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

1. 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.

2. 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?

3. 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)

4. 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

5. 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

6. 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.

7. 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

8. 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

9. 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.

10. 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

11. Armchair and zigzag edge armchair zigzag As the growth proceeds, the proportion of zigzag edge will increase

12. 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

13. 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

14. 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

15. 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

16. 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.

17. 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.

19. 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

20. 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