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Congresso del Dipartimento di Fisica Highlights in Physics 2005 11–14 October 2005, Dipartimento di Fisica, Università di Milano High intensity cluster.

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Presentation on theme: "Congresso del Dipartimento di Fisica Highlights in Physics 2005 11–14 October 2005, Dipartimento di Fisica, Università di Milano High intensity cluster."— Presentation transcript:

1 Congresso del Dipartimento di Fisica Highlights in Physics –14 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, Universit à degli Studi di Milano, Via Celoria 16, I 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 1cm 2 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 cluster/cm 3 ) has been employed in order to perform mass resolved X-ray absorption experiments on free titanium clusters (mass distribution range atoms/cluster with a maximum at 320 atoms/cluster) in PEPICO mode at the Ti L-edge. Supersonic 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 HV pulsed P ulsed M icroplasma C luster S ource : Principle Of Operation 1 mm Ø 6.3 mm Erosion performances with graphite target: Localized erosion: FWHM < 0.7 mm C: ~ 2·10 -4 mm 3 /pulse C: ~ 2·10 16 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 Source Developed at Laboratorio Getti Molecolari e Materiali Nanocristallini, Department of Physics, University of Milano (Italy) PMCS SOURCE CHAMBER DEPOSITION CHAMBER cluster beam Substrate Deposition Apparatus source  >> 1 Fragmentation  << 1 Memory effect P. Milani, S. Iannotta, Cluster Beam Synthesis of Nanostructured Materials, Springer Verlag, Berlin 1999 Deposition Regime E. Barborini, P. Piseri, P.Milani, J. Phys. D, Appl. Phys. 32, L105 (1999) Pulsed valve Cathode Gas stagnation point Aerodynamic 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 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 nozzle Mass selection mechanism Stream Lines St~1 St>>1 St<<1 P. Piseri, et al., Rev. Sci. Instrum. 72, 2261 (2001) H. Vahedi Tafreshi et al., Aerosol Sci. Technol. 36, 593 (2002) aerodynamic lens systems Nanoparticle focusing in aerodynamic lens systems P 0 = 2.6 Torr d p = 15nm P 0 = 2.6 Torr d p = 1000nm PMCS with an aerodynamic lenses system Skimmer Substrate 40 mm500 mm Area 75 mm 2 Source Rate 5-10 Hz Deposition Rate  m/h 15 mm 5 mm Source performance for ns-C deposition Performance: low divergence and high deposition rate Cluster beam Mask Substrate E. Barborini et al. Appl. Phys. Lett. 77, 1059 (2000) 20  m 5  m High resolution patterning by means of stencil masks Microfabrication of nanostructured 3D-objects 2mm 400  m Ns-C tower created by depositing an highly collimated beam produced by means of a 5 lenses aerodynamic system  Source 5 Hz;  Ti Cluster density (peak): cl/(cm 3 s)  Pulse length: ~ 50 ms;  Beam velocity: ~ 1000 m/s; 1 kV/cm Beamline Differential vacuum chamber: P = mbar Interaction chamber: P = mbar Cluster source Photodiode Piezo Channeltrons CESyRa 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 of free Titanium clusters Mass spectra of C clusters Standard cylindrical nozzle Focusing nozzle Aerodynamic lens assembly 5nm 2.5nm Cluster assembled ns-C F.J. de la Mora, P. Riesco-Chueca, J. Fluid. Mech. 195, 1 (1988) Ns-C patterned film Chemistry in the PMCS 100 nm Molybdenum Carbon composite cathode 5nm composite cathode Anode Rotating catode Aerodinamical lenses He Composite cathode COUPLED 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-C ns-CN x NH 3 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 Capacitive Humidity Sensor (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. Sketch of the top view Prototype realized in collaboration with Maxwell Technologies.Inc Capacitance C = 0.2 F Specific capacitance C s = 12.7 F/g Resistance ESR = 24 Ohm Energy density E = 0.03 Wh/kg Power density P = 10 kW/kg Electrode width25 mm Electrode length125 mm Thickness5  m ns-C coated Al electrodes (double side) collector electrode separtor Electrical contact Winding technology Supercapacitors (ns-C) Pt:ns-C film deposited on both sides of Nafion membranes (area: 16 cm 2 ; thickness: from few tens of nanometers to 500nm). Air H2H2 Electrical contact Graphite charge collector Pt:ns-C film Nafion Membrane Electrical contact Graphite charge collector H+H+ Air+H 2 O e-e- e-e- PEM Fuel Cell (Pt:ns-C) Cell performance: Surface exposed: 4.8 cm 2 H 2 pressure: 2 bar Air pressure: 2 bar Cell voltage 800mV (open circuit) Power: mW (depending on sample) Specific power ~300W/g Pt (best performance up to date) Pt:ns-C film Electrochemical applications


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