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 ( 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 10−6mbar, 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 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 (Vacsin (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 withFωis nullified by applying the appropriateVdcat 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 tip-sample interaction which is proportional to the force gradient.

Figure S1

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).

Figure S2

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.

Figure S3

FIGURE S3a) 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 imagewe 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).

1W. 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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