Coherent Excitation of Acoustic Breathing Modes in Bimetallic Core−Shell Nanoparticles

Hodak, José H.; Henglein, and Arnim; Hartland, Gregory V.

doi:10.1021/jp000578v

Cited by 90 publications

(109 citation statements)

References 27 publications

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“…The damping feels the existence of a viscous shell as well and will increase. Hodak et al 36 have performed pump-probe experiments on gold core particles with a thin lead shell from 4 to 20 monolayers. These results show the modification of the frequency as calculated later by Sader et al 37 and (although not explicitly mentioned) a change of the phase upon lead addition on the surface.…”

Section: I(τ)mentioning

confidence: 99%

A Surface Phase Transition of Supported Gold Nanoparticles

et al. 2007

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A thermal phase transition has been resolved in gold nanoparticles supported on a surface. By use of asynchronous optical sampling with coupled femtosecond oscillators, the Lamb vibrational modes could be resolved as a function of annealing temperature. At a temperature of 104°C the damping rate and phase changes abruptly, indicating a structural transition in the particle, which is explained as the onset of surface melting.Metallic nanoparticles are in the focus of fundamental and applied research. They show an optical response in the visible region that depends strongly on size, shape, and local dielectric environment, 1 making them appealing for nanoscale sensor applications. The interaction with light shows interesting peculiarities, such as ultrafast relaxation dynamics, coupling to nonradiative processes, or near field enhancement. 2 In particular nanoscale gold particles have received strong attention due to their versatile synthesis protocols, 3 shape transformations, or binding to biological molecules. 4 They served also to understand the generic structural properties that scale with size or shape. The melting point depression with decreasing particle size represents one of the prominent features, arising from the change in ratio between surface and bulk atoms and the reduction in binding energy. Since the seminal publication of Buffat and Borel 5 several improvements of the description have been made, while the general relation of the melting point depression ∆T mp ∝ 1/D as a function of size D remains valid. One of these points is the existence of surface melting, i.e., a coexistence of a liquid-like layer of atoms at the surface with the solid core material at temperatures below the respective melting point. 6,7 While this effect seems to be very basic, there is still a lack of unambiguous experimental proof regarding the details, in particular for gold nanoparticles.Inasawa et al. 8 have tried to investigate the shape transformation of elongated particles on a surface induced by static heating (ex situ) and concluded that a transformation into spheres takes places at temperatures as low as 400°C. A similar, very recent study has shown that nanorods with a large aspect ratio of the axes can be transformed at 200°C into spheres and partial relaxation occurs much earlier (100°C). 9 Surface melting is taken as the origin for these relaxations.It should also be borne in mind that the relaxation of rods could also be caused by other forces, as has been shown by laser-induced melting, 10 where the onset correlates with dislocations in the interior, as the rods are subjected to a large uniaxial strain due to surface tension.Hartland et al. have also reported on several experiments, which aimed at the structural transformations on a very short time scale, initiated by femtosecond excitation. 11,12 The vibrational response of the particles has been recorded by optical pump-probe techniques, reflecting the mechanical properties of the particles. While the change of the elastic constants of the spheres ...

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Section: I(τ)mentioning

confidence: 99%

A Surface Phase Transition of Supported Gold Nanoparticles

et al. 2007

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“…Thus, the vibrational modes constitute an additional source of information that is particularly important for hybrid or layered systems with structurally distinct constituents, such as, e.g., bimetallic nanoparticles and nanoshells. Recent pump-probe spectroscopy measurements of fundamental ͑breathing͒ mode frequency and damping in bimetallic particles [20][21][22] and gold nanoshells 23 revealed significantly different, from solid nanoparticles, acoustical signatures that could be used as a tool complimentary to optical methods.…”

Section: Introductionmentioning

confidence: 99%

“…Acoustic vibrational modes in nanoparticles can be impulsively launched by a laser pulse and monitored using the pump-probe spectroscopic technique. [15][16][17][18]20,21 After initial stage of rapid expansion, the nanoparticle undergoes radial oscillations around its new equilibrium size. The periodic change in volume translates into a modulation, in time, of the SPR energy and manifests itself in the coherent oscillations of differential transmission.…”

Section: Introductionmentioning

confidence: 99%

Vibrational modes of metal nanoshells and bimetallic core-shell nanoparticles

Kirakosyan

Shahbazyan

2008

The Journal of Chemical Physics

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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 ͑breathing͒ 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.

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“…Several groups have reported on the acoustic vibrations of bimetallic particles with different coating thicknesses. 14,20,21 However, these measurements were conducted on ensembles of nanoparticles and do not provide information on particle-to-particle variations or on possible additional damping mechanisms for example due to the creation of the metal−metal interface or the presence of crystal defects in the coating.…”

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confidence: 99%