Chemistry Reference
In-Depth Information
3. Experimental Section
3.1.
Sample preparation and characterization
(b)
(a)
Fig. 2. (a) Raman spectra for SLG, BLG, FLG, and MLG showing the characteristic
G and 2D peaks (b) Gate voltage characteristics of graphene devices: for comparison,
the ratio of resistivity (
ρ
) and the resistivity at the CNP (
ρ
D
) are plotted as a function
of (
V
bg
−
V
D
) at T = 100 K. The inset shows the optical micrograph and outline of a
typical graphene device. The scale bar is 5
µ
m.
n
++
doped silicon
substrate (the backgate) by micromechanical exfoliation of highly oriented
pyrolytic graphite (HOPG). All flakes were characterized by Raman spec-
troscopy,
56
and subsequent atomic force microscopy indicated the FLG and
MLG devices in the present case to consist of 3
Graphene flakes were prepared on 300 nm SiO
2
on
−
4and
∼
14 layers, respec-
tively. 40 nm gold (99
99%) contacts were defined using standard electron
beam lithography technique. To keep the disorder level comparable, we
have cleaned the Si/SiO
2
substrates in standard RCA solution (1:1:5 so-
lution of NH
4
OH (ammonium hydroxide) + H
2
O
2
(hydrogen peroxide) +
H
2
O (water) at 75 or 80
0
C) followed by acetone and isopropyl alcohol
prior to the graphene deposition. Figure 2(a) shows the characteristic Ra-
man spectra for different graphene flakes where the intensity ratio of the G
peak to the 2D peak can be seen to increase with increasing layer number.
Figure 2(b) shows the gate voltage characteristics of the devices. In all the
cases, charge neutrality point (CNP) was shifted to a finite gate voltage
due to the intrinsic doping (see Fig. 3 also), primarily arising from the im-
purities inside the SiO
2
. Hence in order to compare the influence of gating
we have plotted the ratio of resistivity to that at the CNP as a function of
.
