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Paper   IPM / P / 9560
School of Physics
  Title:   Strain-Induced Instability of Spherical Nanodiamond Hydrocarbons: Effect of Surface CHn
1.  M . Heidari Saani
2.  T. Ghodselahi
3.  K. Esfarjani
  Status:   Published
  Journal: Phys. Rev. B
  Vol.:  79
  Year:  2009
  Pages:   125429
  Supported by:  IPM
The structural stability and vibrational properties of different sizes and charge states of the hydrogen saturated spherical and cagelike C nanoparticles were investigated by means of ab initio density-functional theory. The cagelike nanoparticles remain stable and stiff under structural relaxation, whereas specific sizes and charge states of spherical C nanoparticles deform dramatically and dissociate under the relaxation. In order to see the possibility of observation this effect in other semiconductor nanoparticles, two smallest sizes of spherical Si nanoparticles were also examined and similar instabilities were obtained. We explain the induced large amount of positive strain of unstable spherical C and Si nanoparticles by the presence of CH3 and SiH3 terminations on the surface. These terminations result in a very packed arrangement of polar C-H bonds and consequently a large value of Coulombic potential on the surface and corresponding strain in the volume of the clusters, respectively. We demonstrate that the variation in formation and electronic properties with size correlate with the variation in CH/CH2+CH3 ratio of the surface with size in agreement with the recently reported empirical correlation suggested by x-ray absorption spectroscopy measurements. Likewise, we discuss the reported empirical correlation between H coverage and strain in porous Si and some other corresponding experimental evidences in porous Si. The calculation of the vibrational properties supports the reported structural instabilities.

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