energy band gap engineering of graphene nanoribbons melinda y han et al prl 98 206805 2007
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Energy Band-Gap Engineering of Graphene Nanoribbons Melinda Y. Han et al, PRL 98, 206805 (2007). Yusung Kim 9/3/2014. Outline. Background General Band gap engineering Band gap engineering for Carbon family Paper Experiment setup Measurements Conclusion Conclusion and Comments.

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energy band gap engineering of graphene nanoribbons melinda y han et al prl 98 206805 2007

Energy Band-Gap Engineering of Graphene NanoribbonsMelinda Y. Han et al, PRL 98, 206805 (2007)

Yusung Kim

9/3/2014

outline
Outline
  • Background
    • General Band gap engineering
    • Band gap engineering for Carbon family
  • Paper
    • Experiment setup
    • Measurements
    • Conclusion
  • Conclusion and Comments
general bandgap engineering
General Bandgap Engineering
  • Compound Semiconductors (III-V)
  • Changing EG as a function of position by forming a heterojunction
  • Epitaxy  MBE, MOCVD
    • MBE: Bell Lab by J.R. Arthur and Alfred Y. Cho. In 1960
  • Devices
    • HEMT, HBT (Ultrafast circuits)
    • MQW (VCSEL Laser, IR Sensors,etc.)
    • Solar Cells

http://people.seas.harvard.edu/~jones/ap216/images/bandgap_engineering/algaas_qw_2.gif

bandgap engineering for carbon family
Bandgap Engineering for Carbon Family
  • CNT
      • Larger the diameter the smaller the bandgap.
      • C(4,3) largest of all SWCNT EG=1.28 eV
      • d ~ 3nm will have EG about equal to kT @RT
  • Graphene
    • Substrate-induced Bandgap
    • GNRs
      • Experiments
        • Lithography – 3
          • (“P.Kim et al. PRL 2007, 98, 206805)”, “Chen et al., Physica E 2007, 40, 228”, “J.F.Dayen et al, Small 2008,4 , 716.”)
        • Chemical – 3
          • (“H.J.Dai et al., Science 2008 319, 1229”, “H.J.Dai et al., PRL 2008, 100, 206803”, “Yang et al., Am.Chem. Soc. 2008, 130, 4216)
        • Micromechanical Cleavage – 1
          • (M.Moreno-Moreno, Small 2009, x, No. x, 1-4
      • Theoretical Study
        • AGNR (Metallic & Semiconducting depending on width) – TB and 2D dirac eqns
        • ZGNR – Metallic with peculiar edge states
        • H-Passivated AGNR and ZGNR both ALWAYS have EG. (first principle calculation)
          • Energy Gaps in Graphene Nanoribbons , Y. Son et al, PRL 98, 216803, (2006)
gnr fabrication
GNR fabrication
  • Mechanical Exfoliation
      • Ref[3] : Kish Graphite(Toshiba Ceramics) on degenerately doped Si wafers with a 300-nm SiO2 coating layer, by using micromechanical manipulation.
  • Graphene sheets with lateral size ~20µm contacted with Cr/Au(3/50nm) metal electrodes
  • HSQ(negative tone e-beam resist) spun on to the samples and patterned to form an etch mask defining nanoribbons with widths ranging from 10 – 100 nm and lengths of 1—2 µm.
  • Oxygen Plasma used to etch away the unprotected graphene
graphene nanoribbons
Graphene Nanoribbons
  • P1-P4 Parallel sets – Width (24 ± 4, 49 ± 5, 71 ± 6)
    • HSQ mask not removed
    • Width measured after the performance test
  • D1-D2 sets – Same Width with varying relative orientation

Set P3 covered by a protective HSQ etch mask

P1 each contain many ribbons of varying width running parallel

D2 have ribbons of uniform width and varying relative orientation

conductance measurement
Conductance Measurement
  • Lock-in Technique with (100µV @ 8Hz)

Bulk Graphene Conductance

Gmin = 1.86µS

Gmin = 3.715µS

Gmin = 5.5µS

GNR Conductance (W<100nm)

At Low Temp, Gmin < 10-8

At RoomTemp,Gmin on the order of 4 e2/h(W/L)

Depressed G with respect to Vg band gap

Bulk Graphene

Gmin = 4e2/h happens at Vg=Vdirac

Gmin changes less than 30% (30mK—300K)

Stronger T-dependence for larger Vg region -> narrower ribbon suggesting larger

band gap

conductance measurement1
Conductance Measurement
  • Vg=Vdirac-50V
  • n=3.6X1012/cm2 (hole density)
  • G= σ(W-W0)/L
  • σ = sheet conductivity
  • W0 = inactive GNR width
  • 10nm @ RT
  • 14nm @ 1.6K
  • In epitaxial graphene, W0 was found to be 50nm.
  • Explanation for the difference in W0
  • Contribution from localized edge states due to structural disorder caused by the etch process
  • inaccurate width determination due to over-etching underneath the HSQ etch mask.
  • Found the actual width of the ribbon to be 10nm narrower than the HSQ mask.
  • - The localized edge states is small (< 2nm) at RT and spreads to as much as ~5nm at low temperatures.

σ = sheet conductivity

W0 = inactive GNR width

Sqaure T=300K

Triangle T=1.6K

scaling of the energy gap as a function of the ribbon width
Scaling of the Energy Gap As a function of the Ribbon Width

T=1.6 K

EG/e

Differential Conductance

dI/dVb

EG=0.4meV

EG scaling

EG= α/(W-W*)

α =0.2eV

W*=16nm

EG= 20meV

band gap dependence on crystallographic direction
Band-Gap dependence on Crystallographic direction
  • No sign of crystallographic dependence
  • D1 and D2 fits the linear relationship
  • Edge structure plays a more important role than the overall crystallographic direction in determining the properties of the GNRs.
  • Reasons for not observing any effect
  • Lack of precise control of
  • width
  • edge orientation
  • edge structure
  • chemical termination of the edges

For GNR with W ~ 15nm  ~ 0.2 eV

conclusion
Conclusion
  • Energy gap can be tuned during fabrication process by controlling the width of the ribbon.
  • Fabrication of well-defined edges is still a challenge
  • Recent published paper: “Ultralong Natural Graphene Nanoribbons and Their Electrical Conductivity DOI:10:1002/smll.200801442” 37nm width 1-2 nm thickness and 24um of length (No use of chemical)
conclusion and comments
Conclusion and Comments
  • Bandgap due to the confinement
  • Origin of the band gap in etched GNRs

“Energy Gaps in Etched Graphene Nanoribbons”, Phys. Rev. Lett. 102, 056403 (2009)

    • The charging energy of local resonances or quantum dots forming along the ribbon
    • the strength of the disorder potential
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