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

1. An ab-initio Study of the Growth and the Field Emission of CNTs : Nitrogen Effect Hyo-Shin Ahn§, Tae-Young Kim§, Seungwu Han†, Doh-Yeon Kim§ and Kwang-Ryeol Lee Future Technology Research Division Korea Institute of Science and Technology § also at the Division of Materials Science, Seoul National University † also at the Department of Physics, Ehwa Women’s University Korea-UK Forum of Nanotechnology, 2004.8.26-27, COEX, Seoul

2. CNT Growth by CVD H2, Ar, N2, NH3 40㎚

3. 3.0 mm In H2 , N2 or Ar Environment CNT Growth by Thermal CVD In NH3 Environment

4. Synthesis condition CNT Morphology Citation method Temperatue(oC) Reaction Gas Catalyst PE-CVD 666 C2H2+NH3 Ni Aligned CNT Science 282, 1105 (1998) PE-CVD 660 C2H2+NH3 Ni Aligned CNT APL 75 1086 (1999) PE-CVD 825 C2H2+NH3 Co Aligned CNT APL 77 830 (2000) Thermal-CVD 750~950 C2H2+NH3 Fe Aligned CNT APL 77 3397 (2000) PE-CVD 825 C2H2+NH3 Co Aligned CNT APL 77 2767 (2000) Thermal-CVD 800 C2H2+NH3 Fe Aligned CNT APL 78 901 (2001) Thermal-CVD 950 C2H2+NH3 Ni, Co Aligned CNT TSF 398-399 150 (2001) 850 C2H2+H2, C2H2+N2 Tangled CNT Thermal-CVD 950 C2H2+NH3 Ni Aligned CNT DRM 10 1235 (2001) 950 C2H2+H2, C2H2+N2 Tangled CNT Thermal-CVD 800~900 C2H2+NH3 Ni Aligned CNT JAP 91 3847 (2002) 600~900 C2H2+H2 Tangled CNT PE-CVD 660< C2H2+NH3 Ni Aligned CNT APL 80 4018 (2002) Thermal-CVD 850~900 C2H2+Ar Ni, Co Tangled CNT APL 75 1721 (1999) PE-CVD 500 CH4+N2 Fe, Ni Aligned CNT APL 75 3105 (1999) PE-CVD 550 CH4+N2 Fe Aligned CNT JAP 89 5939 (2001) PE-CVD 700 CH4+H2 Ni Aligned CNT APL 76 2367 (2000) Thermal-CVD 800 ferrocene+xylene Fe Aligned CNT APL 77 3764 (2000)

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

6. XPS Analysis of CNT N with sp2 C N in sp3 environ.

7. Enhanced CNT Growth by Nitrogen Incorporation 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?

8. reactant product Calculation of Growth Kinetics • Kinetic barrier calculation by DMol3 for each reaction step. • Assumptions • Flat graphitic plate represents the large radius CNTs. • Reduction of the kinetic barrier by the catalyst is not affected by the existence of nitrogen.

9. Growth of Pure Carbon Zigzag Edge Energy 176 meV hexagon tetragon pentagon Reaction path

10. Growth of Pure Carbon Armchair Edge Energy 160 meV 64 meV hexagon pentagon Reaction path

11. Nitrogen Incorporation on Zigzag Edge 152meV a,b a 0meV c b c 538meV 154meV 176 meV Nitrogen incorporation Pure C 153 meV Energy hexagon tetragon pentagon Reaction path

12. Nitrogen in valley site 333meV Nitrogen in top site Pure C Energy 176meV No barrier hexagon tetragon pentagon Reaction path Growth with Incorporated Nitrogen No barrier 333meV No barrier No barrier No barrier No barrier

13. Nitrogen and CNT Growth • Nitrogen can be incorporated to the CNT wall and cap from the background gas. • The incorporated nitrogen enhances the growth of CNTs.  Most commercialized CNTs prepared by CVD method might be the nitrogen doped one.  Nitrogen can affect the physical and chemical properties of CNTs.

14. Reactivity of Curved C-N Structures S. Stafstrom, Appl. Phys. Lett. 77 (24), 3941 (2000)

15. Field Emission from CNT CNT is a strong candidate for field emission cathod materials 1. Structural advantage 2. Low turn-on voltage What’s the role of incorporated nitrogen in the field emission?

16. (5,5) Caped CNT, 250atoms Localized basis • Ab initio tight binding calc. To obtain self-consistent potential and initial wave function • Relaxation of the wave function • Basis set is changed to plane wave to emit the electrons • Time evolution • Evaluation of transition rate by time dependent Schrödinger equation Plane wave Calculation Method

17. Energy states (eV, E-EF) A B C D Emitted current(μA) Emission from Pure CNT Cutoff radius 80Ry, bias 0.7V/Å Band selection : E-Ef= -1.5eV ~ 0.5V

18. Emission from Pure CNT A State C state D state B State p* and p bonds: Extended states Localized states, Large emission current

19. Emission from Pure CNT

20. Emission from N doped CNT Cutoff radius 80, bias 0.7V/Å Band selection : E-Ef= -1.5eV ~ 0.5V Energy states (eV, E-EF) A B C D Emitted current(μA)

21. Nitrogen doped CNT Undoped CNT Energy states (eV, E-EF) Energy states (eV, E-EF) Emitted current(μA) Emitted current(μA) Total current: 13.2mA Total current: 8.8mA Enhanced Field Emssion by Nitrogen Incorporation A B C D

22. Emission from N doped CNT B state D state C state A state πbond: Extended state Localized state π*+localized state Coupled states between localized and extended states contribute to the field emssion.

23. Localized state - Undoped CNT - N-doped CNT EF Doped Nitrogen Position Nitrogen Effect 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.

24. Field Emission from N-doped CNT • 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.

25. Role of extrinsic atoms on the morphology and field-emission properties of carbon nanotubes L.H.Chan et al., APL., Vol.82, 4334(2003) N B Experimental Results

26. Boron Doped CNT BORON DOPED NITROGEN DOPED Doped Atom Position

27. 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 and corresponding structure and chemical composition of the nanotube.

29. Reactivity of Curved C-N Structures S. Stafstrom, Appl. Phys. Lett. 77 (24), 3941 (2000)

30. A Theoretical Study Effect of substitutional atoms in the tip on field-emission properties of capped carbon nanotubes G.Zhang et al., APL., Vol.80, 2589(2002) 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

31. Nitrogen in CNT Kim et al, Chemical Physics Letters, Vol. 372, 603(2003)

32. B A C B C Relative charge density w.r.t. undoped cnt A Position in CNT Emission current depends on how many electrons are accumulated at the tip.

33. Possible Nitrogen Effects Reduction in the strain energy of CNT Change in the Growth Kinetics

34. Bulk design Cluster design Energy of flat graphite plate ~10Å DE(eV/atom) ~30Å Radius(Å) Strain Energy Due to Curvature 10nm No Significance in Strain Energy Reduction

35. Possible Nitrogen Effects Reduction in the strain energy of CNT Change in the Growth Kinetics

36. reactant product Calculation 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

37. Computational Method • Dmol3: ab-initio calculation based on DFT • Known to be very accurate • Strong in energy calculation – energetics • Transition state calculation – growth kinetics

38. The Growth of CNT Edge armchair zigzag

39. Growth of Pure Carbon Zigzag Edge Energy 176 meV hexagon tetragon pentagon Reaction path

40. Growth of Pure Carbon Armchair Edge Energy 160 meV 64 meV hexagon pentagon Reaction path

41. The Growth of CNT Edge armchair zigzag

42. Nitrogen Incorporation on Armchair Edge 137meV 160meV 303meV 5455meV Nitrogen incorporation Pure C 137meV 160meV Energy 64meV hexagon pentagon Reaction path

43. Growth with Incorporated Nitrogen 152meV 87meV 179meV 96meV Nitrogen at vortex site 179meV Nitrogen at valley site 160meV Pure C 152meV Energy 96meV 87meV 64meV hexagon pentagon Reaction path

44. Growth with Incorporated Nitrogen No barrier No barrier growth near the nitrogen incorporated region. growth of C Energy 176 meV No barrier hexagon tetragon Pentagon

45. Electron Density

46. 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.

47. Possible Nitrogen Effects Reduction in the strain energy of CNT Change in the Growth Kinetics

48. Bulk design Cluster design Energy of flat graphite plate ~10Å DE(eV/atom) ~30Å Radius(Å) Strain Energy Due to Curvature 10nm No Significance in Strain Energy Reduction

49. Possible Nitrogen Effects Reduction in the strain energy of CNT Change in the Growth Kinetics

50. Growth of Pure Carbon Zigzag Edge Energy 176 meV hexagon tetragon pentagon Reaction path