First-Principles Study of Structural, Electronic, and Magnetic Properties

Nov 17, 2016 - Department of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan Unive...
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First-Principles Study of Structural, Electronic, and Magnetic Properties of One-Dimensional Transition Metals Incorporated Vinylnaphthalene Molecular Wires on Hydrogen-Terminated Silicon Surface Xia Liu,† Yingzi Tan,‡ Zhongyun Ma,† and Yong Pei*,† †

Department of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Hunan Province Xiangtan 411105, People’s Republic China ‡ Department of Biology and Chemistry, Hunan University of Science and Engineering, Hunan Province Yongzhou 425199, People’s Republic China S Supporting Information *

ABSTRACT: Gas-phase one-dimensional organometallic sandwich nanowires (SWNs) made by transition metal atoms and π-conjugated hydrocarbon rings have been subjected to intensive experimental and theoretical studies. In this work, we theoretically predicted the structures and studied the electronic and magnetic properties of a new type of surface confined 3d transition metal (TM = Sc, Ti, V, Cr, and Mn) decorated vinylnaphthalene molecular wires anchored on the hydrogenterminated Si(100)-(2 × 1) surface, denoted as Si-[Vinylnaphthalene-TM2]∞. On basis of the spin-polarized density functional theory (DFT) calculations, we find these surface confined SWNs possess novel electronic and magnetic properties in contrast with their gas-phase analogues. Because of one end of the naphthalene rings is fixed on the silicon surface through a vinyl group, the TM atoms cannot efficiently coordinate to the naphthalene rings. Even though, the Si[vinylnaphthalene-TM2]∞ SWNs still show large binding energies for metal atoms. The electronic structure calculations demonstrate that the Si-[vinylnaphthalene-TM2]∞ SWNs can be ferromagnetism (TM = Cr and Mn), nonmagnetism (TM = Sc and V), or antiferromagnetism (TM = Ti). The half metallic property is found for the Mn-decorated SWNs. big ring complexes LnnCOTm52−57 (TM = transition metal atoms, Ln = lathanide, Cp = C5H5, Bz = C6H6, COT = C8H8). However, despite significant experimental and theoretical efforts, the practical applications of these gas-phase molecular nanowires in spin-devices were lagged seriously. To date, the synthesis of organometallic multidecker SWNs commonly used laser vaporization and molecular beam experiments. It is not easy to move these fragile molecular wires into devices. Therefore, in order to apply these sandwich complexes to practical devices, fabrication of these sandwich nanowires on suitable semiconducting or insulating surfaces should be a critical step. The direct fabrication of sandwich nanowires on a solid substrate will undoubtedly accelerate their practical applications. Recently, based on progresses of fabrication of ordered low-dimensional self-assembly organic molecular nanostructures on the hydrogen-terminated silicon surface,58−64 Lu et al. proposed a two-step assembly of sandwich

1. INTRODUCTION The sandwich transition metal atom/π-conjugated organic molecule complexes have attracted great research interests because of their novel properties originating from intermetal communication,1−8 possessing potential applicability to molecular magnets,9−12 molecular catalysis,13,14 and luminescent devices.15−17 Thus far, the organometallic multidecker sandwich nanowires (SWNs) have been successfully synthesized in vacuo and have undergone intensive experimental and theoretical studies. The properties of these sandwiched multideckers are found to be affected by many factors, such as the type of sandwiched transition metal atom, the property of π-conjugated organic molecule, as well as the substituent groups on the π-conjugated deckers. Depending on the types of metal atoms and πconjugated deckers, many nanowires demonstrated halfmetallic properties and are potential candidates for application in spintronics devices. Some representative organometallic multidecker SWNs are cyclopentadienyl ligand complexes TMnCpm,17−30 benzene ligand complexes TMnBzm,31−51 and © XXXX American Chemical Society

Received: August 28, 2016 Revised: November 17, 2016 Published: November 17, 2016 A

DOI: 10.1021/acs.jpcc.6b08683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Schematic View of Structure of Si-[Vinylnaphthalene-TM2]∞ Nanowires on H−Si(100)-(2 × 1) Surfacea

a

(a) Perspective view, (b) front view, and (c) side view. Blue, grey, white, and purple balls denote Si, C, H, and transition metal atoms, respectively. The label c denotes the lattice constant along the molecular wire direction.

of [NpTM2]∞ (Np = C10H8, i.e. naphthalene) SWNs with the 3d TMs ranged from Sc to Cu and a 4d metal Nb. They found that the [NpTM2]∞ SWNs are AFM for TM = Ti, V, and Nb; FM for TM = Mn; and NM for TM = Sc. The properties of one-dimensional bimetallic TM-naphethalene SWNs, denoted as [Np2V2TM2]∞ (TM = Ti, Cr, Mn, and Fe), were also investigated theoretically. 72 Half-metals are found for [Np2V2TM2]∞ with TM = Cr, Mn, and Fe. At present, we show that when the naphthalene molecules are immobilized on the silicon substrate via a vinyl group linkage, the insertion of 3d transition metal atoms into the spaces between adjacent naphthalene units may lead to a novel type of surface confined Si-[vinylnaphthalene-TM2]∞ wires, which possess structural and electronic properties in contrast with the gas-phase [NpTM2]∞ SWNs. From Scheme 1a, the individual molecules in the single vinylnaphthalene wire are spaced by 3.86 Å, corresponding to the distance between Si−Si dimers on the Si(100)-(2 × 1) surface. Without inserting TM atoms, the naphthyl rings are parallel to each other and tilted to the axial direction of the molecular line. The intermolecular spacing of 3.86 Å is comparable to that in molecular crystals such as metal phthalocyanines and polycyclic aromatic hydrocarbons, which show substantial effects of intermolecular π-coupling. After inserting the 3d transition metal atoms, the molecular wires showed interesting electronic and magnetic properties. Depending on the types of TM atoms, the SWNs may be either ferromagnetism (TM = Cr and Mn), antiferromagnetism (TM = Ti), or nonmagnetism (TM = Sc and V). Particularly, the Mn decorated molecular wires showed half metallic properties. To the best of our knowledge, this is the first report of the half-metallic single-type metal atomdecorated TM-naphethalene SWN. Previously, the half-metallic properties are only seen in the bimetallic [Np2V2TM2]∞ SWNs with TM = Cr, Mn, or Fe. The current theoretical results suggest a possible way to fabricate half-metallic Np-TM2 molecular wires on a semiconducting substrate.

transition-metal/organic molecular wires on a silicon surface. They found that the combination of Mo atoms and covalently attached borine molecules on the H-terminated Si(100) surface led to stable half-metallic [Mo-borine]∞ wires.65 Our group also systematically investigated the electronic and magnetic properties of 3d transition metal (TM = Sc, Ti, V, Cr and Mn) atom incorporated single and double one-dimensional styrene molecular wires confined on the H-terminated Si(100) surface.66 Caused by the surface confinement effects, these molecular wires possessed much different electronic and magnetic properties in contrast with the free stranding gasphase wires. In this work, we reported a density functional theory (DFT) study on the geometric structure, electronic, and magnetic properties of a novel type of 3d transition metal atom (TM = Sc, Ti, V, Cr, and Mn) decorated vinylnaphthalene wires on the H-terminated Si(100)-(2 × 1) surface, denoted as Si-[Vinylnaphthalene-TM2]∞. To date, the molecular wires made by single-ring conjugated deckers have been subjected to great studies.17−57 By contrast, experimental and theoretical efforts on molecular wires constructed from metal atoms and polycyclic hydrocarbon rings are very limited. Moreover, unlike the rich half-metallic properties in sandwiched single-ring/ transition metal complexes, the molecular complexes or molecular wires made by polycyclic rings and sandwiched transition metal atoms commonly exhibited diamagnetism or antiferromagnetism. Katz et al. reported the synthesis of first pentalene-based sandwich complexes Pn2M2 (Pn = C8H6; M = Co, Ni,) and observed their diamagnetism to be in contrast with their bis(cyclopentadienyl) counterparts.67,68 The synthesis of Pn*2M2 complexes (Pn* = C8Me6; M = V, Cr, Mn, Co, and Ni) were also accomplished by Ashley et al.69 The metal centers in Pn*2Co2 exhibited different bonding to each side of the Pn* ligand. On the theoretical side, Wu et al.70 computationally studied the structures and magnetic properties of infinite [PnTM2]∞ (TM = V, Cr, Mn, Co, and Ni) wires. They found that only for TM = Mn was the molecular wire ferromagnetism (FM); other nanowires are either antiferromagnetism (AFM) (TM = V and Cr) or nonmagnetism (NM) (TM = Co, Ni). Zhang et al.71 studied the magnetic properties

2. COMPUTATIONAL METHOD AND DETAILS All calculations are performed within the framework of spinpolarized density functional theory (DFT) as implemented in the Vienna Ab initio Simulation Package (VASP).73,74 The B

DOI: 10.1021/acs.jpcc.6b08683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. (a) Front view and side view of th optimized geometric structures of Si-[vinylnaphthalene-TM2]∞ wires (TM = Sc, Ti, V, Cr, and Mn atoms) on the H−Si(100)-(2 × 1). Nine layers of bottom Si atoms are hided for clarity. (b) Schematic illustration of orientation of naphthyl rings in five types of Si-[vinylnaphthalene-TM2]∞ wires. (c) Definition of atomic numbers (1−4) in the SWNs.

methods are used. Ueff = U − J was used instead of setting individual U and J values. In the current study, Ueff = 3 eV was adopted from previous theoretical works.78,79 For FM Cr and Mn decorated molecular wires, the hybrid functional based on a screened Coulomb potential (HSE06)77 is also applied for energy band structure calculations. The calculation parameters and slab models had been carefully tested in our previous work.66 We model the H-

exchange-correlation interactions are treated by generalized gradient approximation (GGA) parametrized by Perdew, Burke, and Ernzerhof (PBE).75 We describe the interaction between ions and electrons using the projected augmented wave (PAW) with an energy cutoff of 500 eV.76,77 The atomic positions are optimized by the conjugate gradient algorithm until the force on each atom is less than 0.01 eV/Å. In calculating the energy band structures, both GGA and GGA+U C

DOI: 10.1021/acs.jpcc.6b08683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Table 1. Properties of the Silicon Supported Si-[Vinylnaphthalene-Ti2]∞ SWNsa magnetic moment on TM atoms (1, 2, 3, 4) TM

dM‑Mb

Ebb

Sc Ti V Cr Mn

2.867 2.647 2.403 3.020 2.688

−3.611 −3.573 −3.860 −2.180 −1.856

PBE results 0, 0, −0.783,0.768, 0, 0, 3.116, 3.132, 3.000, 2.978,

0, 0 −0.783,0.768 0, 0 3.108, 3.125 3.000, 2.977

PBE+U results 0, 0, −1.342,1.320, 0, 0, 3.623,3.454, 3.578,3.476,

0, 0 −1.342,1.320 0, 0 3.616,3.398 3.578,3.476

Bader chargeib (TM atoms 1 to 4) 1.470,1.476, 1.518,1.505, 1.474,1.504, 1.276,1.281, 1.124,1.159,

1.470,1.476 1.518,1.505 1.474,1.503 1.276,1.281 1.104,1.138

ΔEb

GSc

/ 0.115 / −0.021 −0.098

NM metal AFM metal NM metal FM metal FM half-metal

a

The average metal−metal distance (dM‑M, in unit of Å) and the average binding energy per metal atom Eb (in unit of eV), the local magnetic moment (μB) on TM atom numbered 1−4 (as shown in Figure 1c), Bader atomic charges, electronic ground state (GS), and the energy difference between the FM and AFM ground state (ΔE, in unit of eV per metal atom, ΔE = [E(FM) − E(AFM)]/4). bPBE results. cPBE+U results.

tion to the two adjacent naphthalene rings, respectively (see Figure 1, and the bond lengths between TM and C atoms are given in Figure S1 in the Supporting Information). The tilt angles between the naphthalene ring plane and axial direction of molecular wire were also measured. Caused by the smaller atomic radius of V, Cr, and Mn atoms, the interactions between these metal atoms and naphthalene rings induce structural distortion of naphthalene molecular planes. From Figure 1b, the tilt angles (θ) of naphthalene ring planes are 102.1°, 101.2°, and 104.1°, and the distance between the centers of two neighboring naphthalene units are 3.77 Å, 3.79 Å, and 3.74 Å for TM = V, Cr, and Mn incorporated SWNs, respectively. For SWNs consisting of early transition metal elements such as Sc and Ti, the tilt angles of naphthalene molecular planes are near to 90° and the axial distances between two naphthalene units are 3.86 Å, which is the same as the ones without decorated TM, indicating very small structural distortion of naphthalene rings. In Table 1, geometric, electronic, and magnetic properties of five types of Si-[Vinylnaphthalene-TM2 ] ∞ systems are summarized. In order to estimate the relative stabilities of these silicon supported SWNs, the metal atom binding energies (Eb) are calculated. The binding energy is defined as follows: Eb = {E(Si-[vinylnaphthalene-TM2]∞) − E(Si-[vinylnaphthalene]∞) − nE(TM)}/n, where E(Si-[vinylnaphthalene]∞) is the electronic energy of the silicon supported vinylnaphthalene molecular line without TM doping, E(TM) and n are the electronic energy of the isolated TM atom and the number of TM atoms in the supercell, and the E[Si-[vinylnaphthaleneTM2]∞ is the electronic energy of the metal atom incorporated molecular wires. From Table 1, the early transition metal elements (TM = Sc and Ti) and V have remarkably larger binding energies than the middle transition metal elements (TM = Cr and Mn). The calculated binding energies of TM atoms in different kinds of SWNs are −3.611 eV, − 3.573 eV, − 3.860 eV, − 2.180 eV, and −1.856 eV for TM = Sc, Ti, V, Cr, and Mn, respectively. This binding energy trend partially agrees with the coordination numbers of TM atoms. The early transition metal elements (TM = Sc and Ti) and V formed more metal-C bonds than the middle transition metal elements (TM = Cr and Mn) and hence show larger binding energies. The largest metal atom binding energy is found in the Si[vinylnaphthalene-V2]∞ system. This is most likely caused by the formation of V−V multipole bond. The distance of two neighboring V atoms is measured to be 2.40 Å, which is significantly shorter than a regular V−V single bond. The average length of a V−V bond was found to be 2.83 Å (according to the Cambridge Structure Database). This distance is also shorter than that in vanadium pentalene complex (CpV2 and PnV2, in which the V−V distance is 2.54

terminated Si(100) surface with a 12-layer slab model, as shown in Scheme 1. The Si atoms on both sides of slab are passivated with H atoms. The bottom five layers of Si atoms are fixed during geometrical optimization. In order to explore ferromagnetic (FM) and antiferromagnetic (AFM) configurations, we used an orthogonal supercell containing four metal atoms and two vinylnaphthalene units (see Figure 1c). The cell parameters (a, b, c) are 40.0, 15.45, and 7.72 Å, respectively, where molecular wire is grown along the c direction. The Brillouin-zone is sampled by gamma-centered Monkhorst− Pack method with 1 × 2 × 5 k-point mesh for geometric optimizations and by that of 1 × 1 × 40 k-point mesh for the static total energy calculations. The HSE06 energy band calculations include two steps. A self-consistent HSE06 calculation is first performed with the Brillouin zone being sampled using a 1 × 1 × 10 gamma-centered Monkhorst−Pack grid. Afterward, 20 k-points along the Γ to X direction are added for the second-step band structure calculation.

3. RESULTS AND DISCUSSION The optimized structures of [vinylnaphthalene-TM2]∞ wires on H−Si(100) with TM = Sc, Ti, V, Cr and Mn are displayed in Figure 1a. The five types of metal-atom decorated molecular wires have consistent lattice constant (7.72 Å along the molecular wire direction). For the gas phase [NpTM2]∞ complexes, each sandwiched metal atom is equivalently bonded to 12 C atoms in two neighboring naphthalene rings, resulting in a η6:η6-coordination. At present, owing to the fixed positions of vinylnaphthalene molecules, the naphthalene rings can only rotate their orientations to reach maximum bonding interactions with the TM atoms. This leads to reduced coordination to the TM atoms and hence much different electronic and magnetic properties in contrast with the gas phase [NpTM2]∞ SWNs. From Figure 1a, SWNs confined on silicon substrate containing smaller radius metal atoms (i.e., Mn etc.) show larger structural deformation of naphthalene rings than those consisting of larger radius metal atoms (i.e., Sc, etc.). Moreover, the TM atoms in these surface confined SWNs demonstrate different coordination modes. For Sc and Ti-] decorated SWNs, each Sc or Ti atom is bonded to 12 C atoms of the naphthalene ring (η6:η6-coordination). The naphthalene rings in these two SWNs are nearly vertical to the axial direction of the molecular wire and exhibit planar structures, similar to the geometric configuration of gas phase [NpSc2]∞ and [NpTi2]∞ wires.71 In the case of Si-[vinylnaphthalene-TM2]∞ (TM = V, Cr, and Mn) systems, the coordination number of metal atoms are reduced. The V atoms formed differential η6:η6- and η6:η3coordinations to the two sides of naphthalene rings. The Cr and Mn atoms formed η6:η5/η6:η4- and η6:η6/η6:η3-coordinaD

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Table 2. Local Magnetic Moment on TM Atoms (μB) and Electronic Ground State (GS) of Gas-Phase [NpTM2]∞ Systemsa TM

Sc

Ti

V

Cr

Mn

magnetic moment GS

0 NM metal

0.90 AFM semiconductor

0.67 AFM metal

0 NM semiconductor

1.02 FM metal

a Because the TM atoms have equivalent bonding in the [NpTM2]∞ systems, only one value of local magnetic moment is displayed. These results are predicted from the PBE calculations.

Figure 2. Isosurface contour of electron charge density difference for the Si-[vinylnaphthalene-TM2]∞ systems. The silicon substrate is not displayed. The yellow and blue areas denote the accumulation and depletion of electronic charge density. The isovalues for plotting is 0.075 e/Å3.

have similar electronic and magnetic properties in gas-phase or confined on the silicon substrate, while for SWNs containing V, Cr, and Mn atoms, the surface confined wires possess much different magnetic properties in contrast with their gas phase analogues. From Table 2, for both gas-phase and silicon supported Sc decorated molecular wires, they have the same NM ground states. For the Si-[vinylnaphthalene-Ti2]∞ system, the local magnetic moment on Ti atom is slightly decreased to 0.78 μB, but the AFM ground state is unchanged. For the V atom incorporated SWNs, the local magnetic moment on V atom has decreased from 0.67 μB to 0 μB upon confining on the silicon substrate, and the ground state of Si-[vinylnaphthaleneV2]∞ is NM metal in contrast with the AFM metal properties of [NpV2]∞. Noticeable difference is also seen in magnetic properties of SWNs containing Cr and Mn atoms. As shown in Table 2, the gas-phase [NpCr2]∞ and [NpMn2]∞ possess NM semiconducting and FM metal ground state, and the local magnetic moments on Cr and Mn atoms are 0 and 1.02 μB, respectively. When the SWNs are confined on the silicon substrate, the local magnetic moments on Cr and Mn atoms increased to ∼3.11 μB and ∼3.00 μB, and the SWNs possess FM metal and half metallic FM ground state, respectively. The fundamental reason for the significant increase of local magnetic moments on Cr and Mn atoms in the surface confined SWNs is ascribed to the reduction of the coordination number of TM atoms. The deformation charge densities (Δρ) of five Si-[vinylnaphthalene-TM2]∞ systems are calculated and displayed in Figure 2. The deformation charge density is defined as Δρ = ρ(Si-[vinylnaphthalene-TM2]∞) − ρ(Si-[vinylnaphthalene]∞) − ρ(TM). From Figure 2, five SWNs demonstrated different isosurface contours of deformation charge densities, suggesting different bonding natures between TM atoms and naphthalene rings. For Si-[vinylnaphthalene-TM2]∞ (TM = Sc, Ti) systems, the interactions between TM atoms and naphthalene rings are dominated by the TM-3dz2 and 2pz orbitals of naphthalene rings. Twelve TM-C bonds are formed for each TM atom. In the case of Si-[vinylnaphthalene-TM2]∞ (TM = V, Cr, and Mn) systems, the deformation charge densities are rather asymmetric, caused by the asymmetric coordination of TM atoms.

Å). The shortest V−V bond for any divanadium organometallic compound is that reported in (μ:η5,η6-Ind)2V2 complex (2.35 Å),80 which had been proposed to be a V−V triple bond. The currently discovered short V−V bond length and large binding energy may attribute to the effect of bridging ligands, where the metals can be forced into close proximity in order to maximize favorable metal−ligand interactions without formally populating metal−metal bonding orbitals.81 In order to assess the energy barrier of the metal atom insertion into the vinylnaphthalene wire anchored on silicon surface, the insertion energy of Sc atom is calculated because of the Sc atom has the largest atomic radius in five types of metal atoms. As shown in Figure S2 in the Supporting Information, we consider that three Sc atoms have been bonded with the ligand. The insertion of the fourth Sc atom should be the most difficult step because the neighboring sites are already occupied by the other three TM atoms and the vinylnaphthalene become less flexible. The energy profile of Sc atom insertion is calculated. As shown in Figure S2, the whole atom insertion process is an exothermic process. An energy barrier of 0.65 eV is found. We think this energy barrier is not too large and is entirely possible to be overcome in such an exothermic process. The electronic structure calculations using the GGA+U method indicate that all SWNs are metallic, but possess much different magnetic properties. For the Si-[vinylnaphthaleneTi2]∞, it has an AFM ground state. The energy difference between the FM and AFM states is 0.115 eV per Ti atom. The magnetic moments on Ti atoms favor FM coupling in the axial direction but AFM coupling between nearest neighboring Ti atoms. The Si-[vinylnaphthalene-Sc2]∞ and Si-[vinylnaphthalene-V2]∞ systems have the nonmagnetic ground state, and the Si-[vinylnaphthalene-Cr2]∞ and Si-[vinylnaphthalene-Mn2]∞ show FM ground state, respectively. In two FM SWNs, the local magnetic moments on the Cr and Mn atoms are about 3.1 and 3.0 μB, respectively. The energy differences between the FM and AFM state are −0.021 and −0.098 eV per metal atoms for Cr and Mn decorated SWNs, respectively. The electronic and magnetic properties of silicon-supported SWNs are further compared with the gas-phase [NpTM2]∞ nanowires. From Table 2, the Sc and Ti incorporated SWNs E

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The Journal of Physical Chemistry C The Bader population analysis was performed to explore the amount of charge transfer between TM atom and naphthalene rings. From Figure 2, it can be found that in all five systems, the charge transfer is from TM atoms to naphthalene rings. The amount of charge transfer as summarized in Table 1 is consistent with the tendency of binding energy for Si[vinylnaphthalene-TM2]∞ wires. That is, the larger the electron transfer between transition metal atom and naphthalene rings, the larger the metal atom binding energies. Also, the Bader population analysis indicated that the charge transfer happens between the silicon substrate and surface molecular wires. The amount of charge transfer from the silicon substrate to the [vinylnaphthalene-TM2]∞ wires is 1.74 e, 1.78 e, 1.84 e, 1.79 e, 1.82 e for TM = Sc, Ti, V, Cr, and Mn, respectively. For comparison, without the insertion of the TM atoms, the charge transfer from silicon substrate to vinylnaphthalene nanowire is about 2.09 e. These results indicate that the insertion of metal atom into the molecular wires slightly decreases the charge transfer between substrate and molecular wires. In Figure 3, the spin-charge density isosurfaces of Si[vinylnaphthalene-TM2]∞ are plotted. In the Si-[vinylnaph-

Figure 4. (a) The energy band structure of Si-[vinylnaphthaleneMn2]∞ from PBE+U calculations. (b) The PDOS of Si-[vinylnaphthalene-Mn2]∞ from PBE+U calculations. (c) The LDOS for 3d orbitals of two kind of Mn atoms in the Si-[vinylnaphthalene-Mn2]∞.

Figure 3. Plotted spin density isosurface of various Si-[vinylnaphthalene-TM2]∞ systems. The isovalues for plotting are 0.0004, 0.0181, and 0.0185 e/Å3 for Ti, Cr and Mn systems, respectively. Spin densities in different directions are distinguished by yellow and blue colors, respectively.

ferromagnetic Si-[vinylnaphthalene-Cr2]∞ and Si-[vinylnaphthalene-Mn2]∞ systems (see Figure S5). For both systems, the HSE06 energy band structures are qualitatively consistent with the PBE+U results. The HSE06 calculations predict that the Si[vinylnaphthalene-Mn2]∞ system is a quasi-half-metal with the spin up channel having a very narrow indirect gap of ∼0.03 eV and the spin down channel having a moderate gap of 0.42 eV. In order to gain more insights into the electronic structure properties of these silicon supported SWNs, we computed the spin-polarized projected density of state (PDOS) and local density of states (LDOS) for Si-[vinylnaphthalene-Mn2]∞ systems (as shown in Figure 4b,c). The PDOS and LDOS near to the Fermi level show that the ferromagnetism of the Mn decorated nanowires stems mainly from the 3d orbitals of Mn atoms. Due to the reduced coordination symmetry of Mn atoms, Mn-3d orbitals are fully splitted. We find that the DOS near to the Fermi level are mainly composed of the 3dyz, 3dz2 atomic orbitals of Mn1,3 and 3dyz, 3dxz atomic orbitals of Mn2,4 atoms. From the energy band structure analysis, the Si[vinylnaphthalene-Mn2]∞ is half-metallic with one band crossing the Fermi energy level in the spin-up channel and a direct gap of about 0.2 eV in the spin-down channel (see Figure 4a). The strong couplings between Mn and naphthalene rings are observed in the region below the Fermi level. Hence, the

thalene-Ti2]∞ system, the Ti atoms show FM coupling in the axial direction but AFM coupling between nearest-neighbors. This kind of magnetic coupling is equivalent to two parallel AFM coupled metal-benzene wires. In the Si-[vinylnaphthalene-Cr2]∞ and Si-[vinylnaphthalene-Mn2]∞ systems, TM atoms favor FM coupling. In addition, we find that the energy difference between FM and AFM states of Si-[vinylnaphthalene-Mn2]∞ system is about 0.035 eV per atom larger than that of the gas-phase [PnMn2]∞ SWN, which indicates that the Si[vinylnaphthalene-Mn2]∞ system can be more stable in FM state than the gas-phase wires. The energy band structures of five types of Si-[vinylnaphthalene-TM2]∞ SWNs are calculated using both PBE and PBE+U functional (Figure 4a, and Figures S3 and S4 in the Supporting Information). We find that the PBE and PBE+U calculations predicted similar energy band structures for Sc, Ti, V, and Mn decorated molecular wires. While for the Si[vinylnaphthalene-Cr2]∞ system PBE+U calculations predicted a FM metallic ground state, the PBE calculations give rise to half-metallic energy band structures. The HSE06 functional is further used to investigate the energy band structures of F

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ferromagnetism of the Si-[Vinylnaphthalene-Mn2]∞ system is ascribed to the double exchange (DE) mechanisms.82,83 From LDOS we could also found that the Mn1,3 atoms near the vinyl groups have a much greater contribution to spin polarizations than the Mn2,4; this is correlated with the fact that the Mn1,3 atoms possess a little bigger magnetic moment than the Mn2,4 atoms. Furthermore, the Mn decorated SWNs shows nearly 100% spin polarization at Fermi energy level, which is promising for application in spin transport devices.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08683. The distance between TM atoms (TM = V, Cr, and Mn) and C atoms in adjacent naphthalene rings and the energy band structure of Si-[vinylnaphthalene-TM2]∞ (TM = Sc, Ti, V, Cr, and Mn) SWNs on H−Si(100) surface by PBE functional and PBE+U methods, and HSE06 for Cr and Mn decorated SWNs (PDF)

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REFERENCES

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4. CONCLUSIONS Using the spin-polarized DFT, we have systematically investigated the structure and electronic structure properties of a new type of infinite one-dimensional sandwich vinylnaphthalene-TM molecular wires confined on H−Si(100)-(2 × 1) surface (TM = Sc, Ti, V, Cr and Mn). The results indicate that the building of these molecular wires on H−Si(100) surface are energetically favorable. The binding energies of TM to organic rings may be up to 3.86 eV. These surface confined single molecular wires possess novel electronic and magnetic properties. In contrast to the gas-phase [NpTM2]∞ nanowires, because of the fixed positions of naphthalene rings, the metal atoms cannot efficiently coordinate to the naphthalene units and thus show asymmetric bonding. These lead to the increased local magnetic moments of metal atoms. We predict that the Cr and Mn decorated nanowires are ferromagnetic, the local magnetic moments on Cr and Mn are 3.12 μB and 3.00 μB, respectively, which are much larger than those in gas-phase nanowires. The Sc and V decorated molecular wires are nonmagnetism and the Ti atoms in the molecular wires have antiferromagnetic coupling. The current theoretical study may provide a guide for future experimental synthesis of silicon supported transition metal incorporated single organometallic molecular wires, which has great potential for exploiting spin transport devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y. P.); Tel: 86-18173220488. 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 National Natural Science Foundation of China (21373176, 21422305, 21503182). G

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