140
G. Compagnini et al.
to significant advantages for quality demanding nanotechnology applications in
biomedicine and catalysis.
(d) Electroaffinity: laser-generated (noble) metal colloids are electron acceptors
because of surface atom oxidation resulting in a relatively high particle surface
charge. If this charge is not screened by impurities, the particles attract oxygen
species and the resulting surface charge triggers electrostatic repulsion. At the
same time, the Lewis-acidity of the inorganic nanoparticle makes it possible to
achieve efficient electron-donative ligand adsorption.
(e) Defects: under controlled conditions, defect-rich materials and suboxides can
be sythesized, potentially broadening the range of optical, semiconducting, or
catalytic properties.
Besides the formation of nanomaterials by using an ablation process, power laser
beams can be used to modify plenty of nanostructures which have been previously
obtained using other methods. In this case, the laser energy triggers many possible
phenomena such as fragmentation, melting, or surface functionalization.
4.2.2 Irradiation of Nanoparticles Colloids
Pulsed laser selective heating of existing nanoparticle colloids has been used by
several authors in order to modify the structure and/or the chemical nature of a
colloidal dispersion [27–30]. The case of noble metal colloids has been frequently
treated. As an example, shape modification of gold NPs with pulsed lasers involves
the interaction of laser radiation at a wavelength that is close to the localized surface
plasmon resonance of the nanocrystals and/or the interband transitions.
The first investigations on the effect of nanosecond laser pulses on aqueous
colloidal dispersions of spherical metal NPs showed that fragmentation and reshaping
were the two main effects at high and low laser fluences, respectively. Later on,
the influence of nanosecond and femtosecond laser pulses at different energies on
colloidal gold nanorods, as well as the corresponding structural transformations,
were studied. Femtosecond laser pulses at 800 nm led to reshaping of the anisotropic
NPs into spheres while keeping constant the initial volume, whereas nanosecond
pulses resulted predominantly in fragmentation of the Au NRs.
Recently Koshizaki has reported that suitable irradiation conditions produce
spherical submicrometer particles of different materials. With this method, previously prepared NPs were irradiated by unfocused nanosecond laser pulses in different
liquids. In the following Fig. 4.5 we report the result of our experiment which
indicates that submicrometric spherical TiO 2 particles can be obtained in water
suspension starting from random sized titania nanostructures.
The sphere formation mechanism is pretty intuitive [31]. When a colloidal solution
is irradiated with a pulsed laser, only the solid particles are heated. The selective
heating process depends on the laser energy absorption efficiency of the single solid
particles, as well as that of the environment.
G. Compagnini et al.
to significant advantages for quality demanding nanotechnology applications in
biomedicine and catalysis.
(d) Electroaffinity: laser-generated (noble) metal colloids are electron acceptors
because of surface atom oxidation resulting in a relatively high particle surface
charge. If this charge is not screened by impurities, the particles attract oxygen
species and the resulting surface charge triggers electrostatic repulsion. At the
same time, the Lewis-acidity of the inorganic nanoparticle makes it possible to
achieve efficient electron-donative ligand adsorption.
(e) Defects: under controlled conditions, defect-rich materials and suboxides can
be sythesized, potentially broadening the range of optical, semiconducting, or
catalytic properties.
Besides the formation of nanomaterials by using an ablation process, power laser
beams can be used to modify plenty of nanostructures which have been previously
obtained using other methods. In this case, the laser energy triggers many possible
phenomena such as fragmentation, melting, or surface functionalization.
4.2.2 Irradiation of Nanoparticles Colloids
Pulsed laser selective heating of existing nanoparticle colloids has been used by
several authors in order to modify the structure and/or the chemical nature of a
colloidal dispersion [27–30]. The case of noble metal colloids has been frequently
treated. As an example, shape modification of gold NPs with pulsed lasers involves
the interaction of laser radiation at a wavelength that is close to the localized surface
plasmon resonance of the nanocrystals and/or the interband transitions.
The first investigations on the effect of nanosecond laser pulses on aqueous
colloidal dispersions of spherical metal NPs showed that fragmentation and reshaping
were the two main effects at high and low laser fluences, respectively. Later on,
the influence of nanosecond and femtosecond laser pulses at different energies on
colloidal gold nanorods, as well as the corresponding structural transformations,
were studied. Femtosecond laser pulses at 800 nm led to reshaping of the anisotropic
NPs into spheres while keeping constant the initial volume, whereas nanosecond
pulses resulted predominantly in fragmentation of the Au NRs.
Recently Koshizaki has reported that suitable irradiation conditions produce
spherical submicrometer particles of different materials. With this method, previously prepared NPs were irradiated by unfocused nanosecond laser pulses in different
liquids. In the following Fig. 4.5 we report the result of our experiment which
indicates that submicrometric spherical TiO 2 particles can be obtained in water
suspension starting from random sized titania nanostructures.
The sphere formation mechanism is pretty intuitive [31]. When a colloidal solution
is irradiated with a pulsed laser, only the solid particles are heated. The selective
heating process depends on the laser energy absorption efficiency of the single solid
particles, as well as that of the environment.