4.4 Laser-Ablation Process 53
of the fast evaporation in the high-power laser pulse, even the stoichiometry of a
complex mixed target may be preserved in the vapor phase. During the pulse, a
plume, a supersonic jet of evaporated material, is ejected perpendicular to the
surface into the surrounding gas and expands adiabatically. Immediately after the
laser pulse, the temperature in the plume reaches values of a few thousand Kelvin
[4]. During the adiabatic expansion of the plume, the temperature decreases and
particles are formed. The continuous stream of carrier gas transports the particles
to the powder collector. The carrier gas may also contain reactive gas components;
to obtain oxides, oxygen, for carbides methane, CH 4 , and for nitrides ammonia,
NH 3 , is added. Due to the supersaturation in the vapor caused by decreasing
temperature within the plume, formation of particles occurs. The duration of the
supersaturated conditions is limited by the adiabatic expansion of the plume;
therefore, the gas pressure in the reaction vessel plays a crucial role in particle
nucleation and particle growth. At low gas pressure, the plume expands very
rapidly. Therefore, the concentration of reactive species in the plume decreases
very rapidly, a process limiting particle growth. At higher gas pressure, the supersaturation is higher. The higher the supersaturation, the smaller the size of the
nuclei required for condensation. This leads to a large number of nuclei and, and
as consequence, to smaller particle sizes. This simplified description of the
complex processes in the plume is well supported by experimental results. As an
example, Figure 4.12 displays the dependency of Co 3 O 4 particle size on the gas
pressure [5]. This figure shows small particles are formed at low and at higher gas
pressure, in an intermediate range the particles are largest.
Figure 4.11 Schematic drawing of the
experimental setup for nanoparticle synthesis
applying laser ablation. The pulsed laser
beam is focused at the surface of the
precursor target that may be a metal or an
oxide. The high-intensity laser beam causes a
plume, a supersonic jet of evaporated
material, which is ejected perpendicular to
the target surface, expanding into the gas
space above the target. The particles formed
by condensation in the plume are
transported with the carrier gas to the
powder collector.
Laser beam
OpƟcal system
ReacƟon and
carrier gas in
Plume
Precursor target
Vacuum vessel
NanoparƟcles
To powder
collector
of the fast evaporation in the high-power laser pulse, even the stoichiometry of a
complex mixed target may be preserved in the vapor phase. During the pulse, a
plume, a supersonic jet of evaporated material, is ejected perpendicular to the
surface into the surrounding gas and expands adiabatically. Immediately after the
laser pulse, the temperature in the plume reaches values of a few thousand Kelvin
[4]. During the adiabatic expansion of the plume, the temperature decreases and
particles are formed. The continuous stream of carrier gas transports the particles
to the powder collector. The carrier gas may also contain reactive gas components;
to obtain oxides, oxygen, for carbides methane, CH 4 , and for nitrides ammonia,
NH 3 , is added. Due to the supersaturation in the vapor caused by decreasing
temperature within the plume, formation of particles occurs. The duration of the
supersaturated conditions is limited by the adiabatic expansion of the plume;
therefore, the gas pressure in the reaction vessel plays a crucial role in particle
nucleation and particle growth. At low gas pressure, the plume expands very
rapidly. Therefore, the concentration of reactive species in the plume decreases
very rapidly, a process limiting particle growth. At higher gas pressure, the supersaturation is higher. The higher the supersaturation, the smaller the size of the
nuclei required for condensation. This leads to a large number of nuclei and, and
as consequence, to smaller particle sizes. This simplified description of the
complex processes in the plume is well supported by experimental results. As an
example, Figure 4.12 displays the dependency of Co 3 O 4 particle size on the gas
pressure [5]. This figure shows small particles are formed at low and at higher gas
pressure, in an intermediate range the particles are largest.
Figure 4.11 Schematic drawing of the
experimental setup for nanoparticle synthesis
applying laser ablation. The pulsed laser
beam is focused at the surface of the
precursor target that may be a metal or an
oxide. The high-intensity laser beam causes a
plume, a supersonic jet of evaporated
material, which is ejected perpendicular to
the target surface, expanding into the gas
space above the target. The particles formed
by condensation in the plume are
transported with the carrier gas to the
powder collector.
Laser beam
OpƟcal system
ReacƟon and
carrier gas in
Plume
Precursor target
Vacuum vessel
NanoparƟcles
To powder
collector