36
K. Illath et al.
in a container along with massive balls. By using a high-speed rotating ball, high
mechanical energy is applied. Particle size is reduced in the desired form with the
help of various high energy mills. The particles synthesized in this manner possess
superior physical properties such as enhanced solubility, and some new features to the
component are relying on the material and grain size. The requirement of high energy,
more prolonged time consumption, contamination of powder, and synthesis of sensitive nanostructures are disadvantages of the ball milling method. In mechanochemical based milling, the initial bulk material is added stoichiometrically and milled.
During milling, it undergoes repeated deformation, welding, and fracture of reactants mixture. Several chemical reactions are also evolved at the substrate and reagent
interface. Even though the method is simple, synthesized NPs are highly sensitive
to grinding conditions and require long term milling in the case of smaller particles
[34].
2.2 Laser Ablation
In this method, laser irradiation is used to synthesize NPs from bulk material. Due
to the application of laser energy, fragmentation of solid particles in the form of
NPs are formed, and they remain in liquid which surrounds the bulk material known
as a colloidal solution. The relative number of ablated atoms and generated NPs
depends on the pulse duration and energy of the laser. Further, ablation efficiency
and characteristics of formed particles rely on the wavelength of the laser, pulse
duration, ablation time, and laser fluence. It has several advantages such as simplicity
and effectiveness, properties can be changed by tuning the parameters, and particles
can be formed without adding surfactant also. The major drawback is that exposure
to laser for a long duration may reduce the overall ablation rate [34].
2.3 Sputtering
In sputtering, the bulk solid target material is vaporized with the help of inert gas
ions. The whole procedure is carried out in a vacuum chamber, and sputtering gas
is supplied and is kept under working pressure. By introducing high voltage into
the cathode, free electrons are generated. It follows a spiral path using a magnetic
system, where they collide with sputter gas atoms and leads to ionization of the gas.
Positively charged ions move towards the target, where they continuously impinge.
This process repeats, and if it occurs at an energy level higher than the surface binding
energy of target, an atom can be removed. This scattering of atoms forms a diffused
cloud in the form of NPs. This method is suitable for the synthesis of alloy NPs with
better control [34].
All the top-down approaches produce MNPs in naked form, which can further
agglomerate and hence not suitable for biomedical applications.
K. Illath et al.
in a container along with massive balls. By using a high-speed rotating ball, high
mechanical energy is applied. Particle size is reduced in the desired form with the
help of various high energy mills. The particles synthesized in this manner possess
superior physical properties such as enhanced solubility, and some new features to the
component are relying on the material and grain size. The requirement of high energy,
more prolonged time consumption, contamination of powder, and synthesis of sensitive nanostructures are disadvantages of the ball milling method. In mechanochemical based milling, the initial bulk material is added stoichiometrically and milled.
During milling, it undergoes repeated deformation, welding, and fracture of reactants mixture. Several chemical reactions are also evolved at the substrate and reagent
interface. Even though the method is simple, synthesized NPs are highly sensitive
to grinding conditions and require long term milling in the case of smaller particles
[34].
2.2 Laser Ablation
In this method, laser irradiation is used to synthesize NPs from bulk material. Due
to the application of laser energy, fragmentation of solid particles in the form of
NPs are formed, and they remain in liquid which surrounds the bulk material known
as a colloidal solution. The relative number of ablated atoms and generated NPs
depends on the pulse duration and energy of the laser. Further, ablation efficiency
and characteristics of formed particles rely on the wavelength of the laser, pulse
duration, ablation time, and laser fluence. It has several advantages such as simplicity
and effectiveness, properties can be changed by tuning the parameters, and particles
can be formed without adding surfactant also. The major drawback is that exposure
to laser for a long duration may reduce the overall ablation rate [34].
2.3 Sputtering
In sputtering, the bulk solid target material is vaporized with the help of inert gas
ions. The whole procedure is carried out in a vacuum chamber, and sputtering gas
is supplied and is kept under working pressure. By introducing high voltage into
the cathode, free electrons are generated. It follows a spiral path using a magnetic
system, where they collide with sputter gas atoms and leads to ionization of the gas.
Positively charged ions move towards the target, where they continuously impinge.
This process repeats, and if it occurs at an energy level higher than the surface binding
energy of target, an atom can be removed. This scattering of atoms forms a diffused
cloud in the form of NPs. This method is suitable for the synthesis of alloy NPs with
better control [34].
All the top-down approaches produce MNPs in naked form, which can further
agglomerate and hence not suitable for biomedical applications.