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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.)
高科技研究院( cast.dlut.edu.cn)简介
高科技研究院的纳米研究
计算纳米科学,团簇,纳米线 /管(赵纪军) 纳米力学、纳米尺度的生物仿生力学(郭旭) 纳米尺度生物大分子模拟和谱学研究(陈茂笃) 新型碳材料,纳米金刚石、高分子和吸波材料(温斌) 纳米催化剂,计算纳米化学(田东旭)