Spectroscopic determination of the melting energy of a gold nanorod

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.