E. Duval

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.

Correlation effects on Raman scattering from low‐energy vibrational modes in glasses. II. Experimental results

New experimental results of low‐frequency Raman scattering from different glasses are presented. They are compared to the vibration density of states obtained from inelastic neutron scattering to deduce the light‐vibration coupling coefficient C(ω) as a function of the vibration frequency. It was found that C(ω)∝ω2 at very low frequency ω<20 cm−1 in inorganic glasses (SiO2,B2O3) and that C(ω)∝ω at low frequency in polymer glasses and in inorganic glasses for ω≳20 cm−1. Our experimental results for glasses are interpreted by assuming no correlation at very low‐frequency and only radial correlation at low‐frequency. A full correlation would exist in silica‐aerogels in which the effect of random fluctuations would be negligible.

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.

Dynamical structure of water: Low‐frequency Raman scattering from a disordered network and aggregates

Low‐frequency inelastic light scattering of water is observed from a temperature equal to 80 down to −20 °C in the supercooled regime. For energies higher than 3 cm−1 it is shown that the major part of the light scattering is Raman scattering. A broadband with a maximum at 50 cm−1 is interpreted as scattering from transverse acoustic modes of a disordered network. A weaker scattering which shifts towards the Rayleigh line when the temperature decreases is well explained by Raman scattering from the oscillations of water molecule aggregates which grow when the temperature decreases.

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.

Low-Energy Raman Scattering from Silver Particles in Alkali Halides

Raman spectra of silver particles in NaCl and KI are reported, which show the particles to have spheroidal shape and undergo quadrupolar surface vibrations. From the energies of the observed modes the shape and dimension of the metallic particles are deduced.

Laser spectroscopy of spinel microcrystallites in a Cr3+ doped silicate glass

Abstract Laser spectroscopy of Cr 3+ ions makes it possible to follow the crystallization process in a cordiente glass (52 SiO 2 , 347 Al 2 O 3 , 12.5 MgO and 08 Cr 2 O 3 ) Absorption and fluorescence are interpreted by structural considerations showing the variation of Cr 3+ environment during heat treatment Fluorescence line narrowing is performed at 4 2 K giving information on the detailed crystal structure in MgAl 2 O 4 spinel microcrystallites formed during heat treatment The splitting of the 2 E level is close to 70 cm −1 and the ¦2 D ¦ parameter ranges between 0 95 and 1 35 cm −1 This wide distribution is associated with the well known disordered distribution of Mg 2+ and Al 3+ cations in MgAl 2 O 4 spinels.

Connection between quasielastic Raman scattering and free volume in polymeric glasses and supercooled liquids

Quasielastic light scattering (QLS) in the frequency interval 100–1000 GHz is measured in some polymers: polycarbonate, polybutadiene, polystyrene, and poly(methyl methacrylate). To describe the spectra, a model of the fast picosecond relaxation processes responsible for the QLS, which is based on the damping of the boson peak vibrations by the dynamic hole volume fluctuations, is used. Within the frame of the model, the intensity of the fast relaxation process is proportional to the fractional dynamic hole volume (which above the glass transition temperature Tg is known as the fractional free volume). The hole volumes can be measured using the positron annihilation lifetime spectroscopy (PALS). The comparison of the literature PALS data in the four polymers with the QLS shows an apparent correlation between the relaxation strength and the fractional dynamic hole volume in good agreement with the predictions of the model.

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

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.