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A computational NMR study of nitrogen substitutional impurity in the armchair BeO nanotube
Superlattices and Microstructures 51 (2012) 363–371
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
A computational NMR study of nitrogen substitutional impurity in the armchair BeO nanotube Goodarz Mohseni Roozbahani ⇑, Ahmad Seif Department of Chemistry, Boroujerd Branch, Islamic Azad University, Boroujerd, Iran
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 4 December 2011 Accepted 23 December 2011 Available online 29 December 2011 Keywords: Beryllium monoxide nanotubes Density functional theory Nitrogen doped
a b s t r a c t The properties of nitrogen doped model of (5, 5) armchair beryllium monoxide nanotubes (BeONTs) have been investigated by density functional theory (DFT) and chemical shift parameters were calculated. A BeONT consisting of 60 Be, 60 atoms of O, and having a length of 1.67 nm was considered and each end of the nanotube was capped by 10 hydrogen atoms. The calculated results indicate that by replacing an O atom by N atom (NO-doping), the chemical shift (CS) parameters of 9Be and 17O atoms are un-affected but replacing a Be atom with N (NBe-doping) affects the CS parameters of O atoms. These results imply that role of nitrogen as an electron acceptor is more significant in the structure for which it dopes a Be atom. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Since their discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have become one of the most important branches of research in nanomaterial science for their special properties [2,3]. Since then, a considerable number of inorganic nanotubes such as BN [4] and SiC [5], have been prepared and some other ones such as B2O [6], BeO [7,8] have been predicted theoretically. Due to their special physical properties, the nanotubes have potential application in the fields of electronic components, molecular devices, and particle transportation among others [9]. Most of the inorganic nanotubes consist of elements from groups III and V of the periodic table such as BN, BP [10,11] and recently groups II and VI such as BeO [7,8]. Beryllium oxide (BeO) nanotubes, as a new inorganic non-carbon nanotube, has been studied by ab initio calculation and its structure and physical properties (such as binding energy, the electronic band ⇑ Corresponding author. E-mail address: [email protected] (G.M. Roozbahani). 0749-6036/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2011.12.006
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structure, the density of states, the dependence of the strain energy of the nanotube on the nanotube diameter D, and the Young’s modulus Y) have been calculated [7]. The dependence of properties such as atomic relaxation, strain energy, and electronic structure of BeO nanotube on its diameter is also established [8]. Further, it is suggested that BeO nanotubes can be synthesized by plasma-chemical reaction or through chemical vapor deposition [7]. It is known now that the band gap of BeONTs is larger than that of BN and SiC NTs; BeONTs is predicted to be an insulator and BN and SiCNTs are predicted to be semiconductors. Recently, the effect of boron, carbon and nitrogen doping in the electronic structure and magnetization of BeONTs was studied by ab initio band structure calculations [12]. This work showed that the nonmagnetic BeONTs transform into magnetic semiconductors in the presence of those impurities because B, C, and N dopants place additional electrons in the forbidden gap of a BeONT. Nuclear magnetic resonance (NMR) spectroscopy is one of the most important techniques to study the electronic structure properties of matters [13–15]. Nuclei with spin angular momentum greater than zero, for example 29Si, 13C, 11B, 15N, 71Ga, and 27Al are active in NMR spectroscopy and detect changes due to any doping or impurities [16–19]. Here, the influence of nitrogen doping on the electronic structure properties of the (5, 5) armchair single-wall BeONT having a length of 1.67 nm and a total of 140 atoms is investigated by the density functional theory (DFT) calculations of the 9Be and 17O NMR chemical shift (CS) tensors. We considered three models of (5, 5) single-wall BeONTs, the pristine (no. 1), and two other N-doped models:
Fig. 1. Pristine model. (a) Because of mirror structure of armchair nanotube, we have indicated only one part of these nanotubes. (b) Red circles are oxygen and yellow circles are beryllium. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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N replacing a Be (no. 2), or an oxygen (no. 3) in the nanotube. After optimizing the geometry of pristine BeONT, chemical shifts tensors are determined and converted to isotropic chemical shielding (ICS) and anisotropic chemical shielding (ACS) parameters. Doping of N in models no. 2 and 3 were carried out on the optimized geometry of model no. 1. Similar calculations of ICS and ACS are done for models nos. 2 and 3 and comparison of results of doped models vs. pristine model is presented. 2. Models and computational details For this investigation of the pristine and doped BeONTs, we chose the following three atomic models of (5, 5) armchair single-wall BeONTs, each being 1.67 nm in length. Model no.1 is pristine consisting of 60 Be, 60 O and 20 H atoms (Be60O60H20), no. 2 has one Be atom doped by one N atom at the sixth layer (Be59NO60H20), from now on designated as NBe, and no. 3 has one O atom doped by one N atom at the sixth layer (Be60O59NH20), call it NO. These structures are shown in Figs. 1–3. In all the three models two ends of tubes are capped by H atoms (each end by 10 H) to prevent dangling bonds, and each model consists of 12 layers. First, the structures have been allowed to relax by all atomic geometrical optimization at the DFT level B3LYP and 6-31G⁄ standard basis set using GAUSSIAN 09 package [20,21]. The optimization processes have yielded optimized bond distances (Table 1), band gap energies (EG), total energies (ET), dipole moments (DM), and spin multiplicity (Table 2). Then, the CS parameters have been calculated in the optimized structures using the same level of theory and the
61 67
60 66 65
71 70 77 76
75 81 80 86 85 84 93 92
N 95 99
98
101
100 105
107
106 111
110
119 118 117
Fig. 2. NBe model. (a) Because of mirror structure of armchair nanotube, we have indicated only one part of these nanotubes. (b) Red circles are oxygen, yellow circles are beryllium and blue circles are nitrogen. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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60 59
53 52
51
47 46
43 42
41 37 36 34 33 32 27 26
N 24 23
22
20
19
12
11
15 9
10 3
1
2
Fig. 3. NO model. (a) Because of mirror structure of armchair nanotube, we have indicated only one part of these nanotubes. (b) Red circles are oxygen, yellow circles are beryllium and blue circles are nitrogen. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The optimized structural propertiesa.
a b
Property
Pristine
NBe model
NO model
Be–O length/Å N–Be23 length/Å N–Be27 length/Å N–Be33 length/Å N–O85 length/Å N–O93 length/Å N–O99 length/Å O–H length/Å Be–H length/Å Tip diameter/Å
1.53–1.57
1.45–1.66
1.53–1.57 1.62 1.64 1.62
0.98 1.38 7.48
2.51b 1.37 1.37 0.95 1.38 8.16
0.98 1.38 7.43
See Figs. 1–3. This bond is broken.
resulting data are given in Tables 3 and 4. The CS tensors were calculated by the GIAO (gauge included atomic orbital) method [22]. We adopted the same size models as those of Ref. [12]. The quantum chemical calculations yield the CS tensors in the principal axis system (PAS) with the order of
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d33 > d22 > d11: therefore, Eqs. (1) and (2) are used to calculate the isotropic (ICS) and anisotropic (ACS) Parameters [23,24].
ICS ðppmÞ ¼ ðd33 þ d22 þ d11 Þ=3 ACS ðppmÞ ¼ d33 ðd11 þ d22 Þ=2
ð1Þ ð2Þ
3. Results and discussion 3.1. The optimized structures Table 1 presents the optimized structural properties for the pristine and N-doped models of the above mentioned (5, 5) armchair BeONTs. The results indicate that the values of Be–O bond lengths in NBe model are affected by N-doping while in the case of NO model, no such effect is seen. The effects of N-doping on the tip diameters are more obvious for NBe model. In comparison with the pristine model (Table 2), the calculated values of energy band gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) indicate that the values are significantly decreased in both two NBe and NO models. This trend reveals that the N-doping has significant effect on the energy band gap of N-doped armchair BeONT which can lead to better electroconductivity and better magnetization; this result is in agreement with the results found in Ref. [12].In comparison with data in Ref. [12] band gap energy between HOMO and LUMO in the pristine model is decreased while in the doped model is increased, obviously this trend is because in this work, all of the atoms of both mouths of the nanotube were capped by hydrogen atoms to saturate the dangling bonds and to stabilize the tubes, which can lead to better interaction between atomic orbitals. Moreover, the calculated energies also indicate that NBe model is more stable than pristine model because its total energy is less than that of the pristine NT. On the contrary, total energy of NO model is increased in comparison with the pristine model. The latter implies that stability of NO model is less than that of the pristine model. Obviously, one reason for more stability of NBe model is a broken N–O bond which is between layers 6 and 7 (see Table 1 and Fig. 2) which leads to a decrease of strain energy. Another reason for more stability of NBe is that tip diameter of NBe model is larger than that of NO model, and increasing of diameter, at least in a portion of nanotube, could lead to a decrease of strain energy- this is corroborated by the results of Ref. [7]. Also of significance is that two N-doped models have dipole moments while in the pristine model does not. Therefore, insertion of N impurity leads to polarity, meanwhile insertion of N impurity leads to increase of spin from singlet to doublet. 3.2. The 9Be NMR parameters Table 3 shows the calculated NMR parameters for all 9Be atoms in the pristine and N-doped models of BeONTs. Pristine model consists of 60 Be atoms and NBe consist of 59 Be atoms and one N atom, while NO model also consists of 60 Be atoms. The 60 Be atoms are divided into 12 layers (each layer consists of five Be atoms) with equivalent electrostatic properties, for convenience O atoms are omitted (see Figs. 1–3). No significant difference in ICS is observed in NO model, not even in the values of those Be atoms that are directly bonded to N impurity (these Be atoms are numbers 23, 27 and 33), however a little difference is observed in the ACS for Be33, Be27. For comparison purposes, the values Table 2 Band gap energy (EG), total energy (ET), dipole moment (DM) and spin multiplicity for the pristine and N-doped BeONTs. Property
Pristine
NBe model
NO model
HOMO–LUMO Gap/eV Energy (KeV) Dipole momentum (Debye) Spin multiplicity
3.14 64.34 0 Singlet
2.21 64.82 6.43 Doublet
2.33 64.1 0.33 Doublet
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G.M. Roozbahani, A. Seif / Superlattices and Microstructures 51 (2012) 363–371 Table 3 The 9Be NMR parametersa. Nuclei 9Be
ICS (ppm)
ACS (ppm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
105,105,107 105,105,107 105,105,107 105,105,107 105,105,107 107,107,107 107,107,107 107,107,107 107,107,107 107,107,107 107,106,107 107,107,107 107,107,107 107,107,107 107,106,107 106,106,107 106,106,107 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,103 106,106,106 106,107,106 106,106,106 106,106,106 106,106,106 106,106,104 106,106,107 106,107,106 106,107,106 106,107,106 106,107,106 106,107,104 106,107,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 107,107,106 107,107,107 107,107,107 107,107,107 106,107,105 106,107,105 106,107,105 106,107,107 106,107,107 106,107,107 105,105,105
14,16,13 14,15,13 14,16,13 14,16,13 14,16,13 15,16,15 15,16,15 15,16,15 15,16,15 15,17,15 14,16,14 14,14,14 14,13,14 14,13,14 14,15,14 14,16,14 14,14,14 14,14,14 14,14,13 14,14,13 14,12,14 15,27,15 15,16,15 15,14,15 15,14,15 15,14,16 15,14,31 15,23,15 15,28,15 15,14,15 15,14,15 15,14,15 15,14,40 15,14,18 15,22,15 15,14,15 15,14,15 15,14,15 15,31,31 14,24,15 14,17,15 14,12,13 14,14,13 14,14,13 14,14,13 14,16,12 14,13,13 14,13,13 14,13,13 14,16,13 14,16,13 14,16,13 14,16,15 14,16,15 14,16,15 14,16,13
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Table 3 (continued) Nuclei 9Be
ICS (ppm)
ACS (ppm)
57 58 59 60
105,105,105 105,105,105 105,105,105 105,–b,105
14,16,13 14,16,13 14,15,13 14,–,13
a See Figs. 1–3 for details. In each row, the first number is for the pristine model, the second one is for the NBe model, and the third one is for the NO model. The average values of the NMR parameters are reported. b NBe model consist of 59 Be atoms.
for two other models are given in Table 3. Comparison of the ICS results of pristine and NO model reveals that none of the atomic layers feel the effects of N-doping, obviously this is because of the electronegativities of N and O atoms are close to each other (3 and 3.5 eV, respectively) and consequently electronic environment of Be atoms are not changed. Note that the isotropic term ICS indicates the electronic density at the atomic site whereas the anisotropic term (ACS) is indicative of the orientation of the CS tensors in the molecular structure. In NBe model where a Be atom is replaced by a N atom, none of the Be atoms are directly bonded to N atom. Therefore, we do not see any change in the ICS and ACS parameters (see Table 3).
3.3. The
17
O NMR parameters
Table 4 lists the calculated NMR parameters for all O atoms in the pristine and N-doped models. The 60 oxygen atoms are distributed into twelve layers (each layer consist of five O atoms). In the NBe model (Fig. 2), in which one Be atom is doped by one N atom, the changes of CS parameters for those O atoms which are directly bonded to N atom could be expected because of different electronegativities of N (3 eV) and Be (1.5 eV) atoms. Our calculated results indicate that the CS parameters of 6th, 7th and 8th layers of O atoms feel the most significant changes in the NBe model. In these layers O85 (layer 6), O93 (layer 7) and O99 (layer 8) are directly bonded to N impurity; therefore, their electronic environments are changed. Table 4 indicates that the ICS parameters of these three atoms are suddenly decreased. Decrease of ICS parameters indicate a decrease in electron density, therefore, it means that N atom acts as electron receptor in this model and can attract electron density of three O atoms that were mentioned above (see Fig. 2 and Table 4), other O atoms do not indicate significant impact. In NO model where one O atom is substituted by a N atom, the changes of CS tensors of 9Be and 17O atoms are almost negligible. This trend means that the CS tensors at the site of those Be atoms which are directly bonded to the N atom do not indicate significant effect, and those of the O atoms does not show significant changes because they are not directly bonded to the N atom. A comparison of the calculated CS parameters (Tables 3 and 4) reveals that the electronic structure of Fig. 2 where the Be atom is doped by the N atom are more influenced than those of the Fig. 3 where O atom is doped by the N atom. Comparison of the ICS and ACS values for the 15N atoms (Table 4) also indicate the different electronic environments at the sites of these atoms, which reveal that N atom play role of electron acceptor only in NBe model and does not do so in NO model- the 15N ICS is (135 ppm) for N in NBe model and (543 ppm) for N in NO model. Also, dipole moment of the model NO (0.33 Debye) and pristine model (0 Debye) are closer to each other than the dipole moment of the form NBe (6.43 Debye) and the pristine model (Table 2). This indicates a higher polarity for NBe than NO model. This trend again reveals that in NBe model charge distribution is not uniform and electron density pull out toward the N atom which, in turn, reveal the role of electron acceptor for N impurity in NBe model. However, dipole moment is near the zero in the NO model which shows the symmetrical distribution of charge density in that nanotube-nearly the same as the pristine model.
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G.M. Roozbahani, A. Seif / Superlattices and Microstructures 51 (2012) 363–371 Table 4 The 17O NMR parametersc. Nuclei 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116
17
O
ICS (ppm) d
–,260,– 248,259,247 248,259,247 248,259,247 248,259,247 248,236,247 234,235,235 234,235,235 235,236,234 235,236,234 235,248,235 248,248,249 248,248,249 248,248,249 248,248,249 248,248,249 248,244,246 248,248,247 248,248,247 248,248,247 248,248,247 248,248,247 248,250,247 248,249,247 248,248,247 248,175,247 248,249,247 248,250,245 248,248,245 248,248,245 248,248,245 248,248,245 248,248,245 248,68,247 248,249,247 248,250,247 248,248,247 248,248,247 248,248,247 248,77,247 247,248,247 247,248,247 247,248,247 248,249,247 248,243,247 248,248,247 249,248,248 249,248,248 249,249,248 249,250,248 248,236,234 234,236,234 234,236,234 234,236,234 234,237,235 234,260,247 247,260,247
ACS (ppm) –,37,– 41,38,41 41,38,41 41,38,41 41,38,41 41,124,42 126,123,127 126,123,127 128,124,128 128,124,128 128,101,129 102,102,102 102,102,102 102,102,102 102,102,102 102,102,102 101,95,101 101,101,101 101,101,101 101,101,101 101,103,101 101,103,101 101,100,102 101,100,102 101,101,102 101,102,102 101,102,101 101,102,101 101,102,101 101,102,101 101,102,101 101,102,101 101,102,101 101,83,101 101,96,101 101,104,101 101,102,101 101,102,101 101,102,101 101,107,101 101,103,101 101,103,101 101,103,101 101,92,101 102,120,101 101,103,102 102,101,102 102,101,102 102,101,102 102,105,103 102,124,127 126,123,129 127,123,129 127,123,129 126,121,128 127,36,44 41,37,44
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Table 4 (continued) Nuclei 117 118 119 120
17
O
ICS (ppm) 247,260,247 248,260,247 248,261,247 248,–e,–f
ACS (ppm) 41,38,44 41,39,44 41,34,43 41
c See Figs. 1–3 for details. In each row, the first number is for the pristine model, the second one is for the NBe model, and the third one is for the NO model. The average values of the NMR parameters are reported. The underlined numbers show the contribution of O atoms that are directly bonded to the N impurity. d In pristine and NO model first O atom is number 61 but in NBe model the first O atom is number 60 because NBe model consist of 59 Be atoms. e In NBe model the last O atom is number 119. f NO model consists of 59 O atoms.
4. Conclusions The effect of N-doping on the NMR parameters and electronic structure of the armchair (5, 5) BeONT has been studied using DFT calculations. The optimized band lengths indicate that upon doping, the bond lengths are less changed in the NO form than those of the NBe form. The calculated energies show that the NBe form is more stable than the NO form. The changes of the calculated NMR parameters in the form NO and NBe in comparison with the pristine model are also in agreement with the changes of the structural properties such as bond lengths, energies and dipole moment. The changes of the NMR parameters of the NBe form are more pronounced than those of the NO form in comparison with the pristine model. These facts indicate that N atom plays the electron acceptor role only in NBe model and not in the NO model. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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Contents lists available at SciVerse ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
A computational NMR study of nitrogen substitutional impurity in the armchair BeO nanotube Goodarz Mohseni Roozbahani ⇑, Ahmad Seif Department of Chemistry, Boroujerd Branch, Islamic Azad University, Boroujerd, Iran
a r t i c l e
i n f o
Article history: Received 24 October 2011 Received in revised form 4 December 2011 Accepted 23 December 2011 Available online 29 December 2011 Keywords: Beryllium monoxide nanotubes Density functional theory Nitrogen doped
a b s t r a c t The properties of nitrogen doped model of (5, 5) armchair beryllium monoxide nanotubes (BeONTs) have been investigated by density functional theory (DFT) and chemical shift parameters were calculated. A BeONT consisting of 60 Be, 60 atoms of O, and having a length of 1.67 nm was considered and each end of the nanotube was capped by 10 hydrogen atoms. The calculated results indicate that by replacing an O atom by N atom (NO-doping), the chemical shift (CS) parameters of 9Be and 17O atoms are un-affected but replacing a Be atom with N (NBe-doping) affects the CS parameters of O atoms. These results imply that role of nitrogen as an electron acceptor is more significant in the structure for which it dopes a Be atom. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Since their discovery by Iijima in 1991 [1], carbon nanotubes (CNTs) have become one of the most important branches of research in nanomaterial science for their special properties [2,3]. Since then, a considerable number of inorganic nanotubes such as BN [4] and SiC [5], have been prepared and some other ones such as B2O [6], BeO [7,8] have been predicted theoretically. Due to their special physical properties, the nanotubes have potential application in the fields of electronic components, molecular devices, and particle transportation among others [9]. Most of the inorganic nanotubes consist of elements from groups III and V of the periodic table such as BN, BP [10,11] and recently groups II and VI such as BeO [7,8]. Beryllium oxide (BeO) nanotubes, as a new inorganic non-carbon nanotube, has been studied by ab initio calculation and its structure and physical properties (such as binding energy, the electronic band ⇑ Corresponding author. E-mail address: [email protected] (G.M. Roozbahani). 0749-6036/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.spmi.2011.12.006
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structure, the density of states, the dependence of the strain energy of the nanotube on the nanotube diameter D, and the Young’s modulus Y) have been calculated [7]. The dependence of properties such as atomic relaxation, strain energy, and electronic structure of BeO nanotube on its diameter is also established [8]. Further, it is suggested that BeO nanotubes can be synthesized by plasma-chemical reaction or through chemical vapor deposition [7]. It is known now that the band gap of BeONTs is larger than that of BN and SiC NTs; BeONTs is predicted to be an insulator and BN and SiCNTs are predicted to be semiconductors. Recently, the effect of boron, carbon and nitrogen doping in the electronic structure and magnetization of BeONTs was studied by ab initio band structure calculations [12]. This work showed that the nonmagnetic BeONTs transform into magnetic semiconductors in the presence of those impurities because B, C, and N dopants place additional electrons in the forbidden gap of a BeONT. Nuclear magnetic resonance (NMR) spectroscopy is one of the most important techniques to study the electronic structure properties of matters [13–15]. Nuclei with spin angular momentum greater than zero, for example 29Si, 13C, 11B, 15N, 71Ga, and 27Al are active in NMR spectroscopy and detect changes due to any doping or impurities [16–19]. Here, the influence of nitrogen doping on the electronic structure properties of the (5, 5) armchair single-wall BeONT having a length of 1.67 nm and a total of 140 atoms is investigated by the density functional theory (DFT) calculations of the 9Be and 17O NMR chemical shift (CS) tensors. We considered three models of (5, 5) single-wall BeONTs, the pristine (no. 1), and two other N-doped models:
Fig. 1. Pristine model. (a) Because of mirror structure of armchair nanotube, we have indicated only one part of these nanotubes. (b) Red circles are oxygen and yellow circles are beryllium. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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N replacing a Be (no. 2), or an oxygen (no. 3) in the nanotube. After optimizing the geometry of pristine BeONT, chemical shifts tensors are determined and converted to isotropic chemical shielding (ICS) and anisotropic chemical shielding (ACS) parameters. Doping of N in models no. 2 and 3 were carried out on the optimized geometry of model no. 1. Similar calculations of ICS and ACS are done for models nos. 2 and 3 and comparison of results of doped models vs. pristine model is presented. 2. Models and computational details For this investigation of the pristine and doped BeONTs, we chose the following three atomic models of (5, 5) armchair single-wall BeONTs, each being 1.67 nm in length. Model no.1 is pristine consisting of 60 Be, 60 O and 20 H atoms (Be60O60H20), no. 2 has one Be atom doped by one N atom at the sixth layer (Be59NO60H20), from now on designated as NBe, and no. 3 has one O atom doped by one N atom at the sixth layer (Be60O59NH20), call it NO. These structures are shown in Figs. 1–3. In all the three models two ends of tubes are capped by H atoms (each end by 10 H) to prevent dangling bonds, and each model consists of 12 layers. First, the structures have been allowed to relax by all atomic geometrical optimization at the DFT level B3LYP and 6-31G⁄ standard basis set using GAUSSIAN 09 package [20,21]. The optimization processes have yielded optimized bond distances (Table 1), band gap energies (EG), total energies (ET), dipole moments (DM), and spin multiplicity (Table 2). Then, the CS parameters have been calculated in the optimized structures using the same level of theory and the
61 67
60 66 65
71 70 77 76
75 81 80 86 85 84 93 92
N 95 99
98
101
100 105
107
106 111
110
119 118 117
Fig. 2. NBe model. (a) Because of mirror structure of armchair nanotube, we have indicated only one part of these nanotubes. (b) Red circles are oxygen, yellow circles are beryllium and blue circles are nitrogen. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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60 59
53 52
51
47 46
43 42
41 37 36 34 33 32 27 26
N 24 23
22
20
19
12
11
15 9
10 3
1
2
Fig. 3. NO model. (a) Because of mirror structure of armchair nanotube, we have indicated only one part of these nanotubes. (b) Red circles are oxygen, yellow circles are beryllium and blue circles are nitrogen. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 The optimized structural propertiesa.
a b
Property
Pristine
NBe model
NO model
Be–O length/Å N–Be23 length/Å N–Be27 length/Å N–Be33 length/Å N–O85 length/Å N–O93 length/Å N–O99 length/Å O–H length/Å Be–H length/Å Tip diameter/Å
1.53–1.57
1.45–1.66
1.53–1.57 1.62 1.64 1.62
0.98 1.38 7.48
2.51b 1.37 1.37 0.95 1.38 8.16
0.98 1.38 7.43
See Figs. 1–3. This bond is broken.
resulting data are given in Tables 3 and 4. The CS tensors were calculated by the GIAO (gauge included atomic orbital) method [22]. We adopted the same size models as those of Ref. [12]. The quantum chemical calculations yield the CS tensors in the principal axis system (PAS) with the order of
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d33 > d22 > d11: therefore, Eqs. (1) and (2) are used to calculate the isotropic (ICS) and anisotropic (ACS) Parameters [23,24].
ICS ðppmÞ ¼ ðd33 þ d22 þ d11 Þ=3 ACS ðppmÞ ¼ d33 ðd11 þ d22 Þ=2
ð1Þ ð2Þ
3. Results and discussion 3.1. The optimized structures Table 1 presents the optimized structural properties for the pristine and N-doped models of the above mentioned (5, 5) armchair BeONTs. The results indicate that the values of Be–O bond lengths in NBe model are affected by N-doping while in the case of NO model, no such effect is seen. The effects of N-doping on the tip diameters are more obvious for NBe model. In comparison with the pristine model (Table 2), the calculated values of energy band gaps between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) indicate that the values are significantly decreased in both two NBe and NO models. This trend reveals that the N-doping has significant effect on the energy band gap of N-doped armchair BeONT which can lead to better electroconductivity and better magnetization; this result is in agreement with the results found in Ref. [12].In comparison with data in Ref. [12] band gap energy between HOMO and LUMO in the pristine model is decreased while in the doped model is increased, obviously this trend is because in this work, all of the atoms of both mouths of the nanotube were capped by hydrogen atoms to saturate the dangling bonds and to stabilize the tubes, which can lead to better interaction between atomic orbitals. Moreover, the calculated energies also indicate that NBe model is more stable than pristine model because its total energy is less than that of the pristine NT. On the contrary, total energy of NO model is increased in comparison with the pristine model. The latter implies that stability of NO model is less than that of the pristine model. Obviously, one reason for more stability of NBe model is a broken N–O bond which is between layers 6 and 7 (see Table 1 and Fig. 2) which leads to a decrease of strain energy. Another reason for more stability of NBe is that tip diameter of NBe model is larger than that of NO model, and increasing of diameter, at least in a portion of nanotube, could lead to a decrease of strain energy- this is corroborated by the results of Ref. [7]. Also of significance is that two N-doped models have dipole moments while in the pristine model does not. Therefore, insertion of N impurity leads to polarity, meanwhile insertion of N impurity leads to increase of spin from singlet to doublet. 3.2. The 9Be NMR parameters Table 3 shows the calculated NMR parameters for all 9Be atoms in the pristine and N-doped models of BeONTs. Pristine model consists of 60 Be atoms and NBe consist of 59 Be atoms and one N atom, while NO model also consists of 60 Be atoms. The 60 Be atoms are divided into 12 layers (each layer consists of five Be atoms) with equivalent electrostatic properties, for convenience O atoms are omitted (see Figs. 1–3). No significant difference in ICS is observed in NO model, not even in the values of those Be atoms that are directly bonded to N impurity (these Be atoms are numbers 23, 27 and 33), however a little difference is observed in the ACS for Be33, Be27. For comparison purposes, the values Table 2 Band gap energy (EG), total energy (ET), dipole moment (DM) and spin multiplicity for the pristine and N-doped BeONTs. Property
Pristine
NBe model
NO model
HOMO–LUMO Gap/eV Energy (KeV) Dipole momentum (Debye) Spin multiplicity
3.14 64.34 0 Singlet
2.21 64.82 6.43 Doublet
2.33 64.1 0.33 Doublet
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G.M. Roozbahani, A. Seif / Superlattices and Microstructures 51 (2012) 363–371 Table 3 The 9Be NMR parametersa. Nuclei 9Be
ICS (ppm)
ACS (ppm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56
105,105,107 105,105,107 105,105,107 105,105,107 105,105,107 107,107,107 107,107,107 107,107,107 107,107,107 107,107,107 107,106,107 107,107,107 107,107,107 107,107,107 107,106,107 106,106,107 106,106,107 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,103 106,106,106 106,107,106 106,106,106 106,106,106 106,106,106 106,106,104 106,106,107 106,107,106 106,107,106 106,107,106 106,107,106 106,107,104 106,107,106 106,106,106 106,106,106 106,106,106 106,106,106 106,106,106 107,107,106 107,107,107 107,107,107 107,107,107 106,107,105 106,107,105 106,107,105 106,107,107 106,107,107 106,107,107 105,105,105
14,16,13 14,15,13 14,16,13 14,16,13 14,16,13 15,16,15 15,16,15 15,16,15 15,16,15 15,17,15 14,16,14 14,14,14 14,13,14 14,13,14 14,15,14 14,16,14 14,14,14 14,14,14 14,14,13 14,14,13 14,12,14 15,27,15 15,16,15 15,14,15 15,14,15 15,14,16 15,14,31 15,23,15 15,28,15 15,14,15 15,14,15 15,14,15 15,14,40 15,14,18 15,22,15 15,14,15 15,14,15 15,14,15 15,31,31 14,24,15 14,17,15 14,12,13 14,14,13 14,14,13 14,14,13 14,16,12 14,13,13 14,13,13 14,13,13 14,16,13 14,16,13 14,16,13 14,16,15 14,16,15 14,16,15 14,16,13
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Table 3 (continued) Nuclei 9Be
ICS (ppm)
ACS (ppm)
57 58 59 60
105,105,105 105,105,105 105,105,105 105,–b,105
14,16,13 14,16,13 14,15,13 14,–,13
a See Figs. 1–3 for details. In each row, the first number is for the pristine model, the second one is for the NBe model, and the third one is for the NO model. The average values of the NMR parameters are reported. b NBe model consist of 59 Be atoms.
for two other models are given in Table 3. Comparison of the ICS results of pristine and NO model reveals that none of the atomic layers feel the effects of N-doping, obviously this is because of the electronegativities of N and O atoms are close to each other (3 and 3.5 eV, respectively) and consequently electronic environment of Be atoms are not changed. Note that the isotropic term ICS indicates the electronic density at the atomic site whereas the anisotropic term (ACS) is indicative of the orientation of the CS tensors in the molecular structure. In NBe model where a Be atom is replaced by a N atom, none of the Be atoms are directly bonded to N atom. Therefore, we do not see any change in the ICS and ACS parameters (see Table 3).
3.3. The
17
O NMR parameters
Table 4 lists the calculated NMR parameters for all O atoms in the pristine and N-doped models. The 60 oxygen atoms are distributed into twelve layers (each layer consist of five O atoms). In the NBe model (Fig. 2), in which one Be atom is doped by one N atom, the changes of CS parameters for those O atoms which are directly bonded to N atom could be expected because of different electronegativities of N (3 eV) and Be (1.5 eV) atoms. Our calculated results indicate that the CS parameters of 6th, 7th and 8th layers of O atoms feel the most significant changes in the NBe model. In these layers O85 (layer 6), O93 (layer 7) and O99 (layer 8) are directly bonded to N impurity; therefore, their electronic environments are changed. Table 4 indicates that the ICS parameters of these three atoms are suddenly decreased. Decrease of ICS parameters indicate a decrease in electron density, therefore, it means that N atom acts as electron receptor in this model and can attract electron density of three O atoms that were mentioned above (see Fig. 2 and Table 4), other O atoms do not indicate significant impact. In NO model where one O atom is substituted by a N atom, the changes of CS tensors of 9Be and 17O atoms are almost negligible. This trend means that the CS tensors at the site of those Be atoms which are directly bonded to the N atom do not indicate significant effect, and those of the O atoms does not show significant changes because they are not directly bonded to the N atom. A comparison of the calculated CS parameters (Tables 3 and 4) reveals that the electronic structure of Fig. 2 where the Be atom is doped by the N atom are more influenced than those of the Fig. 3 where O atom is doped by the N atom. Comparison of the ICS and ACS values for the 15N atoms (Table 4) also indicate the different electronic environments at the sites of these atoms, which reveal that N atom play role of electron acceptor only in NBe model and does not do so in NO model- the 15N ICS is (135 ppm) for N in NBe model and (543 ppm) for N in NO model. Also, dipole moment of the model NO (0.33 Debye) and pristine model (0 Debye) are closer to each other than the dipole moment of the form NBe (6.43 Debye) and the pristine model (Table 2). This indicates a higher polarity for NBe than NO model. This trend again reveals that in NBe model charge distribution is not uniform and electron density pull out toward the N atom which, in turn, reveal the role of electron acceptor for N impurity in NBe model. However, dipole moment is near the zero in the NO model which shows the symmetrical distribution of charge density in that nanotube-nearly the same as the pristine model.
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G.M. Roozbahani, A. Seif / Superlattices and Microstructures 51 (2012) 363–371 Table 4 The 17O NMR parametersc. Nuclei 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116
17
O
ICS (ppm) d
–,260,– 248,259,247 248,259,247 248,259,247 248,259,247 248,236,247 234,235,235 234,235,235 235,236,234 235,236,234 235,248,235 248,248,249 248,248,249 248,248,249 248,248,249 248,248,249 248,244,246 248,248,247 248,248,247 248,248,247 248,248,247 248,248,247 248,250,247 248,249,247 248,248,247 248,175,247 248,249,247 248,250,245 248,248,245 248,248,245 248,248,245 248,248,245 248,248,245 248,68,247 248,249,247 248,250,247 248,248,247 248,248,247 248,248,247 248,77,247 247,248,247 247,248,247 247,248,247 248,249,247 248,243,247 248,248,247 249,248,248 249,248,248 249,249,248 249,250,248 248,236,234 234,236,234 234,236,234 234,236,234 234,237,235 234,260,247 247,260,247
ACS (ppm) –,37,– 41,38,41 41,38,41 41,38,41 41,38,41 41,124,42 126,123,127 126,123,127 128,124,128 128,124,128 128,101,129 102,102,102 102,102,102 102,102,102 102,102,102 102,102,102 101,95,101 101,101,101 101,101,101 101,101,101 101,103,101 101,103,101 101,100,102 101,100,102 101,101,102 101,102,102 101,102,101 101,102,101 101,102,101 101,102,101 101,102,101 101,102,101 101,102,101 101,83,101 101,96,101 101,104,101 101,102,101 101,102,101 101,102,101 101,107,101 101,103,101 101,103,101 101,103,101 101,92,101 102,120,101 101,103,102 102,101,102 102,101,102 102,101,102 102,105,103 102,124,127 126,123,129 127,123,129 127,123,129 126,121,128 127,36,44 41,37,44
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Table 4 (continued) Nuclei 117 118 119 120
17
O
ICS (ppm) 247,260,247 248,260,247 248,261,247 248,–e,–f
ACS (ppm) 41,38,44 41,39,44 41,34,43 41
c See Figs. 1–3 for details. In each row, the first number is for the pristine model, the second one is for the NBe model, and the third one is for the NO model. The average values of the NMR parameters are reported. The underlined numbers show the contribution of O atoms that are directly bonded to the N impurity. d In pristine and NO model first O atom is number 61 but in NBe model the first O atom is number 60 because NBe model consist of 59 Be atoms. e In NBe model the last O atom is number 119. f NO model consists of 59 O atoms.
4. Conclusions The effect of N-doping on the NMR parameters and electronic structure of the armchair (5, 5) BeONT has been studied using DFT calculations. The optimized band lengths indicate that upon doping, the bond lengths are less changed in the NO form than those of the NBe form. The calculated energies show that the NBe form is more stable than the NO form. The changes of the calculated NMR parameters in the form NO and NBe in comparison with the pristine model are also in agreement with the changes of the structural properties such as bond lengths, energies and dipole moment. The changes of the NMR parameters of the NBe form are more pronounced than those of the NO form in comparison with the pristine model. These facts indicate that N atom plays the electron acceptor role only in NBe model and not in the NO model. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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