Micro%20and%20Nanosciences%20Laboratory

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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
• Graphene based electronic devices still at early
  stage, possibly ultra-narrow electron waveguide
  channels necessary in devices

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Fabrication of graphene sheets

• 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 deposition1
• Electric field used to peel off single-few layers
  from HOPG to SiO/Si wafer
• Epitaxial growth of graphene on SiC2 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
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The exfoliation technique
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 (lt10 nm) attach strongly on the SiO2
Novoselov et al., Science 306, pp. 666 (2004)
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Identification of Graphene

• 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)
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Epitaxial graphene

• 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 gt1300C
• 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)
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UHV epitaxial graphene

• LEED patterns show the changes in surface
  reconstruction and reveal graphite formation 1
• Auger electron spectroscopy (AES) used to
  determine SiC 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)
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UHV epitaxial graphene

• 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 v01.03E8 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

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Transport in epitaxial graphene

• MR of intermediate width (1 mm) Hall bar shows
  SdH oscillations
• Landau plot reveals anomalous Berrys phase
  (Dirac particles), vF0.7x108 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)