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Congresso del Dipartimento di Fisica Highlights
in Physics 2005 1114 October 2005, Dipartimento
di Fisica, Università di Milano High intensity
cluster beams an enabling technology for
nanostructured materials synthesis and
free-cluster experiments G. Bongiorno,
P. Piseri, E. Barborini, S. Vinati, T. Mazza and
P. Milani CIMAINA and CNR-INFM, Dipartimento di
Fisica, Università degli Studi di Milano, Via
Celoria 16, I-20133 Milano, Italy.
Supersonic Cluster Beam Deposition
Abstract Nanostructured cluster assembled
materials are systems of great interest due to
their high porosity and high specific surface.
These properties make these systems interesting
for applications in electrochemistry, catalysis
and gas sensing. In order to deposit thin films
of nanostructured cluster assembled materials for
industrial applications the use of high intensity
cluster beams is mandatory. The physical and
chemical properties of cluster assembled
materials are strictly related to the properties
of the clusters free in the beam. Therefore it is
very important to analyze the clusters prior to
deposition, not only in terms of mass
distribution, but also from the point of view of
their structure, electronic properties, and
thermodynamic state. As a result, high intensity
cluster beams are needed not only to achieve high
deposition rates but also to perform experiments
on free clusters. In this poster we report on
an evolved version of the Pulsed Microplasma
Cluster Source (PMCS), developed at the Molecular
Beams and Nanocrystalline Materials Laboratory in
Milano, which is able to deliver highly
collimated and intense pulsed cluster beams of
refractory materials (in the case of carbon
cluster beams the deposition rate is about
100µm/h at 500mm source-substrate distance and
with a 1cm2 of covered area). The mass
distribution of the produced beams is lognormal
in the range 0-few thousands of atoms/cluster,
with an average size of few hundreds of
atoms/cluster depending on the source operation
conditions. By means of aero-dynamical effects is
possible to operate mass selection on the
produced clusters (aero-dynamical nozzles can be
used as band-pass filters) and to greatly
collimate the beams. Nanostructured thin films
prepared with this approach have been used as
active components in gas and humidity sensors and
fuel cells. The high intensity of this source
(up to 1013 cluster/cm3) has been employed in
order to perform mass resolved X-ray absorption
experiments on free titanium clusters (mass
distribution range 0-1000 atoms/cluster with a
maximum at 320 atoms/cluster) in PEPICO mode at
the Ti L-edge.
Pulsed Microplasma Cluster Source Principle Of
Operation
Deposition Regime
Cathode
2
1
3
4
Pulsed valve
Nozzle
Anodes
Injection of an highly collimated gas pulse
Thermalization of the ablated material and
cluster aggregation
Supersonic expansion of the mixture gas-clusters
Microplasma formation due to an intense electric
discharge and ion sputtering of cathode surface
P. Milani, S. Iannotta, Cluster Beam Synthesis of
Nanostructured Materials, Springer Verlag, Berlin
1999
Aerodynamic confined target erosion

• Erosion performances with graphite target
• Localized erosion FWHM lt 0.7 mm
• C 210-4 mm3/pulse
• C 21016 atoms C / pulse
• No contamination from the source body

H. Vahedi-Tafreshi et al., Aerosol Sci. Technol.
36, 593 (2002) H. Vahedi-Tafreshi et al., J.
Nanoparticle Res. 4, 511 (2002)
E. Barborini, P. Piseri, P.Milani, J. Phys. D,
Appl. Phys. 32, L105 (1999)
Pulsed Microplasma Cluster Source
Source-nozzle mass selection and inertial
focusing
the essential action of a gas centrifuge could
be reproduced without any moving parts by
allowing gas to expand at high velocity into a
jet having curved lines of flow. P.A.M. Dirac,
Rep. Of U.K.A.E.A. declassified in 1953

• Control on clusters
• Dimensions
• Position
• Chemical reactivity
• Coalescence

Mass selection mechanism
Focusing nozzle
Developed at Laboratorio Getti Molecolari e
Materiali Nanocristallini,Department of Physics,
University of Milano (Italy)
P. Piseri, et al., Rev. Sci. Instrum. 72, 2261
(2001) H. Vahedi Tafreshi et al., Aerosol Sci.
Technol. 36, 593 (2002)
Stokes number is defined as the ratio between
particle stopping distance and a characteristic
length of the system. It depends of upstream
pressure, nozzle diameter, particle size and
density. Exists a critical Stokes number St, at
which particles cross the jet axis at infinity,
corresponding to zero divergence angle downstream
of the nozzle. Particles with a Stokes number
smaller than St do not have enough inertia to
cross the jet axis, while particles with a Stokes
number larger than St cross the axis at finite
distances and the divergence angle increases
asymptotically as St increases.
Performance low divergence and high deposition
rate
High resolution patterning by means of stencil
masks
Nanoparticle focusing in aerodynamic lens systems
P0 2.6 Torr dp 15nm
Ns-C patterned film
P0 2.6 Torr dp 1000nm
Substrate
Mask
E. Barborini et al. Appl. Phys. Lett. 77, 1059
(2000)
F.J. de la Mora, P. Riesco-Chueca, J. Fluid.
Mech. 195, 1 (1988)
Microfabrication of nanostructured 3D-objects
CESyRa project _at_ Gasphase
Total Ion Yield NEXAFS spectrum of free Titanium
clusters
Cluster assembled ns-C
Photodiode

• Source working _at_ 5 Hz
• Ti Cluster density (peak) 1013 cl/(cm3s)
• Pulse length 50 ms
• Beam velocity 1000 m/s

Light clusters
Heavy clusters
Piezo
Ti L-edge
Electron counting 1 kHz
Ns-C tower created by depositing an highly
collimated beam produced by means of a 5 lenses
aerodynamic system
Ions are in the mass range 80 1960 Ti atoms
Channeltrons
Electrochemical applications
Chemistry in the PMCS
Mons-C
Ptns-C
ns-CNx
NH3 as carrier gas
metallorganic precursor
composite cathode