Vibrational Response of Au−Ag Nanoboxes and Nanocages to Ultrafast Laser-Induced Heating

Abstract

Time-resolved spectroscopy has been used to investigate the vibrational properties of hollow cubic nanoparticles: Au-Ag nanoboxes and nanocages. In these experiments laser-induced heating was used to coherently excite the breathing vibrational modes of the particle. The vibrational periods scale with the edge length of the particle, and the nanocages and nanoboxes showing equivalent responses despite a large difference in their morphology. The measured vibrational periods are compared to finite element calculations, where the particles are modeled as a hollow cube, with the principle crystal axes parallel to the sides of the cube. Very good agreement is obtained between the calculations and the experimental data, with the experimental frequencies being slightly lower than the calculated values (by ∼ 7 %). These results demonstrate the importance of accurately modeling the particles in order to interpret experimental data.Advances in synthetic techniques have made it possible to produce high quality samples of metal and semiconductor nanoparticles in a variety of different shapes, such as rods, 1,2,3 triangles, 4,5 cubes and boxes, 6 and branched structures. 7,8 These particles have unique optical properties, and are finding use in a variety of applications, from molecular sensing 9 to biological labeling 10 and photothermal therapy. 11,12 For metal particles the optical response is dominated by the surface plasmon resonance (SPR), which is a coherent oscillation of the conduction electrons across the particle surface. 13 Recent research has focused on understanding how the position and width of the SPR depends on the size and shape of the particles. 14 In this regard, single particle spectroscopy has been particularly useful. 15,16 These experiments have shown, for example, how radiation damping, 15,17 electron-surface scattering, 17,18 and the "lightning-rod effect" 19 control the width of the plasmon resonance.Time-resolved experiments also provide information about the properties of nanoparticles. For metals, transient absorption experiments conducted on a sub-picosecond timescale give information about electron-electron 20 and electron-phonon coupling. 21 At longer timescales (tens to hundreds of picoseconds) transient absorption measurements give data about how energy relaxes from the particle to the environment. 22,23 This information is important to photothermal therapy, where laser-induced heating of nanoparticles is used to kill selected cells. 11,12 For high quality samples, modulations due to coherently excited vibrational modes To obtain quantitative information from the vibrational spectroscopy experiments, an expression is needed that relates the measured frequencies to the particle dimensions and elastic constants. For spheres (both homogeneous and core-shell) and rods, analytic expressions are available for the important modes observed in the transient absorption experiments. 24,25,26, 27 However, for other shapes the vibrational modes must be calculated numerically. 28 In our recent stud...

Plasmonic Optical Properties and Applications of Metal Nanostructures

“…It has been found that the relaxation time is similar to that in spherical nanoparticles and bulk. [180][181][182][183][184] This indicates that the electron-phonon coupling is not affected by the shape of the nanostructure in any significant manner.…”

Probing the acoustic vibrations of single metal nanoparticles by ultrashort laser pulses

“…The period of the vibrations displayed a linear increase with particle size, and the extracted sound velocities were found to be consistent with those of the bulk material. Subsequently, coherently excited acoustic vibrations in metal nanoparticles were studied in ensembles of spheres [23,36,37,[54][55][56][57], rods [23,[58][59][60][61][62], core-shell spherical particles [63], ellipsoids [28], disks [64], cubes [65], boxes and cages [66], triangles [67], columns [68] and bipyramids [69]. These investigations yielded valuable information on the frequency of the fundamental breathing vibration modes of these particles, from which in some cases the elastic constants of the particles could be extracted [61,70].…”

Vibrational Dynamics of Silver Nanocubes and Nanowires Studied by Single-Particle Transient Absorption Spectroscopy

“…The frequencies of both the plasmon oscillation [15][16][17] and the vibrational resonances [12,[18][19][20][21][22][23] are extremely sensitive to the size and shape of the particles. Thus, very high quality samples, and good sample characterization, are required to study these features.…”

“…In general, the samples produced by chemical synthesis are good enough that ensemble measurements can be used to determine how the positions of the LSPR and vibrational resonances depend on size and shape. [17][18][19][20][21][22][23][24] However, the distribution of sizes and shapes invariably present in even the best nanoparticle samples means that the resonances are inhomogeneously broadened, making it impossible to extract meaningful lifetime information. [7,12,13] One way to overcome this problem is to study these resonances at the single particle level.…”