We investigate the vacancy defect reconstruction and its effect on I-V characteristics in a (4, 0) zigzag and (5, 5) armchair silicon-carbide nanotubes (SiCNTs) by applying self consistent non-equilibrium Green’s function formalism in combination with the density-functional theory to a two probe molecular junction constructed from SiCNTs. The results show that single vacancies and di-vacancies in SiCNTs have different reconstructions. A single vacancy when optimized, reconstructs into a 5-1DB configuration in both zigzag and armchair SiCNTs, and a di-vacancy reconstructs into a 5-8-5 configuration in zigzag and into a 5-2DB configuration in armchair SiCNTs. Introduction of vacancy increases the band gap of (4, 0) metallic SiCNT and decreases the bandgap of (5, 5) semiconducting SiCNT, bias voltage dependent current characteristic show reduction in overall current in metallic SiCNT and an increase in overall current in semiconducting SiCNT.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/2790

Choudhary, S.

Qureshi, S.

SocioEconomic Challenges Journal (Sumy State University)

 
 
 
J. Nano- Electron. Phys. 
3 (2011) No1, P. 1035-1040 
 2011 SumDU 
(Sumy State University) 
 
 
1035 
PACS numbers: 81.07.De, 73.22. – f, 71.15.Mb, 71.20.Nr 
 
VACANCY DEFECT RECONSTRUCTION AND ITS EFFECT ON 
ELECTRON TRANSPORT IN SI-C NANOTUBES 
 
S. Choudhary*, S. Qureshi# 
 
 IIT Kanpur, 
 Department of Electrical Engg., Kanpur, India 
 E-mail: *suds@iitk.ac.in, #qureshi@iitk.ac.in  
 
We investigate the vacancy defect reconstruction and its effect on I-V characteristics 
in a (4, 0) zigzag and (5, 5) armchair silicon-carbide nanotubes (SiCNTs) by applying 
self consistent non-equilibrium Green’s function formalism in combination with the 
density-functional theory to a two probe molecular junction constructed from SiCNTs. 
The results show that single vacancies and di-vacancies in SiCNTs have different 
reconstructions. A single vacancy when optimized, reconstructs into a 5-1DB 
configuration in both zigzag and armchair SiCNTs, and a di-vacancy reconstructs 
into a 5-8-5 configuration in zigzag and into a 5-2DB configuration in armchair 
SiCNTs. Introduction of vacancy increases the band gap of (4, 0) metallic SiCNT and 
decreases the bandgap of (5, 5) semiconducting SiCNT, bias voltage dependent current 
characteristic show reduction in overall current in metallic SiCNT and an increase in 
overall current in semiconducting SiCNT.  
 
Keywords: NANOTUBE, SiCNT, ARMCHAIR-ZIGZAG, DEFECTS, VACANCY. 
 
(Received 04 February 2011) 
 
1. INTRODUCTION 
 
The high energy difference between the sp2 and sp3 bonds (1.25 eV per Si-C 
pair) [1], which makes the existence of a silicon carbide (SiC) graphitic phase 
impossible, has not obstructed the scientists from synthesizing SiC 
nanotubes (SiCNTs) [2] [3]. Recently, the scientists have suggested to make 
silicon carbide nanotubes (SiCNTs) as a possible alternative to carbon 
nanotubes (CNTs) for field emitting applications [2] [3], which has boosted 
the interest for SiCNTs and several studies on the electronic properties of 
SiCNTs can be found in the literature [4-7]. Vacancy reconstruction is well 
known for CNTs [8-9], band gap of semiconducting CNT reduces and 
conducting CNT increases when a vacancy is introduced [10]. Therefore, it is 
of interest to understand how the vacancy reconstruction takes place in 
SiCNTs and what its effect on electron transport properties is. 
 In the process of the growth of CNTs or SiCNTs, a variety of structural 
defects inevitably occur, which can affect their electronic properties. 
Vacancies are the most common defects in CNTs, and have drawn much 
research attention in recent years [8] [11-12]. An ideal single vacancy 
containing three hanging bonds is unstable [8], two of the three dangling 
bonds form a new C–C bond and the structure becomes a pentagonal ring 
coupled with one dangling bond, which forms a so-called 5-1DB defect [8] 
[11] [13]. The reconstruction of di-vacancy in CNTs results into an octagonal 
ring coupled to two pentagonal rings [8] [13], the new 5-8-5 structure does 
 
 
 
1036 S. CHOUDHARY, S. QURESHI  
not have a hanging bond. Another study [10], suggests that an introduction 
of vacancy on single wall carbon nanotubes (SWCNTs) can have a large 
impact on electronic properties, vacancy reconstruction and its effects on 
electronic properties in SiCNTs is still an open question. 
In the present work, we investigate the single and di-vacancy 
reconstruction in (4, 0) and (5, 5) SiCNTs by using steepest descent method 
of geometry optimization. Electronic properties of optimized structures were 
also investigated by studying the transmission energy spectrum T(E), 
density of states (DOS) and current-voltage (I-V) characteristics. The 
calculations were carried out on an ab initio based simulator called 
Atomistix [14] which uses density functional theory (DFT) and non 
equilibrium Green’s functions formulations (NEGF) together for obtaining 
electronic transport properties of molecules and devices. More details about 
the method and software could be found in previous reports [15-17].  
 
2. SIMULATION SETUP AND VACANCY RECONSTRUCTION RESULTS 
 
A perfect (no defects) (4, 0) and (5, 5) SiCNTs are shown in Fig. 1, SiCNT 
structures with single and di-vacancy defects are also shown, Si-C bond 
length of 1.78 Å [3] [7] and 1:1 Si-C ratio was considered. A two probe 
geometry [18] that consists of a left electrode, right electrode and a central 
region (scattering region) was constructed.  
 For studying vacancy reconstruction, the whole setup was optimized 
(electrodes fixed) using steepest descent method of geometry optimization. 
In the case of (4, 0) zig-zag SiCNT, a single vacancy reconstructs into 5-1DB 
defect similar to that in CNTs [11], here two of the three dangling bonds 
(marked as 2 and 3 in Fig. 1a) form a new Si-Si bond (2.36 Å bond length) 
and the structure becomes a pentagonal ring coupled with one dangling 
bond. The reconstruction of di-vacancy in (4, 0) SiCNT is also similar to that 
in CNTs [8] [13], here the reconstruction results into an octagonal ring 
coupled to two pentagonal rings (see Fig. 1a), the new 5-8-5 structure has a 
C-C and Si-Si bond, it does not have a hanging bond. 
 In the case of (5, 5) armchair SiCNT, a single vacancy reconstructs into 
5-1DB defect similar to that in (4, 0) SiCNT and CNTs [11]. However, the 
reconstruction of a di-vacancy forms a new 5-2DB defect in (5, 5) armchair 
SiCNT, two of the four dangling bonds (marked as 3 and 4 in Fig. 1b) form 
a new C–C bond and the structure becomes a pentagonal ring coupled with 
two dangling bonds (marked as 1 and 2 in Fig. 1b). The separation width 
between these two Si atoms associated with hanging bonds is 2.55 Å. 
The electron transport characteristic of a SiCNT may depend on its length, 
resistance, contacts and applied bias. We neglect length dependency by 
considering a minimal length segment of nanotube in the central region. In 
order to minimize the contact dependency, SiCNT electrodes similar to the 
central region were used to construct two probe system. The lengths of the 
central regions, screening layers and the electrodes of the simulated sections 
were taken as 8 periods (32 C atoms and 32 Si atoms), 2 periods (8 C atoms 
and 8 Si atoms) and 2 periods (8 C atoms and 8 Si atoms), for (4, 0) SiCNT. 
In case of (5, 5) SiCNT, central region was 4 periods (40 C and 40 Si atoms), 
screening layers and electrodes were taken as one period each (20 C and 20 
Si atoms). Hence, a central region width (SiCNT length) of 22.25 Å for (4, 0) 
and 13.87 Å for (5, 5) SiCNT were used in the simulation.  
 
 
 
 VACANCY RECONSTRUCTION & IT’S EFFECT ON ELECTRONIC… 1037 
 
 
Fig. 1 – (a) 5-1DB defect in single vacancy optimization of (4, 0) zig-zag SiCNT and 
(b) 5-2DB defect in di-vacancy optimization of (5, 5) armchair SiCNT 
 
 The simulation parameters were selected to provide accurate measurements 
as reported for CNTs [18] and are following: mesh cut-off energy was 400 Ry, 
basis set was double zeta polarized with 0.001 Bohr radial sampling, exchange 
correlation functional was set to local density approximation (LDA) type with 
double zeta polarized (DZP) basis set, Brillouin zone integration parameters of 
electrodes are taken as (1, 1, 500). Electrode temperature was set to 1000 K 
which makes the convergence easier; it has no effect on the overall 
measurement which was also verified at lower electrode temperatures. Due to 
the requirement of heavy computing resources, we used the above parameters 
which shall ensure simulation convergence. 
 
3. ELECTRON TRANSPORT RESULTS AND ANALYSIS 
 
Simulation results are presented in Fig. 2, where I-V characteristic is shown 
and in Fig. 3, where zero bias transmission energy spectrum T(E) and density 
of states DOS results are shown. Let us first look at the bias voltage 
dependent current of Fig. 2, a (4, 0) SiCNT is metallic by nature [4] [6], small 
reduction in current can be observed for both single and di-vacancy defects. 
This is due to a small increase in band gap in vacancy defected metallic 
SiCNTs (band gap with no defect is 0 eV and with double vacancy is 0.074 eV 
for bulk system, see Fig. 4a), an increase in bandgap was also reported in case 
of metallic CNTs in the literature [10]. However, introduction of vacancy 
 
 
 
1038 S. CHOUDHARY, S. QURESHI  
reduces the bandgap of semiconducting (5, 5) SiCNT (band gap with no 
defect is 1.91 eV and with double vacancy is 0.203 eV for bulk system, see 
Fig. 4b), similar to bandgap reduction in semiconducting CNTs [10], which 
results in a large increase in current in semiconducting SiCNTs (see 
Fig. 2b). The band gap for bulk system was calculated from the energy band 
plot similar to Fig. 4, by extracting the HOMO (Highest Occupied Molecular 
Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) bands (i.e. the 
lowest conduction band and highest valence band) and taking the difference 
in all k-points and selecting the smallest value.  
 
 
 
Fig. 2 – Bias voltage dependent current characteristic (I-V characteristic) of (4, 0) 
and (5, 5) SiCNT two probe system. Current is shown to vary in perfect and vacancy 
defected SiCNTs 
 
 To gain more insight of I-V characteristic, we study T(E) and DOS 
results of the ideal and deformed SiCNTs, and compare them. The 
transmission spectrum describes the probability for electron with incident 
energy E to transfer from the left electrode to the right electrode and the 
integral of the transmission spectrum yields the current through the system 
[15]. The equation for calculating current from the transmission spectrum 
can be found in literature [15]. 
 The equilibrium transport properties (no bias voltage applied) of the two-
probe model were investigated firstly. T(E) at zero-bias (equilibrium 
transport) is shown in Fig. 3 by black curve, here the Fermi energy is set as 
0 eV [15], DOS is shown by red curve. T(E) of (4, 0) SiCNT in Fig. 3a, 
reflects the conducting nature of SiCNT with a finite transmission around 
0 eV (Fermi level), which is also confirmed by the overlapping of HOMO and 
LUMO bands in energy band plot of Fig. 4a. The semiconducting nature of 
(5, 5) SiCNT is confirmed by a wide transmission gap seen as a flat curve 
around 0 eV in Fig. 3b and also by inspecting the energy band plot where a 
band gap of 1.91 eV could be observed. The I-V characteristic of metallic (4, 
0) SiCNT is in confirmation with T(E) and DOS results. Bandgap of metallic 
(4, 0) SiCNT increases with the introduction of vacancy (see Fig. 4b), which 
results in an increase in transmission gap (see Fig. 3a) and a reduction in 
current (see Fig. 2a). In the case of (5, 5) semiconducting SiCNT, 
introduction of vacancy reduces the bandgap (see Fig. 4b) resulting in 
increase in current, which is also confirmed by the reduction in transmission 
gap around Fermi level (see Fig. 3b). 
 
 
 
 VACANCY RECONSTRUCTION & IT’S EFFECT ON ELECTRONIC… 1039 
 
 
Fig. 3 – Zero bias transmission spectra T(E) and density of states DOS profile of (a) 
(4, 0) SiCNT and (b) (5, 5) SiCNT. DOS is number of electrons/energy, i.e. unit 1/eV. 
[Note: a.u. is arbitrary units and E is energy in electron volts] 
 
We have shown energy band plot of only di-vacancy defect in Fig 4, the 
band gap with single vacancy defect and with geometry optimization was 
calculated separately from their respective energy band plots.  
 
4. CONCLUSION 
 
In summary, the results show that reconstruction of di-vacancy in armchair 
SiCNT forms a 5-2DB defect, which is different from the reconstruction of 
CNTs and zig-zag SiCNT. Vacancy introduction reduces the bandgap of 
semiconducting SiCNTs and increases the bandgap of conducting SiCNTs  
(converts them to semi-metallic nanotubes), which results in large increase 
in current in semiconducting SiCNTs and a small reduction in current in 
conducting SiCNTs, respectively. 
 
  
 
 
 
1040 S. CHOUDHARY, S. QURESHI  
 
 
 
Fig. 4 – Energy band [k-point, energy] plot of (a) zig-zag (4, 0) SiCNT and (b) 
armchair (5, 5) SiCNT with no defect and with di-vacancy defect 
 
REFERENCES 
1. Y. Miyamoto, B.D. Yu, Appl. Phys. Lett. 80, 586 (2002). 
2. T. Taguchia, N. Igawaa, H. Yamamotoa, S. Shamotoa, S. Jitsukawa, Physica E 
28, 431 (2005).  
3. G. Alfieri, T. Kimoto, Appl. Phys. Lett. 97, 043108 (2010). 
4. I.J. Wu, G.Y. Guo, Phys. Rev. B. 76, 035343 (2007).  
5. M. Zhao, Y. Xia, F. Li, R.Q. Zhang, S.T. Lee, Phys. Rev. B. 71, 085312 (2005).  
6. E.V. Larina, V.I. Chmyrev, V.M. Skorikov, P.N. Dyachkov, D.V. Makaev, Inorg. 
Mater. 44, 823 (2008).  
7. Z. Wang, X. Zu, H. Xiao, F. Gao, W.J. Weber, Appl. Phys. Lett. 92, 183116 (2008). 
8. J. Yuan, K.M. Liew, Carbon 47, 1526 (2009). 
9. Z. Zanolli and J.C. Charlier, Phys. Rev. B. 81, 165406 (2010). 
10. L. Vincent Liu, W.Q. Tian, Y.A. Wang, Int. J. Quant. Chem. 109, 3441 (2009). 
11. A.J. Lu and B.C. Pan, Phys. Rev. Lett. 92,105504 (2004). 
12. J. Rossato, R.J. Baierle, A. Fazzio, R. Mota, Nano. Lett. 5, 197 (2005). 
13. P.M. Ajayan, V. Ravikumar, and J.C. Charlier, Phys. Rev. Lett. 81, 1437 (1998). 
14. Atomistix, QuantumWise A/S (www.quantumwise.com). 
15. M. Brandbyge, J.L. Mozos, P. Ordejón, J. Taylor, K. Stokbro, Phys. Rev. B. 65, 
165401 (2002). 
16. J. Taylor, H. Guo, J. Wang, Phys. Rev. B. 63, 245407 (2001). 
17. J.M. Soler, E. Artacho, J.D. Gale, A. Garcia, J. Junquera, P. Ordejón, 
D. Sanchez-Portal, J. Phys.: Condens. Matter. 14, 2745 (2002). 
18. S. Yamacli, M. Avci, Phys. Lett. A. 374, 297 (2009). 


Vacancy defect reconstruction and its effect on electron transport in Si-C nanotubes

Electronic Sumy State University Institutional Repository

   J. Nano- Electron. Phys. 3 (2011) No1, P. 1035-1040  2011 SumDU (Sumy State University)   1035 PACS numbers: 81.07.De, 73.22. – f, 71.15.Mb, 71.20.Nr  VACANCY DEFECT RECONSTRUCTION AND ITS EFFECT ON ELECTRON TRANSPORT IN SI-C NANOTUBES  S. Choudhary*, S. Qureshi#   IIT Kanpur,  Department of Electrical Engg., Kanpur, India  E-mail: *suds@iitk.ac.in, #qureshi@iitk.ac.in   We investigate the vacancy defect reconstruction and its effect on I-V characteristics in a (4, 0) zigzag and (5, 5) armchair silicon-carbide nanotubes (SiCNTs) by applying self consistent non-equilibrium Green’s function formalism in combination with the density-functional theory to a two probe molecular junction constructed from SiCNTs. The results show that single vacancies and di-vacancies in SiCNTs have different reconstructions. A single vacancy when optimized, reconstructs into a 5-1DB configuration in both zigzag and armchair SiCNTs, and a di-vacancy reconstructs into a 5-8-5 configuration in zigzag and into a 5-2DB configuration in armchair SiCNTs. Introduction of vacancy increases the band gap of (4, 0) metallic SiCNT and decreases the bandgap of (5, 5) semiconducting SiCNT, bias voltage dependent current characteristic show reduction in overall current in metallic SiCNT and an increase in overall current in semiconducting SiCNT.   Keywords: NANOTUBE, SiCNT, ARMCHAIR-ZIGZAG, DEFECTS, VACANCY.  (Received 04 February 2011)  1. INTRODUCTION  The high energy difference between the sp2 and sp3 bonds (1.25 eV per Si-C pair) [1], which makes the existence of a silicon carbide (SiC) graphitic phase impossible, has not obstructed the scientists from synthesizing SiC nanotubes (SiCNTs) [2] [3]. Recently, the scientists have suggested to make silicon carbide nanotubes (SiCNTs) as a possible alternative to carbon nanotubes (CNTs) for field emitting applications [2] [3], which has boosted the interest for SiCNTs and several studies on the electronic properties of SiCNTs can be found in the literature [4-7]. Vacancy reconstruction is well known for CNTs [8-9], band gap of semiconducting CNT reduces and conducting CNT increases when a vacancy is introduced [10]. Therefore, it is of interest to understand how the vacancy reconstruction takes place in SiCNTs and what its effect on electron transport properties is.  In the process of the growth of CNTs or SiCNTs, a variety of structural defects inevitably occur, which can affect their electronic properties. Vacancies are the most common defects in CNTs, and have drawn much research attention in recent years [8] [11-12]. An ideal single vacancy containing three hanging bonds is unstable [8], two of the three dangling bonds form a new C–C bond and the structure becomes a pentagonal ring coupled with one dangling bond, which forms a so-called 5-1DB defect [8] [11] [13]. The reconstruction of di-vacancy in CNTs results into an octagonal ring coupled to two pentagonal rings [8] [13], the new 5-8-5 structure does    1036 S. CHOUDHARY, S. QURESHI  not have a hanging bond. Another study [10], suggests that an introduction of vacancy on single wall carbon nanotubes (SWCNTs) can have a large impact on electronic properties, vacancy reconstruction and its effects on electronic properties in SiCNTs is still an open question. In the present work, we investigate the single and di-vacancy reconstruction in (4, 0) and (5, 5) SiCNTs by using steepest descent method of geometry optimization. Electronic properties of optimized structures were also investigated by studying the transmission energy spectrum T(E), density of states (DOS) and current-voltage (I-V) characteristics. The calculations were carried out on an ab initio based simulator called Atomistix [14] which uses density functional theory (DFT) and non equilibrium Green’s functions formulations (NEGF) together for obtaining electronic transport properties of molecules and devices. More details about the method and software could be found in previous reports [15-17].   2. SIMULATION SETUP AND VACANCY RECONSTRUCTION RESULTS  A perfect (no defects) (4, 0) and (5, 5) SiCNTs are shown in Fig. 1, SiCNT structures with single and di-vacancy defects are also shown, Si-C bond length of 1.78 Å [3] [7] and 1:1 Si-C ratio was considered. A two probe geometry [18] that consists of a left electrode, right electrode and a central region (scattering region) was constructed.   For studying vacancy reconstruction, the whole setup was optimized (electrodes fixed) using steepest descent method of geometry optimization. In the case of (4, 0) zig-zag SiCNT, a single vacancy reconstructs into 5-1DB defect similar to that in CNTs [11], here two of the three dangling bonds (marked as 2 and 3 in Fig. 1a) form a new Si-Si bond (2.36 Å bond length) and the structure becomes a pentagonal ring coupled with one dangling bond. The reconstruction of di-vacancy in (4, 0) SiCNT is also similar to that in CNTs [8] [13], here the reconstruction results into an octagonal ring coupled to two pentagonal rings (see Fig. 1a), the new 5-8-5 structure has a C-C and Si-Si bond, it does not have a hanging bond.  In the case of (5, 5) armchair SiCNT, a single vacancy reconstructs into 5-1DB defect similar to that in (4, 0) SiCNT and CNTs [11]. However, the reconstruction of a di-vacancy forms a new 5-2DB defect in (5, 5) armchair SiCNT, two of the four dangling bonds (marked as 3 and 4 in Fig. 1b) form a new C–C bond and the structure becomes a pentagonal ring coupled with two dangling bonds (marked as 1 and 2 in Fig. 1b). The separation width between these two Si atoms associated with hanging bonds is 2.55 Å. The electron transport characteristic of a SiCNT may depend on its length, resistance, contacts and applied bias. We neglect length dependency by considering a minimal length segment of nanotube in the central region. In order to minimize the contact dependency, SiCNT electrodes similar to the central region were used to construct two probe system. The lengths of the central regions, screening layers and the electrodes of the simulated sections were taken as 8 periods (32 C atoms and 32 Si atoms), 2 periods (8 C atoms and 8 Si atoms) and 2 periods (8 C atoms and 8 Si atoms), for (4, 0) SiCNT. In case of (5, 5) SiCNT, central region was 4 periods (40 C and 40 Si atoms), screening layers and electrodes were taken as one period each (20 C and 20 Si atoms). Hence, a central region width (SiCNT length) of 22.25 Å for (4, 0) and 13.87 Å for (5, 5) SiCNT were used in the simulation.      VACANCY RECONSTRUCTION & IT’S EFFECT ON ELECTRONIC… 1037   Fig. 1 – (a) 5-1DB defect in single vacancy optimization of (4, 0) zig-zag SiCNT and (b) 5-2DB defect in di-vacancy optimization of (5, 5) armchair SiCNT   The simulation parameters were selected to provide accurate measurements as reported for CNTs [18] and are following: mesh cut-off energy was 400 Ry, basis set was double zeta polarized with 0.001 Bohr radial sampling, exchange correlation functional was set to local density approximation (LDA) type with double zeta polarized (DZP) basis set, Brillouin zone integration parameters of electrodes are taken as (1, 1, 500). Electrode temperature was set to 1000 K which makes the convergence easier; it has no effect on the overall measurement which was also verified at lower electrode temperatures. Due to the requirement of heavy computing resources, we used the above parameters which shall ensure simulation convergence.  3. ELECTRON TRANSPORT RESULTS AND ANALYSIS  Simulation results are presented in Fig. 2, where I-V characteristic is shown and in Fig. 3, where zero bias transmission energy spectrum T(E) and density of states DOS results are shown. Let us first look at the bias voltage dependent current of Fig. 2, a (4, 0) SiCNT is metallic by nature [4] [6], small reduction in current can be observed for both single and di-vacancy defects. This is due to a small increase in band gap in vacancy defected metallic SiCNTs (band gap with no defect is 0 eV and with double vacancy is 0.074 eV for bulk system, see Fig. 4a), an increase in bandgap was also reported in case of metallic CNTs in the literature [10]. However, introduction of vacancy    1038 S. CHOUDHARY, S. QURESHI  reduces the bandgap of semiconducting (5, 5) SiCNT (band gap with no defect is 1.91 eV and with double vacancy is 0.203 eV for bulk system, see Fig. 4b), similar to bandgap reduction in semiconducting CNTs [10], which results in a large increase in current in semiconducting SiCNTs (see Fig. 2b). The band gap for bulk system was calculated from the energy band plot similar to Fig. 4, by extracting the HOMO (Highest Occupied Molecular Orbital) and LUMO (Lowest Unoccupied Molecular Orbital) bands (i.e. the lowest conduction band and highest valence band) and taking the difference in all k-points and selecting the smallest value.     Fig. 2 – Bias voltage dependent current characteristic (I-V characteristic) of (4, 0) and (5, 5) SiCNT two probe system. Current is shown to vary in perfect and vacancy defected SiCNTs   To gain more insight of I-V characteristic, we study T(E) and DOS results of the ideal and deformed SiCNTs, and compare them. The transmission spectrum describes the probability for electron with incident energy E to transfer from the left electrode to the right electrode and the integral of the transmission spectrum yields the current through the system [15]. The equation for calculating current from the transmission spectrum can be found in literature [15].  The equilibrium transport properties (no bias voltage applied) of the two-probe model were investigated firstly. T(E) at zero-bias (equilibrium transport) is shown in Fig. 3 by black curve, here the Fermi energy is set as 0 eV [15], DOS is shown by red curve. T(E) of (4, 0) SiCNT in Fig. 3a, reflects the conducting nature of SiCNT with a finite transmission around 0 eV (Fermi level), which is also confirmed by the overlapping of HOMO and LUMO bands in energy band plot of Fig. 4a. The semiconducting nature of (5, 5) SiCNT is confirmed by a wide transmission gap seen as a flat curve around 0 eV in Fig. 3b and also by inspecting the energy band plot where a band gap of 1.91 eV could be observed. The I-V characteristic of metallic (4, 0) SiCNT is in confirmation with T(E) and DOS results. Bandgap of metallic (4, 0) SiCNT increases with the introduction of vacancy (see Fig. 4b), which results in an increase in transmission gap (see Fig. 3a) and a reduction in current (see Fig. 2a). In the case of (5, 5) semiconducting SiCNT, introduction of vacancy reduces the bandgap (see Fig. 4b) resulting in increase in current, which is also confirmed by the reduction in transmission gap around Fermi level (see Fig. 3b).     VACANCY RECONSTRUCTION & IT’S EFFECT ON ELECTRONIC… 1039   Fig. 3 – Zero bias transmission spectra T(E) and density of states DOS profile of (a) (4, 0) SiCNT and (b) (5, 5) SiCNT. DOS is number of electrons/energy, i.e. unit 1/eV. [Note: a.u. is arbitrary units and E is energy in electron volts]  We have shown energy band plot of only di-vacancy defect in Fig 4, the band gap with single vacancy defect and with geometry optimization was calculated separately from their respective energy band plots.   4. CONCLUSION  In summary, the results show that reconstruction of di-vacancy in armchair SiCNT forms a 5-2DB defect, which is different from the reconstruction of CNTs and zig-zag SiCNT. Vacancy introduction reduces the bandgap of semiconducting SiCNTs and increases the bandgap of conducting SiCNTs  (converts them to semi-metallic nanotubes), which results in large increase in current in semiconducting SiCNTs and a small reduction in current in conducting SiCNTs, respectively.       1040 S. CHOUDHARY, S. QURESHI     Fig. 4 – Energy band [k-point, energy] plot of (a) zig-zag (4, 0) SiCNT and (b) armchair (5, 5) SiCNT with no defect and with di-vacancy defect  REFERENCES 1. Y. Miyamoto, B.D. Yu, Appl. Phys. Lett. 80, 586 (2002). 2. T. Taguchia, N. Igawaa, H. Yamamotoa, S. Shamotoa, S. Jitsukawa, Physica E 28, 431 (2005).  3. G. Alfieri, T. Kimoto, Appl. Phys. Lett. 97, 043108 (2010). 4. I.J. Wu, G.Y. Guo, Phys. Rev. B. 76, 035343 (2007).  5. M. Zhao, Y. Xia, F. Li, R.Q. Zhang, S.T. Lee, Phys. Rev. B. 71, 085312 (2005).  6. E.V. Larina, V.I. Chmyrev, V.M. Skorikov, P.N. Dyachkov, D.V. Makaev, Inorg. Mater. 44, 823 (2008).  7. Z. Wang, X. Zu, H. Xiao, F. Gao, W.J. Weber, Appl. Phys. Lett. 92, 183116 (2008). 8. J. Yuan, K.M. Liew, Carbon 47, 1526 (2009). 9. Z. Zanolli and J.C. Charlier, Phys. Rev. B. 81, 165406 (2010). 10. L. Vincent Liu, W.Q. Tian, Y.A. Wang, Int. J. Quant. Chem. 109, 3441 (2009). 11. A.J. Lu and B.C. Pan, Phys. Rev. Lett. 92,105504 (2004). 12. J. Rossato, R.J. Baierle, A. Fazzio, R. Mota, Nano. Lett. 5, 197 (2005). 13. P.M. Ajayan, V. Ravikumar, and J.C. Charlier, Phys. Rev. Lett. 81, 1437 (1998). 14. Atomistix, QuantumWise A/S (www.quantumwise.com). 15. M. Brandbyge, J.L. Mozos, P. Ordejón, J. Taylor, K. Stokbro, Phys. Rev. B. 65, 165401 (2002). 16. J. Taylor, H. Guo, J. Wang, Phys. Rev. B. 63, 245407 (2001). 17. J.M. Soler, E. Artacho, J.D. Gale, A. Garcia, J. Junquera, P. Ordejón, D. Sanchez-Portal, J. Phys.: Condens. Matter. 14, 2745 (2002). 18. S. Yamacli, M. Avci, Phys. Lett. A. 374, 297 (2009). 

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Vacancy defect reconstruction and its effect on electron transport in Si-C nanotubes

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