Role of Nitrogen in the Synthesis of Vertically Aligned Carbon Nanotube

1. Role of Nitrogen in the Synthesis of Vertically Aligned Carbon Nanotube Tae-Young Kim*, Minjae Jung, Kwang-Ryeol Lee, Seung-Cheol Lee, Kwang Yong Eun & Kyu-Hwan Oh* Korea Institute of Science & Technology * Also at Seoul National University The Korea-US Symposium of Phase Transformations of Nano-Materials, Seoul, Korea (2002. 10. 25)

2. Carbon Nano-Tubes (CNT) • Unique Structure and Properties • Suggested Potential Applications • Cold Cathode for FED • Hydrogen Storage Materials • Electrode for Fuel Cell • Nanoscale Transistors 12.5㎛

3. Synthesis of CNT • Arc Discharge, Plasma CVD, Laser Ablation, Thermal CVD • Thermal CVD • Thermal decomposition of hydrocarbon gas with Ni, Co, Fe catalyst • Advantages • Relatively easy to obtain vertically aligned CNTs. • Can be employed for large scale production system. • Easy to understand the reaction behavior (Near Equilibrium). Reaction kinetics and the growth mechanism are not fully understood, yet.

4. CNT Growth by Thermal CVD 40㎚

5. CNT Growth by Thermal CVD In H2 , N2 or Ar Environment In NH3 Environment

6. Evolution of Vertically Aligned CNT 70sec (9.8㎛/min) 4min (1.1㎛/min) 7min(0.8㎛/min ) at 950℃ with 16.7 vol. % C2H2 in pure NH3 Environment Intimate Relationship Between the Growth Rate and the Vertically Aligned CNT

7. 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)

8. CNT Growth by Thermal CVD In H2 , N2 or Ar Environment In NH3 Environment

9. SiO2 Si(100) Formation of Catalyst Particles Ni, Co film deposition Agglomeration of the film Heat treatment @ 800oC H2 300nm 300nm 3.4nm Ni 6.8nm Ni

10. Furnace Hood Substrate holder Loading system H2O Gas inlet • Tube type reactor with quartz tube (50F, 800L) at 1 atm. • Procedure:  Sample loading after increasing temperature in Ar •  Pretreatment for 1hr in H2, N2, H2+N2, H2+Ar, NH3 • Total gas flow : 200sccm (NH3 : 100sccm) •  Add C2H2 to the environmental gas •  Cooling in Ar

11. 300nm NH3 Environment Effect at 950℃ with 16.7 vol. % C2H2 in pure NH3 at 950℃ with 2.4 vol. % C2H2 in N2+H2 : H/(H+N)=0.75

12. Catalyst Surface after Pretreatment 300nm In pure NH3 300nm In H2+N2

13. Ease of Decomposition of NH3 Source : CRC Handbook of Chemistry and Physics 1999-2000 • NH3 is much easier to be decomposed than N2 •  Activated nitrogen in the environment

14. Role of Activated Nitrogen • Two possibilities can be suggested. • Catalyst surface modification by nitrogen may enhance the nucleation of graphite layer on the surface and their separation.  Importance of pretreatment in NH3 environment • Activated nitrogen may play a significant role during CNT growth  Importance of NH3 environment during growth

15. Reaction Reaction Pretreatment Pretreatment NH3 + C2H2 H2 + C2H2 NH3 1h NH3 4h Pretreatment Effect I C2H2 : 16.7 vol.%

16. Pretreatment Effect II Reaction Pretreatment Pretreatment Reaction NH3 + C2H2 H2 1h 0h NH3 + C2H2 C2H2 : 16.7 vol.%

18. Two possibilities can be suggested. • Catalyst surface modification by nitrogen may enhance the nucleation of graphite layer on the surface and their separation.  Importance of pretreatment in NH3 environment • Activated nitrogen may play a significant role during CNT growth  Importance of NH3 environment during growth Role of Activated Nitrogen

19. 0.015 0.05 0.231 0.17 Reaction Pretreatment NH3 + C2H2 H2 X = C2H2 / (NH3+C2H2)

20. Nitrogen in CNT

21. Evolution of Vertically Aligned CNT 70sec (9.8㎛/min) 4min (1.1㎛/min) 7min(0.8㎛/min ) at 950℃ with 16.7 vol. % C2H2 in pure NH3 Environment Intimate Relationship Between the Growth Rate and the Vertically Aligned CNT

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

23. Strain Energy of Tubular Form Ab initio Pseudopotential Total Energy Calculation Y. Miyamoto et al, Solid State Comm. 102, 605 (1997)

24. Nitrogen Incoporation Enhancesthe Pentagon Strucutre H. Sjostrom et al, Phys. Rev. Lett. 75, 1336 (1995). N. Hellgren et al, Phys. Rev. B 59, 5162 (1999). • Energy for Pentagon Formation only with carbon atoms= 73.8kcal/mole • Energy for Pentagon Formation if nitrogen substitute two carbon atoms= 26.2kcal/mole

25. Role of Activated Nitrogen

26. EELS Analysis of CNT W.-Q. Han et al, Appl. Phys. Lett. 77, 1807 (2000).

27. Nanotube Junctions PECVD X. Ma et al, Appl. Phys. Lett. 78, 978 (2001).

28. Nitrogen in CNT

29. Nitrogen Incorporation

30. Conclusions • Enhanced CNT growth in an N2 or NH3 environment is due to nitrogen incorporation into the CNT wall or cap. • Nitrogen incorporation can reduce the strain energy required for the tubular graphitic layer, which decrease the activation energy for both the nucleation and growth. • Nitrogen in the CNT would affect their electronic structure, electron transport behavior and chemical activity of the CNTs.

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