18
2 Instrumentation for Cluster Science
flows over the surface will be ionized due to quantum effects or background radiation. Consequently, the ejected electrons display helical motion under the magnetic
field restriction; simultaneously, the positively charged Ar
+ suddenly feels a large
coulombic attraction towards the cathode target. As the Ar
+ ions impact the surface
at high speed, enough energy is transferred to eject some atoms, electrons, and/or
nascent clusters off the target. The ejected target clusters then expand outward and
accompany with the buffer gas (e.g., He) to get through the nozzle. The electrons
ejected from the target surface could collide with additional argon atoms, causing
further ionization and hence intensifying the sputtering process [107].
Similar to other cluster sources, the plasma of atoms/clusters created near the
magnetron head can also cause aggregation before they exit from the source via an
adjustable iris. Analogous to Eq. 2.1, the sizes of clusters formed by such a MagS
source are determined by the aggregation time and the speed of the initial aggregation.
In general, the aggregation time can be adjusted by increasing or decreasing the
flow inside the source, including the backing gas helium (also working gas Ar), the
distance between the magnetron head and the exit iris, as well as the hole size of
the iris [107, 112]. It is worth mentioning that an RF power supply instead of the
DC power supply could make the MagS source applicable to non-conductive target
maerials [113] Recently, there has been combined applications of the MagS-source
with a thermal vaporization source (TVa-source), as shown in Fig. 2.5C, enabling
to syntheisize metal-organic complexes and clusters directly from the gas phase
reactions [111].
2.2 Cluster Growth and Statistical Principles
The growth of clusters is an important research scheme, but it is still open to academic
debate on several issues, for instances, whether the monomers can be simply considered as single atoms or initially small clusters/molecules, and to what extent cluster
may be meticulously tuned and dissociated [114, 115]. In general, cluster growth is
initated by atoms, followed by three-body collisions that grow dimers/trimers in a
sequential process. Simply, this process can be expressed as,
M + M + L → M 2 + L
(2.1)
where “M” aims at a monomer; “L” refers to a collision gas, single or multiple
molecules, providing a higher density of third bodies and it promotes condensation.
This is why the condensation efficiency depends on collision gas pressures and it
can be optimized by using a proper nozzle to adjust the density in the condensation
process.
Besides the promotion of condensation, the collision gas can take place in other
excitation and relaxation processes in the plasma, where collision-induced energy
transfer or quenching removes translation, electronic, vibrational and rotational
energy from the metal atoms or growing clusters, expressed as:
2 Instrumentation for Cluster Science
flows over the surface will be ionized due to quantum effects or background radiation. Consequently, the ejected electrons display helical motion under the magnetic
field restriction; simultaneously, the positively charged Ar
+ suddenly feels a large
coulombic attraction towards the cathode target. As the Ar
+ ions impact the surface
at high speed, enough energy is transferred to eject some atoms, electrons, and/or
nascent clusters off the target. The ejected target clusters then expand outward and
accompany with the buffer gas (e.g., He) to get through the nozzle. The electrons
ejected from the target surface could collide with additional argon atoms, causing
further ionization and hence intensifying the sputtering process [107].
Similar to other cluster sources, the plasma of atoms/clusters created near the
magnetron head can also cause aggregation before they exit from the source via an
adjustable iris. Analogous to Eq. 2.1, the sizes of clusters formed by such a MagS
source are determined by the aggregation time and the speed of the initial aggregation.
In general, the aggregation time can be adjusted by increasing or decreasing the
flow inside the source, including the backing gas helium (also working gas Ar), the
distance between the magnetron head and the exit iris, as well as the hole size of
the iris [107, 112]. It is worth mentioning that an RF power supply instead of the
DC power supply could make the MagS source applicable to non-conductive target
maerials [113] Recently, there has been combined applications of the MagS-source
with a thermal vaporization source (TVa-source), as shown in Fig. 2.5C, enabling
to syntheisize metal-organic complexes and clusters directly from the gas phase
reactions [111].
2.2 Cluster Growth and Statistical Principles
The growth of clusters is an important research scheme, but it is still open to academic
debate on several issues, for instances, whether the monomers can be simply considered as single atoms or initially small clusters/molecules, and to what extent cluster
may be meticulously tuned and dissociated [114, 115]. In general, cluster growth is
initated by atoms, followed by three-body collisions that grow dimers/trimers in a
sequential process. Simply, this process can be expressed as,
M + M + L → M 2 + L
(2.1)
where “M” aims at a monomer; “L” refers to a collision gas, single or multiple
molecules, providing a higher density of third bodies and it promotes condensation.
This is why the condensation efficiency depends on collision gas pressures and it
can be optimized by using a proper nozzle to adjust the density in the condensation
process.
Besides the promotion of condensation, the collision gas can take place in other
excitation and relaxation processes in the plasma, where collision-induced energy
transfer or quenching removes translation, electronic, vibrational and rotational
energy from the metal atoms or growing clusters, expressed as: