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Experimental Study of the Intrinsic and Exxpetriminenstaicl StTudryaonf tshepInotrintsiPc arnodpExetrintsieic sTraonsfpoGrt rParopehrtietseofaGnradphiMte aundltMigurltiagrpaphenne SeamSpalesmples 1217 The true temperature dependence of the resistivity in an ideal, defect-free multigraphene sample should be semiconducting-like, as it is expected for an ideal semimetal with zero or small gap. Dusari, Barzola-Quiquia, Esquinazi & García (2011) noted that the temperature dependence of several multigraphene samples shows a semiconducting-like behavior with a saturation or a weak maximum at low temperatures, this last behavior due to the contribution of interfaces, internal as well as with the substrate or at the free sample surface, parallel to the graphene layers, see also Section 4. It becomes now clear that the contribution of the interfaces to the measured conductance of large samples can overwhelm the one coming from the intrinsic graphene layers in thick enough samples. Recently done studies on several graphite samples of different thickness indicate the existence of an intrinsic energy gap of the order of 40 meV in the graphene layers within ideal Bernal graphite (García et al., 2011). We note that experiments on samples with carrier concentration 109 cm−2 are hardly reported in literature. At such low-carrier densities, as it appears to manifest in graphite, electron correlations and possible localization effects should be considered. Electron interactions are large and for a small-enough carrier density, the expected screening will be very weak promoting therefore the existence of an energy gap (Drut & Lähde, 2009). According to literature the structural quality of graphite samples can be partially resolved by investigating the Raman D-line at ∼ 1350 cm−1. However, also the edges of the samples as well as the borderlines between regions of different thicknesses may contribute to the D-band signal. The Raman spectra between 1300 and 1700 cm−1 have been measured at different positions of several multigraphene samples by Barzola-Quiquia et al. (2008). Those results show that in thin multigraphene samples with similar semiconducting-like behavior in the temperature dependence of the resistivity, different amplitudes of the Raman D-line peak are measured. This indicates that the disorder related to this Raman line does not appear to affect strongly the temperature dependence of the resistivity. As the absolute value of the electrical resistivity depends also on the measured sample thickness, see Section 2.4, it seems clear to assume that the metallic-like behavior is not intrinsic to ideal graphite but it is due to the influence of the interfaces inside the graphite samples with large enough thickness. Magnetoresistance (MR) results follow the behavior observed in the electrical resistivity. If a metallic behavior is obtained (thicker samples) then the MR decreases with increasing temperature and it shows SdH oscillations, see Fig. 4(c). If the graphite sample shows a semiconducting-like behavior (thinner samples) then the MR is clearly smaller in magnitude as well as it decreases decreasing temperature for fields above 1 T, see Fig. 4(b). This figure also shows the absence of the SdH oscillations when the thickness of the graphite sample is small enough. We note that Kohler’s rule does not apply in multigraphene and HOPG samples and one of the reasons might be the contribution of the sample internal structure and interfaces. Defects and interfaces may influence the dimensionality of the transport and might be the origin of localized granular superconductivity. Results presented so far correspond to the situation in which the field is parallel to the c-axis. In the case of having magnetic field applied parallel to the graphene planes nearly no MR is observed. This fact speaks for a huge anisotropy of the metallic or superconducting regions and it suggests that those are within the interfaces found by TEM, see Fig. 2. We note that the field-induced metal-insulator transition (MIT) found in bulk HOPG samples, see Section 4.2, vanishes for samples thinner than 50 nm, which also indicates that certain regions parallel to graphene planes are related to the origin of this MIT (Barzola-Quiquia et al., 2008).PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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