Supplemental Material
Step like surface potential on few layered graphene oxide
M. Jaafar1, G. López-Polín1, C. Gómez-Navarro1* J. Gómez-Herrero1
1
Dpto. Física de la Materia Condensada, Universidad Autónoma de Madrid, 28049, Madrid, Spain.
Sample preparation
The GO (Graphene Oxide) flakes used in this work were obtained from NanoInnova Technologies,
oxidized by Hummers method1. The initial water dispersion contained typical flake sizes of around
150 microns. In order to obtain more optimal sizes for Atomic Force Microscopy (AFM)
characterization (15-40 μm) 2 minutes of sonication was applied to the dispersion.
GO flakes on Silicon substrates were adsorbed by deep coating. The sample was vertically removed
from the GO dispersion at an approximate speed of 10mm per minute. Finally, the samples were flow
with nitrogen. GO flakes were deposited on HOPG and Au by drop casting. Deposition time of the
drop of dispersion was 7 and 20 minutes respectively. The result of both methods was samples with
about 50% coverage of GO layers. The layers presented several folds achieving coverages up to 4-5
layers. In order to obtain higher coverage on silicon substrates another method was employed. At
first, a line was drawn with a permanent marker on the substrate, then a drop of GO solution was dried
on it, and finally the marker line was removed with acetone. The result was very sharp borders of
multilayered GO structures. Afterwards mono layers were deposited by deep coating to obtain the
desired coexistence of high and low structures of GO.
HV- AFM imaging
All of the experiments described in this work have been carried out with Nanotec Electronica
(http://www.nanotec.es) microscopes controlled with the software package WSxM.
Olympus OMLC-AC240TM Pt covered tips with a force constant of K~1.5 N/m were used.
The experimental setup of the High Vacuum measurements consists of a home-made high-vacuum
chamber with a base pressure of 10−6 mbar, equipped with an AFM head.
The scanning AFM mode used in this work is Drive Amplitude Modulation (DAM- AFM). This
method has a similar feedback scheme than Frequency Modulation (FM): two nested feedback loops
give the topography in DAM. The first loop adjusts the driving force in order to maintain the
oscillation amplitude. The driving force needed to sustain this oscillation amplitude is related to the
energy dissipated in the system. By adjustment of the position of the scanner in the z-direction the
driving force is kept constant at the set-point value. A Phase Locked Loop, which tracks the effect
resonance frequency, can operate as parallel feedback loop in DAM. Topography images in DAM
1
represent maps of constant dissipation. The frequency shift controlled by the PLL provides a
spectroscopic image. The scanning speed in vacuum is comparable to that in Frequency Modulation.
Kelvin Probe Force Microscopy (KPFM) imaging
In KPFM, an ac bias voltage (Vac sin (ωt)) is applied to the tip. In order to cancel the electrostatic
interaction between the tip and the sample, the component of the force that oscillates with Fω is
nullified by applying the appropriate Vdc at each tip position; this is the output of a Kelvin Force
measurement, i.e. surface potential. There are two common operation modes in KPFM. In the first one
the bias is oscillated out of resonance (typically at a few KHz, well below the fundamental resonance
frequency). In the second operational mode the bias is oscillated at a frequency corresponding to the
second bending mode of the cantilever. The latter case offers more sensitivity and, due to resonant
enhancement only very small AC voltages are needed to obtain a sufficiently large KPFM signal. This
is the method used in this paper. For our tips the second bending mode was around 450 KHz. The AC
bias voltage used in our experiments is typically between 100-500 mV. KPFM can be operated
simultaneously with the topography measurement in a single scan but in some cases, due to tip
instabilities KPFM is performed in lift-mode (or retrace) at about 15 nm from the surface.
EFM imaging
The EFM maps were acquired in retrace mode. Topography is acquired at the first pass. For the
second pass a DC bias voltage is applied to the tip as it follows the topographic profile taken in the
first pass. In this second pass the tip is oscillated at its free resonance frequency and a DC voltage is
applied. The measured feature is the shift in resonance frequency introduced by electrostatic tipsample interaction which is proportional to the force gradient.
2
Figure S1
(a)
(a)
(b)
1 ML
165.27
25mVmV
HOPG
3 ML
4 ML
2 ML
(d)
(c)
20 ML
4 ML
2 ML 3 ML
16 ML
HOPG
-450
-366.11 mVmV
|Surface Potential Difference(mV)|
500
(e)
Sample A
Sample B
400
300
200
100
0
0
2
4
15
20
Number of layers
FIGURE S1. (a) and (c) AFM topographic images (edge enhancement) of GO layers on HOPG
substrates and its corresponding surface potential maps ((b) and (d)) of the same area acquired in high
vacuum conditions for two samples with different level of oxidation of the layers in solution. Sample
A (a)-(b) presents higher differences in the surface potential maps than sample B (c)-(d) between the
substrate and the first layer and between the different layers. Notice that the colour scale is the same
in both cases. Figure (e) summarizes the two situations: plot of the surface potential difference (in
absolute value) as a function of the number of layers of GO flakes with different oxidation level on
the same substrate (HOPG).
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6.19 mV
- 900 mV 37 mV
(c)
348.74 mV
348.74 mV
500 mV
(b)
-327 mV
(d)
6.19 mV
37 mV
348.74 mV
- 180 mV
Air
High Vacuum
600
0
-600
-1200
0
50
100
Air
High Vacuum
(h)
(g)
150
L (m)
-307 mV
Surface Potential (mV)
(f)
6.19 mV
50 mV
(e)
348.74 mV
Surface Potential (mV)
(a)
6.19 mV
Figure S2
0
-100
-200
-300
0
30
60
90
L (m)
FIGURE S2. a) AFM topographic image (edge enhancement) of GO layers on a HOPG substrate.
Panel b depicts a surface potential map of the same area acquired in ambient conditions. c) Surface
potential map of the same area acquired in high vacuum conditions. (d) Profile performed on the
KPFM map following the schematic paths in (b) and (c) respectively. e) AFM topographic image
(edge enhancement) of GO layers on a Si substrate. Panel f depicts a surface potential map of the
same area acquired in ambient conditions. g) Surface potential map of the same area acquired in high
vacuum conditions. (h) Profile performed on the KPFM map following the schematic paths in (f) and
(g) respectively.
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(b)
6.19 mV
-50 Hz 15 mV
(c)
348.74 mV
-21.82 Hz
-22 Hz
(a)
-48.68 Hz
Figure S3
ML1
-291 mV
ML1
ML2
ML2
HOPG
HOPG
FIGURE S3 a) AFM topographic image (edge enhancement) of GO layers on a HOPG substrate, the
experiment was carried out in vacuum. Panel b depicts the EFM map (represented as a frequency shift
map) of the same area acquired at a tip sample distance of 50 nm and bias voltage of 3V. In the image
we can clearly distinguish the covered and uncovered areas. Moreover, the electrostatic contrast also
allows discriminating between different numbers of layers on the substrate. Panel c corresponds to the
surface potential map on the same area as panels (a) and (b).
1
W. S. Hummers and R. E. Offeman, "Preparation Of Graphitic Oxide," Journal Of
The American Chemical Society 80 (6), 1339-1339 (1958).
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