40 research outputs found
Simple model for the vibrations of embedded elastically cubic nanocrystals
The purpose of this work is to calculate the vibrational modes of an
elastically anisotropic sphere embedded in an isotropic matrix. This has
important application to understanding the spectra of low-frequency Raman
scattering from nanoparticles embedded in a glass matrix. First some low
frequency vibrational modes of a free cubically elastic sphere are found to be
nearly independent of one combination of elastic constants. This is then
exploited to obtain an isotropic approximation for these modes which enables to
take into account the surrounding isotropic matrix. This method is then used to
quantatively explain recent spectra of gold and copper nanocrystals in glasses.Comment: 6 pages, 5 figure
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
Correlated evolution of structure and mechanical loss of a sputtered silica film
Energy dissipation in amorphous coatings severely affects high-precision
optical and quantum transducers. In order to isolate the source of coating
loss, we performed an extensive study of Raman scattering and mechanical loss
of a thermally-treated sputtered silica coating. Our results show that loss is
correlated with the population of three-membered rings of Si-O tetrahedral
units, and support the evidence that thermal treatment reduces the density of
metastable states separated by a characteristic energy of about 0.5 eV, in
favour of an increase of the states separated by smaller activation energies.
Finally, we conclude that three-fold rings are involved in the relaxation
mechanisms only if they belong to more complex chain-like structures of 10 to
100 tetrahedra.Comment: 5 pages, 3 figure
Inelastic neutron scattering due to acoustic vibrations confined in nanoparticles: theory and experiment
The inelastic scattering of neutrons by nanoparticles due to acoustic
vibrational modes (energy below 10 meV) confined in nanoparticles is calculated
using the Zemach-Glauber formalism. Such vibrational modes are commonly
observed by light scattering techniques (Brillouin or low-frequency Raman
scattering). We also report high resolution inelastic neutron scattering
measurements for anatase TiO2 nanoparticles in a loose powder. Factors enabling
the observation of such vibrations are discussed. These include a narrow
nanoparticle size distribution which minimizes inhomogeneous broadening of the
spectrum and the presence of hydrogen atoms oscillating with the nanoparticle
surfaces which enhances the number of scattered neutrons.Comment: 3 figures, 1 tabl
Long-lived submicrometric bubbles in very diluted alkali halide water solutions
Solutions of LiCl and of NaCl in ultrapure water were studied through
Rayleigh/Brillouin scattering as a function of the concentration (molarity, M)
of dissolved salt from 0.2M to extremely low concentration (2.10^-17 M ). The
Landau-Placzek ratio, R/B, of the Rayleigh scattering intensity over the total
Brillouin, was measured thanks to the dynamically controlled stability of the
used Fabry-Perot interferometer. It was observed that the R/B ratio follows two
stages as a function of increasing dilution rate: after a strong decrease
between 0.2M and 2.10^-5 M, it increases to reach a maximum between 10^-9 M and
10^-16 M. The first stage corresponds to the decrease of the Rayleigh
scattering by the ion concentration fluctuations with the decrease of salt
concentration. The second stage, at lower concentrations, is consistent with
the increase of the Rayleigh scattering by long-lived sub-microscopic bubbles
with the decrease of ion concentration. The origin of these sub-microscopic
bubbles is the shaking of the solutions which was carried out after each
centesimal dilution. The very long lifetime of the sub-microscopic bubbles and
the effects of aging originate in the electric charge of bubbles. The increase
of R/B with the decrease of the low salt concentration corresponds to the
increase of the sub-microscopic bubble size with the decrease of concentration,
that is imposed by the bubble stability due to the covering of the surface
bubble by negative ions.Comment: 6 figures. To be published in Physical Chemistry and Chemical Physics
(PCCP
Modes de vibration de nano-objets (des nanoparticules métalliques aux virus biologiques)
Comparée à leur structure généralement très bien caractérisée, la dynamique collective des nano-objets est assez mal connue. En une vingtaine d'années, la diffusion inélastique de lumière est devenue un outil puissant dans l'étude de la dynamique des nano-objets. Ce travail traite de la transposition de cette technique habituellement appliquée à l'étude des modes propres de vibration de nanoparticules solides à la recherche de modes similaires chez leur équivalent biologique, les virus. Nous revenons dans un premier temps sur les expériences Brillouin (ou Raman basse fréquence) sur des verres dopés à l'or et discutons de l'utilité du tandem Fabry-Pérot en particulier par l'apport de la haute résolution dans la détection des levées de dégénérescence associées à l'anisotropie des paramètres élastiques des nanoparticules d'or facettées. La deuxième partie de ce travail est consacrée à la recherche de modes de vibration d'ensemble sur des virus à morphologie sphérique à travers l'étude de différents types d'échantillons : solutions, couches bidimensionnelles et cristaux tridimensionnels de virus. Cette partie rend compte d'une première détermination des constantes élastiques globales d'un cristal de phytovirus. Alors que pour les virus de petite taille (15-30nm) aucune preuve indiscutable de l'existence d'un mode de vibration de virus n'a pu être trouvée, pour un virus de plus grande taille (environ 200nm), un signal s'apparentant à un mode de vibration de virus a été observéCompared to their structure which is in general very well characterized, the collective dynamics of nano-objects are quite poorly documented. Over the last two decades, inelastic light scattering has become a powerful tool in studying the dynamics of nano-objects. This work is about the application of this technique usually dedicated to the study of the vibration eigenmodes of solid nanoparticles (metallic clusters, silica nanospheres) to the investigation of similar modes in their biological counterparts, i. e. viruses. The first part of this work deals with the Brillouin (or low-frequency Raman) experiments on gold nanoparticles embedded in glasses. The advantage of using the Tandem Fabry-Perot Interferometer is then discussed : thanks to the high resolution of this setup, it is possible to observe the lift of degeneracy associated with the elastic anisotropy of the faceted gold nanoparticles. In the second part of this work we try to investigate the overall vibration modes of sphere-like viruses through the study of different kinds of samples : solution, two-dimensional layers and three-dimensional virus crystals. The search for this modes leads to a first estimate of the elastic constants of a phytovirus crystal. For small viruses (15-30nm), no firm evidence of eigenmodes is found, while for a bigger virus (200 nm) we observe a signal which could be related to a vibration mode of the virusLYON1-BU.Sciences (692662101) / SudocSudocFranceF
Boson peak and hybridization of acoustic modes with vibrations of nanometric heterogeneities in glasses
9 pagesInternational audienceThe low-frequency dynamics in glasses is compared with that in icosahedral quasicrystals. For both arrangements of matter, the existence of nanometric heterogeneities, implying the existence of a nanometric inhomogeneous elastic network, is expected to play a crucial role. Thanks to this comparison, mostly based on inelastic x-ray (neutron) scattering data, it is proposed that the excess of vibrational density of states observed in both materials is due to the hybridization of longitudinal and transverse acoustic modes with modes localized around the heterogeneities
Impact of nanocrystallinity segregation on the growth and morphology of nanocrystal superlattices
International audienceA colloidal solution of 5 nm Au tetradecanethiol-coated nanoparticles is synthesized. After fast evaporation of one drop, ordered monolayers both composed of single domain and polycrystalline nanocrystals are obtained. On increasing the amount of materials and the evaporation time, nanocrystal films with irregular outlines are produced together with close-packed 3D superlattices exhibiting a truncated-tetrahedral shape. Using low-frequency micro-Raman scattering spectroscopy and electron microscopy the building block nanocrystallinity is characterized. Spontaneous nanocrystallinity segregation is revealed: the truncated-tetrahedral supracrystals are shown to mainly contain single domain building blocks while the supracrystalline films are composed of a mixture of single domain and polycrystalline nanocrystals. This observation points out the correlation between the nanocrystallinity segregation involved in the growth of the nanocrystal superlattices and their morphology