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 : 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

40㎚

CNT Growth by CVD

H2, Ar, N2, NH3

300nm300nm

at 950 ℃ with 16.7 vol. % C2H2

in N2:H2 = 1:3at 950℃ with 16.7 vol. % C2H2

in pure NH3

CNT Growth by Thermal CVD

Jung et al, Diam. Rel. Mater. 12, 1235 (2001).

Tangled CNTC2H2+H2600~900

Tangled CNTC2H2+H2, C2H2+N2950

Tangled CNTC2H2+H2, C2H2+N2850

method

ferrocene+xylene

CH4+H2

CH4+N2

CH4+N2

C2H2+Ar

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

Reaction Gas CatalystTemperatue(oC)

APL 77 3764 (2000)Aligned CNTFe800 Thermal-CVD

APL 76 2367 (2000)Aligned CNTNi700 PE-CVD

JAP 89 5939 (2001)Aligned CNTFe550 PE-CVD

APL 75 3105 (1999)Aligned CNTFe, Ni500 PE-CVD

APL 75 1721 (1999)Tangled CNTNi, Co850~900Thermal-CVD

APL 80 4018 (2002)Aligned CNTNi660< PE-CVD

JAP 91 3847 (2002)Aligned CNT

Ni

800~900

Thermal-CVD

DRM 10 1235 (2001)Aligned CNT

Ni950

Thermal-CVD

TSF 398-399 150 (2001)Aligned CNT

Ni, Co950

Thermal-CVD

APL 78 901 (2001)Aligned CNTFe800 Thermal-CVD

APL 77 2767 (2000)Aligned CNTCo825 PE-CVD

APL 77 3397 (2000)Aligned CNTFe750~950Thermal-CVD

APL 77 830 (2000)Aligned CNTCo825 PE-CVD

APL 75 1086 (1999)Aligned CNTNi660 PE-CVD

Science 282, 1105 (1998)Aligned CNTNi666PE-CVD

CitationCNT MorphologySynthesis condition

Deposition

PretreatmentH2+C2H2 NH3 +C2H2

H2 XO

NH3 + H2O

NH3X O

Kim et al, Chem. Phys. Lett. 372, 603 (2003)

Nitrogen Incorporation into CNTs

N with sp2 C

N in sp3 environ.

XPS EELS

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


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


Cutoff radius 80Ry, Electric field at the tip 0.7V/ÅBand selection : E-Ef= -1.5eV ~ 0.5V


-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0 1 2


Ene

rgy

stat

es (

eV

, E

-EF)

AB

C

D


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)


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


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


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.



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




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


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


armchair

zigzag

Energy

Nitrogen incorporationPure C

pentagonhexagon

Reaction path

137meV

64meV

160meV

Nitrogen Incorporation on Armchair Edge

160meV 137meV

303meV 5455meV


152meV 87meV

179meV 96meV

Energy

Nitrogen at vortex site

Pure C

pentagonhexagon

Nitrogen at valley site

64meV

152meV160meV

179meV

96meV87meV

Reaction path


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.




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




Reaction path

Energy

176 meV


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


armchair

zigzag


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).



Documents

An ab-initio Study of the Growth and the Field Emission of CNTs : Nitrogen Effect