Carbon Nanotubes

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Carbon Nanotubes

• New Materials for the Twenty-first Century

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Introduction
Carbon Nanotubes
Introduction

• What are CNTs ?

Fullerene
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Overview
Carbon Nanotubes
Overview

• Characterization
• Synthesis
• Electronic Properties
• Current and Future Applications

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Different Structures
Carbon Nanotubes
Characterization

• Two symmetric structures

• Many chiral structures

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Carbon Nanotubes
Characterization

• graphene layer
• a1, a2 unit vectors
• (n,m) notation

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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

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Symmetry
Carbon Nanotubes
Characterization

• 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

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MWNT vs. SWNT
Carbon Nanotubes
Characterization

• 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

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Arc-evaporation
Carbon Nanotubes
Synthesis

• 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

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Carbon Nanotubes
Synthesis
Cathodic soot at different gas pressures

• (a) 20 Torr
• (b) 100 Torr
• (c) 500 Torr

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Laser-vaporisation
Carbon Nanotubes
Synthesis
Catalytic methods
M
M
M
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Electronic properties
Carbon Nanotubes
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

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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)

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Heterojunctions
Carbon Nanotubes
Electronic properties

• 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

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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

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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

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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

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Applications
Carbon Nanotubes
Current and Future 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

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Carbon Nanotubes
Current and Future Applications
Nanomachines

• bearing

gear
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Carbon Nanotubes
Current and Future Applications
Nanoelectronics

• nanowires
• heterojunctions
• diodes
• inverters

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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

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Recap
Carbon Nanotubes

• different structures of CNTs
• various synthesis methods and their different
  results
• interesting electronic properties
• manifold and usefull applications

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Carbon Nanotubes
Thank you for your attention
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Literature
Carbon Nanotubes

• 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