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Silicene is emerging as a graphene like two-dimensional silicon material with very attractive electronic and thermal properties for a wide range of applications in nanoelectronics and thermoelectrics, and thus has attracted increasing research attention in recent years. In this chapter, we briefly review the theoretical research progress in the phonon thermal transport of pristine silicene sheet and silicene nanoribbons (SNRs). We then review our recent study on the defect effects of phonon thermal conductivity in silicene and SNRs and reveal the corresponding underlying physical mechanisms. First, the effect of vacancy defects induces significant diminution in thermal conductivity, which is due to remarkable phonon-defect scattering. In addition, thermal transport in a silicene sheet is strongly affected by concentration, size, and boundary shape of the vacancy. Second, isotope doping generates mass disorder in the lattice that results in increased phonon scattering, thereby reducing thermal conductivity. The phonon thermal conductivity of isotopically doped SNRs is dependent on the concentration and arrangement pattern of dopants. A maximum reduction of about 15% is obtained at the 50% random isotopic doping with 30Si. However, ordered doping (i.e., isotope superlattice) leads to a much larger decrease in thermal conductivity than random doping for the same doping concentration. This study highlights the importance of vacancy and isotopic doping in tuning the thermal properties of silicene, thus guiding defects engineering of thermal properties of two-dimensional silicon materials.

Understanding thermal transport in silicon nanowires (SiNWs) has practical and academic importance given the modern electronic and thermoelectric applications of these materials. Adjustment of the phonon thermal conductivity of SiNWs through surface effects has become a research hotspot in recent years. In this chapter, we briefly review the recent progress made in the investigation of phonon thermal transport properties in one-dimensional SiNWs through experiments and theoretical calculations. We emphasize the surface effects of tunable phonon thermal conductivity, including surface roughness, surface functionalization, surface/shell doping, surface disorder, and surface softening. This chapter concludes that surface engineering methods are effective for tuning nanoscale thermal transport and may foster further advancements in this field.

Silicon nanoclusters/nanoparticles have attracted increasing attention in innovative technology due to their unique properties, which differ from those of bulk materials. Structural and thermodynamic properties of nanoclusters are fundamentally important for the performance and stability of nanocluster-based devices. Unlike the homogeneous melting of bulk silicon, the melting of crystalline silicon nanospheres proceeds over a finite temperature range due to surface effects, which shows the heterogeneous melting of nanoclusters. The melting temperature of silicon nanoclusters is lower than that of bulk silicon and rises with the increase of the cluster size. Structure changes upon heating indicates that the melting of silicon nanospheres is progressively developed from the surface and into the core. The structure of the spherical silicon nanocluster can change gradually from the bulk diamond structure to a non-diamond structure with the decrease in the cluster size. Hydrogenated silicon nanoclusters are thermally stable if the hydrogen coverage is more than 50%. The thermal conductivity of the spherical silicon nanoclusters shows a size-dependent effect arising from the remarkable phonon-boundary scattering and can be about three orders of magnitude lower than that of bulk silicon. The thermal conductivity of crystalline nanospheres also decreases as the temperature increases from 50 to 1000 K because of the stronger phonon-phonon scattering at higher temperatures.

Phonon thermal transport in low-dimensional materials has recently attracted considerable attention because of their potential applications in energy harvesting and generation and thermal management. Significant progress has been achieved for one-dimensional and two-dimensional nanostructures, both theoretically and experimentally. This chapter reviews the advances in the theoretical and experimental methods for determining the thermal conductivity of nanostructures in the literature. We outlined the bases of theoretical approaches and experimental techniques for determining nanoscale thermal conductivity. We then discussed the problems and challenges of each method and provided concluding remarks.

We investigate the size effects on the structures and thermal conductivity of silicon nanoclusters (SiNCs) using molecular dynamics simulations. We demonstrate that as the diameter of the SiNCs increases from 1.80 nm to 3.46 nm, the cluster structure changes from an amorphous state to a crystalline state at 300 K, which is in good agreement with the experimental findings. Our calculated thermal conductivity of the SiNCs shows a size-dependent effect due to the remarkable phonon-boundary scattering and can be about three orders of magnitude lower than that of bulk Si.