Graphene Tianyu Ye November 30th, 2011
Outline What is graphene? How to make graphene? (Exfoliation, Epitaxial, CVD) Is it graphene? (Identification methods) Transport properties; Other properties; Applications; Summery
Outline What is graphene? How to make graphene? (Exfoliation, Epitaxial, CVD) Is it graphene? (Identification methods) Transport properties; Other properties; Applications; Summery.
Birth Proceedings of the National Academy of Sciences
What is graphene? Wiki/Geim: Graphene is an allotrope of carbon, whose structure is one-atom-thick planar sheet of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. Geim, A. K. and Novoselov, K. S. (2007). "The rise of graphene". Nature Materials 6 (3): 183 191.
History & Milestone More than 70 years ago, Landau and Peierls argued that strictly 2D crystals were thermodynamically unstable and could not exist. Their theory pointed out that a divergent contribution of thermal fluctuations in low-dimensional crystal lattices should lead to such displacements of atoms that they become comparable to interatomic distances at any finite temperature. 2010 Nobel Prize in Physics
Why it is interesting? Naturally two dimensional; Dirac-like linear dispersion at low energies; Half-quantized quantum Hall effect; Excellent mechanical, optical and electronic properties....
More importantly it provide scientists a new capacious field to work.
Outline What is graphene? How to make graphene? (Exfoliation, Epitaxial, CVD) Is it graphene? (Identification methods) Transport properties; Other properties; Applications; Summery
Mechanical exfoliation Optical microscope image Use scotch tape peel again and again and again of HOPG (highly oriented pyrolytic graphite). Then put it on substrate. 1 mm μ= 15,000 cm 2 /V -1 s -1
Thermal sublimation of SiC AFM image Ultra-high vacuum 1000~1500 o C Steady throughput, good for industrial production Substrate effects μ= 2,000 cm 2 /V -1 s -1
Chemical vapor deposition (CVD) Use Nickel or copper foil as substrates and catalyzer. Annealing with methane-argon-hydrogen environment at 1000 o C and cool down slowly with gas protection. Then etch metal. Large area~1cm 2 μ= 16,000 cm 2 /V -1 s -1
Other methods Electrostatic deposition Unzipping carbon nanotubes Molecular beam deposition
Outline What is graphene? How to make graphene? (Exfoliation, Epitaxial, CVD) Is it graphene? (Identification methods) Transport properties; Other properties; Applications; Summery
Graphene s Identification Optical microscope High throughput, good for preliminary measurement Low accuracy
Graphene s Identification Atomic Force Microscope (AFM) Atomic force as a function of distance in Angstrom High accuracy, good for single layer graphene. Not measuring intrinsic property Low throughput.
Graphene s Identification Raman spectroscopy 1580 cm -1 2670 cm -1 in-plane vibration of sp2 carbon atoms two phonon double resonance Raman process
Graphene s Identification Raman spectroscopy (i) A laser induced excitation of an electron-hole pair (ii) Electron-phonon scattering with an exchanged momentum q close to K (iii) Electron-phonon scattering with an exchanged momentum -q (iv) Electron-hole recombination.
Graphene s Identification Raman spectroscopy Intrinsic, accurate Imaging has low throughput G band Raman spectrum is consistent with optical microscope image
Other tools Scanning electron microscope Scanning tunneling microscope Low energy electron microscope Transmission electron microscopy Angel-resolved photoemission spectroscopy
Outline What is graphene? How to make graphene? (Exfoliation, Epitaxial, CVD) Is it graphene? (Identification methods) Transport properties; Other properties; Applications; Summery
Experiment Exfoliate graphene; Hall Bar; Measure ρ,σ; Quantum Hall effect; Etc.
Interesting results Ambipolar field effect ρ = ( W / L)( V / I 23 14 ) E
Interesting results Ballistic transport Diffusive transport Mean free path l > L Affect by: defects and impurities; interacts with the substrate; edges and ripples phonons σ L 4e 2 ball = T n W h n= 1 Transport mode l < L Transmission probability
Interesting results Minimum conductivity Ballistic: σ = 4 min e 2 / πh σ min = 4e 2 / h
Interesting results Integer Quantum Hall effect V xy, R xy ρ xy,σ xy V xx, R xx ρ xx,σ xx
Interesting results Integer Quantum Hall effect Localized states Impurities Energy DOS: Increasing B E = hϖ, ϖ eb / m c c = * Extend states
Interesting results Integer Quantum Hall effect (GaAs/AlGaAs) 1 h ρ xy = ν e 2 e σ xy =ν h Spin up and down: Zeeman effect in high M field: 2 2e σ xy =ν h 2 4e σ xy =ν h 2
Interesting results Half quantized Quantum Hall effect (graphene)
Outline What is graphene? How to make graphene? (Exfoliation, Epitaxial, CVD) Is it graphene? (Identification methods) Transport properties; Other properties; Applications; Summery
Mechanical property Spring constant ~1-5 N/m and breaking strength is 42 N/m about 200 times greater than steel.
Optical property Single layer graphene absorbance of visible light is 2.3%. From UV to IR it keeps 2-3% Low reactance ~0.1% Because of its high transparency, low reactance, high carrier mobility and near ballistic transport at room temperature, make it promising choice for transparent electrodes. ( may substitute ITO ( Indium tin oxide ))
Outline What is graphene? How to make graphene? (Exfoliation, Epitaxial, CVD) Is it graphene? (Identification methods) Transport properties; Other properties; Applications; Summery
Applications graphene plane can be formed into a single-electron (or quantum-dot) transistor. The island, which appears in the center of an electron micrograph is too small to accommodate more than one new electron at a time; any second electron is kept away by electrostatic repulsion. An electron from the source tunnels quantum mechanically to the island, then leaves by tunneling on to the drain. The gate voltage is applied to a third electrode. It controls whether a single electron can enter or exit the island, thereby registering either a 1 or a 0.
Summery 1. Give a general back ground about graphene (definition, synthesis and identification); 2. Experimentally introduce transport properties of graphene; 3. Other properties and application of graphene.
Mark 11/30/2011 Thank you