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CARBON-BASED ALLOTROPES AND THEIR PROPERTIES

CARBON-BASED ALLOTROPES AND THEIR PROPERTIES. A.A. 2011-2012. Fullerenes. The compounds of pure Carbon. Carbon is a non-metallic element whose most abundant pure allotropes are graphite and diamond , two hypothetically infinite lattices.

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CARBON-BASED ALLOTROPES AND THEIR PROPERTIES

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  1. CARBON-BASED ALLOTROPES AND THEIR PROPERTIES A.A. 2011-2012 Fullerenes.

  2. The compounds of pure Carbon Carbon is a non-metallic element whose most abundant pure allotropes are graphite and diamond, two hypothetically infinite lattices. Only synthetic diamonds are free from impurities.

  3. Graphite • The most abundant natural allotrope of C. • Color and aspect: Black or gray, • opaque, non crystalline, non fluorescent • Number of interatomic bonds: 3 (sp2 • orbitals, lenght: 1.421 Å) • Atomic density: 1.14 * 1023 • Distance between sheets: • 335 nm, no covalent bonds. • The arrangement in multiple • sheets confers to the graphite high anisotropy, and explains its optical, acoustic and magnetic properties.

  4. Diamond • The second most abundant natural allotrope form of C. • Color and aspect: colourless if • pure, transparent, fluorescent and • phosphorescent. • Number of interatomic bonds: • 4 (sp3 orbitals, length: 1.54 Å, • angle: 109.47°) • Atomic density: 1.77*1023 • Diamond crystal have tetrahedral symmetry (A). • When hexagonal symmetry is displayed, the allotrope is called lonsdaleite (B).

  5. Other carbon-based allotropes: nanomaterials. Nanodiamonds and buckyballs are discrete nanosized molecules Graphene, a single layer of graphite with the thickness of a single atom Carbon nanotubes, Single Walled (SWCNT) or MultiWalled (MWCNT). SWCNTs can be considered as the result of the folding of a graphene sheet. With the only exception of graphene, these compounds are found in traces in nature, as in vulcanic rocks and soots. Despite their chemical similarities, the physical chemistry of the nanosized allotropes of the C, changes dramatically in relation to properties like simmetry and size. Fullerenes, graphene and CNTs are compared herein.

  6. Nanodiamonds Nanodiamonds generate when TNT/RDX detonate in the absence of oxygen. Their size if up to 5 nm (1.5 nm in the exemple here shown). Thanks to their properties, and their propensity to be functionalized at the surface, the nanodiamonds act very well as drug carriers. Their potential toxicity in living systems is under study, and seems to be low.

  7. Buckyballs fullerenes are closed cells of carbon clusters, with icosahedral cubic symmetry. Found in small amounts in soots They are odourless, generally soluble at room temperature in organic solvent, not in water. The smallest stable representative is C20, while bucky balls larger than C100 are known. C atoms are arranged in series of hexagonal and pentagonal faces. The hexagonal faces are diamagnetic and aromatic, The fullerenes the pentagonal ones are paramagnetic and antiaromatics: their properties mutually cancell inside the cluster. The functionalization of the surface is easy.

  8. Solubility of bucky balls fullerenes. The smallest buckyball is C20, however the smallest stable fullerene is C36. Bucky balls larger than C100 are known. At room temperature, the fullerenes are soluble in organic solvent, not in water. The colour of the solution varies for different clusters, according to their symmetry. C60 is purple, C70 is reddish brown, C76 and C84 have different colours for their different isomers. Small band gap fullerenes lack solubility, when pure. These allotropes, including C36, C50 and C72, are highly reactive and bind to other fullerenes, to soot particles, or can be functionalized with a lantanide in their core.

  9. Solubility of C60 and C70 related to properties of the solvent. The solubility of C60 and C70 in various solvents (see the previous slide) is related to the surface tension of the solvent (graph), not to its octanol/water partition coefficient, nor to its specific gravity/density.

  10. Physical properties of C60 Black solid, odourless. Density: 1.65 g cm-3 Standard heat of formation: 9.08 k cal mol-1 Index of refraction: 2.2 (600nm) Boiling point: Sublimes at 800K Resistivity: 1014 ohms m-1 Vapour density: N/A Aromatic, Superconductor, Ferromagnetic (polarized at room T°C)

  11. Graphene. Graphene is a compound of pure carbon, arranged as an hexagonal lattice in which the C atoms are bound by sp2. The sheet has the thickness of a single atom; in this respect graphene differentiates from graphite.

  12. Graphene and CNTs as derivatives of graphite Diamond and graphite are stable structures, at least at room temperature and atmospheric pressure. To transform them into each other high energy is requiered. The production of graphene, a single sheet of the graphene allotrope of C, requires instead the separation of only weak, non-covalent bonds. Graphene is an hexagonal lattice of carbon atoms, bound each other as in graphite, whose thickness is that of a single atom. Though a graphene sheet is potentially infinite in the other two dimensions, its thermodynamic stability depends on the number of atoms and on the shape.

  13. Thermodynamic stability of graphene To be thermodynamically stable the graphene must reach a minimum size of 6000 atoms, that is a sheet of at least 20 x 20 nm. Most stable structures are larger than 24,000 C. The sheet’s shape is curled, not flat, as shown in the picture, which was obtained “in silico” by using the tool for mimicking the energy minimization, implemented in the NanoRex software.

  14. Electrical properties of graphene Graphene is a conductor, even powerful than Cu

  15. Engineered nanomaterials: Single Walled Carbon NanoTubes (SWCNT). Engineered SWCNTs can have different symmetries: Armchair, m=n; n,n = 5,5 Chiral: mn: m,n = 10,5 (in the example, see next slides) Zigzag, m,n = 9,0 The three allotropes can be conceived as generating from a graphene ribbon, rolled up with different axes of symmetry.

  16. A B C SWCNT simmetry…. A: Armchair (m,n=5,5), B: Zigzag (m,n=9,0), C: Chiral (m,n=10,5)

  17. Anti-Bonding Bonding and some consequences. C B A A: Armchair (m,n=5,5), B: Zigzag (m,n=9,0), C: Chiral (m,n=10,5)

  18. Scanning Tunnelling Microscope and conductance measurement. Keeping the tip of the STM close to the surface of CNTs, electrons from the tip can jump to the nanotube. A s.c. "tunnelling current“ establishes. The plot of dI/dV by the voltage measures the number of electronic states available for electrons to tunnel at a certain energy (V).

  19. Metallic or semiconducting SWCNTs. • The chemicals deals into two main classes for their electrical properties: • metals, in which the electric current generally flows freely and there is no energy gap between the valence and the conducting states. • semiconductors, in which an energy gap exists and therefore a higher voltage is needed to make electric current flow. • For most materials the metallic or semiconducting nature • depends on the chemical composition and 2-D arrangement of • atoms and molecules. • SWCNTs can show metallic or semiconducting properties in • relation to their chirality (n,m) and diameter.

  20. Acceptor (porphyrine) Transmitters (short nucelotides) Electrochemical solar cells mimicking photosynthesis. SWCNT Choi et al., 2010, doi:DOI: 10.1117/2.1201007.003130 A new exploitation attempt.

  21. Fullerene-like structures not Carbon-based

  22. Boron bucky balls B112, isomers B16N16 • De et al., 2011, DOI: 10.1103/PhysRevLett.106.225502 • Muya et al., Phys. Chem. Chem. Phys., 2011, 13, 7524–7533 B80

  23. Boron-nitride nanotubes Armchair simmetry (n,m=5,5) Boron: rosa, Nitride: blu.

  24. Aknowledgments • All the figures, if not otherways indicated, have been constructed with the help of the following softwares, freely distributed: • Nanoegineering 1, version 1.1.1, by Nanorex • Ninithi 1, by Lanka Software Foundation • UCSF Chimera, by the University of California. • Further readings • Makarova T. 2004. Magnetism in polymerized fullerenes. In “Frontiers of • Multifunctional Integrated Nanosystems” (E. Buzaneva and P. Scharff eds.), • Kluwer Academic Publishers, the Netherlands, pag. 331-342. • 2.Małolepsza E, Witek HA. 2007. Comparison of Geometric, Electronic, and • Vibrational Properties for Isomers of Small Fullerenes C20-C36. J. Phys. • Chem. A 2007, 111, 6649-6657

  25. Armchair SWCNT (m,n = 5,5):Electrical properties

  26. Chiral SWCNT (m,n = 10,5):Electrical properties

  27. Chiral SWCNT (m,n = 10,5):Electrical properties

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