First principles calculation on field emission of boron/nitrogen doped

carbon nanotube

First principles calculation on field emission of boron/nitrogen doped

carbon nanotube

2004.11.29Hyo-Shin Ahn1,2, Seungwu Han3, Kwang –Ryeol Lee1 and Doh-Yeon

Kim2

1 Korea institute of science and technology2 Department of materials science and engineering, Seoul national

university3 department of physics, Ehwa womans university

Field emissionField emission

Definition:the emission of electrons stripped from parent atoms by a high electric field via quantum mechanical tunneling  

Carbon nanotubefor field emission device- Structural advantage - Property modification by doping

CNT-FED by Samsung

•“Role of extrinsic atoms on the morphology and field-emission properties of carbon nanotubes”L.H.Chan et al., APL., Vol.82, 4334(2003)

B/N doping on multiwall Carbon nanotube Nitrogen doping on CNT enhances emission, while boron suppresses

N

B

Experimental measurementExperimental measurement

Plane wave

(5,5) Caped CNT, 250atoms

•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

• Ab initio tight binding calc. To obtain self-consistent potential and initial wave function

Calculation method –fist stepCalculation method –fist step

Localized basis

Information of wave function shapes and state energy under applied electric field

“First-principles study of field emission of carbon nanotubes”, S. Han et al., PRB, Vol.66, 241402 (2002)

Localized state: Due to the defective structure of nanotube cap

Electronic states of Carbon nanotube Electronic states of Carbon nanotube

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

EF

Localized states

Ene

rgy

<No bias>

Extended states

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

<shapes of orbital>

<Under bias>

Plane wave

Localized basis

(5,5) Caped CNT, 250atoms

•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

• Ab initio tight binding calc. To obtain self-consistent potential and initial wave function

Calculation method – second stepCalculation method – second step

Emission current of undoped CNTEmission current of undoped CNTTotal current: 67.17A

Cutoff radius 80Ry, Electric field: 1.0V/Å, Energy selection : E-Ef= -1.5eV ~ 0.5V

Localized states

Extended states

EF

Emission current vs. Bias voltageEmission current vs. Bias voltage

Cutoff radius 80, Applied field 1.0V/Å, Energy selection : E-Ef= -1.5eV ~ 0.5V

Emission current of N doped CNTEmission current of N doped CNT

Total current: 87.59μA

Localized state

Extended state

mixing of localized and extended states; large contribution to electron emission

π*+localized stateLocalized stateπ extended state

Shape of wave functions in N-doped CNT Shape of wave functions in N-doped CNT

Emission current vs. Bias voltageEmission current vs. Bias voltage

Total current: 87.59μA

Total current: 67.17A

23% increase by N doping

Increase of emission currentIncrease of emission current

Undoped CNT

N-doped CNT

8

10

12

14

16

18

20

22

Emission Current

Em

ission current (A)

Nitrogen position vs. emission currentNitrogen position vs. emission current

Applied electric field : 0.7V/Å, Energy selection : E-Ef= -1.5eV ~ 0.5V

undoped CNT

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

Boron dopingBoron doping

3

4

5

6

7

8

9

10

Curre

nt(A

)

exactly opposite effect - raising the localized state energy350atoms, (5,5) armchair-type, applied electric field: 0.5V/Å

undoped CNT

ConclusionConclusion

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

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

– lowers the energy of localized state

– emission current increase

• Boron doping : – no hybridization of states

– raises localized state energy

– emission current decrease