Iron Endohedral-Doped Boron Fullerene: A Potential Single Molecular

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Iron Endohedral-Doped Boron Fullerene: A Potential Single Molecular Device with Tunable Electronic and Magnetic Properties J. L. Li and G. W. Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Institute of Optoelectronic and Functional Composite Materials, Nanotechnology Research Center, School of Physics and Engineering, Zhongshan (Sun Yat-sen) UniVersity, Guangzhou 510275, Guangdong, People’s Republic of China ReceiVed: July 8, 2009; ReVised Manuscript ReceiVed: September 7, 2009

We have theoretically performed that Fe endohedral-doped boron fullerene (B80) is a potential single molecular device with tunable electronic and magnetic properties. Both the energy gap and magnetic moment of the Fe endohedral-doped B80 can be greatly tuned, simultaneously by changing the position of the Fe atom inside the hollow cage of B80. In comparison with that of the Fe endohedral-doped B80 with Fe atom located at center-at, the energy gap decreases half and the magnetic moment decreases zero for the case of the Fe endohedral-doped B80 with the Fe atom located at hexagon-in in the hollow cage. These fascinating findings imply that the Fe endohedral-doped B80 with tunable electronic and magnetic properties can be expected to be applicable as a single molecular device. 1. Introduction Recently, using molecular-scale conductors such as single molecules with special electronic and magnetic properties as functional units of electronic devices has received a new impetus. One of the most intensively studied single-molecule devices is the fullerene of C60 that is expected to replace the silicone-based semiconductor as scales decrease.1-8 For example, endohedral metallofullerenes represent a novel functionalized molecular compound by encapsulating one or more metal atoms inside a hollow cagelike structure, in which the metal atoms are completely shielded by a robust carbon cage.9,10 Interestingly, this type of molecular compound has been the subject of much scientific as well as technological interest, promising a variety of important applications such as superconductors, organic ferromagnets, laser medium, and ferroelectrics. For example, several studies on spintronics using C60 fullerenes have been reported, and some interesting conclusions were obtained.11-13 Motivated by the extensive technical applications of carbon fullerenes, there is thus a quickly growing interest in exploring novel noncarbon fullerenes in very recent years.14-17 As the nearest neighbor of carbon in the element periodic table, boron has exceptional properties of low volatility and high melting point, and it is stronger than steel, harder than corundum, and lighter than aluminum.18 As a result of this thrust, in the past decade, wide attention is paid to various low-dimensional boron nanostructures. Among them, boron clusters are expected to have broad applications in various circumstances. There are many experimental and theoretical studies that have been performed on small clusters such as the icosahedral B12.19,20 However, to our knowledge, there have been no experimental reports about the existence of free large boron fullerenes so far. Importantly, Gonzalez Szwacki et al.21 predicted a stable atomic arrangement of the fullerene of B80 that is structurally similar in shape and symmetry to C60, which offers a broad map for the experimental investigations in this direction. If this proposed inorganic B80 is confirmed experimentally, it would be the second example in nature after C60. Although there have been no experimental * Corresponding author. E-mail: [email protected].

reports about the existence of free boron fullerenes with exactly 80 atoms, the further explorations theoretically and experimentally on this special boron cluster are timely and desirable. In this contribution, a systematical theoretical study of the Fe endohedral-doped B80 is carried out by using spin-polarized total-energy first-principles calculations, and the geometric, electronic, and magnetic properties of the Fe endohedral-doped metallofullerene are performed in detail. Very interestingly, we find that the Fe endohedral-doped B80 is actually a potential single molecular device with tunable electronic and magnetic properties, i.e., its energy gap and magnetic moment can be greatly tuned simultaneously by changing the position of the Fe atom inside the hollow cage of B80. Therefore, these fascinating findings provide the valuable information for future applications of B80 in single molecular devices. 2. Calculation Method We employ the first-principles spin-polarized density functional theory (DFT) on the basis of an efficient computer code, SIESTA,22-24 in our studies, which performs fully self-consistent calculations for solving the standard Kohn-Sham equations. A flexible linear combination of numerical atomic orbital basis sets is used for the description of valence electrons, and normconserving nonlocal pseudopotentials are adopted for the atomic cores. The pseudopotentials are constructed by using the Troullier-Martins scheme25 to describe the interaction of valence electrons with the atomic cores. The nonlocal components of pseudopotentials are expressed in the fully separable formofBylanderandKleinman.26,27 ThePerdew-Burke-Ernzerhof (PBE) form generalized gradient approximation (GGA) corrections are adopted for the exchange-correlation potential.27 The atomic orbital set employed throughout is a double-ζ plus polarization (DZP) function. The 3d electron orbitals of the Fe atom are taken into account as well. The numerical integrals are performed on a real space grid with an equivalent cutoff of 120 Ry. We use periodic boundary conditions and a supercell large enough to keep a distance of at least of 15 Å between neighboring images. To determine the equilibrium configurations of the Fe endohedral-doped metallofullerenes, we relax all the

10.1021/jp9064592 CCC: $40.75  2009 American Chemical Society Published on Web 09/28/2009

Iron Endohedral-Doped Boron Fullerene

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18293 TABLE 1: Binding Energies Eb for All the Fe Endohedral-Doped Metallofullerenes Configurationsa Fe position

Eb (eV)

M (µB)

m4s (µB)

m3d (µB)

m4p (µB)

center pentagon-in hexagon-in

3.78 3.48 2.28

4.00 (4.15) 4.00 (4.13) 0.00 (0.00)

0.20 0.20 0.00

3.76 3.74 0.00

0.18 0.19 0.00

a The total magnetic moment (M), as well as its contribution from 4s (ms), 3d (md), 4p (mp) orbitals.

TABLE 2: Energy Gaps (∆E) between the HOMO and LUMO of the Fe Endohedral-Doped Metallofullerenes

Figure 1. Optimized configurations of the pristine B80 and a single Fe endohedral-doped metallofullerene: (a) the pristine B80, (b) the center-at metallofullerene, (c) the hexagon-in metallofullerene, and (d) the pentagon-in metallofullerene. The gray color is for B atoms, the red color is for Fe atoms, and the purple color is marked to denote the facets opposite to the endohedral-doped Fe atom.

atomic coordinates involved by using a conjugate gradient (CG) algorithm, until the maximum atomic forces are less than 0.02 eV/Å. Binding energies (Eb) are calculated according to the following expression as Eb ) EB80 + EFe - Etotal where Etotal is the calculated total energy of the Fe endohedral-doped metallofullerene, EB80 is the total energy of the isolated B80, and EFe is the total energy of the isolated Fe atom. 3. Results and Discussion 3.A. Structural Stability. We first calculate the stable configuration and electronic and magnetic structures of B80. After the structure optimization, we obtain the equilibrium configuration of B80 as shown in Figure 1a. In detail, the boron fullerene cage consists of 20 hexagons (filled by a B atom at the center) and 12 pentagons uniformly distributing on the surface. This optimized structure of B80 has the diameter of 8.44 Å, much larger than that of C60 (6.83 Å).21 Additionally, there are three groups of B-B bonds in the cage: 60 between the hexagon and pentagon (LHP), 30 between the hexagon and hexagon (LHH), and 120 between the atom at the center of each hexagon and its nearest neighbors (LBB). The average bond lengths of the three groups are 1.74, 1.70, and 1.73 Å, respectively. According to the special symmetry of B80, several possible representative sites are considered for the endohedrally doped of a single Fe atom in the cage of B80. Meanwhile, for these configurations, we perform a complete geometry optimization including spin polarization. Our calculations reveal that there are three stable configurations of the Fe endohedral-doped metallofullerenes with the different sites of the Fe atom. In detail, the Fe atom is near the inner centers of the hexagon and pentagon and the center position of the cage, respectively. The corresponding Fe endohedral-doped metallofullerenes are denoted by the hexagon-in, pentagon-in, and center-at metallofullerene, respectively, and their equilibrium configurations are shown in Figure 1b-d, respectively.

Fe position

∆E ) EHOMO - ELUMO (eV)

isolated B80 center pentagon-in hexagon-in

1.02 0.98 1.02 0.54

We then investigate the stability of the configurations above by determining the binding energies, and the results are listed in Table 1. A positive binding energy value indicates that the chemical energies process is exothermic. Therefore, the Fe atom prefers to bind strongly at the center of B80 with the relatively large binding energy of 3.78 eV/atom, which is similar to the case of the Fe atom interaction with the fullerene-like cage B36N36.29 For all the three Fe endohedral-doped metallofullerenes, from the viewpoint of binding energy, we predict the stability ordering: the center-at metallofullerene, the pentagonin one, and the hexagon-in one. Since the binding energies of the Fe endohedral-doped metallofullerenes are all very large, we conclude that a single Fe atom can be chemically doped in the hollow cage of B80 to form the stable Fe endohedral-doped metallofullerenes. Note that obvious local distortions occur in the boron cage near the Fe sites as shown in Figure 1. For the pentagon-in metallofullerenes, the boron atoms on the cages connect with the Fe atom with the average bonds of 2.20 Å, accompanying with the B-B bond lengths elongated to 5%. The corresponding values for the hexagon-in metallofullerenes are 2.15 Å and 4.1%, respectively. The structural distortion and the hybridization interaction between Fe and B atoms will dramatically affect the electronic and magnetic properties of the Fe endohedral-doped metallofullerenes. 3.B. Electronic Structure. The electronic structures of all configurations above are calculated in our studies. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy gaps are listed in Table 2. Clearly, for the pentagon-in and center-at metallofullerenes, the endohedral doping of Fe nearly does not change the energy gap of B80 since they almost have the same HOMO-LUMO gaps with that of B80. However, for the hexagon-in metallofullerene, the Fe endohedral-doped greatly modifies the electronic structure of metallofullerene. The dopant introduces states in the gap between the HOMO and LUMO of fullerene and reduces the energy gap to 0.54 eV that is only half of that of B80. Although DFT calculations always underestimate the energy gaps of materials, the relative change of the energy gap between B80 and hexagon-in metallofullerene is still significant. Therefore, we can tailor the energy gap of the Fe endohedral-doped metallofullerenes by changing the position of the doped Fe atom. Additionally, the HOMO-LUMO energy separation generally serves as a simple measure of chemical stability. Therefore, the calculated energy gaps indicate the consistent stability order with that from the viewpoint of binding energy above. To understand the dramatically modified energy gap in the hexagon-in metallofullerene, we analyze atoms and quantum

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Li and Yang TABLE 3: Mulliken Population Analysis of the 4s, 3d, 4p Orbitals and the Total Valent Electrons of the Fe Atom Endohedral-Doped within the Boron Fullerene B80 spin up Fe position

isolated atom center pentagon-in hexagon-in

Figure 2. Local configurations of the optimized hexagon-in metallofullerene and the DOS and PDOS for the pristine B80 and the hexagonin metallofullerene: (a) the top view of the local configuration of the hexagon-in metallofullerene, (b) the side view of the local configuration of the hexagon-in metallofullerene, (c) DOS for the pristine B80, and (d) DOS for the hexagon-in metallofullerene. Panels f, g, h are the PDOS on B for B80, and panels e, f′, g′, h′ are the PDOS, respectively, on Fe and B for the hexagon-in metallofullerene. The corresponding atoms are denoted in the local configurations. The red and blue lines stand for spin up (majority) and spin down (minority), respectively. The vertical dashed lines are the HOMO and LUMO of the isolated B80, and the arrow denotes the Fermi level. The gray color is for B atoms, the red color is for Fe atoms, and the blue color is used to mark the B atoms to be projected the DOS.

orbitals of the Fe endohedral-doped metallofullerene contributing to the top of the valence band and the bottom of the conduction band (HOMO and LUMO) near the Fermi levels. Figure 2 gives the density of states (DOS) and projected density of states (PDOS) on the endohedral-doped Fe atom and the B atoms for the pristine B80 (Figure 2c) and the hexagon-in metallofullerene (Figure 2d), in which the Gaussian widths employed in plotting all the PDOS are 0.02 eV, and PDOS for up and down spins only on Fe and B atoms shown within this local configuration. Clearly, we can see the same DOS features in both spin and down states for the hexagon-in metallofullerene in Figure 2d. The PDOS calculations show that only several atoms on special sites are contributing to the gap states. Thus, these corresponding atoms are denoted in the local configuration marked as B1, B2, B3 in Figure 2b. The vertical dashed lines are the HOMO and LUMO of the isolated B80, and the arrow denotes the Fermi level. Clearly, both the HOMO and LUMO states are stemming

spin down

4s

3d

4p

4s

3d

4p

1.00 0.61 0.60 0.27

5.00 4.83 4.83 3.23

0.42 0.42 0.27

1.00 0.41 0.40 0.27

1.00 1.07 1.09 3.23

0.24 0.23 0.27

total valent electrons 8.00 7.59 7.56 7.73

from the Fe atom and B atoms that sit at the center of the nearest hexagon and the hexagon opposite to the endohedral-doped Fe atom, from the comparison of the PDOS for B80 and hexagonin metallofullerene. In detail, the HOMO and LUMO states are mainly from the Fe atom, with a minority of them introduced by the B atoms sitting at the center of the nearest hexagon of the hexagon opposite to the endohedral-doped Fe atom. Consequently, the free valence electrons of the Fe atom can be mobile and give a contribution to the electronic properties of the Fe endohedral-doped metallofullerenes. The electrons transfer from the Fe to B atoms, and the local strain-induced deformation causes the states introduced to form the narrow energy gap. Importantly, this great modification of electronic structure can be used to tailor the electronic and optoelectronic properties of the Fe endohedral-doped B80. 3.C. Magnetic Properties. Since Fe is a typical magnetic element, the magnetic moments of the Fe endohedral-doped metallofullerenes are calculated in our studies. The calculated total magnetic moments of three Fe endohedral-doped metallofullerenes are listed in Table 1. Additionally, the detailed descriptions of the magnetic moment contributing from different electronic orbitals are listed in Table 1. For comparison, in the parentheses, we give the magnetic moment of the endohedraldoped Fe atom for distinct positions relative to the fullerene. It is obvious that for the center-at and pentagon-in Fe endohedraldoped metallofullerenes, the total magnetic moment of the systems is essentially the same with that of the Fe atom. In other words, the magnetism is mainly contributed from the endohedral-doped Fe atom. In contrast, for the hexagon-in metallofullerene, the magnetic moment of the endohedral-doped Fe atom is completely disappeared, i.e., the hexagon-in metallofullerene has no magnetic moment. Therefore, we can tune the magnetic moments of the Fe endohedral-doped metallofullerenes by changing the doped position of the Fe atom in the cage of B80. That is to say, the magnetic moments of the Fe endohedral-doped metallofullerenes are mobile with the positions of the Fe atom changing. The key to understand why the hexagon-in metallofullerene has zero magnetic moment is the depletion of 4s orbitals due to the confinement. From Table 3, we can see that the electrons transfer mainly from 4s to 4p orbitals. However, for the cases of the center-at and pentagon-in Fe endohedral-doped metallofullerene, the 4s-4p electron transfer is not effective in changing the total magnetic moment of the endohedral-doped Fe atom. It is because the transfer involves at most one electron, which allows 4s-4p electron transfer between minority spin electrons. This is true for small and moderate confinement. With increasing the confinement such as the hexagon-in Fe endohedral-doped metallofullerene, the other 4s and 3d electrons (majority spin) are also transferred to the 3d orbitals (minority spin), which results in the completely disappeared magnetic moments of the endohedral-doped Fe atom. This can be visually

Iron Endohedral-Doped Boron Fullerene

J. Phys. Chem. C, Vol. 113, No. 42, 2009 18295 hollow cage of B80 at different positions due to the hybridization interaction between Fe and B atoms; (ii) three stable configurations of the Fe endohedral-doped metallofullerenes have the endohedral-doped Fe atom at the center of the cage, pentagonin and hexagon-in, respectively; (iii) the energy gap of the hexagon-in Fe endohedral-doped metallofullerene is dramatically modified, only half of that of B80; (iv) the different hybrid structures have different magnetic moments, which depend on the doped sites of Fe, either the same as the isolated Fe atom or nonmagnetic with the inherent magnetic moments of Fe completely disappeared. These fascinating findings theoretically predict valuable directions for applications of the boron fullerene in single molecular devices. Acknowledgment. This work was supported by the National Natural Science Foundation of China (50525206 and U0734004) and the Ministry of Education. References and Notes

Figure 3. PDOS of the considered Fe endohedral-doped metallofullerenes: (a) the isolated B80 and the isolated Fe atom, (b) the pentagonin metallofullerene, (c) the center-at metallofullerene, and (d) the hexagon-in metallofullerene. Positive and negative values represent the DOS projected on the spin up and down, respectively. The red and black solid lines represent the DOS projected on B and Fe atoms, respectively. The vertical dashed lines are the HOMO and LUMO for the pristine B80; the arrow denotes the Fermi level.

presented by the PDOS of the Fe endohedral-doped metallofullerenes in Figure 3. From Figure 3d, we can see that the hexagon-in Fe endohedral-doped metallofullerene is spincompensated with spin-up and spin-down states equally occupied, which is the same as that of Figure 2d. Therefore, the symmetry of DOS results in the zero magnetic moment of the hexagon-in Fe endohedral-doped metallofullerene. However, for the pentagon-in and center-at Fe endohedral-doped metallofullerenes, the DOS features are asymmetry as shown in Figure 3, parts b and c, i.e., the majority (spin-up) and minority (spindown) states are not compensated, which results in the residue of net spin states as shown in Figures 3, parts b and c. Simultaneously, there are small magnetic moments induced in the boron cage. Therefore, the completely alterable magnetic properties of the Fe endohedral-doped metallofullerene are dependent on the sites of Fe, which is definitely different from that of B-N fullerene-like cage B36N36 and C nanostructures interacting with a transition metal.29-31 Interestingly, this unique characteristic would bring the Fe endohedral-doped metallofullerenes into single molecular devices. 4. Conclusion In summary, we have performed the atomic, electronic, and magnetic structures of the Fe endohedral-doped B80 by using spin-polarized first-principles calculations. On the basis of our calculations (i) the Fe atom can be chemically doped in the

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