Picosecond Dynamics of Silver Nanoclusters. Photoejection of Electrons and Fragmentation

Abstract

Silver colloids of particle diameter 40−60 nm have been synthesized using a chemical reduction method in aqueous medium. These nanoclusters are photoactive and exhibit transient bleaching in the 400−500 nm region followed by a strong absorption in the visible−near-infrared region when subjected to 355 nm laser-pulse excitation. The transient bleaching of the surface plasmon absorption band is a monophotonic process, while the absorption growth in the red region is a biphotonic process arising from the photoejection of electrons. The 40−60 nm clusters were observed to break up into smaller clusters (5−20 nm) with 355 nm laser-pulse excitation. The choice of excitation wavelength provides the size selectivity in the fragmentation of the clusters. For example, when the excitation wavelength was switched to 532 nm, only larger (or irregularly shaped) particles were found to break up.

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

“…[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.…”

“…However, if the pump power is increased significantly, the temperature of the metal nanoparticle lattice can be raised to values well above its melting temperature within a few picoseconds (when using ultrashort laser pulses) while the temperature of the environment initially remains constant. This can then lead to size [14][15][16][17] and shape [18][19][20] changes of the nanoparticles before the deposited laser energy can be released to the surrounding medium by phonon-phonon interactions which are usually on the order of 100 ps. 4,7 It was recently reported 29 that gold nanorods prepared by an electrochemical method 30 and encapsulated in micelles in aqueous solution undergo a rod-to-sphere shape transformation within about 30 ps due to melting of the nanorods.…”

“…A size reduction of silver nanoparticles in a glass matrix induced by excimer laser irradiation at 248 nm was also explained by a thermal model. 16 On the other hand, in a study of silver nanoparticles, Kamat et al 17 proposed that the initial ejection of photoelectrons after excitation with 355 nm picosecond laser pulses causes the particles to become positively charged. The repulsion between the charges then leads to fragmentation.…”

Biogenic synthesis of antimicrobial silver nanoparticles capped with l-cysteine

“…For example, to prepare silver nanoparticles, in general ionic silver (Ag + ) contained in a salt, generally AgNO 3 , is reduced to metal silver (Ag 0 ) through a reducing agent such as citrate [22], Al-alkoxide [23], NaBH 4 [24], N,N-dimethyl formamide [25] or sugars [26,27]. The nanoparticles can also be stabilized using chelating substances such as:…”

Microwave-assisted synthesis of silver nanoparticles using benzo-18-crown-6 as reducing and stabilizing agent

“…The position and shape of plasmon absorption of noble metal nanoclusters are strongly dependent on particle size, dielectric medium and surface-adsorbed species (Kamat et al 1998;Heilmann et al 1999). The formation process and the optical properties of the AgNPs can also be identified from both the colour change and UV-visible spectra of the solutions.…”