The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors
We present ab initio calculations on the band structure and density of states of single wall semiconducting carbon nanotubes with high degrees (up to 25%) of B, Si and N substitution. The doping process consists of two phases: different carbon nanotubes (CNTs) for a constant doping rate and differen...
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Tetik, E. 2019-06-14T10:34:18Z 2019-06-14T10:34:18Z 2014 The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors / E. Tetik // Condensed Matter Physics. — 2014. — Т. 17, № 4. — С. 43301: 1–12. — Бібліогр.: 32 назв. — англ. 1607-324X DOI:10.5488/CMP.17.43301 arXiv:1501.02339 PACS: 31.15.A-, 61.48.De, 07.07.Df, 74.62.Dh https://nasplib.isofts.kiev.ua/handle/123456789/153474 We present ab initio calculations on the band structure and density of states of single wall semiconducting carbon nanotubes with high degrees (up to 25%) of B, Si and N substitution. The doping process consists of two phases: different carbon nanotubes (CNTs) for a constant doping rate and different doping rates for the zigzag (8, 0) carbon nanotube. We analyze the doping dependence of nanotubes on the doping rate and the nanotube type. Using these results, we select the zigzag (8, 0) carbon nanotube for toxic gas sensor calculation and obtain the total and partial densities of states for CNT (8, 0). We have demonstrated that the CNT (8, 0) can be used as toxic gas sensors for CO and NO molecules, and it can partially detect Cl₂ toxic molecules but cannot detect H₂S. To overcome these restrictions, we created the B and N doped CNT (8, 0) and obtained the total and partial density of states for these structures. We also showed that B and N doped CNT (8, 0) can be used as toxic gas sensors for such molecules as CO, NO, Cl₂ and H₂S. Представлено ab initio обчислення зонної структури та густини станiв напiвпровiдникових вуглецевих нанотрубок з однiєю стiнкою, що володiють високими ступенями (аж до 25%) замiщення B, Si i N. Процес легування складається з двох етапiв, а саме, рiзнi вуглецевi нанотрубки для сталої швидкостi легування та рiзнi швидкостi легування для зигзагоподiбної (8, 0) вуглецевої нанотрубки. Проаналiзовано залежнiсть легування нанотрубок вiд швидкостi легування i вiд типу нанотрубки. На основi цих результатiв вибрано зигзагоподiбну (8, 0) вуглецеву нанотрубку для обчислення датчика токсичного газу та отримано повнi i парцiальнi густини станiв вуглецевих нанотрубок (8, 0). Показано, що вуглецева нанотрубка (8, 0) може бути використана в якостi датчикiв токсичних газiв для молекул CO i NO; вона здатна частково виявляти токсичнi молекули Cl₂, але не здатна виявляти H₂S. Щоб подолати цi обмеження, створено B i N леговану вуглецеву нанотрубку (8, 0) та отримано повну та парцiальну густини станiв цих структур. Показано, що B i N легованi вуглецевi нанотрубки можуть бути використанi в якостi датчикiв токсичних газiв для таких молекул як CO, NO, Cl₂ i H₂S. en Інститут фізики конденсованих систем НАН України Condensed Matter Physics The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors Електроннi властивостi легованих одностiнкових вуглецевих нанотрубок та датчики на вуглецевих нанотрубках Article published earlier |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine |
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DSpace DC |
| title |
The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors |
| spellingShingle |
The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors Tetik, E. |
| title_short |
The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors |
| title_full |
The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors |
| title_fullStr |
The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors |
| title_full_unstemmed |
The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors |
| title_sort |
electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors |
| author |
Tetik, E. |
| author_facet |
Tetik, E. |
| publishDate |
2014 |
| language |
English |
| container_title |
Condensed Matter Physics |
| publisher |
Інститут фізики конденсованих систем НАН України |
| format |
Article |
| title_alt |
Електроннi властивостi легованих одностiнкових вуглецевих нанотрубок та датчики на вуглецевих нанотрубках |
| description |
We present ab initio calculations on the band structure and density of states of single wall semiconducting carbon nanotubes with high degrees (up to 25%) of B, Si and N substitution. The doping process consists of two phases: different carbon nanotubes (CNTs) for a constant doping rate and different doping rates for the zigzag (8, 0) carbon nanotube. We analyze the doping dependence of nanotubes on the doping rate and the nanotube type. Using these results, we select the zigzag (8, 0) carbon nanotube for toxic gas sensor calculation and obtain the total and partial densities of states for CNT (8, 0). We have demonstrated that the CNT (8, 0) can be used as toxic gas sensors for CO and NO molecules, and it can partially detect Cl₂ toxic molecules but cannot detect H₂S. To overcome these restrictions, we created the B and N doped CNT (8, 0) and obtained the total and partial density of states for these structures. We also showed that B and N doped CNT (8, 0) can be used as toxic gas sensors for such molecules as CO, NO, Cl₂ and H₂S.
Представлено ab initio обчислення зонної структури та густини станiв напiвпровiдникових вуглецевих
нанотрубок з однiєю стiнкою, що володiють високими ступенями (аж до 25%) замiщення B, Si i N. Процес
легування складається з двох етапiв, а саме, рiзнi вуглецевi нанотрубки для сталої швидкостi легування та
рiзнi швидкостi легування для зигзагоподiбної (8, 0) вуглецевої нанотрубки. Проаналiзовано залежнiсть
легування нанотрубок вiд швидкостi легування i вiд типу нанотрубки. На основi цих результатiв вибрано
зигзагоподiбну (8, 0) вуглецеву нанотрубку для обчислення датчика токсичного газу та отримано повнi i
парцiальнi густини станiв вуглецевих нанотрубок (8, 0). Показано, що вуглецева нанотрубка (8, 0) може
бути використана в якостi датчикiв токсичних газiв для молекул CO i NO; вона здатна частково виявляти
токсичнi молекули Cl₂, але не здатна виявляти H₂S. Щоб подолати цi обмеження, створено B i N леговану
вуглецеву нанотрубку (8, 0) та отримано повну та парцiальну густини станiв цих структур. Показано, що
B i N легованi вуглецевi нанотрубки можуть бути використанi в якостi датчикiв токсичних газiв для таких
молекул як CO, NO, Cl₂ i H₂S.
|
| issn |
1607-324X |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/153474 |
| citation_txt |
The electronic properties of doped single walled carbon nanotubes and carbon nanotube sensors / E. Tetik // Condensed Matter Physics. — 2014. — Т. 17, № 4. — С. 43301: 1–12. — Бібліогр.: 32 назв. — англ. |
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2025-11-24T11:38:37Z |
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2025-11-24T11:38:37Z |
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| fulltext |
Condensed Matter Physics, 2014, Vol. 17, No 4, 43301: 1–12
DOI: 10.5488/CMP.17.43301
http://www.icmp.lviv.ua/journal
The electronic properties of doped single walled
carbon nanotubes and carbon nanotube sensors
E. Tetik
Mustafa Kemal University, Computer Science Research and Application Center, Serinyol Campus,
31100 Hatay, Turkey
Received March 23, 2014, in final form September 29, 2014
We present ab initio calculations on the band structure and density of states of single wall semiconducting
carbon nanotubes with high degrees (up to 25%) of B, Si and N substitution. The doping process consists of two
phases: different carbon nanotubes (CNTs) for a constant doping rate and different doping rates for the zigzag
(8, 0) carbon nanotube. We analyze the doping dependence of nanotubes on the doping rate and the nanotube
type. Using these results, we select the zigzag (8, 0) carbon nanotube for toxic gas sensor calculation and obtain
the total and partial densities of states for CNT (8, 0). We have demonstrated that the CNT (8, 0) can be used
as toxic gas sensors for CO and NO molecules, and it can partially detect Cl2 toxic molecules but cannot detect
H2S. To overcome these restrictions, we created the B and N doped CNT (8, 0) and obtained the total and partial
density of states for these structures. We also showed that B and N doped CNT (8, 0) can be used as toxic gas
sensors for such molecules as CO, NO, Cl2 and H2S.
Key words: ab initio calculations, carbon nanotubes structure, gas sensors, doping and substitution effects
PACS: 31.15.A-, 61.48.De, 07.07.Df, 74.62.Dh
1. Introduction
The physics of carbon nanotubes (CNTs) has evolved into a research field since their discovery [1, 2].
It is well known that carbon nanotubes could have either metallic or semiconducting properties depend-
ing on their diameters and chiralities [3–9]. In this regard, carbon nanotubes have been recommended
for several potential applications such as molecular sensors, hydrogen storage and nano-electronic de-
vices. In view of incorporating CNTs into real operational nano-devices (diodes, transistors), CNT-based
intra-molecular junctions have been studied for a long time [10–13]. The most important point in these
applications of carbon find efficient ways to control and regulate their structural and electronic proper-
ties. Foreign atom doping can be used as a feasible way to control the electrical and electronic properties
of nanotubes.
Starting from the discovery of nanotubes, many papers have been published reporting the effects of
their electronic properties by direct adsorption of atoms on CNTs [14–16]. Zhang et al. systematically stud-
ied the isomers of BN doped on (5, 5) armchair carbon nanotubes and found that the doping increases the
redox and electron excitation properties [17]. Krainara et al. investigated the quantum chemical calcula-
tions to study the electronic properties of BN-doped carbon nano-materials grafted with n-nucleophiles.
They used used the PBE density functional theory (DFT) method with the double-con with polarization
basis set and the RI approximation and examined that the charge redistribution shows charge transfer
when the BN-doped SWCNT is grafted with m-nitroaniline and pyridine [18]. Recently the researchers re-
vealed many new research areas on the CNTs. One of them contained the studies which were performed
using doped CNTs to detect the presence of some toxic chemical gases. CNTs have the potential to be
developed as a new gas sensing material due to their inherent properties such as their small size, great
strength, high electrical and thermal conductivity, high surface-to-volume ratio, and hollow structure of
nanomaterials. These advances have led to the design of a new breed of sensor devices. In addition, the
© E. Tetik, 2014 43301-1
http://dx.doi.org/10.5488/CMP.17.43301
http://www.icmp.lviv.ua/journal
E. Tetik
characteristics of CNTs have been investigated for gas molecules adsorption, such as boron doped and
boron nitride CNT.
In this work, we have reported a detailed study on the doping effect on the CNTs using ab initio DFT.
We have investigated the calculations in three steps. In the first step, we changed the carbon nanotube
chirality for the constant doping rate and found out how electronic properties of CNTs affect different
types of constant doping nanotubes. In the second step, we changed the doping rate for a specific nan-
otube and examined the effect of doping rate on the nanotube. We used the atoms which are commonly
selected such as B, Si and N for the doping process. Thus, we performed a comprehensive analysis ex-
amining the effects of doping on carbon nanotubes. In the final step, we identified a suitable nanotube
for doping and obtained a toxic nano-gas-sensor that can detect the presence of H2S, Cl2, CO and NO
molecules. Then, we examined the adsorption behaviors of the proposed pure and doped CNTs, in this
regard, calculated the adsorption energy and binding distance using the total and partial densities of
states for the CNT structures considered. We have demonstrated that these gas sensors are capable not
only of detecting the presence of these toxic gases but also the sensitivity of these sensors can be con-
trolled by the doping level of impurity atoms in a CNT.
2. The theory and computational method
In our work we used two programs which are the Tubegen code [19] and SIESTA ab initio package
[16, 20]. We relaxed the nanotubes by using the radius_conv, error_conv and gamma_conv which are
parameters in the Tubegen code and obtained the atomic position and the lattice parameters which were
used in the SIESTA input file.
Total energy and electronic structure calculations are performed via first principles density func-
tional theory, as implemented in the SIESTA. For the solution method we used the diagon which is a
parameter in the SIESTA. For the exchange and correlation terms, the local density approximation (LDA)
was used [21] through the Ceperley and Alder functional [22] as parameterized by Perdew and Zunger
[23]. The interactions between electrons and core ions are simulated with separable Troullier-Martins
[24] norm-conserving pseudo-potentials. We generated atomic pseudopotentials separately for atoms, C,
B, N, Si and O by using the 2s22p2, 2s22p1, 2s22p3, 3s23p2 and 2s22p4 configurations, respectively. For
carbon atoms, 1s state is the core state while the 2s and 2p states form the valence states. The valence
electrons were described by localized pseudo-atomic orbitals with a double-con singly polarized (DZP)
basis set [25]. Basis sets of this size have been shown to yield structures and total energies in good agree-
ment with those of standard plane-wave calculations [26]. Real-space integration was performed on a
regular grid corresponding to a plane-wave cutoff around 300 Ry, for which the structural relaxations
and the electronic energies are fully converged. We used 25 k points for the total energy calculations of
pure and doped CNTs. We relaxed the isolated CNTs until the stress tensors were below 0.04 eV/Acon3 and
calculated the theoretical lattice constant. In the band structure calculations we used 161 band k vectors
between the con and A high-symmetry points.
3. Results and discussion
We can say that all physical properties are related to the total energy. For example, the equilibrium
lattice constant of a structure is the lattice constant that minimizes the total energy. If the total energy
is calculated, any physical property related to the total energy can be determined. In this regard, we
have relaxed the zigzag (8, 0), (10, 0) and (16, 0) and obtained their equilibrium lattice parameters which
have been computed minimizing the total energy of crystals calculated for different values of lattice
constant. The calculation results of the zigzag (8, 0), (10, 0) and (16, 0) CNTs are shown in figure 1, and the
equilibrium lattice parameters for these CNTs are found to be aCNT(8,0) = 25.0227 Å, aCNT(10,0) = 24.9851 Å
and aCNT(16,0) = 24.9644 Å, respectively.
Then, we created the B, Si and N doped CNTs for constant and different doping rate. We obtained
the electronic properties of doped CNTs that were classified according to the doping rate. We discussed
the band structure and density of state of pure and doped CNTs. The properties of doped CNTs changed
43301-2
The electronic properties of doped SWCNT and CNT sensor
Figure 1. The graphics of equilibrium lattice constant and 3D diagram for CNT (8, 0) (a), CNT (10, 0) (b)
and CNT (16, 0) (c).
according to the doped atoms and the doping rate. Our ultimate goal here is to predict the exact behavior
of nanotubes depending on the doping rate and the doped atom. According to these results, the most
appropriate nanotube can be determined for other studies. Finally, we studied the interaction of CO, NO,
H2S and CI2 molecules with CNT (8, 0). We found that the partial densities of states of the pure and doped
zigzag CNT (8, 0) altered considerably after detecting the CO, NO, H2S and CI2 molecules.
3.1. Different carbon nanotubes for a constant doping rate
In figure 2 we show the band structure and density of state (DOS) for pure zigzag (8, 0), (10, 0) and
(16, 0) nanotubes that include 32, 40 and 64 carbon atoms, respectively. The Fermi level is set to zero
in figure 2 and in all other graphics of band structure and density of state. We can see that all zigzag
nanotubes exhibit semiconductor properties. Their semiconducting energy gaps are Eg = 0.6643, 0.8927
and 0.5914 eV, respectively (table 1). Pure zigzag (10, 0) nanotube has the largest band gap.
Figure 3 contains the electronic dispersion and DOS for B doped zigzag (8, 0), (10, 0) and (16, 0) nan-
otubes that include 4, 5 and 8 B atoms, respectively (table 1). Doping rate of these nanotubes is 12.5%.
We have used a high doping rate because we can see more clearly the effects of doping. If we examine
the DOS for three different semiconducting nanotubes, we show that the peaks are near the Fermi level.
Those peaks display the 1-D Van Hove Singularities (VHSs) pattern. For undoped nanotubes, the DOS is
symmetric around the band gap for the 1st and 2nd VHSs. However, beyond this small energy regime
Figure 2. Band structure and density of state of zigzag (a) (8, 0), (b) (10, 0) and (c) (16, 0) nanotubes.
43301-3
E. Tetik
Figure 3. Band structure and density of state of 12.5% B doped zigzag (a) (8, 0), (b) (10, 0) and (c) (16, 0)
nanotubes.
around the band gap, asymmetries arise due to the mixing of p and s orbitals. B doping leads to low-
ering of the Fermi level into the valence band of the undoped tube. Above the highest occupied bands
of an undoped tube, new bands are formed. These bands correspond to the formation of an acceptor
level in semiconductors having very low dopant concentration. The shift of the Fermi level for the zigzag
(8, 0), (10, 0) and (16, 0) nanotube is around 1.36076 eV, 1.43003 eV and 1.55763 eV, respectively (table 2).
Due to Fermi shift, the number of free electrons in a conduction band of B doped nanotubes increases.
Therefore, conductivity of these nanotubes increases as well.
Table 1. The band gap (eV) of pure and doped (B, Si and N) zigzag (8, 0), (10, 0) and (16, 0) nanotube.
Material Pure (present) Pure (reference) B Doped Si Doped N Doped CNT/Doped Atoms
CNT (8, 0) –0.6643 –0.47 [27] –0.7761 –0.4929 –0.8263 32/4
CNT (10, 0) –0.8927 –0,75 [28], –0.98 [28] –0.6872 –1.0838 –0.7702 40/5
CNT (16, 0) –0.5914 –0.62 [29] –0.5634 –1.0089 –0.6430 64/8
To investigate the effects of doping on the electronic properties of nanotubes, we have performed
similar calculations for zigzag (8, 0), (10, 0) and (16, 0) nanotube using Si and N atoms. We show the
electronic band structure and DOS of Si and N doped nanotubes in figures 4 and 5, respectively. Carbon
and silicon are in the same periodic group and the electron configurations are very close to each other.
Therefore, the Si doped nanotubes do not have a shift of the Fermi level. We show that the band gap of
the Si doped nanotubes changed significantly (table 2). The band gap of these nanotubes is −0.4929 eV,
−1.0838 eV and −1.0089 eV and is fixed around 1 eV. There is a great change in the band gap of the N
doped nanotubes. As we can see in figure 5, the shift of the Fermi level for the N doped tubes is around
0.95805 eV, 1.03256 eV and 1.23198 eV, respectively (table 2), which is a clear indication that the number
of free free electrons of N doped semiconducting nanotubes decreases. As a result, these nanotubes are a
finite conductance and form N-type
Table 2. The shift of the Fermi level for the B, Si and N doped zigzag (8, 0), (10, 0) and (16, 0) nanotube.
Material B Doped (A) Si Doped (A) N Doped (A)
CNT (8, 0) 1.36076 0.0 –0.95805
CNT (10, 0) 1.43003 0.0 –1.03256
CNT (16, 0) 1.55763 0.0 –1.23198
43301-4
The electronic properties of doped SWCNT and CNT sensor
Figure 4. Band structure and density of state of 12.5% Si doped zigzag (a) (8, 0), (b) (10, 0) and (c) (16, 0)
nanotubes.
Figure 5. Band structure and density of state of 12.5% N doped zigzag (a) (8, 0), (b) (10, 0) and (c) (16, 0)
nanotubes.
3.2. Different doping rates for zigzag (8, 0) carbon nanotubes
Secondly, we have studied the electronic band structure and DOS for a zigzag (8, 0) nanotube using
different doping rates (3.125%, 6.25%, 12.5% and 25%). We have calculated that the numbers of B atoms
corresponding to these doping rates are 1, 2, 4 and 8, respectively. The results for the doped zigzag (8, 0)
nanotubes which include 1, 2, 4 and 8 B atoms (according to the doping rate) are presented in table 3.
Moreover, we show the band structure and DOS of the B doped zigzag (8, 0) nanotubes in figure 6 accord-
ing to the doping rate. If we analyze these results, the number of free electrons of nanotubes increases
when the doping rate is increased. On the other hand, the Fermi level shifts downward into the con-
duction band of the doped semiconducting zigzag (8, 0) nanotubes according to the doping rate. Due to
the shift of the Fermi level the boron levels hybridize with the carbon levels forming highly dispersive
acceptor-like bands and consequently lending a metallic property to the nanotubes. In addition to this
study, we use Si and N atoms which are commonly used in the doping process for nanotubes since we
can show the effect on the doping rate of different atoms. The calculation results regarding the band gap
of CNT-Si and CNT-N are shown in table 1. Then, we obtain the changes in the band structure and DOS of
Si doped nanotube for different doping rates in figure 7. We show that the band gap of the Si doped (8, 0)
nanotubes increases according to the doping rate. When the doping rate becomes closer to 25%, the band
gap greatly increases. Although the doping rate is changed, it is seen that the Fermi level of the Si doped
Table 3. The band gap of doped (B, Si and N) zigzag (8, 0) nanotube for Different Doping Rates.
Material 3.125% (A) (1 atom) 6.25% (A) (2 atoms) 12.5% (A) (4 atoms) 25% (A) (8 atoms)
CNT-B (8, 0) –0.2862 –0.3831 –1.3984 –2.0600
CNT-Si (8, 0) –0.4529 –0.4529 –0.4929 –0.9292
CNT-N (8, 0) –0.3973 –0.2729 –0.8263 –0.9521
43301-5
E. Tetik
Figure 6. Band structure and density of state of (a) 3.125%, (b) 6.25%, (c) 12.5% and (d) 25% B doped zigzag
(8, 0) nanotubes.
(8, 0) nanotubes does not shift. Since C and Si atoms belong to the same group, this effect has arisen.
The band structure and DOS of N doped nanotubes for different doping rates is evaluated and graph-
ical results are presented in figure 8. In the N doped nanotubes, the Fermi level shifts upward into the
valence band according to the doping rate. When the doping rate increases, the number of free electrons
of these nanotubes decreases and, as a result, the conductance becomes weak. We can say that the shift
Figure 7. Band structure and density of state of (a) 3.125%, (b) 6.25%, (c) 12.5% and (d) 25% Si doped
zigzag (8, 0) nanotubes.
43301-6
The electronic properties of doped SWCNT and CNT sensor
Table 4. The shift of the Fermi level of a doped (B, Si and N) zigzag (8, 0) nanotube for Different Doping
Rates.
Material 3.125% (A) (1 atom) 6.25% (A) (2 atoms) 12.5% (A) (4 atoms) 25% (A) (8 atoms)
CNT-B (8, 0) 0.30635 0.74464 1.35138 1.71388
CNT-Si (8, 0) 0.0 0.0 0.0 0.0
CNT-N (8, 0) –0.58724 –0.85205 –0.95805 –1.69396
of the Fermi level strongly depends on the geometry of the structure and the dopant concentration. The
shifts of the Fermi level for all nanotubes are presented in table 4. As a result, the group of the doped
atom affects the position of the Fermi level. We have compared the calculation results with the SWCNT
upon boron substitution research by G. Fuentes et al. [30]. G. Fuentes examined the (16, 0) nanotube for
different doping rates (0%, 6.25%, 12.5% and 25%) in the theoretical part of the research. According to
the calculation results, the stronger the doping, the bigger the shift of the Fermi level is. For B doping rate
between 0 and 25%, the shift of the Fermi level reaches 2.2 eV. In this study, the level shifts for (8, 0) and
(16, 0), CNT reaches 1.71 eV (25%) and 1.55 eV (12.5%), respectively. We show that both studies indicate
similar results according to the shift level.
Figure 8. Band structure and density of state of (a) 3.125%, (b) 6.25%, (c) 12.5% and (d) 25%Ndoped zigzag
(8, 0) nanotubes.
3.3. Zigzag (8, 0) carbon nanotube sensors
In this section, it is proved that zigzag (8, 0) CNT is a suitable nanotube for gas sensor applications.
We relaxed the zigzag (8, 0) and obtained their equilibrium lattice parameters for all toxic molecules
(figure 9). Since the changing properties of a nanotube after doping are quite stable, zigzag (8, 0) nanotube
can be used as a gas sensor. In this respect, we investigate the interaction of CO, NO, Cl2 and H2S toxic
molecules with a zigzag (8, 0) nanotube. The total and partial densities of states of toxic molecule and
pure zigzag (8, 0) nanotube are illustrated in figures 9 and 10.
Partial densities of states consist of the s, p and d states of CNT (8, 0) and toxic molecule. In figure 9,
we show a remarkable peak in the Fermi level of CO and NO molecules. This peak is generated by the
43301-7
E. Tetik
Figure 9. (Color online) The graphics of equilibrium lattice constant for pure CNT [CO (a), NO (b), Cl2 (c)
and H2S (d)] and BN doped CNT [CO (e), NO (f), Cl2 (g) and H2S (h)].
CO (a) and NO (b) toxic molecules. As we can see in figure 9 (a), the lowest valence bands that occur
between about −20 and −11 eV are dominated by CO (C and O total) 2s and 3d states while valence
bands occurring between about −10 and −2 eV are dominated by CO 2p states. The highest occupied
valence bands are essentially dominated by CO 2p states occurring between about −2 and +2 eV. Similar
properties also apply to NO doped CNT (8, 0) nanotube in figure 9 (b). For this nanotube, the highest
occupied valence bands are dominated by NO 2p states occurring between about −2.1 and +1.2 eV. Since
interaction nanotubes with CO and NO have a high peak in the Fermi level due to CO and NO 2p states,
the conductivity of CNT (8, 0) nanotube considerably increases.
Secondly, we examine the interaction of Cl2 and H2S toxic molecules with a zigzag (8, 0) nanotube
in figure 10. In both studies, the lowest valence bands occur between about −20 and −1 eV which are
dominated by 2s, 2p and 3d states (see figure 10) of Cl2 and molecules. The states of Cl2 molecule also
contribute to the valence bands in the Femi level, but the values of densities of these states are very
small compared to CO and NO 2p states. Therefore, we think that the possibility of applying the CNT (8,
0) nanotube as toxic gas sensor for Cl2 is very low [figure 10 (a)]. Figure 10 (b) contains the interaction
of H2S toxic molecules with a zigzag (8, 0) nanotube. The states of H2S molecule do not contribute to the
valence bands in the Femi level. For this reason, CNT (8, 0) nanotube is not applicable for toxic gas sensors
Figure 10. The total and projected density of states for adsorption of (a) CO and (b) NO on pure zigzag (8,
0) nanotube. The position of the Femi level is set to zero.
43301-8
The electronic properties of doped SWCNT and CNT sensor
Figure 11. The total and projected density of states for adsorption of (a) Cl2 and (b) H2S on pure zigzag (8,
0) nanotube. The position of the Femi level is set to zero.
for H2S.
To investigate the effects of doping on toxic molecules, we optimize the B and N doped CNT (8, 0) and
realize a similar calculation for this structure. According to researches, undoped nanotubes cannot detect
all toxic gas molecules because some toxic molecules do not absorb on the surface of a nanotube [31]. To
overcome these limitations of pure SWCNTs, diverse external or internal process schemes can be used
[32]. We examine the interaction of CO, NO, Cl2 and H2S toxic molecules with the B and N doped zigzag (8,
0) nanotube. The total and projected density of states for these structures can be observed in figures 11
and 12, respectively. The results of calculation are close to the results of a pure nanotube, but the peaks
of a doped nanotube are more distinct and higher than pure CNT (8, 0). Remarkable peaks in the Fermi
level of Cl2 and H2S molecules are shown in these figures. Similarly, conductivity of a doped CNT (8, 0)
nanotube considerably increases. This is due to the interaction between the toxic molecules of a doped
CNT. The 2p orbital of the nanotube and toxic molecules create bonds which cause degenerate levels in a
nanotube. In this case, the acceptor levels of a nanotube occur, and it is concluded that doped nanotubes
can be used as effective toxic gas sensors for CO, NO, Cl2 and H2S (see figures 11 and 12). Consequently,
CNT (8, 0) nanotube can be used as toxic gas sensors for CO and NO due to their electronic structure
and chemical properties. This nanotube can be used in part for Cl2 molecule, because contribution to
the valence band is very small. For H2S toxic molecule, contribution to the valence band of CNT (8, 0)
nanotube is zero. Therefore, pure CNT (8, 0) cannot be used as toxic gas sensors while the B and N doped
CNT (8, 0) can be used as toxic gas sensors for H2S molecule.
The geometrical and chemical characteristics of atoms in the CO and NO molecule are very close to
the properties of a carbon atom. Hence, toxic gases such as CO and NO can be detected by the CNT (8, 0)
Figure 12. The total and projected density of states for adsorption of (a) CO and (b) NO on a doped zigzag
(8, 0) nanotube. The position of the Femi level is set to zero.
43301-9
E. Tetik
Figure 13. The total and projected density of states for adsorption of (a) Cl2 and (b) H2S on a doped zigzag
(8, 0) nanotube. The position of the Femi level is set to zero.
nanotube. Atoms in the Cl2 and H2S molecules are far from the carbon atom. In this regard, nanotubes
cannot detect such molecules as Cl2 and H2S or can detect them partially. To overcome these restrictions,
we can implement different doping procedures (such as B and N) for nanotubes.
Moreover, we have investigated the optimum adsorption behaviors and an optimum binding distance
of the proposed CNT structures sincewe finalize better the physical mechanism of the operation principle
of the CNT gas sensors for toxic chemical gases. In this regard, the adsorption energy (Ead) is defined as
the difference between the energy of the CNT + molecule(s) system, minus the sum of the energies of
the CNT and of isolated molecule(s) in the same conditions. We have calculated the adsorption energy of
the system as a function of the distance between the gas molecule and the surface of the CNT wall. The
adsorption energy is calculated using the absorption equation [Ead = (Ecnt +Egas)−Ecnt-gas]. In this way,
we have obtained the adsorption energy and the binding distance of the pure and BN doped CNT (8, 0).
In our calculations we show that the pure CNT exhibits a much enhanced binding with NO and CO,
the adsorption energy and the binding distance between CNT and molecules being −0.394, −0.243 eV
and 2.87, 2.92 Å, respectively. However, calculations for Cl2 and H2S show a low adsorption energy and
binding distance between CNT and molecules which are −0.095, −0.071 eV and 2.98, 3.12 Å, respectively.
We can say that a pure CNT (8, 0) is suitable for NO and CO and can be used as a gas sensor. If we examine
the doped CNT, our calculations show that the BN-CNT exhibits large adsorption energy values for all
proposed toxic chemical gases. The adsorption energy and binding distance are −6.284, −5.591, −0.131,
−0.121 eV and 1.63, 1.71, 1.85, 2.23 Å for NO, CO, H2S and Cl2 gases, respectively. Compared to a pure
CNT, BN-CNT shows a higher adsorption energy with respect to all the gas molecules considered. These
results are in good agreement with the graphics of total and partial densities of states and show that the
the BN-CNT can be used as a gas sensor for the proposed gases.
4. Conclusions
In the present work, we have made a detailed investigation of structural and electronic properties of
pure and doped SWCNTs consisting of zigzag (8, 0), zigzag (10, 0) and zigzag (16, 0) nanotubes using the
density functional methods. The results of structural optimization implemented using the LDA are in good
agreement with the other theoretical results. At the first stage, we calculated pure and doped nanotubes
and investigated the doping effects on carbon nanotubes. At the next step, we found out that the total
and partial densities of states of the SWCNT (8, 0) nanotube altered considerably after detecting the CO,
NO, Cl2 and H2S toxic molecules. Since we can clearly see the effects of doping, the doping process was
performed in two stages: different carbon nanotubes for a constant doping rate and for different doping
rates for zigzag (8, 0) carbon nanotubes. Since carbon nanotubes have been recommended for several
potential applications such as molecular sensors and nano-electronic devices, we think that the results on
the doping rate and type of nanotubes can contribute to subsequent theoretical and experimental studies.
Later on, we chose a zigzag (8, 0) nanotube for toxic molecules and investigated the interaction of these
43301-10
The electronic properties of doped SWCNT and CNT sensor
toxic molecules with a zigzag (8, 0) nanotube. There is a transfer of charge from carbon to toxic molecules
according to the calculation results, but this condition is valid for the atoms close to a carbon atom.
Hence, we can say that nanotubes can be used as toxic gas sensors for CO and NO molecules. However,
nanotubes cannot detect H2S and can detect Cl2 toxic molecules partially. Therefore, we created the B
and N doped CNTs and obtained the total and partial densities of states of these structures. According to
these results, B and N doped CNT (8, 0) can be used as toxic gas sensors for such molecules as Cl2 and
H2S. In the final stage, we investigated the adsorption behaviors for all toxic molecules. We show that the
adsorption results are in good agreement with the graphics of total and partial densities of states. Since
there are no experimental data available for the doping rate and toxic gas sensors, we think that an ab
initio theoretical estimation is the only reasonable tool to obtain such important data. Consequently, the
proposed doping rate methodology and toxic gas sensors can be used in experimental studies and for the
purpose of fabricating gas sensors.
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http://dx.doi.org/10.1038/363603a0
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http://dx.doi.org/10.1103/PhysRevLett.68.1579
http://dx.doi.org/10.1103/PhysRevB.45.6234
http://dx.doi.org/10.1103/PhysRevB.47.5485
http://dx.doi.org/10.1021/ar010155r
http://dx.doi.org/10.1016/S0301-0104(02)00376-2
http://dx.doi.org/10.1103/PhysRevB.49.5643
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http://dx.doi.org/10.1021/jp0358578
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http://dx.doi.org/10.1063/1.2201738
E. Tetik
Електроннi властивостi легованих одностiнкових
вуглецевих нанотрубок та датчики на вуглецевих
нанотрубках
Е. Тетiк
Унiверситет Мустафи Кемаля, Центр дослiджень i застосування комп’ютерних наук,
31100 Хатай, Туреччина
Представлено ab initio обчислення зонної структури та густини станiв напiвпровiдникових вуглецевих
нанотрубок з однiєю стiнкою, що володiють високими ступенями (аж до 25%) замiщення B, Si i N. Процес
легування складається з двох етапiв, а саме, рiзнi вуглецевi нанотрубки для сталої швидкостi легування та
рiзнi швидкостi легування для зигзагоподiбної (8, 0) вуглецевої нанотрубки. Проаналiзовано залежнiсть
легування нанотрубок вiд швидкостi легування i вiд типу нанотрубки. На основi цих результатiв вибрано
зигзагоподiбну (8, 0) вуглецеву нанотрубку для обчислення датчика токсичного газу та отримано повнi i
парцiальнi густини станiв вуглецевих нанотрубок (8, 0). Показано, що вуглецева нанотрубка (8, 0) може
бути використана в якостi датчикiв токсичних газiв для молекул CO i NO; вона здатна частково виявляти
токсичнi молекули Cl2, але не здатна виявляти H2S. Щоб подолати цi обмеження, створено B i N леговану
вуглецеву нанотрубку (8, 0) та отримано повну та парцiальну густини станiв цих структур. Показано, що
B i N легованi вуглецевi нанотрубки можуть бути використанi в якостi датчикiв токсичних газiв для таких
молекул як CO, NO, Cl2 i H2S.
Ключовi слова: ab initio обчислення, структура вуглецевих нанотрубок, датчики газу, ефекти легування
та замiщення
43301-12
Introduction
The theory and computational method
Results and discussion
Different carbon nanotubes for a constant doping rate
Different doping rates for zigzag (8, 0) carbon nanotubes
Zigzag (8, 0) carbon nanotube sensors
Conclusions
|