The anchoring effect on the spin transport properties and I-V characteristics of pentacene molecular devices suspended between nickel electrodes.

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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 13191

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The anchoring effect on the spin transport properties and I–V characteristics of pentacene molecular devices suspended between nickel electrodes S. Caliskana and A. Laref*bc Spin-polarized transport properties are determined for pentacene sandwiched between Ni surface electrodes with various anchoring ligands. These calculations are carried out using spin density functional theory in tandem with a non-equilibrium Green’s function technique. The presence of a Se atom at the edge of the pentacene molecule significantly modifies the transport properties of the device because Se has a different electronegativity than S. Our theoretical results clearly show a larger current for spin-up electrons than for spin-down electrons in the molecular junction that is attached asymmetrically across the Se linker at one side of the Ni electrodes (in an APL magnetic orientation). Moreover, this molecular junction exhibits pronounced NDR as the bias voltage is increased from 0.8 to 1.0 V. However, this novel NDR behavior is only detected in this promising pentacene molecular device. The NDR in the current–voltage (I–V) curve results from the narrowness of the density of states for the molecular states. The feasibility of controlling the TMR is also predicted in these molecular device nanostructures. Spin-dependent transmission calculations show that the sign and strength of the current–bias voltage characteristics and the TMR could be tailored for the organic molecule devices.

Received 12th October 2013, Accepted 12th May 2014 DOI: 10.1039/c3cp54319f

These molecular junctions are joined symmetrically and asymmetrically between Ni metallic probes across the S and Se atoms (at the ends of the edges of the pentacene molecule). Our theoretical findings show that spin-valve phenomena can occur in these prototypical molecular junctions. The TMR and NDR results show that nanoscale junctions with spin valves could play a vital role in the production of

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novel functional molecular devices.

1. Introduction The rapidly evolving field of molecular spintronics1–16 and quantum transport properties of different small conjugated organic molecules,17–30 graphene12–18 and magnetic molecules21–46 has recently become immensely important for various technological applications. Molecular electronics are being specifically used to develop innovative devices based on molecular magnets. Organic spin electronics have emerged as exceptional potential candidates for high-density information storage and quantum computing that can be used in the next generation of molecular devices, which are characterized by intensified functionality and improved performance.6–8 The capability to tailor the electron spin in organic molecular materials represents a novel and

a

Department of Physics, Fatih University, 34500, Buyukcekmece, Istanbul, Turkey Department of Physics and Astronomy, College of Science, King Saud University, Riyadh 11451, Kingdom of Saudi Arabia c Department of Physics, National Taiwan University, Taipei 106, Taiwan. E-mail: [email protected]; Fax: +886 2 33665136; Tel: +886 2 33665156 b

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highly attractive direction for spin electronics, both from basic and technological perspectives. This capability derives from the indisputable benefits of harnessing weak spin–orbit and hyperfine interactions in organic molecules, which make it feasible to sustain spin-coherence over distances and times that are much longer than those in common metals or semiconductors. Thus, it is useful to learn how to tailor and enhance the control of electron transport that is determined by the spin degrees of freedom. Controlling electron transport adds an alternative dimension to the emerging area of molecular scale electronics, and this valuable expertise can be used for potential applications in spin-based molecular electronics (spintronics). These applications include hot electron coherent spin transfer across the spin-injection in p-conjugated molecules, molecular bridges,22–47 and organic tunneling junctions.32–46 However, previous studies have shown conclusively that spin-polarized currents can be injected into organic materials with a fairly high efficiency.38–46 A few seminal theoretical studies on electron transport through molecular magnets21–46 have appeared concurrently. Challenges are typically encountered

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in trying to control the magnetic response of these devices. Extreme temperatures, huge resistances,48–52 short drives in device fabrication,53–60 and the effects of bias on the magnetotransport properties34–40 present severe shortcomings for future applications. These disruptive effects are caused by an insufficient expertise in manipulating the binding properties between the organic materials and the magnetic electrodes and their responses to external electric and magnetic fields. Thus, advances in discerning and engineering these components could produce a useful strategy for chemically controlling the desired magnetic response. Thus, the investigation of spin transport is to develop an ultimate device miniaturization scheme using organic molecules. Molecular electronics are receiving considerable attention because of timely advances in microscale fabrication technology. A wide variety of researchers have expressed substantial interest in these organic molecules. Organic pentacene compounds are used in technological applications for the development and use of carbon-based materials in microelectronics. In recent decades, pentacene single crystals have been increasingly used in basic research, particularly for investigating its intrinsic electronic properties.4–14 This molecule has been used as an active semiconducting material in electronic devices, such as organic field effect transistors (OFET), because of its exceptional high chargecarrier mobility.2–10,47 These devices are expected to significantly promote the arena of micro- and nano-electronics. Thus far, distinctive and promising organic electronic devices have been manufactured using pentacene crystals or thin films. In this context, many experimental studies have been conducted to investigate electron transport in pentacene.2–19 This system has also been used as a medium for electron spin-polarized transport in composite junctions24–27 with applications to spin-injection,40–44 while the solid phase of pentacene is non-magnetic.18,19 Moreover, if all of the elements consisted of a molecular bridge, such as a molecule and electrodes that do not possess intrinsic charge density localization and magnetism, their insignificant coupling would facilitate the localization of spin density in the molecule. In regular tunnel junctions, the spin polarization of the device is conferred through the electronic states at the interface within the magnetic electrodes and the insulator.40–50 These molecular objects are sensitive to the contact geometry and the molecular end-groups and features that create enormous variation in the magneto-resistance. To this end, we investigate the spin transport properties of pentacene molecular spin valves which originate from the orbital characteristics of the states near the Fermi level. A major feature of spintronics is that different conductances have been observed for electrons with opposite spin orientations in spin valves.21,44–46 Electron tunneling occurs from one magnetic electrode toward another across a barrier. The magnitude of the tunneling current depends on the relative magnetic direction of the two electrodes. Here, switching between parallel (PL) and antiparallel (APL) terminal Ni spin configurations occurs in a magnet under an external magnetic field with uniaxial anisotropy. A collection of stable low- and high-resistance states can be observed, and this phenomenon is known as the tunneling

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magneto-resistance (TMR) effect.42–45 Magnetic random access memories and modern hard-drive read heads are based on this phenomenon. Exhaustive experimental and theoretical studies45–52 have been conducted to obtain a significant TMR ratio to improve the structural quality of the interfaces in magnetic tunnel junctions. From first principles spin transport calculations, the conductance depends on the geometry and orbital characteristics of the atoms at the junction.36–46 To study TMR in both metallic and tunneling regimes, we analyze pentacene molecule sandwiched between Ni electrodes. Standard techniques can be used to investigate magneto-transport in a spin valve. The organic molecule exhibits tunneling behavior with the spin polarization of the current, which depends strongly on the contact surface states at the interface between the Ni electrodes and the pentacene molecule linked across sulfur (selenium) anchoring groups. In this study, we investigate the transport properties of Ni–pentacene–Ni junctions by varying the effect of the anchoring group in the molecular devices. The geometric, electronic and magnetic properties of these molecular devices can be effectively modulated by varying the ligands. Research on the transport properties of these systems is still in its infancy. To address the aforementioned issues and thereby improve our knowledge of spin-dependent transport properties in these molecular leads, we use ab initio modeling based on spin-polarized density functional theory (DFT) in conjunction with a nonequilibrium Green’s function (NEGF) approach. The firstprinciples electronic transport schemes are designed to capture the electronic and magnetic structures at interfaces (i.e., the pentacene molecule with Ni surface contacts). Our aim is to address the following theoretical issues. (i) What is the effect of Ni metallic electrodes on the transmission, I–V characteristics and TMR of the molecular nano-contacts? (ii) How do organic molecules, such as pentacene, and the electronic structure of nano-contacts affect transport properties? In this study, we account for the magnetic semi-infinite fcc (111) Ni surface electrodes joined by the nano-contacts of the pentacene molecule. The thiol groups connecting the organic molecule and the electrodes are modeled as S or Se atoms to facilitate substantial spin transport calculations. We predict a small magnetic moment for the C atoms of the pentacene molecule because of the interaction of pentacene with the Ni ferromagnetic (FM) and Ni anti-ferromagnetic (AFM) contacts. Then, the Ni magnetic moment in the scattering region is decreased by incorporating the Ni surface in both the PL and APL states. Thus, the Ni atoms positioned on the surface or axis of the pentacene contacts induce an insignificant magnetic moment on the carbon atoms. The orbital overlap is modified by a series of anchoring ligands of S and Se atoms joined at the edge of the pentacene molecule by two Ni magnetic contacts. A specific contact geometry can change the chemical bonding in the pentacene rings, thereby critically affecting the local electronic structure. This can significantly modify the electronic states of the devices. In particular, the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are affected by the various occupancies of the 3d Ni surface atoms and the 2p C atoms. In this study, the potential


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of using magnetic Ni junctions as spin valves is demonstrated by analyzing the spin-dependent transmission spectra under a zero bias for the pentacene molecule with different anchors coupled to these junctions. We also compute the I–V characteristics of the pentacene molecule in contact with the Ni electrodes and the two magnetic contacts, which are used as current and voltage probes. Here, we demonstrate theoretically that the appropriate selection of organic molecules and anchoring groups can be used to engineer the TMR of the organic spin valves. We find that the two anchoring groups have clearly different effects on the transport properties of the organic molecule. Under a bias voltage, the I–V curves of the Ni–pentacene–Ni junctions coupled to a S atom exhibit metal-like characteristics. Conversely, one Se atom linker significantly alters the transport properties of the Ni–pentacene–Ni junctions. Our results show that the current–voltage features are altered depending on whether the thiol groups consist of S or Se atoms and the chemical bonds between the Ni contact and the pentacene molecule. Hence, a novel negative differential resistance (NDR) is predicted for the Ni–pentacene–Ni junctions (in the APL geometry), where the Se atom is coupled asymmetrically to one end of the edge of the pentacene molecule. This nonlinear behavior is observed under high bias, implying that this molecular junction could serve as a potential candidate for NDR molecular devices. Our results show that the magnitude of the TMR depends on the details of the molecular junctions that can occur in the metallic and tunneling regimes. The magnitude of the TMR is found to be a little large for a device attached asymmetrically via a Se linker at one side of the edge of the pentacene molecule, making this system a promising candidate for realistic applications in molecular spintronics. Thus, anchoring groups deserve ample consideration for molecular devices.

2. Computational methods The spin-resolved transport properties of the molecular junctions are determined by implementing a fully spin-polarized DFT with a state-of-the-art NEGF scheme in an ATK package.61 The electronic structure properties are then computed using DFT, where the wave functions are represented using a linear combination of atomic orbitals (LCAO) as a basis set. The behavior of the core electrons is described by the norm-conserving Troullier– Martins nonlocal pseudopotential.62 We use the generalized

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gradient approximation (GGA) in the Perdew–Burke–Ernzerhof (PBE) form62 in the simulations to describe the exchange and correlation energy. In the ATK-based simulations, a double zeta (z) plus polarization (DZP) basis set is employed to represent the electronic structure of the valence electrons of the C, H, S, and Se atoms and the Ni electrodes. We apply periodic boundary conditions in the direction orthogonal to transport with a uniform 3 3 100 k-point grid. Here, the potentials and charge density are obtained using a real-space sampling with a mesh cutoff energy of 150 Ry, which is sufficient to reach a total energy convergence with less than 0.04 meV per unit cell in the self-consistent loop. All of the atoms of the free pentacene molecules are fully relaxed before the transport calculations are performed. The molecular size for the relaxed geometry is 8 Å, which is close to the experimentally measured value.47 The overall molecular structural relaxation is determined from the standard conjugate gradient with a force tolerance of 0.05 eV Å1. A well-relaxed interfacial geometry is crucial for obtaining reliable electronic structure results. We optimize the molecular devices to obtain the equilibrium distances between the central molecules and the electrodes, where the S–Ni and Se–Ni separations are approximately 1.25 Å and 1.66 Å, respectively, and the C–S and C–Se bond lengths are 1.1 Å and 1.53 Å, respectively. We adopt the following simulation model: the central molecule is composed of five acene rings suspended between two Ni contacts in a fcc [111] orientation. The pentacene molecule is sandwiched between two semi-infinite Ni surfaces with periodicity in the x–y plane. Thus, the transport junction is constructed by coupling the terminated atoms of the molecules attached symmetrically and asymmetrically via the S and Se atoms at the hollow sites above the Ni(111) surface. We considered three different binding geometries using the thiol groups to bind the pentacene to the Ni surface. Fig. 1 shows these prototypical model devices that are associated with several anchoring geometries for pentacene and the Ni electrodes (i.e., the left Ni electrode is oriented in the up spin direction, and the right Ni electrode is oriented in the spin up/down direction). Panel (a) shows the symmetric configuration in which only the two ends of the upper edges of the pentacene molecule are connected to the Ni surface across the S atom. Fig. 1(b) and (c) show the asymmetric configuration, where the lower edge on the right side and the upper edge on the left side of the pentacene molecule are connected to the electrodes via the S/Se and S atoms. Here, the organic molecule, namely, pentacene,

Fig. 1 Computational models for the proposed molecular junctions. The pentacene is sandwiched between two Ni electrodes. The system is presented by a schematic structure of the various anchoring geometry of pentacene attached to Ni via a thiol group: (a) symmetrically S linkers; (b) asymmetrically S linkers; (c) asymmetric configuration, where the lower edge on the right side and the upper edge on the left side are connected to the electrodes via Se and S atoms. In the molecule structure, the large dark grey spheres are for C, the light yellow spheres are for S, the dark yellow spheres are for Se, the green spheres are for Ni, and the small grey ones are for H. The Ni electrodes are in PL and APL orientations of the magnetic moments.



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is composed of 5 linearly fused benzene rings, which are denoted as 5-acenes. Thus, these molecules can be regarded as short hydrogen-terminated graphene nano-ribbons. These molecules form bulk crystals with a herringbone structure, where the face of one molecule is located near the edge of another molecule. The sequential analysis of the molecular contact, which consists of two-terminal molecular electronic devices, involves coupling the 5-acenes molecule to the Ni electrodes. We adopt PL and APL magnetic configurations for the two semi-infinite Ni(111) bulk contacts that are connected by the scattering region. For the left and right electrodes, the unit cells in the x and y directions are selected to prevent the molecules from interacting with their mirror images. The extended molecule consists of the bridging pentacene molecule together with a sulfur (selenium) atom attached symmetrically and anti-symmetrically to both the left and right electrodes and four Ni atoms from the first surface layer of each electrode with the respective geometry. The NEGF-based formalism is employed to compute the electron transport properties in a low bias regime for both the PL and APL spin directions of the systems. For this molecular junction, the metal–molecule–metal interface is expected to play a crucial role in simulating the I–V characteristics. The total current in this system is determined by summing both the spin-up (Im) and spin-down (Ik) electrons. In our calculations, the spin-resolved I–V curves of the molecular junctions are estimated as follows: Is ðVÞ ¼

ð e Ts ðE; VÞ½ f ðE mL Þ f ðE mR ÞdE; h

where Ts(E,V) is the transmission coefficient for a spin-up/ spin-down electron (s = m/k) and is denoted as Ts(E,V) = Tr[GLGsGRG+s]. Here, Gs(G+s) represents the spin-dependent retarded (advanced) Green’s function of the central scattering region, GL/R is the coupling matrix between the scattering region and the left (right) electrode, f (E mL(R)) is the Fermi function, and mL/R denotes the chemical potential of the left/ right electrode, respectively. The system is divided into three sections, the left electrode (L), the molecular region (M), and the right electrode (R). The molecular region corresponds to a portion of the physical electrodes, i.e., the extended molecule. The electrostatic potential energy reference is chosen at the middle of the bimetallic junction. The electrochemical potential of the two electrodes depends on the applied bias voltage V as mL = EF eV/2 and mR = EF + eV/2, where EF (which is set to zero) is the Fermi energy. The spin-polarized conductance is computed between the two atomic planes positioned in the perfect leads. We estimate the TMR from the spin-polarized conductance results as follows: TMR = (GPL GAPL)/GAPL. Here, GPL and GAPL are the conductances for the PL and APL magnetic configurations of the Ni contacts attached to the molecular junctions across the thiol groups in both the symmetric


and asymmetric cases, respectively. The spin polarization of the current for each bias voltage is evaluated as follows: P = (Im Ik)/(Im + Ik), where Im and Ik are the currents from the spin-up and spindown electrons, respectively, at a specific applied voltage.

3. Results and discussion We model a spin valve by sandwiching the pentacene molecule between two semi-infinite nickel contacts with magnetization vectors that are either PL or APL to each other via a thiol group (i.e., S and Se atoms). We analyze the effect of the different linking geometries on the electronic structure and transport properties of the Ni–pentacene–Ni junctions. Before analyzing the results given below, we first discuss the spectra for the total density of states (TDOS) of the devices and the partial density of states (PDOS) projected onto the 3d Ni atoms and the 2p C atoms for all of the considered configurations. We identify these states by following the evolution of the orbital-resolved DOS for a pentacene molecule attached to nickel as a function of the Ni–S or Ni–Se separations. In particular, we investigate the dependence of the C 2p DOS, which are the relevant orbitals for the nickel surface leads. Fig. 2–4 illustrate the electronic structure results for the Ni/pentacene/Ni interface. In the 3 to 3 eV energy range, the corresponding electronic states are traced back to the Ni 3d and C 2p orbitals. Fig. 2(a–d) display that the apparent features of the DOS profiles are similar to those of the fcc Ni bulk, indicating the metallic character of these systems. In Fig. 3(a–d), the spinpolarized PDOS exhibits the contribution of the Ni 3d atoms (which form the layers of Ni leads near the pentacene molecule, joined symmetrically and asymmetrically across the S and Se atoms). A high spin-polarized DOS is observed near the Fermi energy in the Ni–pentacene–Ni devices. Accordingly, the spin-up PDOS for the Ni contacts in the PL and APL alignments are governed by a sharp peak produced by the Ni 3d states and are centered roughly at 0.5 and 0.6 eV, respectively. The Ni 3d states in the spin-down direction are formed from sharp resonances located at 0.2 eV around the Fermi level (Fig. 3(a–d)). For the Ni–pentacene–Ni junction aligned in the PL configuration, the majority spin levels are positioned on the upper edge of the molecule and are coupled to the electrodes symmetrically via the linker S thiol group. Conversely, the minority spin levels localized on the lower edge can be accessed from the electrodes only through tunneling in the vacuum because there is no direct binding atom. Next, we consider the C 2p sites affected by S–C or Se–C and C–C bonding. These states belong to the s bonding orbitals with a p symmetry. Fig. 4(a–d) display that at lower energies, changing the thiol group results in sharp C-p HOMO states of the pentacene molecule that are steadily spin-split. The energy levels in the molecule widen substantially through a strong coupling between the molecule and the electrodes. However, we deduce that the transport is correlated with the HOMO states. It is clear that the C 2p PDOS exhibit half-metallic behavior (HM) for the majority spin density channel from 0.4 to 0.2 eV


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Fig. 2 Spin resolved density of states (DOS) of the devices. Positive DOS are for the majority spins and the negative are for the minority spins for the pentacene suspended between the Ni leads (PL and APL configurations) and the symmetric and asymmetric anchoring configurations via S and Se atoms. TDOS: (a) Ni–pentacene–Ni (PL), (b) Ni–pentacene–Ni (APL), (c) Ni–pentacene–asym-Se–Ni (PL) and (d) Ni–pentacene–asym-Se–Ni (APL).

and 0.2 to 0.2 eV in the Ni–pentacene–Ni junctions (in the PL and APL alignments, respectively) attached asymmetrically via the Se atom (at the end of one edge). Although there is significant coupling across the s bonding of the Se 4p and C 2p orbitals, the hybridization between the p bonding orbitals at the neighboring carbon sites is almost suppressed. This suppression is caused by strong Se–C bonding, which is typical for the pentacene molecule. The corresponding resonances clearly appear as sharp peaks in the C 2p PDOS (Fig. 4(a–d)). Note that the isolated pentacene molecule exhibits an insulating state, which is in stark contrast to our theoretical finding for the Ni–pentacene–Ni junctions linked to the Se atom (at the side of one edge of the pentacene molecule). The s bonding orbitals intercept the primary region of orbital overlap within the pentacene ring and with the adjacent Se atom. The C 2p DOS exhibits characteristic peaks near 2.5 eV. The s states are subject to strong bonding–antibonding splitting more than the p states that appear at lower energies because the s states mediate direct orbital overlap. Then, the hybridization between the p bonding orbitals at the sites of the C atoms is almost insignificant. Thus, the p-type coupling to the acene ring is reduced in the Ni–pentacene–Ni junctions, which is attributed to the strong Se–C bonding. This behavior can drive to rather spin-polarized DOS at energies near EF because of the p and p* orbitals of the C atoms that are affected by the shift of these


associated states. The reduced C–C coupling corresponds to a HM system, whereas the p bonding states are associated with the electrical conductance of an acene ring. The minority and majority PDOS (Fig. 4(a–d)) associated with the two edges of the molecule are slightly different: thus, the spin polarization for the transmission coefficient can be determined from Fig. 5. In all of the cases considered, the spinsplitting of the states occurs at the C sites of the pentacene molecule, originating from their higher coupling via the S and Se atoms to the Ni electrodes, as shown in Fig. 4(a–d). This result corresponds to a strong coupling for the upper edge state and a weak coupling, i.e., stronger localization, for the lower edge state, as anticipated from the bonding geometry. Moreover, quite significant electron coupling enhances the partial charge transfer from the Ni electrodes: the charge transfer is approximately 0.08 for all of the configurations. It is well recognized that the ground state of the isolated thiolated molecules is non-magnetic for pentacene. Thus, the coupling to the electrodes is indicative of the final magnetic state for the considered configurations. In the coupling to both the symmetric and asymmetric geometries (Fig. 4(a–c)), the systems acquire fractional magnetic moments. The coupling to the electrodes usually enlarges the pentacene levels, upon which charge is transferred from the Ni to the molecule HOMO. Although the majority HOMO is completely occupied, the minority HOMO may only be partially filled,


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Fig. 3 Spin resolved projected density of states (PDOS) projected over the d states of Ni atoms of the leads (for both PL and APL spin orientations). Positive PDOS are for the majority spins and the negative are for the minority spins. EF is the position of the Fermi level of the nickel leads. PDOS: (a) Ni–pentacene–Ni (PL), (b) Ni–pentacene–Ni (APL), (c) Ni–pentacene–asym-Se–Ni (PL) and (d) Ni–pentacene–asym-Se–Ni (APL).

causing the total spin polarization to develop a magnetic moment. Thus, a spin-polarized current in pentacene and the effects of the magnetic leads can be used to monitor asymmetric bonding to a metallic surface. When the linker S thiol is joined symmetrically to the edges of the pentacene molecule (see Fig. 4(a) and (b)), the majority and minority states will develop different coupling strengths to the electrodes. The two edges of the molecule are prevailed by the Ni coupling: the average magnetic moment per atom is approximately 0.47 mB (where mB is the Bohr magnetron), and the average magnetic moment per Ni atom is approximately 0.70 mB; these values are reasonably close to the previous literature values.44–46 The coupling of the molecule to the


electrodes moves the states slightly to the Fermi level, which induces a nonzero magnetic moment in the C atom. Thus, the d orbitals may play a major role in the transport properties. Note that the experimentally measured gap for pentacene in the gas phase is approximately 5.54 eV.47 This gap can be made to vanish by coupling the pentacene molecule to the Ni metallic leads. The resulting total magnetic moment of the molecule is a common characteristic of such an interface and is entirely dependent on the various electronic couplings of the two spin channels of the molecule with the Ni atoms. The magnetic moment can potentially be tailored across the bias or gate voltage, which presents intriguing prospects for device applications.


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Fig. 4 Projected density of states (PDOS) projected over the p states of C atoms of pentacene molecular junctions. In all panels positive values correspond to m spins and negative values to k spins PDOS: (a) Ni–pentacene–Ni (PL), (b) Ni–pentacene–Ni (APL), (c) Ni–pentacene–asym-Se–Ni (PL) and (d) Ni–pentacene–asym-Se–Ni (APL).

It is well recognized that the HOMO and LUMO analyses are salient for understanding the electronic properties because electron transitions appear naturally between these two orbitals. To elucidate the origin of the spin transport properties, we compute the electronic spatial distribution in the framework of the HOMO and LUMO for two spin channels at a zero bias voltage. Note that the spatial distribution of electrons for the molecular junction can be used to determine the source of the spin transport characteristics. The HOMO and LUMO eigenstates can be determined by renormalization of molecular orbitals through the molecule–electrode interaction. Fig. 5 and 6 depict the isosurfaces spatial distribution plots for pentacene involving four geometries. Several HOMO, HOMO 1,


LUMO and LUMO + 1 orbitals are obtained near EF, where the spatial distribution is spread over the orbitals of the molecule. Note that the typical PL and APL coupling between the spins of the two edge states can be determined from these figures. The typical behavior of the molecule in the gas phase is somewhat preserved during the construction of the molecular junction. In some situations, the coupling to the electrodes breaks the axial symmetry of the molecule, producing a magnetic moment. Fig. 5 and 6 illustrate that the HOMO and LUMO are delocalized across the entire scattering region, which results from the strong coupling between the electrodes and the scattering region. The HOMO of all of the configurations is almost thoroughly localized at the central ring of the pentacene molecule because of


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Fig. 5 Spin-resolved spatial distribution plots for the relevant molecular states of the HOMO and HOMO 1 of Ni–pentacene–Ni linked to S and Se atoms. The various panels are, respectively, for pentacene majority spins and minority spins. Positive isosurfaces are represented in light grey (red) and negative ones in dark grey (blue).

substantial electron backscattering. The chemical bonds composed from the HOMO orbitals are designated by s bonds (single bonds) and are localized between the bonding C atoms. The orbital pz remains unchanged, and its direction is perpendicular to the plane of the sp2 orbitals and that of the C atoms. The pz orbitals of the adjacent atoms overlap, producing another bond, the so-called p bond (a double bond), and delocalizing the density of the electrons on the upper and lower planes of the molecule. They are denoted as the nodal plane for the p electron density. This p bonding is significantly weaker than the s bonds that make up the molecular backbone. Therefore, the lowest electronic distributions of the LUMO of the conjugated molecules at the p–p* transitions are localized at the edge of the molecule (see Fig. 5 and 6). The linearly fused benzene rings exhibit remarkable electronic properties because of the conjugated p-electron system. These electronic properties affect the molecular conductivity of pentacene. It is significant that the majority spins HOMO for pentacene are located on the upper edge of the molecule, whereas the minority spins are positioned at the lower edge


of the molecule. The HOMO and LUMO spatial distributions for the spin-up and spin-down carriers exhibit peculiar patterns with approximate double bonds at the apexes and C–C bonds with alternating lengths that spread from the apex to the middle of the edges of the molecules. The degree of bond interchange diminishes in moving towards the center of each edge and enhances the electron delocalization in the chain. Note that the isosurface of the HOMO states has a negligible amplitude around the S or Se atoms of the thiol group and has a substantial density on the middle carbon backbone. We therefore expect the pentacene molecular junctions to develop tunneling and metal-like spin valves. The electrons of the LUMO are depleted between the central C atoms, primarily because of resonant tunneling into the anti-bonding states or the p bonding orbitals. The electron distribution can be recovered in the central C bonds when the resonant-tunneling condition does not occur. The spatial distribution of the HOMO 1 and LUMO + 1 orbitals is confined at the right interface between the molecule and the electrode over the acene ring or at the double C bond. The localization of the electronic distribution is more


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Fig. 6 Spin-resolved spatial distribution plots for the relevant molecular states of the LUMO and LUMO + 1 of Ni–pentacene–Ni linked to S and Se atoms. The various panels are, respectively, for pentacene majority spins and minority spins. Positive isosurfaces are represented in light grey (red) and negative ones in dark grey (blue).

noticeable along the upper and lower chains. Fig. 5 and 6 display that these orbitals are delocalized along the molecular backbone and are orthogonal to the molecular plane. The electrons in the occupying p-orbitals are less tightly bound than the electrons in the s-orbitals. For all of the geometries considered, the HOMO 1 for the majority and minority spins exhibits electron localization along the edge of the molecule perpendicular to the Ni surface. Nevertheless, in all of these configurations, the LUMO isosurfaces for the majority and minority spins are concentrated along the z axis perpendicular to the chain. The LUMO + 1 isosurfaces related to the electron distribution of the minority and majority spins are more localized on the rings parallel to the Ni surface. Thus, the edge of the molecules attached asymmetrically across the S and Se atoms exhibits electron delocalization along the central ring. The p* states are closer to EF than the p states, in accordance with a transfer of approximately 0.08e from the electrodes to the molecule.


It is known that the primary contribution to the transport arises from the frontier molecular orbitals of the HOMO and LUMO attached to the Ni electrodes via the thiol group. The spatial distribution extends to the unoccupied orbitals because the delocalization is significant in electron transport. Nevertheless, the different spatial extension of the C-p and S/Se-p orbitals and the significant increase in localization of the Ni-d states leads us to expect that the tunneling regime is dominated primarily by tunneling electrons with sp symmetry. However, the electronic states with d symmetry have the greatest contribution in the contact limit. The LUMO exhibits charge depletion between the central C atoms, primarily because of resonant tunneling into the anti-bonding states or the p bonding orbitals. All of the charge transferred from the Ni atoms to the molecule is taken from the Ni and directly transferred to the pentacene molecule when a small charge is extracted from or joined to the Ni bonded to the S and Se. Thus, the electron can


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tunnel through the pentacene molecule. Therefore, the conductibility of pentacene molecular devices should be enhanced by substitution with a thiol group atom. To gain more insight into the relationship between the electronic structure and the transport properties of the molecular junctions, we consider the energy-dependent zero-bias transmission coefficient through the Ni electrodes. The transmission functions at the Fermi level for the four devices are computed under a zero bias. Fig. 7(a–d) display the total spin zero-bias transmission coefficient, T(E) = SsTs(E), as a function of energy for all four configurations of the junction (where s denotes the spin). The effect of the electrodes on the transmission can be observed by comparing the results of the Ni–pentacene–Ni in the PL and APL junctions attached symmetrically and asymmetrically across the S and Se atoms to the electrodes. Overall, there is good agreement between the peaks in the transmission spectra and those in the DOS. The electron transmission is somehow different in the two spin directions. In the Ni–pentacene–Ni (in the PL configuration) and Ni–pentacene–Ni (in the APL configuration) joined via the S atoms to the electrodes, T(E) decreases from 0.25 to 0.1 eV and from 0.2 to 0.2 eV, respectively. This decrease can be attributed to the low density around EF (see Fig. 7(a) and (b)). The distinctive features of the Ni–pentacene–Ni transmission spectra correspond to the appearance of three resonance peaks,

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which are close to 0.5 and 1.8 eV above EF and at 0.6 eV below EF. Fig. 7(c) and (d) depict the transmission spectra of the Ni–pentacene–Ni junctions in the PL and APL configurations, showing the effect of the coupling of the Se atomic anchor on these systems. In these two cases, T(E) decreases at approximately 0.3 and 0.2 eV, indicating high spin-polarized behavior. Further examination of Fig. 7(c) and (d) reveals that in the tunneling regime with energy values higher than 0.1 eV, the transmission coefficient in the APL configuration is larger than that in the PL configuration. However, at energies lower than 0.1 eV, the electron transmission behavior is reversed. At EF, the transmission coefficient in the PL configuration is larger than that in the APL configuration (which is attached via the Se anchor at one side of the edge of the molecule). The transmission spectrum of pentacene coupled to Ni(111) surfaces aligned in APL configuration is dominated by three peaks far from EF: two peaks at 0.6 and 1.7 eV above EF and one peak at 0.7 eV below EF. Hence, the transmission spectrum in this system is ‘robust’ and possesses the following characteristics: (1) very broad resonances that diminish the effect of current-induced fluctuations at the interface and (2) a large transmission probability in the vicinity of EF that can be used to build a good conductor from a molecular junction. A significant peak for the spin up carriers at 0.2 eV occurs in the PL configuration because of the interaction between the 3d states

Fig. 7 The zero-bias spin-resolved transmission coefficient in the contact regime for the four-configurations sandwiched between two Ni terminal leads. The transmission curves of the spin-up and spin-down electrons are labeled by the black and red lines, respectively. T(E): (a) Ni–pentacene–Ni (PL), (b) Ni–pentacene–Ni (APL), (c) Ni–pentacene–asym-Se–Ni (PL) and (d) Ni–pentacene–asym-Se–Ni (APL).



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of the Ni contacts and the molecule (see Fig. 7(c) and (d)). The transmission coefficients for both configurations increase linearly above EF. The most striking feature of the spectrum is the radical change in the position of various resonant peaks upon changing the thiol group atom. Without showing the detailed structure of the PDOS near EF, we mention that the high density of the d states near EF for various symmetries is always related to the resonance-like states (see Fig. 4, for example, which shows the C 2p atom when the system is linked via the Se atom at one end of the edge of the molecule). In general, the orbitals around the Fermi level responsible for the transmission have Ni-d characteristics that mix with those of the C orbitals via the S/Se p states. When some of these resonances approach the Fermi level, the transmission of the corresponding channel reaches its highest peak. There is a clear correlation between the partial Ni 3d PDOS at EF and the transmission probabilities. Consequently, the calculated difference in the transmission of the two spins is primarily caused by the Ni leads (see Fig. 3). In particular, the state located near the Fermi level (Fig. 4) will produce a high transmission of the corresponding channel. The increase in the PDOS for the Ni–pentacene–Ni junction linked via S atoms is accompanied by a reduction in the transmission. That is, the hybridization between the C 2p states and the Ni 3d resonance (Fig. 5–7) causes the wave function to become more localized at the Ni atom. The transport features for the minority spins are dominated by the p orbitals of the C atoms and some of the contributions from the Ni d orbitals on the surface, which produce several resonant peaks. Additionally, a smaller number of resonant peaks for the majority spins appear between 1 and 1 eV for the PL and APL systems. Comparing the energies of the peaks in the transmission spectrum to the eigenvalues of the molecular orbitals shows that the primary contributions to the transmission peaks near EF are from the HOMO 1, HOMO, LUMO, and LUMO + 1. Note that the transmission peaks correspond to the molecular orbitals and that this phenomenon does not appear in the isolated molecules. In the contact geometries, the peaks are associated with the so-called metal-induced gap state, which is a mixture of the metallic surface state and the extended molecular state. The band lineup in the extended molecule is sensitive to the electronic structure of the molecule and the magnetic metallic electrodes. However, using metallic electrodes will modify the Fermi levels, the band lineup, the expected conductance and the I–V characteristics. Note that the transmission probability is defined by the spectrum of the molecular bridge and influenced (broadened and renormalized) by the coupling to the electrodes. For higher or lower energies, the induced resonances are associated with ‘‘metal-induced gap states’’ that originate from the hybridization of the magnetic Ni metal and the interfacial thiol group. Note that an electron can tunnel through the tail of the extended state or tunnel through the localized state. In pentacene, electrons can travel across the molecule through either localized or extended states: thus, the transmission is energy sensitive, and the transmission spectra exhibit sharp peaks. The relative contribution and number of


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the resonant peaks of the two spin species is a simple consequence of the position of the Fermi level and the magnitude of the spin splitting. Note that changing the spin orientation of the Ni metallic electrodes produces more frequent shifts in the position of the Fermi energy that are accompanied by the C p and Ni d states for the majority and minority spins. The current in the molecular junctions depends on several factors, including the nature of the molecular bridge and the contacts, the strength of the electrode–molecule interaction, and the position of the Fermi energy level in the metal–molecule–metal junction across the thiol group. To investigate the structural features that affect the molecular conductance for the spin up electron (Gm), the spin down electron (Gk), and the TMR, we study the effect of the thiol group (i.e., the S or Se atoms) on pentacene molecular junctions with Ni surfaces. The two different anchoring groups (S and Se) that terminate the pentacene molecule can produce different magneto-transport properties and a TMR that enhances the magnetoresistance. One interesting aspect is that the TMR can be tailored to the anchoring group. Both spin channels promote the conductance that produces only moderate tunneling magneto-resistance values. The spin-polarized conductances are calculated at a zero bias voltage. The amplitude of the majority T(E) is nearly the same in the two configurations. In the APL configuration, the magnitude of the minority spin is larger than that in the PL configuration. Thus, the minority spins supply most of the transmission in the APL configuration because of the significance of the Ni d and C p states in producing electron transport. In the APL configuration, the total transmission near the Fermi level is slightly smaller than that in the PL configuration, which results in a positive TMR ratio. For the Ni–pentacene–Ni junctions connected symmetrically via S atoms (i.e., the left and right Ni contacts are aligned in the PL configuration), Gm = 8.94 107 Siemens, Gk = 2.35 106 Siemens, and Gtot = 3.25 106 Siemens. However, Gm = 1.46 106 Siemens, Gk = 1.31 106 Siemens, and Gtot = 2.78 106 Siemens for the Ni–pentacene–Ni junctions connected asymmetrically across the S atoms (i.e., the left and right Ni contacts are aligned in the APL configuration). For the atomic contact with a PL geometry, the minority spin channel induces a sum of fractional contributions from many modes because of the p and d states of the C and Ni atoms that are always present near the Fermi level. The PL and APL conductances for the spin-down carriers are most likely of almost the same order of magnitude. Note that the conductance is not simply controlled by the DOS at the Fermi energy but that the precise coupling between the orbitals of the molecular junctions with metallic electrodes plays a crucial role in molecular spintronic devices. The calculated TMR is 16.90%. It weakens the electron scattering, which then further reduces the TMR. However, for the Ni–pentacene– Ni junctions (in PL configuration) coupled asymmetrically via Se atoms at one side of the edge of the molecule, Gm = 4.83 106 Siemens, Gk = 1.48 105 Siemens, and Gtot = 1.96 105 Siemens. Thus, only one spin direction for the conducting electrons transmits in the PL state. This inference is supported by the spin-up conductance, which is smaller than that of the


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spin-down conductance. Thus, the molecular junction can serve as a molecular spin valve. A basic TMR device is characterized by a tunneling barrier that separates two magnetic metal layers and this TMR contributes to the device leads. The tunnel conductance is related to the relative direction of the magnetic moments and tends to decrease if the directions are reversed, inducing a spin valve effect.22,23 The resulting conductances for the Ni–pentacene–Ni junctions (in APL configuration) attached asymmetrically across the Se anchoring group at one end of the edge of the molecule are as follows: Gm = 6.37 106 Siemens, Gk = 8.71 106 Siemens, and Gtot = 1.51 105 Siemens. We predict that this system should exhibit a quite larger conductance than a system in which the molecules are joined via S atoms (by almost a factor of 6). The increase in the conductance can be attributed to the charge transfer between the molecule and the electrodes that bring the HOMO level near to the Fermi level. So-called tunneling behavior is observed in the intermediate coupling regime. The TMR obtained using the relaxation of the scattering region is approximately 29.83%. Thus, the molecule is conducting for both PL and APL alignments of the electrodes linked to a S atom. This metallic behavior prevents the molecule from charging once a bias is applied. The zero-bias TMR is changed from 16.90% to 29.83% when the anchoring group is changed from S to Se. The aforementioned results show that the relaxation of the scattering region reduces the Ni–S bond length relative to the Ni–Se bond length and perturbs the fcc stacking layers within the scattering region, thereby decreasing the conductance. Replacing the thiol group S atom by a Se atom in the contact area can also augment the TMR of the device. Spin-polarized quantum transport through a Ni–benzenedithiol (BDT)–Ni molecular magnetic tunnel junction (MTJ) has been reported in previous theoretical studies. A magnetoresistance ratio of B27% was found for the Ni–BDT–Ni MTJ. However, the spin currents were found to be nonlinear functions of the bias voltage, despite the variation in the sign at certain voltages because of the specific characteristics of the coupling between the magnetic electrodes and the molecular states.22,23 The spin-dependence of the I–V curve is a significant feature for designing a molecular device and determining the charge transport characteristics of the molecular junctions. Therefore, it is imperative to investigate the change in the transport properties as a function of the applied bias voltage. We first discuss the origin of the shape of I–V curves and then address the transport properties along with the absolute value of the spin-polarized current. There is a substantial difference when connecting a Se atom and Ni atoms to one end of the pentacene molecule. The bias voltage is varied from 1.0 to 1.0 V. In Fig. 8(a–d), the I–V curves exhibit linear and nonlinear characteristics. For the Ni–pentacene–Ni devices (in a PL configuration) linked to both ends of the frontier molecule with S atoms, the I–V features show two increasing nonlinear bias regimes. For a low bias (0–0.5 V), the I–V profile varies almost linearly, indicating metallic transport characteristics. In the second regime, there is a monotonic increase in the I–V profile from 0.5 to 1.0 V. Thus, the current for the spin majority reaches


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its largest value at 5000 nA, whereas the current for the down spins attains its maximum value at approximately 6000 nA. The difference between the current of the spin minority and the spin majority is clearly insignificant over the entire bias voltage range. Therefore, the system is driven out of the equilibrium condition under the applied bias voltage, and the electrode potential changes. Augmenting the bias causes the molecule to gain extra charges from the electrodes relative to the neutral molecule. Consequently, the electron scattering process is extended, and tunneling from one electrode to another decreases. It is well recognized that the transmission coefficients are correlated to the degree of coupling between the molecular orbitals and the incident states of the electrodes. Generally speaking, the transmission coefficient can be associated with the molecular orbitals that have been modified by the electrodes. In this case, the I–V curve of the molecular contact exhibits ohmic behavior because there is a smooth density of states near EF. The HOMO contributes to the initial augmentation of the current under a small applied voltage near EF. Meanwhile, the LUMO controls molecular conduction near EF for a low negative bias of 0.5 V. Thus, the electron flows out of the molecule to the lead. The bias successfully modulates the I–V characteristics of pentacene. The aforementioned transmission peaks result from the augmentation of the bias. The negative region is clearly symmetric with respect to the positive region. This result can be attributed to the essential resonant tunneling-like behavior across either the p or (at a higher bias) p* orbitals and the fact that a large portion of the transmission spectrum contributes to the current. However, when the molecule is incorporated asymmetrically in a two-terminal device (in the APL configuration) via one Se atom linker, the I–V curve increases nonlinearly over three regimes. In the first regime, the current gradually increases from 0 to 0.3 V. In the next regime, the current increases between 0.3 and 0.75 V, and the current varies more noticeably between the minority spin and the majority spin. Then, an augmentation is detected between 0.75 and 1.0 V. The highest spin-down current is 7000 nA; however, the highest spin-up current is 4000 nA. In this configuration, the spin-up current has the same topology as the spin-down current between 0 and 0.3 V. The PL and APL configurations connected to the thiol group (with an S atom) have the same threshold voltage (0.5 V) because they have fairly similar transmissions. The I–V curve is linear for low bias voltages (i.e., over a 0.5 to 0.5 V range). Therefore, the current continues to increase. This result implies that the Ni–pentacene–Ni systems (in both the PL and APL configurations) attached across the S atom behave as metals over this voltage range. For a bias beyond 0.5 or 0.5 V, the I–V curve corresponding to the Se linker atom exhibits nonlinearity, which changes the behavior of the molecular device. Three regimes are observed for the I–V curve for the Ni–pentacene–Ni junctions (in the PL configuration) connected across one Se linker. In the first regime, there is a rapid increase in the bias between 0 and 0.5 V; in the second regime, the current saturates from 0.5 to 0.75 V. In the third regime, the current increases monotonically with the bias from 0.75 to 1 V.


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Fig. 8 Current–voltage (I–V) characteristics for PL and APL configurations of a pentacene-based nickel spin-valve. Up and down indicate, respectively, the majority and minority spin contributions. (I–V): (a) Ni–pentacene–Ni (PL), (b) Ni–pentacene–Ni (APL), (c) Ni–pentacene–asym-Se–Ni (PL) and (d) Ni–pentacene–asym-Se–Ni (APL).

The spin-down and spin-up currents reach their maximum value at approximately 11 500 nA and 9000 nA, respectively. The magnitudes of the currents depend on the distance between the two metallic electrodes and the coupling strength between the electrodes and the molecule via the thiol group. The difference between the currents obtained for pentacene linked with S and Se contact atoms appears to be the shift in the transmission peaks, which are due to the HOMO shift from negative to positive energies. Thus, the most suitable anchoring atom depends on the bias voltage range. The average value of the spin polarization for zero and low bias voltages with Se anchoring atoms is different from that when S atoms are used. However, at higher voltages, S is less conductive than Se. Thus, Se is a better electron coupling group than S. Fig. 8 shows that the Ni/pentacene/Ni device (in an APL orientation for the Ni magnetizations) linked via one Se atom exhibits metallic behavior at low voltages. The total spin current first increases and then decreases as the bias voltage grows from 0 to 0.75 V.


Note that the spin-up current curve has the same shape as the spin-down current curve between 0 and 0.2 V. An interesting observation is that the spin-up current is approximately 11 000 nA for a voltage at approximately 0.75 V, which is associated with strong NDR phenomena. After the bias is augmented at approximately 0.75 V, there is an abrupt decline in the bias that results in a maximum spin-up current at approximately 9000 nA. The voltage is 0.9 V, which is the crossover point between the spin-up and spin-down current carriers. This voltage can be controlled using a biased current modulation for the Ni–pentacene–Ni junction in which the Ni electrodes are aligned anti-parallel across one Se atom linker. The I–V curve for this system exhibits NDR behavior between 0.7 and 1 V. The highest peak for the spin-down current at approximately 0.75 V shows that the current gradually decreased because of the strong chemical bonding coupling to the pentacene molecule. Note that a high conductance is expected at low voltages, followed by a sudden drop in the conductance.


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Briefly speaking, the induced NDR mechanism is usually associated with a large LUMO-like resonance peak at approximately EF, which also manifests in the transmission coefficient of this system. Then, a large resonance state for a spin-down channel at approximately EF is induced at a zero bias because the Se atom bonds quite strongly to the Ni electrodes and the pentacene molecule. In the APL state, the transmission spectrum near EF is dominated by the minority-spin electrons, which yield a minority-spin current much larger than the majority-spin current. However, beyond 0.75 V, the electron tunneling from the molecule decreases from one electrode to the other. This tunneling causes the transmission resonance peaks to decrease, resulting in NDR behavior. At a high bias, the spin-up current of the APL configuration is much larger than that for the PL configuration, indicating a change in the current characteristics. The NDR behavior causes the spin-down current to decrease below the spin-up current, which reaches a value of 0.95 V. This behavior may indicate a negative magnetoresistance effect. The transmission resonances should drop abruptly over the bias window from 0.75 to 1 V. This decrease in the resonances should obviously decrease the tunneling probability of the electrons and diminish the current through the Ni–pentacene–Ni junction in the APL configuration. When the bias reaches large values beyond 1.0 or (1.0 V), more resonances enter into the bias window and contribute to the current. The NDR phenomenon is not detected in the other systems studied. Applying a bias voltage across the molecule attached to metallic contacts has two effects. First, the bias voltage changes the Fermi level of the contacts. Second, the electrons lower their energy at the positive electrode and increase their energy at the negative electrode. Thus, the electrons available at the negatively polarized contact have energies corresponding to the empty states available at the positive electrode. If these electrons find a delocalized molecular level that connects both ends to the leads, electron transport occurs. Thus, the current grows rapidly at low bias when there is a transition from tunneling to resonant transport and then saturates. This large difference in the I–V characteristics of the molecule linked via different thiol groups is caused by the largest resistance in the system. Clearly, a system linked via one Se atom to the Ni electrodes has a considerably larger current than a system joined by S atoms. Linking the pentacene molecule to the probing electrodes mediated by one Se atom plays a dominant role in the current flow from one electrode to the other electrode. Thus, molecules with highly nonlinear I–V curves are preferred because they can create switchable states. The H atom contribution is negligible to the PDOS at the Fermi level, i.e., the H atom acts as a barrier, and the electronic transport occurs via a resonant tunneling mechanism. S and Se have similar chemical properties and are both non-metals. However, Se is closer to a metal than S, i.e., the valence electrons of Se are less tightly held than for S. Thus, Se forms more delocalized bonds with metals (e.g., Ni) than S. The extended electron state affects the electrical and transport properties of the element. Then, substituting Se by S significantly affects electron transport. The amplitude of the I–V curves for the APL systems varies over


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more than two orders of magnitude compared to PL systems. Obviously, this results from the contact with S or Se linking the pentacene molecule and the electrodes. Note that the current– voltage characteristics are symmetric upon voltage reversal, as expected, provided that the junction is geometrically symmetric. The I–V asymmetry of the metal–molecule–metal junction results from a dissimilar shift in the energy levels in pentacene for positive and negative voltages. Thus, it is very important to understand how different linking atoms can control the relative variation in the current through the same molecule suspended between metallic probes. However, it has been shown that the shape of I–V curves changes radically when a S atom linker is substituted by a Se atom, which significantly modifies the frontier molecular orbitals. The substituent atom determines the properties of the molecular device. The existence of electronic pathways in the molecule depends strongly on the charge that enables and disables charge transport under a bias. When the charge is almost zero, there is no LUMO connection between the two terminals of the molecules, and no conduction channels exist. However, as the voltage increases, the molecule becomes charged, the LUMO orbital extends over the two ends of the molecules, and the electronic pathways connect the two electrodes, thereby maximizing electron transport. Thus, the transport properties depend much more on the coupling effect between pentacene and the metal electrodes. We show that these nonlinear phenomena can be tailored by choosing a suitable separation between the thiolated ends of the molecule and the metal contacts. We predict that some bias values produce nonlinear transport phenomena, implying NDR for a pentacene molecular junction (APL configuration) asymmetrically thiolated to a Se atom. Thus, the mechanism for both NDR and spin valves in molecular junctions is resonant interface state transmission. Note that a negative differential resistance is expected when sharp transmitting states of such molecular junctions are resonantly shifted under a bias. Conversely, for molecular junctions terminated with S atoms, the predicted sharp transmitting states are caused by interface states and hybridization between molecular sulfur states and Ni d-electron states. NDR is one of the most interesting effects observed in the system studied. NDR corresponds to a negative slope of the I–V curve, which can occur for two primary reasons. In charge or conformation-induced NDR at a given voltage, the molecule may become charged or alter its structure such that the current diminishes after a voltage increment. The other intrinsic NDR is produced by a change in the molecular electronic structure that reduces the electron transport capabilities of the molecular orbitals involved in conduction. The NDR behavior for a pentacene molecule linked to Ni electrodes asymmetrically via a Se thiol atom at one side at the edge of the molecule is caused by the asymmetric response of the charge distribution of the molecular orbitals to positive and negative voltages. The NDR in the I–V curve is caused by tunneling via discrete quantum-mechanical states and a narrow DOS of the molecular levels. The current increases because of the increasing width and number of the transmission resonances. Thus, the NDR behavior depends on the type of coupling to the contacts and the Coulomb interaction. Transmission near the Fermi level


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for these molecular junctions is only by the minority spin, which results in the majority spin current being much smaller than the minority spin current, indicating spin valve character. When the molecules are bonded to the Ni electrodes through a Se atom, a large resonance state for a spin-down channel at approximately the Fermi level is induced at a zero bias. However, this novel NDR feature is exclusively detected in this promising pentacene molecular device. Our results indicate that the NDR effect offers excellent potential for spintronics of nanodevices. We now discuss the effect of different anchoring groups that depend on transport properties such as the spin polarization (P) of the current in molecular devices. The current polarization can be more clearly and intuitively described than quantum transport characteristics under a finite bias voltage. Here, we discuss the current polarization versus the bias voltage for all of the considered configurations (i.e., the left Ni electrode is spin up and the right Ni electrode is spin down or spin up) for both symmetric and asymmetric cases linked via the thiol group atoms S and Se. To determine the physical origin of the

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spin-valve effect in pentacene, which is mediated by the spinpolarized electron transport between two PL and APL electrodes, we plot the polarization of the current for the Ni–pentacene–Ni systems in Fig. 9(a–d). These figures show the polarization of the current at a finite bias voltage for a bias window over 1 to 1 V, where the pentacene molecule is connected to the Ni leads in the PL and APL orientations. Several important features are clearly visible in the evolution of the polarization of the currents. Note that applying low voltages to these systems produces a current for the electrons in both spin orientations. Changing the relative orientation of the magnetic polarization of the electrodes induces a slight variation in the conductance. For Ni–pentacene–Ni leads joined symmetrically and asymmetrically across S and Se atoms to Ni surfaces in PL configurations, we clearly observe a smooth increment in the polarization of the current, and the current saturates at a value of P = 0.33 at a bias voltage of 0.25 V. Then, the current polarization decreases at a bias of approximately 0.7 V. Thus, a positive bias of V = 0.75 V, where P(APL) 4 P(PL), contributes significantly to the tunnel current. Therefore, the sign

Fig. 9 The spin polarization for a pentacene molecule attached to Ni electrodes in either PL or APL states. (a) Ni–pentacene–Ni (PL), (b) Ni–pentacene–Ni (APL), (c) Ni–pentacene–asym-Se–Ni (PL) and (d) Ni–pentacene–asym-Se–Ni (APL).



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of the TMR is expected to invert as the bias is applied. Meanwhile, the exact values for the transmission probabilities and the TMR at the Fermi level reported in this study for PL configurations could change dramatically. Obviously, a local minimum could originate from a small accession at 1 V. This behavior is in sharp contrast to that of pentacene sandwiched between Ni electrodes aligned in an APL configuration and attached via S atom linkers to the electrodes. Note that the polarization of the current displays two local minima at bias voltages of approximately 0.1 and 0.3 V in a Ni–pentacene–Ni system (for an APL configuration). The subsequent sudden increase results from the highest value for the current polarization of 0.4, which occurs at the bias voltage near 0.8 V. However, for a Ni–pentacene–Ni junction (in the APL configuration) joined asymmetrically across one Se atom to Ni electrodes, there is a local minimum in the bias at approximately 0.17 V. Note that this minimum is followed by a sharp increase in the current polarization, which grows continuously up to a limiting value of 0.4. Finally, P dramatically decreases to a value of 0.08 at a bias of 1 V. Notice that P is altered in the APL system compared to the PL configuration. Fig. 8(d) shows that the APL configuration is reasonably asymmetric. Charge effects for the molecular states combined with asymmetric coupling to the electrodes can lead to asymmetries in the I–V curve. The spin current is found to vary nonlinearly with the bias, the up and down spin currents can be larger or smaller than each other, and the spin polarization changes sign (see Fig. 8(a–c)). These features are caused by transport resonances that are mediated by a combination of molecular states and the Ni surface electronic structure. The overall increase in P can primarily be attributed to an increase in the radius of the anchoring group. Then, the current through the minority states is reduced more drastically than that through the majority spin states. The spin-polarized electrons traveling across the Ni– molecule–Ni device are controlled by the molecule–Ni contact as well as by the anchoring group. The contact can control the current amplitude and the spin polarization, as well as the transmission channel, depending on the degree of localization of the states at the Fermi level. The prototypical structure of a spin valve device is that the conduction electrons experience a lower resistance, although the relative orientations of the magnetizations in the two Ni contacts are aligned in PL rather than in APL. This intriguing result yields the so-called spin-valve or magnetoresistive effect.

4. Conclusions We highlight several conclusions from this study in which analyzing the electronic density of states provided insight into significant difference in the transmission spectra and I–V characteristics of the considered devices. We perform spinpolarized electron transport calculations for a pentacene molecule coupled to Ni electrodes (in PL and APL alignments) via an anchoring group. In each case, the electronic structure and spin transport properties are studied using first-principles DFT combined with the NEGF scheme. We explain our results in


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terms of the spin valve performances correlated to the magnetic electrodes and the anchoring group atoms. The electronic interaction of the molecule with the electrodes is different between the two edges of the molecule: therefore, a fractional charge transfer induces a total net spin. Although this molecule is attached to Ni electrodes via thiol linkers, a transition from a non-magnetic to a magnetic ground state occurs. The presence of a Se atom at one side of the edge of the pentacene molecule significantly modifies the behavior of the transport properties of the device because Se has a different electronegativity than S. We also demonstrate that the spin polarization of the PL and APL leads induces a spin polarization for the current, whereas the induced magnetic moments in the molecule can be rather small. Both the values of the conductance and the current polarization are highly sensitive to the contact geometry and the type of thiol group. All of these results originate from the p and d states of the molecule and the metallic surface contact, which play a critical role in electrical conduction. The difference in the transmission function and the current–voltage features for the PL and APL spin configurations of the two-terminal molecular junctions are clearly observed. The results show that a system linked through a Se atom to one Ni surface contact exhibits a much higher current than that connected across a S atom. The mechanism of the NDR phenomenon is associated with the asymmetric coupling of a Se atom between pentacene and one Ni metallic electrode. A TMR of up to 29% is predicted for a pentacene molecule attached between Ni leads via a Se atom (at one end of the edge of the molecule). The non-trivial characteristics of the DOS of the devices show that it is critical to tailor the end-groups and then it is viable to engineer magnetoresistance. The negative differential resistance in the I–V features results from tunneling via discrete quantum-mechanical states and the narrowness of the DOS of the molecular levels. This behavior is observed for Ni electrodes (in an APL orientation) linked to a pentacene molecule via a Se atom (at one side of the edge of the molecule). The effect of the degree of coupling between the orbitals of the pentacene molecule and the Ni electrodes is attributed to the variation in the transferred charge in the molecule at various biases, which may produce NDR behavior. The characteristic NDR observed in the I–V curve results from the narrowness of the density of states of the molecular states. Our findings show that the crystallographic orientation of the electrodes should be considered in designing molecular electronic devices. The I–V curve exhibits a linear region at low bias, nonlinear behavior under higher bias voltages and a conspicuous NDR phenomenon over a certain bias voltage range. Our results provide a formal basis for using Se as an anchoring group in experiments to produce strong coupling between molecules and Ni electrodes. Thus, this system can also be used as a molecular spin valve. The effect predicted here could be confirmed experimentally by fabricating molecular junctions in contact with Ni surfaces across various thiol groups, which could play a significant role in spintronic devices based on magnetic-organic systems. This spin-valve property can be established by binding the molecule to Ni leads via thiol linkers either symmetrically or asymmetrically. Studying the chemical bonding


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at the interface of Ni–pentacene–Ni in greater depth may shed light on the physical origin of the spin valve effect. There is already some experimental evidence for this effect, which could be exploited to fabricate spin-transistors at the molecular level.


Acknowledgements S. C. acknowledges support from TUBITAK under Grant No. 108T710. A. L. would like to extend its sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding to this research through the Research Group Project no. RGP-VPP-233.

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