Joaquin Rodriguez-Nieva, Millie S. Dresselhaus
Ph.D. student: 2011-2016
We studied the Double-Resonant (DR) Raman scattering process in disordered graphene, and showed the dependencies of the D and D’ band Raman intensities on laser energy, defect concentration and electronic lifetime. Several important features, which were contrasted with experiments, were discussed:
the laser energy dependencies of both the D and D’ band intensities are sensitive to the scattering potentials, thus providing detailed information about defects.
the D and D’ bands show a different laser energy dependence.
when the defect collision rate becomes faster than the electron-phonon collision, the ratio ID/IG saturates as a function of defect concentration.
Numerous theoretical and experimental works on the DR process are available, yet some of the most interesting and potentially useful questions remain to be answered:
distinguishing signatures of the different types of defects on the Raman spectra.
edges vs. point defects?
laser energy dependence?
D vs. D’ bands
Why ID≫ ID’???
INTEGRATED RAMAN INTENSITY
The Raman intensity is concentrated for backscattering of the photoexcited electron-hole pair:
Momentum transfer given by:
Raman Intensity can be integrated analytically using:
γ(~10meV) ≪ ωq(~0.2eV) ≪ EL(~eV)
ROLE OF THE SCATTERING POTENTIAL
Experiments typically show ID≫ ID’
What determines the ratio ID/ID’???
Two effects mainly determine ID/ID’:
Long wavevector scattering
Suppression of Backscattering
By (indirectly) measuring λ, we can identify the nature of the defects.
LASER ENERGY DEPENDENCE
Several laser energy dependencies of the integrated D and D’ band intensities are obtained in experiments... why?
a) Raman spectra is defect sensitive: potential probed at backscattering
b) γ ∝ EL
Dispersive behavior of D and D’ bands explain their different laser energy.
ID/ID’ ∝ (ωq≈κ/ωq≈Γ)3
Edge-Induced D Band :
DEPENDENCE ON DEFECT CONCENTRATION
Saturation of the D band Intensity with defect concentration is controlled by the electronic lifetime due to electron-phonon (ep) and electron-defect (d) scattering:
γep ~ 15 meV
Raman Spectroscopy can provide detailed information about the elastic scattering potential due to impurities, allowing to identify the nature of defects by using the laser energy dependence of the D and D’ bands, or the ID/ID’ ratio. Several experiments can be used to test our predictions, such as correlations with transport measurements or doping effects. Further computational work is required to model more accurately the scattering potential introduced by the different types of defects.
JFRN, et al., PRB 90, 235410 (2014)