1. The first principles calculation of nitrogen effect on the growth of carbon nanotube
2004 대한금속재료학회
상변태, 열역학 분과 심포지엄
7.1.~7.2.
Thank you chairman.
I think I have to tell you that The title of my presentation is changed.
Fortunately, I could get more results after submitting the abstract
But unfortunately this work is finished just before the conference, I had no time to tell to secretariat.
Anyway, In this talk, I’m going to talk about “Nitrogen effect on CNT growth by ab-initio calculation”
Hyo-Shin Ahn1,2, Tae-Young Kim1,2, Seung-Cheol Lee1, Seungwu Han3,
Kwang –Ryeol Lee1 and Doh-Yeon Kim2
1 Korea institute of science and technology
2 Department of materials science and technology, seoul national university
3 Department of physics, Ehwa womans university

2. What is the role of Nitrogen in CNT growth?1
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)
Carbon nanotube is the most famous structure in carbon nanotechnology.
Since the discovery of CNT, tremendous researches on this unique material have been done and still in progress.
the application of CNT is also an important and big job.
Due to the electron emission property,
Vertically aligned CNT, became a promising structure for filed emitter or display
To fabricate VACNT, it is reported that Nitrogen atmosphere is a necessary condition.
With this condition, Nitrogen is incorporated into CNT.
this nitrogen incorporation accelerates the growth rate and makes CNTs aligned.
Through the observation, We know that nitrogen incorporation enhances CNT growth.
But, in what way, the growth rate becomes higher, we have no idea
In this presentation, I will talk about the role of nitrogen in CNT growth
What is the role of Nitrogen in CNT growth?

3. Calculation of growth kinetics2
Calculation of growth kinetics
Dmol3: commercial package of DFT (density functional theory) Ab-initio calculation
Transition state(TS) calculation - Finding the energy barrier for the reaction
- TS calculation on each step of atom attachment
reactant
product
Assumptions
Flat graphitic plate represents large radius CNT
No catalytic effect
Considering the 1st stage of nucleation on each edge
Only one nitrogen atom in one carbon hexagon ring

4. The growth on the edge of CNT3
The growth on the edge of CNT
CNT growth front will have two kinds of structures
: Armchair and zigzag edge
armchair
zigzag
Actually, these two structures are not separated, rather have very close relationship.
Think about the growth on armchair structure, Because it needs lower energy for the growth, structure will be changed like this, filling every kinks.
After several steps, some part of armchair edge would have zigzag edge. After more steps, zigzag edges will be extend and some part will have armchair structure.
Because zigzag edge is rate determining, zigzag edge will be remained.
Nano tube of pure carbon will grow like this.
At this point if nitrogen is introduced to this structure, what will happen?
As the growth proceeds, the proportion of zigzag edge will increase

5. Growth reaction on zigzag edge
4-1
Growth reaction on zigzag edge
Energy
176 meV
tetragon
pentagon
hexagon
Reaction path
Total energy for the zigzag edge growth is 176meV

6. Growth reaction on armchair edge
4-2
Growth reaction on armchair edge
Energy
160 meV
64 meV
pentagon
hexagon
Reaction path
Total energy for the armchair edge growth is 160meV, lower than that of zigzag edge. Thus the zigzag edge growth is rate determining in undoped CNT.

7. 5-1 The reaction path of the lowest energy
Nitrogen incorporation on zigzag
No barrier
152meV
No barrier
154meV
176 meV
Nitrogen incorporation
~26meV
Pure C
Energy
150 meV
hexagon
tetragon
pentagon
Reaction path
Total energy for the growth is about 150meV.
Nitrogen incorporation lowers kinetic barrier by ~26meV.

8. The reaction path of the lowest energy
5-2
The reaction path of the lowest energy
Nitrogen incorporation on armchair
137meV
160meV
64meV
Nitrogen incorporation
Pure C
137meV
160meV
Energy
~70meV
64meV
pentagon
hexagon
Reaction path
Total energy for the growth is 160meV, no nitrogen incorporation is preferred.
No significant advantage by nitrogen incorporation.

9. 6-1 Growth on nitrogen doped armchair edge 152meV 87meV 179meV 96meV
Nitrogen at top site
Nitrogen at valley site
160meV
96meV
Pure C
Energy
152meV
87meV
64meV
pentagon
hexagon
Reaction path
total energy is slightly lower in 1st case (red box). However, no characteristic nitrogen effect on growth of armchair edge.

10. 6-2 Growth on nitrogen doped zigzag edge No barrier 333meV
No more reaction is considered
No barrier
No barrier
No barrier
333 meV
Nitrogen in valley site
Nitrogen in top site
176 meV
Energy
Pure C
No barrier
hexagon
tetragon
pentagon
Reaction path
Nitrogen at valley site makes reaction difficult. However, nitrogen at top site eliminates the kinetic barrier for the growth.

11. 6-3 Growth on nitrogen doped zigzag edge
The formation of carbon hexagon ring near the nitrogen doped edge
No barrier
No barrier
growth near the nitrogen incorporated region.
growth of C
Energy
176 meV
The left picture is the same one of previous page.
Carbon atom is approached to the tetragon and makes pentagon.
And activation energy is eliminated in case of nitrogen atom on top site
Here, I put carbon atom on different site in right picture.
Carbon atom is put next to the tetragon.
If this were pure carbon system, 176meV energy would be required, however, calculation result shows that no energy is needed.
Nitrogen on top site of hexagon ring, energy barriers of pure carbon atoms to grow is eliminated near to the nitrogen.
No barrier
hexagon
tetragon
pentagon
Near the nitrogen incorporated region (top site), the activation energy for carbon growth disappears.

12. Summary – growth kinetics7
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 site of zigzag edge, makes all energy barriers for the growth disappear.
Nitrogen enhances the growth by lowering the kinetic barrier.
Let me summarize the growth kinetic part and make it clear the role of nitrogen.
In pure carbon system
Armchair edge grows faster, then growth on zigzag edge is rate determining step.
If we consider nitrogen atom as an additives to the carbon network there would be two ways
:the step of nitrogen incorporation into edges and the effect of incorporated nitrogen on carbon atom attachment
Nitrogen incorporation
-lowers energy barrier
-makes the growth rates of zigzag edge similar to that of armchair.
Incorporated nitrogen lowers the activation energy for next atom to attach
Especially, nitrogen in top site of zigzag edge of, all energy barriers disappear for the growth.
The Nitrogen effect in growth of CNT is the enhancement of the growth of zigzag edge.