Gold nanorods in colloidal solution can be melted into spherical nanopartic
les by excitation with intense femtosecond laser pulses of the proper energ
y. The threshold of the laser pulse energy for the complete melting of the
nanorods with a mean aspect ratio of 4.1 in solution is determined by obser
ving the change in the absorption intensity of the longitudinal absorption
band (measure of the rod concentration) at 800 nm with increasing number of
laser pulses of known energy. The number of laser pulses needed to reduce
the band intensity (rod concentration) by 1/e of its initial value is deter
mined as the laser energy per pulse increases. For pulses of lower energy t
han threshold, it is found that the number of pulses required to melt the g
old nanorods present in solution increases significantly with decreasing la
ser pulse energy. Above threshold, this number is constant since the additi
onal absorbed laser energy will only further heat the particles to temperat
ures above their melting point. The gold concentration in the colloidal sol
ution is measured using inductively coupled plasma atomic emission spectros
copy (ICP-AES), from which the gold nanorod concentration is determined fro
m the known shape and size distribution obtained from transmission electron
microscopy (TEM) results. A simple analysis using the determined threshold
energy and the nanorod concentration showed that it takes an average of si
milar to 60 femtojoule (fJ) to melt a single gold nanorod. Experiments usin
g 820 nm as well as 410 nm femtosecond laser pulses yield similar values, i
ndicating that the laser induced shape transformation of the nanorods is in
dependent of the irradiation wavelength and that this process is therefore
photothermal in origin. (C) 2001 American Institute of Physics.