Lucien Saviot

Size dependence of acoustic and optical vibrational modes of CdSe nanocrystals in glasses

Abstract The size dependence of resonant Raman scattering from acoustic and optical vibrational modes in CdSe nanocrystals (size less than 10 nm) embedded in a glassy matrix has been investigated. In the low-frequency Raman scattering range ( −1 ) a fine structure due to the confinement of acoustic modes is observed. The Raman line corresponding to the interaction with the optical modes (new 210 cm −1 ) shifts and widens with a decreasing of the particle size. These experimental results can be described by a single model based on the size dependence of the eigenvibration modes of spherical particles in a matrix.

Resonant Raman scattering by breathing modes of metal nanoparticles

Low-frequency Raman scattering experiments have been performed on metal nanoparticles embedded in two different thermally treated matrices. In addition to the well-known Raman scattering by the nanoparticle quadrupolar vibrational mode, the spectra measured in the 3–40 cm−1 frequency range exhibit several new bands. They are ascribed to resonant scattering by the nanoparticle breathing mode and its harmonics, in very good agreement with time-resolved measurements.

Resonant Raman scattering by quadrupolar vibrations of Ni-Ag core-shell nanoparticles

Low-frequency Raman-scattering experiments have been performed on thin films consisting of nickel-silver composite nanoparticles embedded in alumina matrix. It is observed that the Raman scattering by the quadrupolar modes, strongly enhanced when the light excitation is resonant with the surface dipolar excitation, is mainly governed by the silver electron contribution to the plasmon excitation. The Raman results are in agreement with a core-shell structure of the nanoparticles, the silver shell being loosely bonded to the nickel core.

Probing atomic ordering and multiple twinning in metal nanocrystals through their vibrations

Control of nanocrystal (NC) crystallinity currently raises great interest because of its potential benefits in both physics modeling and technological applications. Advances in methods for synthesizing perfect single-crystalline NCs are recent, so that the effect of crystallinity on NC properties has received only limited study and still needs to be properly investigated. Here, we report that crystallinity of gold NCs dramatically modifies their vibrations. Using low-frequency Raman scattering, we clearly demonstrate that single-domain NCs vibrate differently than their multiply twinned counterparts, through the splitting of the quadrupolar vibrations, which is only observed for the former. Using the resonant ultrasound approach, we calculate the vibrational frequencies of a gold sphere and show that elastic anisotropy induces a lift of degeneracy of the quadrupolar mode in good agreement with our experimental measurements. These findings open up challenging perspectives on using Raman spectroscopy to characterize nanocrystallinity.

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

Phonons in an inhomogeneous continuum: Vibrations of an embedded nanoparticle

{\mathrm{SiO}}_{2}

Crystallinity Dependence of the Plasmon Resonant Raman Scattering by Anisotropic Gold Nanocrystals

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.

Acoustic vibrations of anisotropic nanoparticles

The spectrum of acoustic vibrational modes of an inhomogeneous elastic continuum is analyzed with application to a spherical nanoparticle embedded in an infinite glass block. The relationship of these modes to the discrete vibrational spectrum of a free sphere is studied. The vibrational modes of a sphere with a fixed surface are relevant in some situations. Comparisons are also made to calculations of mode frequency and damping based on complex-valued frequency.

Structural, optical and electronic properties of hydrogenated polymorphous silicon films deposited at 150°C

Au nanocrystals (NCs) with different crystalline structures and related morphologies are unselectively synthesized using an organometallic route. The acoustic vibrations of these NCs are studied by plasmon mediated low-frequency Raman scattering (LFRS). A splitting of the quadrupolar vibration mode is pointed out in the LFRS spectrum. Comparison of the measured frequencies with calculations and careful examination of the NCs morphologies by transmission electron microscopy ascertain this splitting as being an effect of crystallinity. The excitation dependence of the LFRS spectra is interpreted by the shape-selection of the NCs via plasmon-vibration coupling. These results give new insights into the crystallinity influence on both the vibrations of the NCs and their coupling with plasmons and demonstrate the relevance of elastic anisotropy in monodomain NCs.

Nanovectorization of TRAIL with single wall carbon nanotubes enhances tumor cell killing

Acoustic vibrations of nanoparticles made of materials with anisotropic elasticity and nanoparticles with nonspherical 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.