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

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?

Calculation of growth kinetics 2 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

The growth on the edge of CNT 3 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

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

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.

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.

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.

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.

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.

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.

Summary – growth kinetics 7 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.