Raman Enhancement of a Dipolar Molecule on Graphene
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Abstract
We show a large enhancement in the
Raman signal from a highly polarizable
molecule attached to single layer graphene. Through spatial mapping
of the Raman signal and wavelength-dependent Raman measurements from
a dipolar chromophore latched to a graphene/SiO<sub>2</sub> substrate
and to a bare SiO<sub>2</sub> substrate, we show that strong electronic
coupling in the hybrid structure contributes to the enhancement. The
dipolar molecule is a pyrene tethered Disperse Red 1 (DR1P) that noncovalently
binds to graphene. Upon comparison of the Raman signal of DR1P on
single layer graphene with that on a bare SiO<sub>2</sub>/Si substrate,
we found that the enhancement factor is in the range 29–69
at 532 nm excitation. As the surface coverage of DR1P on graphene
increases, Raman intensity also increases and saturates at a certain
concentration. The saturation of the Raman signal intensity at higher
DR1P concentrations were accompanied by shifts in the G band and the
2D band of graphene due to p-doping. We further show that the Raman
enhancement that occurs on single layer is larger than on few layer
graphene. Quantitative analysis on the Raman scattering cross section
of DR1P on graphene shows a higher Raman scattering cross section
compared to that in solution confirming a strong electronic coupling.
A series of all-electron ab initio calculations using density functional
theory (DFT) modeled the noncovalent binding of DR1P on a large graphene
fragment where the pyrene tether is interacting with the graphene
fragment via π–π stacking interactions. The DR1P
molecule has occupied energy levels that are close to the Fermi level
of graphene, and these interact strongly with the semimetallic nature
of graphene. As a consequence, in complete contrast to the isolated
DR1P molecule, our time-dependent DFT calculations show that the orbital
energies and densities for DR1P are significantly modified by the
graphene substrate