**5. Concluding remarks and perspectives**

and the in-plane modes. This mechanism is further linked to the different atomic structure, i.e., for graphene, it is planar (no buckling distance), while silicene has a buckling distance of about 0.42 Å. By performing phonon polarization and spectral energy density (SED) analysis, the authors further revealed the underlying physics of the novel phenomenon in terms of the different impacts on the dominant phonons in the thermal transport of silicene induced by the substrate. These results indicate that by choosing different substrates, the thermal conductivity of 2D silicene can be largely tuned, which paves the way for manipulating the thermal

Very recently, the thermal conductivity of silicene supported in an amorphous silicon dioxide (SiO2) for temperature ranging from 300 to 900 K was studied by Wang et al. from MD simulations [69]. They found that the thermal conductivity of silicene has a substantial reduction with increasing temperature, and putting silicene on amorphous SiO2 leads to 78% reduction in the overall thermal conductivity of silicene at room temperature. They further compared model-level phonon properties, such as phonon relaxation times and phonon mean free paths (MFPs) of freestanding and supported silicene at 300 K. It is found that the phonon relaxation time in the case of supported silicene is reduced from 1–13 ps to 1 ps, and corre‐ sponding MFPs decrease from 10–120 nm to 0–20 nm. The thermal conductivities of free‐ standing and supported silicene are mainly (more than 85%) contributed by the longitudinal and transverse acoustic phonons, while the out-of-plane acoustic phonons have a negligible contribution of less than 3%. These results are in line with those found previously [68].

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

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

bonding density on the thermal conductivity of bilayer graphene,

transport properties of silicene for future emerging applications.

210 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

**4. Heterostructures**

the effect of interlayer *sp*<sup>3</sup>

The present chapter has surveyed the recent advancements in understanding the phonon transport properties of some representative two-dimensional materials, which are one of the fastest growing emerging fields in nanostructured semiconductors and nanocrystals. The thriving expansion of new capabilities of two-dimensional semiconductors has progressed rapidly during the last few years. While physical fundamentals for electronic properties of this class of nanomaterials have been well understood and explored extensively, limited under‐ standing has been achieved in thermal transport properties. Although most of two-dimen‐ sional materials have as simple as honeycomb lattice structure, revealing the phonon transport mechanism in such atomic thin materials seems not an easy task. Phonon transport in graphene is the first success in this line. However, the previous understanding achieved for graphene cannot be straightforwardly extended to other similar two-dimensional materials. In this chapter, we demonstrated a comparative study of the phonon transport properties between graphene, silicene, and phosphorene and found that these three materials could have funda‐ mentally different governing mechanism in heat conduction. We also analyze the detailed mechanism from different aspects.

Despite the significant accomplishment that has been gained in understanding different behavior of phonon transport in two-dimensional materials in the last few years, some important physical fundamentals still remain to be clarified. For example, the thicknessdependent thermal conductivity of few-layer two-dimensional structures and phonon interactions between two-dimensional monolayer and substrate demand further systematic study. In addition, the two-dimensional materials-based heterostructures could possess even more diverse and fantastic phonon transport behavior, which could open up new building blocks for the next generation of advanced functional 3D devices.
