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

Abstract

Gold nanorods have been found to change their shape after excitation with intense pulsed laser irradiation. The final irradiation products strongly depend 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 producing 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 absorption spectroscopy by monitoring the changes in the plasmon absorption bands characteristic for gold nanoparticles; (2) transmission electron microscopy (TEM) in order to analyze the final shape and size distribution. While at high laser fluences (∼1 J cm -2 ) the gold nanoparticles fragment, a melting of the nanorods into spherical nanoparticles (nanodots) is observed when the laser energy is lowered. Upon decreasing the energy of the excitation 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, φ-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), it 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 nanosecond laser pulses are used. For nanosecond pulse excitation, the absorption of the nanorods decreases during the laser pulse because of the bleaching of the longitudinal plasmon band. In addition, the cooling of the lattice 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 the femtosecond pulse excitation. When the excitation source is a femtosecond laser pulse, the involved processes (absorption of the photons by the electrons (100 fs), heat transfer between the hot electrons and the lattice (<10 ps), melting (30 ps), and heat loss to the surrounding solvent (>100 ps) are clearly separated in time.

Femtosecond-pulsed optical heating of gold nanoparticles

“…16 Several experimental and numerical approaches aimed at studying the internal processes of heat generation under pulsed illumination and the subsequent effects observed in the surrounding medium, e.g. temperature and pressure variations, [17][18][19] acoustic wave generation, 18 vibration modes, [20][21][22] , cell apoptosis, 11 drug release 9,23 nanosurgery, 15,24 bubble formation, [25][26][27][28] NP shape modification 29 and melting, [29][30][31] nanosecondpulses for biomedical applications, 32,33 extreme thermodynamics conditions. [33][34][35][36][37] In this paper, we present and use a versatile numerical framework to investigate theoretically and numerically the evolution of the temperature distribution of a gold nanoparticle immersed in water when shined by a femtosecond-pulsed laser.…”

Laser-assisted synthesis, and structural and thermal properties of ZnS nanoparticles stabilised in polyvinylpyrrolidone

“…Laser irradiation as a method of nanoparticle synthesis was initially introduced into materials chemistry in the synthesis of nanoparticles of noble metals via ablation techniques [25,26], and this method has also been extended to the preparation of compound nanocrystalline materials [27,28]. Laser synthesis has the advantage of a high reaction rate and thus shorter reaction times.…”

In-situ bioconjugation in stationary media and in liquid flow by femtosecond laser ablation

“…Furthermore, femtosecond pulses has been already found to melt and photothermally reshape ablated gold nanoparticles in water [14] and also to induce certain degradation to gold conjugates, however the degradation mechanism still remains unclear [15].…”