An ab-initio Study of the Growt h and the Field Emission of CNT s : Nitrogen Effect Hyo-Shin Ahn § , Seungwu Han † , Seung-Chul Lee, Ky u-Hwan Lee and Kwang-Ryeol Lee Future Technology Research Division, KIST, Seoul, Korea § also at the Division of Materials Science, Seo ul National University, Seoul, Korea † at the Department of Physics, Ehwa Women’s Uni versity, Seoul, Korea Korea-RIKEN Workshop on Nanoscienc and Nanotechnology, 2004.10.1-2, Hanyang Univ. Seoul
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An ab-initio Study of the Growth and the Field Emission of CNTs : Nitrogen Effect
An ab-initio Study of the Growth and the Field Emission of CNTs : Nitrogen Effect. Hyo-Shin Ahn § , Seungwu Han † , Seung-Chul Lee, Kyu-Hwan Lee and Kwang-Ryeol Lee Future Technology Research Division, KIST, Seoul, Korea - PowerPoint PPT Presentation
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An ab-initio Study of the Growth and the Field Emission of CNTs : Nitr
ogen Effect
Hyo-Shin Ahn§, Seungwu Han†, Seung-Chul Lee, Kyu-Hwan Lee and Kwang-Ryeol Lee
Future Technology Research Division, KIST, Seoul, Korea
§ also at the Division of Materials Science, Seoul National University, Seoul, Korea † at the Department of Physics, Ehwa Women’s University, Seoul, Korea
Korea-RIKEN Workshop on Nanoscienc and Nanotechnology, 2004.10.1-2, Hanyang Univ. Seoul
W.-Q. Han et al, Appl. Phys. Lett. 77, 1807 (2000).Kim et al, Chem. Phys. Lett. 372, 603 (2003)
Nitrogen incorporation significantly enhances the CNT growth resulting in vertically aligned CNTs.
16.7 vol. % C2H2 in NH3, CVD process
• What is the role of nitrogen in the CNT growth?• What is the effect of the incorporated nitrogen? • What is the role of nitrogen in the CNT growth?• What is the effect of the incorporated nitrogen?
Nitrogen Incorporation into CNTs
Zigzag Edge Armchair Edge
Growth Kientics of CNT
152meV
154meV
Pure C
Nitrogen incorporation
tetragon pentagonhexagon
Energy
Reaction path
153 meV
176 meV
Nitrogen Incorporation on Zigzag Edge
0meVa
b c
a,b
538meVc
Growth with Incorporated Nitrogen
No barrier
No barrierNo barrier No barrier
333meV
Energy Pure C
Nitrogen in valley site
tetragon pentagonhexagon
Nitrogen in top site
No barrier176meV
333meV
Reaction path
No barrier
Nitrogen and CNT Growth
1. Nitrogen can be incorporated to the CNT wall and cap from the background gas.
2. The incorporated nitrogen can reduce the kinetic barrier for the growth of CNTs.
Most commercialized CNTs prepared by CVD method might be the nitrogen doped one.
Reactivity of Curved C-N Structures
S. Stafstrom, Appl. Phys. Lett. 77 (24), 3941 (2000)
CNT is a strong candidate for field emission cathod materials
1. Structural advantage 2. Low turn-on voltage
Field Emission from CNT
What’s the effect of incorporated nitrogen?What’s the effect of incorporated nitrogen?
Calculation Method
Plane wave
Localized basis
(5,5) Caped CNT, 250atoms
• Ab initio tight binding calc. To obtain self-consistent potential and initial wave function
• Relaxation of the wave functionBasis set is changed to plane wave to emit the electrons
• Time evolutionEvaluation of transition rate by time dependent Schrödinger equation
Cutoff radius 80Ry, Electric field at the tip 0.7V/ÅBand selection : E-Ef= -1.5eV ~ 0.5V
Emission from Pure CNT
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0 1 2
Emitted current(μA)
Ene
rgy
stat
es (
eV
, E
-EF)
AB
C
D
A State B State D stateC state
Localized states, Large emission current
* and bonds:Extended states
Emission from Pure CNT
Cutoff radius 80Ry, Electric field at the tip 0.7V/ÅBand selection : E-Ef= -1.5eV ~ 0.5V
Emission from Pure CNT
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0 1 2
Emitted current(μA)
Ene
rgy
stat
es (
eV
, E
-EF)
AB
C
D
Emission from Pure CNT
S. Han et al, Phys. Rev. B 66, 241402(R) (2002).
Cutoff radius 80, Electric field at the tip 0.7V/ÅBand selection : E-Ef= -1.5eV ~ 0.5V
Emission from N doped CNT
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0 1 2 3 4
Ene
rgy
stat
es (
eV
, E
-EF)
Emitted current(μA)
AB
C
D
Enhanced Field Emssion by Nitrogen Incorporation
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0 1 2 3 4
Pure CNT
Emitted current(μA)
Total current: 8.8A
En
erg
y st
ate
s (e
V,
E-E
F)
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0 1 2 3 4
Nitrogen doped CNT
Emitted current(μA)
En
erg
y st
ate
s (e
V,
E-E
F)
Total current: 13.2A
AB
C
D
Coupled states between localized and extended states contribute to the field emssion.
B stateA state C state D state
π*+localized stateLocalized stateπ bond:Extended state
Emission from N doped CNT
Nitrogen Effect
EF
- N-doped CNT
- Undoped CNTLocalized state
The nitrogen has lower on-site energy than that of carbon atom.T. Yoshioka et al, J. Phys. Soc. Jpn., Vol. 72, No.10, 2656-2664 (2003).
The lower energy of the localized state makes it possible for more electrons to be filled in the localized states.
Doped Nitrogen Position
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
Ban
d sh
ift (
eV)
8
10
12
14
16
18
20
22
Em
ission current (A
)
• Doped nitrogen enhances the field emission of CNT.
• In addition to localized state, hybrid states of the extended and localized states play a significant role.
• Doped nitrogen lowers the energy level of the localized state, which makes electrons more localized to the tip of nanotube.
Field Emission from N-doped CNT
Experimental Results
Role of extrinsic atoms on the morphology and field-emission properties of carbon nanotubesL.H.Chan et al., APL., Vol.82, 4334(2003)
N
B
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8 PW-B ABTB-B PW-N ABTB-N
Ban
d sh
ift (
eV)
BORON DOPED
NITROGEN DOPED
Boron Doped CNT
Doped Atom Position
Conclusions
• Nitrogen incorporation in CNT– Enhances the growth rate of CNT.– Significantly affects the electron field emission.
• For the CNT applications, one should understand more about the CNTs to be used. – One should carefully consider the deposition condition a
nd corresponding structure and chemical composition of the nanotube.
Reactivity of Curved C-N Structures
S. Stafstrom, Appl. Phys. Lett. 77 (24), 3941 (2000)
Energy levels around the Fermi level for (a) the tube with substitutional boron, (b) the pure carbon nanotube and (c) the tube with substitutional nitrogen
Effect of substitutional atoms in the tip on field-emission properties of capped carbon nanotubesG.Zhang et al., APL., Vol.80, 2589(2002)
A Theoretical Study
Nitrogen in CNT
Kim et al, Chemical Physics Letters, Vol. 372, 603(2003)
Emission current depends on how many electrons are accumulated at the tip.
C
A
B
Relative charge density w.r.t. undoped cnt
8
10
12
14
16
18
20
22
Emission Current
Em
ission cu
rrent (A
)
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
Band shift by Nitrogen
Ba
nd
sh
ift (
eV
) A
B
CPosition in CNT
Possible Nitrogen Effects
Reduction in the strain energy of CNT
Change in the Growth Kinetics
Radius(Å)
E(e
V/at
om)
Cluster design
~10Å
Bulk design
Energy of flat graphite plate
~30Å
Strain Energy Due to Curvature
No Significance in Strain Energy ReductionNo Significance in Strain Energy Reduction
10nm
Possible Nitrogen Effects
Reduction in the strain energy of CNT
Change in the Growth Kinetics
Calculation of Growth KineticsCalculation of Growth Kinetics
Assumptions• Flat graphitic plate represents large radius CNT • Catalyst metals assist formation of carbon precursor and provide a diffusion path to the reaction front
reactant product
Computational Method
Dmol3: ab-initio calculation based on DFT
• Known to be very accurate
• Strong in energy calculation – energetics
• Transition state calculation – growth kinetics
The Growth of CNT EdgeThe Growth of CNT Edge
armchair
zigzag
zigzag armchair
Reaction path
Energy
176 meV
tetragon pentagonhexagon
Growth of Pure Carbon Zigzag EdgeGrowth of Pure Carbon Zigzag Edge
Growth of Pure Carbon Armchair Edge
Energy
pentagonhexagon
160 meV
64 meV
Reaction path
The Growth of CNT EdgeThe Growth of CNT Edge
armchair
zigzag
Energy
Nitrogen incorporationPure C
pentagonhexagon
Reaction path
137meV
64meV
160meV
Nitrogen Incorporation on Armchair Edge
160meV 137meV
303meV 5455meV
Growth with Incorporated Nitrogen
152meV 87meV
179meV 96meV
Energy
Nitrogen at vortex site
Pure C
pentagonhexagon
Nitrogen at valley site
64meV
152meV160meV
179meV
96meV87meV
Reaction path
Growth with Incorporated Nitrogen
No barrier
Energy
growth of C
tetragon Pentagon
hexagon
growth near the nitrogen incorporated region.
No barrier
176 meV
No barrier
Electron Density
Summary – Growth Kinetics
Pure CNT Growth - Growth of zigzag edge is the rate determining step, since the armchair edge growth has lower kinetic barrier.
Nitrogen Incorporation- Growth of armchair edge becomes the rate determining step.
Growth with Incorporated Nitrogen - Nitrogen enhances the growth by lowering the kinetic barrier.- Under a certain coordination of nitrogen on zigzag edge, energy barrier for the growth disappears.
Possible Nitrogen Effects
Reduction in the strain energy of CNT
Change in the Growth Kinetics
Radius(Å)
E(e
V/at
om)
Cluster design
~10Å
Bulk design
Energy of flat graphite plate
~30Å
Strain Energy Due to Curvature
No Significance in Strain Energy ReductionNo Significance in Strain Energy Reduction
10nm
Possible Nitrogen Effects
Reduction in the strain energy of CNT
Change in the Growth Kinetics
Reaction path
Energy
176 meV
tetragon pentagonhexagon
Growth of Pure Carbon Zigzag EdgeGrowth of Pure Carbon Zigzag Edge
Growth of Pure Carbon Armchair Edge
Energy
pentagonhexagon
160 meV
64 meV
Reaction path
The Growth of CNT EdgeThe Growth of CNT Edge
armchair
zigzag
The Growth of CNT EdgeThe Growth of CNT Edge
armchair
zigzag
In NH3 Environment
CNT Growth by Thermal CVD
In H2 , N2 or Ar Environment
3.0 m
EELS Analysis of CNT
W.-Q. Han et al, Appl. Phys. Lett. 77, 1807 (2000).
Reactivity of Curved C-N Structures
S. Stafstrom, Appl. Phys. Lett. 77 (24), 3941 (2000)