Presentation at National Center for Theoretical Sciences

& National Cheng Kung University

8/25/2006

First-principles study of chemically First-principles study of chemically

modified carbon nanotubesmodified 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

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

Structure of different carbon allotropes

Multiwall nanotubes: S. Iijima, 1991 Single-wall nanotubes (SWNT): S. Iijima, D. Bethune et al., 1993

• 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)

Discovery of carbon nanotube and bundles

STM image, C. Dekker, 1998

Nanorope, mass production, R. Smalley, 1996

Atomic and electronic structures of carbon nanotubes

(5, 5)

Γ Z

MetallicΓ Z

(10, 0)

Semiconducting

armchair zigzag

Folding of graphene sheet leads to single-walled nanotube (SWNT). Nano

tube chirality depend on the folding angle Chirality dependent: all armchair (n,n) tubes are metallic; zigzag (n,0) tu

bes are metallic if n=3m, otherwise, semiconductor.

-2 -1 0 1 20

2

4

6

8

-2 -1 0 1 20

2

4

6

8

Density of states: van-Hove singularities

Conductance: ballistic transport, quantized in unit of G0=2e2/h

(5, 5) (10, 0)

Electronic states and conductance of carbon nanotubes

armchair zigzagG

(2e

2 /h

)

G (

2e2 /

h)

-2 -1 0 1 20

10

20

30

40

(5, 5)

DO

S

-2 -1 0 1 20

10

20

30

40

(10, 0)

DO

S

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)

• 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

Summary of our theoretical efforts & Outline of this talk

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

Brief overview of our computational methods

Work function of pristine carbon nanotubes

Work function (WF): important parameter for electronic properties of CNT; usefu

l 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) ar

e ~ 5 eV (Photoemission spectrum experi

ment: ~5 eV), slightly higher than and in

dividual tube (~4.75 eV). Phys. Rev. B 65, 193401 (2002)

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)

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.

(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)

Electronic states of alkali metal doped nanoropes

MaterialsStorage capacity

Li/C ratio

Graphite 372 mAh/g LiC6

MWNT 450 mAh/g Li1.2C6

SWNT, as prepared 600 mAh/g Li1.6C6

SWNT, etched 740 mAh/g Li2C6

SWNT, ball-milled 1000 mAh/g Li2.7C6

Li /Metal Oxides Li / Nanotubes

Cell Phone/Laptop

Li ion diffusion

Li battery based on carbon nanotubes

L=3-4m

L=0.5m

L=~10mclosed

Experiment by O. Zhou, PRL (2001)

Phys. Rev. Lett. 85, 1706 (2000)

Li intercalation in carbon nanotube bundle

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 semic

onducting tubes into metallic

(10,0) CNT Li5C40

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

Phys. Rev. Lett. 85, 1706 (2000)

SaturationLiC2

Ball-millednanotubes

Li intercalated at both interstitial sites and inside nanotubes

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

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).

CNT with substitutional doping by boron

Chem. Mater. 17, 992 (2005)

(4,0) BC3 tube, based on (8,0) C tube

(3,3) BC3 tube, based on (6,6) C tube

-4

-2

0

Ene

rgy/

eV

Z

-4

-2

0

2

Ene

rgy/

eV

Z

(8,0) C tube: conjugate

electron density on

hexagonal carbon ring

(4,0) BC3 tube: reduced

electron density on B site

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.

Experimentally, BCx composite tubes are synthesized.

Barrier for Li penetrating through tube wall

Chem. Mater. 17, 992 (2005) Chem. Phys. Lett. 415, 323 (2005)

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.

Nanopeapod: a novel one-dimensional hybrid structure

Smith, Monthioux, Luzzi, Nature 296, 323 (1998)

Encapsulated C60 and other cage-like molecules in carbon nanotubes: “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.

Why peapod?

Encapsulating C48N12/C48B12 inside nanotube

C48N12/C48B12 pair in

semiconductor (17,0) tube

Insert energy: ~ 2.4 eV per cluster

C48N12: -0.39 |e| on tube, donor, n-type

C48B12: 0.67 |e| on tube, acceptor, p-type

Nanotube-based p-n junction by C48N12/C48B12 peapods

Phys. Rev. Lett. 90, 206602 (2003)

Fermi level of carbon nanotube is around -4.8 eV

C48N12C48B12

HOMO: -5.58 eVHOMO: -4.38 eV

electronelectron

Na6Pb clusters encapsulated inside nanotubes

Phys. Rev. B 68, 035401 (2003).

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.

Experiment: CPL 237, 334 (1995) Na6Pb clusters can be inserted into

nanotubes with diameter > 1.0 nm, insertion energy about 1.2-2.8 eV per

clusterMagic cluster

Vol. 3, 459 (2003)

Noncovalent functionalization

Vol. 6, 598 (2005)

Covalent functionalization

Chemical functionalization of nanotubes

Vol. 13, 195 (2002)

Gas adsorption

Importance of gas environment of carbon nanotubes

Sensitivity of tube conductance to gas, exposure: Dai, (NO2, NH3); Zettl (O2) bo

th on Science, (2000).

Long-term stability of field-emission currentdue to residential gas, e.g., Dean, APL (1999)

Nanotechnology 13, 195 (2002)

Interaction between CNT and gas molecules

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.

1.5 2.0 2.5 3.0 3.5 4.0 4.5

-0.5

0.0

0.5

1.0

(10,0) tube-NO2

(17,0) tube-NO2

(5,5) tube-NO2

Ads

orpt

ion

ener

gy (

eV)

Tube-molecule distance (A)

1.5 2.0 2.5 3.0 3.5 4.0 4.5

0.0

0.5

1.0 (10,0) tube-H

2O

(17,0) tube-H2O

(5,5) tube-H2O

Tube-molecule interaction:

Van der Waals force,

insensitive to tube type

LDA used in calculation,

overestimate the adsorption

energy and charge transfer

Nanotechnology 13, 195 (2002)

Hybridization between molecular orbital of the charge acceptors (NO2, O2)

and tube valence band transform semiconductor tube into p-type conductor.

-3 -2 -1 0 1 20

2

4 NO2-(10,0) tube

Den

sity

of

Stat

e (a

rb. u

nit)

Energy (eV)

-3 -2 -1 0 1 20

2

4

(10,0) tube

-3 -2 -1 0 1 20

2

4

NH3-(10,0) tube

Electron density for top

nine valence bands shows

weak coupling between

NO2 and (10,0) nanotube

Electronic properties of gas adsorbed semiconductor tubes

-3

-2

-1

0

1

2

Ban

d en

ergy

(eV

) (10,0) tube

ZZ

-3

-2

-1

0

1

2

(10,0) tube + NO2

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-(5,5)

SWNT

N2-(5,5)

SWNT

Increase of tube resistance by various

gases, Eklund’s group, PRL (2000). Mat. Res. Soc. Symp. Proc. 644, A13.48 (2001)

O2 on (10,10) tube: resistance increase by 0.25 per molecule

Appl. Phys. Lett. 82, 3746 (2003)

Noncovalent functionalization: role of aromaticity

Eklund, PRL (2002).

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

DDQ (C8N2O2Cl2) on (10,0) tube

Appl. Phys. Lett. 82, 3746 (2003)

Adsorption energy ~3 times larger than O2

Hybridization due to existence of molecular lev

el 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

CNT with noncovalent functionalization by DDQ

Experimental progresses after our theoretical work

A. Star, Nano Lett. (2003)

Field-effect transistor with semiconducting SWNT

Gas sensitivity on gate voltage shift Vg

Y. P. Sun, JACS, (2004)

DomP

In solution Solid state

S22 S11

Diminishing of band-gap transition due to DomP Chemical senor for organic compound!

Covalent functionalization of CNT: background

M. S. Strano et al., Science 301, 1519 (2003)

Monovalent Divalent

K. Kamaras et al., Science 301, 1501 (2003)

Electronic structures of carbon nanotubes can be modified by covalent functionalization in different ways

Divalent-COOH =CCl2

J. Phys. Chem. B 108, 4227 (2004) ChemPhysChem 6, 598 (2005)Nano Letters 6, 916 (2006)

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

Type of covalent functionalization on nanotube sidewall

Nanotechnology 16, 635 (2005)

Binding energy: 1.2~1.8 eV

Binding energy: 0.7~1.4 eV

Monovalent

Binding energy of addends: effects of size & concentration

0 5 10 15 20 25

15

20

25

30

35

Bin

ding

ene

rgy

(kca

l/mol

)

% ratio of modification

(6,6) tube (9,0) tube (10,0) tube

0.1 0.2 0.30.5

1.0

1.5

2.0

2.5

3.0

3.5

(9,0)

(11,0)(10,0)

(8,0)

(5,5)

(6,6)(12,0)

(7,7)(8,8)

Semiconducting

Metallic

: SWNT + F: SWNT + OH: SWNT + COOH

Bin

ding

Ene

rgy

(eV

)

1/R (Å-1)

Nanotechnology 16, 635 (2005) ChemPhysChem 6, 598 (2005)

• 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

J. Phys. Chem. B 108, 4227 (2004)

CNT with monovalent functionalization

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)…

(6,6) tube -COOH-NH2

(10,0) tube-H CN nanotub

enanotu

be

C

N

H

isoelectron

COOH - (6,6) SWNT)

Nanotechnology 16, 635 (2005)

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 spectra is molecule-

dependent: single molecule detectors?

Conductance of CNT with monocovalent addends

Metallic (8,8) tube with different addends

CNT with divalent functionalization

ChemPhysChem 6, 598 (2005)

(a): two separated H atoms

(b): two H atoms on nearby C

(c): CCl2 on closed sidewall

(d): CCl2 on opened sidewall

(6,6) SWNT

nanotubeC

H

C

H(a)

(b)

nanotubeC

H

C

H

(c)

nanotubeC C

Cl Cl

C

nanotubeC C

Cl Cl

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)

Tube conductance vs. concentration of addends

Nano Letters 6, 916 (2006)

Extend Hückel Hamiltonian,

30 configuration for each plot,

length for central part of nan

otube over 6nm

Tube conductance vs. concentration of addends

• Monovalent functionalizations decrease the conductance rapidly, CNT lose metalli

city around 25% modification ratio,

• For divalent addition, conductive properties of CNT remains robust up to 25%

Summary

Chemical modification provides pathways for tuning electronic properties of nanotube Alkali-metal intercalation: charge transfer from metal to nanotube and shift Fer

mi 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 arou

nd 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

Thank you for your attentions!

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 Georgi

a)

Prof. Z. Zhou (Nankai Univ.)

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