Effects of morphology on phonons in nanoscopic silver grains
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Publisher:American Physical Society
Citation:Gustavo A. Narvaez, Jeongnim Kim, John W. Wilkins, "Effects of morphology on phonons in nanoscopic silver grains," Physical Review B 72, no. 15 (2005), doi:10.1103/PhysRevB.72.155411
The morphology of nanoscopic Ag grains significantly affects the phonons. Atomistic simulations show that realistic nanograin models display complex vibrational properties. (1) Single-crystalline grains. Nearly pure torsional and radial phonons appear at low frequencies. For low-energy, faceted models, the breathing mode and acoustic gap (lowest frequency) are about 10% lower than predicted by elasticity theory (ET) for a continuum sphere of the same volume. The sharp edges and the atomic lattice split the ET-acoustic-gap quintet into a doublet and triplet. The surface protrusions associated with nearly spherical, high-energy models produce a smaller acoustic gap and a higher vibrational density of states (DOS) at frequencies ν<2 THz. (2) Twined icosahedra. In contrast to the single-crystal case, the inherent strain produce a larger acoustic gap, while the core atoms yield a DOS tail extending beyond the highest frequency of single-crystalline grains. (3) Mark's decahedra, in contrast to (1) and (2), do not have a breathing mode; although twined and strained, do not exhibit a high-frequency tail in the DOS. (4) Irregular nanograins. Grain boundaries and surface disorder yield nondegenerate phonon frequencies, and significantly smaller acoustic gap. Only these nanograins exhibit a low-frequency ν^2 DOS in the interval 1–2 THz.
Rights:©2005 The American Physical Society
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