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Gallium doped in armchair and zigzag models of boron phosphide nanotubes (BPNTs): A NMR study
Physica B 407 (2012) 3717–3721
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Physica B journal homepage: www.elsevier.com/locate/physb
Gallium doped in armchair and zigzag models of boron phosphide nanotubes (BPNTs): A NMR study M. Rezaei-Sameti n Department of Applied Chemistry, Faculty of Science, Malayer University, Malayer 65174, Iran
a r t i c l e i n f o
abstract
Article history: Received 31 March 2012 Received in revised form 21 May 2012 Accepted 22 May 2012 Available online 30 May 2012
The electrical properties and NMR parameters of the pristine and Ga-doped structures of two representative (8, 0) zigzag and (4, 4) armchair of boron phosphide nanotubes (BPNTs) have been investigated. The structural geometries of above nanotubes have been allowed to relax by optimization and then the isotropic and anisotropic chemical shielding parameters (CSI and CSA) of 11B and 31P have been calculated based on DFT theory. The results reveal that the influence of Ga-doping was more significant on the geometries of the zigzag model than the armchair one. The difference of band gap energies between the pristine and Ga-doped armchair BPNTs was larger than the zigzag model. Significant differences of NMR parameters of those nuclei directly contributed to the Ga-doping atoms have been observed. & 2012 Elsevier B.V. All rights reserved.
Keywords: BPNTs NMR DFT Ga-doped
1. Introduction In the past decade, the significant research efforts to synthesize and theoretical study of nanometer-scale tubular forms of various materials have been done [1–9]. Among these materials, semiconductors of group III and V, e.g., boron nitride (BN), aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) have more been studied [10–16] than the group III-phosphide, e.g., boron phosphide (BP), aluminum phosphide (AlP), gallium phosphide (GaP), and indium phosphide (InP). Among the group III-phosphide, much more attention has recently been focused on the determination and characterization of BP material [17,18] because of the known similarity between the properties of the electronic structures of BP and silicon carbide (SiC). In particular, the BPNTs have attracted much attention, due to their unique properties and promising applications, optical, mechanical properties, electronics and optoelectronics applications and nanocomposites that can be operated at extreme environment such as high temperature, high power and radiation, and in harsh environments. In a recent study [19,20], we have investigated the properties of boron phosphide nanotube (BPNTs) and AlNNTs by quantum chemical calculations of chemical shielding (CS) parameters. In another recent study [21], our calculated CS parameters indicated that the electronic and structural properties of boron phosphide nanotube (BPNT) detect the effects of Al and N doped. In current research, we have investigated the electronic structures of the Gallium-doped (Ga-doped) models of the zigzag and armchair BPNTs by density functional theory (DFT) calculations of the CS parameters for 11B and 31P atoms of the optimized
n
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structures. The Ga-doped BPNT could be expected as n- or p-type semiconductor which depends on the substitution of the B atom by the Ga atom; therefore, the electronic properties of these models of BPNTs are important. The calculations of CS parameters could reveal insightful trends about the electronic properties of matters because any effects on the electronic densities at the atomic sites could properly be detected by these parameters [22,23]. Since nuclear magnetic resonance (NMR) spectroscopy is an insightful technique to study the properties of the electronic structure of matters [18,19], chemical shielding (CS) tensors have also been calculated for the optimized structures of the investigated BPNTs. The results of the pristine and the Ga-doped models have been compared to indicate the influences of Ga-doping on the properties of the electronic structures of the BPNTs. To this point, there have not been any available experimental data for the BPNTs. In this work the structures of the (4, 4) armchair and (8, 0) zigzag single-walled BPNTs and Ga-doped in these models (Figs. 1 and 2) have been optimized by performing density functional theory (DFT) (see Table 1). The isotropic and anisotropic chemical shielding of B and P have been calculated (see Tables 2 and 3).
2. Computational methods In this computational, the representative models of (4, 4) armchair and (8, 0) zigzag single-walled BPNTs in which the ends of nanotubes are saturated by hydrogen atoms (see Figs.1 and 2). All models are individually optimized by using density functional theory (DFT) at B3LYP [24] level of theory using the Gaussian 03 set of programs [25]. The standard 6–31Gn basis set was used for all models. The CS tensors are calculated in the optimized structures using the same level of the theory. Furthermore, the chemical
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M. Rezaei-Sameti / Physica B 407 (2012) 3717–3721
Fig. 1. 3D views of the undoped and Ga doped of (4, 4) armchair model of BPNTs.
shielding (CS) tensors at the sites of 11B, 31P nuclei are calculated based on the gauge included atomic orbital (GIAO) approach [26]. The calculated CS tensors in principal axes system (PAS) (s33 4 s22 4 s11 ) are converted to measurable NMR parameters, chemical shielding isotropic (CSI) and chemical shielding anisotropic (CSA) by using Eqs. (1) and (2), respectively [19,20]. The evaluated NMR parameters at the sites of 11B, 31P nuclei are presented in Tables 2 and 3. CSIðppmÞ ¼ 1=3ðs11 þ s22 þ s33 Þ
ð1Þ
CSAðppmÞ ¼ s33 ðs11 þ s22 Þ=2
ð2Þ
3. Results and discussions 3.1. The structural geometry of BPNTs and Ga-doped The bond lengths of (B–P) and bond angles (B–P–B) of the (4, 4) armchair and (8, 0) zigzag forms of BPNTs (Figs. 1 and 2) and Gadoped on boron site of presented BPNTs (Figs. 1 and 2) are listed in
Table 1. The average B–P bond length for armchair and zigzag forms of BPNTs is 1.89 A˚ which is in agreement with other studies [17,20]. By doping Gallium on the site of B42 of (4, 4) armchair form of BPNTs the bond lengths between P41–Ga, P52–Ga and P32–Ga are 2.26, 2.25 and 2.25 A˚ respectively and by doping Gallium on the site of B52of (8, 0) zigzag forms of BPNTs the bond lengths between P42–Ga, P62–Ga and P63–Ga are 2.23, 2.26 and 1.92 A˚ respectively. The electronegativity of Ga (eGa¼1.80) is slightly smaller than that of B (eB¼2.04) and P (eP¼2.20) leading a charge transfer from Ga to B and P yielding asymmetric electronic charge density distribution along Ga–P bond. By doping Gallium on the site of B42 on (4, 4) armchair form of BPNTs the bond anglesoB31–P41–GaoP32–Ga– P52oGa–P52–B52 andoGa–P32–B32 are decreased from original values. And by doping of Gallium on the site of B52 of (8, 0) zigzag forms of BPNTs the bond anglesoP62–Ga, B52–P42oP42–Ga, B42– P63oGa, B52–P63–B53oGa, B52–P42–B33oGa, B52–P42–B32 and oGa, B52–P62–B51 are decreased too. From the geometrical results we calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The value of differences between HOMO and LUMO energies, band gap energies, in the optimized
M. Rezaei-Sameti / Physica B 407 (2012) 3717–3721
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Fig. 2. 3D views of the undoped and Ga doped of (8, 0) Zigzag model of BPNTs.
Table 1 The optimized geometries of (4, 4) armchair and (8, 0) Zigzag form of BPNTs, and Ga-doped. Armchair form
Zigzag form a,b
Undoped
Ga-doped
1.89 A˚ 1.89 A˚
2.26 A˚ 2.25 A˚
1.89 A˚ 1.89 A˚
2.25 A˚ 1.90 A˚
P63–B52/Ga
1.89 A˚ 1.89 A˚
1.90 A˚ 1.90 A˚
P42–B33
1.89 A˚ 1.89 A˚
1.90 A˚ 1.90 A˚
P63–B73
P41–B51 P32–B22
1.89 A˚
1.90 A˚
Bond angle o B31–P41–Ga,B42 o P41–Ga,B42–P52 o P32–Ga,B42–P52 o Ga,B42–P52–B52 o Ga,B42–P32–B32 o P32–B32–P42 o P41–B31–P31 o P41–B51–P61
1111 1221 1171 1111 1111 1211 1211 1171
981 1231 1131 1001 1081 1251 1191 1191
Properties ˚ Bond length (A) P41–B42/Ga P52–B42/Ga P32–B42/Ga P52–B52 P52–B32 P52–B62 P41–B31
a b
See Figs. 1 and 2 for details. Undoped form (Ref. 17 and 20).
Properties
Undoped
Ga-doped
1.89 A˚ 1.89 A˚
2.23 A˚ 2.26 A˚
1.89 A˚ 1.89 A˚
1.92 A˚ 1.92 A˚
1.89 A˚ 1.89 A˚
1.92 A˚ 1.92 A˚
P62–B51
1.89 A˚ 1.89 A˚
1.92 A˚ 1.89 A˚
P62–B72
1.89 A˚
1.92 A˚
Bond angle o P62–Ga,B52–P63 o P62–Ga,B52–P42 o P42–Ga,B42–P63 o Ga,B52–P63–B53 o Ga,B52–P42–B33 o Ga,B52–P42–B32 o Ga,B52–P62–B51
1221 1191 1191 1101 1161 1161 1151
1251 1171 1171 1031 1061 1061 981
˚ Bond length (A) P42–B52/Ga P62–B52/Ga P42-B32 P63–B53
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M. Rezaei-Sameti / Physica B 407 (2012) 3717–3721
Table 2 The NMR parameters of the 9B and
31
P nuclei in (4, 4) armchair BPNTs.
B-15 nuclei
CSI (ppm)
CSA (ppm)
P-31 nuclei
CSI (ppm)
CSA (ppm)
B11 B12 B21 B22 B31 B32 B41 B42 B51 B52 B61 B62 B71 B72 B81 B82 Ga
36a 36 35 35 40 40 42 42 42 42 40 40 35 35 36 36 –
86a 86 85 85 73 73 91 91 82 82 89 89 99 99 112 112
P11 P12 P21 P22 P31 P32 P41 P42 P51 P52 P61 P62 P71 P72 P81 P82
414a 414 360 360 358 358 359 359 359 359 358 358 360 360 414 414
115a 115 124 124 233 233 139 139 238 238 124 124 235 235 103 103
35 37 35 35 38 45 42 – 41 38 40 40 32 36 36 39 1484
123 127 51 41 107 107 40 – 26 113 30 32 119 47 55 120 85
416 414 235 343 359 346 351 432 361 348 350 434 362 348 418 406
69 81 361 233 116 153 249 228 137 142 241 209 107 139 124 109
a
See Fig. 1 for details, in each row, the first number is undoped form (Ref. [17,20]); the second one is for Ga-doped BPNTs model.
Table 3 The NMR parameters of the 9B and 31P nuclei in (8, 0) Zigzag BPNTs. B-15 nuclei CSI (ppm) CSA (ppm) B11 B12 B13 B14 B31 B32 B33 B34 B35 B51 B52 B53 B54 B71 B72 B73 B74 B75 Ga
29 29 29 29 45 45 45 45 45 41 41 41 41 47 47 47 47 47 –
32 30 26 26 52 44 42 44 53 42 – 39 39 47 47 47 47 47 1484
P-31 nuclei CSI (ppm) CSA (ppm)
29 120 29 120 84 84 84 84 84 22 111 22 111 94 94 94 94 94
90 100 106 106 110 31 105 31 110 90 – 81 81 44 87 44 102 44 85
P21 P22 P23 P24 P25 P41 P42 P43 P44 P61 P62 P63 P64 P65 P81 P82 P83 P84
404 404 404 404 404 371 371 371 371 345 345 345 345 345 239 239 239 239
406 405 406 405 406 353 338 374 339 374 338 353 339 426 340 340 340 340
105 105 105 105 105 77 237 77 237 195 195 195 195 195 90 277 90 277
105 82 115 73 105 224 253 192 80 192 255 224 108 79 194 230 194 230
See Fig. 2 for details, in each row, the first number is undoped form (Ref. [17]); the second one is for Ga-doped BPNTs model.
structures yielded 2.95 eV for the armchair BPNTs and by doping Gallium the band gap is decreased to 2.91 eV; on the other hand the band gap energies in pristine and Ga-doped of zigzag form are increased from 2.57 to 2.62 eV. This trend revealed that by doping of Ga, the band gap energy of the armchair model is larger than the zigzag and the variation of band gap energy in Zigzag model is larger than armchair. 3.2. The NMR parameters of
11
B
The NMR (CSI and CSA) parameters of 11B nuclei for the (4, 4) armchair BPNTs (Fig. 1) and (8, 0) zigzag forms of BPNTs (Fig. 2) and Ga-doped on boron sites of presented BPNTs (Figs. 1 and 2) are given in Tables 2 and 3. A look at results for the (4, 4) armchair BPNTs show that various 11B nuclei are divided into four layers with equivalent calculated CSI and CSA parameters [17,20], the similar results are shown for (8, 0) zigzag forms of BPNTs, which means that the nuclei in each layer have equivalent electrostatic properties. The direction of changes for isotropic and anisotropic chemical shielding because of difference in physical concept of
these parameters [17–23] is different. The CSI values for B nuclei at the end layer are maximum and the first layer minimum. This meaning that boron nuclei in the end layer have maximum electron shielding. By doping Ga in the B41 site of (4, 4) armchair BPNTs the CSI values of 11B nuclei undergo a small change. But by doping Ga in the B52 site of (8, 0) zigzag forms of BPNTs the CSI value of the B31 and B35 sites are significantly increased from original value of 45. On the other hand the CSA values of (4, 4) armchair BPNTs at the sites of B21, B22, B41, B51, B61, B62, B72 and B81 are decreased and other sites are increased from original valves. The CSA values of (8, 0) zigzag BPNTs at the sites of B32, B34, B71, B72, B73 and B75 are decreased and the other sites are increased. This results show that by doping Ga the directions of CS tensors orientations are changed in these atoms. 3.3. The NMR parameters of
31
P
The evaluated 31P NMR parameters (CSI and CSA) for the considered models of (4, 4) armchair BPNTs and (8, 0) zigzag forms of BPNTs and Ga-doped on boron sites are given in Tables 2 and 3. Similar results of 11B NMR parameters the CSI values for (4, 4) armchair and (8, 0) zigzag form of BPNTs are divided into four layers with equivalent 31P. In the (4, 4) armchair model of BPNTs the CSI values of the layers (1, 8), (2, 7), (3, 6) and (4, 5) are 414, 360, 358 and 359 ppm respectively. And in the (8, 0) zigzag model of BPNTs the CSI values of the layers 2, 4, 6 and 8 are 404, 371, 345 and 239 ppm respectively. The comparison between the B and P NMR parameters show that the CSI values of P sites are larger than B sites, because of the electronegativity of P a charge transfer from B sites to P sites and yielding asymmetric electronic charge density distribution along B–P bond. By doping Ga in the B42 site of (4, 4) armchair model of BPNTs the CSI values of 31P nuclei at the P21, P22, P32, P41, P42, P52, P61, P62, P72 and P82 are largely decreased from original values and the other sites are increased. On other hand in the (8, 0) zigzag model of BPNTs by doping Ga in the B52 nuclei the CSI values of P41, P42, P44, P62 and P64 are significantly decreased and other sites are increased. The CSA values of 31P nuclei in (4, 4) armchair model of BPNTs at sites P11, P12, P31, P32, P51, P52, P71 and P72 are significantly decreased and other sites are increased. The CSA values for (8, 0) zigzag model of BPNTs at the sites P22, P24, P44, P64 and P65 are largely decreased and other sites increased. The results indicated that the atoms of different layers have different values of NMR properties which mean that these atoms can be used to detect different electronic environments in the structure of nanotube. Due to this perturbation to electronic structure properties, the NMR parameters of Ga-doped model differ from those of the undoped model. However, the changes of NMR parameters are just viewed on the first neighbors of Ga atom. Especially, for those atoms which are directly connected to the Ga atom.
4. Conclusions The electronic structure properties of armchair and zigzag models of BPNTs and doping of Ga in the B nuclei have been investigated. The geometrical structures results show that by the Ga-doping, the values of band gap energies in armchair form is decreased and in zigzag form is increased; the band gap energy of the armchair model was larger than the zigzag one. The NMR parameters of two models of BPNTs show that by doping Ga at neighbor site of doping the CSI and CSA values are decreased and other sites increased. The results show that CSI values are directly proportional to the electronic density at the
M. Rezaei-Sameti / Physica B 407 (2012) 3717–3721
atomic sites. It is worth noting that the electronic densities at the atomic sites of nanotubes are very important for interactions occurring between nanotube and other molecules or atoms.
Acknowledgment The authors would like to thank the Malayer University for providing the necessary facilities to carry out the research. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
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Physica B journal homepage: www.elsevier.com/locate/physb
Gallium doped in armchair and zigzag models of boron phosphide nanotubes (BPNTs): A NMR study M. Rezaei-Sameti n Department of Applied Chemistry, Faculty of Science, Malayer University, Malayer 65174, Iran
a r t i c l e i n f o
abstract
Article history: Received 31 March 2012 Received in revised form 21 May 2012 Accepted 22 May 2012 Available online 30 May 2012
The electrical properties and NMR parameters of the pristine and Ga-doped structures of two representative (8, 0) zigzag and (4, 4) armchair of boron phosphide nanotubes (BPNTs) have been investigated. The structural geometries of above nanotubes have been allowed to relax by optimization and then the isotropic and anisotropic chemical shielding parameters (CSI and CSA) of 11B and 31P have been calculated based on DFT theory. The results reveal that the influence of Ga-doping was more significant on the geometries of the zigzag model than the armchair one. The difference of band gap energies between the pristine and Ga-doped armchair BPNTs was larger than the zigzag model. Significant differences of NMR parameters of those nuclei directly contributed to the Ga-doping atoms have been observed. & 2012 Elsevier B.V. All rights reserved.
Keywords: BPNTs NMR DFT Ga-doped
1. Introduction In the past decade, the significant research efforts to synthesize and theoretical study of nanometer-scale tubular forms of various materials have been done [1–9]. Among these materials, semiconductors of group III and V, e.g., boron nitride (BN), aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN) have more been studied [10–16] than the group III-phosphide, e.g., boron phosphide (BP), aluminum phosphide (AlP), gallium phosphide (GaP), and indium phosphide (InP). Among the group III-phosphide, much more attention has recently been focused on the determination and characterization of BP material [17,18] because of the known similarity between the properties of the electronic structures of BP and silicon carbide (SiC). In particular, the BPNTs have attracted much attention, due to their unique properties and promising applications, optical, mechanical properties, electronics and optoelectronics applications and nanocomposites that can be operated at extreme environment such as high temperature, high power and radiation, and in harsh environments. In a recent study [19,20], we have investigated the properties of boron phosphide nanotube (BPNTs) and AlNNTs by quantum chemical calculations of chemical shielding (CS) parameters. In another recent study [21], our calculated CS parameters indicated that the electronic and structural properties of boron phosphide nanotube (BPNT) detect the effects of Al and N doped. In current research, we have investigated the electronic structures of the Gallium-doped (Ga-doped) models of the zigzag and armchair BPNTs by density functional theory (DFT) calculations of the CS parameters for 11B and 31P atoms of the optimized
n
Tel./fax: þ98 851 3339843. E-mail addresses: [email protected], [email protected]
0921-4526/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2012.05.047
structures. The Ga-doped BPNT could be expected as n- or p-type semiconductor which depends on the substitution of the B atom by the Ga atom; therefore, the electronic properties of these models of BPNTs are important. The calculations of CS parameters could reveal insightful trends about the electronic properties of matters because any effects on the electronic densities at the atomic sites could properly be detected by these parameters [22,23]. Since nuclear magnetic resonance (NMR) spectroscopy is an insightful technique to study the properties of the electronic structure of matters [18,19], chemical shielding (CS) tensors have also been calculated for the optimized structures of the investigated BPNTs. The results of the pristine and the Ga-doped models have been compared to indicate the influences of Ga-doping on the properties of the electronic structures of the BPNTs. To this point, there have not been any available experimental data for the BPNTs. In this work the structures of the (4, 4) armchair and (8, 0) zigzag single-walled BPNTs and Ga-doped in these models (Figs. 1 and 2) have been optimized by performing density functional theory (DFT) (see Table 1). The isotropic and anisotropic chemical shielding of B and P have been calculated (see Tables 2 and 3).
2. Computational methods In this computational, the representative models of (4, 4) armchair and (8, 0) zigzag single-walled BPNTs in which the ends of nanotubes are saturated by hydrogen atoms (see Figs.1 and 2). All models are individually optimized by using density functional theory (DFT) at B3LYP [24] level of theory using the Gaussian 03 set of programs [25]. The standard 6–31Gn basis set was used for all models. The CS tensors are calculated in the optimized structures using the same level of the theory. Furthermore, the chemical
3718
M. Rezaei-Sameti / Physica B 407 (2012) 3717–3721
Fig. 1. 3D views of the undoped and Ga doped of (4, 4) armchair model of BPNTs.
shielding (CS) tensors at the sites of 11B, 31P nuclei are calculated based on the gauge included atomic orbital (GIAO) approach [26]. The calculated CS tensors in principal axes system (PAS) (s33 4 s22 4 s11 ) are converted to measurable NMR parameters, chemical shielding isotropic (CSI) and chemical shielding anisotropic (CSA) by using Eqs. (1) and (2), respectively [19,20]. The evaluated NMR parameters at the sites of 11B, 31P nuclei are presented in Tables 2 and 3. CSIðppmÞ ¼ 1=3ðs11 þ s22 þ s33 Þ
ð1Þ
CSAðppmÞ ¼ s33 ðs11 þ s22 Þ=2
ð2Þ
3. Results and discussions 3.1. The structural geometry of BPNTs and Ga-doped The bond lengths of (B–P) and bond angles (B–P–B) of the (4, 4) armchair and (8, 0) zigzag forms of BPNTs (Figs. 1 and 2) and Gadoped on boron site of presented BPNTs (Figs. 1 and 2) are listed in
Table 1. The average B–P bond length for armchair and zigzag forms of BPNTs is 1.89 A˚ which is in agreement with other studies [17,20]. By doping Gallium on the site of B42 of (4, 4) armchair form of BPNTs the bond lengths between P41–Ga, P52–Ga and P32–Ga are 2.26, 2.25 and 2.25 A˚ respectively and by doping Gallium on the site of B52of (8, 0) zigzag forms of BPNTs the bond lengths between P42–Ga, P62–Ga and P63–Ga are 2.23, 2.26 and 1.92 A˚ respectively. The electronegativity of Ga (eGa¼1.80) is slightly smaller than that of B (eB¼2.04) and P (eP¼2.20) leading a charge transfer from Ga to B and P yielding asymmetric electronic charge density distribution along Ga–P bond. By doping Gallium on the site of B42 on (4, 4) armchair form of BPNTs the bond anglesoB31–P41–GaoP32–Ga– P52oGa–P52–B52 andoGa–P32–B32 are decreased from original values. And by doping of Gallium on the site of B52 of (8, 0) zigzag forms of BPNTs the bond anglesoP62–Ga, B52–P42oP42–Ga, B42– P63oGa, B52–P63–B53oGa, B52–P42–B33oGa, B52–P42–B32 and oGa, B52–P62–B51 are decreased too. From the geometrical results we calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The value of differences between HOMO and LUMO energies, band gap energies, in the optimized
M. Rezaei-Sameti / Physica B 407 (2012) 3717–3721
3719
Fig. 2. 3D views of the undoped and Ga doped of (8, 0) Zigzag model of BPNTs.
Table 1 The optimized geometries of (4, 4) armchair and (8, 0) Zigzag form of BPNTs, and Ga-doped. Armchair form
Zigzag form a,b
Undoped
Ga-doped
1.89 A˚ 1.89 A˚
2.26 A˚ 2.25 A˚
1.89 A˚ 1.89 A˚
2.25 A˚ 1.90 A˚
P63–B52/Ga
1.89 A˚ 1.89 A˚
1.90 A˚ 1.90 A˚
P42–B33
1.89 A˚ 1.89 A˚
1.90 A˚ 1.90 A˚
P63–B73
P41–B51 P32–B22
1.89 A˚
1.90 A˚
Bond angle o B31–P41–Ga,B42 o P41–Ga,B42–P52 o P32–Ga,B42–P52 o Ga,B42–P52–B52 o Ga,B42–P32–B32 o P32–B32–P42 o P41–B31–P31 o P41–B51–P61
1111 1221 1171 1111 1111 1211 1211 1171
981 1231 1131 1001 1081 1251 1191 1191
Properties ˚ Bond length (A) P41–B42/Ga P52–B42/Ga P32–B42/Ga P52–B52 P52–B32 P52–B62 P41–B31
a b
See Figs. 1 and 2 for details. Undoped form (Ref. 17 and 20).
Properties
Undoped
Ga-doped
1.89 A˚ 1.89 A˚
2.23 A˚ 2.26 A˚
1.89 A˚ 1.89 A˚
1.92 A˚ 1.92 A˚
1.89 A˚ 1.89 A˚
1.92 A˚ 1.92 A˚
P62–B51
1.89 A˚ 1.89 A˚
1.92 A˚ 1.89 A˚
P62–B72
1.89 A˚
1.92 A˚
Bond angle o P62–Ga,B52–P63 o P62–Ga,B52–P42 o P42–Ga,B42–P63 o Ga,B52–P63–B53 o Ga,B52–P42–B33 o Ga,B52–P42–B32 o Ga,B52–P62–B51
1221 1191 1191 1101 1161 1161 1151
1251 1171 1171 1031 1061 1061 981
˚ Bond length (A) P42–B52/Ga P62–B52/Ga P42-B32 P63–B53
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M. Rezaei-Sameti / Physica B 407 (2012) 3717–3721
Table 2 The NMR parameters of the 9B and
31
P nuclei in (4, 4) armchair BPNTs.
B-15 nuclei
CSI (ppm)
CSA (ppm)
P-31 nuclei
CSI (ppm)
CSA (ppm)
B11 B12 B21 B22 B31 B32 B41 B42 B51 B52 B61 B62 B71 B72 B81 B82 Ga
36a 36 35 35 40 40 42 42 42 42 40 40 35 35 36 36 –
86a 86 85 85 73 73 91 91 82 82 89 89 99 99 112 112
P11 P12 P21 P22 P31 P32 P41 P42 P51 P52 P61 P62 P71 P72 P81 P82
414a 414 360 360 358 358 359 359 359 359 358 358 360 360 414 414
115a 115 124 124 233 233 139 139 238 238 124 124 235 235 103 103
35 37 35 35 38 45 42 – 41 38 40 40 32 36 36 39 1484
123 127 51 41 107 107 40 – 26 113 30 32 119 47 55 120 85
416 414 235 343 359 346 351 432 361 348 350 434 362 348 418 406
69 81 361 233 116 153 249 228 137 142 241 209 107 139 124 109
a
See Fig. 1 for details, in each row, the first number is undoped form (Ref. [17,20]); the second one is for Ga-doped BPNTs model.
Table 3 The NMR parameters of the 9B and 31P nuclei in (8, 0) Zigzag BPNTs. B-15 nuclei CSI (ppm) CSA (ppm) B11 B12 B13 B14 B31 B32 B33 B34 B35 B51 B52 B53 B54 B71 B72 B73 B74 B75 Ga
29 29 29 29 45 45 45 45 45 41 41 41 41 47 47 47 47 47 –
32 30 26 26 52 44 42 44 53 42 – 39 39 47 47 47 47 47 1484
P-31 nuclei CSI (ppm) CSA (ppm)
29 120 29 120 84 84 84 84 84 22 111 22 111 94 94 94 94 94
90 100 106 106 110 31 105 31 110 90 – 81 81 44 87 44 102 44 85
P21 P22 P23 P24 P25 P41 P42 P43 P44 P61 P62 P63 P64 P65 P81 P82 P83 P84
404 404 404 404 404 371 371 371 371 345 345 345 345 345 239 239 239 239
406 405 406 405 406 353 338 374 339 374 338 353 339 426 340 340 340 340
105 105 105 105 105 77 237 77 237 195 195 195 195 195 90 277 90 277
105 82 115 73 105 224 253 192 80 192 255 224 108 79 194 230 194 230
See Fig. 2 for details, in each row, the first number is undoped form (Ref. [17]); the second one is for Ga-doped BPNTs model.
structures yielded 2.95 eV for the armchair BPNTs and by doping Gallium the band gap is decreased to 2.91 eV; on the other hand the band gap energies in pristine and Ga-doped of zigzag form are increased from 2.57 to 2.62 eV. This trend revealed that by doping of Ga, the band gap energy of the armchair model is larger than the zigzag and the variation of band gap energy in Zigzag model is larger than armchair. 3.2. The NMR parameters of
11
B
The NMR (CSI and CSA) parameters of 11B nuclei for the (4, 4) armchair BPNTs (Fig. 1) and (8, 0) zigzag forms of BPNTs (Fig. 2) and Ga-doped on boron sites of presented BPNTs (Figs. 1 and 2) are given in Tables 2 and 3. A look at results for the (4, 4) armchair BPNTs show that various 11B nuclei are divided into four layers with equivalent calculated CSI and CSA parameters [17,20], the similar results are shown for (8, 0) zigzag forms of BPNTs, which means that the nuclei in each layer have equivalent electrostatic properties. The direction of changes for isotropic and anisotropic chemical shielding because of difference in physical concept of
these parameters [17–23] is different. The CSI values for B nuclei at the end layer are maximum and the first layer minimum. This meaning that boron nuclei in the end layer have maximum electron shielding. By doping Ga in the B41 site of (4, 4) armchair BPNTs the CSI values of 11B nuclei undergo a small change. But by doping Ga in the B52 site of (8, 0) zigzag forms of BPNTs the CSI value of the B31 and B35 sites are significantly increased from original value of 45. On the other hand the CSA values of (4, 4) armchair BPNTs at the sites of B21, B22, B41, B51, B61, B62, B72 and B81 are decreased and other sites are increased from original valves. The CSA values of (8, 0) zigzag BPNTs at the sites of B32, B34, B71, B72, B73 and B75 are decreased and the other sites are increased. This results show that by doping Ga the directions of CS tensors orientations are changed in these atoms. 3.3. The NMR parameters of
31
P
The evaluated 31P NMR parameters (CSI and CSA) for the considered models of (4, 4) armchair BPNTs and (8, 0) zigzag forms of BPNTs and Ga-doped on boron sites are given in Tables 2 and 3. Similar results of 11B NMR parameters the CSI values for (4, 4) armchair and (8, 0) zigzag form of BPNTs are divided into four layers with equivalent 31P. In the (4, 4) armchair model of BPNTs the CSI values of the layers (1, 8), (2, 7), (3, 6) and (4, 5) are 414, 360, 358 and 359 ppm respectively. And in the (8, 0) zigzag model of BPNTs the CSI values of the layers 2, 4, 6 and 8 are 404, 371, 345 and 239 ppm respectively. The comparison between the B and P NMR parameters show that the CSI values of P sites are larger than B sites, because of the electronegativity of P a charge transfer from B sites to P sites and yielding asymmetric electronic charge density distribution along B–P bond. By doping Ga in the B42 site of (4, 4) armchair model of BPNTs the CSI values of 31P nuclei at the P21, P22, P32, P41, P42, P52, P61, P62, P72 and P82 are largely decreased from original values and the other sites are increased. On other hand in the (8, 0) zigzag model of BPNTs by doping Ga in the B52 nuclei the CSI values of P41, P42, P44, P62 and P64 are significantly decreased and other sites are increased. The CSA values of 31P nuclei in (4, 4) armchair model of BPNTs at sites P11, P12, P31, P32, P51, P52, P71 and P72 are significantly decreased and other sites are increased. The CSA values for (8, 0) zigzag model of BPNTs at the sites P22, P24, P44, P64 and P65 are largely decreased and other sites increased. The results indicated that the atoms of different layers have different values of NMR properties which mean that these atoms can be used to detect different electronic environments in the structure of nanotube. Due to this perturbation to electronic structure properties, the NMR parameters of Ga-doped model differ from those of the undoped model. However, the changes of NMR parameters are just viewed on the first neighbors of Ga atom. Especially, for those atoms which are directly connected to the Ga atom.
4. Conclusions The electronic structure properties of armchair and zigzag models of BPNTs and doping of Ga in the B nuclei have been investigated. The geometrical structures results show that by the Ga-doping, the values of band gap energies in armchair form is decreased and in zigzag form is increased; the band gap energy of the armchair model was larger than the zigzag one. The NMR parameters of two models of BPNTs show that by doping Ga at neighbor site of doping the CSI and CSA values are decreased and other sites increased. The results show that CSI values are directly proportional to the electronic density at the
M. Rezaei-Sameti / Physica B 407 (2012) 3717–3721
atomic sites. It is worth noting that the electronic densities at the atomic sites of nanotubes are very important for interactions occurring between nanotube and other molecules or atoms.
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