Presentation at National Center for Theoretical Sciences & National Cheng Kung University

1. First-principles study of chemically modified carbon nanotubes Jijun Zhao State Key Laboratory of Materials Modification by Laser, Electron, and Ion Beams & College of Advanced Science and Technology Dalian University of Technology Presentation at National Center for Theoretical Sciences & National Cheng Kung University 8/25/2006

2. Structure of different carbon allotropes Diamond:sp3 bonding, hard and insulating Graphite: sp2 bonding, soft between graphene planes C60 (buckyball):hollow sphere ~0.7 nm in diameter Carbon nanotube:0.5-50nm in diameter 10-100micron long

3. Discovery of carbon nanotube and bundles Single-wall nanotubes (SWNT): S. Iijima, D. Bethune et al., 1993 Multiwall nanotubes: S. Iijima, 1991 STM image, C. Dekker, 1998 Nanorope, mass production, R. Smalley, 1996 • Growth methods: arc-discharge, Laser ablation, CVD • Single-wall (SWNT) or multi-wall (MWNT) • micrometers in length; 0.7-30 nm in diameter • SWNTs form 2-D lattice: nanotube bundle (nanorope)

4. (10, 0) (5, 5) Γ Z Semiconducting Γ Z Metallic Atomic and electronic structures of carbon nanotubes armchair zigzag • Folding of graphene sheet leads to single-walled nanotube (SWNT). Nanotube chirality depend on the folding angle • Chirality dependent: all armchair (n,n) tubes are metallic; zigzag (n,0) tubes are metallic if n=3m, otherwise, semiconductor.

5. (5, 5) (10, 0) DOS DOS Electronic states and conductance of carbon nanotubes zigzag armchair Density of states: van-Hove singularities Conductance: ballistic transport, quantized in unit of G0=2e2/h (5, 5) (10, 0) G (2e2/h) G (2e2/h)

6. Chemical modification of carbon nanotubes (CNT) (a) intercalation; (b) substitutional doping; (c) encapsulating clusters; (d) metal coating/filling; (e) molecule adsorption; (f) covalent functionalization For details, see our review article: J. Nanosci. Nanotech. 3, 459(2003)

7. Summary of our theoretical efforts & Outline of this talk • Alkali-metal intercalation Alkali-metal intercalation and work functions; Li intercalation and battery • Substational doping BC2N tube; BC3 tubes, Li adsorption and diffusion • Encapsulating fullerenes or clusters: peapods C48N12/C48B12, Na6Pb, Au32 • Gas adsorption and noncovalent functionalization NO2, O2, NH3, CO2, CH4, H2O, N2, H2; C6H6, C6H12, C8N2O2Cl2 (DDQ) • Covalent sidewall functionalization COOH, F, H, OH, NH2, CH3; CCl2, NCOOC2H5 • Transition-metal coating or filling Ti, V, Cr, Fe, Co, etc

8. Brief overview of our computational methods • Electronic structure and total energy • density functional theory (LDA & GGA-PW91) • All electron LCAO, numeric basis (DMol) • plane-wave pseudopotential (CASTEP) • Finite k-point sampling of 1-D Brillouin zone • Dynamic simulation & Structural optimization • Molecular dynamics simulation with empirical force field • Numeric minimizations (conjugate gradient, BFGS) • Conductance • Green’s function within tight-binding approximations

9. Work function of pristine carbon nanotubes Work function (WF): important parameter for electronic properties of CNT; useful for designing of CNT-based nanodevices and NEMS; a critical parameter for field emission of CNT (Field emission can be enhanced by reducing work function) • WF is not sensitive to size & chirality • WFs for all tube bundles (nanoropes) are ~ 5 eV (Photoemission spectrum experiment: ~5 eV), slightly higher than and individual tube (~4.75 eV). Phys. Rev. B 65, 193401 (2002)

10. Work function of alkali-metal doped nanotubes Photoemission spectra by Suzuki, APL (2000).(a) to (c): increasing Cs concentrations. Experiment by S. Suzuki, PRB (2003): WF=3.3 eV for KC10, confirm our theoretical prediction~3.6 eV. • WF decreases with doping concentration, insensitive to tube type • Reduced WF indicates enhanced field emission, experimentally observed by A. Wadhawan, APL (2001). Phys. Rev. B 65, 193401 (2002)

11. Electronic states of alkali metal doped nanoropes (10,10) tube (17,0) tube • Valence bands: almost not affected by alkali-metal doping. • Conduction bands: new peaks associated with alkali-metal atoms. • The density of states near Fermi level is significantly enhanced. • No difference between (10,10) and (17,0) tube bundles for DOS at Fermi level (indistinguishable), supported by Wu’s NMR experiment (UNC) Phys. Rev. B 65, 193401 (2002)

12. L=~10mm closed Cell Phone/Laptop L=3-4mm Li /Metal Oxides Li / Nanotubes L=0.5mm Li ion diffusion Li battery based on carbon nanotubes Experiment by O. Zhou, PRL (2001)

13. Li intercalation in carbon nanotube bundle (10,0) CNT Li5C40 • Li intercalation induce small deformation of SWNTs (~10% by aspect ratio) • Hybridization between Li and nanotube modifies tube conduction bands • Nearly complete charge transfer from Li to nanotube, transforming the semiconducting tubes into metallic Phys. Rev. Lett. 85, 1706 (2000)

14. Saturation Ball-milled nanotubes LiC2 Capacity for Li intercalation inside nanorope Experiment: O. Zhou, CPL, (2000) • Intercalation potential of nanotubes comparable to that of graphite • Saturation Li density (~LiC2) in nanotube bundles is much higher than graphite, due to lower carbon density Li intercalated at both interstitial sites and inside nanotubes Phys. Rev. Lett. 85, 1706 (2000)

15. Li diffusion behavior inside nanotube bundle • Intercalation energies inside tube comparable to interstitial sites • Li ions are impossible to penetrate the tube wall • Energy barrier between two interstitial sites is high (~1.5 eV) • 1-D diffusion behavior of Li ions along tube axis is expected

16. Li diffusion behavior inside nanotube bundles • Li ions form layered structures around tubes. • The 1-D Li diffusion behavior (along tube axis ). • Diffusion in nanotube is faster than in graphite. • As Li density increases, diffusion becomes slower. • The diffusion at room T up to LiC2 is still fast enough to allow Li go through the tube. (1s for 1 m tube).

17. CNT with substitutional doping by boron (4,0) BC3 tube, based on (8,0) C tube Experimentally, BCx composite tubes are synthesized. (8,0) C tube: conjugate electron density on hexagonal carbon ring (4,0) BC3 tube: reduced electron density on B site (3,3) BC3 tube, based on (6,6) C tube • Semiconducting zigzag CNT: with B-doping, remain semiconductor with slightly lower gap, from 0.71 eV to 0.66 eV for (4,0) BC3 tube. • Metallic armchair CNT: with B-doping, become semiconductor, small gap ~0.45 eV. Chem. Mater. 17, 992 (2005)

18. Barrier for Li penetrating through tube wall Chem. Mater.17, 992 (2005) Chem. Phys. Lett.415, 323 (2005)

19. Reduced Li diffusion barriers in BC3 composite tubes • Defect formation energies lower in BC3 tube than in C tubes • Li penetration barriers for BC3 tubes much lower than CNT with same defect, due to electron deficient of boron  BCx composite nanotubes are good candidates for Li battery.

20. Nanopeapod: a novel one-dimensional hybrid structure Encapsulated C60and other cage-like molecules in carbon nanotubes: “peapod” Smith, Monthioux, Luzzi, Nature 296, 323 (1998) Why peapod? • The interior hollow space of a carbon nanotube provides a 1D container for encapsulating a variety of nanomaterials. • CNTs serve as a highly confining reaction vessel, modifying the stability and reactivity of the encapsulated molecules. • It is possible to engineer the Fermi level of the peapods by controlling the space in the tube and the species of the encapsulated fullerenes/clusters.

21. C48B12 C48N12 HOMO: -5.58 eV HOMO: -4.38 eV electron electron Fermi level of carbon nanotube is around -4.8 eV Encapsulating C48N12/C48B12 inside nanotube C48N12: -0.39 |e| on tube, donor, n-type C48B12: 0.67 |e| on tube, acceptor, p-type C48N12/C48B12 pair in semiconductor (17,0) tube Insert energy: ~ 2.4 eV per cluster Nanotube-based p-n junctionbyC48N12/C48B12 peapods Phys. Rev. Lett. 90, 206602 (2003)

22. Na6Pb clusters encapsulated inside nanotubes Na6Pb clusters can be inserted into nanotubes with diameter > 1.0 nm, insertion energy about 1.2-2.8 eV per cluster Experiment: CPL 237, 334 (1995) Magic cluster Incorporating Na6Pb array inside (8,8) tube • Delocalized electron density of conduction bands: hybridization between cluster and nanotube. • Increase number of conduction channels of armchair nanotube from two to three. Phys. Rev. B 68, 035401 (2003).

23. Covalent functionalization Gas adsorption Noncovalent functionalization Vol. 13, 195 (2002) Vol. 6, 598 (2005) Vol. 3, 459 (2003) Chemical functionalization of nanotubes

24. Importance of gas environment of carbon nanotubes Sensitivity of tube conductance to gas, exposure: Dai, (NO2, NH3); Zettl (O2) both on Science, (2000). Long-term stability of field-emission current due to residential gas, e.g., Dean, APL (1999)

25. Interaction between CNT and gas molecules Tube-molecule interaction: Van der Waals force, insensitive to tube type LDA used in calculation, overestimate the adsorption energy and charge transfer • Most gas molecules (NH3, N2, CO2, CH4, H2O, H2, Ar) are charge donors and interact very weakly: binding energy 0.05~0.15 eV, charge transfer 0.01~0.035 e. • Charge acceptor found for NO2 and O2, with relatively stronger interaction: binding energy 0.3~0.8 eV, charge transfer -0.06~ -0.14 e. Nanotechnology 13, 195 (2002)

26. Electronic properties of gas adsorbed semiconductor tubes • Hybridization between molecular orbital of the charge acceptors (NO2, O2) and tube valence band transform semiconductor tube into p-type conductor. Electron density for top nine valence bands shows weak coupling between NO2 and (10,0) nanotube Nanotechnology 13, 195 (2002)

27. N2-(5,5) SWNT O2-(5,5) SWNT Electronic properties of gas adsorbed metallic tubes • Molecule-induced charge fluctuation acts as scattering center and lead to increases of tube resistance • Nanotube-based gas senor becomes a highly active field since then O2 on (10,10) tube: resistance increase by 0.25  per molecule Increase of tube resistance by various gases, Eklund’s group, PRL (2000). Mat. Res. Soc. Symp. Proc. 644, A13.48 (2001)

28. Noncovalent functionalization: role of aromaticity • Noncovalent functionalization preserve the tube structure, thus maintain the superior mechanical properties. • Coupling of  electrons between aromatic molecules and nanotube (- stacking) modify the electronic and transport properties. Resistances of SWNTs are modified by the adsorption of C6H6, but not by C6H12 Aromatic C6H6 delocalization of  conduction electron Nonaromatic C6H12 conduction electron localized on SWNT Appl. Phys. Lett. 82, 3746 (2003) Eklund, PRL (2002).

29. CNT with noncovalent functionalization by DDQ DDQ (C8N2O2Cl2) on (10,0) tube • Adsorption energy ~3 times larger than O2 • Hybridization due to existence of molecular level near tube valence band edge; molecular level delocalized over SWNT. • Charge transfer from DDQ to tube makes (10,0) SWNT p-type conductor. • J. Liu (Duke) observed dramatic decrease of SWNT film resistance upon exposure to DDQ, effect much stronger than oxygen Appl. Phys. Lett. 82, 3746 (2003)

30. DomP Experimental progresses after our theoretical work Y. P. Sun, JACS, (2004) A. Star, Nano Lett. (2003) Field-effect transistor with semiconducting SWNT In solution Solid state S22 S11 Gas sensitivity on gate voltage shift Vg Chemical senor for organic compound! Diminishing of band-gap transition due to DomP

31. Covalent functionalization of CNT: background Divalent Monovalent M. S. Strano et al., Science 301, 1519 (2003) Electronic structures of carbon nanotubes can be modified by covalent functionalization in different ways K. Kamaras et al., Science 301, 1501 (2003)

32. Type of covalent functionalization on nanotube sidewall =CCl2 -COOH Divalent Monovalent Binding energy: 0.7~1.4 eV Binding energy: 1.2~1.8 eV Local carbon bonding changes from sp2 to sp3: significant disruption on nanotube electronic states Local carbon bonding remains sp2, less disruption on tube electronic states. Local C-C bond on tube opens J. Phys. Chem. B 108, 4227 (2004) ChemPhysChem 6, 598 (2005) Nano Letters 6, 916 (2006) Nanotechnology 16, 635 (2005)

33. Binding energy of addends: effects of size & concentration • Smaller tube has larger binding energy (more reactive) due to curvature effect • Metallic tubes are more reactive, observed experimentally: Smalley, Science (2003); Haddon, Science (2003); Hirsch, JACS (2003); Wong, JACS (2004)… • Binding energy decrease as concentration increases ChemPhysChem 6, 598 (2005) Nanotechnology 16, 635 (2005)

34. nanotube nanotube CNT with monovalent functionalization COOH - (6,6) SWNT) • Radical addition lead to local sp3 bonding and induce half-occupied impurity state near EF . • Different from substitutional doping & topological defect; similar to effect by vacancy defect. • Disruption of tube sp2 electron states found by experimental UV spectra: Smalley, CPL (1998)… (10,0) tube -H CN -NH2 (6,6) tube -COOH C H isoelectron N J. Phys. Chem. B 108, 4227 (2004)

35. Conductance of CNT with monocovalent addends Metallic (8,8) tube with different addends • Addend-induced state acts as scattering center, hinders tube ballistic conductions and increases tube resistance: agree with experiments (-F, -H), Smalley, CPL (1998); Kim, Adv. Mater. (2002) ... • Modification on conductance spectraismolecule-dependent: single molecule detectors? Nanotechnology 16, 635 (2005)

36. H H H H C C C C Cl Cl Cl Cl C C nanotube nanotube nanotube nanotube C C C C CNT with divalent functionalization (a) (a): two separated H atoms (b): two H atoms on nearby C (c): CCl2 on closed sidewall (d): CCl2 on opened sidewall (b) (6,6) SWNT (c) (d) Similar to case (b): pyrrolidine ring functionalized SWNTs at low modification ratio showed that metallicity of pristine SWNTs was retained,experiment by Franco et al., JACS (2004) ChemPhysChem 6, 598 (2005)

37. Tube conductance vs. concentration of addends Extend Hückel Hamiltonian, 30 configuration for each plot, length for central part of nanotube over 6nm Nano Letters 6, 916 (2006)

38. Tube conductance vs. concentration of addends • Monovalent functionalizations decrease the conductance rapidly, CNT lose metallicity around 25% modification ratio, • For divalent addition, conductive properties of CNT remains robust up to 25%

39. Summary • Chemical modification provides pathways for tuningelectronic properties of nanotube • Alkali-metal intercalation: charge transfer from metal to nanotube and shift Fermi level into conduction band, reduce work function • Molecule adsorption:very weak interaction for charge donor molecules; coupling of tube valence bands and molecular level for stronger acceptors. • Noncovalent functionalization: - stacking modifies electronic properties. • Chemical functionalization • monovalent addition induces sp3 local hybridization and impurity states around Fermi level • divalent addition doesn’t disrupt sp2 electron state at low concentration but will lead to metal-nonmetal transition at high concentration. • Chemically modified nanotubes might lead to many applications, such as: • Li battery with high capacity • enhanced field emission • gas sensors and molecule detectors • nanoelectronics and spintronics devices

40. Acknowledgements Collaborators: • Prof. J.P. Lu, Dr. A. Buldum, Dr. H. Park (Univ. of North Carolina) • Dr. J. Han (NASA, Ames Research Center) • Prof. C.K. Yang (Chang Gung Univ.) • Dr. R.H. Xie, Dr. G.W. Bryant (NIST) • Prof. P.R. Schleyer, Prof. R. B. King, Dr. Z.F. Chen (Univ. of Georgia) • Prof. Z. Zhou (Nankai Univ.) Thank you for your attentions!

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