Theoretical Studies of Sandwich Molecular Wires with Europium and

Mar 22, 2016 - spin-polarized density functional theory. Both SMWs are of high stability and ultrahigh magnetic moments, and the [Eu-COTB-. Eu-COT]∞...
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Theoretical Studies of Sandwich Molecular Wires with Europium and Boratacyclooctatetraene Ligand and the Structure on a H‑Ge(001)2×1 Surface Xiaojing Yao,† Shijun Yuan,*,† and Jinlan Wang*,†,‡ †

Department of Physics, Southeast University, Nanjing 211189, China Synergetic Innovation Center for Quantum Effects and Applications (SICQEA), Hunan Normal University, Changsha 410081, China

‡

S Supporting Information *

ABSTRACT: The structural, electronic, and magnetic properties of two kinds of boron-doped europium cyclooctatetraene sandwich molecular wires (SMWs), [EuCOTB]∞ and [EuCOTB-Eu-COT]∞ (Eu = europium, COT = cyclooctatetraene = C8H8, COTB = boratacyclooctatetraene), are investigated with spin-polarized density functional theory. Both SMWs are of high stability and ultrahigh magnetic moments, and the [Eu-COTBEu-COT]∞ SMW even owns half-metallic characteristics. Our calculations further reveal that the [Eu-COTB-Eu-COT]∞ SMW anchored on a semiconductor germanium surface is a quasi-halfmetallic ferromagnet, and it can be tuned into full half-metallicity under a mild external electric field. The unveiled intriguing properties here suggest that the boron-doped europium cyclooctatetraene SMWs may be compelling candidates for future spintronics devices.

1. INTRODUCTION

On the other hand, sandwich complexes comprised of europium atoms with cyclooctatetraene ligands, that is, EunCOTn+1, exhibit better stability and much higher magnetic moments18−21 than transition metal sandwich complexes. The infinite [EuCOT]∞ SMW also has an ultrahigh magnetic moment of ∼7μB per unit cell with semiconducting characteristics.22 It is natural to seek the possibility of heterocyclic EuCOT based SMWs that may possess the ultrahigh magnetic moments of Eu atoms as well as the high reactive activity of the functional atoms and bring new opportunity for the application in molecular electronics. In fact, tracing back 2003, the boratacyclooctatetraene (COTB) ligand was successfully synthesized.23 It is thus expected to synthesize Eu-COTB heterocyclic sandwich complexes in experiment. In this work, we explore the electronic and magnetic properties of [EuCOTB]∞ and its hybrid SMW [Eu-COTB-Eu-COT]∞ by employing spin-polarized density functional theory (DFT). Our calculations show that both of them are thermally stable and possess high magnetic moments. More importantly, the [EuCOTB-Eu-COT]∞ SMW bonded on the hydrogen-terminated Ge(001) surface is predicted to be a quasi-half-metallic ferromagnet and can be tuned into full half-metallicity under an electric field of 0.2 V/Å.

Organometallic sandwich molecular wires (OSMWs), of which metal atoms are sandwiched by organic molecules, have attracted persistent attention for years because of their unique structures, fascinating electronic and magnetic properties, and promising applications in electronics and spintronics devices.1−5 Various transition metal organic infinite SMWs, such as vanadium−benzene ([VBz]∞),6 vanadium−cyclooctatetraene ([(VCOT)]∞),7 iron−cyclopentadienyl ([FeCp]∞),8 and vanadium−cyclopentadienyl ([VCp]∞)9 SMWs, and their derivatives10 have been predicted to be ferromagnetic (FM) halfmetals with excellent spin filter effect. Hybrid SMWs with different metals and different ligands have shown wide-ranged tunable electronic and magnetic properties by altering metal elements or ligands. For example, the [FeCpVCp]∞ wire11 is a FM semiconductor, while the [FeCpCrCp]∞ and [FeCpCoCp]∞ and [TiCpCrCp]∞ wires are FM half-metals, and their magnetic moments can be engineered in the range of 1−5μB per unit cell.12 In contrast, the hybrid ligands can greatly enhance both the structural and magnetic stabilities.13,14 The heterocyclic sandwich structures have also been reported.15,16 The biggest advantage of heterocyclic SMWs is the introduction of doped atoms (like boron) in cyclic hydrocarbons as functional atoms could assist the anchoring of SMWs on semiconducting substrates in the nanoelectronic industry.17 © 2016 American Chemical Society

Received: November 29, 2015 Revised: March 16, 2016 Published: March 22, 2016 7088

DOI: 10.1021/acs.jpcc.5b11660 J. Phys. Chem. C 2016, 120, 7088−7093

Article

The Journal of Physical Chemistry C

atom. Under a ligand field of D8h symmetry, the seven degenerate 4f orbitals of Eu atoms are split into f0, f±1, f±2, and f±3 orbitals, the spin-up orbitals are fully occupied, and the f states are mainly localized in the energy window [0, −0.7 eV] in the partial density of states (PDOS), as shown in Figure 1b. The SMW is semiconducting, with band gaps of 1.92 and 2.94 eV for the spin-up and spin-down channels, respectively. These results are in good agreement with previous results,22 indicating that our computational method is indeed reliable. We then study the B-doped europium cyclooctatetraene SMWs, [EuCOTB]∞. A number of staggered configurations having different rotation angles between the neighboring COTB rings are computed. The most stable structure of [EuCOTB]∞ is a transoid conformation with the neighboring COTB rings rotated by 180°; the energy differences of other low-lying isomers are smaller than 0.14 eV, as shown in Figure 2a. The optimized lattice constant of the most stable

2. METHODOLOGY All calculations were carried out within the framework of spinpolarized DFT implemented in the Vienna ab initio simulation package (VASP).24,25 We adopted the Perdew−Burke− Ernzerholf26 generalized gradient approximation (GGA) as the exchange−correlation potential. The electron−ion interaction was described by the projector augmented wave potential.27 The electronic wave functions were expanded in a plane-wave basis with a cutoff energy of 400 eV. To treat Eu 4f states properly, GGA+U28 was employed to include the orbital dependence of the Coulomb and exchange interaction, and the on-site Coulomb repulsion parameter U was set to 3.7 eV according to earlier studies.19,29 Periodic boundary conditions were applied along the molecular wire with the unit cell containing two Eu atoms and two ligands for the computation of [EuCOTB]∞ and [Eu-COTB-Eu-COT]∞. The vacuum region between wires was larger than 10 Å, and all atoms were fully relaxed until the Hellmann−Feynman force on each atom was less than 0.01 eV/Å. The reciprocal space was sampled by 7 × 1 × 1 grid meshes using the Monkhorst−Pack scheme30 for geometry optimization. A very dense k-point grid (41 × 1 × 1) was used for electronic structure calculations. Regarding of calculations of SMW on a substrate, we used the periodic slab model of the Ge(001)-2×1 substrate including six Ge layers with a (2 × 2) unit cell. Each Ge atom in the bottom layer was passivated by two hydrogen atoms to mimic the bulk germanium, and the bottom two Ge layers and the passivated H atoms were fixed. At the top surface, the Ge−Ge dimers sat along the [110] direction, and these Ge atoms were saturated by H atoms and the [Eu-COTB-Eu-COT]∞ SMW. The vacuum space above the surface was about 11 Å, and the Brillouin-zone was sampled by Monkhorst Pack meshes of 5 × 3 × 1 and 11 × 7 × 1 for geometry optimization and electronic structure calculation, respectively. The external electric field was simulated via an artificial dipole sheet in the middle of the vacuum among the periodic supercell,31 with the direction perpendicular to the Ge(001) surface. 3. RESULTS AND DISCUSSION We first consider the [EuCOT]∞ SMW for comparison; the optimized lattice constant is 8.64 Å, while the C−C and C−H bond lengths in COT rings are 1.42 and 1.09 Å, respectively (see Figure 1a). The Eu atoms in this SMW are ferromagnetically coupled with a local magnetic moment of 7.0μB on the Eu

Figure 2. (a) Energy differences and optimized structures of [EuCOTB]∞ with various configurations. Blue, dark gray, red, and white spheres denote Eu, C, B, and H atoms, respectively. (b) Charge density difference (CDD) of [EuCOTB]∞; the isovalue is 0.03 e/Å3. Yellow and cyan colors indicate the accumulation and depletion of electrons, respectively. (c) Spin density isosurfaces of [EuCOTB]∞; the isovalues for up and down spins are 0.1 and 0.017 e/Å3, respectively. (d) Band structures and PDOS of [EuCOTB]∞. Red and blue colors denote B,C-px and Eu-f orbitals, respectively.

[EuCOTB]∞ is 8.34 Å, smaller than that of [EuCOT]∞ (8.64 Å). The average C−C and Eu−C bond lengths in [EuCOTB]∞ are 1.42 and 2.79 Å, shorter than the B−C (1.54 Å) and Eu−B bonds (2.85 Å), respectively, which arises from the larger atomic radius of B than that of C. To evaluate the thermodynamic stabilities of SMWs, we calculated the binding energies (Eb), defined as Eb =

Figure 1. (a) Optimized structure of the [EuCOT]∞ SMW. Blue, dark gray, and white spheres denote Eu, C, and H atoms, respectively. (b) Band structure and PDOS of [EuCOT]∞. Red and blue colors denote C-px and Eu-f orbitals, respectively. The size of the circles in each band denotes the contributions from different states. The dashed green line is the Fermi level, which is shifted to zero.

(2E ligand + 2E Eu − ESMW )

(1) 2 where Eligand, EEu, and ESMW are the total energies of the ligand (COT or COTB), Eu atom, and SMW, respectively. As shown in Table 1, the Eb of [EuCOTB]∞ is 5.59 eV, even higher than that of [EuCOT]∞, 5.34 eV, indicating the high stability of 7089

DOI: 10.1021/acs.jpcc.5b11660 J. Phys. Chem. C 2016, 120, 7088−7093

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The Journal of Physical Chemistry C

The ground spin state of [EuCOTB]∞ is FM with a total magnetic moment of 13.2μB per unit cell. It is more stable than the antiferromagnetic (AFM) state by 27.4 meV, which is 22 times larger than that in [EuCOT]∞ (1.2 meV between FM and AFM). This implies that the B doping can enhance the FM stability of the SMW. We also consider other possible spin states like a total magnetic moment of 12μB, in which the Eu atom transfers one 4f electron to COTB to form Eu3+ and COTB3−. However, it is found to be less stable at 0.14 eV higher per unit cell in energy than the ground state. Spin density analysis reveals that the magnetism of [EuCOTB]∞ is mainly from the Eu atom, in which the local spin is about 6.78μB; the COTB ring possesses a small negative magnetic moment of −0.18μB (Figure 2c). Charge densities of COTB ligands in [EuCOTB]∞ presented in Figure S1 in the Supporting Information reveal that the conjugated π bond of the COTB ligand can still be seen on the eight-number ring and is partially broken at the place of the B atom. Because the spin polarization in the itinerant π electron is normally much less than that in an isolated electron (1μB), the partially broken conjugated π bond leads to a total magnetic moment on COTB as small as −0.18μB. Therefore, the formation of the ionic− covalent bonding and the AFM coupling between Eu and COTB in [EuCOTB]∞ are responsible for the relatively small magnetic moment with respect to [EuCOT]∞ (14.0μB). The magnetic nature of [EuCOTB]∞ can be further understood in terms of the band structure and density of states. As shown in Figure 2d, The 4f orbitals of Eu atoms are mainly localized in the lower-energy level, at around [−1.3, −0.9 eV], and are all located in the spin-up channels, which induces a large spin split and thereby a high magnetic moment in the sandwich wire. Moreover, significant down-spin states of the COTB ligands are present around the Fermi level, corresponding to the AFM coupling nature between the Eu atoms and COTBs and leading to smaller total magnetic moments in the B doping SMW. In addition, different from

Table 1. Lattice Constants (a), Binding Energies (Eb), Total Magnetic Moments (MMs), and Energy Differences between FM and Antiferromagnetic (AFM) States (EAFM − EFM) of [EuCOT]∞, [EuCOTB]∞, [Eu-COTB-Eu-COT]∞, and [EuCOTB-Eu-COT]/Ge(001) Per Unit Cell SMWs

a (Å)

Eb (eV)

MM (μB)

EAFM − EFM (meV)

[EuCOT] ∞ [EuCOTB] ∞ [Eu-COT-Eu-COTB] ∞ [Eu-COT-Eu-COTB]@Ge

8.64 8.34 8.50

5.34 5.59 5.45

14.0 13.2 13.1 13.1

1.2 27.4 71.0 60.0

[EuCOTB]∞ and feasible synthesis of the SMW [EuCOTB]∞ in experiment. The high stability of [EuCOTB]∞ is ascribed to the unique ionic−covalent bonding between Eu and the COTB ligand. According to the Hückle rule, in [EuCOT]∞, each COT ring needs to capture two extra electrons to form a stable planar ring, two 6s electrons of Eu atom are transferred to the COT ligand, and a pure ionic structure between Eu2+ ions and COT2− rings is thus formed and responsible for the high stability of [EuCOT]∞. In [EuCOTB]∞, because the B atom only has three valence electrons (2s22p1), the COTB ring needs to obtain one more electron from Eu than COT to form a planar structure. Nevertheless, the Eu-4f electrons are strongly localized, and the Eu atom is unwilling to fully transfer a 4f electron to COTB. As a compromise, Eu and the COTB ring will share this electron and form a covalent bond. The ionic bonding characteristics can be vividly seen from the charge density difference (CDD) with charges accumulating around the COTB ligands and depleting around Eu atoms (see Figure 2b), while the covalent bond nature is evidently shown by the overlapping Eu-4f and COTB-p orbitals (Figure 2d). Therefore, we argue that the ionic−covalent bonding makes [EuCOTB]∞ extremely stable.

Figure 3. (a) Optimized structure for [Eu-COTB-Eu-COT]∞. (b) Band structures and PDOS of [Eu-COTB-Eu-COT]∞. Red and blue colors denote B,C-px and Eu-f orbitals, respectively. (c) CDD for [Eu-COTB-Eu-COT]∞ SMW. Yellow and cyan colors indicate the accumulation and depletion of electrons, respectively. (d) Spin density isosurfaces of [Eu-COTB-Eu-COT]∞. 7090

DOI: 10.1021/acs.jpcc.5b11660 J. Phys. Chem. C 2016, 120, 7088−7093

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The Journal of Physical Chemistry C [EuCOT]∞, the Fermi level shifts downward in [EuCOTB]∞, owing to the decrease in the amount of total electrons upon B replacement. As a result, [EuCOTB]∞ has metallic features, with the bands crossing the Fermi level in both spin-up and -down channels. We further study the [Eu-COTB-Eu-COT]∞ SMW, that is, two Eu atoms are alternately sandwiched between a COTB ring and a COT ring (Figure 3a). The optimized lattice constant is 8.50 Å, which is between that of [EuCOT]∞ (8.64 Å) and that of [EuCOTB]∞ (8.34 Å). Similar to the case of [EuCOTB]∞, the Eu−B bond (2.84 Å) is longer than the Eu−C bond (2.79 Å), and the B−C bond (1.53 Å) is longer than the C−C bond (1.43 Å) in [Eu-COTB-Eu-COT]∞. The binding energy of [Eu-COTB-Eu-COT]∞ is 5.45 eV per unit cell, which is between that of [EuCOT]∞ (5.34 eV) and [EuCOTB]∞ (5.59 eV). The FM state is about 71.0 meV lower than the AFM state, indicating that the Eu atoms favor FM coupling in [Eu-COTB-Eu-COT] ∞ as well. Moreover, compared with [EuCOTB]∞, the hybrid px bands of B and C atoms in spin-up and spin-down channels present a larger split in [Eu-COTB-Eu-COT]∞ (see Figure 3b). As a result, [EuCOTB-Eu-COT]∞ is a half-metal with two bands crossing the Fermi level in the spin-up channel and a direct gap of about 2.6 eV in the spin-down channel. In the FM ground state of [EuCOTB-Eu-COT]∞, the total magnetic moment is 13.1μB per unit cell, and the local magnetic moment on the Eu atom is 6.81μB. The magnetic moments on COTB and COT ligands are −0.33μB and −0.14μB, respectively (Figure 3d). Furthermore, it is evident from the CDD plot (Figure 3c) that charges accumulate around the COTB and COT ligands and deplete around Eu atoms, suggesting significant charge transfer from Eu to COTB or COT. Moreover, the overlapping Eu-4f and COTB-p orbitals can also be seen in Figure 3b, implying the formation of the covalent bond nature between Eu and COTB. Therefore, we argue that the ionic bonding between Eu and COT and the ionic−covalent bonding between Eu and COTB make [Eu-COTB-Eu-COT]∞ highly stable. Very recently, two-step reaction pathways were proposed to fabricate SMWs or organic polymer chains on semiconductor surfaces along the desired direction for the sake of application of molecular electronics.17,32 For the hydrogen-terminated surface Si(001)-2×1, a line of H atoms along the [110] direction on the surfaces can be removed using ultrahigh vacuum scanning tunneling microscopy. The borines can be anchored along the trench on the Si(001)-2×1 surface, and the metal atoms are then inserted between the borines and thereby form the SMW on the surface Si substrate.17 In this way, we can anchor the [EuCOTB]∞ and [Eu-COTB-Eu-COT]∞ onto a suitable semiconductor substrate via the functional B atoms. Considering the lattice matching, we use a hydrogenterminated Ge(001) substrate. The structure of [Eu-COTBEu-COT]/Ge(001) is shown in Figure 4a, in which [EuCOTB-Eu-COT]∞ is anchored on the hydrogen-terminated Ge(001)2×1 surface with a row of H atoms removed and the B and C atoms bonded with the Ge substrate directly. The distance between two Ge dimers is 8.17 Å; thus, the lattice constant of [Eu-COTB-Eu-COT]∞ is compressed by 4% to make the wire and the substrate commensurate. The ground spin state of [Eu-COTB-Eu-COT]/Ge(001) is FM, with a total magnetic moment of 13.1μB per unit cell, and the energy is 60.0 meV lower than the AFM state. Band structure calculation shows that this surface molecular wire is conductive; more precisely, it is quasi-half-metallic, in which

Figure 4. (a) Optimized structure for the [Eu-COTB-Eu-COT]/ Ge(001) system. Blue, dark gray, red, white, and green spheres denote Eu, C, B, H, and Ge atoms, respectively. (b) Band structure and PDOS of [Eu-COTB-Eu-COT]/Ge(001); in the band structure, gray, red, and blue dotted lines are the contributions from different Ge-s+p, B,Cpx, and Eu-f. The size of the circles in each band denotes the contributions from different states. (c) Isodensity surface of the two bands across the Fermi level in spin-up channels.

two bands in the spin-up channel cross the Fermi level while very few densities cross the Fermi level in the spin-down channel. Component analysis of bands and the PDOS further reveals that these two bands across the Fermi level in the spinup channel are mainly from the hybrid px orbitals of COT and COTB ligands and Eu-f orbitals. That is, the band structure close to the Fermi level is actually decided by the [Eu-COTBEu-COT]∞ SMW (see Figure 4b and 4c). The Ge(001) surface still maintains the semiconducting characteristic. Similar to the free-standing wire, the Eu-4f orbitals in [Eu-COTB-Eu-COT]/ Ge(001) are also strongly localized with a large spin split, which is responsible for the high magnetic moment of the surface molecular wire. Moreover, the molecular wire is covalently bonded with the substrate due to significant electron transfer (Figure S2) and great hybridization between the molecular wire and the substrate (PDOS in Figure 4b), which makes the [EuCOTB-Eu-COT]/Ge(001) system very stable. We further explore the electronic properties of the [EuCOTB-Eu-COT]/Ge(001) system under external electric fields. When a 0.2 V/Å gate electric field is applied perpendicular to the Ge substrate (shown in Figure 5a), the

Figure 5. Schematic of applying the electric field (a) and band structures of the [Eu-COTB-Eu-COT]/Ge(001) under an electric field of 0.2 V/Å (b).

electrons will transfer from the substrate to the molecular wire. The px bands of the spin-down channel are filled preferentially and shift below the Fermi level, meanwhile the px bands of the spin-up channel are still half-filled and cross the Fermi level (Figure 5b). These indicate that the external electric field can efficiently tune the quasi-half-metallic [Eu-COTB-Eu-COT]/ Ge system into a fully half-metallic ferromagnet, suggesting that 7091

DOI: 10.1021/acs.jpcc.5b11660 J. Phys. Chem. C 2016, 120, 7088−7093

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Conductors for Spin Transport. J. Am. Chem. Soc. 2006, 128, 2310− 2314. (7) Zhu, S.; Fu, H.; Gao, G.; Wang, S.; Ni, Y.; Yao, K. A First Principles Study of Novel One-Dimensional Organic Half-Metal Vanadium-Cyclooctatetraene Wire. J. Chem. Phys. 2013, 139, 024309. (8) Zhou, L.; Yang, S. W.; Ng, M. F.; Sullivan, M. B.; Tan, V. B. C.; Shen, L. One-Dimensional Iron-Cyclopentadienyl Sandwich Molecular Wire with Half Metallic, Negative Differential Resistance and HighSpin Filter Efficiency Properties. J. Am. Chem. Soc. 2008, 130, 4023− 4027. (9) Shen, L.; Yang, S. W.; Ng, M. F.; Ligatchev, V.; Zhou, L.; Feng, Y. Charge-Transfer-Based Mechanism for Half-Metallicity and Ferromagnetism in One-Dimensional Organometallic Sandwich Molecular Wires. J. Am. Chem. Soc. 2008, 130, 13956−13960. (10) Zhu, L.; Wang, J. Ab Initio Study of Structural, Electronic, and Magnetic Properties of Transition Metal-Borazine Molecular Wires. J. Phys. Chem. C 2009, 113, 8767−8771. (11) Da, H.; Jin, H. M.; Lim, K. H.; Yang, S. W. Half-Metallic Spintronic Switch of Bimetallic Sandwich Molecular Wire via the Control of External Electrical Field. J. Phys. Chem. C 2010, 114, 21705−21707. (12) Zhang, X.; Tian, Z.; Yang, S. W.; Wang, J. Magnetic Manipulation and Half-Metal Prediction of One-Dimensional Bimetallic Organic Sandwich Molecular Wires [CpTM1CpTM2]∞ (TM1 = Ti, Cr, Fe; TM2 = Sc-Co). J. Phys. Chem. C 2011, 115, 2948−2953. (13) Wang, L.; Cai, Z.; Wang, J.; Lu, J.; Luo, G.; Lai, L.; Zhou, J.; Qin, R.; Gao, Z.; Yu, D.; et al. Novel One-Dimensional Organometallic Half Metals: Vanadium-Cyclopentadienyl, Vanadium-Cyclopentadienyl-Benzene, and Vanadium-Anthracene Wires. Nano Lett. 2008, 8, 3640−3644. (14) Zhang, X.; Wang, J. Ab Initio Study of Bond Characteristics and Magnetic Properties of Mixed-Sandwich VnBzmCpk Clusters. J. Phys. Chem. A 2010, 114, 2319−2323. (15) Tan, W. B.; Jin, H.; Yang, S. W.; Xu, G. Q. BoratabenzeneVanadium Sandwich Molecular Wire and Its Properties. Nanoscale 2012, 4, 7557−7562. (16) Zhang, X.; Cao, M.; Liu, L.; Liu, Y. Tunable Electronic and Magnetic Properties of Boron/Nitrogen- Doped BzTMCp*TMBz/ CpTMCp*TMCp Clusters and One-Dimensional Infinite Molecular Wires. J. Phys. Chem. C 2014, 118, 11620−11627. (17) Lu, Y. H.; Jin, H.; Zhu, H.; Yang, S. W.; Zhang, C.; Jiang, J. Z.; Feng, Y. P. A Possible Reaction Pathway to Fabricate a Half-Metallic Wire on A Silicon Surface. Adv. Funct. Mater. 2013, 23, 2233−2238. (18) Hosoya, N.; Takegami, R.; Suzumura, J.; Yada, K.; Koyasu, K.; Miyajima, K.; Mitsui, M.; Knickelbein, M. B.; Yabushita, S.; Nakajima, A. Lanthanide Organometallic Sandwich Nanowires: Formation Mechanism. J. Phys. Chem. A 2005, 109, 9−12. (19) Zhang, X.; Ng, M. F.; Wang, Y.; Wang, J.; Yang, S. W. Theoretical Studies on Structural, Magnetic, and Spintronic Characteristics of Sandwiched EunCOTn+1 (n = 1−4) Clusters. ACS Nano 2009, 3, 2515−2522. (20) Miyajima, K.; Knickelbein, M. B.; Nakajima, A. Stern-Gerlach Study of Multidecker Lanthanide-Cyclooctatetraene Sandwich Clusters. J. Phys. Chem. A 2008, 112, 366−375. (21) Miyajima, K.; Knickelbein, M. B.; Nakajima, A. Magnetic Properties of Lanthanide Organometallic Sandwich Complexes Produced in a Molecular Beam. Polyhedron 2005, 24, 2341−2345. (22) Xu, K.; Huang, J.; Lei, S. L.; Su, H.; Boey, F. Y. C.; Li, Q.; Yang, J. Efficient Organometallic Spin Filter Based on Europium-Cyclooctatetraene Wire. J. Chem. Phys. 2009, 131, 104704. (23) Fang, X.; Woodmansee, D.; Bu, X.; Bazan, G. C. The Boratacyclooctatetraene Ligand: An Isoelectronic Trianionic Analogue of the Cyclooctatetraene Dianion. Angew. Chem., Int. Ed. 2003, 42, 4510−4514. (24) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for OpenShell Transition Metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 13115−13118.

the [Eu-COTB-Eu-COT]/Ge system can be a good candidate for molecular electronics and spintronics devices.

4. CONCLUSIONS In summary, we have studied the structural, electronic, and magnetic properties of boron-doped europium cyclooctatetraene sandwich molecular wires, [EuCOTB]∞ and [EuCOTB-Eu-COT]∞ SMWs. Our calculations show that both boron-doped SMWs possess ultrahigh magnetic moments of about 13μB per unit cell and the doping can enhance the structural stability and spin stability of the SMWs simultaneously. The alternative doping can even turn the FM semiconductor [EuCOT]∞ into a FM half-metal in [EuCOTB-Eu-COT]∞. Moreover, the active functional B atoms in heterocyclic ligands can facilitate the SMWs in practical nanoelectronics by anchoring the wires chemically bonded on the semiconductor surface. The [Eu-COTB-Eu-COT]∞ wire confined on the Ge(001) substrate is a quasi-half-metallic ferromagnet, and it can be tuned into full half-metallicity under a mild electric field of 0.2 V/Å. The unusual properties may promote the applications of the SMWs in spintronics devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11660. Charge density of the ligands in the molecular wires (Figure S1) and charge density difference of [Eu-COTBEu-COT]/Ge(001) (Figure S2) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.W.). *E-mail: [email protected] (S.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (21525311, 21173040, 21373045), NSF of Jiangsu (BK20130016), SRFDP (20130092110029), and the Scientific Research Foundation of Graduate School of Southeast University (YBJJ1563) in China. The authors thank the computational resources at the SEU and National Supercomputing Center in Tianjin.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b11660 J. Phys. Chem. C 2016, 120, 7088−7093

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DOI: 10.1021/acs.jpcc.5b11660 J. Phys. Chem. C 2016, 120, 7088−7093