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21236 Graphene – Synthesis, Characterization, Properties anWdill-bAe-psept-lbiyc-IaN-tTiEoCnHs Fig. 15. Normalized Josephson critical current Ic/Ic(0) as a function of the normalized temperature T/Tc obtained for different samples. The values obtained for Ic(0) range from 0.9 to 7 μA and for Tc ∼ 23 k to 175 K for different samples. The solid line is the theoretical curve taken from the work of Hagymási et al. (2010) without free parameters. 5. Conclusion The main messages of the work described in this chapter are: (1) Several of the transport properties measured in large graphite samples are not intrinsic of graphite but are influenced in a large extent by interfaces and other defects and impurities. Taking into account that one defect or impurity in a graphene layer inside graphite can contribute with one carrier, it is clear that the carrier densities are not intrinsic but are due to defects and to a large extent probably to highly conducting interfaces, internal as well as at the sample boundaries. We note also that, as the large experimental evidence in semiconductors indicates, even the quantum Hall effect observed in graphite is actually not intrinsic of ideal graphite but can be attributed to the highly conducting quasi-two dimensional electron systems at the interfaces. Clearly, the details of this effect will depend on the sample dimensions and quality, providing us a way to understand experimental discrepancies found in literature. Our work indicates that the intrinsic mobility of the carriers in the graphene layers inside graphite are extraordinarily large μ > 106 cm2/Vs and these carriers show ballistic transport in the micrometer range even at room temperature. (2) The first observation of superconductivity in doped graphite goes back to 1965 when it was observed in the potassium graphite intercalated compound C8K (Hannay et al., 1965). A considerable amount of studies has reported this phenomenon in intercalated graphite compounds or doped graphite (Emery et al., 2005; Kopelevich et al., 2004; Weller et al., 2005), however the superconducting properties of pure graphite are still under discussion. In this chapter we have given a possible clarification of this topic. By reducing the dimensions of the samples, the intrinsic properties of graphite have been deeper investigated. The overall idea coming out from all the experimental results can be resumed as follows: bulk graphite samples cannot be considered as a uniform electronic system, but it must be considered as a semiconducting matrix with metallic as well as superconducting domains in it. We note that already Lu et al. (2006) realized the coexistence of insulating andPDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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