Hot Electron Cooling in Silicon Nanoclusters via Landau-Zener Nonadiabatic Molecular Dynamics: Size Dependence and Role of Surface Termination.

We develop a new express methodology for modeling excited-state dynamics occurring in dense manifolds of electronic states in atomistic systems. The approach leverages a modified Landau-Zener formula, the neglect of a back-reaction approximation, and the highly efficient density functional tight-binding method. We study the hot electron dynamics in a series of H- and F-terminated silicon nanocrystals (NCs) containing up to several hundred atoms. We explain the slower electron cooling dynamics in F-terminated NCs by the larger energy gaps between the adjacent electronic states in these systems as well as their slower fluctuations. We conclude that both the mass and chemical identity of the surface termination groups equally influence the electron dynamics, on average. However, the mass effect becomes dominant for higher-energy excitations. We find that the electron decay dynamics in F-terminated NCs has a greater sensitivity to the mass of the surface ligands than do the H-terminated NCs and explain this observation by the details of the electron-phonon coupling in the systems. We find that in the H-terminated NCs, electronic transitions in the cooling process occur predominantly between the surface states, whereas in F-terminated Si NCs, both surface and NC core states are coupled to the nuclear vibrations. We find that electron energy relaxation is accelerated in larger NCs and attribute this effect to the higher densities of states and smaller energy gaps in these systems.