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

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

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

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

Identification Optical AFM Si SiO2 White light Single layerBi layer

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

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

Setup Spectrometer Laser 633nm or 514nm mW Sample

Raman effect in Graphene

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 Single phonon process Induced by defects Two phonon process

D-Band 4 th order transition 1.e excitation 2.e-phonon scattering 3.defect scattering 4.E-hole recombination To mention: influence of number of layers ~1eV>>E(phonon)

2D-Band Wavelength dependent Dependent on number of layers 4 th order transition 1.e excitation 2.e-phonon scattering 3.Phonon with opposite momentum 4.E-hole recombination 2 phonon process 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