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The investigation of nitrogen effect on Carbon nanotube growth by ab-initio calculation

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The investigation of nitrogen effect on Carbon nanotube growth by ab-initio calculation

2003.10.11

Hyo-Shin Ahn*,**, T.Y. Kim*,**, S.-C. Lee*, K.-R. Lee* and D.-Y. Kim***Future Technology Research Division, KIST

**School of Materials Science and Engineering, Seoul Nat’l Univ.

Effect of Nitrogen on CNT growth

Experimental result

Nitrogen incorporation enhances CNT growth drastically that vertically aligned CNT can be fabricated

16.7 vol. %C2H2 in NH3, CVD process

Vertically aligned multi-wall CNT of 30~40nm in diameter Very high growth rate

Chemical Physics Letters, Vol. 372, 603(2003)

What is the role of Nitrogen in CNT growth?

Nitrogen effect: possibility 1

Nitrogen effect:

Reduction in the strain energy of CNT

Due to the strain energy, growth rate can be retarded.

When nitrogen atoms locate on defect or strained site of carbon network, system energy lowers.

Illustration of the defect stabilization by nitrogen

PRB. 59, No. 7, 5162(1999)

Nitrogen effect: possibility 2

- Nitrogen effect: Change of growth kinetics
- During the growth, nitrogen will change the growth behavior
- No remarkable experimental results of nitrogen effect on CNT growth
- We do not even know the exact mechanism of the CNT growth in atomistic scale

Method :Computational calculation

- Dmol3: commercial package of DFT (density functional theory) Ab-initio calculation
- Very accurate calculation results
- Strong in energy calculation - energetics
- Transition State calculation – growth kinetics
Calculating activation energy for chemical reaction

Strain energy 1/R2

40nm

Calculated strain energy by ab-initio : up to 6Å

Energetics on CNT wall

Strain energy due to curved wall

Conventional design for the calculation: CNT unit cell -

Larger radius CNT needs more atoms

Real CNT

Due to the computing power, ab-initio calculation cannot describe real size system.

Attach Hydrogen

Cut out

Cluster design / Curved clusters

Journal of Computer-Aided Materials Design, 5, 279 (1998)

By Calculating the energies of curved pieces of graphite (cluster), the energy of CNT with corresponding radius can be calculated

Strain energy of CNT 1/R2

Bulk design

Energy of flat graphite plate

Cluster design

~10Å

DE(eV/atom)

~35Å

- Introducing cluster design calculation, the energies of large size CNT can be calculated

Radius(Å)

- Over the radius of 35Å strain energy disappears

Expanding the scale by cluster design

Nitrogen effect on Strain energy

- Strain energy becomes negligible when radius of CNT is larger than 3~4nm.
- Radius of the vertically aligned CNTs is typically 15~20 nm, thus CNT has no strain energy.
- Strain energy reduction by the Nitrogen incorporation would be negligible.

No barrier

160meV

176meV

64meV

No barrier

Growth kinetics

Two kinds of edges

armchair

zigzag

Growth on zigzag edge:

Rate determining step!

Calculation of transition state (activation barrier of reaction) on each step of atom attachment

Armchair and zigzag edge

armchair

zigzag

As the growth proceeds, the proportion of zigzag edge will increase

Nitrogen incorporation into zigzag edge

No barrier

No barrier

152meV

154meV

176meV

No barrier

No barrier

No barrier

88meV

No barrier

538meV

Required energy for reaction

Nitrogen incorporation: 176meV

Nitrogen incorporation:154meV

Nitrogen incorporation:152meV

Pure Carbon: 176meV

Nitrogen incorporation: ~153meV

Smaller than armchair edge growth

Growth on nitrogen doped armchair

152meV

179meV

160meV

87meV

64meV

96meV

No remarkable effect

Required energy for reaction

Pure Carbon: 160meV / Nitrogen incorporated C: 152meV

No barrier

No barrier

333meV

No barrier

Growth on nitrogen doped zigzag

No barrier

176meV

No reaction!

No barrier

When nitrogen locates at the top of the hexagon ring, energy barrier for the growth vanishes

Growth on nitrogen doped carbon system

zigzag

No barrier

In pure C system

:176meV

No barrier

No barrier

No barrier

Near the nitrogen incorporated region (top site), the activation energy for carbon network growth disappears

Summary – growth kinetics

In pure carbon system

Armchair edge can grow faster, then growth on zigzag edge is rate determining step.

Nitrogen incorporation into zigzag edge

- -lowers energy barrier
- -makes the growth rates of zigzag edge similar to that of armchair.
Incorporated nitrogen effect on carbon attachment

Activation energy becomes lower.

nitrogen in top siteof zigzag edge, makes all energy barriers for the growth disappear.

Nitrogen enhances the growth of zigzag edge.

Conclusion

- Multi-wall CNT with tens of nanometer size has no excess strain energy.
- Reducing the strain energy is not a major reason for the enhanced CNT growth by nitrogen incorporation in large CNT.
- Nitrogen incorporation significantly affect the growth kinetics by lowering the activation barriers for the growth.

Nitrogen incorporation into armchair edge

No Reaction

160meV

303meV

137meV

64meV

5455meV

<pure carbon>

Energy required for the growth : 160meV

No remarkable effect of nitrogen incorporation

C150H30

C24H12

C54H18

C96H24

C54H18

C24H12

C96H24

C150H30

← Cluster size increases

(graphite)

Expanding the scale of calculation

Calculation by Cluster

Journal of Computer-Aided Materials Design, 5, 279 (1998)

Linear relationship

Relatively small size graphite cluster can reflect whole graphite sheet/CNT