The bleaching recovery of the pristine Au colloids consists of a fast (t ¼ 2:5 ps)
and a slow process (t > 50 ps) [71]. The slower component of the recovery had a
lifetime of 170 ps. These fast and slow recoveries correspond to the relaxation of
‘‘hot’’ electrons via electron–phonon coupling and phonon–phonon relaxation of
the lattice, respectively. Dumping thermal energy into the solvent causes the
dielectric of the surrounding medium to change, which in turn, influences the
plasmon resonance frequency of the metal nanoclusters.
19.3.2
Laser Induced Fusion and Fragmentation of Metal Nanoclusters
Silver and gold nanoparticles show significant photoactivity under high intensity
UV and visible irradiation. These nanoparticles undergo morphological changes
under laser irradiation [73, 77–82]. Whereas photofragmentation and photofusion
processes are found to be biphotonic and require high intensity laser excitation,
simple shape changes can occur even at low intensity excitation [83].
The morphological changes caused by the 532 nm laser pulse irradiation of
thionicotinic acid-capped gold nanoparticle samples are shown in transmission
electron microscopy (Figure 19.4). Native gold colloids prepared by the citric acid
reduction method are nearly spherical in shape with a particle diameter of 15–
20 nm. Surface neutralization of citrate-stabilized gold nanoparticles with thiol
readily yields clustered aggregates with broad absorption in the infrared. The purple colored gold colloidal solution turns blue if one adds small amounts of thionicotinamide (TNA). The transmission electron micrograph of TNA-capped gold
nanoparticles (Figure 19.4A) shows the presence of cluster islands, each consisting
of several nanoparticles that are in close contact. After 1 min laser pulse irradiation
(532 nm, 10 Hz, 1.5 mJ) the aggregate sample transforms to form large size
spherical particles (Figure 19.4B). Pumping the laser pulses (532 nm) for a longer
duration (15–30 min) results in the fragmentation of these fused particles (Figure
19.4C).
A
B
C
100
100
100
Fig. 19.4. Thionicotinamide capped gold nanoparticles (A)
before, (B) after 1 min and (C) after 30 min of 532 nm laser
excitation. (From [78].)
19 Photochemistry and Electrochemistry of Nanoassemblies
624
and a slow process (t > 50 ps) [71]. The slower component of the recovery had a
lifetime of 170 ps. These fast and slow recoveries correspond to the relaxation of
‘‘hot’’ electrons via electron–phonon coupling and phonon–phonon relaxation of
the lattice, respectively. Dumping thermal energy into the solvent causes the
dielectric of the surrounding medium to change, which in turn, influences the
plasmon resonance frequency of the metal nanoclusters.
19.3.2
Laser Induced Fusion and Fragmentation of Metal Nanoclusters
Silver and gold nanoparticles show significant photoactivity under high intensity
UV and visible irradiation. These nanoparticles undergo morphological changes
under laser irradiation [73, 77–82]. Whereas photofragmentation and photofusion
processes are found to be biphotonic and require high intensity laser excitation,
simple shape changes can occur even at low intensity excitation [83].
The morphological changes caused by the 532 nm laser pulse irradiation of
thionicotinic acid-capped gold nanoparticle samples are shown in transmission
electron microscopy (Figure 19.4). Native gold colloids prepared by the citric acid
reduction method are nearly spherical in shape with a particle diameter of 15–
20 nm. Surface neutralization of citrate-stabilized gold nanoparticles with thiol
readily yields clustered aggregates with broad absorption in the infrared. The purple colored gold colloidal solution turns blue if one adds small amounts of thionicotinamide (TNA). The transmission electron micrograph of TNA-capped gold
nanoparticles (Figure 19.4A) shows the presence of cluster islands, each consisting
of several nanoparticles that are in close contact. After 1 min laser pulse irradiation
(532 nm, 10 Hz, 1.5 mJ) the aggregate sample transforms to form large size
spherical particles (Figure 19.4B). Pumping the laser pulses (532 nm) for a longer
duration (15–30 min) results in the fragmentation of these fused particles (Figure
19.4C).
A
B
C
100
100
100
Fig. 19.4. Thionicotinamide capped gold nanoparticles (A)
before, (B) after 1 min and (C) after 30 min of 532 nm laser
excitation. (From [78].)
19 Photochemistry and Electrochemistry of Nanoassemblies
624