Molecular spintronics: spin-dependent electron transport in molecular wires

Intramolecular Magnetic Exchange Interaction in Dichalcogenide Substituted Organic Diradical Dications

Organic diradical dications, due to reduced intermolecular interactions, exhibit a greater tendency to adopt high spin states in the solid phase compared to their neutral diradical counterparts. This characteristic makes them promising candidates for applications involving organic electronics. We present a theoretical study of a recently synthesized sulfur-based diradical dication, a unique system exhibiting a robust triplet ground state. Using a number of density functional theory (DFT)-based methods (e.g., standard broken-symmetry DFT, constrained DFT, spin-flip TDDFT) and wave function-based multireference CASSCF+NEVPT2 methods, we investigate its magnetic properties and explore the influence of chalcogen substitution on magnetic exchange coupling. An active space scanning method was adopted to overcome the difficulties in choosing the correct active space for multireference calculation. Our findings highlight the critical role of multireference methods in accurately capturing the magnetic behavior of highly π-conjugated systems. The study reveals a surprising variation in magnetic properties among sulfur, selenium, and tellurium-based diradical dications despite being elements of the same group. These results offer valuable insights into the design and tuning of magnetic properties in organic diradical dications.

Spin-crossover in iron(II)-phenylene ethynylene-2,6-di(pyrazol-1-yl) pyridine hybrids: toward switchable molecular wire-like architectures

Luminescent oligo(p-phenylene ethynylene) (OPE) and spin-crossover (SCO) active Fe(II)-2,6-di(pyrazol-1-yl) pyridine (BPP) systems are prominent examples proposed to develop functional materials such as molecular wires/memories. A marriage between OPE and Fe(II)-BPP systems is a strategy to obtain supramolecular luminescent ligands capable of metal coordination useful to produce novel spin-switchable hybrids with synergistic coupling between spin-state of Fe(II) and a physical property associated with the OPE skeleton, for example, electronic conductivity or luminescence. To begin in this direction, two novel ditopic ligands, namely L¹ and L², featuring OPE-type backbone end-capped with metal coordinating BPP were designed and synthetized. The ligand L² tailored with 2-ethylhexyloxy chains at the 2 and 5 positions of the OPE skeleton shows modulated optical properties and improved solubility in common organic solvents relative to the parent ligand L¹. Solution phase complexation of L¹ and L² with Fe(BF₄)₂·6H₂O resulted in the formation of insoluble materials of the composition [Fe(L¹)] _n (BF₄)_2n and [Fe(L²)] _n (BF₄)_2n as inferred from elemental analyses. Complex [Fe(L¹)] _n (BF₄)_2n underwent thermal SCO centred at T _1/2  =  275 K as well as photoinduced low-spin to high-spin transition with the existence of the metastable high-spin state up to 52 K. On the other hand, complex [Fe(L²)] _n (BF₄)_2n , tethered with 2-ethylhexyloxy groups, showed gradual and half-complete SCO with 50% of the Fe(II)-centres permanently blocked in the high-spin state due to intermolecular steric interactions. The small angle x-ray scattering (SAXS) pattern of the as-prepared solid complex [Fe(L¹)] _n (BF₄)_2n revealed the presence of nm-sized crystallites implying a possible methodology towards the template-free synthesis of functional-SCO nanostructures.

Effect of the linkage modes of thiolated ethynyl groups on the spin-dependent electronic transport properties in transition metal porphyrin molecular junctions

Using density functional theory and nonequilibrium Green's function method, the spin-dependent electronic transport properties of six transition metal porphyrin molecules (VP, CrP, MnP, FeP, CoP, and NiP), which are linked to gold electrodes through the thiolated ethynyl groups, are investigated. Two different linkage modes (beta linkage and meso linkage) of the substituted ethynyl groups on the porphyrin macrocycle are considered. The results show that the linkage mode of ethynyl groups plays an important role on the spin transport properties of the molecular junctions and the beta linkage is more favorable for the spin filtering efficiency of current than the meso linkages. The spin-up and spin-down energy levels show the different evolutions which is responsible for the difference of spin filtering efficiency between the two linkage modes. The computational results of total current show that the meso-linked molecular junctions have the better conductive performances than the beta-linked ones which may be caused by the different electronic transport paths.

Tuning spin filtering by anchoring groups in benzene derivative molecular junctions

One of the important issues of molecular spintronics is the control and manipulation of charge transport and, in particular, its spin polarization through single-molecule junctions. Using ab initio calculations, we explore spin-polarized electron transport across single benzene derivatives attached with six different anchoring groups (S, CH₃S, COOH, CNH₂NH, NC and NO₂) to Ni(1 1 1) electrodes. We find that molecule-electrode coupling, conductance and spin polarization (SP) of electric current can be modified significantly by anchoring groups. In particular, a high spin polarization (SP  >  80%) and a giant magnetoresistance (MR  >  140%) can be achieved for NO₂ terminations and, more interestingly, SP can be further enhanced (up to 90%) by a small voltage. The S and CH₃S systems, on the contrary, exhibit rather low SP while intermediate values are found for COOH and CNH₂NH groups. The results are analyzed in detail and explained by orbital symmetry arguments, hybridization and spatial localization of frontier molecular orbitals. We hope that our comparative and systematic studies will provide valuable quantitative information for future experimental measurements on that kind of systems and will be useful for designing high-performance spintronics devices.

Thermoelectric efficiency of single-molecule junctions with long molecular linkers

We report results of theoretical studies of thermoelectric efficiency of single-molecule junctions with long molecular linkers. The linker is simulated by a chain of identical sites described using a tight-binding model. It is shown that thermoelectric figure of merit ZT strongly depends on the bridge length, being controlled by the lineshape of electron transmission function within the tunnel energy range corresponding to HOMO/LUMO transport channel. Using the adopted model we demonstrate that ZT may significantly increase as the linker lengthens, and that gateway states on the bridge (if any) may noticeably affect the length-dependent ZT. Temperature dependences of ZT for various bridge lengths are analyzed. It is shown that broad minima emerge in ZT versus temperature curves whose positions are controlled by the bridge lengths.

Theoretical Studies of the Spin-Dependent Electronic Transport Properties in Ethynyl-Terminated Ferrocene Molecular Junctions

The spin-dependent electron transport in the ferrocene-based molecular junctions, in which the molecules are 1,3-substituted and 1,3′-substituted ethynyl ferrocenes, respectively, is studied by the theoretical simulation with nonequilibrium Green’s function and density functional theory. The calculated results suggest that the substitution position of the terminal ethynyl groups has a great effect on the spin-dependent current-voltage properties and the spin filtering efficiency of the molecular junctions. At the lower bias, high spin filtering efficiency is found in 1,3′-substituted ethynyl ferrocene junction, which suggests that the spin filtering efficiency is also dependent on the bias voltage. The different spin-dependent transport properties for the two molecular junctions originate from their different evolutions of spin-up and spin-down energy levels.

Remarkable Differences in Spin Couplings for Various Self-Paired Dimers of Ring-Expansion-Radicalized Uracil: A Basis for the Design of Magnetically Anisotropic Assemblies

The spin-coupling properties of a series of radicalized uracil (rU) dimer diradicals with different H-bonding modes is examined. Each rU has four double H-bonding sites [the amide units: two at the Watson-Crick face (upper site WC₁ and lower site WC₂ ), a Hoogsteen site (HO), and a minor-groove site (MI)], and ten homogeneous dimers (rU-rU) can self-pair with well-defined diradical characters and comparable stability to the native U dimers. More interestingly, all these dimers exhibit distinctly different spin-coupling characters (ferromagnetic (FM) versus antiferromagnetic (AFM) and large- versus small-magnitude spin couplings), indicative of remarkable magnetic-coupling anisotropy of rU. This observation originates from the fusion of a cyclopentadienyl radical to U, which leads to uneven spin-density distribution. In rU, the fused five-membered radical ring can spin-polarize to the edge in the minor groove, and thus dimerization of rU leads to different H-bonded structures with remarkably different magnetic couplings. The calculated larger magnetic coupling constants J are 1003.7 and 540.2 cm^-1 for the WC₂ -HO and MI-HO H-bonding modes between rU, which exhibit considerably large FM couplings, the MI-MI, WC₁ -WC₂ and WC₂ -WC₂ modes show moderate FM couplings (J=0.4-77 cm^-1 ), and the other modes exhibit moderate or weak AFM couplings. These observations indicate that the HO and MI sites are favorable spin-coupling sites. In addition, the H-bond lengths and electronic structures of the H-bonding sites, proton transfer, and extra H-bonding interaction with the surroundings can also affect the magnetic couplings of the base pairs. Clearly, the unique magnetic coupling anisotropy of rU provides a promising application basis for the design and assembly of bio-inspired anisotropically magnetic membranes and even magnetism-tunable building blocks for novel magnetic nanoscale devices.

Magnetic and transport properties of conjugated and cumulated molecules: the π-system enlightens part of the story

We have investigated the intramolecular magnetic exchange coupling constants (J) for a series of nitronyl nitroxide diradicals connected by a range of linear conjugated and cumulene couplers focusing on the unusual π-interaction properties within the couplers.

Diradicalized biphenyl derivative carbon-based material molecules: exploring the tuning effects on magnetic couplings

While the conductance behavior of carbon-based couplers has been successfully investigated, insight into the magnetic properties of such carbon-based molecule coupled diradical systems is still scarce, and especially the structural effect of such couplers on the magnetic properties is poorly understood. The present work reports three different interference effects on the magnetic properties of carbon-based molecule coupled nitroxide diradicals: twisting, sideways group, and position effects. DFT calculations reveal that (i) torsion does not change their broken-symmetry singlet ground state and antiferromagnetic coupling, but decreases their magnetism; (ii) different linkages of two radical moieties result in different ground states and thus different magnetisms, depending on a combination of meta-sites and para-sites; (iii) the antiferromagnetic coupling with a broken-symmetry singlet ground state is not changed by adding sideways groups, but the coupling magnitude can be tuned by modifying the side-bridge. Discussions on geometries, magnetic properties, SOMO-SOMO splittings, and spin density distributions are made to clarify relevant magnetic behaviors. Clearly, the findings concerning the regulation of the diradicalized material molecules through modifying the carbon-based bridges provide a comprehensive understanding of the magnetism of such carbon-based diradicals and new prospects for the design of building blocks of magnetic functional molecular materials.

DNA bases ring-expanded with a cyclopentadiene free radical: a theoretical investigation of building blocks with diradical character

In this work, we computationally design radical nucleobases which possess improved electronic properties, especially diradical properties through introducing a cyclopentadiene radical. We predict that the detailed electromagnetic features of base assemblies are based on the orientation of the extra five-membered cyclopentadiene ring. Broken symmetry DFT calculations take into account the relevant structures and properties. Our results reveal that both the radicalized DNA bases and the base pairs formed when they combine with their counterparts remain stable and display larger spin delocalization. The mode of embedding the cyclopentadiene free radical in the structures has some influence on the degree of π-conjugation, which results in various diradical characteristics. Single-layered radical base pairs all have an open-shell singlet ground state, but the energy difference between singlet and triplet is not significant. For two-layered radical base pairs, the situation is more complex. All of them have an open-shell state as their ground state, including an open-shell singlet state and an open-shell triplet state. That is, the majority of radical base pairs possess anti-ferromagnetic or ferromagnetic characteristics. We present here a more in-depth discussion and analyses to study the magnetic characteristics of radical bases and base pairs. As an important factor, two-layered radical base pairs also have been carefully analyzed. We hope that all the measurements and results presented here will stimulate further detailed insights into the related mechanisms in modified DNA bases and the design of better ring-expanded DNA magnetic materials.

Toward interfacing organic semiconductors with ferromagnetic transition metal substrates: enhanced stability via carboxylate anchoring

The controlled growth of chemically and structurally well-defined as well as thermally stable carboxylate-anchored self-assembled monolayers on ferromagnetic 3d transition metal substrates provides promising model systems for advanced studies of spin-dependent electron transport.

Role of the coupler to design organic magnetic molecules: LUMO plays an important role in magnetic exchange

The occupation number and spatial position of the LUMO play an important role in magnetic exchange in diradicals.

Spin transport properties of n-polyacene molecules (n = 1-15) connected to Ni surface electrodes: theoretical analysis

Using non-equilibrium Green function formalism in conjunction with density functional theory, we explore the spin-polarized transport characteristics of several planar n-acene molecules suspended between two semi-infinite Ni electrodes via the thiol group. We examine the spin-dependence transport on Ni-n-acenes-Ni junctions, while the number of fused benzene rings varies between 1 and 15. Intriguingly, the induced magnetic moments of small acene molecules are higher than that of longer acene rings. The augmentation of fused benzene rings affects both the magnetic and transport features, such as the transmission function and conductance owing to their coupling to the Ni surface contacts via the anchoring group. The interplay between the spin-polarized transport properties, structural configuration and molecular electronic is a fortiori essential in these attractive molecular devices. Thus, this can conduct to the engineering of the electron spin transport in atomistic and molecular junctions. These prominent molecules convincingly infer that the molecular spin valves can conduct to thriving molecular devices.

Aromatic molecules as spintronic devices

In this paper, we study the spin-dependent electron transport through aromatic molecular chains attached to two semi-infinite leads. We model this system taking into account different geometrical configurations which are all characterized by a tight binding Hamiltonian. Based on the Green's function approach with a Landauer formalism, we find spin-dependent transport in short aromatic molecules by applying external magnetic fields. Additionally, we find that the magnetoresistance of aromatic molecules can reach different values, which are dependent on the variations in the applied magnetic field, length of the molecules, and the interactions between the contacts and the aromatic molecule.

Theoretical studies of electronic structures, magnetic properties and electron conductivities of one-dimensional Ni(n) (n = 3, 5, 7) complexes

Electronic structures, magnetic properties and electron conductivities of linearly aligned one-dimensional (1-D) Ni(II)3, Ni(II)5 and Ni(II)7 complexes, i.e. [Ni3(dpa)4NCS2], [Ni5(tpda)4X2] (X = Cl, CN, N3, NCS) and [Ni7(teptra)4Cl2], are systematically investigated by the broken-symmetry B3LYP calculations and simulations based on an elastic scattering Green's function theory. Calculated spin densities appear only at terminal Ni ions, while inner Ni ions are the closed-shell. The calculated effective exchange integrals (J(ab)) values reproduce well the experimental results that indicate anti-ferromagnetic (AF) interactions between two terminal Ni ions. Natural orbitals and their occupation numbers show that a change in the weak AF couplings by axial ligands in penta-nickel complexes originates in σ-type orbitals. Simulated electron conductivities of [Ni3(dpa)4NCS2] and [Ni5(tpda)4NCS2] semi-quantitatively correspond to the experimental results. By the analyses, it is elucidated that electrons are mainly transmitted by σ-type orbitals, but the bonds between Au and axial ligands are also dominant factors for conductivity.

Quantum-chemical investigation of the spin crossover complex [Fe(mbpzbpy)(NCS)2]

The geometries of the spin crossover complex [Fe(mbpzbpy)(NCS)2] were optimized by several density functionals and basis sets. In addition, the vibrational modes and IR spectra, excited states and UV/vis absorption spectra and spin splittings energies were also predicted by DFT/TD-DFT calculations.

DFT/TD-DFT study of the spin transition complex [Fe(pmea)(NCS)2]

The spin crossover (SCO) compound [Fe(pmea)(NCS)(2)] (where pmea symbolizes the ligand bis[(2-pyridyl)methyl]-2-(2-pyridyl)ethylamine) has been studied by DFT/TD-DFT methods. Several density functionals and basis sets were used in the calculations to obtain optimized geometries of the compound in the low-(LS), intermediate-(IS) and high-spin (HS) states. The vibrational modes and IR spectra, spin splittings energies, excited states and UV/vis absorption spectra were calculated. From the TD-DFT calculations, it can be inferred that this complex may act as a reversible optical switch via the LIESST effect and its reverse process.

QUANTUM TRANSPORT AND CURRENT-TRIGGERED DYNAMICS IN MOLECULAR TUNNEL JUNCTIONS

The modelling of nanoelectronic systems has been the topic of ever increasing activity for nearly two decades. Yet, new questions, challenges and opportunities continue to emerge. In this article we review theoretical and numerical work on two new developments in the theory of molecular-scale electronics. First we review a density functional theory analysis within the Keldysh non-equilibrium Green function formalism to predict nonlinear charge transport properties of nanoelectronic devices. Next we review a recently developed quantum mechanical formalism of current-triggered nuclear dynamics. Finally we combine these theories to describe from first principles the inelastic current and the consequent molecular dynamics in molecular heterojunctions.

Molecular electronic junction transport: some pathways and some ideas

When a single molecule, or a collection of molecules, is placed between two electrodes and voltage is applied, one has a molecular transport junction. We discuss such junctions, their properties, their description, and some of their applications. The discussion is qualitative rather than quantitative, and focuses on mechanism, structure/function relations, regimes and mechanisms of transport, some molecular regularities, and some substantial challenges facing the field. Because there are many regimes and mechanisms in transport junctions, we will discuss time scales, geometries, and inelastic scattering methods for trying to determine the properties of molecules within these junctions. Finally, we discuss some device applications, some outstanding problems, and some future directions.

Electrochemically gated oligopeptide nanowires bridging gold electrodes: novel bio-nanoelectronic switches operating in aqueous electrolytic environments

We present electronic structure and transport calculations that reveal that oligopeptide based molecular nanowires support unoccupied extended electronic states that span the length of the nanowire and are resistant to disorder. Electrochemical gating in aqueous electrolytes is shown to bring these extended states into resonance with the Fermi level of gold electrodes, transforming these nanowires from insulators into conductors. Thus oligopeptide nanowires are promising candidates for bionanoelectronic switches operating in the aqueous electrolytic environments native to biological systems.

Spin routes in organic semiconductors

Organic semiconductors are characterized by a very low spin-orbit interaction, which, together with their chemical flexibility and relatively low production costs, makes them an ideal materials system for spintronics applications. The first experiments on spin injection and transport occurred only a few years ago, and since then considerable progress has been made in improving performance as well as in understanding the mechanisms affecting spin-related phenomena. Nevertheless, several challenges remain in both device performance and fundamental understanding before organic semiconductors can compete with inorganic semiconductors or metals in the development of realistic spintronics applications. In this article we summarize the main experimental results and their connections with devices such as light-emitting diodes and electronic memory devices, and we outline the scientific and technological issues that make organic spintronics a young but exciting field.

Quantitative analysis of nonequilibrium spin injection into molecular tunnel junctions

We report quantitative analysis of nonequilibrium spin injection from Ni contacts to the octanethiol molecular spintronic system. Our calculation is based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism. The first principles results allow us to establish a clear physical picture on how spins are injected from the Ni contacts through the Ni-molecule linkage to the molecule, why tunnel magnetoresistance is rapidly reduced by the applied bias in an asymmetric manner, and to what extent ab initio transport theory can make quantitative comparisons to the corresponding experimental data. We found that extremely careful sampling of the two-dimensional Brillouin zone of the Ni surface is crucial for accurate results.

Mechanism for negative differential resistance in molecular electronic devices: local orbital symmetry matching

A new mechanism for negative differential resistance (NDR) originating from local orbital symmetry matching between an electrode and a molecule in a single molecular electronic device is proposed and demonstrated by a joint experimental and theoretical scanning tunneling microscope study of a cobalt phthalocyanines (CoPc) molecule on a gold substrate. For two different metal tips used, Ni and W, NDR occurs only with Ni tips and shows no dependence on the geometrical shape of the tip. Calculations reveal that such a behavior is a result of local orbital symmetry matching between the Ni tip and Co atom.

Efficient organometallic spin filter between single-wall carbon nanotube or graphene electrodes

We present a theoretical study of spin transport in a class of molecular systems consisting of an organometallic benzene-vanadium cluster placed in between graphene or single-wall carbon-nanotube-model contacts. Ab initio modeling is performed by combining spin density functional theory and nonequilibrium Green's function techniques. We consider weak and strong cluster-contact bonds. Depending on the bonding we find from 73% (strong bonds) up to 99% (weak bonds) spin polarization of the electron transmission, and enhanced polarization with increased cluster length.

Electron transport in molecular junctions

Building an electronic device using individual molecules is one of the ultimate goals in nanotechnology. To achieve this it will be necessary to measure, control and understand electron transport through molecules attached to electrodes. Substantial progress has been made over the past decade and we present here an overview of some of the recent advances. Topics covered include molecular wires, two-terminal switches and diodes, three-terminal transistor-like devices and hybrid devices that use various different signals (light, magnetic fields, and chemical and mechanical signals) to control electron transport in molecules. We also discuss further issues, including molecule-electrode contacts, local heating- and current-induced instabilities, stochastic fluctuations and the development of characterization tools.

Size-dependent alternation of magnetoresistive properties in atomic chains

Spin-polarized electronic and transport properties of carbon atomic chains are investigated when they are capped with magnetic transition-metal (TM) atoms like Cr or Co. The magnetic ground state of the TM-C(n)-TM chains alternates between the ferromagnetic (F) and antiferromagnetic (AF) spin configurations as a function of n. In view of the nanoscale spintronic device applications the desirable AF state is obtained for only even-n chains with Cr; conversely only odd-n chains with Co have AF ground states. When connected to appropriate metallic electrodes these atomic chains display a strong spin-valve effect. Analysis of structural, electronic, and magnetic properties of these atomic chains, as well as the indirect exchange coupling of the TM atoms through non-magnetic carbon atoms are presented.

Nonlinear spin current and magnetoresistance of molecular tunnel junctions

We report on a theoretical study of spin-polarized quantum transport through a Ni-bezenedithiol(BDT)-Ni molecular magnetic tunnel junction (MTJ). Our study is based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism, so that microscopic details of the molecular MTJ are taken into account from first principles. A magnetoresistance ratio of approximately 27% is found for the Ni-BDT-Ni MTJ which declines toward zero as bias voltage is increased. The spin currents are nonlinear functions of bias voltage, even changing sign at certain voltages due to specific features of the coupling between molecular states and magnetic leads.