First Principles Calculation of the Field Emission of Nitrogen/Boron Do

ped Carbon Nanotubes

Hyo-Shin Ahn§, Seungwu Han†, Kwang-Ryeol Lee,

Do Yeon Kim§

Future Technology Research Division, KIST, Seoul, Korea

§ also at the Division of Materials Science, Seoul National University, Seoul, Korea † at the Department of Physics, Ehwa Women’s University, Seoul, Korea

2005. 5. 3, ICMCTF 2005, San Diego, CA, USA

10nm

CNT is a strong candidate for field emission cathode materials

1. Structural advantage : Highly enhanced electric field

2. Materials property Low turn-on voltage

Field Emission from CNT

What’s the effect of doping?What’s the effect of doping?CNT-FED by Samsung

CNT Growth by CVD

H2, Ar, N2, NH3

10nm

Tangled CNTC2H2+H2600~900

Tangled CNTC2H2+H2, C2H2+N2950

Tangled CNTC2H2+H2, C2H2+N2850

method

ferrocene+xylene

CH4+H2

CH4+N2

CH4+N2

C2H2+Ar

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

C2H2+NH3

Reaction Gas CatalystTemperatue(oC)

APL 77 3764 (2000)Aligned CNTFe800 Thermal-CVD

APL 76 2367 (2000)Aligned CNTNi700 PE-CVD

JAP 89 5939 (2001)Aligned CNTFe550 PE-CVD

APL 75 3105 (1999)Aligned CNTFe, Ni500 PE-CVD

APL 75 1721 (1999)Tangled CNTNi, Co850~900Thermal-CVD

APL 80 4018 (2002)Aligned CNTNi660< PE-CVD

JAP 91 3847 (2002)Aligned CNT

Ni

800~900

Thermal-CVD

DRM 10 1235 (2001)Aligned CNT

Ni950

Thermal-CVD

TSF 398-399 150 (2001)Aligned CNT

Ni, Co950

Thermal-CVD

APL 78 901 (2001)Aligned CNTFe800 Thermal-CVD

APL 77 2767 (2000)Aligned CNTCo825 PE-CVD

APL 77 3397 (2000)Aligned CNTFe750~950Thermal-CVD

APL 77 830 (2000)Aligned CNTCo825 PE-CVD

APL 75 1086 (1999)Aligned CNTNi660 PE-CVD

Science 282, 1105 (1998)Aligned CNTNi666PE-CVD

CitationCNT MorphologySynthesis condition

EELS Analysis of CNT

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

Calculation Method

Plane wave

Localized basis

(5,5) Caped CNT, 250atoms

1. Ab initio tight binding calc.To obtain self-consistent potential and initial wave function with applied electric field

2. Relaxation of the wave functionBasis set is changed to plane wave to emit the electrons

• Time evolutionEvaluation of transition rate by time dependent Schrödinger equation

S. Han et al., PRB, 66, 241402 (2002).

Electronic Structure of Pure CNT

A State B State D stateC state

Localized states Due to the defective structure of CNT cap

and * bonds, Extended statesDue to the graphene structure of nanotube wall

S. Han et al., PRB, 66, 241402 (2002).

Energy of the States in pure CNT

EF

Localized statesE

nerg

y

<No bias>

Extended states

<Under bias>

Emission from Pure CNT : 67.17A

Localized states

Extended states

EF

Cutoff radius 80Ry, Electric field: 1.0V/Å

Emission from Pure CNT

S. Han et al, PRB 66, 241402 (2002).

Emission from N-doped CNT : 87.59A

Cutoff radius 80Ry, Electric field: 1.0V/Å

Total current: 87.59μA

Total current: 67.17mA

23% Increase

Undoped CNT

N-doped CNT

Emission from N-doped CNT : 87.59A

Extended state

Cutoff radius 80Ry, Electric field: 1.0V/Å

AB

D

C

Coupled states between localized and extended states signigicantly contribute to the field emssion.

B stateA state C state D state

π*+localized stateLocalized stateπ bond:Extended state

Electronic Structure of N doped CNT

Emission from N-doped CNT

Nitrogen Doping Effect

EF

- N-doped CNT

- Undoped CNTLocalized 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 filled in the localized states.

Doped Nitrogen Position

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

Ban

d sh

ift (

eV)

8

10

12

14

16

18

20

22

Em

ission current (A

)

Emission from B Doped CNT350atoms, (5,5) armchair-type, applied electric field: 0.5V/Å

0.0

0.2

1.0

1.2

1.4

1.6

1.8

Energ

y of l

oca

lized s

tate

s, E

-EF (eV) Boron doped CNT

Undoped CNT

3

4

5

6

7

8

9

10

Curre

nt(A

)

undoped CNT

Experimental Results

L.H.Chan et al., APL., 82, 4334 (2003).

N

B

Summary

• Emission of undoped carbon nanotube is mainly due to the localized states

• Nitrogen doping : – coupling of the extended and the localized states

– lowers the energy of the localized state

– emission current increased

• Boron doping : – no coupling of the states

– raises the energy of the localized state

– emission current decreased