Carbon Nanotubes and Fullerenes

Document Sample
Carbon Nanotubes and Fullerenes Powered By Docstoc
					Carbon Nanostructures:
Fullerenes/Carbon Nanomaterials

   Nanotechnology ME465, unit 9, 10 and 11
   Peter Filip
   Office hours: T/Th 10 – 12 am
Lecture Overview

 Forms of Carbon/Bonding in Carbon
 Carbon Nanotubes and their Types
 Formation of Nanotubes
 Properties of Nanotubes
 Uses of Nanotubes
Carbon and Forms of Carbon
 Sixth element in the periodic table
 Atomic weight 12.011
 Three isotopes:
   C12 (99% of the naturally occurring carbon -reference
     for relative atomic mass of 12),
   C13 (has magnetic moment, spin=1/2 – used as a
     probe in NMR),
   and C14 (radioactive isotope, half life 5730 years –
     used in dating of artefacts and ‘label’ organic reaction
 Electronic ground state: 1s22s22p2
 C exhibits “catenation” = bonding to itself – limitless
  number of chains, rings and networks
Types of Carbon
 Diamond and Graphite alotropes of C with
  10928’ and 120 bonds until 1964
 Other bond angles:
    C8H8, 90, “cubane” (P. Eaton, University of
    Chicago, 1964)
   C20H20, dodecahedron shape (L. Paquette, Ohio
    State University, 1983)
   Carbon Clusters (3, 11, 15, 19, 23, 40, 50, 60,
    70, 80, 90) – C60: fullerene
   Carbon Nanotubes (S. Iijima, 1991, Japan
Carbon Nanotubes

   What is it?
    Sheet of graphite rolled into a tube
    Single-Walled (SWNT) and Multi-Walled (MWNT)
    Large application potential, metallic, semiconducting


SWNT                    zigzag

     Types of Carbon nanotubes

Two main types of
carbon nanotubes:

nanotubes (SWNTs)
consist of a single
graphite sheet
seamlessly wrapped
into a cylindrical tube.

Multiwalled nanotubes
(MWNTs) comprise an
array of such nanotubes
(more than one wall)
that are concentrically
nested with in.
               Why Carbon Nanotubes ?

                                    Small Dimensions
                                        Chemically Stable
                                          Mechanically Robust

                                          High Thermal Conductivity

                                      High Specific Surface Area (Good
                             Low Resistivity (Ballistic Electron Conduction)
Ideal materials for applications in conductive and high-strength composites; energy
storage and energy conversion devices; sensors; field emission displays and
radiation sources; hydrogen storage media; and nanometer-sized semiconductor
devices, probes, and interconnects.
Fabrication/Nanotube Synthesis
SWNTs and MWNTs are usually made by

   carbon-arc discharge methods
       C electrodes, 20-25V potential, 1mm, 500 torr, C ejected from +
        electrode forms NT on – electrode (Co, Ni or Fe for SWNTs,  1-5nm, 1µm
        length, no catalyst = MWNTs,)

   laser ablation of carbon
       1200C, pulsed laser, catalysts (Co, Ni), condensation (10-20nm, 100µm

   chemical vapor deposition (typically on catalytic particles)
        1100 C, decomposition of hydrocarbon gas (e.g. CH4), open NTs,
        catalyst on substrate, industrial scale up,  and length can vary

Nanotube diameters
   range from 0.4 to > 3 nm for SWNTs and from ~1.4 to at least 100
    nm for MWNTs
     Carbon-arc discharge
                                                                                The schematic diagram of and arc chamber
                Gas Inlet                                                       for CNT production is shown. After evacuating
                                                                                the chamber, an appropriate ambient gas is
                                                                                introduced at the desired pressure, and then
                                                                                a dc arc voltage is applied between the two
                      rod                                                       graphite rods. When pure graphite rods are
                                                                                used, the anode evaporates and the is
                                                                                deposited on the cathode, which contains
                             CNT                                                C N Ts . T h e s e C N Ts , a r e M W N Ts .
                                                                                When a graphite rod containing metal catalyst
                                                                                (Fe, Co, etc.) is used as the anode with a
            Water cooled chamber                                                pure graphite cathode, SWNTs are generated
                                                                                in the form of soot.

Figure shows typical setup used for laser ablation of carbon, which
consists of a furnace, a quartz tube with a window, a target carbon              Gas Inlet
                                                                                                Furnace           Water Inlet
composite doped with catalytic metals, a water-cooled trap, and flow
systems for the buffer gas to maintain constant pressures and flow                                                    Pump
rates. A laser beam (typically a YAG or CO2 laser) is introduced
                                                                        Laser                Target
through the window and focused onto the target located in the center
                                                                                              Rod         SWNT
of the furnace. The target is vaporized in high-temperature Ar buffer
gas and forms SWNTs. The SWNTs produced are conveyed by the
buffer gas to the trap, where they are collected.                                                     Laser
The method has several advantages, such as high-quality SWNT                                          ablation of
production, diameter control, investigation of growth dynamics, and
the production of new materials. High-quality SWNTs with minimal                                      carbon for
defects and contaminants, such as amorphous carbon and catalytic
metals, have been produced using the laser-furnace method.
                                                                                                      CNT growth
     Typical CVD Furnace Schematics

                                             Parameters Used:

                                             Ferrocene/Xylene: 1gm in 100 ml
                                             Gas flow rate: 100 sccm
                                             Growth temperature: 770 0C

                                             Figure 1: Schematics of the nanotube growth

The CVD method can be
used for growing controlled
architectures (aligned as well
as patterned) of carbon                Aligned CNTs
nanotubes on various
substrates.                      40 micron
 Figure 6.2. Illustration of the molecular and supramolecular structures associated with nanotubes at three different
   length scales. (a) shows the wrapping of a graphene sheet into a seamless SWNT cylinder. (b) and (c) show the
  aggregation of SWNTs into supramolecular bundles. The cross-sectional view in (c) shows that the bundles have
triangular symmetry. (d) A MWNT, another nanotube polymorph composed of concentric, nested SWNTs. (e) At the
                                 macromolecular scale, bundles of SWNTs are entangled.
Structure of Single Walled Carbon
 Structure depends on rolling direction
      Metallic
      Semi-conducting
Figure 6.1. Diagram explaining the relationship of a SWNT to a graphene sheet. The wrapping
vector for an (8,4) nanotube, which is perpendicular to the tube axis, is shown as an example.
Those tubes which are metallic have indices shown in red. All other tubes are semiconducting.
Three Forms of CNTs

   Chiral
   Zigzag
   Armchair
   Vectors describe the rolling process
    that occurs when a graphite sheet is
    transformed into a tube
Orbitals with 60 Carbon Atoms

    Figure 5.8. Hückel molecular orbital diagram for C60 in units of . (2 ~ 36 kcal)
Real and Reciprocal Space
Brillouin Zone

Figure 6.3. (a) The unit cell of graphene, and (b) the corresponding reciprocal lattice and Brillouin zone
   construction by the perpendicular bisector method. Dimensions are not to scale, but orientation
between the real and reciprocal lattice is preserved. Important locations within the Brillouin zone are 
            at the zone center, K at the zone corner, and M at the midpoint of the zone edge.
Real and Reciprocal Space
Brillouin Zone

 Figure 6.4. The dispersion surface of two-dimensional graphene in proximity to the Fermi level. The
    valence and conduction bands are tangent at each K point. (From Ref. 48 by permission of the
                                     American Physical Society.)
     Figure 6.5. (a) Wrapping vectors and allowed k-states for (3,0) (zigzag), (4,2), and (3,3) (armchair) SWNTs. The
degeneracy at the K point is allowed only for the (3,0) and (3,3) tubes, which behave like metals. The (4,2) tube does not
contain the degeneracy, so it has a band gap. Note that the lines of allowed k-states are perpendicular to the wrapping
  vector for each tube. (From Ref. 49 by permission of Annual Reviews.) (b) The band structure of a (6,6) SWNT. The
 presence of many overlapping subbands is typical for SWNTs. (From Ref. 14 by permission of the Am. Phys. Society.)
Properties of Nanotubes

 Electrical Properties
   Metallic – armchair structure – conductive
   Semi-conductors – zigzag and chiral
     Depends on diameter (quantum effects)
   Ropes of SWNTs (R=10-4cm-1 at 27C)
   Combinations – transistors
     Bent molecules
     Response to stretching
     Chirality and diameter of nanotubes are
      important parameters!!!
Figure 6.18. Atomic force microscopy image of an isolated SWNT deposited onto seven Pt electrodes by
 spin-coating from dichloromethane solution. The substrate is SiO2. An auxiliary electrode is used for
                electrostatic gating. (Reproduced with kind permission of C. Dekker.)
    Properties of Nanotubes

     Mechanical Properties
            Young’s modulus E = 1.28 – 1.8TPa
             (steel 0.21TPa)
            Strength Rm = 45,000 MPa (high
             strength steel 2,000 MPa)
            Buckling – no fracture – change in
             hybridization (from sp2)

Molecular dynamics simulations of a (10,10) nanotube under axial tension (J. Bernholc, M. Buongiorno Nardelli
and B. Yakobson). Plastic flow behavior is shown after 2.5 ns at T = 3,000 K and 3% strain. The blue area indicates
the migration path (in the direction of the arrow) of the edge dislocation (green). This sort of behavior might help make
composite materials that are really tough (as measured by their ability to absorb energy).
 Some Numbers
Nanotube diameters range from 0.4 to 3 nm for SWNTs and from 1.4 to at least 100
nm for MWNTs.
Phonons also propagate easily along the nanotube: The measured room
temperature thermal conductivity for an individual MWNT (3000 W/m.K) is greater
than that of natural diamond and the basal plane of graphite (both 2000 W/m.K).
Small-diameter SWNTs are quite stiff and exceptionally strong, meaning that they
have a high Young’s modulus and high tensile strength. Young’s modulus for an
individual (10, 10) nanotube is 0.64 TPa, which is consistent with measurements.
0.64 TPa is about the same as that of silicon carbide nanofibers (0.66 TPa) but
lower than that of highly oriented pyrolytic graphite (1.06 TPa).
The density-normalized modulus and strength of this typical SWNT are, respectively,
~19 and ~56 times that of steel wire and, respectively, ~2.4 and ~1.7 times than
silicon nano rod.
Because of the nearly one-dimensional electronic structure, electronic transport in
metallic SWNTs and MWNTs occurs ballistically (i.e., without scattering) over long
nanotube lengths, enabling them to carry high currents with essentially no heating.
 Generally a mixture of NTs is produced
 Impurities are removed by chemicals and
 Separation between electrodes
     Silicon wafer one electrode
     Carbon nanotubes deposited on wafer
     Metal electrode on top of CNTs
     High current – only metallic CNTs conduct –
      heating - evaporation
Derivatization and Functionalization

  Figure 6.19. Two common reaction schemes for the covalent derivatization of SWNTs: (I) carboxylic acid
           derivatization, and (II) fluorination. Many variations on these schemes are possible.
Filling of Nanotubes

 Figure 6.20. Transmission electron micrograph of a MWNT filled with Sm2O3. The interlayer separation
in the MWNT is c.a. 0.34 nm. Lattice planes in the oxide are clearly seen. (From Ref. 55 by permission of
                                    The Royal Society of Chemistry.)
Buckyballs in SWNT

  Figure 6.21. (a) Transmission electron micrograph of C60@SWNT. The nanotube is surrounded by
 vacuum and does not lie on a substrate. The encapsulated fullerenes form a one-dimensional chain
   with a lattice periodicity of c.a. 1.0 nm. It is possible to obtain diffraction signatures from these
   structures. (b) False-color transmission electron micrograph of La2@C80@SWNT. Each C80 cage
      contains two point scattering centers which are the individual La atoms contained within.
       Change of Conductivity

Figure 6.22. Differential conductance spectra of a C60 peapod. (a) Conductance versus position (Å) and sample bias (V) for
 the peapod. Spatially localized modulations are observed only for positive sample bias, i.e. in the unoccupied density of
   states. The periodicity of these modulations matches the periodicity of the encapsulated fullerenes. (b) and (c) show
conductance at constant position and at constant sample bias. (d) Conductance versus position for the same location on
 the SWNT after the C60 molecules have been shuttled into an empty part of the tube by manipulation with the STM tip. No
periodic modulations are observed. (From Ref. 33 by permission of The Am. Association for the Advancement of Science.)
Reinforcement of Composites
critical length interfacial theory (Kelly and Tyson, 1965; Chawla, 1998):

    f  d / 2  Lc  Lc   f  d / 2 

                       FL                  3
                  3 E  I
                        I   r  ri / 4
                                        4       4
                        E v     2
Application of Nanotubes
   Variety of Applications
       Cost dependent
   Field Emission and Shielding
       Flat panel displays TV and computer monitors)
       High electrical conductive armchair SWNTs – shield magnetic fields
   Computers
       Based on conductivity change (small V change can change conductivity
        106 times – switch on of faster than current)
   Fuel Cells
       Storage of charge carriers (Li, H)
   Chemical Sensors
       Sensitivity of vibration modes to the presence of other molecules
   Catalysts
       hydrogenation
   Mechanical Reinforcement
       5% (vol) increases strength of Al by factor 2
CNTs in Electronic Devices

  Figure 5.16. Nanoscale electronic device connected with a nanotube (left). (Reproduced with kind
permission of Ph. Avouris.) La2@C80 trapped inside a single walled carbon nanotube. a.k.a PEAPODS
                     (right). (Reproduced with kind permission of D. E. Luzzi.)
Things to Think About
 How many forms (structural
  modifications) of CNTs exist?
 What is chirality and how it influences
  electrical conductivity?
 How are CNTs made? What is the role
  of Co, Ni, Fe? What is the separation
 Make a CNT with R=na1+ma2 from
  chicken wire
 Obligatory
   Chapters 4 to 6 in required book
 Recommended
   Chapter 5 in recommended book

Shared By: