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Study of Structural and Electronic Properties of Doped Arm Chair Single-Walled Carbon Nanotubes
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ScienceDirect Materials Today: Proceedings 3 (2016) 1820–1827
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Recent Advances In Nano Science And Technology 2015 (RAINSAT2015)
Study of Structural and Electronic Properties of Doped Arm Chair Single-Walled Carbon Nanotubes Shobhna Dhimana*, Ranjan Kumarb and Keya Dharamvirb 1
Department of Applied Science, PEC, University of Technology, Chandigarh, India 2 Department of Physics, Panjab University, Chandigarh-160014, India
Abstract Structural and electronic properties of endohedrally doped armchair single-wall carbon nanotubes (SWCNTs) with a chain of six atoms of Ag and Cu have been studied using ab-initio density functional theory. We investigate the binding energy/atom, ionization potential, electron Affinity and Homo-Lumo gap of doped armchair SWNTs from (4,4) to (6,6) with two ends open. BE/atom is maximum for (5, 5) doped armchair carbon nanotube; suggest that it is more stable than (4, 4) and (6, 6) doped tubes. Whereas ionization potential of Ag doped tubes is more and electron affinity is less with respect to of Cu doped tubes showing that Ag doped tubes are less reactive than Cu doped tubes. Homo- Lumo gap of doped arm chair tubes decreases exponentially with the increase in diameter of the tubes. This shows that confinement induce a strong effect on electronic properties of doped tubes. These combined systems can be used for future Nano electronics. The ab–initio calculations were performed with SIESTA code using generalized gradient approximation (GGA). © 2015Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords:Density functional theory; SIESTA; Single-walled armchair carbon nanotubes; HOMO-LUMO gap.
* Corresponding author. E-mail address: [email protected] 2214-7853© 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].
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1. Introduction Carbon nanotubes (CNTs) have exceptionally high tensile strength, high resilience and semiconductor behavior, high current carrying capacity and high thermal conductivity [1-3]. CNTs can be classified with respect to its structure by means of the roll up or the chirality vector. The chirality vector defines the roll up angle and the circumference of the CNTs. On the basis of classification based on chirality vector there exists three kinds of CNTs: armchair (n, n), chiral (n, m) 0<|m|
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a
b
c
Figure1. The optimized structures of Cu6@CNTs (a) (4, 4), (b) (5,5), (c) (6,6)
3. Results and discussion First of all we have performed structural optimization of undoped (n, n) armchair SWCNTs, n=4-6. We have optimized the structures using SIESTA code. The structures have been allowed to relax until the forces are smaller than 0.04 ev/Å. The obtained C-C bond length for (4,4), (5,5) and (6,6) armchair CNTsare1.44 Å, 1.44 Å and 1.43 Å respectively. The calculated bond length value agrees well with previously reported values of 1.43 Å for (4, 4), (5, 5) and (6, 6) CNTs [46]. In the next step we have considered an endohedralsystem by placing a chain of six dopant atoms inside armchair CNT. The chain of dopant atoms was formed by placing these atoms at a distance of about their bond length. The whole system was allowed to relax with respect to all degrees offreedomwithout additional constraints. The reason for choosing a chain of dopant owe to the cylindrical cavity available inside CNT. We have done endohedral doping in different armchair carbon nanotubes. Armchair nanotubes of fixed length 12.47Å were taken. Initially we have placed one Cu atom inside CNT. The dopant was placed at different positions inside CNT.
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We found that Cu atom at the center give rise the most stable structure. Then we have increased the number of Cu atoms up to six. The BE/dopant atom (EB) of the endohedral armchair carbon nanotube was calculated as follow:
E = (
@
−
−
)/n
a
b
c
Figure2. The optimized structures of Ag6@CNTs (a) (4,4), (b) (5, 5) ,(c) (6,6)
where @ is the total energy of endohedral complex and n is the number of dopant atoms in CNT, is the total energy of undoped CNT and is the total energy of one dopant atom. We have considered (4, 4), (5,5) and (6,6) armchair SWCNTs for endohedral doping of Cu and Ag atoms figures 1, 2 illustrate the optimized structures after doping the chain consisting of six atoms of Cu and Ag in arm chair CNTs. The values of the BE/atom for pure CNTs were found to be in the range -7.745 eV to -7.895 eV. The minimum value of BE/Cu atom is -1.840 eV and for BE/Ag atom is -0.472 eV for Cu and Ag doped (4, 4) CNTs with diameter 5.66 Å and 102 number of atoms(see table1). The maximum value of BE/Cu atom is -2.095 eV and BE/Ag atom is -1.630 eV for Cu and Ag doped (6, 6) CNTs with diameter 8.29 Å and 150 number of atoms respectively. This means that BE/dopant atom increases while going from (4, 4) to (6, 6)atom increases while going
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from (4, 4) to (6, 6) armchair carbon nanotube. Our calculations shows that (6, 6) is most suited for Cu/Ag dopingas BE/atom is maximum for the same. CNT Cu6 @CNT
(5,5)
4.8 Ionization Potential (eV)
Ag6 @CNT
4.7
(6,6)
(5,5)
(6,6)
4.6
(5,5) (6,6)
(4,4)
4.5 (4,4)
4.4
(4,4)
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Diameter of carbon nanotube
Figure3. The variation of ionization potential with diameter for Cu and Ag chain encapsulated in armchair (n,n),n=4-6 CNTs 4.1 4.0 Electron affinity (eV)
(6,6)
C NT C u6@C NT Ag 6@CNT
(6,6) (5,5) (6,6)
3.9 (5,5)
3.8 (5,5)
3.7 (4,4)
3.6
(4,4)
3.5
(4,4)
5.5
6.0 6.5 7.0 7.5 8.0 Diameter of carbon nanotube
8.5
Figure4. The variation of electron affinity with diameter for Cu and Ag chain encapsulated in armchair(n,n),n=4-6 CNTs
The diameter of the tube plays an important role on the stability of the tube. In a finite nanotube, quantum confinement may induce a strong effect on the electronic properties of nanotubes. In order to have a clear insight of electronic properties, we have studied IP and EA of pure and doped CNTs. We put one extra electron on Cu6@CNT, Ag6@CNT molecule and compute the total energy. Subtracting the total energy of neutral Cu6@CNT, Ag6@CNT from total energy obtained yields electron affinity of Cu6@CNT and. Ag6@CNT. Similarly we have computed IP by making Cu6@CNT, Ag6@CNT one electron deficit. Subtracting the total energy from total energy of neutral molecule gives ionization potential of the compound. These quantities give us an idea about the chemical reactivity of endohedral CNTs. The variation of IP of and EA with tube diameter is shown in figures 3, 4. The maximum value of IP in pristine CNT is 4.823 eV and decreases with increasing diameter of tube. EA increases monotonically with diameter of the tube. The endohedral doping of linear atomic chain changes IP and EA significantly. We have found that (4, 4) armchair carbon nanotube doped with chain of Cu and Ag atoms have less values of IP and EA than pristine (4, 4) tube. The minimum value of IP and EA has been observed for doped (4, 4) carbon nanotube. Whereas maximum value of IP is observed for pristine (5, 5) carbon nanotube tube, but the doped (5, 5) CNT with metals have low value of ionization potential and higher values of electron affinity. This means that doped (5, 5) tube is more reactive as compared to pristine CNT. In case of (6, 6) CNT the IP of pure tube is greater than of doped Cu tube, but less than Ag doped tube. Whereas electron affinity for both doped tubes is greater than pristine. This suggests that Cu doped (6, 6) CNT is chemically more reactive. The concept of highest occupied molecular orbital
Shobhna Dhiman, et al./ Materials Today: Proceedings 3 (2016) 1820–1827
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(HOMO) and lowest unoccupied molecular orbital (LUMO) is an important factor in understanding the charge transfer, stability and reactivity of many organic molecules. The calculated HOMO-LUMO gap of pure and doped armchair nanotubes is shown in figure 5. On doping of Cu and Ag, HOMO-LUMO gap decreases as we move from (4, 4) to (6, 6) CNTs. 0.70 (4,4)
CNT Cu6 @CNT
Homo-Lumo gap (eV)
0.65 0.60
Ag6 @CNT
(4,4)
0.55 0.50 0.45
(4,4)
(5,5)
0.40 (5,5)
(6,6)
0.35 (5,5)
0.30 5.5
6.0
6.5
7.0
(6,6)
7.5
8.0
8.5
Diameter of carbon nanotube Figure5. The variation of Homo-Lumo gap with Diameter for Cu and Ag chain encapsulated in armchair (n, n), n=4-6 CNTs
Table1. The variation of BE/atom, diameter (Å) and HOMO-LUMO gap (eV) for pristine and doped Cu and Ag armchair (n, n), n=4-6 CNTs.. S.NO 1. 2. 3. 4. 5. 6. 7. 8. 9.
Arm Chair (CNT) 4,4 5,5 6,6 Cu6@4,4 Cu6@5,5 Cu6@6,6 Ag6@4,4 Ag6@5,5 Ag6@6,6
No. of atoms
Diameter (Å)
BE/dopant atom
Gap (eV)
96 120 144 102 126 150 102 126 150
5.66 6.90 8.29 5.66 6.90 8.29 5.66 6.90 8.29
-1.840 -2.260 -2.095 -0.472 -1.679 -1.630
0.67 0.43 0.32 0.64 0.34 0.33 0.43 0.41 0.34
Also it is clear from figure 5 that values of the HOMO-LUMO gap are approximately equal for pristine and doped (6,6) CNTs. In general, from Figure 5, it can be inferred that doping has not much effect on the HOMOLUMO gap of armchair nanotubes. Endohedral doping of Si, Ge, Au, and Tl in armchair (n, n), n=4-6) has been reported by our group [47]. 4. Conclusions Structural and electronic properties of endohedrally doped armchair (n, n), n=4-6 single wall carbon nanotubes (SWCNTs) with chain of Cu and Ag atoms have been studied using ab-initio density functional theory. We have considered doping of a linear chain of six atoms inside SWCNTs of different diameters, but of same length. We have calculated BE/dopant atom, ionization potential, electron affinity and HOMO-LUMO gap of doped armchair carbon nanotubes. For undoped armchair CNTs, the BE/atom increases with the increase in diameter of tubes. In case of Cu doped CNTs BE/Cu atom increases with the increase in diameter of tubes. But in case of Ag doped CNTs
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the BE/Ag atom is maximum for (5, 5) CNT. It is observed that pristine (5, 5) CNT has a maximum value of IP. EA of undoped armchair carbon nanotubes increases with the diameter of tubes. For doped armchair CNTs, maximum IP and EA is observed for doped (6, 6) tube. The study of HOMO-LUMO gap reveals that doping has not much effect on the HOMO-LUMO gap of armchair carbon nanotubes. Acknowledgement Authors are thankful to the SIESTA group for providing the computational code and greatly acknowledge the HPCC facility provided by the Department of Physics, Panjab University, Chandigarh
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ScienceDirect Materials Today: Proceedings 3 (2016) 1820–1827
www.materialstoday.com/proceedings
Recent Advances In Nano Science And Technology 2015 (RAINSAT2015)
Study of Structural and Electronic Properties of Doped Arm Chair Single-Walled Carbon Nanotubes Shobhna Dhimana*, Ranjan Kumarb and Keya Dharamvirb 1
Department of Applied Science, PEC, University of Technology, Chandigarh, India 2 Department of Physics, Panjab University, Chandigarh-160014, India
Abstract Structural and electronic properties of endohedrally doped armchair single-wall carbon nanotubes (SWCNTs) with a chain of six atoms of Ag and Cu have been studied using ab-initio density functional theory. We investigate the binding energy/atom, ionization potential, electron Affinity and Homo-Lumo gap of doped armchair SWNTs from (4,4) to (6,6) with two ends open. BE/atom is maximum for (5, 5) doped armchair carbon nanotube; suggest that it is more stable than (4, 4) and (6, 6) doped tubes. Whereas ionization potential of Ag doped tubes is more and electron affinity is less with respect to of Cu doped tubes showing that Ag doped tubes are less reactive than Cu doped tubes. Homo- Lumo gap of doped arm chair tubes decreases exponentially with the increase in diameter of the tubes. This shows that confinement induce a strong effect on electronic properties of doped tubes. These combined systems can be used for future Nano electronics. The ab–initio calculations were performed with SIESTA code using generalized gradient approximation (GGA). © 2015Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015.]. Keywords:Density functional theory; SIESTA; Single-walled armchair carbon nanotubes; HOMO-LUMO gap.
* Corresponding author. E-mail address: [email protected] 2214-7853© 2015 Elsevier Ltd.All rights reserved. Selection and Peer-review under responsibility of [Conference Committee Members of Recent Advances In Nano Science and Technology 2015. ].
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1. Introduction Carbon nanotubes (CNTs) have exceptionally high tensile strength, high resilience and semiconductor behavior, high current carrying capacity and high thermal conductivity [1-3]. CNTs can be classified with respect to its structure by means of the roll up or the chirality vector. The chirality vector defines the roll up angle and the circumference of the CNTs. On the basis of classification based on chirality vector there exists three kinds of CNTs: armchair (n, n), chiral (n, m) 0<|m|
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a
b
c
Figure1. The optimized structures of Cu6@CNTs (a) (4, 4), (b) (5,5), (c) (6,6)
3. Results and discussion First of all we have performed structural optimization of undoped (n, n) armchair SWCNTs, n=4-6. We have optimized the structures using SIESTA code. The structures have been allowed to relax until the forces are smaller than 0.04 ev/Å. The obtained C-C bond length for (4,4), (5,5) and (6,6) armchair CNTsare1.44 Å, 1.44 Å and 1.43 Å respectively. The calculated bond length value agrees well with previously reported values of 1.43 Å for (4, 4), (5, 5) and (6, 6) CNTs [46]. In the next step we have considered an endohedralsystem by placing a chain of six dopant atoms inside armchair CNT. The chain of dopant atoms was formed by placing these atoms at a distance of about their bond length. The whole system was allowed to relax with respect to all degrees offreedomwithout additional constraints. The reason for choosing a chain of dopant owe to the cylindrical cavity available inside CNT. We have done endohedral doping in different armchair carbon nanotubes. Armchair nanotubes of fixed length 12.47Å were taken. Initially we have placed one Cu atom inside CNT. The dopant was placed at different positions inside CNT.
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We found that Cu atom at the center give rise the most stable structure. Then we have increased the number of Cu atoms up to six. The BE/dopant atom (EB) of the endohedral armchair carbon nanotube was calculated as follow:
E = (
@
−
−
)/n
a
b
c
Figure2. The optimized structures of Ag6@CNTs (a) (4,4), (b) (5, 5) ,(c) (6,6)
where @ is the total energy of endohedral complex and n is the number of dopant atoms in CNT, is the total energy of undoped CNT and is the total energy of one dopant atom. We have considered (4, 4), (5,5) and (6,6) armchair SWCNTs for endohedral doping of Cu and Ag atoms figures 1, 2 illustrate the optimized structures after doping the chain consisting of six atoms of Cu and Ag in arm chair CNTs. The values of the BE/atom for pure CNTs were found to be in the range -7.745 eV to -7.895 eV. The minimum value of BE/Cu atom is -1.840 eV and for BE/Ag atom is -0.472 eV for Cu and Ag doped (4, 4) CNTs with diameter 5.66 Å and 102 number of atoms(see table1). The maximum value of BE/Cu atom is -2.095 eV and BE/Ag atom is -1.630 eV for Cu and Ag doped (6, 6) CNTs with diameter 8.29 Å and 150 number of atoms respectively. This means that BE/dopant atom increases while going from (4, 4) to (6, 6)atom increases while going
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from (4, 4) to (6, 6) armchair carbon nanotube. Our calculations shows that (6, 6) is most suited for Cu/Ag dopingas BE/atom is maximum for the same. CNT Cu6 @CNT
(5,5)
4.8 Ionization Potential (eV)
Ag6 @CNT
4.7
(6,6)
(5,5)
(6,6)
4.6
(5,5) (6,6)
(4,4)
4.5 (4,4)
4.4
(4,4)
5.5
6.0
6.5
7.0
7.5
8.0
8.5
Diameter of carbon nanotube
Figure3. The variation of ionization potential with diameter for Cu and Ag chain encapsulated in armchair (n,n),n=4-6 CNTs 4.1 4.0 Electron affinity (eV)
(6,6)
C NT C u6@C NT Ag 6@CNT
(6,6) (5,5) (6,6)
3.9 (5,5)
3.8 (5,5)
3.7 (4,4)
3.6
(4,4)
3.5
(4,4)
5.5
6.0 6.5 7.0 7.5 8.0 Diameter of carbon nanotube
8.5
Figure4. The variation of electron affinity with diameter for Cu and Ag chain encapsulated in armchair(n,n),n=4-6 CNTs
The diameter of the tube plays an important role on the stability of the tube. In a finite nanotube, quantum confinement may induce a strong effect on the electronic properties of nanotubes. In order to have a clear insight of electronic properties, we have studied IP and EA of pure and doped CNTs. We put one extra electron on Cu6@CNT, Ag6@CNT molecule and compute the total energy. Subtracting the total energy of neutral Cu6@CNT, Ag6@CNT from total energy obtained yields electron affinity of Cu6@CNT and. Ag6@CNT. Similarly we have computed IP by making Cu6@CNT, Ag6@CNT one electron deficit. Subtracting the total energy from total energy of neutral molecule gives ionization potential of the compound. These quantities give us an idea about the chemical reactivity of endohedral CNTs. The variation of IP of and EA with tube diameter is shown in figures 3, 4. The maximum value of IP in pristine CNT is 4.823 eV and decreases with increasing diameter of tube. EA increases monotonically with diameter of the tube. The endohedral doping of linear atomic chain changes IP and EA significantly. We have found that (4, 4) armchair carbon nanotube doped with chain of Cu and Ag atoms have less values of IP and EA than pristine (4, 4) tube. The minimum value of IP and EA has been observed for doped (4, 4) carbon nanotube. Whereas maximum value of IP is observed for pristine (5, 5) carbon nanotube tube, but the doped (5, 5) CNT with metals have low value of ionization potential and higher values of electron affinity. This means that doped (5, 5) tube is more reactive as compared to pristine CNT. In case of (6, 6) CNT the IP of pure tube is greater than of doped Cu tube, but less than Ag doped tube. Whereas electron affinity for both doped tubes is greater than pristine. This suggests that Cu doped (6, 6) CNT is chemically more reactive. The concept of highest occupied molecular orbital
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(HOMO) and lowest unoccupied molecular orbital (LUMO) is an important factor in understanding the charge transfer, stability and reactivity of many organic molecules. The calculated HOMO-LUMO gap of pure and doped armchair nanotubes is shown in figure 5. On doping of Cu and Ag, HOMO-LUMO gap decreases as we move from (4, 4) to (6, 6) CNTs. 0.70 (4,4)
CNT Cu6 @CNT
Homo-Lumo gap (eV)
0.65 0.60
Ag6 @CNT
(4,4)
0.55 0.50 0.45
(4,4)
(5,5)
0.40 (5,5)
(6,6)
0.35 (5,5)
0.30 5.5
6.0
6.5
7.0
(6,6)
7.5
8.0
8.5
Diameter of carbon nanotube Figure5. The variation of Homo-Lumo gap with Diameter for Cu and Ag chain encapsulated in armchair (n, n), n=4-6 CNTs
Table1. The variation of BE/atom, diameter (Å) and HOMO-LUMO gap (eV) for pristine and doped Cu and Ag armchair (n, n), n=4-6 CNTs.. S.NO 1. 2. 3. 4. 5. 6. 7. 8. 9.
Arm Chair (CNT) 4,4 5,5 6,6 Cu6@4,4 Cu6@5,5 Cu6@6,6 Ag6@4,4 Ag6@5,5 Ag6@6,6
No. of atoms
Diameter (Å)
BE/dopant atom
Gap (eV)
96 120 144 102 126 150 102 126 150
5.66 6.90 8.29 5.66 6.90 8.29 5.66 6.90 8.29
-1.840 -2.260 -2.095 -0.472 -1.679 -1.630
0.67 0.43 0.32 0.64 0.34 0.33 0.43 0.41 0.34
Also it is clear from figure 5 that values of the HOMO-LUMO gap are approximately equal for pristine and doped (6,6) CNTs. In general, from Figure 5, it can be inferred that doping has not much effect on the HOMOLUMO gap of armchair nanotubes. Endohedral doping of Si, Ge, Au, and Tl in armchair (n, n), n=4-6) has been reported by our group [47]. 4. Conclusions Structural and electronic properties of endohedrally doped armchair (n, n), n=4-6 single wall carbon nanotubes (SWCNTs) with chain of Cu and Ag atoms have been studied using ab-initio density functional theory. We have considered doping of a linear chain of six atoms inside SWCNTs of different diameters, but of same length. We have calculated BE/dopant atom, ionization potential, electron affinity and HOMO-LUMO gap of doped armchair carbon nanotubes. For undoped armchair CNTs, the BE/atom increases with the increase in diameter of tubes. In case of Cu doped CNTs BE/Cu atom increases with the increase in diameter of tubes. But in case of Ag doped CNTs
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the BE/Ag atom is maximum for (5, 5) CNT. It is observed that pristine (5, 5) CNT has a maximum value of IP. EA of undoped armchair carbon nanotubes increases with the diameter of tubes. For doped armchair CNTs, maximum IP and EA is observed for doped (6, 6) tube. The study of HOMO-LUMO gap reveals that doping has not much effect on the HOMO-LUMO gap of armchair carbon nanotubes. Acknowledgement Authors are thankful to the SIESTA group for providing the computational code and greatly acknowledge the HPCC facility provided by the Department of Physics, Panjab University, Chandigarh
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