Congresso del Dipartimento di Fisica Highlights in Physics 2005

11–14 October 2005, Dipartimento di Fisica, Università di Milano

High intensity cluster beams: an enabling technology for nanostructured materials synthesis and free-cluster experimentsHigh intensity cluster beams: an enabling technology for nanostructured materials synthesis and free-cluster experimentsG. 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.

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.

Supersonic Cluster Beam DepositionSupersonic Cluster Beam Deposition

Thermalization of the ablated material and cluster aggregation

Supersonic expansion of the mixture gas-clusters

Injection of an highly collimated gas pulse

Pulsed valve

Anodes

Cathode

Nozzle

Microplasma formation due to an intense electric

discharge and ion sputtering of cathode surface

HVpulsed

PPulsedulsed M Microplasmaicroplasma C Cluster luster SSourceource:: Principle Of Operation Principle Of Operation

1 mm

Ø 6.3 mm

Erosion performances with graphite target: • Localized erosion: FWHM < 0.7 mm• C: ~ 2·10-4 mm3/pulse• C: ~ 2·1016 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)

Control on clusters:

Dimensions

Position

Chemical reactivity

Coalescence

Anode

Rotating cathode

Aerodinamical lenses

GAS

Pulsed Microplasma Cluster SourcePulsed Microplasma Cluster Source

Developed at Laboratorio Getti Molecolari e Materiali Nanocristallini,Department of Physics, University of Milano (Italy)

PMCSPMCS

SOURCE CHAMBERSOURCE CHAMBER

DEPOSITION CHAMBER

cluster beam

cluster beam

SubstrateSubstrateDeposition ApparatusDeposition Apparatus

source

>> 1

Fragmentation << 1

Memory effect

ClCoh

ClClKin

E

NE /

P. Milani, S. Iannotta, Cluster Beam Synthesis of Nanostructured Materials, Springer Verlag, Berlin 1999

ClKin

Cl

ClCoh

E KineticEnergy

N Atoms

E CohesionEnergy

Deposition RegimeDeposition Regime

E. Barborini, P. Piseri, P.Milani, J. Phys. D, Appl. Phys. 32, L105 (1999)

Pulsed valve Cathode

Gas stagnation point

Aerodynamic confined target erosionAerodynamic confined target erosion

“…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

Source-nozzle: mass selection and inertial focusingSource-nozzle: mass selection and inertial focusing

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.

Focusing nozzleFocusing nozzle

Mass selection mechanism

Stream Lines

St~1

St>>1St<<1

P. Piseri, et al., Rev. Sci. Instrum. 72, 2261 (2001)H. Vahedi Tafreshi et al., Aerosol Sci. Technol. 36, 593 (2002)

Distance from inlet lens (cm)

Rad

iald

ista

nce

(mm

)

0 5 10 15 20 25 30

-5

0

5

10

15

20

25Po = 345 Pa (2.6 torrs)dp = 15 nm

Nanoparticle focusing in aerodynamic lens systemsaerodynamic lens systems

Distance from inlet lens (cm)

Radia

ldis

tance

(mm

)

0 5 10 15 20 25 30

-5

0

5

10

15

20

25Po = 345 Pa (2.6 torrs)dp = 1000 nm

P0 = 2.6 Torrdp = 15nm

P0 = 2.6 Torrdp = 1000nm

PMCS with an aerodynamic lenses system

Skimmer

Substrate

40 mm 500 mm

Area 75 mm2 Source Rate 5-10 HzDeposition Rate 50-150 m/h

15

m

m

5 mm

Source performanceSource performancefor ns-C depositionfor ns-C deposition

Performance: Performance: low divergencelow divergence and and high deposition ratehigh deposition rate

Cluster beam

MaskSubstrate

E. Barborini et al. Appl. Phys. Lett. 77, 1059 (2000)

20 m

5 m

High resolution patterning by High resolution patterning by means of stencil masksmeans of stencil masks

Microfabrication of Microfabrication of nanostructured 3D-objectsnanostructured 3D-objects

2mm

400m

Ns-C tower created by depositing an highly collimated beam produced by means of a 5 lenses aerodynamic system

Source working @ 5 Hz;

Ti Cluster density (peak): 1013 cl/(cm3s)

Pulse length: ~ 50 ms;

Beam velocity: ~ 1000 m/s;

1 kV/cm

Beamline

Differential vacuum chamber:P = 10-6mbar

Interaction chamber:P = 10-8mbar

Cluster source

Photodiode

Piezo

Channeltrons

CESyRa project @ GasphaseCESyRa project @ Gasphase

Electron counting: ~ 1 kHz

Heavy clusters

Light clusters

Ions are in the mass range 80 – 1960 Ti atoms

Ti L-edge

Total Ion Yield NEXAFS spectrum Total Ion Yield NEXAFS spectrum of free Titanium clustersof free Titanium clusters

Mass spectra of C clustersMass spectra of C clustersStandard cylindrical nozzleFocusing nozzleAerodynamic lens assembly

5nm2.5nm

Cluster assembled ns-CCluster assembled ns-C

F.J. de la Mora, P. Riesco-Chueca, J. Fluid. Mech. 195, 1 (1988)

Ns-C patterned film

Chemistry in the PMCSChemistry in the PMCS

100 nm

Molybdenum

Carbon

composite cathode

5nmcomposite cathode

AnodeAnode

Rotating catodeRotating catode

Aerodinamical Aerodinamical lenseslenses

HeHe

Composite cathodeCOUPLED CATHODE: qualitative control on

composition modifying the position of the interface between the two materials relative to the ablation point

SINTERED or COMPRESSED CATHODE: absolute control on composition

G. Bongiorno et al., J. Nanosci. Nanotech., 5, 1072, 2005<

Pt:ns-C

metallorganic precursor

Mo:ns-Cns-CNx

NH3 as carrier gas

E. Barborini et al., APL 81 , 3359 (2002) G. Bongiorno et al., Carbon 43, 1460 (2005)

Inert gas input

PMCS

Metallorganic precursor bubbler

Gas-phase injection

Cap

acit

ive

Hu

mid

ity

Sen

sor

(ns-

C)

Cap

acit

ive

Hu

mid

ity

Sen

sor

(ns-

C)

Fast and reversible changes in the capacitance have been observed as the relative humidity is cyclically varied.

Ambient air RH ~ 40%

Vacuum

Sensor Concept: two serial capacitors with two Au rear electrodes,

the ns-C film as the dielectric and a thin Au layer electrode on top.

1.5 m m0.8 m m

4.0

mm

1.0

mm

1 .0 m m

3.8 m m

5.0

mm

4 .5 m mSketch of the top viewSketch of the top view

Prototype realized in collaboration with Maxwell Technologies.Inc

Capacitance C = 0.2 FSpecific capacitance Cs = 12.7 F/gResistance ESR = 24 OhmEnergy density E = 0.03 Wh/kgPower density P = 10 kW/kg

Electrode width 25 mmElectrode length 125 mmThickness 5 m

ns-C coated Al electrodes ns-C coated Al electrodes (double side)(double side)

collector

electrode

separtor

Electrical contact

Winding technologyWinding technology

Su

per

cap

acit

ors

(n

s-C

)S

up

erca

pac

ito

rs (

ns-

C)

Pt:ns-C film deposited on both sides of Nafion membranes (area: 16 cm2; thickness: from few tens of nanometers to 500nm).

AirH2

Electrical contact

Graphite charge collector

Pt:ns-C film

Nafion Membrane

Electrical contact

Graphite charge collector

H+

Air+H2O

e- e-

PE

M F

uel

Cel

l (P

t:n

s-C

)P

EM

Fu

el C

ell

(Pt:

ns-

C)

Cell performance:

Surface exposed: 4.8 cm2

H2 pressure: 2 bar

Air pressure: 2 bar

Cell voltage 800mV (open circuit)

Power: 30-50 mW (depending on sample)

Specific power ~300W/gPt (best performance up to date)

Pt:ns-C film

Electrochemical applicationsElectrochemical applications