Resonant Raman scattering by breathing modes of metal nanoparticles

Sensitivity of Localized Surface Plasmon Resonance and Acoustic Vibrations to Edge Rounding in Silver Nanocubes

Precise knowledge of the dependence of nano-object properties on their structural characteristics such as their size, shape, composition, or crystallinity, in turn, enables them to be finely characterized using appropriate techniques. Spectrophotometry and inelastic light scattering spectroscopy are noninvasive techniques that are proving highly robust and efficient for characterizing the optical response and vibrational properties of metal nano-objects. Here, we investigate the optical and vibrational properties of monodomain silver nanocubes synthesized by the chemical route, with edge length ranging from around 20 to 58 nm. The synthesized nanocrystals are not perfectly cubic and exhibit rounded edges and corners. This rounding was quantitatively taken into account by assimilating the shape of the nanocubes to superellipsoids. The effect of rounding on their optical response was clearly evidenced by localized surface plasmon resonance spectroscopy and supported by calculations based on the discrete dipole approximation method. The study of their acoustic vibrations by high-resolution low-frequency Raman scattering revealed a substructure of the T2g band, which was analyzed as a function of rounding. The measured frequencies are consistent with the existence of an anticrossing pattern of the two T2g branches. Such an avoided crossing in the T2g modes is clearly evidenced by calculating the vibrational frequencies of silver nanocubes using the Rayleigh-Ritz variational method that accounts for both their real size, shape, and cubic elasticity. These results show that it is possible to assess the rounding of nanocubes, including by means of ensemble spectroscopic measurements on well-calibrated particles.

Optical measurement of the picosecond fluid mechanics in simple liquids generated by vibrating nanoparticles: a review

Standard continuum assumptions commonly used to describe the fluid mechanics of simple liquids have the potential to break down when considering flows at the nanometer scale. Two common assumptions for simple molecular liquids are that (1) they exhibit a Newtonian response, where the viscosity uniquely specifies the linear relationship between the stress and strain rate, and (2) the liquid moves in tandem with the solid at any solid-liquid interface, known as the no-slip condition. However, even simple molecular liquids can exhibit a non-Newtonian, viscoelastic response at the picosecond time scales that are characteristic of the motion of many nanoscale objects; this viscoelasticity arises because these time scales can be comparable to those of molecular relaxation in the liquid. In addition, even liquids that wet solid surfaces can exhibit nanometer-scale slip at those surfaces. It has recently become possible to interrogate the viscoelastic response of simple liquids and associated nanoscale slip using optical measurements of the mechanical vibrations of metal nanoparticles. Plasmon resonances in metal nanoparticles provide strong optical signals that can be accessed by several spectroscopies, most notably ultrafast transient-absorption spectroscopy. These spectroscopies have been used to measure the frequency and damping rate of acoustic oscillations in the nanoparticles, providing quantitative information about mechanical coupling and exchange of mechanical energy between the solid particle and its surrounding liquid. This information, in turn, has been used to elucidate the rheology of viscoelastic simple liquids at the nanoscale in terms of their constitutive relations, taking into account separate viscoelastic responses for both shear and compressible flows. The nanoparticle vibrations have also been used to provide quantitative measurements of slip lengths on the single-nanometer scale. Viscoelasticity has been shown to amplify nanoscale slip, illustrating the interplay between different aspects of the unconventional fluid dynamics of simple liquids at nanometer length scales and picosecond time scales.

Raman Spectroscopic Fingerprints of Atomically Precise Ligand Protected Noble Metal Clusters: Au38 (PET)24 and Au38-x Agx (PET)24

Distinct Raman spectroscopic signatures of the metal core of atomically precise, ligand-protected noble metal nanoclusters are reported using Au₃₈ (PET)₂₄ and Au_38-x Ag_x (PET)₂₄ (PET = 2-phenylethanethiolate, -SC₂ H₄ C₆ H₅ ) as model systems. The fingerprint Raman features (occurring <200 cm^-1 ) of these clusters arise due to the vibrations involving metal atoms of their Au₂₃ or Au_23-x Ag_x cores. A distinct core breathing vibrational mode of the Au₂₃ core has been observed at 90 cm^-1 . Whereas the breathing mode shifts to higher frequencies with increasing Ag content of the cluster, the vibrational signatures due to the outer metal-ligand staple motifs (between 200 and 500 cm^-1 ) do not shift significantly. DFT calculations furthermore reveal weak Raman bands at higher frequencies compared to the breathing mode, which are associated mostly with the rattling of two central gold atoms of the bi-icosahedral Au₂₃ core. These vibrations are also observed in the experimental spectrum. The study indicates that low-frequency Raman spectra are a characteristic fingerprint of atomically precise clusters, just as electronic absorption spectroscopy, in contrast to the spectrum associated with the ligand shell, which is observed at higher frequencies.

Terahertz Raman Spectroscopy of Ligand-Protected Au8 Clusters

For ligand-protected gold clusters, geometrical differences of gold cores and/or the presence of secondary gold core-ligand interactions influence their unique optical and electronic properties and can, in principle, be detected by spectral changes of gold core vibrations (phonon modes) in ultralow-frequency Raman spectroscopy. We report experimental and theoretical Raman spectra of Au₈ clusters protected by phosphine ligands particularly in the "gold cluster fingerprint" region from 50 to 150 cm^-1 Raman shift (1.5 to 4.5 terahertz, THz). A characteristic core breathing mode observed at ca. 123 cm^-1 was sensitive to differences of core geometries. A new band was found at ca. 150 cm^-1, originating from a local strain on a polyhedral gold core caused by weak Au···π interactions. THz Raman spectroscopy can be utilized for metal nanoclusters to visualize core structural changes and Au···π interactions, which cannot be captured by single crystal X-ray analysis.

Electronic and vibrational surface-enhanced Raman scattering: from atomically defined Au(111) and (100) to roughened Au

In surface-enhanced Raman spectra, vibrational peaks are superimposed on a background continuum, which is known as one major experimental anomaly. This is problematic in assessing vibrational information especially in the low Raman-shift region below 200 cm⁻¹, where the background signals dominate. Herein, we present a rigorous comparison of normal Raman and surface-enhanced Raman spectra for atomically defined surfaces of Au(111) or Au(100) with and without molecular adsorbates. It is clearly shown that the origin of the background continuum is well explained by a local field enhancement of electronic Raman scattering in the conduction band of Au. In the low Raman-shift region, electronic Raman scattering gains additional intensity, probably due to a relaxation in the conservation of momentum rule through momentum transfer from surface roughness. Based on the mechanism for generation of the spectral background, we also present a practical method to extract electronic and vibrational information at the metal/dielectric interface from the measured raw spectra by reducing the thermal factor, the scattering efficiency factor and the Purcell factor over wide ranges in both the Stokes and the anti-Stokes branches. This method enables us not only to analyse concealed vibrational features in the low Raman-shift region but also to estimate more reliable local temperatures from surface-enhanced Raman spectra.

Both electronic and vibrational information at the metal/dielectric interface were explicitly extracted from surface-enhanced Raman spectra.

The elastic Rayleigh drop

Bioprinting technologies rely on the formation of soft gel drops for printing tissue scaffolds and the dynamics of these drops can affect the process. A model is developed to describe the oscillations of a spherical gel drop with finite shear modulus, whose interface is held by surface tension. The governing elastodynamic equations are derived and a solution is constructed using displacement potentials decomposed into a spherical harmonic basis. The resulting nonlinear characteristic equation depends upon two dimensionless numbers, elastocapillary and compressibility, and admits two types of solutions, (i) spheroidal (or shape change) modes and (ii) torsional (rotational) modes. The torsional modes are unaffected by capillarity, whereas the frequency of shape oscillations depend upon both the elastocapillary and compressibility numbers. Two asymptotic dispersion relationships are derived and the limiting cases of the inviscid Rayleigh drop and elastic globe are recovered. For a fixed polar wavenumber, there exists an infinity of radial modes that each transition from an elasticity wave to a capillary wave upon increasing the elastocapillary number. At the transition, there is a qualitative change in the deformation field and a set of recirculation vortices develop at the free surface. Two special modes that concern volume oscillations and translational motion are characterized. A new instability is documented that reflects the balance between surface tension and compressibility effects due to the elasticity of the drop.

Mass loading effects in the acoustic vibrations of gold nanoplates

The breathing modes of single suspended gold nanoplates have been examined by transient absorption microscopy. These vibrational modes show very high quality factors which means that their frequencies can be accurately measured. Measurements performed before and after removing the organic layer that coats the as synthesized nanoplates show significant increases in frequency, which are consistent with removal of a few nm of organic material from the nanoplate surface. Experiments were also performed after depositing polymer beads on the sample. These measurements show a decrease in frequency in the region of the beads. This implies that adding a localized mass to the nanoplate hybridizes the vibrational normal modes, creating a new breathing mode which has a maximum amplitude at the bead. The nanoplate resonators have a mass sensing detection limit of ca. 10 attograms, which is comparable to the best results that have been achieved with plasmonic nanoparticles.

Electrochemical THz-SERS Observation of Thiol Monolayers on Au(111) and (100) Using Nanoparticle-assisted Gap-Mode Plasmon Excitation

Surface-enhanced Raman scattering (SERS) microscopy using nanoparticle-assisted gap-mode plasmon excitation, which enables us to observe an atomically defined planar metal surface, was combined with THz-Raman spectroscopy to observe ultra-low-frequency vibration modes under electrochemical conditions. This combination helps us to gain deeper insights into electrode/electrolyte interfaces via direct observation of extramolecular vibrations including information on intermolecular and substrate/molecule interactions. Electrochemical reductive desorption of benzenethiol derivatives from Au(111) and (100) was monitored to demonstrate the power of this spectroscopy. Structural differences of the monolayers between these surfaces were seen only in the extramolecular vibration modes such as a large-amplitude hinge-bending motion of the phenyl ring. On the Au(111), where hollow-site and bridge-site adsorption coexisted, the electrochemical reductive desorption was preferentially induced at the hollow sites.

Time-domain investigation of the acoustic vibrations of metal nanoparticles: size and encapsulation effects

The acoustic vibrations of single-metal and multi-material nanoparticles are studied by ultrafast pump-probe optical spectroscopy and described in the context of the continuous elastic model. The applicability of this model to the small size range, down to one nanometer, is discussed in the light of recent experimental data and ab initio calculations. Investigations of multi-material nano-objects stress the impact of the intra-particle interface on the characteristics of their vibrational modes, also yielding information on the composition and spatial distribution of the constituting materials.

Vibrational and electronic excitations in gold nanocrystals

An experimental analysis of all elementary excitations--phonons and electron-holes--in gold nanocrystals has been performed using plasmon resonance Raman scattering. Assemblies of monodisperse, single-crystalline gold nanoparticles, specific substrates and specific experimental configurations have been used. Three types of excitations are successively analyzed: collective quasi-acoustical vibrations of the particles (Lamb's modes), electron-hole excitations (creating the so-called "background" in surface-enhanced Raman scattering) and ensembles of atomic vibrations ("bulk" phonons). The experimental vibrational density of states extracted from the latter contribution is successfully compared with theoretical estimations performed using atomic simulations. The dominant role of surface atoms over the core ones on lattice dynamics is clearly demonstrated. Consequences on the thermodynamic properties of nanocrystals such as the decrease of the characteristic Debye temperature are also considered.

Acoustic vibrations of Au nano-bipyramids and their modification under Ag deposition: a perspective for the development of nanobalances

We investigated the acoustic vibrations of gold nanobipyramids and bimetallic gold-silver core-shell bipyramids, synthesized by wet chemistry techniques, using a high-sensitivity pump-probe femtosecond setup. Three modes were observed and characterized in the gold core particles for lengths varying from 49 to 170 nm and diameters varying from 20 to 40 nm. The two strongest modes have been associated with the fundamental extensional and its first harmonic, and a weak mode has been associated with the fundamental radial mode, in very good agreement with numerical simulations. We then derived linear laws linking the periods to the dimensions both experimentally and numerically. To go further, we investigated the evolution of these modes under silver deposition on gold core bipyramids. We studied the evolution of the periods of the extensional modes, which were found to be in good qualitative agreement with numerical simulations. Moreover, we observed a strong enhancement of the radial mode amplitude when silver is deposited: we are typically sensitive to the deposition of 40 attograms of silver per gold core particle. This opens up possible applications in the field of mass sensing, where metallic nanobalances have an important role to play, taking advantage of their robustness and versatility.

Nanocrystallinity and the ordering of nanoparticles in two-dimensional superlattices: controlled formation of either core/shell (Co/CoO) or hollow CoO nanocrystals

Here it is demonstrated that the diffusion process of oxygen in Co nanoparticles is controlled by their 2D ordering and crystallinity. The crystallinity of isolated Co nanoparticles deposited on a substrate does not play any role in the oxide formation. When they are self-assembled in 2D superlattices, the oxidation process is slowed and produces either core/shell (Co/CoO) nanoparticles or hollow CoO nanocrystals. This is attributed to the decrease in the oxygen diffusion rate when the nanoparticles are interdigitated. Initially, polycrystalline nanoparticles form core/shell (Co/CoO) structures, while for single-domain hexagonal close-packed Co nanocrystals, the outward diffusion of Co ions is favored over the inward diffusion of oxygen, producing hollow CoO single-domain nanocrystals.

Radial vibration of free anisotropic nanoparticles based on nonlocal continuum mechanics

Radial vibration of spherical nanoparticles made of materials with anisotropic elasticity is theoretically investigated using nonlocal continuum mechanics. The anisotropic elastic model is reformulated using the nonlocal differential constitutive relations of Eringen. The nonlocal differential equation of radial motion is derived in terms of radial displacement. Cubic, hexagonal, trigonal and tetragonal symmetries of the elasticity are discussed. The suggested model is justified by a good agreement between the results given by the present model and available experimental data. Furthermore, the model is used to elucidate the effect of small scale on the vibration of several nanoparticles. Our results show that the small scale is essential for the radial vibration of the nanoparticles when the nanoparticle radius is smaller than 1.5 nm.

Laser ultrasonics detection of an embedded crack in a composite spherical particle

Laser ultrasonics was applied to the manufacturing control of the integrity (no failure) of coated spherical particles designed for High Temperature Reactors (HTR). This control is of major importance, since the coating of the nuclear fuel kernel is designed to prevent from the diffusion of fission products outside the particle during reactor operation. The SiC layer composing the coating is particularly important, since this layer must be an impenetrable barrier for fission products. The integrity of the SiC shell (no crack within the shell) can be assessed by the ultrasonic vibration spectrum of the HTR particle, which is significantly changed, compared to the reference spectrum of a defect-free particle. Spheroidal vibration modes of defect-free dummy particles with a zirconium dioxide (ZrO(2)) core were observed in the 2-5MHz range. A theoretical analysis is presented to account for the observed vibration spectra of defect-free or cracked HTR particles.

Supra- and nanocrystallinities: a new scientific adventure

Nanomaterials exist in the interstellar medium, in biology, in art and also metallurgy. Assemblies of nanomaterials were observed in the early solar system as well as silicate particle opals. The latter exhibits unusual optical properties directly dependent on particle ordering in 3D superlattices.The optical properties of noble metal nanoparticles (Ag, Au and Cu) change with the ordering of atoms in the nanocrystals, called nanocrystallinity. The vibrational properties related to nanocrystallinity markedly differ with the vibrational modes studied. Hence, a drastic effect on nanocrystallinity is observed on the confined acoustic vibrational property of the fundamental quadrupolar modes whereas the breathing acoustic modes remain quasi-unchanged. The mechanical properties characterized by the Young's modulus of multiply twinned particle (MTP) films are markedly lower than those of single nanocrystals.Two fcc supracrystal growth mechanisms, supported by simulation, of Au nanocrystals are proposed: heterogeneous and homogeneous growth processes. The final morphology of nanocrystal assemblies, with either films by layer-by-layer growth characterized by their plastic deformation or well-defined shapes grown in solution, depends on the solvent used to disperse the nanocrystals before the evaporation process.At thermodynamic equilibrium, two simultaneous supracrystal growth processes of Au nanocrystals take place in solution and at the air-liquid interface. These growth processes are rationalized by simulation. They involve, on the one hand, van der Waals interactions and, on the other hand, the attractive interaction between nanocrystals and the interface.Ag nanocrystals (5 nm) self-order in colloidal crystals with various arrangements called supracrystallinities. As in bulk materials, phase diagrams of supracrystals with structural transitions from face-centered-cubic (fcc) to hexagonal-close-packed (hcp) and body-centered-cubic (bcc) structures are observed. They depend on the chain length of the coating agent and on the solvent used to disperse the nanocrystals before evaporation. The transition from fcc to hcp is attributed to specific stacking processes depending on evaporation kinetics whereas the formation of bcc supracrystals is attributed to van der Waals attractions.These results open up a new research area, which currently suffers from an extensive lack of knowledge.

Low sensitivity of acoustic breathing mode frequency in Co nanocrystals upon change in nanocrystallinity

Cobalt nanocrystals (NCs) with narrow size distribution and polycrystalline structure in their native form are synthesized in reverse micelles. After annealing at 350 °C, these NCs are transformed into single crystalline phase with hexagonal close-packed structure. The vibrational dynamics of NCs differing by their nanocrystallinity is studied by femtosecond pump-probe spectroscopy. By recording the differential reflectivity signal in the native and annealed Co NCs, the frequency of their fundamental breathing acoustic mode can be measured in the time domain. A small decrease of the breathing mode frequency is observed in single crystalline Co NCs compared to that measured in polycrystals, indicating low sensitivity of their fundamental radial mode upon change in crystallinity. This result is in agreement with predictions from calculations using the resonant ultrasound approach.

Modulation of breathers in the three-dimensional nonlinear Gross-Pitaevskii equation

In this paper we present analytical breather solutions of the three-dimensional nonlinear generalized Gross-Pitaevskii equation. We use an Ansatz to reduce the three-dimensional equation with space- and time-dependent coefficients into a one-dimensional equation with constant coefficients. The key point is to show that both the space- and time-dependent coefficients of the nonlinear equation can contribute to modulate the breather excitations. We briefly discuss the experimental feasibility of the results in Bose-Einstein condensates.

Optically excited acoustic vibrations in quantum-sized monolayer-protected gold clusters

We report a systematic investigation of the optically excited vibrations in monolayer-protected gold clusters capped with hexane thiolate as a function of the particle size in the range of 1.1-4 nm. The vibrations were excited and monitored in transient absorption experiments involving 50 fs light pulses. For small quantum-sized clusters (< or =2.2 nm), the frequency of these vibrations has been found to be independent of cluster size, while for larger clusters (3 and 4 nm), we did not observe detectable optically excited vibrations in this regime. Possible mechanisms of excitation and detection of the vibrations in nanoclusters in the course of the transient absorption are discussed. The results of the current investigation support a displacive excitation mechanism associated with the presence of finite optical energy gap in the quantum-sized nanoclusters. Observed vibrations provide a new valuable diagnostic tool for the investigations of quantum size effects and structural studies in metal nanoclusters.

The synthesis of single layers of Ag nanocrystals by ultra-low-energy ion implantation for large-scale plasmonic structures

Single layers of silver (Ag) nanoparticles embedded in silica (SiO2) have been fabricated by ultra-low-energy ion implantation. The distance between the Ag particles and the free SiO2 surface is controlled with nanometer precision. Raman scattering and reflectivity measurements strongly correlate to transmission electron microscopy analyses, allowing the use of these non-invasive techniques to monitor structural and dynamical properties. These results open up new opportunities to manipulate electromagnetic near-field interactions on wafer-scale plasmonic devices.

Ion beam sputtering nanopatterning of thin metal films: the synergism of kinetic self-organization and coarsening

Creation of self-organized surface nanostructures by ion beam sputtering (IBS) has strong potential for use in a broad range of technologies, from nanoelectronics and photonics to sensing and catalysis. Recently, we have developed a simple two-stage process for fabricating self-assembled arrays of Cu dots and lines on Si and SiO(2) substrates employing IBS of thin Cu films. We found that the self-assembled structures on the substrate result from a complex interaction between the structure-forming kinetic instability and various outcomes of the surface diffusion and coarsening, which tend to drive the surface pattern towards a thermodynamic equilibrium. Here, we analyze in detail the interplay of the kinetic nanopatterning and coarsening, in order to better understand the mechanisms defining the IBS-generated metallic structures on substrates of a different material. By means of kinetic Monte Carlo (KMC) modeling we investigate the pertinent trends of the self-organization at the surface of a metallic film. In the light of this discussion, we review the fabricated nanostructures. Finally, we present a KMC model of the two-stage IBS process and analyze the stability of the fabricated metal patterns at the surface of a substrate. We discuss the opportunities and challenges of this technique, concluding that the IBS creation of surface heterostructures provides considerable room for future numerical and experimental studies.

Supracrystals of inorganic nanocrystals: an open challenge for new physical properties

When naturally occurring spherical objects self-organize, the physical properties of the material change. For example, a colorless opal is the result of a disordered aggregate of silica particles. When the silica particles are ordered, however, the opal takes on color, which is determined by the size of the self-assembled particles. In this Account, we describe how these 3D arrangements of nanomaterials can self-organize in 3D arrays called supracrystals; the 3D arrays can fall into the familiar categories of face-centered cubic (fcc), hexagonal compact packing (hcp) crystals, and body-centered (bcc) crystals. The collective properties of these 2D and 3D arrangements are different from the properties of individual nanoparticles and from particles in bulk. Comparison between the approach to saturation of the magnetic curve for supracrystals and disordered aggregates produced from the same batch of nanocrystals is similar to that observed with films or nanoparticles, either highly crystallized or amorphous. We also demonstrate by two various processes and with two types of nanocrystals (silver and cobalt) that when nanocrystals are self-ordered in 3D superlattices, they exhibit a coherent breathing mode vibration of the supracrystal, analogous to a breathing mode vibration of atoms in a nanocrystal. Furthermore, we used 10 nm gamma-Fe(2)O(3) nanocrystals to gain new insight into the scaling law of crack patterns. We found that isotropic and directional crack patterns follow the same universal scaling law over a film height varying by 3 orders of magnitude. These data have led us to propose general analogies between supracrystals of nanocrystals, individual nanocrystals, and the molecules in the bulk phase for certain physical properties based on the ordering of the material. As we continue to study the physical properties of the ordered and disordered arrangements of nanomaterials, we will be able to go further in these analogies. And this exploration leads to new questions: first and foremost, is this behavior general?

Surface enhanced Raman scattering of silver sensitized cobalt nanoparticles in metal-dielectric nanocomposites

We report the preparation of a new type of nanocomposite containing cobalt and silver nanoparticles organized in parallel layers with a well controlled separation. This arrangement allows the observation of an enhanced low-frequency Raman signal at the vibration frequency of cobalt nanoparticles excited through the surface plasmons of silver nanoparticles. Numerical simulations of the electric field confirm the emergence of hot spots when the separation between silver and cobalt nanoparticles is small enough.

Vibrational modes of metal nanoshells and bimetallic core-shell nanoparticles

We theoretically study the spectrum of radial vibrational modes in composite metal nanostructures such as bimetallic core-shell particles and metal nanoshells with dielectric core in an environment. We calculate frequencies and damping rates of fundamental (breathing) modes for these nanostructures along with those of two higher-order modes. For metal nanoshells, we find that the breathing mode frequency is always lower than the one for solid particles of the same size, while the damping is higher and increases with a reduction in the shell thickness. We identify two regimes that can be characterized as weakly damped and overdamped vibrations in the presence of external medium. For bimetallic particles, we find periodic dependence of frequency and damping rate on the shell thickness with period being determined by the mode number. For both types of nanostructures, the frequency of higher modes is nearly independent of the environment, while the damping rate shows a strong sensitivity to the outside medium.

Acoustic vibration modes and electron-lattice coupling in self-assembled silver nanocolumns

Using ultrafast spectroscopy, we investigated electron-lattice coupling and acoustic vibrations in self-assembled silver nanocolumns embedded in an amorphous Al2O3 matrix. The measured electron-lattice energy exchange time is smaller in the nanocolumns than in bulk silver, with a value very close to that of isolated nanospheres with comparable surface to volume ratio. Two vibration modes were detected and ascribed to the breathing and extensional mode of the nanocolumns, in agreement with numerical simulations.

Self-assembly of inorganic nanocrystals: fabrication and collective intrinsic properties

In this Account, we demonstrate that the ordering of nanocrystals over long distances in 3D superlattices, called supracrystals, can lead to unexpected results: the emergence of collective intrinsic properties. The shape of the nanocrystal organization at the mesoscopic scale also induces new physical properties. In addition, we show that nanocrystals can be used as masks for lithography.

Surface plasmons and vibrations of self-assembled silver nanocolumns

Optical and vibrational properties of novel self-assembled silver nanocolumns are studied experimentally and theoretically. The split of the surface plasmon resonance into transverse and longitudinal modes verifies the one-dimensional character of the nanocolumns. In this work, we have identified the acoustic vibration modes of the nanocolumns using Raman scattering, as spheroid-like modes (l = 2, m = +/-2) involving vibrations of the nanocolumns along their minor axes and the existence of surface plasmon-vibration coupling mechanisms.

Optical control of the coherent acoustic vibration of metal nanoparticles

Optical control of the coherent breathing vibrations of silver nanospheres is demonstrated using a high-sensitivity femtosecond pump-probe technique in a double-pump pulse configuration. Oscillation of the fundamental mode that usually dominates the time-domain vibrational response can thus be stopped, permitting observation of the first order radial mode and determination of its properties. These are found to be in agreement with the predictions of the model of an elastic sphere embedded in an elastic matrix.

Intrinsic behavior of face-centered-cubic supra-crystals of nanocrystals self-organized on mesoscopic scale

We describe intrinsic behavior due to the high ordering of nanocrystals at the mesoscopic scale. The first example shows well-defined columns in the formation of cobalt nanocrystals when an applied magnetic field is applied during the evaporation process. Collective breathing properties between nanocrystals are demonstrated. In both cases, these features are observed when the nanocrystals are highly ordered in fcc supra-crystals.

Intrinsic Vibrational Coherence in Face-Centered Cubic Supra-Crystals of Silver Nanocrystals: Raman Scattering Measurements

The ordering of silver nanocrystals is tuned from amorphous aggregates to highly well-ordered, face-centered cubic supra-crystals, using various substrates and controlling their temperature to obtain this. Low-frequency Raman scattering, for the first time, demonstrates vibrational coherence in fcc supra-crystals of nanocrystals. This is shown by a narrowing of the peak corresponding to the quadrupolar modes of the nanocrystals. However, this is obtained when the supra-crystals are smaller than the excitation wavelength. When the supra-crystals are larger, the narrowing cannot be observed. Furthermore, for any size of the supra-crystals, a shift to low frequency of the Raman peak due to the Lorentz field effect is seen.

Vibrational coherence of self-organized silver nanocrystals in f.c.c. supra-crystals

Fabrication of devices from inorganic nanocrystals normally requires that they are self-organized into ordered structures. It has now been demonstrated that nanocrystals are able to self-organize in a 'supra'-crystal with a face-centred cubic (f.c.c.) structure. The physical properties of nanocrystals self-organized into compact arrays are quite different from those of both isolated nanocrystals and the bulk phase. The collective optical and magnetic properties of these nanocrystal assemblies are governed mainly by dipolar interactions. Here, we show that nanocrystals vibrate coherently when they are self-organized in f.c.c. supra-crystals. Hence, a phase relation exists between the vibrations of all of the nanocrystals in a supra-crystal. This vibrational coherence can be observed by a substantial change of the quadrupolar low-frequency Raman scattering peak. Although a change in electronic transport properties has previously been observed on self-organization of silver nanocrystals, vibrational coherence represents the first intrinsic property of f.c.c. supra-crystals.