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Báo cáo hóa học: " Tuning the electronic properties of boron nitride nanotube by mechanical uni-axial deformation: a DFT study"

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  1. Ju et al. Nanoscale Research Letters 2011, 6:160 http://www.nanoscalereslett.com/content/6/1/160 NANO EXPRESS Open Access Tuning the electronic properties of boron nitride nanotube by mechanical uni-axial deformation: a DFT study Shin-Pon Ju*, Yao-Chun Wang, Ting-Wei Lien Abstract The effect of uni-axial strain on the electronic properties of (8,0) zigzag and (5,5) armchair boron nitride nanotubes (BNNT) is addressed by density functional theory calculation. The stress-strain profiles indicate that these two BNNTS of differing types display very similar mechanical properties, but there are variations in HOMO-LUMO gaps at different strains, indicating that the electronic properties of BNNTs not only depend on uni-axial strain, but on BNNT type. The variations in nanotube geometries, partial density of states of B and N atoms, B and N charges are also discussed for (8,0) and (5,5) BNNTs at different strains. Introduction As BNNTs have many special mechanical, thermal, electrical, and chemical properties and have a large In nanoscale materials, especially for nanotubes, numer- number of potential applications, such as in composite ous special properties depend on their ultra-small sizes. materials, hydrogen storage, and force sensors [10-13], Carbon nanotubes (CNTs), discovered by Iijima in 1991 many scientists have studied the properties of BNNTs [1], have been a very promising one-dimensional mate- and related material [2,14-18]. The hydrogen storage rial in nanoscience. Theoretical calculations and experi- attracted much attention in recent years especially. Ma mental measurements on carbon nanotubes have shown et al. [16] found that the structure of BNNTs is better many exceptional properties that make CNTs promising for several proposed applications, such as high Young’s able to store hydrogen at high temperature than CNTs, such that BNNTs can store 1.8 to 2.6 wt% at 10 MPa. modulus and electronic properties [1-6]. Boron nitride In theoretical studies, Cheng et al. obtained that capabil- nanotubes (BNNTs) were theoretically predicted in 1994 ity of hydrogen storage in single-walled boron nitride and were synthesized experimentally in the following nanotube arrays (SWBNNTA) can be increased with the year [7]. BNNTs are a structural analogy to CNTs that increase of distance between BNNTs. Zhao and Ding instead alternate boron and nitride atoms to replace the [11] indicated that several gas molecules (H2, O2, and carbon atoms in the hexagonal structure. Although CNTs and BNNTs have similar structures, their proper- H 2 O) dissociate and chemisorb on BNNT edges, and ties are quite different. For example, electronic proper- the adsorption of these molecules induces a charge ties of CNT are distinctly different from those of transfer. Yuan and Liew [18] reported that boron nitride impurities will cause a decrease in Young’s moduli of BNNTs because of the large ionicity of B-N bonds [2]. Another difference is that BNNTs have a much better SWCNTs. Moreover, the effect of these impurities in resistance to oxidation in high temperature systems zigzag SWCNTs is more significant because of the link- than CNTs [8]. Moreover, the BNNT is independent of ing characteristics of an increase in electrons. Mpourm- the chirality and diameter and is a semiconductor with pakis and Froudakis [19] discovered that BNNTs are a wide band gap [9]. preferable to CNTs for hydrogen storage because of the ionic character of BNNTs bonds which can increase the binding energy of hydrogen. In addition, some methods have been shown to improve the efficiency of storage. * Correspondence: jushin-pon@mail.nsysu.edu.tw Department of Mechanical and Electro-Mechanical Engineering, Center for An increase in the diameter of BNNT can increase the Nanoscience and Nanotechnology, National Sun Yat-sen University, efficiency of hydrogen storage [20]. Further, Tang et al. Kaohsiung, 804, Taiwan © 2011 Ju et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
  2. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 2 of 11 http://www.nanoscalereslett.com/content/6/1/160 adjusted by applying the mechanical deformation. Since [21] improved the concentration of hydrogen storage to BNNTs have some material properties superior to CNT, 4.6 wt% by bending the BNNTs. BNNTs also have many it is worth understanding how to adjust the electronic great physical and chemical properties. Zhi et al. [14] properties of BNNT by the mechanical deformation for found that MWBNNTs have the ability to form covalent further applications, such as hydrogen storage for fuel bonds with ammonia and can act as a solute in an cell. Therefore, this study utilizes DFT to investigate organic solution. Chen et al. [15] obtained the result armchair (5,5) and zigzag (8,0) single-wall BNNTs under that field-emission current density of an Au-decorated different uni-axial loadings. The HOMO-LUMO gap, boron nitride nanotube (Au-BNNT) is significant radial bucking variety, and bond length are adopted to enhanced in contrast to pure BNNTs. Chen et al. [22] discuss the relationship between the mechanical defor- used ball milling-annealing to synthesize BNNTs and mation and electronic properties for the two different found that the average resistivity of that is 7.1 ± 0.9 × 1012 Ω. Chopra and Zettl [2] observed that the BNNT chiralities. has the highest elastic modulus of 700-900 GPa in one- dimensional fibers. Simulation model Recent studies have shown that applying strain to a In this study, DFT methods are adopted to study the one-dimensional material will affect its electrical prop- relationship between strain and electronic properties of erty. Shiri et al. discovered that the band gap of silicon single-wall armchair and zigzag BNNT. This method nanowire (SiNW) can be affected under uni-axial tensile has been widely used in theoretical calculations of nano- strain. They also found that the strain induced direct-to- tube systems, including structural and electronic proper- indirect transition in the band gap of SiNW with differ- ties. Density functional semi-core pseudo-potentials ent diameters [23]. Tombler et al. used theoretical and (DSPP) [29] calculations were employed with double experimental approaches to study the effect of single- numerical basis sets plus d-functions (DND) and gener- walled carbon nanotubes (SWNTs) with deformation on alized gradient approximation (GGA) [30] with the its electrical conductance. They found the electrical con- Perdew-Wang 1991 (PW91) generalized gradient ductance of SWNT is obviously reduced as compared to approximation correction [31]. Mulliken population ana- SWCNT without deformation [24]. For the theoretical lysis was used to obtain both the charge and net spin studies, Li et al. [25] demonstrated that the transport population on each atom. We chose the finite cluster property of CNT with double vacancy is reduced under (8,0) BNNT with length of 18.11 Å including totally 64 external force. The stress-strain curve of armchair CNTs boron, 64 nitrogen, and 16 hydrogen atoms, and (5,5) shows a step-by-step increasing behavior, and the C-C BNNT with length of 18.25 Å including totally 70 bond length varies significantly at specific strain during boron, 70 nitrogen, and 20 hydrogen atoms as the stu- the tensile process. Those changes are more apparent died systems. Table 1 lists the simulation result and for the smaller-sized armchair CNT. Wang reported a compares it to the previous studies, Ref. [20]. The differ- structural transformation from zigzag (Z-type) to an ent profiles of bond type in (8,0) and (5,5) BNNT are unusual type of fourfold-coordinated (H-type) and to shown in Figure 1a,b. The simulation result is close to armchair (A-type) structure in the ultrathin SiCNTs other studies and means that our results are accurate. under uni-axial compression [26]. Wu et al. [27] found Results and discussion that the radial deformation of BNNT significantly affects the H2 adsorption energy on BNNT. They presented the In order to investigate material properties for armchair relationship between the H2 adsorption energy at differ- and zigzag BNNTs at different strains, (8,0) and (5,5) ent adsorption sites and the extent of radial deformation BNNTs of close radii are used. Although the results of of BNNT. other armchair and zigzag BNNTs are not shown in this In experimental part, Kaniber et al. [28] utilized the study, the results are very similar for BNNTs of the piezoelectric device to apply different uni-axial strains to same type. Figure 2 shows the profiles of axial stress CNT. They mounted the CNT on two Au pads (source and HOMO-LUMO (highest occupied molecular orbital and drain) of a piezoelectric stack. When different vol- and lowest unoccupied molecular orbital) gap at differ- tages were applied to the piezoelectric device, the axial ent strains for (8,0) armchair and (5,5) BNNTs. The stress on the m plane of the nanotube in the n-direction length of CNT can be adjusted. For CNT with different uni-axial strains, they found that the electronic proper- is calculated by [32]. ties of CNT can be affected by the uni-axial mechanical ⎡ mv im v in ⎤ 1   Int ∑ deformation. From this experiment and references 1  mn = − ri ⋅ Fi ⎢ ⎥ (1) [23-28] it is obvious that besides the size and shape of ⎢ vi ⎥ Ns 2v i ⎣ ⎦ i nanomaterials, the electronic properties can be further
  3. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 3 of 11 http://www.nanoscalereslett.com/content/6/1/160 The normal strain in the axial direction of the BNNT Table 1 Diameter, bond length, and binding energy for different BNNTs is given by Nanotube and Tube Bond length Binding energy (eV/ l z ( t ) − l z ( o) stoichiometry diameter distribution each atom) = (3) (Å) l z ( o) Type Type I II BNNT (4,4) 5.657 1.461 1.456 6.361 where l z(t ) is the length of the BNNT in the axial 5.49a 1.440b 1.444b direction following elongation and l z ( o ) is the initial BNNT (5,5) 7.043 1.457 1.455 6.422 length, which the axial stress is zero after a complete a b b 6.87 1.439 1.442 geometry optimization by DFT. The stress-strain rela- BNNT (6,6) 8.43 1.462 1.458 6.457 tionship of the BNNT can then be obtained from Equa- 8.23a 1.439b 1.440b tions 1 and 3. BNNT (7,7) 9.49 1.462 1.459 6.476 The lengths of both (8,0) and (5,5) BNNTs after the 9.59a 1.438b 1.439b relaxation by the DFT method are defined as the refer- BNNT (8,8) 11.21 1.459 1.457 6.490 enced lengths at strain of 0, where the axial stresses are 10.95a 1.439b 1.439b 0 after calculation by Equation 1. As we focus on the BNNT (4,0) 3.556 1.499 1.439 6.187 electronic properties of the intact BNNTs at different 3.35a 1.476b 1.423b strains without bond breakage, the maximal strains BNNT (5,0) 4.191 1.473 1.442 6.371 shown in Figure 2 before significant necking and some 4.08a 1.460b 1.429b bond breakage are 21.5 and 27% for (8,0) and (5,5) BNNT (8,0) 6.263 1.460 1.454 6.606 BNNTs, respectively; the corresponding maximal stres- 6.37a 1.445b 1.434b ses are about 0.526 and 0.511 TPa. For the stress-strain a From Ref. [20]; bfrom Ref. [40]. profiles, it is apparent that the stresses increase with an increase in strain in both cases. The profiles of HOMU- where m is the mass of atom i ; v im and v in are the LUMO gaps, where the gap value for the (8,0) BNNT velocity components of atom i in the m- and n-direc- remains at a constant of 3.7 eV, are close to the refer- tions, respectively; v i is the volume assigned around ence value [34] from strain 0 to 5%, and then displays a atom i; Ns is the number of particles contained within parabolic decrease when the strain increases from 5 to region S , where S is defined as the region of atomic 12.5%. As the strain is larger than 12.5%, the gap interaction; r is the position of atom i; and FiInt is the decreases linearly with the increase of strain. For (5,5) internal force acting on atom i. BNNT, the HOMO-LUMO gap is 4.65 eV at strain 0, The first term on the right-hand side of Equation 1 which is close to the reference value [35], the gap line- describes the kinetic effect of the atomic motion and is arly decreases with the increase of strain until the strain dependent on the temperature. This term is not consid- reaches 20%. When the strain is larger than 20%, the ered for our current DFT calculation. The second term profile displays a parabolic decrease. Although the expresses the effect of the interactive forces and is deter- stress-strain profiles of (8,0) and (5,5) BNNTs seem very mined by the distance between the atoms. In Equation 2, similar, the variations of HOMO-LUMO gaps at differ- Vi is the Voronoi volume of atom i and is constructed by ent strains are clearly different. Accordingly, Figure 2 the perpendicular planes bisecting the lines between this clearly demonstrates that the mechanical deformations atom and all of its neighboring atoms. Clearly, it is time- of BNNTs significantly influence their electronic proper- consuming to compute the Voronoi volume of each atom ties, with the electronic properties of different chirality in the simulation system. Accordingly, Srolovitz et al. pro- BNNTs displaying different responses to the strains. posed the following formulation to obtain a sphere whose Further, different levels of strain may produce either lin- volume is equal to the original Voronoi volume [33]: ear or non-linear electronic property profiles. The variations of bond lengths and bending angles of ∑r −1 (8,0) BNNT at different strains are shown in Figure 3b, ij 4 3 c, with the corresponding bond lengths and bending j Vi = = (2) ai ai 2∑ r −2 3 angles depicted in Figure 3a. The B-N bonds parallel to ij the axial direction are designated as Bond-II, and the B- j N bonds slanted from the axial direction are labeled as where ai is the average radius of atom i and rij is the Bond-I. According to the bending angles formed by dif- distance between atom i and its neighboring atom j. ferent bond types and the central atom type, four angles
  4. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 4 of 11 http://www.nanoscalereslett.com/content/6/1/160 (A) (B) B63 Type Type I N61 Figure 1 Cross-section and side views of (a) single wall (8,0) BNNT and (b) (5,5) BNNT. Gray, white, and blue beads stand for boron, nitrogen, and hydrogen atoms, respectively. E, F, G, and H are used to indicate different bending labeled as A, B, C, and D are used to indicate different angle types in Figure 4c. In Figure 4b, the lengths of bending angle types used in Figure 3c. In Figure 3b, the Bond-IV significantly increase with the increase of lengths of Bond-I slightly increase with the increase of strain, but the lengths of Bond-III remain constant strain, but the lengths of Bond-II display a significant when the strain is smaller than 5% and slightly decrease increase with the increase of strain. As shown in Figure when the strain is larger than 5%. As shown in Figure 3c, angles B and C increase when the strain increases, 4c, angles F and G increase when the strain increases, whereas decreases in angles A and D can be seen as the whereas decreases in angles E and H with the increase strain increases. Consequently, the elongation of (8,0) of strain can also be seen. Consequently, the elongation BNNT is mainly due to the altering of bond angles and of the (5,5) armchair BNNT is mainly due to the alter- the elongation of Bond-II, which is parallel to the axial ing of bond angles and the elongation of Bond-IV which direction. is slanted from the axial direction. The relationship of bond lengths and bending angles In Figures 3c and 4c, at strain of 0 the bending angles of (5,5) BNNT at different strains are shown in Figure D and H are about 113.9° and 116.5° for (8,0) and (5,5) 4b,c, with the corresponding bond lengths and bending BNNTs, respectively. The other three angles A, B, and angles depicted in Figure 4a. The B-N bonds normal to C of (8,0) BNNT are close to 118.5° and angles E, F, the axial direction are designated as Bond-III, and those and G of (5,5) BNNT are about 120°. N atoms and their slanted from the axial direction are labeled as Bond-IV. nearest three B atoms form local pyramid structures and According to the bending angles formed by different are not located on the same cylindrical surface, with N bond types and central atom type, four angles labeled as
  5. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 5 of 11 http://www.nanoscalereslett.com/content/6/1/160 (A) (A) 0.6 4 Stress-strain curve HOMO-LUMO Gap 0.5 3.5 0.4 HOMO-LUMO Gap 3 Stress (TPa) 0.3 (B) 2.5 0.2 1.7 Bond-I 2 Bond-II 0.1 1.65 Bond length(angstrom) 0 1.5 1.6 0 5 10 15 20 25 strain % 1.55 (B) 1.5 0.6 Stress-strain curve HOMO-LUMO Gap 4.6 1.45 0.5 0 5 10 15 20 25 30 strain(%) 0.4 HOMO-LUMO Gap (C) 4.4 Stress (TPa) 0.3 AngleA AngleB 130 4.2 AngleD 0.2 AngleC Angle(anstrom) 0.1 4 120 0 0 5 10 15 20 25 30 Strain % 110 Figure 2 Stress-strain profiles for (a) (8,0) Zigzag BNNT and (b) (5,5) Armchair BNNT. The red line shows HOMO-LUMO gap variation at different strains. 0 5 10 15 20 25 strain(%) Figure 3 Simulation model and definitions for (a) bond angles a nd B atoms occupying the outer and inner shells, and bond lengths of (8,0) BNNT are shown. Bonds parallel to respectively, as reported in previous studies [36]. This axial are shown as Bond-II, and other ones slanted to the axial are shown as Bond-I. Bond angles are labeled as A, B, C, and D. phenomenon is called radial buckling and can also be Variation of (b) the radial buckling and bond lengths of (8,0) BNNT seen for SiC nanotubes and ZnO nanotubes [20,37]. To at different strains and (c) the radial buckling and bond angles of investigate the variation of radial bucking at different (8,0) BNNT at different strains. strains for (8,0) and (5,5) BNNTs, Figure 5 shows the
  6. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 6 of 11 http://www.nanoscalereslett.com/content/6/1/160 that the BNNT consists of two cylindrical surfaces with (A) N atoms situated on the outer surface [36]. At strain of 0, the values of radial buckling are about 0.02 and 0.074 for (8,0) and (5,5) BNNTs, indicating the radial buckling is less significant for a zigzag BNNT. In Figure 5, the radial buckling of the (5,5) BNNT dramatically decreases with an increase in strain, indicating that the B and N atoms are gradually forced to the same cylindrical sur- face when the (5,5) armchair BNNT is subjected to an increasing uni-axial external stress. However, for the (8,0) zigzag BNNT, the value of radial buckling remains (B) at an almost constant 0.02 when the strain continuously 1.7 increases. Bond-III Bond-IV Figure 6 shows the Mulliken charges at different 1.65 strains for the B63 and N61 atoms of the (8,0) BNNT Bond length(angstrom) and for the B68 and N35 atoms of the (5,5) BNNT. 1.6 These B and N atoms are located in the central sections 1.55 of the BNNTs, as shown in Figure 1; it is clear that the charge variations of B and N atoms at different strains 1.5 are very similar for (8,0) and (5,5). At strain 0, the charges of B and N atoms are about 0.465 and -0465 1.45 eV, respectively, which are in agreement with previous 1.4 studies [38]. When the strain becomes larger, the B and 0 5 10 15 20 25 30 N atoms appear more ionic. strain(%) The partial density of states (PDOS) profiles for B68 (C) and N35 atoms of the (8,0) BNNT and for B63 and N61 atoms of the (5,5) BNNT, as shown in Figures 7 and 8, Angle E Angle F 130 respectively, are further studied to demonstrate the Angle G Angle H strain effect on the electronic structures of BNNTs. Fig- ure 7a,b,c,d,e shows the PDOS of s and p orbitals of Angle(anstrom) B68 and N35 atoms as well as the summation of these 120 orbitals for the (8,0) BNNT. At strain of 0, there is no contribution to the total DOS from B68 2s and N35 2s 110 0.03 0.09 0 5 10 15 20 25 (8,0)BNNT Radial Buckling strain(%) (5,5)BNNT Radial Buckling Figure 4 Simulation model and definitions for (a) bond angles (8,0)BNNT Radial Buckling (5,5)BNNT Radial Buckling and bond lengths of (5,5) BNNT are shown. Bonds normal to axial are shown as Bond-III, and other ones slanted to the axial are 0.02 0.08 shown as Bond-IV. Bond angles are labeled as E, F, G, and H. Variation of (b) the radial buckling and bond lengths of (5,5) BNNT at different strains and (c) the radial buckling and bond angles of (5,5) BNNT at different strains. 0.01 0.07 r adial buckling at different strains. The definition of radial buckling b is as shown in Equation 4:  = rN − rB (4) 0 0.06 where r B and r N represent the radii of the B and N 0 5 10 15 20 25 30 Strain% cylinders. If the value of radial buckling approaches Figure 5 Radial Buckling of (8,0) and (5,5) BNNT at different zero, the B and N atoms will be located on the cylindri- strains. cal surface of the BNNT, while a positive value indicates
  7. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 7 of 11 http://www.nanoscalereslett.com/content/6/1/160 0.6 -0.44 Bs Density of States (electrons/eV) Density of States (electrons/eV) Density of States (electrons/eV) Density of States (electrons/eV) Density of States (electrons/eV) Strain 0% B68 Charge 1.6 Bp Ns N35 Charge Np B63 Charge Bs+Bp+Ns+Np 1.2 N61 Charge 0.56 -0.48 0.8 Nitrigen Charge (e) Boron Charge (e) 0.4 0.52 -0.52 0 Strain 5% 1.6 0.48 -0.56 1.2 0.8 0.44 -0.6 0.4 0 5 10 15 20 25 30 strain% 0 Strain 8% 1.6 Figure 6 Variation of the calculated atom charge of (8,0) and (5,5) BNNT. The solid and dashed lines show charge variation of boron and that of nitrogen, respectively. 1.2 0.8 orbitals around the Fermi level. It should be noted that the total DOS strength of empty states near Fermi level 0.4 mainly comes from N35 2p electron and to a lesser 0 degree B68 2p electron. The N35 2p orbital contributes 1.6 Strain 13% more to the total DOS of occupied states near the Fermi level, and grabs electron from nearby B atoms. 1.2 Moreover, the LUMO mainly comes from the B68 2p orbital and to a lesser degree N35’s 2p orbital. Conse- 0.8 quently, N atoms have negative charges and B atoms possess positive charges, which can be seen in Figure 6. 0.4 At strain of 5%, the unoccupied state is split into two 0 states, resulting in a significant decrease in the HOMO- Strain 21% 1.6 LUMO gap when the strain is larger than 5%, as shown in Figure 2a. When the strain increases from 5 to 13%, 1.2 the relative strengths of two split states become more dramatic, which can be seen in Figure 7b,c,d. At strain 0.8 of 21%, both the occupied and unoccupied states display a significant left-shift and the two split unoccupied 0.4 states merge into one unoccupied state, as shown in 0 Figure 7e. The contribution from the B68 2p to the -20 -15 -10 -5 0 5 10 HOMO becomes less significant when the strain Energy (eV) becomes larger, which can be seen at the peak indicated Figure 7 PDOS profiles of B68 and N35 atoms. by arrows in Figure 7a,b,c,d,e. This reveals that N atoms will grab more electrons from B atoms when the strain becomes larger, and B and N atoms become more ionic, states near the Fermi level, and grabs electron from as was shown in Figure 6. nearby B atoms. Consequently, N atoms have negative Figure 8a,b,c,d shows the PDOS of s and p orbitals of charges and B atoms possess positive charges, as was B63 and N61 atoms as well as the summation of those shown in Figure 6. For the empty states, the total DOS orbitals for the (5,5) BNNT. At strain of 0, there is strength mainly comes from the B63 2p electrons and almost no contribution to the total DOS from 2s orbi- to a lesser degree the N61 2p electron. As the strain tals of B63 and N61 around the Fermi level. The N61 increases to 8, 17, and 25%, the occupied states undergo 2p orbital contributes more to the DOS of occupied a slight right-shift toward the Fermi level and the
  8. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 8 of 11 http://www.nanoscalereslett.com/content/6/1/160 Mayer ’ s study [39]. The BO values are calculated 1.6 Bs Density of States (electrons/eV) Density of States (electrons/eV) within the first nearest neighbor atoms around a refer- Strain 0% Bp Ns enced atom, and this value becomes very small when 1.2 Np the distance between the reference atom and its near- Bs+Bp+Ns+Np est neighbor atom is beyond the stable bond length. In 0.8 Figure 9a, the distribution of positive iso-value around the B68 atom indicates that the extra electron will be 0.4 accumulated between the B-N bond after the B and N atoms form the (8,0) BNNT at strain of 0. The BO 0 values of two slanted B-N bonds are very close to that Strain 8% of the B-N bond parallel to the axial direction, indicat- 1.2 ing that the bond strengths of these two bond types are very close. Although the summation of the three 0.8 BO values decreases from 3.216 to 3.099 as the strain continuously increases from 0 to 21%, the BO value of 0.4 slanted bonds gradually increases from about 1.073 to 1.101, indicating the bonding strength will slightly 0 increase under the larger strain. However, the BO of the Density of States (electrons/eV) Strain 17% B-N bond parallel to the axial becomes smaller at larger 1.2 strains. The increase and decrease in the BO values for the slanted and parallel bonds become more consider- 0.8 able as the strain becomes larger than 5%, which is con- sistent with the variation of HOMO-LUMO gaps shown 0.4 in Figure 2a. As the strain increases from 0 to 21%, the distributions of electron differences along the slanted 0 bonds become wider, whereas that of the parallel bond Density of States (electrons/eV) Strain 25% turns out to be narrower. According to the result of the 1.2 Mulliken charge analysis shown in Figure 6, B and N atoms become more ionic under the larger strain. 0.8 Although the electrons transfer more from B atoms to N atoms at larger strain, the electron accumulation 0.4 along the slanted bonds will become more significant. In Figure 10a, the distribution of positive iso-value 0 around the B63 atom indicates that the extra electron -20 -15 -10 -5 0 5 10 will accumulate between the B-N bond after the B and Energy (eV) N atoms form the (5,5) BNNT at strain of 0. The BO Figure 8 PDOS profiles of B63 and N61 atoms. values of two slanted B-N bonds are slightly smaller than that of the B-N bond normal to the axial direction, unoccupied states left-shift, resulting in a decrease of indicating that the slanted bond strength is slightly the HOMO-LUMO gap, which can be seen from Figure weaker than that of the bond normal to the axial direc- 2b. During the tensional process, the unoccupied state is tion. The summation of three BO values decreases from not split into two states. 3.23 to 3.079 as the strain continuously increases from 0 The electron differences at the iso-value of 0.15 and to 25%, but the BO value of the normal bond gradually the Mayer bond orders (BO) of three B-N bonds at increases from 1.110 to 1.289, indicating the bonding different strains for (8,0) and (5,5) BNNTs are shown strength of the normal bond will slightly increase under in Figures 9 and 10. The electron difference is defined the larger strain. However, the BO values of two slanted as the electron density distribution of BNNT minus bonds become smaller at larger strains. As the strain the electron density distributions of isolated B atoms increases from 0 to 25%, the distributions of electron and isolated N atoms which constitute this BNNT. differences along the slanted bonds become narrow, The value of the Mayer bond order between two whereas that of the normal bond turns out to be wider, atoms is very close to the corresponding classical bond indicating that the electron accumulation along the number between these two atoms, and the detailed slanted bonds will become more significant when the introduction of Mayer bond order can be found in BNNT is under larger strain.
  9. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 9 of 11 http://www.nanoscalereslett.com/content/6/1/160 Figure 9 Deformation density and Mayer bond orders are shown for boron on (8,0) BNNT at different strains. The iso-value is 0.15. (a) Strain = 0%, (b) strain = 5%, (c) strain = 8%, (d) strain = 13%, (e) strain = 21%. Figure 10 Deformation density and Mayer bond orders are shown for boron on (5,5) BNNT at different strains. The iso-value is 0.15. (a) Strain = 0%, (b) strain = 8%, (c) strain = 17%, (d) strain = 25%.
  10. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 10 of 11 http://www.nanoscalereslett.com/content/6/1/160 11. Zhao JX, Ding YH: The effects of O-2 and H2O adsorbates on field- Conclusion emission properties of an (8,0) boron nitride nanotube: a density This study utilizes DFT calculation to address the influ- functional theory study. Nanotechnology 2009, 20:085704. ence of axial tensions on the electronic properties of 12. Golberg D, Bando Y, Tang CC, Zhi CY: Boron nitride nanotubes. Adv Mater 2007, 19:2413. (8,0) zigzag and (5,5) armchair BNNTs. Although the 13. Enyashin AN, Ivanovskii AL: Mechanical and electronic properties of a C/ stress-strain profiles indicate the mechanical properties BN nanocable under tensile deformation. Nanotechnology 2005, 16:1304. of these two BNNTs are very similar, the variations of 14. Zhi CY, Bando Y, Tang CC, Huang Q, Golberg D: Boron nitride nanotubes: functionalization and composites. J Mater Chem 2008, 18:3900. electronic properties at different uni-axial strains are 15. Chen H, Zhang HZ, Fu L, Chen Y, Williams JS, Yu C, Yu DP: Nano Au- drastically different. At strain lower than 5%, the decorated boron nitride nanotubes: Conductance modification and HOMO-LUMO gap of (8,0) BNNT remains at a con- field-emission enhancement. Appl Phys Lett 2008, 92:243105. 16. Ma RZ, Bando Y, Zhu HW, Sato T, Xu CL, Wu DH: Hydrogen uptake in stant value, but decreases at a larger strain. For the (5,5) boron nitride nanotubes at room temperature. J Am Chem Soc 2002, BNNT, the gap monotonically decreases when the strain 124:7672. becomes larger. The changes in nanotube geometries, 17. Rajeswaran M, Blanton TN, Zumbulyadis N, Giesen DJ, Conesa-Moratilla C, Misture ST, Stephens PW, Huq A: Three-dimensional structure PDOS of B and N atoms, B and N charges also indicate determination of N-(p-tolyl)-dodecylsulfonamide from powder diffraction the uni-axial deformation definitely influences the elec- data and validation of structure using solid-state NMR spectroscopy. tronic properties of (8,0) and (5,5) BNNTs. J Am Chem Soc 2002, 124:14450. 18. Yuan JH, Liew K: Effects of boron nitride impurities on the elastic properties of carbon nanotubes. Nanotechnology 2008, 19:445703. Abbreviations 19. Mpourmpakis G, Froudakis GE: Why boron nitride nanotubes are Au-BNNT: Au-decorated boron nitride nanotube; BNNT: boron nitride preferable to carbon nanotubes for hydrogen storage?: An ab initio nanotubes; CNTs: carbon nanotubes; DFT: density functional theory; DSPP: theoretical study. Catal Today 2007, 120:341. density functional semi-core pseudo-potentials; GGA: generalized gradient 20. Baumeier B, Kruger P, Pollmann J: Structural, elastic, and electronic approximation; PDOS: partial density of states; PW91: Perdew-Wang 1991; properties of SiC, BN, and BeO nanotubes. Phys Rev B 2007, 76:085407. SWBNNTA: single-walled boron nitride nanotube arrays; SWNTs: single-walled 21. Tang CC, Bando Y, Ding XX, Qi SR, Golberg D: Catalyzed collapse and carbon nanotubes. enhanced hydrogen storage of BN nanotubes. J Am Chem Soc 2002, 124:14550. Acknowledgements 22. Chen H, Chen Y, Liu Y, Fu L, Huang C, Llewellyn D: Over 1.0 mm-long The authors would like to thank the (1) National Science Council of Taiwan, boron nitride nanotubes. Chem Phys Lett 2008, 463:130. under Grant No. NSC98-2221-E-110-022-MY3, (2) National Center for High- 23. Shiri D, Kong Y, Buin A, Anantram MP: Strain induced change of bandgap performance Computing, Taiwan, and (3) National Center for Theoretical and effective mass in silicon nanowires. Appl Phys Lett 2008, 93:073114. Sciences, Taiwan, for supporting this study. 24. Tombler TW, Zhou CW, Alexseyev L, Kong J, Dai HJ, Lei L, Jayanthi CS, Tang MJ, Wu SY: Reversible electromechanical characteristics of carbon Authors’ contributions nanotubes under local-probe manipulation. Nature 2000, 405:769. TWL carried out the density functional theory simulation and performed the 25. Li Z, Wang CY, Ke SH, Yang W: First-principles study for transport data analyze. YCW drafted the manuscript and participated in its design. SPJ properties of defective carbon nanotubes with oxygen adsorption. Eur participated in the design of the study and conceived of the study. All Phys J B 2009, 69:375. authors read and approved the final manuscript. 26. Wang XQ, Wang BL, Zhao JJ, Wang GH: Structural transitions and electronic properties of the ultrathin SiC nanotubes under uniaxial Competing interests compression. Chem Phys Lett 2008, 461:280. The authors declare that they have no competing interests. 27. Wu XJ, Yang JL, Hou JG, Zhu QS: Deformation-induced site selectivity for hydrogen adsorption on boron nitride nanotubes. Phys Rev B 2004, Received: 14 July 2010 Accepted: 21 February 2011 69:153411. Published: 21 February 2011 28. Kaniber SM, Song L, Kotthaus JP, Holleitner AW: Photocurrent properties of freely suspended carbon nanotubes under uniaxial strain. Appl Phys Lett 2009, 94:261106. References 29. Delley B: Hardness conserving semilocal pseudopotentials. Phys Rev B 1. Iijima S: Helical microtubules of graphitic carbon. Nature 1991, 354:56. 2002, 66:155125. 2. Chopra NG, Zettl A: Measurement of the elastic modulus of a multi-wall 30. Perdew JP, Burke K, Ernzerhof M: Generalized gradient approximation boron nitride nanotube. 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  11. Ju et al. Nanoscale Research Letters 2011, 6:160 Page 11 of 11 http://www.nanoscalereslett.com/content/6/1/160 38. Nirmala V, Kolandaivel P: Structure and electronic properties of armchair boron nitride nanotubes. Theochem J Mol Struct 2007, 817:137. 39. Mayer I: Bond orders and valences from abinito wave-functions. Int J Quantum Chem 1986, 29:477. 40. Jia JF, Wu HS, Jiao H: The structure and electronic property of BN nanotube. Physica B 2006, 381:90. doi:10.1186/1556-276X-6-160 Cite this article as: Ju et al.: Tuning the electronic properties of boron nitride nanotube by mechanical uni-axial deformation: a DFT study. Nanoscale Research Letters 2011 6:160. Submit your manuscript to a journal and benefit from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the field 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com
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