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Carbon Nanostructures:Fullerenes/Carbon Nanomaterials Nanotechnology ME465, unit 9, 10 and 11 Peter Filip A108, firstname.lastname@example.org Office hours: T/Th 10 – 12 am
Lecture Overview • Forms of Carbon/Bonding in Carbon Materials • 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 mechanisms) • 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 10928’ 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 [Ref.1])
Carbon Nanotubes Types Fabrication Structure Properties Applications
What is it? • Sheet of graphite rolled into a tube • Single-Walled (SWNT) and Multi-Walled (MWNT) • Large application potential, metallic, semiconducting armchair SWNT zigzag MWNT chiral
Types of Carbon nanotubes Two main types of carbon nanotubes: Single-walled 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 Adsorbents) • 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 • 1200C, pulsed laser, catalysts (Co, Ni), condensation (10-20nm, 100µm length) • 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
Gas Inlet Graphite rod CNT Deposit Water cooled chamber Gas Inlet Furnace Water Inlet Figure shows typical setup used for laser ablation of carbon, which consists of a furnace, a quartz tube with a window, a target carbon composite doped with catalytic metals, a water-cooled trap, and flow systems for the buffer gas to maintain constant pressures and flow rates. A laser beam (typically a YAG or CO2 laser) is introduced through the window and focused onto the target located in the center 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. The method has several advantages, such as high-quality SWNT production, diameter control, investigation of growth dynamics, and the production of new materials. High-quality SWNTs with minimal defects and contaminants, such as amorphous carbon and catalytic metals, have been produced using the laser-furnace method. Pump Target Rod Laser SWNT Laser ablation of carbon for CNT growth Carbon-arc discharge The schematic diagram of and arc chamber 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 graphite rods. When pure graphite rods are used, the anode evaporates and the is deposited on the cathode, which contains CNTs. These CNTs, are MWNTs. When a graphite rod containing metal catalyst (Fe, Co, etc.) is used as the anode with a pure graphite cathode, SWNTs are generated in the form of soot.
Aligned CNTs 40 micron Parameters Used: Ferrocene/Xylene: 1gm in 100 ml Gas flow rate: 100 sccm Growth temperature: 770 0C Figure 1: Schematics of the nanotube growth apparatus Typical CVD Furnace Schematics The CVD method can be used for growing controlled architectures (aligned as well as patterned) of carbon nanotubes on various substrates.
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 Nanotubes • Structure depends on rolling direction (chirality) • 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 SpaceBrillouin 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 SpaceBrillouin 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.)
Zigzag Properties of Nanotubes • Electrical Properties • Metallic – armchair structure – conductive • Semi-conductors – zigzag and chiral • Depends on diameter (quantum effects) • Ropes of SWNTs (R=10-4cm-1 at 27C) • 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.
Separation • Generally a mixture of NTs is produced • Impurities are removed by chemicals and filtration • 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 criticallength interfacial theory (Kelly and Tyson, 1965; Chawla, 1998):
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 (protection) • 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 (Raman) • 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 process. • Make a CNT with R=na1+ma2 from chicken wire
Reading • Obligatory • Chapters 4 to 6 in required book • Recommended • Chapter 5 in recommended book