Eur. Phys. J. B 76, 289–299 (2010) DOI: 10.1140/epjb/e2010-00218-y
THE EUROPEAN PHYSICAL JOURNAL B
Regular Article
Structural, electronic and magnetic properties of the 3d transition metal atoms adsorbed on boron nitride nanotubes J.-M. Zhang1,a , S.-F. Wang1 , L.-Y. Chen1 , K.-W. Xu2 , and V. Ji3 1 2 3
College of Physics and Information Technology, Shaanxi Normal University, Xian 710062, Shaanxi, P.R. China State Key Laboratory for Mechanical Behavior of Materials, Xian Jiaotong University, Xian 710049, Shaanxi, P.R. China ICMMO/LEMHE UMR CNRS 8182, Universit´e Paris-Sud 11, 91405 Orsay Cedex, France Received 25 November 2009 / Received in final form 13 March 2010 c EDP Sciences, Societ` Published online 2 July 2010 – a Italiana di Fisica, Springer-Verlag 2010 Abstract. Adsorption configurations for a series of transition metal (TM) 3d atoms adsorbed on the zigzag (8, 0) BNNT at five different sites have been investigated using the first-principles PAW potential within DFT under GGA. The most stable adsorption sites are different for different TM atoms. Partially filled 3d metals V, Cr and Mn can bind strongly with zigzag (8, 0) BNNT, and Sc, Ti, Co and Ni can be chemically adsorbed on the (8, 0) BNNT. The binding between the Fe or Cu atom and the BNNT is only marginal. One unusual case is Zn. Its zero binding energy independent of the adsorption sites implies it can only physically adsorbed on the BNNT mainly stemmed from the van de Waals interaction. Electronic structure analyses show that: (1) for each TM atom adsorbed at five different sites, the total DOS curves of both majority and minority spins make a slightly relative shift along the energy axis, and for each site the total DOS of the minority spin shifts slightly in high energy direction with respect to that of the majority spin lead to a exchange splitting, except fully filled 3d metals Cu and Zn; (2) total DOS curves of both the majority and minority spins for the adsorbed systems shift to the lower energy region compared with that of the pristine (8, 0) BNNT. And the smaller 3d electrons number of the TM atom, the larger shift to the lower energy region of its DOS curves; (3) for V-, Mn- and Fe-adsorbed (8, 0) BNNT, only one type of electrons (either majority spin or minority spin) passes through the Fermi level implies these adsorbed systems are all half-metals.
1 Introduction Because of their unique structures, the carbon nanotubes (CNTs) have been shown to exhibit interesting optical, electronic, superconducting and magnetic properties, and also provide a unique opportunity for fabricating novel one dimensional system [1–4]. Experimental and theoretical studies show that most transition metal (TM) atoms can be adsorbed onto CNTs [5–17] thereby the adsorption systems have further applications in many areas such as catalysis [11], hydrogen storage [12], sensing [13–16] and the fabrication of magnetic nanodevices [17]. Advances in both experimental and theoretical investigations have inspired the development of novel nanodevices through TM atom adsorption on nanotubes. Soon after the discovery of CNTs, one of their isoelectronic structures, boron nitride nanotubes (BNNTs) have been predicted by theoretical calculation [18,19] and synthesized experimentally [20,21]. Different from CNTs, BNNTs are insulators independent of their chirality and diameter and possess the potential for nanoscale electronic devices owing to their special properties such as a
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strong hardness, high thermal stability and chemical inertness [18,22–24]. Experimental and theoretical investigations have also shown that BNNTs prefer a zigzag orientation during the growth [23–27]. The adsorbed behavior (for example, endohedral, substitutional, and exohedral doping) of BNNTs is important since it may create the acceptors or donors for the use in the nanoscale electronic device, such as nanomagnets, metallic connects, and spintronic devices, as well as for further studies of nanotubebased sensors, catalysts, and storage materials [28–46]. For instances, BNNTs were proposed as potential hydrogen storage media by Ma et al. [44] and Tang et al. [45]; either single boron or single nitrogen atom substituted in the C-doped BNNTs was found to induce spontaneous magnetization [46]; the adsorption of toxic gaseous molecules, such as formaldehyde (HCOH), on BNNTs was studied by Wang et al. [34] and so on. Therefore the studies for the adsorbing behavior of BNNTs have the most important significance in the coming days. In this paper, the structural, electronic and magnetic properties of each 3d transition-mental (TM) atom adsorbed on the outer surface of the zigzag (8, 0) BNNT have been investigated using the projector-augmentedwave (PAW) potential approach to the density-functional
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theory (DFT) within the generalized-gradient approximation (GGA) implemented in Vienna ab-initio simulation package (VASP). The rest of the paper is organized as follows. Section 2 gives the details of our DFT calculation method and models of the adsorption configurations. In Section 3, we firstly present the binding energies of the adsorption configurations, then the results for electronic structures mainly including total density of states (DOS), projected density of states (PDOS), magnetic moments, charge density and band structures. Finally, the conclusions of the work are given in Section 4.
2 Calculation methods and model All the calculations are performed within the DFT using the plane-wave basis VASP code [47–52]. The 2s2 2p1 and 2s2 2p3 electrons are taken as the valence electrons for boron and nitride atoms, and the 3d1 4s2 , 3d2 4s2 , 3d3 4s2 , 3d5 4s1 , 3d5 4s2 , 3d6 4s2 , 3d7 4s2 , 3d8 4s2 , 3d10 4s1 and 3d10 4s2 electrons are taken as the valence electrons for 3d TM atoms Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn atoms, respectively. The electron-ionic core interaction is represented by the PAW potentials [53] which are more accurate than the ultrasoft pseudopotentials. As it is known that the GGA is an improvement over local density approximation (LDA) in describing the electronic properties of transition metals [54,55] and is more accurate for inhomogeneous electron densities and the calculation of binding energies [56]. We use GGA since it considers not only the charge density as in LDA, but also the gradient of the charge density which is more important for the cases of the nanobutes with adsorption of the transition metal atoms and thus large gradient of the charge density where. To treat electron exchange and correlation, we chose the Perdew-Burke-Ernzerhof [56] formulation of the GGA, which yields the correct ground-state structure of the combined systems. The cutoff energy for the planewaves is chosen to be 400 eV, which is much higher than the 250 eV cutoff typically used for PAW potentials, and the supercell is large enough to ensure that the vacuum space is 18 ˚ A to eliminate the interaction between periodic images. The Brillouin zone integration is performed within the Gamma centered Monkhorst-Pack scheme [57] using 1 × 1 × 10 k-points. To avoid the numerical instability due to level crossing and quasi-degeneracy near the Fermi level, we use a method of Methfessel-Paxton order N (N = 1) with a width of 0.2 eV. Geometric structures of the adsorption systems are fully relaxed to minimize the total energy of the system until a precision of 10−4 eV is reached. The conjugate gradient minimization is used for optimization of the atom coordinates until the forces acting on each atom are smaller than 0.02 eV/˚ A. The zigzag (8, 0) BNNT is chosen here because the zigzag orientation is a preferred growth orientation [23–27] and the (8, 0) BN tube has a moderate diameter. Considering the reliability of the calculations, we chose a tetragonal supercell, who contains 32 boron and 32 nitride atoms as well as a single TM atom, the length of c (in the axial direction) is twice of the periodicity of the (8, 0) BNNT. The
b
b' Z
a'
c
a
H
N
A
B c'
Fig. 1. (Color online) Side views of different adsorption sites of a single TM atom adsorbed on the zigzag (8, 0) BNNT. Yellow and blue balls denote boron and nitride atoms, respectively. The five different sites are denoted by the red letters.
˚) between two neighboring adsorbed TM distance (8.64 A atoms is much larger than the nearest-neighbor spacing in bulk structure. So the interaction between two neighboring TM atoms is weak enough to be neglected. Figure 1 shows the side views of different adsorption sites of a single TM atom adsorbed on the zigzag (8, 0) BNNT. Yellow and blue balls denote boron and nitride atoms, respectively. The five different sites are (1) the top site of the boron atom (B), (2) the top site of the nitrogen atom (N), (3) the hollow site of the B3 N3 hexagon ring (H), (4) the bridge site over an axial BN bond (A), and (5) the bridge site over a zigzag BN bond (Z).
3 Results and discussions Firstly the pure zigzag (8, 0) BNNT is fully optimized, and the electronic band structures and total density of states (DOS) are calculated considering spin as polarized. The calculated B−N bond length is about 1.44 ˚ A and the average diameter about 6.35 ˚ A, in accordance with previously reported values [32]. These results suggest that the method used in the present calculations is suitable for describing the behavior of BNNTs. The calculated band structures (left two panels) and total DOS (right two panels) are shown in Figure 2. The Γ and Z represent two highly symmetric points in the Brillouin zone of the supercell, that is (0, 0, 0) and (0, 0, 0.5), respectively. The Fermi level is set to zero energy and indicated by the horizontal red dotted lines. One readily identifies the symmetry in the proximity of the Fermi level between majority spin and minority spin for both the band structures and the total DOS. This indicates pure zigzag (8, 0) BNNT has no spin splitting in the proximity of the Fermi level and thus no magnetic moment. Secondly, the lowest unoccupied conduction band (LUCB) and the highest occupied valence band (HOVB) appear at Γ point shows pure zigzag (8, 0) BNNT is a direct semiconductor with a band gap of about 3.565 eV closing to the value of 3.65 eV reported early [18,58]. One similar band gap also can be gained from the total DOS.
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10
Majority Spin
Minority Spin
5
4 Binding Energy (eV)
Energy (eV)
B N A Z H
5
Minority Spin
Majority Spin
0
-5
3 2 1
-10 Γ
Ζ Γ
Ζ -15
-10
-5
0
5
10
15
DOS (1/eV)
Fig. 2. (Color online) Band structure (left two panels) and total DOS (right two panels) for the pure zigzag (8, 0) BNNT. The Γ and Z represents two highly symmetric points in the Brillouin zone of the supercell, that is, (0, 0, 0) and (0, 0, 0.5), respectively. The Fermi level is set to zero energy and indicated by the horizontal red dotted lines.
Ten 3d TM atoms (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) have been considered to be adsorbed on the outer surface of the pure zigzag (8, 0) BNNT. For each species of the adsorbate, five different initial adsorption sites (as shown in Fig. 1) are selected to examine the interaction between the (8, 0) BNNT and single TM atom. In order to investigate the stability of different adsorption configurations, we have calculated the binding energy (Eb ) of all adsorption configurations. Here, the binding energy is given by the equation Eb = E (BNNT) + E (TM) − E (BNNT + TM)
(1)
where E(BNNT), E(TM), and E(BNNT + TM) are the spin-polarized total energies (per supercell) of the pristine BNNT, a single TM atom, and the BNNT with adsorbed TM atom, respectively. So a positive binding energy indicates an attractive interaction between the (8, 0) BNNT and the single TM atom and thus the adsorption process is exothermic. On the contrary, a negative binding energy indicates a repulsive interaction between them and thus the adsorption processes is endothermic. The calculated binding energies for each TM atom adsorbed at five sites on the (8, 0) BNNT are compared in Figure 3 as a function of the number of 3d electrons (Nd ) of the considering TM atoms, where the red square, green circular, blue triangular, cyan diamond and magenta star symbols denote the binding energies of each TM atom adsorbed at the B, N, A, Z and H sites, respectively, and the points connected by black line represent the binding energies of the most stable configurations. In addition, all calculated values of the binding energies are also listed in the fifth column of Table 1 together with the corresponding bond distances between boron atom and TM atom (dB−TM ) as well as between nitrogen atom and TM atom (dN−TM ) as well as the net magnetic moment (μa ) of the adsorption atoms compared with that (μi ) of the isolated atoms. It can be seen that, firstly, the most stable adsorption sites are different for different TM atoms. The Sc, Ti, Cr and Zn atoms are H site, V atom is B site, Mn atom is N
0 1 Sc
2 Ti
3 V
54 Cr
5 Mn
6 Fe
7 Co
8 Ni
10 9 Cu
10 Zn
Nd
Fig. 3. (Color online) Binding energy (Eb ) for a single TM atom adsorbed at five different sites on outer surface of the zigzag (8, 0) BNNT as a function of the number of 3d electrons (Nd ) of the first-row transition metals.
site, Fe atom is Z site, and the Co, Ni and Cu atoms are A sites. While Wu et al. found the most stable adsorption sites are Z, A, A and H sites for V, Cr, Mn and Co atoms, respectively. This may be because for these four atoms adsorbed at the top sites of the boron and nitrogen atoms were not considered in their paper [30]. Secondly, the larger variations in binding energy for different adsorption sites are obtained for partially filled 3d metals V, Cr and Mn, and the smaller variations in binding energy for different adsorption sites are found for fully filled 3d metals Cu and Zn. However, partially filled 3d metal Fe is an exception, it always has a low binding energy, regardless of adsorption site. The reason is not clear and further investigation is needed. Furthermore, from the maximum of the binding energy, we know that the favorable adsorption TM atoms are partially filled 3d metals Mn, V and Cr at N, H and B sites, respectively. These results show that the numbers of 3d electrons (Nd ) of the TM atoms play an important role in adsorptions but Fe is an exception. Thirdly, according to the relative values of the binding energies (black line), we know that the partially filled 3d metals V, Cr and Mn can bind strongly with zigzag (8, 0) BNNT (with Eb > 4.0 eV), and some others such as Sc, Ti, Co and Ni can be chemically adsorbed on the sidewall of the (8, 0) BNNT (with 1.0 eV < Eb < 2.0 eV). The binding between Fe or Cu atom and the BNNT is only marginal (with Eb < 0.5 eV). One unusual case is Zn. Its zero binding energy independent of the adsorption sites implies it can not be chemically but physically adsorbed on the BNNT mainly stemmed from the van de Waals interaction. Fourthly, the variation tend of the binding energy with Nd for the most stable adsorption configurations (black line) is not only unlike to that in the case of TM atoms adsorbed on C and SiC nanotubes [8,9,59], but also unlike to that in the case of TM atoms adsorbed on BNNT [30]. Finally, comparing with magnetic moment μi of the isolated TM atoms, one can see that a zero magnetic moment is still obtained for Cu and Zn except Cu at H site, the net magnetic moments of the ferromagnetic TM atoms
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Table 1. Calculated binding energies (Eb ) for ten TM atoms adsorbed at five different sites on outer surface of the zigzag (8, 0) BNNT, the corresponding bond distances between boron atom and TM atom (dB−TM ) as well as between nitrogen atom and TM atom (dN−TM ) (the superscripts a, b, c and a , b , c of the bond distances for H site denote the different positions of the nitrogen and boron atoms respectively specified in Fig. 1), and the net magnetic moment (μa ) of the adsorption atoms compared with that (μi ) of the isolated atoms. TM
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Initial site B N A Z H B N A Z H B N A Z H B N A Z H B N A Z H B N A Z H B N A Z H B N A Z H B N A Z H B N A Z H
dB−TM (˚ A) 2.490 2.627 2.499 2.493 2.370a 2.744b 2.746c 2.309 2.553 2.457 2.456 2.412a 2.635b 2.633c 2.418 2.582 2.498 2.464 2.550a 2.682b 2.677c 2.595 2.774 2.566 2.662 2.988a 3.376b 3.363c 3.492 3.763 3.826 3.722 3.902a 4.153b 4.154c 2.977 3.456 3.207 3.114 3.534a 3.760b 3.771c 2.123 2.645 2.092 2.680 3.448a 3.683b 3.683c 1.917 2.333 2.133 2.130 3.409a 3.542b 3.549c 2.213 2.536 2.250 2.319 3.438a 3.630b 3.633c 3.610 3.927 3.773 3.766 3.886a 4.145b 4.143c
dN−TM (˚ A) 2.580 2.198 2.232 2.169 2.183a 2.849b 2.847c 2.409 2.234 2.267 2.213 2.182a 2.680b 2.678c 2.572 2.145 2.177 2.164 2.157a 2.803b 2.801c 2.909 2.395 2.551 2.673 3.097a 3.242b 3.256c 3.739 3.381 3.809 3.654 3.871a 4.087b 4.085c 3.277 3.063 3.155 3.002 3.473a 3.721b 3.709c 2.344 2.217 2.072 2.501 3.391a 3.631b 3.629c 1.972 1.840 1.864 1.867 3.186a 3.573b 3.567c 2.525 2.132 2.198 2.282 3.304a 3.558b 3.561c 3.857 3.580 3.747 3.733 3.881a 4.062b 4.074c
Eb (eV) 0.386 0.859 0.505 1.283 1.625 0.711 0.782 0.801 0.786 1.357 4.286 1.029 3.749 2.232 3.824 3.243 1.112 2.367 2.671 4.006 4.602 5.193 1.853 2.191 3.480 0.064 −0.126 0.034 0.140 −0.184 0.993 0.792 1.191 0.669 0.441 1.442 1.490 1.649 1.546 0.211 0.520 0.457 0.561 0.417 0.224 0.007 −0.001 0.005 0.005 0.009
μa (μB ) 1.224 0.713 1.192 0.484 0.481 2.200 2.178 2.183 2.084 1.693 3.325 3.259 3.283 3.271 3.211 4.438 4.366 4.401 4.421 4.457 4.429 4.562 4.582 4.463 4.566 3.479 3.452 3.492 3.473 3.464 2.157 2.074 1.894 2.330 2.476 −0.200 −0.108 −0.113 −0.133 1.175 0.000 0.000 0.000 0.000 0.206 0.000 0.000 0.000 0.000 0.000
μi (μB )
0.979
2.061
3.220
3.966
4.150
3.511
2.505
1.178
0.000
0.000
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Fig. 4. (Color online) The stable adsorption configurations for a single Sc atom adsorbed on the zigzag (8, 0) BNNT initially at (a) B, (b) N, (c) A, (d) Z and (e) H sites, where the yellow, blue and gray balls denote the B, N and Sc atoms, respectively.
Fe, Co and Ni are reduced after adsorbing on BNNT, but those of anti-ferromagnetic TM atoms Cr and Mn are increased after adsorption. The net magnetic moments of the remaining TM atoms Sc, Ti and V at H site are decreased due to having more coordinations. Using the Sc case as an example, the stable adsorption configurations of Sc atom on the zigzag (8, 0) BNNT at the B, N, A, Z and H sites initially are shown in Figures 4a−4d and 4e, where the yellow, blue and gray balls denote the boron, nitride and Sc atoms, respectively. It can be seen that the adsorption configuration at the B site is similar to that at the A site. In detail, at the B site, two bond lengths dB−Sc and dN−Sc are 2.490 and 2.580 ˚ A, respectively. At the A site, the two bond lengths
change to 2.499 and 2.232 ˚ A, respectively. At the N site, the two bond lengths are 2.627 and 2.198 ˚ A, respectively. At the Z site, the two bond lengths are 2.493 and 2.169 ˚ A, close to those at the A site. At the H site, the Sc atom no longer locates above the center of the B3 N3 hexago nal ring, the three B−Sc bonds lengths daB−Sc , dbB−Sc and dcB−Sc are 2.370, 2.746 and 2.744 ˚ A, and three N−Sc bonds a b c lengths dN−Sc , dN−Sc and dN−Sc are 2.183, 2.849 and A 2.847 ˚ A, respectively. The results dbB−Sc ≈ dcB−Sc ≈ 2.74 ˚ and dbN−Sc ≈ dcN−Sc ≈ 2.85 ˚ A satisfy the tube symmetry requirement, and the former is shorter than the latter as well as daN−Sc < daB−Sc indicate the Sc atom tends to locate closer to the Na atom than to Ba atom. The calculated
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B N A Z H
(a) Sc 10 0
(b) Ti
-10 20 -20 (c) V
(d) Cr
(e) Mn
(f) Fe
(g) Co
(h) Ni
(i) Cu
(j) Zn
10 0 -10
DOS (1/eV)
20 -20 10 0 -10 20 -20 10 0 -10 20 -20 10 0 -10 -20
-5
0
-5
5
0
5
Energy (eV)
Fig. 5. (Color online) The total densities of states DOS with the upper (lower) panel representing the majority (minority) spin for the stable adsorption configurations of the single TM atoms at five different sites on the (8, 0) BNNT. The Fermi level is set to zero energy and indicated by the vertical red dashed lines.
binding energies range from 0.386 to 1.625 eV for the five stable adsorption configurations of Sc atom on the (8, 0) BNNT, all these positive binding energy values indicate that the adsorption processes are all exothermic or spontaneous. The H site corresponds to the highest binding energy of 1.625 eV means it is the most stable adsorption site for Sc atom adsorbed on the (8, 0) BNNT. Better insight into the distribution of the electrons with energy can be gained from an analysis of electronic density of states (DOS) and band structures. So the total DOS with the upper (lower) panel representing the majority (minority) spin for a single TM atom at five different stable sites, as indicated in Figure 5a, on the zigzag (8, 0)
BNNT are presented in Figures 5a Sc, 5b Ti, 5c V, 5d Cr, 5e Mn, 5f Fe, 5g Co, 5h Ni, 5i Cu and 5j Zn. The Fermi level is set to zero energy and indicated by the vertical red dashed lines. It can be seen that, for each TM atom adsorbed at five different sites, the total DOS curves of both majority and minority spins make a slightly relative shift along the energy axis, and for each site the total DOS of the minority spin shifts slightly in high energy direction with respect to that of the majority spin lead to a exchange splitting. Two exception cases are observed for fully filled 3d metals Cu and Zn. For Cu adsorption, except at H site having an exchange splitting and thus magnetism, at each of the remaining four sites the total
S.-F. Wang et al.: Structural, electronic and magnetic properties of the 3d transition metal atoms... 20 Majority Spin
15
Sc Ti V Cr Mn Fe Co Ni Cu Zn
DOS (1/eV)
10 5 0 -5 -10 Minority Spin
-15 -20
0
2
4 Energy (eV)
6
8
Fig. 6. (Color online) The spin-polarized total densities of states DOS with the upper (lower) panel representing the majority (minority) spin for the most stable adsorption configurations of the single TM atoms adsorbed on the (8, 0) BNNT together with that of pure (8, 0) BNNT (dashed line).
DOS curves of the majority and minority spins are still symmetry and thus no magnetism. For Zn adsorbed at five different sites, all total DOS curves are not only superposition for either majority spin or minority spin, but also symmetry between majority spin and minority spin and thus no magnetism, and all adsorption configurations are semiconductor due to a broad zero DOS appeared above the Fermi level. In addition, the total DOS with the upper (lower) panel representing the majority (minority) spin of the most stable adsorption configurations for ten different TM atoms adsorbed on the (8, 0) BNNT are compared in Figure 6 together with that of pure (8, 0) BNNT (dashed line). It can be seen that the DOS curves of both the majority and minority spins for the adsorbed systems shift to the lower energy region compared with that of the pristine BNNT. And the smaller 3d electrons number Nd of the TM atom, the larger shift to the lower energy region of its DOS curves. This may be because the bonding of the TM atoms to BNNT is via a Dewar coordination [60] donating a 3d electron from the TM atom to the BNNT leaving the paired d electrons to reside on the TM atom. So the Highest Occupied Molecular Orbital (HOMO) states that cross the Fermi level have no significant overlap with that of the pristine BNNT (dashed line) which also implies these electrons in these states are localized on the TM atom. In Figure 7, we present the total DOS (black lines) and PDOS (red lines) onto the TM atoms with the upper (lower) panel representing the majority (minority) spin for the most stable configurations of a single TM atom adsorbed on the (8, 0) BNNT. The Fermi level is set to zero energy and indicated by the vertical red dashed lines. It can be seen that firstly, comparing the DOS at the Fermi level of majority spin (upper panel) with that of minority spin (lower panel), one readily identifies the adsorbed systems have a high spin polarization and magnetic moment except for fully filled 3d metals Cu and Zn cases. Secondly, comparing the total DOS of each adsorbed system with the PDOS onto the corresponding TM atom, we know that the peaks on the total DOS curves near the Fermi
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level are contributed to the 3d electrons of the TM atom, except for fully filled 3d metals Cu and Zn their 3d states form two symmetry narrow DOS peaks located far below the Fermi level and thus without magnetism. To some extent, the s electrons of the TM atom also contribute to the density of states near the Fermi level, but the contribution is much less than that from the d electrons. Consequentially, the high spin polarization and magnetic moment of the adsorbed systems are mainly coming from the 3d electrons of the partial filled 3d TM atoms. In particular, for V-, Mn- and Fe-adsorbed (8, 0) BNNT, only one type of electrons (either majority spin or minority spin) passes through the Fermi level implies the V-, Mn- and Fe-adsorbed (8, 0) BNNT are all half-metals and can be used in the field of spintronics for producing nearly 100% spin polarized currents. Using the Ti case as an example, the charge density of the most stable adsorption configuration of Ti atom at H site on the zigzag (8, 0) BNNT further illuminates the effect of spin polarization. Shown in Figures 8a−8c are the respective charge density for the majority spin, minority spin, and the difference of the two spins on the layer through the tube axis and Ti atom. Magnitude of the charge density is expressed by colors, with the red at one end of the spectrum being the smallest and the blue at the other end the largest. Firstly, small overlap between Ti atom and B atom as well as N atom can be seen from both Figures 8a and 8b. This indicates there is the weak interaction between Ti atom and boron atom as well as nitride atom. Secondly, Figure 8c indicates that the magnetism is confined within the Ti atom, in consistent with the conclusion obtained from the DOS analysis in Figure 7. Understandably, because the ground state of the pristine (8, 0) BNNT is nonmagnetic, the net magnetic moment should origin from the magnetism of the adsorbed TM atom. Thirdly, charge density around the Ti atom in Figure 8a for majority spin is much larger than that in Figure 8b for minority spin shows the Ti atom is responsible for high spin polarization of the Ti adsorbed system. This high spin polarization ensures a very high degree of passage of the preferred spin. Therefore, the Ti atom adsorbed on the zigzag (8, 0) BNNT could be of interest for the use in electron spin injection. Band structures of both majority spin (left panel) and minority spin (right panel) for the most stable adsorption configurations of the TM atoms on the zigzag (8, 0) BNNT are also shown in Figures 9a Sc, 9b Ti, 9c V, 9d Cr, 9e Mn, 9f Fe, 9g Co, 9h Ni, 9i Cu and 9j Zn. The Γ and Z represent two highly symmetric points in the Brillouin zone of the supercell, i.e., (0, 0, 0) and (0, 0, 0.5), respectively. The Fermi level is set to zero energy and indicated by the horizontal red dashed lines. Comparing with Figure 2 for the band structures of the pristine (8, 0) BNNT in which there is a large band gap of about 3.565 eV, one can see that there are many new bands appeared in the band gap region and even a few pass through the Fermi level. We introduce a concept “impurity state”. The PDOS in Figure 7 shows these impurity states are mainly contributed from the TM atoms. Secondly, comparing the band structures
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DOS and PDOS (1/eV)
296 20 15 10 5 0 -5 -10 -15 20 -20 15 10 5 0 -5 -10 -15 20 -20 15 10 5 0 -5 -10 -15 20 -20 15 10 5 0 -5 -10 -15 -20 20 15 10 5 0 -5 -10 -15 -20
(a) Sc
(b) Ti
(c) V
(d) Cr
(e) Mn
(f) Fe
(g) Co
(h) Ni
(i) Cu
-10
(j) Zn
-5
0
5
10 -10 Energy (eV)
-5
0
5
10
Fig. 7. (Color online) The total DOS (black lines) and PDOS (red lines) projected onto the TM atoms with the upper (lower) panel representing the majority (minority) spin for the most stable configurations of a single TM atom adsorbed on the (8, 0) BNNT. The Fermi level is set to zero energy and indicated by the vertical red dashed lines.
of the majority spin (left panel) with those of minority spin (right panel), one readily identifies the asymmetry at the Fermi level (red dashed lines) for Ti-, V-, Mn-, Feand Co-adsorbed (8, 0) BNNT. In detail, there are more bands crossing the Fermi level for the majority spin than those of the minority spin for the Ti- and V- adsorbed systems, while a reverse phenomenon is observed for the Mn-, Fe- and Co-adsorbed (8, 0) BNNT. This indicates a spin polarization transport process can be achieved in these adsorbed systems. In particular, for V-, Mn- and Fe-adsorbed (8, 0) BNNT, only one type of bands (either
majority spin or minority spin) crosses the Fermi level also implies the V-, Mn- and Fe-adsorbed (8, 0) BNNT are all half-metals and can be used in the field of spintronics for producing nearly 100% spin polarized currents.
4 Conclusions A detailed analysis of the adsorption configurations for a series of 3d TM atoms adsorbed on the zigzag (8, 0) BNNT at five different sites have been investigated systematically
S.-F. Wang et al.: Structural, electronic and magnetic properties of the 3d transition metal atoms...
(3)
(4)
(5)
(6)
(7) Fig. 8. (Color online) Charge density for (a) majority spin, (b) minority spin, and (c) difference of the two spins on the layer through the tube axis and the adsorption atoms of the most stable adsorption configurations of Ti atoms at the H site on the zigzag (8, 0) BNNT. Magnitude of the charge density is expressed by colors, with the red at one end of the spectrum being the smallest and the blue at the other end the largest.
using the first-principles PAW potential within DFT under GGA. Following conclusions are obtained: (1) The most stable adsorption sites are different for different TM atoms. The Sc, Ti, Cr and Zn atoms are H site, V atom is B site, Mn atom is N site, Fe atom is Z site, and the Co, Ni and Cu atoms are A site. (2) Partially filled 3d metals V, Cr and Mn can bind strongly with zigzag (8, 0) BNNT (with Eb > 4.0 eV), and some others such as Sc, Ti, Co and Ni can be chemically adsorbed on the sidewall of the (8, 0)
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BNNT (with 1.0 eV < Eb < 2.0 eV). The binding between the Fe or Cu atom and the BNNT is only marginal (with Eb < 0.5 eV). One unusual case is Zn. Its zero binding energy independent of the adsorption sites implies it can not be chemically but physically adsorbed on the BNNT mainly stemmed from the van de Waals interaction. For each TM atom adsorbed at five different sites, the total DOS curves of both majority and minority spins make a slightly relative shift along the energy axis, and for each site the total DOS of the minority spin shifts slightly in high energy direction with respect to that of the majority spin lead to a exchange splitting, except fully filled 3d metals Cu and Zn. Total DOS curves of both the majority and minority spins for the adsorbed systems shift to the lower energy region compared with that of the pristine (8, 0) BNNT. And the smaller 3d electrons number Nd of the TM atom, the larger shift to the lower energy region of its DOS curves. Comparing the total DOS at the Fermi level of majority spin with that of minority spin, one readily identifies the adsorbed systems have a high spin polarization and magnetic moment except for fully filled 3d metals Cu and Zn cases. And the peaks on the total DOS curves near the Fermi level are contributed to the 3d electrons of the TM atom, except for fully filled 3d metals Cu and Zn their 3d states form two symmetry narrow DOS peaks located far below the Fermi level and thus without magnetism. More important, for V-, Mn- and Fe-adsorbed (8, 0) BNNT, only one type of electrons (either majority spin or minority spin) passes through the Fermi level implies these adsorbed systems are all half-metals and can be used in the field of spintronics for producing nearly 100% spin polarized currents. The magnetism is confined within the TM atom is obtained from the analyses of the charge density and comparison between the PDOS and total DOS. There are many new “impurity states” appeared in the band gap region and even a few pass through the Fermi level for adsorbed systems. Comparing the band structures of the majority spin with those of minority spin, one readily identifies the asymmetry at the Fermi level for Ti-, V-, Mn-, Fe- and Co-adsorbed (8, 0) BNNT. In particular, for V-, Mn- and Fe-adsorbed (8, 0) BNNT, only one type of bands (either majority spin or minority spin) crosses the Fermi level also implies the V-, Mn- and Fe-adsorbed (8, 0) BNNT are all half-metals.
The results of this study have the potential not only in explaining the character of the bonding between the TM atom and the (8, 0) BNNTs, but also in showing how the physical properties of (8, 0) BNNTs are influenced by the adsorbed a single TM atom. And they might be helpful for fabricating nanodevices, such as nanomagnets, metallic connects, and spintronic devices, as well as for further studies of nanotube-based sensors, catalysts, and storage materials.
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The European Physical Journal B 5
5
Majority spin
4
Minority spin
3
2
2
1
1
0
0
-1
-1
-2
-2
-3
-3
-4
-4
(a) Sc
-5
-5
5 Γ
ΖΓ
Ζ
Majority spin
4
Minority spin
5
3
2
2
1
1
0
0
-1
-1
-2
-2
-3
-3
Majority spin
Ζ
Minority spin
(d) Cr
-5 Γ
ΖΓ
Ζ
Γ
ΖΓ
Ζ
5
5
Majority spin
4
Minority spin
Majority spin
4
3
3
2
2
1
1
0
0
-1
-1
-2
-2
Minority spin
-3
-3 -4
-4
(e) Mn
-5
(f) Fe
-5
5Γ
ΖΓ
Majority spin
4
Ζ
Minority spin
5
Γ
3
2
2
1
1
0
0
-1
-1
-2
-2
-3
ΖΓ
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4
3
Ζ
Minority spin
-3
-4
-4
(g) Co
-5
5
ΖΓ
-4
(c) V
-5
Minority spin
(b) Ti Γ
4
3
-4
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4
3
(h) Ni
-5 Γ
ΖΓ
Majority spin
4
Ζ
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5
3
2
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0
0
-1
-1
-2
-2
-3
-3 -4
(i) Cu
-5
-5 Γ
ΖΓ
Ζ
ΖΓ
Majority spin
4
3
-4
Γ
Ζ
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(j) Zn Γ
ΖΓ
Ζ
Fig. 9. (Color online) Band structures (left, majority spin; right, minority spin) for the most stable adsorption configurations of (a) Sc, (b) Ti, (c) V, (d) Cr, (e) Mn, (f) Fe, (g) Co, (h) Ni, (i) Cu and (j) Zn atoms on the zigzag (8, 0) BNNT. The Γ and Z represent two highly symmetric points in the Brillouin zone of the supercell, i.e., (0, 0, 0) and (0, 0, 0.5), respectively. The Fermi level is set to zero energy and indicated by the horizontal red dashed lines.
S.-F. Wang et al.: Structural, electronic and magnetic properties of the 3d transition metal atoms... The authors would like to acknowledge the State Key Development for Basic Research of China (Grant No. 2004CB619302) for providing financial support for this research.
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