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I. Shyjumon, Rainer Hippler

Structural deformation & Lattice parameter studies of size selected Ag clusters. I. Shyjumon, Rainer Hippler. Institut für Physik, University of Greifswald, Germany. Outline. Introduction Materials & Methods Nanocluster source Characterization techniques (AFM & GIXD)

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I. Shyjumon, Rainer Hippler

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  1. Structural deformation & Lattice parameter studies of size selected Ag clusters I. Shyjumon, Rainer Hippler Institut für Physik, University of Greifswald, Germany I.Shyjumon

  2. Outline • Introduction • Materials & Methods • Nanocluster source • Characterization techniques (AFM & GIXD) • Results & Discussion • Size selection of Ag nanoclusters • Structural deformation of Ag – nanoclusters • Melting point & lattice parameter studies • Summary & Outlook I.Shyjumon

  3. Introduction – What is a cluster ? Clusters – Intermediate range between atoms and bulk matter Aggregates of 2–10n particles (n  6 or 7) I.Shyjumon

  4. Types of Clusters - 1 Based on the type of constituent particles & type of bonding Semiconductor Clusters Rare gas Clusters Metal Clusters S block – Metallic bond (e.g. alkali & alkaline earth) SP metals – Covalent bond (e.g. Al) Bonding -Van der Waals Inter-atomic attraction increases with increasing atomic mass (HeRn). Bonding – Covalent e.g. C, Si & Ge http://www.tc.bham.ac.uk/~roy/Teaching/CHM3T1 I.Shyjumon

  5. Types of Clusters - 2 Ionic Clusters Molecular Clusters Cluster molecules thermodynamically and/or kinetically stable with respect to coalescence e.g. Al13MgAl13 atoms with large difference in electronegativity Bonding – Ionic e.g. (NaCl)n, [MgxOy]2(xy)+ van der Waals, dipole-dipole interactions, and hydrogen bonding e.g. (N2)n, (C6H6)n, (H2O)n I.Shyjumon

  6. Why clusters ? - Applications • Clusters are of fundamental interest: • due to their intrinsic properties and reduced size • because of their central position between molecular and condensed matter science. Technical Importance due to the unique electronic & optical properties • Basic use in the fabrication of thin films • They have potential as a catalyst in certain chemical reactions. • Ag clusters used in photography & in electronic devices • Energetic cluster interaction on surface surface cleaningsurface metallization, precision machining I.Shyjumon

  7. Materials & Methods – I • Nanocluster source • Cluster formation • Cluster growth I.Shyjumon

  8. Nanocluster source • Fast metal atoms are formed by the cathode bombardment of ions • A part of thermalized atoms are transferred into clusters I.Shyjumon

  9. G = ko /(KNa) Growth parameter Cluster formation Cluster formation byGas aggregation Mk+M  Mk+1 2M+A  M2+A, M, A – metal and buffer gas atoms Size of formed cluster n  G3/4 I.Shyjumon

  10. Mechanisms of cluster growth If free atoms (or molecules) dominates Mn + Mq Mn+q Takes place at high temperature I.Shyjumon

  11. Materials & Methods – II • Characterization techniques • Atomic force microscopy (AFM) • X-ray diffraction (XRD) I.Shyjumon

  12. Characterisation Techniques • Atomic Force Microscopy (AFM) • X-Ray Diffraction (XRD) I.Shyjumon

  13. AFM – Width measurement Width = w+2d • deconvolution by Au colloidal particles, d ~ 4 nm I.Shyjumon

  14. Results & Discussion • Size selection of clusters • Structural deformation of Ag nanoclusters • Melting point & lattice parameter studies I.Shyjumon

  15. M ~ 1.2×105 amu Size selection of Ag nanoclusters • Mass selection by mass filter • Clusters are selected according to M/e • Size selection is also possible by varying the aggregation tube length I.Shyjumon

  16. Cluster Flattening m = 2.3×105 amu, size ~ 4.1 nm, N ~2131 atoms t = 240 sT = 198 K I.Shyjumon

  17. Processes in cluster-surface collision W. Harbich, in Metal Clusters at Surfaces, edited by K.H. Meiwes-Broer(Springer, Berlin, 2000). I.Shyjumon

  18. Cluster Flattening mechanism • Cluster surface collision mechanisms depend on different parameters Cluster size, N E = 0.1 to 1 eV/atom Impact energy, E • Front atoms are stopped due to collision I.Shyjumon

  19. Size effect on melting point • expt. with = 0.5°, 2Θ = 36° to 46°; Heating : 373K up to 973K t = 15 min bulk Ag,M.P 1230 K • Small melting temperature due to small cluster size • Theoretical Gibbs-Thomson equation I.Shyjumon

  20. Lattice constant change with size • Strain due to residual tensile stress t = 15 min, Vs = -1.5 KV • Strain as a result of lattice disorders & structural disorders • influence of local melting reduced with increase in size I.Shyjumon

  21. Summary & Outlook • Size selected cluster deposition done in two different ways. • Flattening of clusters occurs with increase in substrate bias. • A proportional increase of cluster melting point with size is observed • Lattice constant increases at small cluster sizes and decreases after a a size of 12 nm. • MD simulation studies on the cluster surface impact processes. • Studies with other substrate conditions such as temperature I.Shyjumon

  22. Acknowledgement I. Shyjumon, M. Gopinadhan, O. Ivanova, M. Quass, H. Wulff, C. A. Helm, R. Hippler, European Physical Journal D,(in print, Nov 2005) DOI:10.1140/epjd/e2005/00319x Thanks to: Prof. Christiane A. Helm & Manesh Gopinadhan, Dr. Harm Wulff & Dr. Marion Quaas, Thank you all…….. I.Shyjumon

  23. Size selection of Ag nanoclusters-I • Size selection by varying the aggregation tube length t = 10 s, T = 148 K • Aggregated clusters are omitted in AFM height analysis I.Shyjumon

  24. Cluster - surface collision In general collision processes can be of 3 types Low energy, Medium energy & High energy The important parameters that control the process are, Cluster sizeN (2  N  105), Impact energyE (10-2 to 108 eV) Impact angleΦin, charge state, cluster & surface temperature Cohesive energies of clusterEclcohand surfaceEscoh I.Shyjumon

  25. Size selection without & with mass filter aggregation tube length = 16 cm t = 55 s t = 25 s Aggregated clusters were omitted in AFM height analysis I.Shyjumon

  26. Quadrupole mass filter V- voltage (1-250 Volt) f – frequency (3-100 kHz) d=25.4 mm, diameter of poles K – Mass calibration factor (0.5 to 2) – calibration of cluster mass with other characterization techniques I.Shyjumon

  27. Quadrupole mass filter Clusters are mass selected by quadrupole mass filter Expected height & measured heights fits well- shows the perfect working of the mass filter I.Shyjumon

  28. Rp+RT RT –h/2 w/2 cluster tip 4 3 w Height [nm] 2 1 0 0 10 20 30 40 50 60 70 X[nm] M= 3 10 Comparison –spherical clusters Si tip (RT~ 6nm) J.Vesenka (et.al), BioPhy.J. 65, 992 (1993) M.Gopinadhan I.Shyjumon

  29. 10 nm Si tip 6 nm Hi‘ res tip 100nm 100nm - 50 nm - - 50 nm - Nioprobe sample With HI’RES tip 15 nm Sharp peaks of less than 12nm Determine the shape of the tip apex 0 nm 250 nm TEM image of Nioprobe AFM analysis (HI’ RES Tip Geometry) Scanning Probe Image Processor (SPIP) I.Shyjumon

  30. Size determination by XRD mean effective particle sizes in direction of the diffraction vector Scherrer equation diffraction line is convolution of physical line profile & instrumental profile L is the domain of definition of the experimental line profile http://www.uni-leipzig.de/~iom/muehlleithen/2003/2003_wulff.pdf I.Shyjumon

  31. GIXD measurements Step size of heating reduced to 20 K/min near the melting temperature I.Shyjumon

  32. Cluster catalysts Supported [Ru6C] cluster catylysts on TiO2 – ammonia synthesis, decomposition of NOx Clusters of Ru3(Co)12 – assymetric transfer hydrogenation of ketones Small Ti clusters for catalysis of hydrogen exchange in NaAlH4 nanoscale silver oxide (Ag2O) - strong photoactivated emission for excitation wavelengths shorter than 520 nm I.Shyjumon

  33. Chemical composition of Ag clusters Ag clusters on Si(100); t = 5 min Ag(111) is the most intensive peak in randomly oriented polycrystalline Ag I.Shyjumon

  34. Comparison of Ti & Ag clusters Sputtering yeild Ti - 0.37 & Ag -1.4 Larger surface coverage of Ag clusters due to high sputter yeild Larger mean size for Ti clusters are due to oxidation I.Shyjumon

  35. The Magnetron • A planar magnetron is a system of ring-magnets, placed under the target • The additional magnetic field causes a Lorentz-force which is forcing the electrons on a circular „race track“ • The field lines are well confined around the target racetrack and electron loss is reduced to minimum • Advantage: a high sputter-rate even in very low pressure conditions • Disadvantage: the target material is sputtered only in the „race track“-region Ref: S.Wrehde I.Shyjumon

  36. Plasma conditions Measurements by V. Stranak & S. Wrehde in Plasma process monitor Energy –resolved mass spectroscopy Energy distribution of 36Ar+, at 0.7 pa R.Hippler et. Al, Contrib.Plasma Phys. 45, No. 5-6, 348 (2005) I.Shyjumon

  37. The Magnetron Unbalanced mode Inner magnet down Outer magnet up The basic principle of the unbalanced magnetron is to allow electron release from the magnetron plasma in order to create ionisation away from the magnetron and at the substrate. This ionisation is then used as an additional form of energy available for the growth of the thin film. Ref: S.Wrehde I.Shyjumon

  38. Formation of clusters-Theory No. Density of ideal gas atom at P=1 atm & T =273 K, N=2.69×1019 cm-3 At T =200 K & P = 0.1 mbar = 10-4 atm, NAr ~ 4×1015 cm-3 , ~ 2×10-11 cm3/s K = 10-32 cm6/s & G ~ 5×105, average size. of cluster, n = 0.3 ×G3/4 = 104 atoms Experimentally for Ti clusters, average size is h =12 nm, r = 60 Å n = 3×104 atoms Clusters flux, N – surface coverage, t –time of deposition I.Shyjumon

  39. Formation of clusters-Theory-II At T =200 K, P = 0.1 mbar, gas flow = 12 Sccm 12 Sccm = 12×1000 mbar. cm3/60s = 0.2 bar. cm3/s 1 bar = 6.023×1023/22400 /cm3 12 Sccm = 0.2 bar. cm3/s × 6.023×1023/22400 /cm3 , argon atoms = 5 ×1018 /s Na at P = 0.1 mar & T =200 K is 4×1015 / cm3, volume of the tube is 1200 cm3 So residence time of Ar atoms τ = 1 s At magnetron voltage V = 225 & I = 0.5 A, sputter yeild for Ti is Y = 0.37 0.5 A/1.6×10-19 A.s, Ar atoms are hitting the target, producing, × Y Ti atoms, ~ 0.12×1019 /s Ti atoms I.Shyjumon

  40. Extraction of +ve & -ve clusters • +ve clusters are found to have more in numbers than –ve ones I.Shyjumon

  41. X-ray reflectivity results Ti (Oxide) clusters on Si wafer Dimater, d = 2π / ΔQ I.Shyjumon

  42. UV-visible spectra of TiO2 in quartz No peak observed at 410 nm – characteristic absorption peak for Ti Concludes that the nanoparticles are fully oxidised (no free electron present) I.Shyjumon

  43. Cluster size dependence on time wall temperature, T = 148 K t = 120 s N/µm2 - 280 to 390 h – 14 to 24 nm Around 30 s, the second layer growth - starts before the first layer is completed I.Shyjumon

  44. Cluster size dependence on temperature time of deposition:- 15 s, P = 100 W, at 0.1 mbar • the temperature dependence of size is due to cluster-Ar interaction, that increases the coagulation processes I.Shyjumon

  45. Ti2p3/2 – 458.8 eV TiO2 Chemical composition • Chemical composition analysed by XPS Ti/O - 1:2 • Clusters on the surface are fully oxidised TiO2 I.Shyjumon

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