases monotonically with the increase of interlayer bonding density, which follows the same trend as that obtained in the literature. However, for the regularly bonded bilayer graphene structure, They observed the unexpected non-monotonic interlayer bonding density dependence of thermal conductivity. The phonon spectral energy density, participation ratio, and mode weight factor analyses were performed to explore the underlying mechanism of this counterintuitive phenomenon. It is found that the lifetimes of low-frequency (<5 THz) phonons for randomly and regularly bonded bilayer graphene are nearly the same, which is consistent with the general knowledge that the low-frequency phonons are not sensitive to the detailed interfacial bonding configuration, considering that the characteristic size of the covalent bonding is in the order of only a few angstroms, which is much shorter than the typical wavelength of the low frequency phonons. In contrast, for medium-frequency (5–30 THz) and high-frequency (30–50 THz) phonons, the lifetimes in the regularly bonded bilayer graphene are quite larger than that of randomly bonded structure, especially for the medium-frequency phonon modes which have a rather large contribution to the higher thermal conductivity of the regularly bonded bilayer graphene than the randomly bonded structure. By evaluating the relative importance of the contributions of phonon lifetime and square of phonon group velocity, we conclude that compared with random bonding, the much higher thermal con‐ ductivity of the regularly bonded bilayer graphene is mainly due to the large enhancement in the group velocity of the phonon modes in the medium-frequency range (5–30 THz). In addition, the thermal conductivity of bilayer graphene depends not only on the interlayer bonding density, but also the detailed topological arrangement of the interlayer *sp*<sup>3</sup> bonds. Comparing the regularly bonded bilayer graphene between different bonding densities, the change of phonon lifetime, instead of the change of group velocity, is the main reason for the non-monotonic thermal conductivity dependence. The phonon mode localization is charac‐ terized by calculating the participation ratio and provides evidence that the participation ratio at different bonding density resembles the non-monotonic dependent thermal conductivity. Again this can be correlated with their atomic structure, where the regular bonding density of 6.25% and 12.5% has more homogeneous *sp*<sup>3</sup> bonding than that of other bonding densities, which should be responsible for their different phonon band structure.
