Raman Scattering from High Frequency Phonons in Supported n-Graphene Layer Films

Raman Scattering from High Frequency Phonons in Supported n-Graphene Layer Films

| A. Gupta, Gugang Chen, P. Joshi, S. Tadigadapa, P.C. Eklund
This study reports room temperature Raman scattering results for ultrathin graphitic films supported on Si (111)/SiO₂ substrates. The results differ significantly from those of bulk graphite. Raman spectra were collected using 514 nm radiation on films with n=1 to 20 graphene layers, determined by AFM. Both first and second order Raman spectra show unique signatures of the number of layers in the film. The nGL film analog of the Raman G-band in graphite exhibits a Lorentzian lineshape whose center frequency shifts linearly with 1/n. Three weak bands, identified with disorder-induced first order scattering, are observed at ~1350, 1450, and 1500 cm⁻¹. The 1500 cm⁻¹ band is weak but sharp and exhibits an interesting n-dependence. The intensity of these D-bands decreases dramatically with increasing n. Three second order bands are also observed (~2450, ~2700, and 3248 cm⁻¹). They are analogs to those observed in graphite. However, the ~2700 cm⁻¹ band exhibits an interesting and dramatic change of shape with n. Interestingly, for n<5 this second order band is more intense than the G-band. The study also investigates the phonon and electronic state dispersion in single graphene layers. The results show that the G-band frequency exhibits an almost linear dependence on 1/n. The nGL films were prepared by mechanically transferring thin flakes of material from HOPG to a SiO₂:Si substrate. The films were characterized using AFM and Raman spectroscopy. The Raman spectra show a strong dependence on the number of layers n, with the G-band frequency downshifting with increasing n. The second order ~2700 cm⁻¹ band exhibits an interesting n-dependence in shape and frequency. The sharp second order feature at ~3248 cm⁻¹ is insensitive to n. For n<5, the second order ~2700 cm⁻¹ band is more intense than the first order G-band. Additional weak features are present in the first order region. The study also discusses the origin of the n-dependence of the D-band scattering in nGLs. One possible explanation is the bending of the graphene layers necessary to accommodate the surface roughness of the substrate. The n-dependence of the D-scattering may stem from the loss in sp² bond bending disorder associated with the increase in rigidity of the nGL as the number of layers n increases. The study concludes that Raman scattering can be used to identify the number of layers in an nGL film. The results suggest that the G-band position follows a simple 1/n behavior, although the exact reason for this behavior remains unexplained. The study also highlights the importance of film rigidity in determiningThis study reports room temperature Raman scattering results for ultrathin graphitic films supported on Si (111)/SiO₂ substrates. The results differ significantly from those of bulk graphite. Raman spectra were collected using 514 nm radiation on films with n=1 to 20 graphene layers, determined by AFM. Both first and second order Raman spectra show unique signatures of the number of layers in the film. The nGL film analog of the Raman G-band in graphite exhibits a Lorentzian lineshape whose center frequency shifts linearly with 1/n. Three weak bands, identified with disorder-induced first order scattering, are observed at ~1350, 1450, and 1500 cm⁻¹. The 1500 cm⁻¹ band is weak but sharp and exhibits an interesting n-dependence. The intensity of these D-bands decreases dramatically with increasing n. Three second order bands are also observed (~2450, ~2700, and 3248 cm⁻¹). They are analogs to those observed in graphite. However, the ~2700 cm⁻¹ band exhibits an interesting and dramatic change of shape with n. Interestingly, for n<5 this second order band is more intense than the G-band. The study also investigates the phonon and electronic state dispersion in single graphene layers. The results show that the G-band frequency exhibits an almost linear dependence on 1/n. The nGL films were prepared by mechanically transferring thin flakes of material from HOPG to a SiO₂:Si substrate. The films were characterized using AFM and Raman spectroscopy. The Raman spectra show a strong dependence on the number of layers n, with the G-band frequency downshifting with increasing n. The second order ~2700 cm⁻¹ band exhibits an interesting n-dependence in shape and frequency. The sharp second order feature at ~3248 cm⁻¹ is insensitive to n. For n<5, the second order ~2700 cm⁻¹ band is more intense than the first order G-band. Additional weak features are present in the first order region. The study also discusses the origin of the n-dependence of the D-band scattering in nGLs. One possible explanation is the bending of the graphene layers necessary to accommodate the surface roughness of the substrate. The n-dependence of the D-scattering may stem from the loss in sp² bond bending disorder associated with the increase in rigidity of the nGL as the number of layers n increases. The study concludes that Raman scattering can be used to identify the number of layers in an nGL film. The results suggest that the G-band position follows a simple 1/n behavior, although the exact reason for this behavior remains unexplained. The study also highlights the importance of film rigidity in determining
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