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Micro and Nanosciences Laboratory

Micro and Nanosciences Laboratory . Fabrication of Graphene sheets. Goal: Large-scale nanoelectronic devices based on patterned and modified graphene sheets Fabrication method for large area single and multi layer graphene sheets needed

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Micro and Nanosciences Laboratory

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  1. Micro and Nanosciences Laboratory MICRONOVA Fabrication of Graphene sheets • Goal: Large-scale nanoelectronic devices based on patterned and modified graphene sheets • Fabrication method for large area single and multi layer graphene sheets needed • Graphene based electronic devices still at early stage, possibly ultra-narrow electron waveguide channels necessary in devices

  2. Fabrication of graphene sheets MICRONOVA • Exfoliation of highly ordered pyrolytic graphite (HOPG) • Easy-to-access (e.g. cleavage by Scotch tape) • Labour-intensive, uncertain placement • Identification of graphene sheets by optical microscopy/AFM • Electrostatic deposition[1] • Electric field used to peel off single-few layers from HOPG to SiO/Si wafer • Epitaxial growth of graphene on SiC[2] and other substrates • Vacuum graphitization by UHV thermal treatment • CVD growth • Layers studied by STM and XPS (LEED, ...) [1] N. Sidorov et al., Nanotechnology 18 (2007), 135301 [2] C. Berger at al., Science 312 (2006), 1191

  3. The exfoliation technique MICRONOVA RIE O2 plasma HOPG mesas Glass substrate + resist Mesas stick on resist Exfoliation w/ Scotch tape to thin sheets Si/SiO2 substrate Sheets dissolved in ace Sheets attach on substrate Si/SiO2 substrate Thin flakes (<10 nm) attach strongly on the SiO2 Novoselov et al., Science 306, pp. 666 (2004)

  4. Identification of Graphene MICRONOVA • flakes on SiO2/Si: interference colors in optical microscopy. • Sheets visible down to thickness ~1.5 nm (few-layer graphene ”invisible”) SEM optical FLG • SEM can distinquish few-layer graphene • AFM can measure the thickness down to single layer graphene (thickness ~4 A) • distance between 1st graphene layer and SiO2 can vary several Angstroms double fold Novoselov et al., Science 306, pp. 666 (2004)

  5. Epitaxial graphene MICRONOVA • motivation: cleaved graphene comes in small dimensions (~10 mm), ”hit-and miss” approach not suitable for applications • epitaxial methods: large, high-quality 2D graphite  also graphene?? • epitaxy of graphite by CVD [1]: • varying substrates; precursors hydrocarbons such as ethane, benzene • growth of ”monolayer graphite” achived • epitaxy of graphite by ultra-high vacuum (UHV) treatment of SiC [2]: • samples typically grown on (6H-)SiC at ~>1300°C • growth on the Si-face (0001) is slow  thin layers • growth on the C-face (000-1) is faster  up to 100 monolayers thickness [1] Oshima et al., J.Phys. Condens. Matter 9, pp. 1 (1997) [2] Review: de Heer et al., Solid State Comm. (2007)

  6. 1050°C 1100°C 1400°C 1250°C UHV epitaxial graphene MICRONOVA • LEED patterns show the changes in surface reconstruction and reveal graphite formation [1] • Auger electron spectroscopy (AES) used to determine Si:C ratios [2] • STM images of surface show graphene atomic lattice [1] [1] Berger et al. J. Phys. Chem. B 108 (2004) [2] de Heer et al., Solid State Comm. (2007)

  7. UHV epitaxial graphene MICRONOVA • ”Dirac cone” dispersion relation properties determined by Landau level spectroscopy (infrared transmission in magnetic field) • inter-Landau level transition energies follow B1/2 chirality • Fermi velocity v0=1.03E8 cm/s de Heer et al., Solid State Comm. (2007) • conclusion: ”multilayered graphene”, not graphite • transmission experiments probe the properties of the ”low charge density” bulk of the epitaxial graphene layer with n= 1.5x1010/cm2 • The interfacial graphene layer is probed by 2D transport measurements and has n= 2x1012/cm2 due to built-in electric field • Graphene grown the on Si face (low growth rate) has low mobility, whereas on the C face (high growth rate) it has high mobility

  8. Transport in epitaxial graphene MICRONOVA • MR of intermediate width (1 mm) Hall bar shows SdH oscillations • Landau plot reveals anomalous Berry’s phase (Dirac particles), vF=0.7x108 cm/s • As the width of the ribbon is decreased, the low-carrier density graphene becomes insulating • MR of 500 nm width Hall bar shows quantum confinement effects • Landau plot deviates from linear as a result of confinement • No Quantum Hall Effect was observed! • This may be linked to the weakness of the SdH oscillations de Heer et al., Solid State Comm. (2007)

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