Chemistry Reference
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with increasing concentration of TTF and TCNE, respectively, with en-
hancement of the intensity ratio of D- to G-band.
44
In our calculations,
45
we have obtained qualitatively similar results. For pure 2D graphene, our
calculations yield the optical phonon frequency at Brillouin zone center
Γ-point corresponding to the Raman active G-band at around 1579 cm
−
1
.
However, in presence of electron acceptors (TCNE and TCNQ), we find that
the G-band frequency is shifted to a higher value (
1599 cm
−
1
and
1596
cm
−
1
for TCNE- and TCNQ-adsorbed graphene, respectively) because of
the nonadiabatic removal of Kohn anomaly at Γ-point, while for the elec-
tron donor (TTF), it goes to a lower frequency region at about 1565 cm
−
1
,
corroborating the experimental findings.
43,44
We also find that in presence
of strong electron acceptors like TCNE and TCNQ, the intensity ratio of
D- to G-band increases three orders of magnitudes (
∼
∼
10
3
), while for TTF
adsorption, the intensity ratio increases only to two orders of magnitude
(
∼
10
2
), smaller than that for strong acceptors as found experimentally.
43,44
We find reasonably good agreement with previous calculations
53,54
and ex-
perimental results.
43,44,55
To compare and contrast the low frequency optical excitations which can
be affected by molecular charge transfer, we plot the low frequency regime
of the optical conductivity for pure graphene along with donor ( TTF) and
acceptor (e.g; TCNE, TCNQ) intercalated graphene in Fig. 6. Note that,
for pure 2D graphene, the Fermi level lies exactly at Dirac point resulting
in only the possibility of inter band electronic transitions, giving rise to
the optical conductivity peak only at above 0.50 eV. However, for molecule
adsorbed on graphene, the shifting of Fermi level towards the valence band
or conduction band from the Dirac point depending on the nature of the or-
ganic dopant as discussed earlier, creates the possibility of Drude like intra
band transitions, resulting in the low frequency optical excitation below
0.50 eV.
45
This can also be accounted for the appearance of flat molec-
ular levels in between the valence and conduction bands. Interestingly,
the significant amount of charge transfer for TCNE- and TCNQ-adsorbed
graphene induces spin-polarization effects on low-frequency optical conduc-
tivity profile. In fact, this induces asymmetry to the population of majority
and minority spins at the Fermi level (see Fig. 4), which result in the differ-
ence in conductivity values for the carriers with different spins in the low
frequency region (see Fig. 6). However, for TTF-adsorbed graphene, due
to comparatively less charge transfer, both the majority and minority spins
show symmetric population (see Fig. 4), with less intense low-frequency
peaks in optical conductivity.
∼
