Size dependent properties of Au particles: Coherent excitation and dephasing of acoustic vibrational modes

Abstract

Ultrafast laser spectroscopy has been used to characterize the low frequency acoustic breathing modes of Au particles, with diameters between 8 and 120 nm. It is shown that these modes are impulsively excited by the rapid heating of the particle lattice that occurs after laser excitation. This excitation mechanism is a two step process; the pump laser deposits energy into the electron distribution, and this energy is subsequently transferred to the lattice via electron–phonon coupling. The measured frequencies of the acoustic modes are inversely proportional to the particle radius; a fit to the data for the different sized particles yields v̄R=0.47cl/Rc, where R is the particle radius, cl is the longitudinal speed of sound in Au, and c is the speed of light. This functional relationship exactly matches the prediction of classical mechanics calculations for the lowest frequency radial (breathing) mode of a free, spherical particle. The inverse dependence of the frequency on the radius means that the modulations are damped for polydisperse samples. Analysis of our data shows that this inhomogeneous decay dominates the damping, even for our high quality samples (8%–10% dispersion in the size distribution). The size dependence of the electron–phonon coupling constant was also examined for these particles. The results show that, to within the signal to noise of our measurements, the electron–phonon coupling constant does not vary with size for particles with diameters between 4 and 120 nm. Furthermore, the value obtained is the same as that measured for bulk gold.

Laser-Induced Shape Changes of Colloidal Gold Nanorods Using Femtosecond and Nanosecond Laser Pulses

“…[25][26][27] The electron-phonon coupling was found to be size 3,7 and shape 7,8 independent for gold nanoparticles in the size range from 8 to 120 nm. The measured electron-phonon relaxation times depend on the laser pump power 2,3,7 and are on the order of a few picoseconds (1-4 ps). [1][2][3][4][5][6][7][8][9][10][11][12][13] The results obtained for the nanoparticles furthermore compare well with the electronphonon coupling constant measured for bulk gold 28 using similar time-resolved laser techniques.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] In particular, silver 1,2,9 and gold [2][3][4][5][6][7][8][10][11][12][13] nanoparticles have been studied intensively as they show a strong absorption band in the visible region, which is due to the excitation of the surface plasmon resonance. [25][26][27] The electron-phonon coupling was found to be size 3,7 and shape 7,8 independent for gold nanoparticles in the size range from 8 to 120 nm. The measured electron-phonon relaxation times depend on the laser pump power 2,3,7 and are on the order of a few picoseconds (1-4 ps).…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] Time-resolved transient absorption spectroscopy allows one to follow the electronphonon relaxation dynamics in thin metal films [21][22][23][24] and small metal particles. [1][2][3][4][5][6][7][8][9][10][11][12][13] In particular, silver 1,2,9 and gold [2][3][4][5][6][7][8][10][11][12][13] nanoparticles have been studied intensively as they show a strong absorption band in the visible region, which is due to the excitation of the surface plasmon resonance. [25]…”

Controllable Cobalt Oxide/Au Hierarchically Nanostructured Electrode for Nonenzymatic Glucose Sensing

“…This is followed by the transfer of the thermalized hot electron gas to the nanoparticle lattice (electron-phonon coupling) where a quasi-equilibrium state is reached in ∼1 ps [9,35,[140][141][142][143]. The energy exchange between the hot electrons and phonons results in hot phonons and can be described by the two-temperature model which is used to determine the electron-phonon coupling constant g [144][145][146]. The final step involves relaxation of phonons on the time scale of hundreds of ps, which leads to energy transfer to the surroundings (phonon-phonon interactions) like a solid matrix or solvent molecules [2,57,134,[145][146][147][148][149][150].…”

High Frequency Acoustics in Colloid-Based Meso- and Nanostructures by Spontaneous Brillouin Light Scattering

“…However, there is a paucity of non-destructive experimental techniques to probe this 'music' of particle vibrations since both high frequency resolution and sensitivity are required to detect the numerous eigenmodes. Raman scattering [83,84] has been utilized to measure few eigenfrequencies of nanoparticles with dimension below 10 nm, whereas Brillouin light scattering (BLS) [45,46,85] and optical pulse-probe techniques [27,28,86] can probe respectively the spontaneous and stimulated vibrations confined in sub-micrometer particles. In the latter technique, the excited acoustic oscillations are observed in the form of modulations of the transient reflectivity of the probe laser, and hence the particles must possess good reflectance, e.g., by introduction of gold shells.…”