Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Ring formation in Single-Wall Carbon NanotubesWilliam Casper-OrtizMechanical Engineering DepartmentUC BerkeleyRichard Martel, Herbert R. Shea, Phaedon AvourisJournal of Physical Chemistry B Vol 103, Num. 36, 1999Outline•Significance•Fabrication process•Ring yield•Theoretical explanation•Electrical conductance•ConclusionsSignificance•Developing molecular superstructures is essential for device miniaturization •Self organization and self assembly are sought as means to replace conventional fabrication techniques•Study the influence of geometrical configuration on transport phenomena in CNT•Carbon nanotubes are promising building blocks•New nanofabrication techniques hold the keyFabrication processUltrasound 40kHz, 190 W 1-3 hr @ 40-50°CSonication •Provides activation energyPreparation and Activation0.4 mgd=1.4nm SWNT0.8 mLH2SO4/H2O2 10-30% ; 9:1 ratioOxidative treatment •Disperses SWNTs•Reduces residual metal catalysts•Shortens nanotube ropes (3-4μm)•Filter through a 0.2μm membrane•Rinse residues with DI water•Dry in airFiltering and suspension•Suspend in 1,2-dichloroethane (solvent) with brief and low power sonification 100WFabrication processRing yield•Ring formation 50%•Most rings are between 300-400nm•Uneven ring thicknessRing yieldCoils??? Tori???•van der Waals•Weaker •C-C covalent bonds•Stronger•Shortening functionalizes the tube ends with carboxylic acid groups•Rings are easily “opened” by manipulation with AFM tipCoils are formed!!!Theoretical explanationThermodynamic analysis•Strain energy (curvature)•Ring formation requires•Hydrophobic effects•Adhesion energy (VDW)Theoretical explanationThermodynamic yield Experimental yieldProcess is kinetically controlled!!!Experimental radii > thermodynamic radiiTheoretical explanationKinetic Process•Activation energy involves mechanical effects produced by cavitationhttp://www.chemsoc.org/ExemplarChem/entries/2004/bristol_eaimkhong/ultrasound.htmTheoretical explanationConceptual ring formationTheoretical explanation•Once a “proto-ring” is formed, other tubes and ropes attach randomly Explains uneven thickness along circumference •No evidence of ring relaxation towards an equilibrium diameter configuration•Short rings are kinetically improbable•Long nanotubes yield tangled structures, not rings.•MWNT are less likely to form rings due to higher flexural rigidity•R increases as T decreases Electrical conductance•Only tubes in direct contact with both electrodes contribute to conduction•I-V curves at low T are linear•No effect of a back gate bias •Ring resistance ranges from 20-50kΩ @ 300K• Metallic or semiconductor conductance?Electrical conductance•Ring resistance decreases as quantum interference is destroyed•Opposite phase shifts are introduced and the standing wave disappears Metallic conductance!Conclusions•Simple technique for fabricating SWNT rings but it fails to achieve small ring diameters•Yield is around fifty percent with peak radii range of 300-400nm•Rings have a coil structure stabilized by van der Waals forces•Resistance ranges between 20-50Ω @300K•Electrical transport is dominated by metallic conductance•Need for other techniques to produce smaller radii and toroidal
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