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Carbon Nanotubes

Carbon Nanotubes. ‘New Materials for the Twenty-first Century‘. Nanotube. Carbon Nanotubes. Introduction. Introduction. What are CNTs ?. Fullerene. Carbon Nanotubes. Overview. Overview. Characterization Synthesis Electronic Properties Current and Future Applications. armchair.

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Carbon Nanotubes

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  1. Carbon Nanotubes ‘New Materials for the Twenty-first Century‘ Poulcheria Christou, University of Stuttgart, 26.4.2007

  2. Nanotube Carbon Nanotubes Introduction Introduction • What are CNTs ? Fullerene Poulcheria Christou, University of Stuttgart, 26.4.2007

  3. Carbon Nanotubes Overview Overview • Characterization • Synthesis • Electronic Properties • Current and Future Applications Poulcheria Christou, University of Stuttgart, 26.4.2007

  4. armchair zig-zag chiral Carbon Nanotubes Characterization Different Structures • Two symmetric structures • Many chiral structures Poulcheria Christou, University of Stuttgart, 26.4.2007

  5. chiral vector Ch = na1 + ma2 • coil up along Ch O = A • m = 0 zig-zag n = m armchair • chiral angle q • archetype CNTs: (5,5) q = 0° (9,0) q = 30° Carbon Nanotubes Characterization • graphene layer • a1, a2 unit vectors • (n,m) notation Poulcheria Christou, University of Stuttgart, 26.4.2007

  6. Carbon Nanotubes Characterization • CNT as one -dimensional crystal • translational unit cell, along the axis and cylindrical • (9,0) lenght of a, (5,5) lenght of a, with a the unit vector of 2D graphite lattice Poulcheria Christou, University of Stuttgart, 26.4.2007

  7. Carbon Nanotubes Characterization Symmetry • armchair and zig-zag tubes are rotation-symmetric • armchair and zig-zag have symmetry-planes • chiral tubes are not rotation-symmetric, basic symmetry operation R = (y,t), with y: rotation angle and t: translation vector Poulcheria Christou, University of Stuttgart, 26.4.2007

  8. Carbon Nanotubes Characterization MWNT vs. SWNT • russian doll (a) or swiss roll (b)? • equal number of walls on either side and internal caps point towards (a) • optimum distance between layers of 0.334 nm Poulcheria Christou, University of Stuttgart, 26.4.2007

  9. Carbon Nanotubes Synthesis Arc-evaporation • cathode gets consumed, CNTs in cathodic soot • structure of CNTs depends on: current I, voltage V, He gas pressure, anode material, distance between the electrodes Poulcheria Christou, University of Stuttgart, 26.4.2007

  10. Carbon Nanotubes Synthesis Cathodic soot at different gas pressures • (a) 20 Torr • (b) 100 Torr • (c) 500 Torr Poulcheria Christou, University of Stuttgart, 26.4.2007

  11. Carbon Nanotubes Synthesis Laser-vaporisation Catalytic methods M M M Poulcheria Christou, University of Stuttgart, 26.4.2007

  12. (5,5) armchair (9,0) zig-zag (10,0) zig-zag Carbon Nanotubes Electronic properties Electronic Properties Predictions • numerical calculations • 1D dispersion relations for electrons and phonons • small-diameter NT, exhibit metallic or semiconducting behaviour, depending on diameter and chiral angle, not on dopants or defects Poulcheria Christou, University of Stuttgart, 26.4.2007

  13. (9,0) zig-zag (10,0) zig-zag Carbon Nanotubes Electronic properties • allowed electronic states are limited: behavior of a quantumwire • finite DOS for the metallic NT (a), vanishing DOS for the semiconducting NT (b) • all armchair NTs are metallic, one-third of the zig-zag and chiral ones are metallic, too, remaining being semiconducting • metallic conduction occurs, when n – m = 3q (q an integer) Poulcheria Christou, University of Stuttgart, 26.4.2007

  14. Carbon Nanotubes Electronic properties Heterojunctions • `elbow` connections between tubes • connection of a metallic and a semiconducting tube: heterojunction • electrons from the semiconducting side flow to the metallic side, but not back • use for example as a diode Poulcheria Christou, University of Stuttgart, 26.4.2007

  15. Carbon Nanotubes Electronic properties Experimental measurements • dispersing NT bundles on a SiO2-waffer and connecting the NT with the pads, using photolithographic methods • results on one NT with diameter 20 nm and lenght 800 nm • resistance rise with falling temperature: proportional to – ln T above 1K, saturation at 0.01 K • magnetic field perpendicular tube axis reduces resistance at all T, causing negative magnetoresistance Poulcheria Christou, University of Stuttgart, 26.4.2007

  16. Carbon Nanotubes Electronic properties • measurements on six tubes, two straight, four curved • measurements at room temperature • resistivity falls with increasing tube diameter • curved tubes higher resistivity than straight • NT on edge of a carbon fibre, can be dipped into heated mercury • conductance quantisation • tubes were undamaged: balistic transport of electrones Poulcheria Christou, University of Stuttgart, 26.4.2007

  17. Carbon Nanotubes Electronic properties • (a) and (d): resistivity rises with falling T • (b) different behaviour within one single tube at different lenghts • (c) behavior anomalous, rises immensely with falling T • very small effect with a magnetic field perpendicular to tube axis • four tungsten wires, each 80 nm wide connecting to a single tube • measurements on eight tubes Poulcheria Christou, University of Stuttgart, 26.4.2007

  18. Carbon Nanotubes Current and Future Applications Applications Filled NTs • protection and storage of substances • filling with radioactive substances • developing new magnetic devices `New` microscopes • CNT on tip of an AFM • finer tip = higher resolution Poulcheria Christou, University of Stuttgart, 26.4.2007

  19. Sensors • CNTs sensitive to gas and pressure: change of their electronic properties Carbon Nanotubes Current and Future Applications Nanomachines bearing gear Poulcheria Christou, University of Stuttgart, 26.4.2007

  20. CNT field-effect transistor Carbon Nanotubes Current and Future Applications Nanoelectronics • nanowires • heterojunctions • diodes • inverters Poulcheria Christou, University of Stuttgart, 26.4.2007

  21. Carbon Nanotubes Current and Future Applications Fuell cells • CNTs as catalyst carrier for DMFC - Electrodes • larger surface area, larger conductivity and excellent mechanical properties • MWNTs researched for both cathode and anode: more efficient than conventional Pt-VulcanXC72R system • Karl-Winnacker-Institute of Dechema e.V. in cooperation with the MPI for Solid State Research and the ICVT Institute of the University of Stuttgart are working on a SWNT-DMFC-Anode Poulcheria Christou, University of Stuttgart, 26.4.2007

  22. Carbon Nanotubes Recap • different structures of CNTs • various synthesis methods and their different results • interesting electronic properties • manifold and `usefull` applications Poulcheria Christou, University of Stuttgart, 26.4.2007

  23. Carbon Nanotubes Thank you for your attention Poulcheria Christou, University of Stuttgart, 26.4.2007

  24. Carbon Nanotubes Literature • Peter Harris: `Carbon Nanotubes and Related Structures`, 1999 • Mildred S. Dresselhaus, Gene Dresselhaus, Peter C. Eklund: `Science of Fullerenes and Carbon Nanotubes`, 1996 • William A. Goddard, Donald W. Brenner, Sergey Edward Lyshevski, Gerald J. Iafrate: `Handbook of Nanoscience, Engineering and Technology`, 2003 Poulcheria Christou, University of Stuttgart, 26.4.2007

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