Selective Chemistry of SWCNT

1. Selective Chemistry of SWCNT Reifenberger Nanophysics Lab Deepak K Pandey: Birck Nanotechnology Center Purdue University, West Lafayette, IN USA 1

2. Some Facts: Carbon Nanotubes • Allotropes of carbon with length-to- diameter ratio 3 x 107 : 1[1] • Member of the fullerene structure family • Can be thought of a sheet of one atom thick carbon layer rolled and capped with hemisphere buckyballs • Nanotube are categorized as single-walled nanotubes (SWNTs) and multi-walled nanotubes MWNTs • Chemical bonding of nanotubes mainly composed of SP2 bonds • Nanotubes naturally align themselves into “ropes” held together by Van der Walls forces • High pressure forces nanotubes to merge together trading some of SP2 bonds for SP3, giving possibility of producing strong unlimited length wires Zhu et al., Ultralongsingle-wall carbon nanotubes, Nature Materials 3, 673 - 676 (2004)

3. Types: Carbon Nanotubes • Wrapping of graphene sheets to make a CNT is represented by pair of indices (m,n) known as chiralverctors • Three types of CNTs depending on chiralindices • m=0: zigzag • n=m: armchair • other wise chiral Figure from: http://upload.wikimedia.org/wikipedia/commons/3/35/CNTnames.png

4. Types: Carbon Nanotubes [3] • Single Walled • Unique electrical properties • Development of first intramolecular logic gate using SWNT FETs[2] • Very expensive to produce • Multi Walled • Multiple layers of graphene rolled together • Interlayer distance between walls of MWCNTs 3.3 Å • Share very similar morphology and properties as SWNT but have improved resistance to chemicals: particularly double walled [4] [2] Martel, R.; V. Derycke, C. Lavoie, J. Appenzeller, K. K. Chan, J. Tersoff, and Ph. Avouris, "Ambipolar Electrical Transport in Semiconducting Single-Wall Carbon Nanotubes“, Physical Review Letters87 (25) 2001 [3] http://www.physics.bc.edu/EMXRD/SWCNT.jpg [4] http://image.mrs.org/2007brazil/monpix/mwcnt.jpg

5. Types: Carbon Nanotubes • SWCNT • Metallic or Semiconducting depending on the chirality [5] • Possible applications • Ultra low resistance material: Metallic SWCNT • FET with semiconducting SWCNT • Problem: SWCNT are produced by synthetic protocols as mixture of all electronic types[5] • Early work to fractionate CTN on electronic type includes selective flocculation with amines[6] and dielectrophoresis[7] • This work by Satrino et al. from University of Illinois Urbana- Champaign used p-Hydroxibenzenediazonium salt to selectively functionalize metallic SWCNT at 450. [5] Michael J. Bronikowski, Peter A. Willis, Daniel T. Colbert, K. A. Smith, and Richard E. Smalley, “Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPco process: A parametric study “, J. Vac. Sci. Technol. 19 pp. 1800-1805 [6] Chattopadhyay, D.; Galeska, L.; Papadimitrakopoulos, F. J. Am. Chem. Soc., 125, 3370-3375, 2003. [7] Tadashi Hasegawa, Takeshi Akasaka ,Shigeru Nagase et al., Large-Scale Separation of Metallic and Semiconducting Single-Walled Carbon Nanotubes, J. Am. Chem. Soc., 127, 10287-10290, 2005.

6. Selective Functionalization and Free SolutionElectrophoresis of SWCNT: Separate Enrichment of Metallic and Semiconducting SWNT Woo-Jae Kim, Monica L. Usrey, and Michael S. Satrano Chem. Mater. 2007, 19, 1571-1576

7. Approach • It has been shown that electron withdrawing reagents such as diazonium salt can selectively react metallic SWNT over semiconducting SWNT under certain condition • In this scheme • Selectively attach p-hydroxybenzene group to matellicnanotubes • Use free solution electrophoresis at high pH to separate reacted metal from unreacted semiconductor Method • Photoabsorption and Raman Spectroscopy are used to benchmark the seperation

8. Mechanism: Diazonium Chemistry • Water soluble diazonimu salt react with CNT via charge transfer • Diazonimu salt make stable covalent bond with CNT • This covalent aryl bond has high electron affinity for electrons with energies near Fermi level, Ef of the nanotube • Metallic SWNT have bigger electron density near Ef resulting in higher reactivity over semiconducting SWNT • In process diazonimu salt forms a charge-transfer complex at the nanotube surface, where electron donation from nanotube stabilizes the transition state which in turn accelerates the reaction • Once the bond symmetry of the CNT disrupted due to formation of this defect, adjacent carbon increases in reactivity and initial selectivity of metallic CNT gets amplified.

9. Selective Reaction Condition • Reactant molecules should be added at a very small rate to SWNT solution for a sufficient long time • Slow addition ensures reaction with only metallic SWNTs and with no SC SWNTs as all the reactant molecules are taken up by the metallic SWNTs • Long exposure ensures that most of the nanotubes are reacted • If the entire diazonium solution is added all instantaneously then SC SWNTs will also react due to presence of excess reactant

10. Properties of SWNT to detect functionalization: Spectroscopy • Unique optical and spectroscopic properties of SWCNT arises due to one dimensional confinement of electronic and phonon states • Ultraviolet-Visible-near-Infrared Spectroscopy: • The UV-vis-nIR spectrum monitors the valence (v) to conduction (c) electronic transitions denoted Enn where n is the band index • The E11 transitions for the metallic nanotubes occur from ~440 to 645 nm. The E11 and E22 transitions for the semiconducting nanotubesare found from 830 to 1600 nm and 600 to 800 nm, respectively • These separated absorption features allow for the monitoring of valence electrons in each distinct nanotube • Reaction at the surface result in localization of valence electrons makes them no longer free to participate in photoabsorption which results in decay of the spectrum features

11. Probing selective Chemistry via UV-vis-nIR Due to diazonium chemistry peak intensities that represent the first Van Hove transition of metallic species (E11,metal) decreases, while second (E22, SC) and first (E11, SC) Van Hove transition of the semiconducting show little or no change.

12. Probing selective Chemistry via UV-vis-nIR • Clear decay of photoabsorption feature for the metallic SWCNT • No noticeable difference for the photoabsorption peaks for semiconducting SWCNT

13. Probing selective Chemistry via UV-vis-nIR • G band frequency can be used (1) to distinguished between metallic and semiconducting SWCNT (2) to probe charge transfer arising from doping a SWCNT • G+ is sensitive to charge transfer and lineshape G- is highly sensitive to whether the SWCNT is metallic (Breit-Wigner-Fanolineshape) of semiconducting (Lorentzianlineshape) • Intensity of D peak measures covalent bond made with the nanotube surface. • Selective functionalization increases the intensity of the D peak due to formation of aryl-nanotube bond

14. Reversibility of Diazonium Chemistry • Nanotubes reacted with the diazonium reagent can be converted back into pristine nanotubes when thermally treated at 300°C in an atmosphere of inert gas. This cleaves the aryl hydroxyl moieties from the nanotubesidewall and restores the spectroscopic feature (Raman and UV-vis-nIRspectra) of pristine nanotube Separation of Metallic and SC SWNTs • Separation can be done in solution by deprotonation (removal of proton from a molecule) of the p-hydroxybenzene group on the reacted nanotubes(metallic) in alkaline solution followed by electrophoretic separation (electrokineticseparation )of these charged species from the neutral species (SC nanotubes). This followed by annealing would give separated pristine SC and metallic SWNT

15. Conclusion • Metallic and semiconducting carbon nanotubes generally coexist in as- grown materials. • To get only SC or only metallic nanotubes selective functionalization of metallic SWNTs via 4-hydroxybenzene diazonium can be used. • Separation can be done in solution by deprotonation of the p- hydroxybenzenegroup on the reacted nanotubes (metallic) in alkaline solution followed by electrophoretic separation of these charged species from the neutral species (SC nanotubes). • This followed by annealing would give separated pristine SC and metallic SWNT