Nucleation and kinetics of shock-induced plastic
deformations for group-IV nanoparticles from first-principles molecular
CEMS, University of Minnesota
Shock-induced structural deformations of isolated semiconductor nanoparticles are investigated via first-principles molecular dynamics using a novel computational approach based on the definition of the “quantum volume” of an isolated system and on the construction of an electronic-enthalpy functional . In particular we focus on the nucleation mechanisms and the kinetic pathways leading to the final structure in dependence of material choice, size and surface passivation. We find that Si and bigger Ge nanoparticles undergo an amorphization process that, in hydrogen-covered systems, is nucleated in the inner core of the particle by the formation of closed-packed clusters of atoms of characteristic shape. In non H-passivated Si systems the structural deformation is instead nucleated on the outer shell suggesting that surface reconstruction weakens the sp3 network of bonds that characterizes the diamond structure and is responsible for its stability and hardness. In unpassivated C nanoparticles, however, the surface reconstruction produces a strong sp2 shell (consisting of graphene sheets and fullerene domes) that proves to be even more incompressible than the inner diamond core.
The kinetic trapping of Si and bigger Ge systems in amorphous higher energy configurations highlights the potential for nanostructured impact-absorbing materials. Ongoing research and future extensions to this study to tune pressure thresholds and switching times through material choice, chemical composition or surface passivation are outlined and discussed.
A straightforward methodological extension of the electronic enthalpy approach is also presented that allows to compute the “quantum surface” of an isolated system and to introduce a surface tension term in the electronic functional that can be used to describe the cavitation energy in solvation models .
 M. Cococcioni, F. Mauri, G. Ceder and N. Marzari, Phys. Rev. Lett 94, 145501 (2001)  D. Scherlis et al. Journ. Chem. Phys. 124, 074103 (2006).