First Principles Calculation of the Field Emission of Nitrogen/Boron Doped 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 , ICMCTF 2005, San Diego, CA, USA

10nm

Field Emission from CNT Emission of electrons stripped from parent atoms by a high electric field through quantum mechanical tunneling CNT-FED by Samsung

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? CNT-FED by Samsung

CNT Growth by CVD H 2, Ar, N 2, NH 3 10nm

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 function Basis set is changed to plane wave to emit the electrons Time evolution Evaluation of transition rate by time dependent Schrödinger equation S. Han et al., PRB, 66, (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 states Due to the graphene structure of nanotube wall S. Han et al., PRB, 66, (2002).

Energy of the States in pure CNT EFEF Localized states Energy Extended states

Emission from Pure CNT :  A Localized states Extended states EFEF Cutoff radius 80Ry, Electric field: 1.0V/Å

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

Emission from N-doped CNT :  A 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 :  A Extended state Cutoff radius 80Ry, Electric field: 1.0V/Å A B D C

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

Emission from N-doped CNT

Nitrogen Doping 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

Emission from B Doped CNT 350atoms, (5,5) armchair-type, applied electric field: 0.5V/Å 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