Enhanced Growth and Field Emission of Carbon Nanotube by Nitrogen Incorporation: The First Principle Study Hyo-Shin Ahn*, Seungwu Han†, Do Yeon Kim§, Kwang-Ryeol Lee Future Technology Research Division, KIST, Seoul, Korea * Presently at Center for Strongly Correlated Materials Research, Seoul National University, Seoul, Korea † Department of Physics, Ehwa Women’s University, Seoul, Korea § Department of Materials Science, Seoul National University, Seoul, Korea Thank you, Prof.Ihm. My presentation is about first principle theoretical analysis of CNT growth and its field emission behavior. Especially, I will focus on the nitrogen effect and would like to show the importance of the incroporated nitrogen in these behaviors. My collaborators are Hyo shin Ahn, he is my former PhD student, and actually this work is part of his phD thesis, Seung-Wu Han at Ehwa University.

Enhanced Growth and Field Emission of Carbon Nanotube by Nitrogen Incorporation: The First Principle Study Hyo-Shin Ahn*, Seungwu Han†, Do Yeon Kim§, Kwang-Ryeol Lee Future Technology Research Division, KIST, Seoul, Korea * Presently at Center for Strongly Correlated Materials Research, Seoul National University, Seoul, Korea † Department of Physics, Ehwa Women’s University, Seoul, Korea § Department of Materials Science, Seoul National University, Seoul, Korea Thank you, Prof.Ihm. My presentation is about first principle theoretical analysis of CNT growth and its field emission behavior. Especially, I will focus on the nitrogen effect and would like to show the importance of the incroporated nitrogen in these behaviors. My collaborators are Hyo shin Ahn, he is my former PhD student, and actually this work is part of his phD thesis, Seung-Wu Han at Ehwa University.

CNT Growth by CVD 40㎚ H2, Ar, N2, NH3 Chemical vapor deposition is one of the most common process to grow CNTs because CVD process has many advantages over other processes. For example, mass production is possible and it is relatively easy to control the growth behavior. The obtained CNT is typically multiwall bamboo-like nanotubes as shown in this TEM microstructure. Typical diameter is a few tens nm. In this method, CNT is grown on the nano-sized transition metal dots by decomposing a hydrocarbon gases. In most cases, the hydrocarbon gas is diluted by the background gas. Hydrogen, argon, nitrogen, or ammonia are typically used as the background gas. The role of the background gas has been consdiered to control the degree of supersaturation of carbon in order to suppress excess carbon deposition or to reduce oxide layer of the catalyst surface to optimize the catalyst effect. However, many experimental works showed that the effect of the background gas is much more than this.

CNT Growth by Thermal CVD 300nm The growth behavior is strongly dependent on the background gas. These two SEM microstructures shows the deposits under the exactly the same deposition condition except the environment gas. Left one is that deposited in the ammonia environment. We can observe the vertically aligned CNTs. Vertically aligned CNT is known to be a consequence of high growth rate and nucleation rate. However, if we use the mixture of nitrogen and hydrogen, all catalysts are passivated by the carbon soot deposition even when the ratio of nitrogen and hydrogen is the same as that in ammonia, one to three. This result clearly shows that the ammonia background gas play a significant role in CNT growth. at 950℃ with 16.7 vol. % C2H2 in pure NH3 at 950℃ with 16.7 vol. % C2H2 in N2:H2 = 1:3 M.-J. Jung et al, Diam. Rel. Mater. 10, 1235 (2001).

T.Y. Kim et al, Chem. Phys. Lett. 372, 603 (2003). Deposition Pretreatment H2+C2H2 NH3 +C2H2 H2 X O NH3 + H2 NH3 Recently, we reported the details of the background gas effect. Because of the limited time, I cannot explain the details of the results in this presentation. However, as shown in this table, vertically aligned CNT was always obtained by using ammonia background gas during the deposition stage. Because ammonia is easy to be decomposed even in thermal CVD environment, we suggest that this result is due to the existence of activated nitrogen in growth environment. T.Y. Kim et al, Chem. Phys. Lett. 372, 603 (2003).

Nitrogen Incorporation into CNTs N with sp2 C N in sp3 environ. And XPS spectra of the vertically aligned CNT showed that nitrogen is actually incorporated in CNT wall or cap and chemically bonded to the carbon atoms, there. This can be also confirmed by high resolution EELS analysis, as shown in this figure. XPS EELS Kim et al, Chem. Phys. Lett. 372, 603 (2003) W.-Q. Han et al, Appl. Phys. Lett. 77, 1807 (2000).

Nitrogen Incorporation into CNTs Nitrogen incorporation significantly enhances the CNT growth resulting in vertically aligned CNTs. 16.7 vol. % C2H2 in NH3, CVD process What is the effect of the incorporated nitrogen?

Enhanced CNT Growth by Nitrogen Incorporation Ab-initio Study of Nitrogen Effect on Carbon Nanotube Growth, Nanotechnology, 17, 909-912 (2006).

Field Emission from CNT CNT-FED by Samsung CNT is a strong candidate for field emission cathode materials 1. Long aspect ratio : Highly enhanced electric field 2. Materials property Low turn-on voltage What’s the effect of nitrogen?

Calculation Method Relaxation of the wave function Localized basis (5,5) Caped CNT, 250atoms 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 Localized basis S. Han et al., PRB, 66, 241402 (2002).

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

Emission from Pure CNT Emitted current(μA) Energy states (eV, E-EF) A A State B State Emitted current(μA) Energy states (eV, E-EF) A B C D D state C state

Emission from Pure CNT S. Han et al., PRB, 66, 241402 (2002).

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

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

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

Emission from N doped CNT B state C state D state A state π bond: Extended state Localized state π*+localized state Hybridized states of the localized and the extended states contribute to the field emssion.

Doped Nitrogen Position Nitrogen Effect Doped Nitrogen Position EF - N-doped CNT - Undoped CNT Localized state 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 in the localized states.

L.H.Chan et al., Appl. Phys. Lett..82, 4334 (2003). Experimental Results N B L.H.Chan et al., Appl. Phys. Lett..82, 4334 (2003).

Boron Doped CNT BORON DOPED NITROGEN DOPED Doped Atom Position

Field Emission from N-doped CNT Nitrogen incorporating enhances the field emission of CNT. In addition to localized state, hybrid states of the extended and the localized states play a significant role. Incorporated nitrogen lowers the energy level of the localized state, which makes electrons more localized to the tip of nanotube. H. S. Ahn et al., Appl. Phys. Lett. 88, 093122 (2006).

Acknowledgement Financial support from Core Capability Enhancement Program of KIST.