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First principles calculation on field emission of boron/nitrogen doped carbon nanotube. 2004.11.29 Hyo-Shin Ahn 1,2 , Seungwu Han 3 , Kwang –Ryeol Lee 1 and Doh-Yeon Kim 2 1 Korea institute of science and technology - PowerPoint PPT Presentation
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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