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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 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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  1. Energy Band-Gap Engineering of Graphene NanoribbonsMelinda Y. Han et al, PRL 98, 206805 (2007) Yusung Kim 9/3/2014

  2. Outline • Background • General Band gap engineering • Band gap engineering for Carbon family • Paper • Experiment setup • Measurements • Conclusion • Conclusion and Comments

  3. 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

  4. 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)

  5. 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

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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

  11. 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)

  12. 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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