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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 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 • 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…
Graphite Graphene • Strong bonds in a particular layer • Weak bonding between layers, which can easily be removed
M Electronic structure • Electrons near the K point show relativistic dispersion • Relativistic behaviour • Graphene band structure is 1st order approximation for Graphite bandstructure
Preparation • Chemical growth: production of multilayers • Micromechanical cleavage Among thicker graphite flakes there are always thin graphite films and single graphene layers
SiO2 Si Identification AFM Single layer Bi layer White light Optical
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
e Raman effect Photon dispersion E Energy and Momentum conservation usually K~0, because BZ>>k Phonon band K(max) k
Setup Spectrometer Laser 633nm or 514nm 0.04-4 mW Sample
G-Band K • Most prominent line • Relative Intensity enhances with the number of layers • shift~1/n;chemical doping?
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
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
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
Graphene bi layer • 2 inequivalent sublattices • Splitting into 4 bands
Graphene bi-layer Difference to single layer and bulk graphite Level splitting due to splitting in electron bands