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Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Electronic Structure and Spin Transport Properties of a New Class of Semiconductor Surface-Confined One-Dimensional Half-Metallic [Eu-(CnHn−2)]N (n = 7−9) Sandwich Compounds and Molecular Wires: First Principle Studies Xia Liu,† Yingzi Tan,‡ Guiling Zhang,§ and Yong Pei*,†

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†

Department of Chemistry, Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan, Hunan Province 411105, China ‡ Department of Biology and Chemistry, Hunan University of Science and Engineering, Yongzhou 425199, Hunan Province, P. R. China § College of Chemical and Environmental Engineering, Harbin University of Science and Technology, Harbin 150080, China S Supporting Information *

ABSTRACT: Transition-metal atom/π-conjugated ring sandwich compounds are promising candidates for application in molecular spintronics. However, a great challenge that has significantly restrained the practical application of these sandwich compounds is their fabrication on a well-characterized solid-state substrate in a controllable manner. In this work, we suggested a two-step self-assemble way to fabricate the Eu-CnHn−2 compounds on the hydrogen-terminated Si(100) surface and theoretically studied the geometric structure and electronic and magnetic properties. Theoretical results indicate that the silicon surface is an ideal substrate to support such kind of metal atom-encapsulated sandwich compounds as the lattice distance of silicon (100) surface is close to the inter-ring distances of freestanding gas-phase sandwich compounds. On the basis of the spin-polarized density functional theory calculations and ab initio molecular dynamics simulations, we find that the silicon surface-supported Si-[EuCh]N, Si-[EuCOT]N, and Si-[EuCnt]N sandwich compounds all process a ferromagnetic ground state. Moreover, the cycloheptatrienyl (Ch) and cyclononatetraenyl (Cnt) Eu sandwich compounds show half-metallic properties. The calculation of electron/spin transport properties using the nonequilibrium Green’s-function method confirms that the Ch Eu sandwich compounds are excellent spin filters, and the spin filter efficiency (SFE) is independent of the cluster size (N), whereas the SFE of Si-[EuCOT]N decreases rapidly with the increase of cluster size. The perfect half-metallic properties of these surface-supported sandwich compounds are promising for future application in spin devices. The present work suggests a way to fabricate the half-metallic sandwich compounds on a semiconductor silicon surface. sandwich compounds have been proposed to be spin filters because of their half-metallic properties. Since the first report of half-metallic properties in an infinite (VBz)∞ sandwiched nanowire,31,32 a lot of half-metallic infinite single molecular wires (SWMs) made of 3d transition-metal atoms and π-conjugated rings such as (MCp)∞ (M = Fe, V, and Cr), (MBz)∞ (M = Mn and Tc),7,8 (FeCpCrCp)∞,10 (Np2V2TM2)∞ (TM = Cr, Mn, and Fe),13,14 (VBzVCp)∞,15 and so forth have been envisioned theoretically. However, although a great deal of efforts had been spent to study and predict the half-metallic properties of SWMs, there are two primary challenges that restrain these compounds seriously in practical application. The first challenge is the fabrication of a

1. INTRODUCTION Transition-metal atom/π-conjugated ring sandwich compounds have attracted great research interest because of their potential applicability in molecular magnets, molecular catalysis, luminescent devices, and molecular spintronics. In the past few decades, a lot of gas-phase organometallic sandwich compounds assembled by cyclopentadiene (Cp) or benzene (Bz) and transition-metal atoms have been synthesized and studied consecutively.1−36 Since the first actinide organometallic uranocene was discovered in 1986,37 chemistry has expanded to include a wide variety of other f-block element sandwich compounds,38−50 such as An(COT)2 (An = Th, Pa, Np, Pu, Am; COT = cyclooctatetraene)38 and Lnn(COT)m (Ln = Ce, Nd, Eu, Ho, Yb).40,41 From studies on properties of these sandwich compounds, such as structure, magnetic properties, charge distribution, ionization energies, valence orbital energies, and photodetachment energies, many © XXXX American Chemical Society

Received: May 10, 2018 Revised: June 20, 2018 Published: June 21, 2018 A

DOI: 10.1021/acs.jpcc.8b04443 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

molecules, which enables us to well control the molecular line lengths as well as their electronic properties. In recent years, the Eun(COT)n+1 cluster compounds have received great research attention because of their ultralarge magnetic moment (∼7 μB per Eu atom) and relative higher stability compared with the 3d-transition-metal−π-ligand sandwich complexes. The huge magnetic moment complexes were successfully synthesized with selective one-dimensional (1D) stacking up to 18 layers; thus, it was suggested as a promising candidate as a spin filter.41−49 However, the 1D infinite [Eu-COT]∞ molecular wire is a ferromagnetic (FM) semiconductor, and the SFE declined rapidly with the cluster size. However, it is noticed that some very small-sized Eu-COT clusters can serve as spin filters, such as Eu2(COT)2 and Eu3(COT)4.50 It is well known that the properties of sandwich nanowires depend significantly on the number of π electrons of the organic coordinating group. The addition or reduction of C atoms within the π-conjugated unit will add or reduce the π electrons of the systems, and the properties of the SWM will change drastically. For example, when replacing the C5H5 group to C6H6, the (MnCp)∞ system would change from metal to half-metal.8 Meanwhile, we found that the theoretical research studies in Eu sandwich complexes usually refer to the aromatic hydrocarbon ligands (CnHn) for n = 8, but for n = 7 or 9, none has been authenticated so far. In fact, several cycloheptatrienyl (Ch) f-block element sandwich compounds had been synthesized or theoretically predicted. Arliguie et al.77 synthesized the first seven-membered ring sandwich compound U(Ch)2−. Li et al.78 discussed the electronic structure of actinide compounds An(Ch)2 (An = Th, Ps, U, Np, Pu, Am). Recently, the cyclononatetraenyl (Cnt) Eu sandwich compound Eu(C9H9)2 also has been reported.79 The test calculation performed in this work indicated that the binding energy (Eb) of (Eu-Ch)∞ and (Eu-Cnt)∞ is as high as 5.10 and 4.30 eV per Eu atom, respectively, which are higher than that of Cp or Bz transition-metal compounds. Both (EuCOT)n (n = 1−18) and (Eu-Cnt)2 were synthesized in the laboratory successfully. We have a reason to believe that the sandwiched Ch and Cnt Eu molecular wires are also highly stable for future experimental synthesis. In this work, we propose a strategy of two-step assembly of Eu-CnHn−2 (n = 7, 8, and 9) sandwiched molecular wires confined on the hydrogen-terminated Si(100) surface. The basic idea of constructing the surface-confined Eu-CnHn-2 system is given in Scheme 1. The CnHn (n = 7, 8, and 9) rings are first anchored between two surface dimer rows on the H−Si(100) surface via the formation of Si−C covalent bonds. Then, the Eu atoms are inserted into the empty space between the two neighboring CnHn−2 units. These surface-confined [Eu-CnHn−2]N SWMs show advantages in both thermodynamic stabilities and size-insensitive spin filter properties. First, it is found that the insertion of Eu atoms into the preconfined [CnHn−2] molecular wires is nearly barrierless, suggesting the spontaneous formation of the metalπ sandwich structure. The ab initio molecular dynamics (MD) simulations confirmed that the molecular wire may retain a stable sandwich structure at room temperature. Second, the electronic and magnetic properties of these surface-confined sandwich molecular wires are less affected by surface confinement. They all process FM ground states (GSs) similar to gas-phase molecular wires, and the Si-[Eu-Ch]∞ and Si-[EuCnt]∞ are half-metals. Third, the investigation of size-

long-enough SWM on well-characterized solid-state substrates in a controlled manner.55 To date, the synthesis of multistacked metal-π sandwich compounds are generally conducted in the gas phase. Because of the poor solubility and high sensitivity to air, it is a great challenge to transfer the assynthesized compounds onto a solid substrate.49 The second is that the intrinsic properties of these sandwich compounds limited their practical applications. It was found by many theoretical studies that the spin filter efficiency (SFE) of the sandwich compounds significantly depends on the cluster size (number of encapsulated metal atom or conjugated rings). For example, the infinite (VBz)∞ shows perfect half-metallic properties. Nonetheless, at the finite size, the VnBzn+1 (n ≤ 8) shows semiconducting properties, except that the V4Bz5 could serve as a spin filter.32 Similarly, the half-metallic Fe-Cp nanowires also show rapid decline of SFE with the increase of n in FenCpn+1 systems.7 The SFE of FeCp2, Fe2Cp3, and Fe3Cp4 is 45.9, 19, and 17.2%, respectively. Hence, it is important to find a new type of sandwich compound which could not only anchor on solid substrate efficiently but also has the SFE that is not seriously dependent on the cluster size. In our previous works, we have proposed a two-step assembly of transition-metal atom sandwich molecular wires confined on the hydrogen-terminated surface,56,57 inspired by the discovery that the organic molecules can be attached to the hydrogen-terminated silicon surface.58−71 The concept of solidifying the organometallic SWMs on the Si substrate was also demonstrated by Lu et al. and Yao et al. Lu et al.72 investigated a series of SWMs attached on a H−Si(100) surface which formed with different transition-metal atoms and borine units. Yao et al.73 investigated two kinds of borondoped europium cyclootatetraene sandwich molecular wires anchored on a Ge(001) surface. Actually, attaching various organic molecules on silicon surfaces has become an increasingly mature technology. Several ways have been developed to assemble organic molecules on a silicon surface, such as wet chemical functionalization strategies, hydrosilylation involving a radical initiator, thermally induced hydrosilylation, photochemical hydrosilylation, hydrosilylation mediated by metal complexes, reactions of alkyl/aryl carbanions, electrochemical diazonium reduction, and so on.58 For anchoring the CnHn−2 on a Si−H-passivated surface, the electrochemically accessible method through treatment of the native Si−H-passivated surface with a methyl Grignard or methyl lithium organometallic under anodic conditions is promising. Hence, we believe that anchoring the CnHn−2 on the H−Si(100) surface could be feasible. Meanwhile, these tactics were believed as an effective way to solve the first challenge of solidifying the SWMs on the solid substrate.74,75 Recently, the development of new synthesis methods has led to the growth of highly ordered Eu-COT nanowires on the graphene surface.76 For the second issue, a number of theoretical studies have shown that the SFE of SWMs depended seriously on the cluster size or molecular wire length; only some specific clusters with a certain length and compositions could serve as a perfect spin filter.7,15,32,35,50−54 The gas-phase wires usually ended with either metal atoms or organic rings. Thus, the gasphase SWMs were not always a half-metal; even the infinite wires are half-metallic. Different from the gas-phase SWMs, the surface-confined SWMs may provide a template to insert specific metal atoms between the neighboring organic B


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basis set. The mesh cutoff is 150 Ry, and the size of the mesh grid in k space for electrode parts is 2 × 2 × 500. The calculation parameters and slab models were carefully tested in our previous work.56 We model the H-terminated Si(100) surface with a 12-layer slab model. The Si atoms on both sides of the slab are passivated with H atoms. The bottom five layers’ Si atoms are fixed during geometrical optimization. To explore both FM and antiferromagnetic (AFM) configurations, we used an orthogonal Si-(Eu-CnHn-2)∞ (n = 7, 8, and 9) sandwich wires supercell, 7.72 × 15.45 × 40 Å3, containing two Eu metal atoms and two CnHn-2 rings alternately sandwiched on the H−Si(100)(2 × 1) surface along the direction shown in (a) (as shown in Scheme 2). The Brillouin zone is meshed by the γ-centered Monkhorst−Pack method with 5 × 2 × 1 k-points for geometry optimizations and by that of 40 × 1 × 1 k-points for the static total energy calculations along the molecular wire axis. The HSE06 energy band calculations include two steps. A self-consistent HSE06 calculation is first performed with the Brillouin zone being sampled using a 10 × 1 × 1 gamma-centered Monkhorst−Pack grid. After that, 20 k-points along the Γ to X direction are added for the second-step band structure calculation. The proposed molecular junction consists of three parts: the left electrode, the central scattering region, and the right electrode (as shown in Scheme 3). We choose the Al(100) nanowires as the electrode with 3 × 3 cross section as reported by Smeu et al.55 The lowest energy lead plane to ring plane distance was found to be 2.5 Å, and this optimized lead−ring orientation was used for all molecular junction systems.

Scheme 1. Schematic Representation of the Two-step Assembly of [Eu-CnHn−2]∞ Sandwich Molecular Wires on the H-Passivated Si(100) Surfacea

a Gas-phase CnHn molecules first attached to the Si substrate along the (100) direction by the removal of two H atoms; then, the Eu atoms are inserted in the spaces between the neighboring organic rings.

dependent SFE of silicon-supported finite Eu-CnHn-2 (n = 7, 8) sandwich clusters using the nonequilibrium Green’s-function (NEGF) method indicates excellent SFE of Eu-Ch sandwich clusters. The SFE is almost insensitive to the cluster size or molecular wire length. The facile preparation procedure, high stabilities, and size-independent excellent SFE suggests that these silicon surface-confined [Eu-CnHn−2]N sandwich molecular wires are promising for future applications in spintronics.

2. COMPUTATIONAL METHOD AND MODEL All structure optimization calculations are performed within the framework of spin-polarized density functional theory as implemented in the Vienna Ab initio Simulation Package (VASP).80,81 The exchange−correlation interactions are treated by generalized gradient approximation parameterized by Perdew, Burke, and Ernzerhof (PBE).82 We describe the interaction between ions and electrons using the projected augmented wave (PAW).83 The periodic unit is optimized by the conjugate gradient algorithm with an energy cutoff of 500 eV until the force on each atom is less than 0.01 eV/Å. The calculated electronic structures were verified by the hybrid functional based on a screened coulomb potential (HSE06).84 The electronic transport calculations of finite Si−SWMs are carried out using the NEGF formalism, which is implemented in the Atomistix Toolkit-Virtual Nanolab package.85,86 The electron wave function is expanded using a double-ζ polarized

3. RESULTS AND DISCUSSIONS 3.1. Eu Atom Binding Energy and Insertion Energy Barriers of Si-[Eu-CnHn−2]∞ (n = 7, 8, and 9). The binding energy (Eb) of a metal atom is an important indicator of stability of sandwiched SWMs. In this work, the binding energy of an Eu atom in the gas-phase [Eu-CnHn]∞ and surfaceconfined Si-[Eu-CnHn−2]∞ is first investigated using eqns 1 and 2, respectively E b = [E[Eu‐CnHn]∞ − E(CnH n) − nE(Eu)] /n

(1)

E b = [E[Si‐(Eu‐CnHn − 2)]∞ − E(Si‐(CnHn − 2)∞ ) − nE(Eu)] /n

(2)

Scheme 2. Schematic View of the Structure of Si-Supported Eu Sandwich Nanowires on the H−Si(100)(2 × 1) Surface (Using Si-[EuCh]∞ as an Example); (a) Perspective View, (b) Front View, and (c) Side View. Blue, Gray, White, and Red Balls Denote Si, C, H, and Eu Atoms, Respectively. The Label c Denotes the Lattice Constant Along the Molecular Wire Direction

C


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The Journal of Physical Chemistry C Scheme 3. Two-Probe Transport Structurea

Semi-infinite leads extending to ±∞ (left and right boxes) are bridged by the Si-(Eu-Ch)N (N = 1−6) line in the scattering region (center box). The atoms in the shaded region were excluded from calculations for simplicity, and dangling bonds were capped with H atoms. The inset figure shows the arrangement of the four lead atoms closest to the Si-supported molecule from a view along the lead.

a

Table 1. Properties of the Eu SWMsa SWMs

cb

Ebb

Sb

Bader chargeb

(Eu-Ch)∞ (Eu-COT)∞ (Eu-Cnt)∞ Si-(Eu-Ch)∞ Si-(Eu-COT)∞ Si-(Eu-Cnt)∞

8.52 8.48 7.35 7.72 7.72 7.72

−5.10 −5.65 −4.30 −4.16 −5.20 −4.22

12.0 14.0 12.8 12.0 13.6 12.6

1.44 1.46 1.37 1.51 1.49 1.43

ΔEb

GSc FM FM FM FM FM FM

quasi-half-metal semiconductor metal half-metal semiconductor half-metal

−80 −3 −146 −28 −6 −64

a

The lattice constant (c, along the molecular direction, in unit of Å) and the average binding energy per metal atom Eb (in unit of eV), the total magnetic moment S in a unit cell (in unit of μB). Bader atomic charges, electronic GS, and the energy difference between the FM and AFM GS (ΔE, in unit of meV per metal atom, ΔE = (E(FM) − E(AFM))/2). bPBE results. cHSE06 results.

where E(CnHn) is the electronic energy of a single freestanding CnHn molecule, E(Eu) and n are the electronic energy of the isolated Eu atom and the number of Eu atoms in the supercell, E[Si-(CnHn−2)∞] is the electronic energy of the siliconsupported CnHn−2 molecular line without Eu doping, and E[Si-(Eu-CnHn−2)∞] is the electronic energy of the Eu atom incorporated in the Si-supported molecular wires. From Table 1, the calculated binding energies of the Eu atom in the surface-confined [Eu-COT]∞, [Eu-Ch]∞, and [EuCnt] ∞ nanowires are −5.20, −4.16, and −4.22 eV, respectively, which is only slightly smaller than the gas-phase ones by 0.45, 0.94, and 0.08 eV, respectively. Of note is that as the inter-ring distance in the Si-supported nanowire is fixed to be 3.86 Å (corresponding to the distance between underlying Si−Si dimers), the inter-ring distance of these surface-confined [Eu-CnHn−2]∞ SWMs was compressed by about 9.39 and 8.96% for n = 7 and 8 systems and stretched by about 5.03% for the n = 9 system in comparison to the gas-phase [EuCnHn]∞. To explore the thermodynamic stability of these surfaceconfined SWMs, ab initio MD simulations are carried out based on a simplified surface model (as shown in Figure S1) on which a [Eu-Ch]∞ molecular wire is anchored. The fluctuations of electronic energies and total magnetic moment during 2 ps MD simulations are displayed in Figure 1. The MD results indicate that the surface-confined molecular wire is stable at 300 K, and the dissociation of the molecular wire is not found within 2 ps simulations. As shown in Scheme 1, the surface-confined sandwich molecular wires may be fabricated via a two-step strategy, in which the insertion of Eu atoms into the preconfined molecular wires is a key step. Although the CnHn−2 are not aromatic and not planar, when the Eu atom is inserted, the

Figure 1. Evolution of temperature (T), total energy (ET), kinetic energy (EK), total magnetic moment of unit cell (S), and temperature during the 2 ps simulations at 300 K. A simplified six-layer silicon substrate model was used in MD simulation. The two bottom silicon layers as well as bottom hydrogen atoms are fixed during MD simulations. The MD simulations are carried out using the PBE/PAW method for electronic energy calculation, and the Nosé−Hoover method is used for temperature control. The time-step in MD simulations is 1 fs.

organic ring gets electron from the Eu atom, forming 10e− aromatic systems. Therefore, when the Eu atom is far away from the CnHn−2 ring, the organic ring is nonplanar, whereas when the Eu atom is near the ring and charge transfer occurred, the ring changed to planar rapidly (as shown in Figure S2). In Figure 2, we assessed the energy barrier of the Eu atoms inserted into the Si-[CnHn−2]∞ wires, and the energy profile of the Eu atom insertion is calculated. As shown in Figure 2a, using a supercell containing two CnHn−2 rings and D


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barrierless. The most difficult step could be the second Eu atom insertion, as two nearest CnHn−2 units have been bound with the first inserted Eu atoms. However, our calculations (Figure 2b) show that the energy barriers of the second Eu atom insertion are as small as 0.32, 0.11, and 0.06 eV for n = 7, 8, and 9, respectively. Such levels of energy barriers are considered to be easy to overcome in such an exothermic reaction. We believe that the Si-supported Eu SWMs systems would be realized easily from the viewpoint of their high stabilities and small Eu atom insertion energy barriers. 3.2. Magnetic and Electronic Properties of Silicon Surface Supported Eu-CnHn-2 Sandwich Compounds. The electronic structure and magnetic properties of these surface-confined SWMs are explored using a supercell containing two Eu atoms and two CnHn−2 units. Both FM and AFM configurations were sampled. In general, we found that the silicon-supported infinite EuCnHn−2 SWMs all process robust FM GSs. The average magnetic moment of each Eu-CnHn-2 unit in the unit cell is as large as 6.0, 6.8, and 6.3 μB (cf. Table 1), respectively. The energy differences between the FM and AFM states are −28, −10, and −64 meV per metal atom for Si-[Eu-CnHn−2]∞ systems with n = 7, 8, and 9, respectively. The magnetic moment values and energy differences between the FM and AFM states of the surface-confined molecular wires are comparable to those of the gas-phase molecular wires as shown in Table 1. For comparison, the FM and AFM energy differences of the

Figure 2. Energy profiles of inserting (a) first and (b) second Eu atoms into the Si-[CnHn−2]∞ array. The corresponding atomic models are shown as insets. Starting from an initial position that is 5 Å away from the final equilibrium position, the Eu atom is moved along a straight path in steps of 1.0 Å. We choose six layers of Si atoms as the substrate. At each step, the position of the Eu atom is fixed, whereas other atoms are fully relaxed, except for the bottom three layers of Si atoms, before the total energy is calculated.

two Eu atoms, we show that insertion of the first Eu atom into the molecular wires is a strongly exothermic process and

Figure 3. Energy band structures of the 12-layer H-terminated Si(100) surface (a) and three SWMs (b−d). The black lines denote the contribution by the H−Si(100) surface, the blue dotted lines denote the contribution by C atoms, and the green and red dotted lines represent the contribution of 5d and 4f orbitals from Eu atoms, respectively. The dashed line at 0 eV indicates the Fermi energy. E


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aromatic Cnt−. As the results, from the Ch to the COT system, the Fermi energy level was across the Eu4f orbital in the Eu-Ch system, whereas it locates above the Eu4f orbital in the EuCOT system. Further increase of the energy levels of the occupied π orbitals in the Eu-Cnt system causes the interactions of Eu5d and C2p orbitals. As shown in Figure 3 and Figure S3, in both gas-phase Eu(C9H9)∞ and Si-Eu-Cnt systems, the Eu5d orbital contributions are seen in the energy bands near the Fermi energy levels, whereas the difference between the freestanding and the silicon-supported SWM systems is that the Si-(EuCnt)∞ manifests unique half-metallic properties and the spin-polarized bands crossing the Fermi energy level are contributed by the C2p orbitals. 3.3. The Electron/Spin Transport Properties of Finite Si-(Eu-CnHn−2)N SWMs (n = 7−8 and N = 1−6). The above discussions showed the possibility of solidifying the Eu-doped SWMs on the silicon surface, and these infinite SWMs show half-metallic and semiconductor properties. In this part of the discussions, the electron/spin transport properties of finitesized surface-confined (Eu-CnHn−2)N (N = 1−6) cluster compounds are investigated. It has been found in many studies that the finite-sized metal atom sandwich compounds may have much different spin filter properties to the infinite system. In other words, several theoretically predicted half-metallic properties in the infinite molecular wires may be not retained in the finite systems. Therefore, it is important to investigate the size-dependent effect of electron/spin transport properties of finite Si-(EuCnHn−2)N systems. To examine the size-dependent electron/spin transport properties of finite silicon-supported [Eu-CnHn−2]N complexes, we carried out electron transport calculations for finite Si[EuCh]N and Si-[EuCOT]N complexes (as shown in Scheme 3) to explore the effect of molecular wire length on the spin transport efficiency, where N is the number of Eu atoms. The spin-dependent transmission spectra and the zero bias SFE are investigated on the basis of a series of models containing different numbers of Eu and organic rings. The SFE is defined as

corresponding freestanding gas-phase SWMs are −80, −3, −146 meV, respectively. The HSE06 functional is used to calculate the energy band structure of SWMs. The Si-[Eu-CnHn−2]∞ system demonstrated an evolution trend of half-metal to semiconductor and then to a half-metal with n being changed from 7 to 9. For three kinds of SWMs, their band structures are generally similar to each other. However, the relative positions of the Eu4f orbitals and C2p orbitals to the Fermi energy changed significantly with the increase of the size of the organic coordinating groups. From the spin-up channel of Figure 3b−d, the Fermi energy level crossed the Eu4f orbitals in the Eu-Ch system, whereas the Eu4f orbitals are about 0.5 and 2.0 eV below the Fermi energy in the Eu-COT and Eu-Cnt systems, respectively. A similar trend was also found for the highest occupied energy bands which are contributed by the C2p orbitals, whose relative positions to the Fermi energy level in the spin-down channel (Figure 3b−d) move down continually with the increase of the size of conjugated rings. As a result, when the number of π electrons increases, the surface-confined Eu-CnHn−2 SWMs change from half-metal to semiconductor and then to halfmetal. Because of the shift of the relative position of Fermi energy levels, the atomic orbital components in the energy bands near the Fermi energy changed correspondingly. As shown in Figure 3b−d, for the Eu-Ch system, the energy bands near the Fermi energy are contributed majorly by the Eu4f and C2p orbitals, whereas for the Eu-Cnt system, with the shift of Fermi energy positions, the highest energy occupied bands in the spin-up and spin-down channels are majorly contributed by the C2p orbitals. It is important to explore the effect of surface confinement on the electronic structure and magnetic properties of molecular wires. Figure S3 displays the energy band structures of three freestanding, gas-phase SWMs. It is found that the electronic and magnetic properties of surface-confined [EuCnHn−2]∞ SWMs are very similar to their gas-phase analogues. A slight difference is found that in the gas-phase systems, SWMs changed from a quasi-half-metal to a semiconductor and then to a metal with the increase of the conjugated ring size from C7H7 to C9H9. The similar geometric structure and electronic and magnetic properties of surface-confined SWMs and the freestanding gas-phase ones indicate the surface confinement effect employs small influences on the overall properties of Eu-sandwiched wires. To deeply understand the half-metal to semiconductor and then to half-metal transitions in three kinds of surface-confined SWMs, we analyzed the atomic and molecular orbital interactions in detail (shown in Figure S4). Xu et al.50 found that in the D8h symmetric Eu-COT complexes, seven Eu 4f orbitals are split into four sets of orbitals with A1u, E1u, E2u, and E3u symmetry. According to the Hückel rule, the COT ring tends to capture two additional electrons to form a planar aromatic COT2− ring. The Eu atom may donate two 6s electrons to the COT ring, leaving seven 4f valence electrons unpaired, which are responsible for the 7 μB magnetic moments and semiconducting property of those Eu-COT clusters. The Ch ring and Cnt ring have either one π electron lesser or greater than the COT unit. The Ch ring tends to capture three electrons from the Eu atoms to form a planar aromatic Ch3− unit, and the Cnt ring tends to capture one additional electron from the Eu atoms to form a planar

SFE =

Tmaj − Tmin Tmaj + Tmin

× 100%

where Tmaj and Tmin indicate the transmission coefficient of the majority and minority spin channel, respectively. To see how the length of the molecular wire affects the SFE, we investigated the number of Eu atom ranges from N = 1−6. As shown in Figure 4a, the SFE of Si-[EuCh]N increases with the number of Eu atoms (N) and approaches 100% rapidly, whereas the Si-[EuCOT]N SFE decreases with the increase of the molecular wire lengths. To gain a deeper understanding of the higher SFE in the Si[EuCh]N, we examined spin contributions of the majority and minority states to the total current (Figure 4b). The spin injection factor (P) defined as (Imaj − Imin)/(Imaj + Imin) is calculated for the Si-[EuCh]6. The inset of Figure 4b shows the variation of P value as a function of applied bias voltage. Within the bias voltage range considered here (0−0.5 eV), a small variation (∼5%) of P value with bias voltage is found. The maximum P value is 99.99%, and the minimum P value is ∼95%. The observed large SFE together with a small variation of P value with the bias voltages suggests that the finite Si[EuCh]N system is also a perfect highly effective spin filter F


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The spin-dependent transmission spectra of finite Si[EuCh]N and Si-[EuCOT]N complexes are also calculated and displayed in Figure 5, which agree with the SFE trends displayed in Figure 4. From Figure 5, it can be seen that for the Si-[EuCOT]N systems, with the increase of the molecular wire length, although the spin-up channel keeps metallic properties, the spin-down channel changes gradually from the metallic to semiconductor properties. In contrast, for the Si-[EuCh]N system, with the increase of N, the metallic property of the spin-up channel is retained all the way, and the spin-down channel maintained the semiconductor property, which means that the Si-[EuCh]N system has a perfect half-metallic property even in finite length.

4. CONCLUSIONS In this work, we theoretically studied the structure, stability, and electronic structure properties of a series of silicon surfaceconfined Eu-CnHn−2 (n = 7, 8, and 9) sandwich compounds and suggested a two-step assembly way to fabricate the EuCnHn sandwich compounds on the hydrogen-terminated Si(100) surface. By means of the first principle calculations

Figure 4. (a) Zero bias SFE with different Eu numbers (N) and (b) the bias-dependent majority and minority channel contributions to current in the Si-(Eu-Ch)6. The inset shows the variation of the magnitude of spin injection coefficient (P) with bias.

candidate, and its spin filter properties are less affected by the length of molecular wires.

Figure 5. Spin-resolved transmission of the Si-(Eu-CnHn−2)N (n = 7 and 8, N = 1−6) sandwich clusters between two Al(100) nanowires. The red dotted line represents the Fermi level. G


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The Journal of Physical Chemistry C and ab initio MD simulations, we find that the siliconsupported Eu-CnHn-2 sandwich compounds process high thermodynamic stabilities and show unique magnetic and electronic properties. When the molecular wires are confined on the silicon substrate, because the surface lattice compressed or stretched the inter-ring distances, the SWMs change from half-metal to semiconductor and then to half-metal with the increase of ring size, different from the gas-phase analogues. The possible usage of these silicon-supported sandwich compounds in the spin devices is also discussed from investigating the SFE of the finite Si-(Eu-CnHn−2)N SWMs (n = 7−8 and N = 1−6). The results show that the Si-(Eu-Ch)N has good spin filter properties, whose SFE is weakly affected by the length of the molecular wire. The Si-(Eu-COT)N system shows high SFE with N in the range of 2−4, whereas the SFE decreases rapidly with the further increase of the molecular wire length. The size-insensitive SFE properties of the Si-(EuCh)N system suggests that it is a promising candidate for the spin filter devices. We expect that the current findings will generate experimental studies toward the realization of a new generation of spin filter device based upon sandwich molecular wires.

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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.8b04443. MD simulation model, structure changes of the Si-EuCh system when the Eu atom is inserted between the organic rings, band structures of three gas-phase Eu SMWs, and orbital interaction diagrams (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yong Pei: 0000-0003-0585-2045 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (21773201 and 21422305) and the project of the innovation team of the Ministry of Education (IRT_17R90).

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REFERENCES

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