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Theoretical characterization of the single walled boron nitrid nanotube in the external electric field, A DFT study.
D. Farmanzadeh and S. Ghazanfary
Faculty of chemistry, University of Mazandaran, Babolsar, P.O. Box: 453, I. R. Iran.
[email protected] and samereh-ghazanfari@ yahoo.ca
Introduction
Boron nitride nanotubes (BNNTs) possess different electronic and chemical properties in compared with their carbon nanotubes (CNTs) analogues. BNNTs are expected to play an important role in manufacturing of nanodevices because its wide band gap [1].
Replacing the C-C pairs in the CNTs with iso-electronic B-N pairs gives BNNTs alternative functional properties such as pyroelectrict and piezoelectric behavior as well as photogalvanic properties which CNTs do not have [2]. These promising properties of BNNTs generate a large of important potential applications including energy storage, nanotube electronic devices and new composite materials [3]. The range of applications of these boron nitride nanotubes would be extended if their band gap can be tuned to desired values in controlled way.
In this research, we apply a theoretical methodology to study the structural and electrical properties, including electrical conductivity of BNNTs using an electric field applied to the tube axis.
Key words: field effect; nanoelectronic; DFT-B3LYP; BNNT.
Computational procedures
Geometry optimization and calculation of the structural and electronic properties of the zigzag models (4,0), single walled boron nitride (BN) nanotube over the ranges of the 0- 1.6×10-2 a.u. (1 a.u.=514.224 V/nm) field strength have been carried out at DFT-B3LYP level of theory with 6-31G* basis set. The standard direction of an electric dipole field is along +x axis, (Figure 1), the left side of the BNNT is directed to the negative (-) pole of the field and right side of the BNNT is directed to the positive (+) pole of the field. G03 program is used throughout for all quantum chemical calculations [4].
2 Results and Discussion
Optimized values of geometric parameters at entire ranges of applied electric field strength i.e E=0-1.6×10-2 a.u. shows slight variations (maximums: bond lengths <0.009 Å, bond and dihedral angles <7º). Also the calculated electronic spatial extent (ESE) values show a small increasing (0.049%) at entire ranges of applied field strength (0- 1.6×10-2 a.u.). Theses feature can be regarded as positive indexes for BNNT as nanodevice in nano circuit.
The calculated values of the size and components of the electric dipoles moment (in Debye) are plotted in Figure 2 as function of the applied electrical field strength. It can be seen from this Figure that both |µx| and µ varies considerably from 2.2197 and 3.9786 Debye at zero field strength to 14.5406 and 15.7443 Debye at 1.6×10-2 a.u. respectively.
This results show that when the BNNT (4,0) subjected to external electric field has a much stronger interaction with the poles (electrodes) of the nano-electronic circuit.
Values of the HOMO–LUMO gap (HLG), HLG= ELUMO–EHOMO, calculated for BNNT (4,0) at different electric field strength and are reported in Table 1. The data in Table 1 show that the HLG values are decrease gradually from 3.163 eV to 1.618 eV (48.84%) by increasing the applied electric field strength from 0 to 1.6×10-2 a.u. This trend shows that application of the external electric field results in more destabilization of frontier orbitals including HOMO and LUMO.
The calculated electronic energies show that the electronic stability of the BNNT (4,0) increases with increasing strength of the applied electric field.
Natural bond orbital (NBO) and Mulliken atomic charges analysis shows that increasing the applied electric field strength from 0 to 1.6×10-2 a.u. increases separation of the positive and negative electric charges (polarization) of BNNT (4,0).
Results of this study show that it is possible to control field–induced charge redistribution over the BNNT by using different electric field strength. The methodology presented in this study can be applied for any molecular scale devices.
Table 1. The HOMO–LUMO energy gap, HLG, (in eV) values calculated at B3LYP/6- 31G* level of theory for the BNNT (4,0) [introduced in Figure 1] under various field strengths (in 10-4 a.u).
Field 0 10 20 30 40 50 60
HLG 3.163 3.091 3.088 2.936 2.8523 2.768 2.680
Field 70 80 90 100 120 140 160
HLG 2.589 2.495 2.397 2.296 2.084 1.857 1.618
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Figure 1. The BNNT (4, 0) structure, which response to the external electric field is studied in this work at DFT-B3LYP/6-31G* level of theory. The frame of axes used in this study is also shown.
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15
10
5
0
-5 0 50 100 150 200
-10
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-20
Field strength(10 -4 au)
Figure 2. Size of the electronic dipole moment vector and its components (in Debye) at various field strengths E calculated at DFT-B3LYP/6-31G* level of theory at the BNNT (4, 0) [introduced in figure 1].
Refrences:
1. M. Terrones, et al., Materials Today 10 (2007) 30.
2. E. G. Mele, P .Kral, Physical Review Letters. 85. (2002), 056803.
3. L.A Chernozatonski, E.G.Galperr, I.V.Stankevich, Y.K.Shimkus,Carbon.37, (1999), 117.
4. M. J. Frisch et al., Gaussian 03, Revision A. 6, Gaussian Inc. Pittsburgh, PA, (2003).
µx µy µz µtot
Dipole moment(Debye)
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