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)

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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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