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Experimental Study of the Intrinsic and Exxpetriminenstaicl StTudryaonf tshepInotrintsiPc arnodpExetrintsieic sTraonsfpoGrt rParopehrtietseofaGnradphiMte aundltMigurltiagrpaphenne SeamSpalesmples 1173 diffraction (EBSD), electronic transport and Raman spectroscopy. We show the correlations between the internal microstructure and sample size – lateral as well as thickness from millimeter size graphite samples to mesoscopic ones, i.e. tens of nanometer thick multigraphene samples – and the temperature (T) and magnetic field (B) dependence of the longitudinal resistivity ρ(T, B). Low energy transmission electron microscopy reveals that the original highly oriented pyrolytic graphite (HOPG) material - from which the multigraphene samples were obtained by exfoliation - is composed of a stack of ∼ 50 nm thick and micrometer long crystalline regions separated by interfaces running parallel to the graphene planes (Barzola-Quiquia et al., 2008). We found a qualitative and quantitative change in the behavior of ρ(T, B) upon thickness of the samples, indicating that their internal microstructure is important. The overall results described in sections 2 and 4 indicate that the metallic-like behavior of ρ(T) at zero magnetic field measured for bulk graphite samples is not intrinsic of ideal graphite. The influence of internal interfaces on the transport properties of bulk graphite is described in detail in Section 4 of this chapter. We will show that in specially prepared multigraphene samples the transport properties show clear signs for the existence of granular superconductivity within the graphite interfaces, which existence was firstly reported by Barzola-Quiquia et al. (2008). Based on the results described in Section 4 we argue that the superconducting-insulator or metal-insulator transition (MIT) reported in literature for bulk graphite is not intrinsic of the graphite structure but it is due to the influence of these interfaces. 2. Samples characteristics 2.1 Sample preparation In order to systematically study the transport properties of ideal graphite and compare them with those of bulk graphite samples measured in the past, we need to perform measurements in different tens of nanometer thick multigraphene samples of several micrometer square area. The samples presented in this chapter were obtained from a highly oriented pyrolytic graphite (HOPG) bulk sample with a mosaicity (rocking curve width) of ∼ 0.35◦ ± 0.1◦ from Advanced Ceramics company. This material does not only guaranty high crystalline quality but also allows us to easily cleave it and obtain up to several hundreds of micrometers large flakes with thickness from a few to several tens of nanometers. The starting geometry of the bulk graphite material for the preparation of the flakes was ∼ 1 mm2 and ∼ 20 μm thickness. The selected piece was located between two substrates and carefully pressed and rubbed. As substrate we used p-doped Si with a 150 nm SiN layer on top. The usefulness of the SiN layer on the Si substrate is twofold: firstly the multigraphene flake on it shines with high contrast by illuminating it with white light allowing us to use optical microscopy to select the multigraphene samples. Immediately after the rubbing process we put the substrate containing the multigraphene films on it in a ultrasonic bath during 2 min using high concentrated acetone. This process cleans and helps to select only the good adhered multigraphene films on the substrate. After this process we used optical microscopy and later scanning electron microscopy (SEM) to select and mark the position of the films. Figure 1 shows four of the investigated samples of different micrometer length and tens of nanometer thickness. For the preparation of the electrical contacts we used conventional electron beam lithography. The contacts were done by thermal deposition of Pd (99.95%) or Pt/Au (99.5%/99.99%) in high vacuum conditions. We have used Pd or Pt/Au because these elements do notPDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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