We present a molecular dynamics investigation on the thermal conductivity of silicon-doped graphene and the resulting change in phonon properties. A significant reduction in the thermal conductivity is observed in the presence of silicon impurity even at a small concentration of silicon atoms. Conductivity values continued to decrease with an increase in silicon concentration. The increase in the scattering rate, which is measured by the reduction or broadening of the peaks of the van Hove singularities, is the most significant factor contributing to the large conductivity reduction. An analysis with scattering time models shows that the mass displaced by the silicon impurity plays a significant role in reducing the conductivity, especially at a moderate concentration. The nonmass effect, which comes from the change of the sp 2 C-C bonds to the sp 3 Si-C bonds, is less strong or comparable with the mass change effect. For high impurity concentrations, the shape of the graphene is severely distorted and the irregularity of the ripples increases, which could contribute to the reduction in conductivity.

The ability to tailor the thermal conductivity of graphene by introducing crystalline defects has attracted considerable research attention. In this study, nonequilibrium molecular dynamics calculation is used to investigate the effect of crystalline defects on the thermal conductivity of 2D graphene. The defects considered include substitutional nitrogen and silicon, pure structural single vacancy and Stone-Wales defects, and structurally different pyridinic nitrogen. In particular, this study focuses on the unique phonon scattering behaviors arising from the low dimensionality of graphene. The results reveal that the low dimensionality of graphene has a negligible effect on phonon scattering in substitutionally defected graphene, for which the Klemens scattering model is accurate without the need for any corrections. The substitutional silicon defect leads to more effective reduction of the thermal conductivity than the structural defects because of the effect of change in the hybridization and the mass on the scattering. Almost equal reductions are observed for the two structural defects, the scattering strengths of which are significantly weakened by the two dimensionality of graphene. Callaway analysis of the vacancy scattering reveals that even with the perturbation of the vacancy, the 2D honeycomb structure preserves considerable phonon stability compared with a 3D material. In addition, the absence of mass deficiency for the Stone-Wales defect suggests that the contribution of mass deficiency is minimized for structural defects of graphene. Finally, opposite to the findings for the substitutional nitrogen defect, the introduction of pyridinic nitrogen leads to further reduction of the thermal conductivity compared with that for a single vacancy defect.

In the molecular dynamics study of short-pulsed laser processing of semiconductors, potential models capable of describing the atomistic behavior during high electronic excitations is the most critical issue at the current stage. This study of the molecular dynamics adopts the Tersoff-potential model to analyze the ultrafast laser processing of silicon. The model was modified to include electronic excitation effects by reducing the attraction of the antibonding state by half. It offers an excellent description of the experimental behavior during nonthermal melting. Subpicosecond melting is achieved above certain threshold levels of superheating and carrier density as required in experiments. Energy conservation is demonstrated with a bandgap energy of the order obtained in experiments. The modification of the potential mimics an absorption of bandgap energy and a subsequent lattice heating on a time scale within 0.3 ps. The melting kinetics establishes a correlation between nonthermal melting and thermal bulk melting. For superheating of less than two, the electronic melting of bond softening proceeds via homogeneous nucleation. The associated thermal theory, corrected with a limit on the nucleus radius to bond length, is still valid for the higher superheating regime. The original Tersoff model shows that this superheating by a factor of two is isothermal for spallation—the lowest-energy ablative mechanism. Its proximity to the evaporating point suggests the role of thermal boiling during spallation.

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Although many researches using nonthermal and thermal models have been conducted on ultrashort pulse laser processing, a promising tool for precision fabrication, an understanding of the underlying mechanism is still needed. The present study, in which molecular dynamics was carried out, accepted a thermal aspect as Tersoff model for silicon does not consider the nonthermal effect due to the electron-hole density. The kinetics of melting from the molecular motions was investigated by Voronoi polyhedron (VP) analysis as a structural tool. The results have shown that the ultrafast melting by the laser with 100 fs pulse is governed by bulk melting homogeneous nucleation. The bulk melting characteristics were revealed in the absence of liquid-solid interface, a melting speed excessively higher than the speed of sound, and kinetics consistent with that of the thermal model on homogeneous nucleation.