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Raman Spectrum of Graphene and Graphene layers

Raman Spectrum of Graphene and Graphene layers. Sebastian Remi Journal Club 11/26/2006. PRL 97 , 187401 (2006). Carbon. Organic molecules, fuel etc. Physical structure 6 electrons 1s2, 2s2, 2p2, “4” unpaired electrons

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Raman Spectrum of Graphene and Graphene layers

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  1. Raman Spectrum of Graphene and Graphene layers Sebastian Remi Journal Club 11/26/2006 PRL 97, 187401 (2006)

  2. Carbon • Organic molecules, fuel etc. • Physical structure 6 electrons 1s2, 2s2, 2p2, “4” unpaired electrons • In molecule crystal structures hybridization: mixing of atomic orbitals in a way which maximizes the binding energy with the neighbour atoms • Is found in a huge variety of different materials among those the hardest (diamond) and the softest…

  3. Graphite Graphene • Strong bonds in a particular layer • Weak bonding between layers, which can easily be removed

  4. M Electronic structure • Electrons near the K point show relativistic dispersion • Relativistic behaviour • Graphene band structure is 1st order approximation for Graphite bandstructure

  5. Preparation • Chemical growth: production of multilayers • Micromechanical cleavage Among thicker graphite flakes there are always thin graphite films and single graphene layers

  6. SiO2 Si Identification AFM Single layer Bi layer White light Optical

  7. Raman spectrum • Raman spectrum shows characteristic dependence on thickness of graphite film • Identification and comparison of single, bi… layers • Evolution of Raman lines is directly connected to electronic structure

  8. e Raman effect Photon dispersion E Energy and Momentum conservation usually K~0, because BZ>>k Phonon band K(max) k

  9. Setup Spectrometer Laser 633nm or 514nm 0.04-4 mW Sample

  10. Raman effect in Graphene

  11. G-Band K • Most prominent line • Relative Intensity enhances with the number of layers • shift~1/n;chemical doping?

  12. D-Band-Double Resonance • Phonon momentum at edge of Brillouinzone • 1 and 2 phonon processes • General character: wavelength dependence and difference for changing number of layers Two phonon process Single phonon process Induced by defects

  13. D-Band ~1eV>>E(phonon) • 4th order transition • e excitation • e-phonon scattering • defect scattering • E-hole recombination To mention: influence of number of layers

  14. 2D-Band Dependent on number of layers • 4th order transition • e excitation • e-phonon scattering • Phonon with opposite momentum • E-hole recombination 2 phonon process Wavelength dependent • Line shape and position sensitive to the number of layers

  15. Graphene bi layer • 2 inequivalent sublattices • Splitting into 4 bands

  16. Graphene bi-layer Difference to single layer and bulk graphite Level splitting due to splitting in electron bands

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