Spectroscopy of vibrational modes in metal nanoshells

Journal of Physical Chemistry B, 2000

The wide variety of applications of metal nanoparticles has motivated many studies of their properties. Some important practical issues are how the size, composition and structure of these materials affect their catalytic and optical properties. In this article we review our recent work on the photophysics of metal nanoparticles. The systems that have been investigated include Au particles with sizes ranging from 2 nm diameter (several hundred atoms) to 120 nm diameter, and bimetallic core-shell particles composed of Au, Ag, Pt and/or Pb. These particles, which have a rather narrow size distribution, are prepared by radiolytic techniques. By performing time-resolved laser measurements we have been able to investigate the coupling between the electrons and phonons in the particles, and their low frequency "breathing" modes. These experiments show that for Au the time scale for electron-phonon coupling does not depend on size, in contrast to metals such as Ga and Ag. On the other hand, the frequency of the acoustic breathing modes strongly depends on the size of the particles, as well as their composition. These modes are impulsively excited by the rapid lattice heating that accompanies ultrafast laser excitation. The subsequent coherent nuclear motion modulates the transmitted probe laser intensity, giving a "beat" signal in our experiments. Unlike quantum-beats in molecules or semiconductors, this signal can be completely understood by classical mechanics.

The Journal of Physical Chemistry A, 2000

Acoustic vibration of silver films and nanoparticles in glass, due to impulsive excitation of their fundamental expansion mode, is investigated using a femtosecond pump-probe technique. The vibrational motion is monitored via the induced modulation of the material optical properties which, in the two systems, is shown to reflect that of the real part of the dielectric function. The observed differences in their time responses are attributed to the strong coupling of the nanoparticles with their glass environment. In both systems, the phase and the amplitude of the observed oscillations are in quantitative agreement with an indirect displacive excitation process associated with lattice expansion. Control of the acoustic vibration by two femtosecond pulses is also discussed.

The Journal of Chemical Physics, 1999

The wide variety of applications of metal nanoparticles has motivated many studies of their properties. Some important practical issues are how the size, composition and structure of these materials affect their catalytic and optical properties. In this article we review our recent work on the photophysics of metal nanoparticles. The systems that have been investigated include Au particles with sizes ranging from 2 nm diameter (several hundred atoms) to 120 nm diameter, and bimetallic core-shell particles composed of Au, Ag, Pt and/or Pb. These particles, which have a rather narrow size distribution, are prepared by radiolytic techniques. By performing time-resolved laser measurements we have been able to investigate the coupling between the electrons and phonons in the particles, and their low frequency "breathing" modes. These experiments show that for Au the time scale for electron-phonon coupling does not depend on size, in contrast to metals such as Ga and Ag. On the other hand, the frequency of the acoustic breathing modes strongly depends on the size of the particles, as well as their composition. These modes are impulsively excited by the rapid lattice heating that accompanies ultrafast laser excitation. The subsequent coherent nuclear motion modulates the transmitted probe laser intensity, giving a "beat" signal in our experiments. Unlike quantum-beats in molecules or semiconductors, this signal can be completely understood by classical mechanics.

Surface Science

a b s t r a c t The theory of sound generation and heat transfer in matrix-embedded metal nanoparticles under ultra-short laser irradiation has been developed. The shape and time dependence of acoustic waves generated by a sharp change in the pressure of electrons have been investigated for a single nanoparticle and for the group of nanoparticles located on a 2-D flat matrix plane. The dependence of the electronic temperature and the temperature of the interface between the dielectric matrix and nanoparticle as a function of time has been derived.

Journal of Physical Chemistry B, 2000

The low-frequency acoustic breathing modes in Au-Pb core-shell nanoparticles have been studied by timeresolved spectroscopy. The frequency of the breathing mode was determined for a series of samples with a 47 nm diameter Au core and Pb shells of different thickness. The measured frequencies decrease with increasing Pb content, but not to the extent expected from a classical model for a homogeneous sphere. These results show that the acoustic breathing modes of metallic core-shell particles are significantly perturbed when the two metals have different elastic properties.

Journal of Physical Chemistry C, 2008

The acoustic vibration of single gold nanoprism pairs on a glass substrate has been investigated in the timedomain combining a spatial modulation spectroscopy microscope with a high-sensitivity femtosecond pump-probe setup. Three modes were observed and ascribed to two in-plane and one out-of-plane vibration of the nanoprisms forming the pair, in agreement with a theoretical analysis. The periods of the two former modes with similar nature show weak (about 10%) and well correlated pair to pair fluctuations that can be unambiguously ascribed to variation of the prism geometry. In contrast, strong fluctuations, by almost a factor of 6, of the mode damping are evidenced with no correlation with their period. This indicates large variations of the prism-substrate coupling, providing a unique way for its local investigation.

Surface Science, 2009

The theory of sound generation and heat transfer in matrix-embedded metal nanoparticles under ultrashort laser irradiation has been developed. The shape and time dependence of acoustic waves generated by a sharp change in the pressure of electrons have been investigated for a single nanoparticle and for the group of nanoparticles located on a 2-D flat matrix plane. The dependence of the electronic temperature and the temperature of the interface between the dielectric matrix and nanoparticle as a function of time has been derived. (N.I. Grigorchuk), ptomchuk@iop.kiev.ua (P.M. Tomchuk).

ChemPhysChem, 2009

Nanoparticles of highly conducting metals interact strongly with light at optical and near-infrared frequencies. Single metal nanoparticles, and in particular their aggregates, can significantly enhance local electromagnetic fields, leading to large non-linear optical effects. Strong absorption of light by metal nanoparticles is due to a resonant excitation of the collective motion of free electrons, called a surface plasmon. This is why metal nanoparticles find important technological applications in sub-wavelength optical devices, optical data storage, and in biological labelling and sensing. Due to strong coupling with light, variations in the optical response of single nanoparticles are measurable on timescales as short as 100 fs. This gives unique access to their elastic properties via optical methods when other (mechanical) methods fail.

2000

The lifetimes of the acoustic vibrations of metal nanostructures depend sensitively on the properties of the environment, such as the acoustic impedance and viscosity. In order to accurately study these effects, they have to be separated from the damping processes that are inherent to the nanostructure. Here we show that this can be done experimentally by investigating individual gold nanowires suspended over a trench in air and liquid environments. The experiments were done by ultrafast pumpprobe microscopy, recording transient absorption traces at the same point on the nanowire in both environments. These first experiments were performed with water, and the measured vibrational quality factors due to the presence of water were compared to continuum mechanics calculations for a cylinder in a homogeneous environment. Good agreement was found between the experimental quality factors and the calculated values. The continuum mechanics analysis shows that damping is dominated by the acoustic impedance of the solvent rather than by its viscosity for the nanowires in the present experiments. This experimental technique opens up the possibility of studying the effect of viscosity on the high frequency vibrational motions of nanostructures for a variety of liquids.

The Journal of Physical Chemistry B

The acoustic vibrational modes of Au nanorods with aspect ratios between 2 and 5 have been investigated by time-resolved laser spectroscopy. The results show that laser excitation launches a coherent vibrational motion that has a period that depends linearly on the length of the rod. Due to polydispersity in the samples, the measured period also depends on the probe wavelength (i.e., different probe wavelengths interrogate different length rods). Analysis of the data provides information about the size distribution of the sample and the homogeneous line width of the longitudinal plasmon band of the rods.

Physical Review B, 2011

The acoustic vibration modes of spherical composite nanoparticles formed by a core and an arbitrary number of shells are computed using a semianalytic approach based on elastic theory. The modes observed in time-resolved pump-probe experiments are identified in the case of metal-dielectric-based spherical composite nanoparticles, and results are illustrated in the case of Ag@SiO 2 core-shell nanoparticles. The presence of a light dielectric shell is shown to only weakly shift the frequency of the dominant mode observed in time-resolved experiments. Moreover, a large impact of the mechanical contact between the different materials forming the particle on the vibrational mode frequency and damping is predicted, offering the possibility of experimentally addressing this parameter.

Applied Optics, 2009

We present a physical model that explains several sequential stages of the conversion of optical to acoustical energy when irradiating diluted suspensions of metal nanoparticles with laser pulses. Optical absorption and scattering of a single particle driven by plasmon resonance interactions in an aqueous medium are considered. Thermal effects produced by laser-irradiated nanoparticles, dynamics of vapor bubble formation, and acoustic signals from expanding bubbles formed around heated nanoparticles are calculated. Stochastic features of the pressure magnitude emitted as a result of low-fluence irradiation of suspensions are also discussed. The probabilistic distribution of pressure magnitude from individual bubbles was found to obey Zipf's law for low concentrations of nanoparticles, while increasing their concentration brings the pressure magnitude distribution into conformance with the Gaussian law.

Physical Review B, 2014

An investigation of the vibrational density of states (VDOS) in silver nanocrystals is performed using Raman scattering. A specific sample architecture, setup configuration, and original elaboration process are used in order to take simultaneously advantage of spectrally and spatially localized surface plasmon resonance, optical amplification, and dark-field spectroscopy. Disentangling the contributions of atom vibrations and electron-hole excitations (i.e., the so-called "background" in surface-enhanced Raman scattering) is performed. The extracted VDOS is successfully compared with theoretical ones obtained by atomic scale simulations. The effects of size, strain, and disorder on the VDOS are analyzed; in particular, the strain effect is investigated experimentally using the geometrical phase analysis coupled with high-resolution transmission electron microscopy. This work offers an opportunity to examine thermodynamic properties, like specific heat, at the nanoscale.

Journal of Physics: Conference Series, 2007

We demonstrate and discuss acoustic vibrations in spherical shell particles. The particles consist of 38-nm-thick gold shells, sandwiched between an inner silica core of 456 nm in diameter and an outer silica shell of 10 nm in thickness. Employing standard and asynchronous optical sampling (ASOPS) ultrafast pump-probe experiments, we excite acoustic vibrations of the shells and detect the oscillations via modulations of the optical reflectivity. Two kinds of vibrations were excited and observed. One oscillation with 400 ps period corresponds to the prediction of Lamb theory for breathing modes of free thin gold shells. The other oscillation with 25ps-period corresponds to resonant longitudinal thickness vibrations of the gold shells. At later times, echoes appear corresponding to the travel time through the silica core.

The intention of this paper is to provide a basic understanding of the information low-frequency Raman Spectroscopy (LFRS) may provide when this characterization tool is applied to the nanomaterials like oxide nanoparticles and nanocomposite glassy materials. A short theoretical introduction on the vibrations of elastic spheres will be described for the free nanoparticles as well as for the nanoparticles embedded in matrix where the proper account on the boundary conditions should be applied. The spherical case is well understood -the normal modes of the sphere are divided into torsional and spheroidal and, experimentally, have been measured in various situations. The application of the theory will be illustrated by the LFRS measurements on quasifree SnO2 nanoparticles and TiO 2 and Ge nanoparicles embadded in glass matrix.