S. Link et al., Laser-induced shape changes of colloidal gold nanorods using femtosecond and nanosecond laser pulses, J PHYS CH B, 104(26), 2000, pp. 6152-6163
Gold nanorods have been found to change their shape after excitation with i
ntense pulsed laser irradiation. The final irradiation products strongly de
pend on the energy of the laser pulse as well as on its width. We performed
a series of measurements in which the excitation power was varied over the
range of the output power of an amplified femtosecond laser system produci
ng pulses of 100 fs duration and a nanosecond optical parametric oscillator
(OPO) laser system having a pulse width of 7 ns. The shape transformations
of the gold nanorods are followed by two techniques: (1) visible absorptio
n spectroscopy by monitoring the changes in the plasmon absorption bands ch
aracteristic for gold nanoparticles; (2) transmission electron microscopy (
TEM) in order to analyze the final shape and size distribution. While at hi
gh laser fluences (similar to 1 J cm(-2)) the gold nanoparticles fragment,
a melting of the nanorods into spherical nanoparticles (nanodots) is observ
ed when the laser energy is lowered. Upon decreasing the energy of the exci
tation pulse, only partial melting of the nanorods takes place. Shorter but
wider nanorods are observed in the final distribution as well as a higher
abundance of particles having odd shapes (bent, twisted, phi-shaped, etc.).
The threshold for complete melting of the nanorods with femtosecond laser
pulses is about 0.01 J cm(-2). Comparing the results obtained using the two
different types of excitation sources (femtosecond vs nanosecond laser), i
t is found that the energy threshold for a complete melting of the nanorods
into nanodots is about 2 orders of magnitude higher when using nanosecond
laser pulses than with femtosecond laser pulses. This is explained in terms
of the successful competitive cooling process of the nanorods when the nan
osecond laser pulses are used. For nanosecond pulse excitation, the absorpt
ion of the nanorods decreases during the laser pulse because of the bleachi
ng of the longitudinal plasmon band. In addition, the cooling of the lattic
e occurring on the 100 ps time scale can effectively compete with the rate
of absorption in the case of the nanosecond pulse excitation but not for th
e femtosecond pulse excitation. When the excitation source is a femtosecond
laser pulse, the involved precesses (absorption of the photons by the elec
trons (100 fs), heat transfer between the hot electrons and the lattice (<1
0 ps), melting (30 ps), and heat loss to the surrounding solvent (>100 ps)
are clearly separated in time.