Computational comparative study of substitutional, endo and exo BN Co-Doped single walled carbon nanotube system

Accepted Manuscript Computational Comparative Study of Substitutional, Endo and Exo BN Co-Doped Single Walled Carbon Nanotube System Khurshed A. Shah, M. Shunaid Parvaiz PII:

S0749-6036(16)30105-7

DOI:

10.1016/j.spmi.2016.03.012

Reference:

YSPMI 4240

To appear in:

Superlattices and Microstructures

Received Date: 7 March 2016 Accepted Date: 8 March 2016

Please cite this article as: K.A. Shah, M.S. Parvaiz, Computational Comparative Study of Substitutional, Endo and Exo BN Co-Doped Single Walled Carbon Nanotube System, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.03.012. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Computational Comparative Study of Substitutional, Endo and Exo BN Co-Doped Single Walled Carbon Nanotube System Khurshed A. Shah* and M. Shunaid Parvaiz

RI PT

Nanomaterials Research Laboratory, Department of Physics, Govt. Degree College for Women, K. P. Road, Anantnag-192101, India Abstract:

In this report we investigate the effect of doping on electronic properties of a zig-zag (4, 0) semi-conducting single walled two probe carbon nanotube system by using substitutional,

SC

endo and exo doping in the configuration. We choose atoms of elements Boron (B) and Nitrogen (N) because of their similar atomic radii to that of carbon. The Atomistic Tool Kit software (Version 13.8.1) and its graphical interface Virtual Nanolab is used in device mode for

M AN U

simulations. The calculations were carried out by using Huckel Parameters and the comparative study of current-voltage characteristics and conductance of the proposed models were done under low bias conditions. The results show that substitution doping has increased the conductance of the model than endo and exo. However, when the concentration of BN dopants is increased from two atom to four atom the endo doping model shows better performance than other two models. Hence the study is very beneficial for designing various CNT devices for

Keywords:

TE D

commercial applications including amplifiers and oscillators.

Introduction:

EP

Two probe CNT system, Substitutional doping, Endo doping, Exo doping, Comparative studies

AC C

Carbon nanotubes (CNTs) have a great potential for various applications due to their

unique electronic transport properties related to their one dimensional character [1]. Since the discovery of CNTs by Sumio Iijima in 1991 [2], CNTs have been intensively studied for various research activities. With the advancements in synthesizing technology, doped CNTs have attracted more and more attention due to their exceptional properties. Different types of techniques including arc discharge [3], chemical vapour deposition [4], pyrolysis [5] and substitution reactions [6] are already being used for B and N doping of CNTs. It is due to B and N doping, that more control is achieved on the electronic transport properties of CNTs [3, 7].

1

ACCEPTED MANUSCRIPT The electronic transport properties of CNTs can be easily played with by making the proper modifications in the dopant positions. It is due to the uniqueness of CNTs that we are able to incorporate different types of doping mechanisms like doping along the tube which alters its electronic properties [8]. Therefore it is of great interest to make the comparative study of the

RI PT

effect of dopant positions and concentrations on electronic transport properties of a co-doped CNT system. Infact several attempts were made in the past with regard to the theoretical study of doped SWCNT devices [9-12], but to the best of our knowledge there is no report which should had aimed at comparative study of transport properties of substitutional, endo and exo doped SWCNT systems.

SC

In this paper, we report the effect of substitutional, endo and exo doping on electronic transport properties of zig-zag (4, 0) SWCNTs two probe systems by doping the configurations

M AN U

exactly at the same positions. The goal is accomplished by using Huckel basis set of Atomistic Tool Kit Software (Version 13.8.1) and its graphical interface. The results show that substitutional doping is comparatively good at low doping levels, however endo doping shows profound transport properties at high doping concentration. Thus the results are very important

Models and methods:

TE D

for designing of future CNT based devices for various industrial applications.

In order to study the transport properties of each substitutional, endo and exo doped CNTs system we modeled a zig-zag (4, 0) two probe SWCNT system that consists of a left gold electrode, right gold electrode and a central scattering region by using ATK (13.8.1) software

EP

and co-doped the SWCNT with one boron (B) atom near the right electrode and one nitrogen (N) atom near the left electrode by substituting the carbon atoms in the case of substitutional doping

AC C

is shown in Figure 1(a). Gold electrodes have been also used in the previous research works to study the electronic properties for both doped as well as undoped CNTs [9, 13-15]. Figure 1(b, c) shows the 2 atom endo and exo BN co-doped configurations respectively.

2

(b)

AC C

EP

TE D

M AN U

SC

(a)

RI PT

ACCEPTED MANUSCRIPT

(c ) Figure 1: Two probe geometry of 2 atom BN co-doped zig-zag (4, 0) SWCNT with Au electrodes (a) substitutional (b) endo (c) exo

3

ACCEPTED MANUSCRIPT In each model shown above both electrodes consist of 25 gold atoms and the central scattering region consists of 164 atoms. The bond length between carbon atoms was taken as 1.42086 A0 and the length between two electrodes was set to be 7.1043 A0. To overcome the scattering losses, 10% of the length of electrodes was considered as scattering region. Thus we modeled a

RI PT

two probe system in order to understand the behavior of the SWCNT device for comparative studies. Here we based our hypothesis on Non Equilibrium Green’s Function (NEGF) formalism and used the single particle approach based on Landauer-Buttiker formalism. The models were simulated using Huckel parameters with the electrode temperature of 300K. The Density mesh cut-off was put on 10 Hartee.The set of k-points were chosen as 1, 1, 100 for appropriate

Basis Type Cedra Gold (fcc) Cedra Boron (BN Hexagonal) Cedra Carbon (Graphite) Cedra Nitrogen (BN Hexagonal)

M AN U

Atom Au

SC

sampling. The Huckel basis parameters used are shown in Table 1.

B C

~ 6 eV ~ 7.36577 eV ~ 6 eV

TE D

N

Vacuum Level ~ 4.8533 eV

Table 1: Huckel Basis Parameters of Proposed Models The maximum range for the interaction was taken as 10 Angstroms and Fourier2D solver was

EP

adopted as Poisson solver for the boundary conditions. The applied bias across the two electrodes was varied from 0-2 Volts in order to measure the values of current and conductance

AC C

effectively. In order to gain much understanding of the modelled two probe systems under low bias conditions we extended our work by increasing the concentration of the dopants in the geometries. As such we doped the (4, 0) zig-zag SWCNT geometries with four dopant atoms i.e, two B atoms near the left electrode and two N atoms near the right electrode as shown in Figure2 (a, b, c) keeping all simulation and calculation parameters same as that of the three models shown in Figure 1(a, b, c).

4

SC

TE D

M AN U

(a)

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

(b

(c) Figure 2: Two probe geometry of 4 atom BN co-doped zig-zag (4, 0) SWCNT with Au electrodes (a) substitutional (b) endo (c) exo 5

ACCEPTED MANUSCRIPT Simulation Results: All the proposed models were simulated in device mode using Atomistic Tool Kit (13.8.1) [16] and its graphical interface Virtual Nanolab to study the effect of different doping methods on the conductance of two probe Zig-Zag (4, 0) SWCNTs. The BN doped structures

RI PT

were fully relaxed in order to make all the residual forces on each atom smaller than 0.05 eV/Å. In order to investigate the effect of doping on electronic transport property of the geometries we analyzed the transmission spectra of all models by plotting I-V and conductance curves under different bias voltages. Figure 3, Figure 4 and Figure 5 shows the I-V and conductance curves of

(a)

TE D

M AN U

SC

two atom substitutional, endo and exo BN co-doped SWCNT systems respectively.

(b)

AC C

EP

Figure 3: Curves of 2 atom substitutional BN doped two probe SWCNT system (a) I-V (b) dI/dV

(a)

(b)

Figure 4: Curves of 2 atom endo BN doped two probe SWCNT system (a) I-V (b) dI/dV

6

RI PT

ACCEPTED MANUSCRIPT

(b)

SC

(a)

M AN U

Figure 5: Curves of 2 atom exo BN doped two probe SWCNT system (a) I-V (b) dI/dV In order to clearly understand the doping effect on electronic properties of CNT geometries we have introduced Table 2 for comparative study and the values were taken from Figure 3(b), Figure 4(b) and Figure 5(b). Figure 6 clearly shows that for two atom doped SWCNT models, the maximum conductance is shown by substitutional BN co-doped model and minimum conductance is shown by endo doped model. Bias (V)

Conductance (nS) 0 162724 163292 178638.33 185202.5 174258 175705 178023.57 171338.75 164269.44 161909 (b)

TE D

AC C

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Conductance (nS) 0 220716.5 200966.5 206201.67 202646.25 200751 192781.67 192257.14 186419.38 179381.67 172164.5 (a)

EP

Bias (V)

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Bias (V) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Conductance (nS) 0 152811.5 152454.25 181566.67 198893.75 197701 194413.33 195275 187684.38 179691.67 178142 (c)

Table 2: Values of conductance for 2 atom BN Co-doped two probe SWCNT systems at different bias voltages (a) substitutional (b) endo (c) exo

7

ACCEPTED MANUSCRIPT

250000

150000

RI PT

Conductance (V)(nS)

200000

100000

Exo Endo Sub.

0 0.0

0.5

1.0

SC

50000

1.5

2.0

M AN U

Bias (V)

Figure 6: Conductance Vs Voltage graph showing doping effect on 2 atom BN co-doped zig-zag (4, 0) SWCNT systems From the above results it is clear that substitutional doping method shows maximum conductance due to the introduction of new electronic states around the Fermi level. Also lowering of orbital localization takes place which helps in tunneling from left to right electrode

TE D

resulting in the increase of conductance and suppression of NDR effect [17, 18]. Furthermore endo and exo methods do not contribute much in the conductivity process as they produce structural defects which alter the density of states. Figures 7, Figure 8 and Figure 9 shows the I-V and conductance curves of four atom

EP

substitutional, endo and exo BN co-doped SWCNT systems respectively. From these curves it is visible that noticeable changes occur in the I-V and conductance curves when we increased the

AC C

dopant concentration from two atoms to four atoms.

(a)

(b) 8

ACCEPTED MANUSCRIPT

SC

RI PT

Figure 7: Curves of 4 atom Substitutional BN co-doped two probe SWCNT system (a) I-V (b) dI/dV

(b)

M AN U

(a)

EP

TE D

Figure 8: Curves of 4 atom endo BN co-doped two probe SWCNT system (a) I-V (b) dI/dV

(a)

(b)

AC C

Figure 9: Curves of 4 atom exo BN co-doped two probe SWCNT system (a) I-V (b) dI/dV For clear understanding we introduced Table 3 showing the values of conductance for 4 atom BN co-doped SWCNT geometries taken from Figure 7(b), Figure 8(b) and Figure 9(b). Figure 10 clearly indicates that for 4 atom doped SWCNT models, the maximum conductance is shown by exo and minimum conductance is shown by endo BN doped SWCNT two probe system.

9

ACCEPTED MANUSCRIPT

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Conductance (nS) 0 177024 179808.5 174663.3 173491.2 168243 162920 162499.3 157513.1 148387.8 143705 (b)

Bias (V) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Conductance (nS) 0 161183.5 161385 185346.7 194038.8 198494 196690.8 191914.3 185671.9 176357.8 170266 (c)

RI PT

Bias (V)

SC

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Conductance (nS) 0 181445 189748.2 188111.7 177623.8 183286 169870 179394.3 174384.4 163506.1 154252.5 (a)

M AN U

Bias (V)

Table 3: Values of conductance for 4 atom BN Co-doped two probe SWCNT systems at different bias voltages (a) Substitutional (b) Endo (c) Exo

TE D

150000

100000

Endo Exo Sub.

EP

Conductance (nS)

200000

AC C

50000

0

0.0

0.5

1.0

1.5

2.0

Bias (V)

Figure10: Conductance vs voltage graph showing doping effect on 2 atom BN co-doped zig-zag (4, 0) SWCNT systems

10

ACCEPTED MANUSCRIPT The above results shows that the substitutional doping method has less conductance particularly at low bias voltages because of the increase in dopant density which moves the electronic states away from the Fermi level. On the other hand the conductivity of exo doped model is almost same to that of its 2 atom model, however the endo doping model shows less conductivity here

RI PT

than that of its 2-atom model due to the increased inter-electronic interaction in the conduction band.

From the above discussion it is clear that the transport properties of all models are dependent on method of doping and concentration of the dopants in the geometry of SWCNT. It is further concluded that the B and N atoms alters the energy levels in the band gap of the

SC

SWCNT device which in turn is dependent on the method of doping and the concentration of the external atoms in the SWCNT geometry. The results are important for exploiting the doped

M AN U

SWCNT devices in various commercial applications in the future. Conclusion:

Comparative study of electronic transport properties of BN co-doped two probe zig-zag (4, 0) SWCNTs is investigated using different doping methods at different dopant concentration levels. For 2 atom BN co-doped SWCNT device, maximum conductance was observed in

TE D

substitutional doped SWCNT two probe system. As the concentration of dopants was increased from 2 atoms to 4 atoms, the maximum conductance shifts from substitutional to exo doped SWCNT two probe device particularly at higher applied voltages keeping all device parameters constant. However, the minimum conductance was observed in endo doped SWCNT for both

EP

concentration levels. From this study it is concluded that the substitutional doping method is favorable at low bias voltages (< 0.5 V) both at high and low doping levels, however for bias

AC C

voltage >0.5 V exo doping method shows good transport properties. The reported results are very important from both basic and applied point of view.

Acknowledgement:

The authors greatly acknowledge the financial support by University Grants Commission

(UGC), New Delhi (grant No: 42-768/2013) which enables this study to occur. References: [1] I. Deretzis, A. La Magna, Nanotechnology 17(2006)5063. 11

ACCEPTED MANUSCRIPT [2] S. Iijima, Nature 354 (1991) 56. [3] O. Stephan, P. M. Ajayan, C. Colliex, et al., Science 266(1994) 1683. [4] J. Yu, J. Ahn, S. F. Yoon, et al., Appl. Phys.Lett. 77 (2000) 1949. [5] M. Terrones, A. M. Benito, C. Manteca-Diego, et al., Chem. Phys.Lett. 257 (1996) 576.

RI PT

[6] W. Han, Y. Bando, K.Kurashima, et al., Chem. Phys. Lett. 299(1999) 368. [7] P. Zhao, P. J. Wang, Z. Zhang, et al., Phys.Lett. A 374 (2010) 1167. [8] K. Hafid, H. Patrick, H. Luc, Phys. Rev. B 81(2010) 193411. [9] Y. T Yang, R. X Ding, J. X. Song, Physica B 406(2010)216. [10] S. Choudary, S. Qureshi, Phys. Lett. A 375 (2011) 3382.

SC

[11] T. Taguchi, N. Igawa, H. Yamamoto, S. Jitsukawa, Journal of the American Ceramic Society 88 (2005) 459.

M AN U

[12] J. Song, Y. Yang, H. Liu, IEEE EDSSC 60(2009) 509.

[13] X. F. Li, K. Q. Chen, L. Wang, et al., Appl. Phys. Lett.91(2007) 133511. [14] Y. T. Yang, J. X. Song, X. H. Liu, C. C. Chai, Chinese Science Bulletin 53(2008) 3770. [15] K. A. Shah, J. R. Dar, IJIRSET 03(2014) 17395.

[16] Atomistic Tool Kit version 13.8.1 currently available online at www.quantumwise.com [17] S. Choudary, S. Qureshi, Phys. Lett. A 377 (2013) 430.

AC C

EP

TE D

[18] S. Choudary, S. Qureshi, Phys. Lett. A 376 (2012) 3359.

12

ACCEPTED MANUSCRIPT

Highlights (for Review) •

Effect of doping on electronic properties of a zig-zag (4, 0) semi-conducting single walled two probe carbon nanotube system by using substitutional, endo and exo doping

RI PT

in the configuration. •

ATK-VNL software was used for calculations.

•

Boron (B) and Nitrogen (N) doping is done because of their similar atomic radii to that of carbon.

Calculations were carried out by using Huckel Parameters and the comparative study of

SC

•

current-voltage characteristics and conductance of the proposed models were done under low bias conditions.

As the concentration of BN dopants is increased from two atom to four atom the endo

M AN U

•

doping model shows better performance than other two models. substitutional doping method is favorable at low bias voltages (< 0.5 V) both at high and low doping levels, however for bias voltage >0.5 V exo doping method shows good

EP

TE D

transport properties.

AC C

•