Using equilibrium molecular dynamic simulations, we calculate the phonon thermal conductivity of a graphene-like silicon nanosheet called silicene at room temperature. We find that the in-plane thermal conductivity of silicene sheets is about one order of magnitude lower than that of bulk silicon. We further investigate the effects of vacancy defects on thermal conductivity and observe its significant diminution owing to the effect of phonon-defect scattering. Our results show that phonon transport in a silicene sheet is strongly affected by vacancy concentration, vacancy size, and vacancy boundary shape; this could be used to guide defects engineering of the thermal properties of low-dimensional silicon materials.

Using molecular dynamics simulations, we have calculated the thermal conductivity of nitrogen-terminated silicon nanowires (SiNWs). We found that nitrogen adsorption can remarkably bring down the thermal conductivity of SiNWs. This nitrogenation-induced drop in thermal conductivity arises mainly from the phonon scattering by defects near the surface and the suppression of some vibrational modes. Our simulation results clearly demonstrate the importance of surface chemistry or functionalization in tuning the thermal conductivity, which has profound implications for thermoelectric applications of SiNWs.

We investigated the electronic properties and second hyperpolarizabilities of hydrogenated silicon nanoclusters (H-SiNCs) by using the density functional theory method. The effects of cluster size, external electric field and incident frequency on the second hyperpolarizability were also examined, respectively. We found that small H-SiNCs exhibit large second hyperpolarizability. With the increase of the number of silicon atoms in H-SiNCs, the frontier molecular orbital energy gap decreases, attributed to the enhancement of the second hyperpolarizability. Interestingly, we also found the electric-field-induced gigantic enhancement of the second hyperpolarizability for H-SiNCs due to the change of electron density distributions. In addition, our results demonstrate a significant dependence on the frequency of incident light.

The second-order nonlinear optical (NLO) effects are usually forbidden in materials with inversion symmetry, which restricts the application of graphene in NLO technologies such as phase-only modulation, second-harmonic generation, and sum/difference frequency generation. Here, we break the centrosymmetry by applying external uniform electrostatic fields across hexagonal graphene quantum dots (GQDs) and induce a second-order NLO response in the centrosymmetric GQDs. Ab initio quantum chemistry methods were performed to investigate the external electric field effects on the electronic structure and the electronic first hyperpolarizability of the hexagonal GQDs. Under the action of an external electric field, centrosymmetries of the geometric structure and electron density distribution of the hexagonal GQD are both broken, thereby resulting in nonzero electronic first hyperpolarizability. The external electric field induced electronic first hyperpolarizability shows remarkable anisotropy for different directional fields. Particularly, under the electric field of a certain direction, the field dependence of electronic first hyperpolarizability shows a nonmonotonic trend. The electron density redistribution induced by a strong electric field can significantly reduce the frontier orbital energy gap of the quantum dot, thereby evidently enhancing the electronic first hyperpolarizability. This study provides an important theoretical guidance for the experimental realization of an electrically tunable second-order NLO response in GQDs.

Heterostructuring, as a promising route to optimize the physical properties of 2D materials, has attracted great attention from the academic community. In this paper, we investigated the room-temperature in-plane and cross-plane phonon thermal transport in silicene/graphene van der Waals (vdW) heterostructures using molecular dynamics simulations. Our simulation results demonstrated that heat current along the graphene layer is remarkably larger than that along the silicene layer, which suggests that graphene dominates the thermal transport in silicene/graphene heterostructures. The in-plane phonon thermal conductivity of the silicene/graphene heterostructures could be a compromise between monolayer graphene and monolayer silicene. Heterostructuring can remarkably reduce the in-plane thermal conductivity of the graphene layer but increase the in-plane thermal conductivity of the silicene layer in heterobilayers compared with the freestanding monolayer counterparts because of their different structures. We also simulated the interlayer interaction strength effect on the in-plane phonon thermal conductivity and cross-plane interfacial thermal resistance of silicene/graphene heterostructures. Total in-plane phonon thermal conductivity and interfacial thermal resistance both decrease with the increase in the interlayer interaction strength in the silicene/graphene heterobilayers. In addition, the calculated interfacial thermal resistance shows the effect of the thermal transport direction across the interface. This study provides a useful reference for the thermal management regulation of 2D vdW heterostructures.