85 research outputs found
Acoustic modes in metallic nanoparticles: atomistic versus elasticity modeling
The validity of the linear elasticity theory is examined at the nanometer
scale by investigating the vibrational properties of silver and gold
nanoparticles whose diameters range from about 1.5 to 4 nm. Comparing the
vibration modes calculated by elasticity theory and atomistic simulation based
on the Embedded Atom Method, we first show that the anisotropy of the stiffness
tensor in elastic calculation is essential to ensure a good agreement between
elastic and atomistic models. Second, we illustrate the reduction of the number
of vibration modes due to the diminution of the number of atoms when reducing
the nanoparticles size. Finally, we exhibit a breakdown of the
frequency-spectra scaling of the vibration modes and attribute it to surface
effects. Some critical sizes under which such effects are expected, depending
on the material and the considered vibration modes are given.Comment: Accepted to Phys. Rev.
Vibrations of weakly-coupled nanoparticles
The vibrations of a coupled pair of isotropic silver spheres are investigated
and compared with the vibrations of the single isolated spheres. Situations of
both strong coupling and also weak coupling are investigated using continuum
elasticity and perturbation theory. The numerical calculation of the eigenmodes
of such dimers is augmented with a symmetry analysis. This checks the
convergence and applicability of the numerical method and shows how the
eigenmodes of the dimer are constructed from those of the isolated spheres. The
frequencies of the lowest frequency vibrations of such dimers are shown to be
very sensitive to the strength of the coupling between the spheres. Some of
these modes can be detected by inelastic light scattering and time-resolved
optical measurements which provides a convenient way to study the nature of the
mechanical coupling in dimers of micro and nanoparticles.Comment: expanded version, 8 pages, 5 figures, 2 table
Acoustic vibrations of anisotropic nanoparticles
Acoustic vibrations of nanoparticles made of materials with anisotropic
elasticity and nanoparticles with non-spherical shapes are theoretically
investigated using a homogeneous continuum model. Cubic, hexagonal and
tetragonal symmetries of the elasticity are discussed, as are spheroidal,
cuboctahedral and truncated cuboctahedral shapes. Tools are described to
classify the different vibrations and for example help identify the modes
having a significant low-frequency Raman scattering cross-section. Continuous
evolutions of the modes starting from those of an isotropic sphere coupled with
the determination of the irreducible representation of the branches permit some
qualitative statements to be made about the nature of various modes. For
spherical nanoparticles, a more accurate picture is obtained through
projections onto the vibrations of an isotropic sphere.Comment: 11 pages, 9 tables, 6 figure
Poisson ratio and excess low-frequency vibrational states in glasses
In glass, starting from a dependence of the Angell's fragility on the Poisson
ratio [V. N. Novikov and A. P. Sokolov, Nature 431, 961 (2004)], and a
dependence of the Poisson ratio on the atomic packing density [G. N. Greaves et
al., Nat. Mater. 10, 823 (2011)], we propose that the heterogeneities are
predominantly density fluctuations in strong glasses (lower Poisson ratio) and
shear elasticity fluctuations in fragile glasses (higher Poisson ratio).
Because the excess of low-frequency vibration modes in comparison with the
Debye regime (boson peak) is strongly connected to these fluctuations, we
propose that they are breathing-like (with change of volume) in strong glasses
and shear-like (without change of volume) in fragile glasses. As a
verification, it is confirmed that the excess modes in the strong silica glass
are predominantly breathing-like. Moreover, it is shown that the excess
breathing-like modes in a strong polymeric glass are replaced by shear-like
modes under hydrostatic pressure as the glass becomes more compact
Vibrations of free and embedded anisotropic elastic spheres: Application to low-frequency Raman scattering of silicon nanoparticles in silica
Vibrational mode frequencies and damping are calculated for an elastic sphere
embedded in an infinite, homogeneous, isotropic elastic medium. Anisotropic
elasticity of the sphere significantly shifts the frequencies in comparison to
simplified calculations that assume isotropy. New low frequency Raman light
scattering data are presented for silicon spheres grown in a SiO2 glass matrix.
Principal features of the Raman spectrum are not correctly described by a
simple model of the nanoparticle as a free, isotropic sphere, but require both
matrix effects and the anisotropy of the silicon to be taken into account.
Libration, not vibration, is the dominant mechanism
Comment on "Estimate of the vibrational frequencies of spherical virus particles"
This comment corrects some errors which appeared in the calculation of an
elastic sphere eigenenergies. As a result, the symmetry of the mode having the
lowest frequency is changed. Also a direction for calculating the damping of
these modes for embedded elastic spheres is given.Comment: comment L. H. Ford Phys. Rev. E 67 (2003) 05192
Far infrared absorption by acoustic phonons in titanium dioxide nanopowders
We report spectral features of far infrared electromagnetic radiation
absorption in anatase TiO2 nanopowders which we attribute to absorption by
acoustic phonon modes of nanoparticles. The frequency of peak excess absorption
above the background level corresponds to the predicted frequency of the
dipolar acoustic phonon from continuum elastic theory. The intensity of the
absorption cannot be accounted for in a continuum elastic dielectric
description of the nanoparticle material. Quantum mechanical scale dependent
effects must be considered. The absorption cross section is estimated from a
simple mechanical phenomenological model. The results are in plausible
agreement with the absorption being due to a sparse layer of charge on the
nanoparticle surface.Comment: 8 pages, 5 figures, submitted to Journal of Nanoelectronics and
Optoelectronic
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