Allotropes of Carbon PHYS 1600 - PowerPoint Presentation

1. Allotropes of CarbonPHYS 1600October 4, 2021

2. Carbon – the “king” of elementsApproximately 18% of human body mass is carbon atomsCO2 (currently 420 ppm of air) is an essential atmospheric gas  mediates between animal and plant life.4th most abundant element in universe, after H, He, and O; 15th most abundant on earth.Atomic number Z = 6Valence is +4, from outermost 2s2 and 2p2 electrons  form sp2 and sp3 hybrid orbitals.allotropes of pure carbonOct. 4, 20212

3. Allotropes of CarbonMore than any other element: diamond graphene carbon nanotubes buckeyballsPhenomenal mechanical strength possible with highly-ordered allotropes – far stronger than steel – AND much lighter.Oct. 4, 20213

4. Magic of carbon bonding4 valence electrons: 2s2 2p2  sp2 and sp3 “hybrid” orbitalsDiamond – tetrahedral arrangement of 4 identical sp3 orbitals in a giant “molecule” of pure C.Silicon has same valence arrangement, but with weaker “𝜋” bonding.Great variety of allotropes does not occur with Si.methanesilane – pyrophoric!Oct. 4, 20214

5. DiamondvsGrapheneDiamond has “tetrahedral” structure – each C atom bonded to four others by sp3 hybrid orbitals.All four valence electrons are (strongly) fixed in 4 equal bonds.Electrons are immobile – insulator.Carbon atoms in graphene sheets bond to only three C neighbors.Each C atom bonds to three others. Three electrons in sp2 hybrid orbitals (𝜎 bonds).Fourth bonding electron (in a 𝜋 bond) is not fixed to a specific C atom  “mobile”.Excellent electrical conductivity.Oct. 4, 20215

6. GrapheneSmall sheets occur naturally, as tiny plates in pencil lead.Graphene sheets are thickness of single C atoms: 0.34 nm.Each C atom bonds to three others – the fourth bonding electron is “unoccupied”  excellent electrical conductivity.Pure sheets (no defects) have extraordinary tensile strength.Oct. 4, 20216

7. GrapheneCan make monolayer sheets, but with impuritiesPure graphene sheets of 1 m2 would have mass 0.77 mg, tensile strength 200 X steel.Immediate applications: nanoelectronics battery technologyOct. 4, 20217

8. Graphenein automotivebatteriesGraphene can accommodate higher lithium atom densities.More stored energy per unit weight.Faster recharging times .Oct. 4, 20218https://www.youtube.com/watch?v=m68izpH-5_0&t=2s

9. Exotic effects with two graphene sheetsSurprising properties: two sheets at small twist angle can produce high-temperature superconductivity. Nature volume 556, 80–84 (2018).Oct. 4, 202191.1∘

10. Graphene SynthesisVery pure, but small monolayer samples can be extracted from graphite samples by “exfoliation,” using Scotch tape. Oct. 4, 202110Large sheets, but with impurities, can be produced by using CVD on a metal substrate, or graphene oxide deposition on a substrate with subsequent removal of oxygen.

11. Carbon NanotubesTwist and roll graphene sheet, to form a tube.Different morphologies, depending on how the twisting and rolling occur.From: Carbon Nanotubes in Biomedicine, 2020.Oct. 4, 202111SWCNT – “single-wall carbon nanotube.”MWCNT - “multiple-wall carbon nanotube.”

12. Properties of Carbon NanotubesLike graphene, excellent electrical conductivity, and tensile strength along tube direction.Manufacture into fibers, and assemble with epoxy resins.Oct. 4, 202112Credit: Hisashi Sugime, Waseda UniversityGrowing “forests” of nanotubes up to 14 cm long.https://www.youtube.com/watch?v=kRHOvyft3Z0&t=4s

13. Properties of Carbon NanotubesLike graphene, excellent electrical conductivity, and tensile strength along tube directionManufacture into fibers, and assemble with epoxy resins. CNT’s in carbon fiber are used all over – very light, and very strong:Oct. 4, 202113Potential uses of CNT’shttps://www.youtube.com/watch?v=yjVXFfQdWN8Utilizing carbon fiberhttps://www.youtube.com/watch?v=yNsjVEm_9TI

14. CNT’s in TransistorsTraditional transistors are predominately MOSFET design:Most common man-made device ever, by number: over 1.3 X 1022 manufactured between 1960 – 2018.FUNCTION – Switching circuit, allowing current to flow between source (S) and drain (D) under the control of voltage applied to the gate (G)Oct. 4, 202114

15. CNTFET’s – faster, less powerSingle carbon nanotubes controlled by gate voltage, same function as MOSFET’s.Oct. 4, 202115https://www.researchgate.net/figure/CNTFET-structure-a-Planar-b-Coaxial_fig2_282366345Ultimate goal – single-electron transistors.

16. Synthesis of Carbon NanotubesArc-discharge method using carbon electrodes in a low-pressure argon or helium tube. Produces both fullerenes, and single-, an multiwall carbon nanotubes.Oct. 4, 202116www.alamy.comLaser ablation method using igh-powered pulses of laser on a graphite/transition metal target in inert gas environment. Produces both fullerenes, and single-wall carbon nanotubes, but more expensive.Chemical vapor deposition (CVD) carbon-containing gas (methane, ethane, …) at high temperature, that grow onto a catalyst, allowing fibers to “grow” to significant lengths.

17. FullerenesC60 molecule is spherical, like a soccer ball, with each hexagonal patch surrounded by three hexagonal and three pentagonal patches.MANY other combinations possible. C70 (shaped like rugby ball), C100, …Have even been observed in interstellar space dust.“Encapsulation” is possible: molecules trapped inside fullerenes (endohedral fullerenes).Oct. 4, 202117EXAMPLE: Analysis of ancient atmospheres by looking at small samples created by meteor impacts (e.g. meteor impact at end of Pleistocene era, 250 million years ago).

18. Applications of FullerenesMedical: light-activation to produce radicals that act as antimicrobial agents, or in attacking cancer cells.Biomedical applications in medical imaging, such as in high-performance MRI contrast agents, X-ray imaging contrast agents, photodynamic therapy.Targeted drug delivery vehicles.Enhanced lubricants for automotive use. Oct. 4, 202118