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, , COEX, Seoul

40 ㎚ CNT Growth by CVD H 2, Ar, N 2, NH 3

In NH 3 Environment CNT Growth by Thermal CVD In H 2, N 2 or Ar Environment 3.0  m

Deposition Pretreatment H 2 +C 2 H 2 NH 3 +C 2 H 2 H2H2 X O NH 3 + H 2 O NH 3 XO Kim et al, Chem. Phys. Lett. 372, 603 (2003)

XPS Analysis of CNT N with sp 2 C N in sp 3 environ.

Nitrogen incorporation significantly enhances the CNT growth resulting in vertically aligned CNTs vol. % C 2 H 2 in NH 3, CVD process What is the role of Nitrogen in the CNT growth? Enhanced CNT Growth by Nitrogen Incorporation

Calculation of Growth Kinetics Kinetic barrier calculation by DMol 3 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. reactant product

Reaction path Energy 176 meV tetragonpentagon hexagon Growth of Pure Carbon Zigzag Edge

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

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

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

Nitrogen and CNT Growth 1.Nitrogen can be incorporated to the CNT wall and cap from the background gas. 2.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.

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

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

Calculation Method Plane wave 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

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

A State B State D stateC state Localized states, Large emission current  * and  bonds: Extended states Emission from Pure CNT

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

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

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

Nitrogen Effect EFEF - 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, (2003). The lower energy of the localized state makes it possible for more electrons to be filled in the localized states. Doped Nitrogen Position

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. Field Emission from N-doped CNT

Experimental Results 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

BORON DOPED NITROGEN DOPED Boron Doped CNT Doped Atom Position

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.

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

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

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

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

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

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

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

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 reactant product

Computational Method Dmol 3 : ab-initio calculation based on DFT Known to be very accurate Strong in energy calculation – energetics Transition state calculation – growth kinetics

The Growth of CNT Edge armchair zigzag

Reaction path Energy 176 meV tetragonpentagon hexagon Growth of Pure Carbon Zigzag Edge

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

The Growth of CNT Edge armchair zigzag

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

Growth with Incorporated Nitrogen 152meV87meV 179meV96meV Energy Nitrogen at vortex site Pure C pentagon hexagon Nitrogen at valley site 64meV 152meV 160meV 179meV 96meV 87meV Reaction path

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

Electron Density

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.

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

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

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

Reaction path Energy 176 meV tetragonpentagon hexagon Growth of Pure Carbon Zigzag Edge

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

The Growth of CNT Edge armchair zigzag

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