Nano Science (Lec7 Bucky Ball-CNT-Graphene)

Nano Science (Lec7 Bucky Ball-CNT-Graphene) - MAE 287/EE...

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MAE 287/EE 257
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Carbon is the most important element found in the living things. Without carbon, there would be no life. Carbon is special because of its ability to bond with other atoms in many different ways. E.g. (a) Diamond forms where each carbon is bonded to four others. (b) Graphite forms in planes that consist of hexagonal arrangement of carbon atoms. (c) Another form of diamond called lonsdaleite has more hexagonal character to its structure. (d)–(f) Fullerenes are carbon’s representative in the zero-dimensional class of nanomaterials. (g) Amorphous carbon does not have any crystalline structure (no long-range order). Coal and soot are amporphous carbons. (h) Carbon nanotubes.
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Carbon Buckyball (0D) Carbon Nanotube (1D) Graphene (2D)
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The molecular orbital energy levels split from the original ones in independent atoms due to orbital configuration interaction, as shown in (a) originally the electrons have the same energy level in both atoms, (b) two different energies in a molecule. 4 The wave functions of the atoms can be added or subtracted in the molecule. The subtracted (antisymmetric) orbital leads to the depleted electron density between the two atoms (top figure) and higher energy. The added (symmetric) orbital leads to the enhanced electron density between the two atoms (bottom figure) and lower energy.
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When two atoms are brought together, inter-atomic electrostatic interactions can form by sharing the electrons between the two atoms. The attraction occurs at short range, and rapidly dies off as the interacting atoms move apart. Repulsion occurs when the distance between interacting atoms becomes even slightly less than the sum of their equilibrium radii. Energy Atomic distance Attractive regime
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C Atom 2s 2p sp 2 2p z One electron is excited from 2s to 2p orbital, the combination of three new sp 2 hybrid orbitals allow the C atom bonds with three neighbor C atoms with symmetric s bonding. The bond angle of sp 2 hybrid orbitals is 120° (e.g., in graphene). The three sp 2 orbitals per carbon form a single plane. 6 Graphene
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The unit cell of graphite (containing two atoms) is shown on the left. On the right, the relationship between two planes of graphite is depicted.
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Three layers of graphite are shown. The d-spacing between layers is 3.34 Å. The lattice parameter a of the unit cell is equal to 2.46 Å. The C–C bond length is 1.42 Å. Graphite is the most stable form of carbon. It forms in planes that consist of hexagonal arrangement of carbon atoms. The bonding in graphite is sp 2 . Van der Waals forces hold the planes together.
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Graphene is a one-atom-thick planar sheet of sp 2 -bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Graphite itself consists of many graphene sheets stacked together.
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a , In graphene, each carbon atom is bonded to three other carbon atoms to form a two-dimensional honeycomb structure that displays a variety of novel electronic properties. b , c , Scanning electron micrographs showing a single graphene sheet ( b ) and thicker layers of graphite ( c ).
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This note was uploaded on 04/17/2011 for the course MAE 287 taught by Professor Yongchen during the Winter '11 term at UCLA.

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Nano Science (Lec7 Bucky Ball-CNT-Graphene) - MAE 287/EE...

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