Camara et al. Nanoscale Research Letters 2011, 6:141 http://www.nanoscalereslett.com/content/6/1/141 Page 3 of 6 Figure 2 AFM images of continuous and almost free standing monolayer graphene islands grown on the C-face of an 8° off-axis 4H-SiC substrate. (a) at a large scale, the zoom in (b) showing the wrinkle and the step bunched character of the SiC surface below and (c) a layer scratched by an AFM tip. Tens of similar monolayer islands grown on, both, on axis and off-axis substrates were probed by Raman spec- troscopy. We used the 514 nm laser line of an Ar-ion laser for excitation and got very similar features. At the micrometer size, all spectra reveal that the islands are of the same nature and very homogeneous. First, the D-band, which usually indicates the presence of disorder or edges defects, is very weak and the Raman signature is extremely close to the one found for exfoliated graphene on SiO2/Si [11]. Second, the 2D-band appears at low fre- quency (2685 cm-1) which is strong evidence that there is no strain at the layer to substrate interface (i.e. almost a free-standing SLEG layer). Third, this 2D-band can be fitted with a single Lorentzian shape with a FWHM of 30 cm-1 [17]. Fourth, the ratio I2D/IG between the inte- grated intensities of the 2D-band and the G-band is high, which suggests weak residual doping in the order of 3 to 6 × 1012 cm-2 [18]. Altogether, these Raman and micro- scopy measurements tend to demonstrate the almost free-standing low-doped and continuous character of the grown layers [12,19]. Electrical transport measurements Gold alignment marks were used to select some SLEG position by OM. Then, they were contacted by e-beam lithography and subsequent deposition of a contact layer made of Cr/Au in Hall bar configuration. A typical example is shown in Figure 3. Then transport measurements were done at low tem- perature on the different samples, using a maximum magnetic field of 13.5 T. The contact geometry allowed simultaneous measurement of, both, the longitudinal and transverse voltages with the current flowing between two injection contacts at the flake extremities. In both series of samples, from the sign of the Hall vol- tage, we found that the carriers were holes (in agree- ment with other results published on the C-face [13,14]). The holes concentration ranged from 1 × 1012 to 1 × 1013 cm-2 at low temperature, with a weak tem- perature dependence. For carrier concentrations larger than 3 × 1012 cm-2, no QHE could be detected and only Shubnikov-de Haas (SdH) oscillations were found. This is shown in Figure 4 for an off-axis sample and, as usual, the plot of the inverse field at which the oscillations maxima occur ver- sus the Landau level index shows a clear linear depen- dence going down to the origin. This is the usual signature of the heavily doped graphene. Figure 3 Optical microscopy of a SLEG grown on 8° off-axis semi-insulating SiC substrate. (a) before contact and (b) after contacting in a Hall Bar configuration for Hall Effect measurement.