4 Laser-Induced Synthesis and Processing of Nanoparticles …
135
received much attention for manipulating the morphology of nano-sized particles
during or after their formation. Laser irradiation has been shown to cause fragmentation of nanometer-size silver and gold particles in aqueous solutions, size enlargement of spherical metal and semiconducting nanoparticles, transformation of gold
nanorods into spheres as well as laser-induced alloying [16, 17]. The result of laser
irradiation of metal nanoparticles is strongly dependent on experimental conditions.
For instance, the Koshizaki group has published many papers showing a method to
produce spherical submicrometer particles of different materials by laser irradiation
of nanocolloids which has been previously grown by PLAL or by classical chemical
syntheses [18, 19]. They have also shown that heating–melting–evaporation model
can be successfully applied for many phenomena arising when colloidal nanoparticle interacts with pulsed laser beams. On the other hand, Compagnini et al. have
successfully synthesized Au/Ag colloidal nano-alloys with a wide range of compositions by laser ablation of single metal targets in water and a re-irradiation of mixed
colloidal suspensions [20, 21]. In this respect, control over the size and polydispersity of NPs can be achieved by modulation of the laser pulse width and fluency, but
also through rational use of selected capping ligands with different affinity toward
the nanoparticles surface.
In the following, we plan to briefly discuss some fundamentals regarding the
interaction of the pulsed laser beam with a target material embedded in liquid environments and then propose a few examples in which nanomaterials are obtained and
applied to biosensing, energy, and environmental applications.
4.2 Fundamentals
4.2.1 Liquid Phase Laser Ablation of Solid Targets
The mechanisms of ablation of any target by laser pulses in a liquid confining medium
have been extensively described in the papers by Fabbro et al. [22, 23] and by Sakka
et al. [24, 25]. Basically, a plume containing the material is produced at the solid–
liquid interface in the place where the incident laser pulses impinge the target. A
sketch of the sequence of this mechanism, with the possible involved reactions, is
depicted in Fig. 4.1.
The laser-induced plasma expands adiabatically at a supersonic velocity,
absorbing the residual part of the laser pulse. This fact determines the creation of
a shock wave confined by the liquid environment, and the shock wave, therefore,
induces extra pressure. Such a phenomenon is currently described as the formation
of the so-called ‘cavitation bubble’. The extra pressure created in the plasma induces,
in turn, an additional temperature increase, and this continuous process determines
the conditions for a continual supply of the vaporizing species coming from the solid
target. In synthesis, a thermodynamic state of higher temperature, higher pressure,
and higher density is determined with respect to that of the initially generated plasma.
135
received much attention for manipulating the morphology of nano-sized particles
during or after their formation. Laser irradiation has been shown to cause fragmentation of nanometer-size silver and gold particles in aqueous solutions, size enlargement of spherical metal and semiconducting nanoparticles, transformation of gold
nanorods into spheres as well as laser-induced alloying [16, 17]. The result of laser
irradiation of metal nanoparticles is strongly dependent on experimental conditions.
For instance, the Koshizaki group has published many papers showing a method to
produce spherical submicrometer particles of different materials by laser irradiation
of nanocolloids which has been previously grown by PLAL or by classical chemical
syntheses [18, 19]. They have also shown that heating–melting–evaporation model
can be successfully applied for many phenomena arising when colloidal nanoparticle interacts with pulsed laser beams. On the other hand, Compagnini et al. have
successfully synthesized Au/Ag colloidal nano-alloys with a wide range of compositions by laser ablation of single metal targets in water and a re-irradiation of mixed
colloidal suspensions [20, 21]. In this respect, control over the size and polydispersity of NPs can be achieved by modulation of the laser pulse width and fluency, but
also through rational use of selected capping ligands with different affinity toward
the nanoparticles surface.
In the following, we plan to briefly discuss some fundamentals regarding the
interaction of the pulsed laser beam with a target material embedded in liquid environments and then propose a few examples in which nanomaterials are obtained and
applied to biosensing, energy, and environmental applications.
4.2 Fundamentals
4.2.1 Liquid Phase Laser Ablation of Solid Targets
The mechanisms of ablation of any target by laser pulses in a liquid confining medium
have been extensively described in the papers by Fabbro et al. [22, 23] and by Sakka
et al. [24, 25]. Basically, a plume containing the material is produced at the solid–
liquid interface in the place where the incident laser pulses impinge the target. A
sketch of the sequence of this mechanism, with the possible involved reactions, is
depicted in Fig. 4.1.
The laser-induced plasma expands adiabatically at a supersonic velocity,
absorbing the residual part of the laser pulse. This fact determines the creation of
a shock wave confined by the liquid environment, and the shock wave, therefore,
induces extra pressure. Such a phenomenon is currently described as the formation
of the so-called ‘cavitation bubble’. The extra pressure created in the plasma induces,
in turn, an additional temperature increase, and this continuous process determines
the conditions for a continual supply of the vaporizing species coming from the solid
target. In synthesis, a thermodynamic state of higher temperature, higher pressure,
and higher density is determined with respect to that of the initially generated plasma.