Graphene – Optical Properties Michael Tsang UC Berkeley Physics 141A Spring 2013 Lawrence Berkeley Lab Manchester Group for monolayers

Graphene is  a single layer of carbon packed in hexagonal (honeycomb) lattice  the first truly 2D material What is Graphene Physics 141A Spring 2013 Michael Tsang UC Berkeley

Graphene is  a single layer of carbon packed in hexagonal (honeycomb) lattice  the first truly 2D material What is Graphene Physics 141A Spring 2013 MoS 2. Michael Tsang UC Berkeley

Graphene has  Covalent Bonding  sp2 hybridization  Remarkably high quality A Closer Look at Graphene Physics 141A Spring nm Michael Tsang UC Berkeley

Familiar Carbon Structures  Carbon Nanotubes, known since 1993  Buckeyballs C60, known since 1996  Graphite, known since … a long time Physics 141A Spring 2013 Michael Tsang UC Berkeley

 Konstantin Novoselov Andre Geim Nobel Prize in Physics (2010) Physics 141A Spring 2013 Isolated large sheets in order to identify and characterize graphene and verify 2D properties Michael Tsang UC Berkeley

1. “You put scotch tape on graphite or mica and peel the top layer. There are flakes of graphite that come off your tape. 2. “Then you fold the tape in half and stick it to the flakes on top and split again. And you repeat this procedure 10 or 20 times. Each time, the flakes split into thinner and thinner flakes.” 3. “At the end you’re left with very thin flakes attached to your tape. You dissolve the tape and everything goes into solution” Methods: Micromechanical Cleavage Physics 141A Spring 2013

The Optical Properties of Graphene Physics 141A Spring 2013 Michael Tsang UC Berkeley

Physics 141A Spring 2013 Optical Measurements  Take prepared macroscopic membranes of graphene  Shine light through the membrane  Detector measures light intensity Michael Tsang UC Berkeley

Physics 141A Spring 2013 Optical Measurements  Ideal Dirac Fermions?  ? Michael Tsang UC Berkeley

Physics 141A Spring 2013 Dirac Fermions?  “Dirac Fermions” refers to electrons that race through the graphene structure  Relativistic Quantum Mechanics becomes important  Dirac Equation Richard Feynman, a founding father of quantum electrodynamics Michael Tsang UC Berkeley

Physics 141A Spring 2013 Understanding origins of πα  The universal conductivity implies that observable, optical transmittance is also universal  It was argued that conductivity for Dirac fermions is a universal constant, G Michael Tsang UC Berkeley

Physics 141A Spring 2013 Ultrafast Nano-Optics Group  Changing the optical reflection of graphene from electrical gating  Optical determination of graphene electronic structure Optical Properties in Berkeley Physics Professor Feng Wang (left) Prof Wang welcomes interested undergraduates and graduates into his lab Michael Tsang UC Berkeley

Other Properties of Graphene  Density of: 0.77 mg/m 2  Breaking Strength: 42 N/m  Conductivity: 0.96x10 6 Ω -1 cm -1 ( > copper)  Thermal Conductivity: 10x greater than copper Physics 141A Spring 2013 According to the Nobel Prize paper, a hammock of graphene could hold a cat (4kg) while weighing the same as one of its whiskers! Michael Tsang UC Berkeley

Physics 141A Spring 2013 Recap Graphene:  2D, honeycomb lattice of carbons  Pure, remarkable strength, conductivity  “Mechanical Cleavage”  πα = 2.3% Light Absorption  First material observed with Dirac Fermions Michael Tsang UC Berkeley

Only 193 US Patents!

Whenever a new material is characterized, entire systems can change Take home message, Physics 141A Spring 2013 Michael Tsang UC Berkeley

 mesoscopic/publications/graphene/Science_2008fsc.p df  aug/08augSWGeim/  stories/2010/10/15/the-noise-about-graphene/  reates/2010/advanced-physicsprize2010.pdf  hene/  Cool-Electronics-with-Great-Efficiency shtml  rbon-wonderland References Physics 141A Spring 2013 Michael Tsang UC Berkeley