Raman Spectrum of Graphene and

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Transcript Raman Spectrum of Graphene and

Raman Spectrum of Graphene and Graphene layers PRL


, 187401 (2006) Sebastian Remi Journal Club 11/26/2006


• 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…


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

Electronic structure

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


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



White light SiO2 Si Single layer Bi layer 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

Raman effect

   

k i i

   


 

s s

     


Energy and Momentum conservation usually K~0, because BZ>>k Photon dispersion E 


e  Phonon band K(max) k

Laser 633nm or 514nm 0.04-4 mW


Spectrometer Sample

Raman effect in Graphene


K •Most prominent line  1580


•Relative Intensity enhances with the number of layers •shift~1/n;chemical doping?

 1

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



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


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 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