First-principles study of chemically modified carbon nanotubes Jijun

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

Structure of different carbon allotropes Diamond: sp 3 bonding, hard and insulating C 60 (buckyball): hollow sphere ~0. 7 nm in diameter Graphite: sp 2 bonding, soft between graphene planes Carbon nanotube: 0. 5 -50 nm in diameter 10 -100 micron long

Discovery of carbon nanotube and bundles Multiwall nanotubes: S. Iijima, 1991 Single-wall nanotubes (SWNT): S. Iijima, D. Bethune et al. , 1993 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)

Atomic and electronic structures of carbon nanotubes armchair (5, 5) Γ (10, 0) zigzag Z Metallic q q Γ Z Semiconducting 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=3 m, otherwise, semiconductor.

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

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)

Summary of our theoretical efforts & Outline of this talk • Alkali-metal intercalation and work functions; Li intercalation and battery • Substational doping BC 2 N tube; BC 3 tubes, Li adsorption and diffusion • Encapsulating fullerenes or clusters: peapods C 48 N 12/C 48 B 12, Na 6 Pb, Au 32 • Gas adsorption and noncovalent functionalization NO 2, NH 3, CO 2, CH 4, H 2 O, N 2, H 2; C 6 H 6, C 6 H 12, C 8 N 2 O 2 Cl 2 (DDQ) • Covalent sidewall functionalization COOH, F, H, OH, NH 2, CH 3; CCl 2, NCOOC 2 H 5 • Transition-metal coating or filling Ti, V, Cr, Fe, Co, etc

Brief overview of our computational methods q Electronic structure and total energy Ø density Ø All functional theory (LDA & GGA-PW 91) electron LCAO, numeric basis (DMol) Ø plane-wave Ø Finite q k-point sampling of 1 -D Brillouin zone Dynamic simulation & Structural optimization Ø Molecular Ø Numeric q pseudopotential (CASTEP) dynamics simulation with empirical force field minimizations (conjugate gradient, BFGS) Conductance Ø Green’s function within tight-binding approximations

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 e. V (Photoemission spectrum experiment: ~5 e. V), slightly higher than and individual tube (~4. 75 e. V). Phys. Rev. B 65, 193401 (2002)

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 e. V for KC 10, confirm our theoretical prediction ~3. 6 e. V. q WF decreases with doping concentration, insensitive to tube type q Reduced WF indicates enhanced field emission, experimentally observed by A. Wadhawan, APL (2001). Phys. Rev. B 65, 193401 (2002)

Electronic states of alkali metal doped nanoropes (10, 10) tube (17, 0) tube q Valence bands: almost not affected by alkali-metal doping. q Conduction bands: new peaks associated with alkali-metal atoms. q The density of states near Fermi level is significantly enhanced. q 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)

Li battery based on carbon nanotubes Cell Phone/Laptop Li /Metal Oxides Li / Nanotubes L=~10 m closed Li ion diffusion Materials Storage capacity Li/C ratio Graphite 372 m. Ah/g Li. C 6 MWNT 450 m. Ah/g Li 1. 2 C 6 SWNT, as prepared 600 m. Ah/g Li 1. 6 C 6 SWNT, etched 740 m. Ah/g Li 2 C 6 SWNT, ball-milled 1000 m. Ah/g Li 2. 7 C 6 L=3 -4 m L=0. 5 m Experiment by O. Zhou, PRL (2001)

Li intercalation in carbon nanotube bundle (10, 0) CNT Li 5 C 40 q Li intercalation induce small deformation of SWNTs (~10% by aspect ratio) q Hybridization between Li and nanotube modifies tube conduction bands q Nearly complete charge transfer from Li to nanotube, transforming the semiconducting tubes into metallic Phys. Rev. Lett. 85, 1706 (2000)

Capacity for Li intercalation inside nanorope Ball-milled nanotubes Li. C 2 Saturation Experiment: O. Zhou, CPL, (2000) q q Li intercalated at both interstitial sites and inside nanotubes Intercalation potential of nanotubes comparable to that of graphite Saturation Li density (~Li. C 2) in nanotube bundles is much higher than graphite, due to lower carbon density Phys. Rev. Lett. 85, 1706 (2000)

Li diffusion behavior inside nanotube bundle q Intercalation energies inside tube comparable to interstitial sites q Li ions are impossible to penetrate the tube wall q Energy barrier between two interstitial sites is high (~1. 5 e. V) q 1 -D diffusion behavior of Li ions along tube axis is expected

Li diffusion behavior inside nanotube bundles Ø Li ions form layered structures around tubes. Ø The 1 -D Li diffusion behavior (along tube axis ). Ø Diffusion in nanotube is faster than in graphite. Ø As Li density increases, diffusion becomes slower. Ø The diffusion at room T up to Li. C 2 is still fast enough to allow Li go through the tube. (1 s for 1 m tube).

CNT with substitutional doping by boron (4, 0) BC 3 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) BC 3 tube: reduced electron density on B site (3, 3) BC 3 tube, based on (6, 6) C tube q Semiconducting zigzag CNT: with B-doping, remain semiconductor with slightly lower gap, from 0. 71 e. V to 0. 66 e. V for (4, 0) BC 3 tube. q Metallic armchair CNT: with B-doping, become semiconductor, small gap ~0. 45 e. V. Chem. Mater. 17, 992 (2005)

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

Reduced Li diffusion barriers in BC 3 composite tubes q Defect formation energies lower in BC 3 tube than in C tubes q Li penetration barriers for BC 3 tubes much lower than CNT with same defect, due to electron deficient of boron BCx composite nanotubes are good candidates for Li battery.

Nanopeapod: a novel one-dimensional hybrid structure Encapsulated C 60 and 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 1 D 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.

Encapsulating C 48 N 12/C 48 B 12 inside nanotube C 48 N 12 C 48 B 12 HOMO: -4. 38 e. V HOMO: -5. 58 e. V electron Fermi level of carbon nanotube is around -4. 8 e. V C 48 N 12/C 48 B 12 pair in semiconductor (17, 0) tube C 48 N 12: -0. 39 |e| on tube, donor, n-type C 48 B 12: 0. 67 |e| on tube, acceptor, p-type Insert energy: ~ 2. 4 e. V per cluster Nanotube-based p-n junction by C 48 N 12/C 48 B 12 peapods Phys. Rev. Lett. 90, 206602 (2003)

Na 6 Pb clusters encapsulated inside nanotubes Experiment: CPL 237, 334 (1995) Magic cluster Na 6 Pb clusters can be inserted into nanotubes with diameter > 1. 0 nm, insertion energy about 1. 2 -2. 8 e. V per cluster Incorporating Na 6 Pb array inside (8, 8) tube q Delocalized electron density of conduction bands: hybridization between cluster and nanotube. q Increase number of conduction channels of armchair nanotube from two to three. Phys. Rev. B 68, 035401 (2003).

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

Importance of gas environment of carbon nanotubes Sensitivity of tube conductance to gas, exposure: Dai, (NO 2, NH 3); Zettl (O 2) both on Science, (2000). Long-term stability of field-emission current due to residential gas, e. g. , Dean, APL (1999)

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 q Most gas molecules (NH 3, N 2, CO 2, CH 4, H 2 O, H 2, Ar) are charge donors and interact very weakly: binding energy 0. 05~0. 15 e. V, charge transfer 0. 01~0. 035 e. q Charge acceptor found for NO 2 and O 2, with relatively stronger interaction: binding energy 0. 3~0. 8 e. V, charge transfer -0. 06~ -0. 14 e. Nanotechnology 13, 195 (2002)

Electronic properties of gas adsorbed semiconductor tubes q Hybridization between molecular orbital of the charge acceptors (NO 2, O 2) and tube valence band transform semiconductor tube into p-type conductor. Electron density for top nine valence bands shows weak coupling between NO 2 and (10, 0) nanotube Nanotechnology 13, 195 (2002)

Electronic properties of gas adsorbed metallic tubes q Molecule-induced charge fluctuation acts as scattering center and lead to increases of tube resistance q Nanotube-based gas senor becomes a highly active field since then O 2 -(5, 5) N 2 -(5, 5) SWNT O 2 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, A 13. 48 (2001)

Noncovalent functionalization: role of aromaticity q Noncovalent functionalization preserve the tube structure, thus maintain the superior mechanical properties. q Coupling of electrons between aromatic molecules and nanotube ( - stacking) modify the electronic and transport properties. Resistances of SWNTs are modified by the adsorption of C 6 H 6, but not by C 6 H 12 Aromatic C 6 H 6 delocalization of conduction electron Nonaromatic C 6 H 12 conduction electron localized on SWNT Eklund, PRL (2002). Appl. Phys. Lett. 82, 3746 (2003)

CNT with noncovalent functionalization by DDQ q Adsorption energy ~3 times larger than O 2 q DDQ (C 8 N 2 O 2 Cl 2) on (10, 0) tube Hybridization due to existence of molecular level near tube valence band edge; molecular level delocalized over SWNT. q Charge transfer from DDQ to tube makes (10, 0) SWNT p-type conductor. q 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)

Experimental progresses after our theoretical work Y. P. Sun, JACS, (2004) A. Star, Nano Lett. (2003) Dom. P Field-effect transistor with semiconducting SWNT In solution Solid state S 22 S 11 Gas sensitivity on gate voltage shift Vg Diminishing of band-gap transition due to Dom. P Chemical senor for organic compound!

Covalent functionalization of CNT: background Monovalent Divalent 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)

Type of covalent functionalization on nanotube sidewall Monovalent -COOH Binding energy: 1. 2~1. 8 e. V Divalent =CCl 2 Binding energy: 0. 7~1. 4 e. V Local carbon bonding changes from sp 2 to sp 3: significant disruption on nanotube electronic states Local carbon bonding remains sp 2, less disruption on tube electronic states. Local C-C bond on tube opens J. Phys. Chem. B 108, 4227 (2004) Nanotechnology 16, 635 (2005) Chem. Phys. Chem 6, 598 (2005) Nano Letters 6, 916 (2006)

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 Nanotechnology 16, 635 (2005) Chem. Phys. Chem 6, 598 (2005)

CNT with monovalent functionalization Radical addition lead to local sp 3 bonding and induce half-occupied impurity state near EF. q Different from substitutional doping & topological defect; similar to effect by vacancy defect. q Disruption of tube sp 2 electron states found by experimental UV spectra: Smalley, CPL (1998)… q -H (10, 0) tube C N nanotube -NH 2 (6, 6) tube -COOH - (6, 6) SWNT) C H isoelectron nanotube N J. Phys. Chem. B 108, 4227 (2004)

Conductance of CNT with monocovalent addends Metallic (8, 8) tube with different addends q 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). . . q Modification on conductance spectra is moleculedependent: single molecule detectors? Nanotechnology 16, 635 (2005)

CNT with divalent functionalization (a) H H C C (a): two separated H atoms nanotube (b) H (b): two H atoms on nearby C (c): CCl 2 on closed sidewall (d): CCl 2 on opened sidewall (6, 6) SWNT H C C nanotube (c) Cl Cl C C C nanotube (d) Cl Cl C C C nanotube 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) Chem. Phys. Chem 6, 598 (2005)

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

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%

Summary v Chemical modification provides pathways for tuning electronic properties of nanotube Ø Alkali-metal intercalation: charge transfer from metal to nanotube and shift Fermi level into conduction band, reduce work function Ø Molecule adsorption: very weak interaction for charge donor molecules; coupling of tube valence bands and molecular level for stronger acceptors. Ø Noncovalent functionalization: - stacking modifies electronic properties. Ø Chemical functionalization ü monovalent addition induces sp 3 local hybridization and impurity states around Fermi level ü divalent addition doesn’t disrupt sp 2 electron state at low concentration but will lead to metal-nonmetal transition at high concentration. v Chemically modified nanotubes might lead to many applications, such as: Ø Li battery with high capacity Ø enhanced field emission Ø gas sensors and molecule detectors Ø nanoelectronics and spintronics devices

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

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