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

Perylene-based molecular device: multifunctional spintronic and spin caloritronic applications

Carbon-based magnetic molecular junctions are promising candidates for nanoscale spintronic applications because they are atomically thin and possess high stability and peculiar magnetism. Herein, based on first-principles and non-equilibrium Green's function, we designed a carbon-based molecular spintronic device composed of carbon atomic chains, zigzag-edged graphene nanoribbon (ZGNR), and a perylene molecule. Our results show that the device exhibits integrated spintronic and spin caloritronic functionalities, such as the bias-voltage driven spin filtering effect, negative differential resistance effect and giant magnetoresistance, temperature-gradient driven spin Seebeck effect, thermal spin filtering effect, high thermal magnetoresistance, and thermal colossal giant magnetoresistance. Furthermore, considering the phonon vibration effect, the spin and charge thermoelectric figure of merits (ZT_sp and ZT_ch) can be enhanced and the peak of ZT_sp is much larger than that of ZT_ch, indicating the excellent thermospin performance. The asymmetrical contact configuration between the carbon atomic chain and perylene/ZGNR inhibits the phonon thermal conductivity significantly, leading to the optimal ZT_sp and ZT_ch of 2.4 and 0.5 at 300 K, respectively. These results suggest multifunctional spintronic and spin caloritronic applications for the perylene-based molecular device.

Design of multifunctional spin logic gates based on manganese porphyrin molecules connected to graphene electrodes

The spin-resolved transport properties of molecular logic devices composed of two Mn porphyrin molecules connected to each other via a six-carbon atomic chain were studied using the non-equilibrium Green's function combined with density functional theory. The molecules were symmetrically connected to armchair graphene nanoribbon electrodes through four-carbon atomic chains on the left- and right-hand sides. Our calculations revealed that the spin-resolved current-voltage curves depend on the initial spin setting of the transition metal Mn atoms and carbon atoms on the zigzag edges where the electrodes come in contact with the molecule. By simultaneously regulating the spin orientations of the intermediate functional molecules and the zigzag edges of the armchair graphene nanoribbon electrodes, seven spin polarization configurations were obtained. These configurations were examined in this study considering the spin-related symmetry of molecular junctions. By meticulously selecting different combinations according to the specific input and output signals, YES, NOT, OR, NOR, and XOR multifarious spin logic devices were created. The findings of this study are expected to contribute toward the extension of molecular junction functions in future spintronic integrated circuit design and further miniaturization.

Effect of Impurity Adsorption on the Electronic and Transport Properties of Graphene Nanogaps

Graphene stands out as a versatile material with several uses in fields that range from electronics to biology. In particular, graphene has been proposed as an electrode in molecular electronics devices that are expected to be more stable and reproducible than typical ones based on metallic electrodes. In this work, we study by means of first principles, simulations and a tight-binding model the electronic and transport properties of graphene nanogaps with straight edges and different passivating atoms: Hydrogen or elements of the second row of the periodic table (boron, carbon, nitrogen, oxygen, and fluoride). We use the tight-binding model to reproduce the main ab-initio results and elucidate the physics behind the transport properties. We observe clear patterns that emerge in the conductance and the current as one moves from boron to fluoride. In particular, we find that the conductance decreases and the tunneling decaying factor increases from the former to the latter. We explain these trends in terms of the size of the atom and its onsite energy. We also find a similar pattern for the current, which is ohmic and smooth in general. However, when the size of the simulation cell is the smallest one along the direction perpendicular to the transport direction, we obtain highly non-linear behavior with negative differential resistance. This interesting and surprising behavior can be explained by taking into account the presence of Fano resonances and other interference effects, which emerge due to couplings to side atoms at the edges and other couplings across the gap. Such features enter the bias window as the bias increases and strongly affect the current, giving rise to the non-linear evolution. As a whole, these results can be used as a template to understand the transport properties of straight graphene nanogaps and similar systems and distinguish the presence of different elements in the junction.

0D/1D organic ferroelectrics/multiferroics for ultrahigh density integration: Helical hydrogen-bonded chains, multi-mode switching, and proton synaptic transistors

In recent years, room-temperature ferroelectricity has been experimentally confirmed in a series of two-dimensional (2D) materials. Theoretically, for isolated ferroelectricity in even lower dimensions such as 1D or 0D, the switching barriers may still ensure the room-temperature robustness for ultrahigh-density non-volatile memories, which has yet been scarcely explored. Here, we show ab initio designs of 0D/1D ferroelectrics/multiferroics based on functionalized transition-metal molecular sandwich nanowires (SNWs) with intriguing properties. Some functional groups such as -COOH will spontaneously form into robust threefold helical hydrogen-bonded chains around SNWs with considerable polarizations. Two modes of ferroelectric switching are revealed: when the ends of SNWs are not hydrogen-bonded, the polarizations can be reversed via ligand reorientation that will reform the hydrogen-bonded chains and alter their helicity; when both ends are hydrogen-bonded, the polarizations can be reversed via proton transfer without changing the helicity of chains. The combination of those two modes makes the system the smallest proton conductor with a moderate migration barrier, which is lower compared with many prevalent proton-conductors for higher mobility while still ensuring the robustness at ambient conditions. This desirable feature can be utilized for constructing nanoscale artificial ionic synapses that may enable neuromorphic computing. In such a design of synaptic transistors, the migration of protons through those chains can be controlled and continuously change the conductance of MXene-based post-neuron for nonvolatile multilevel resistance. The success of mimicking synaptic functions will make such designs promising in future high-density artificial neutral systems.

Spin Transport Properties of One-Dimensional Benzene Ligand Organobimetallic Sandwich Molecular Wires

Organometallic sandwich complexes, composed of cyclic hydrocarbon ligands and transition-metal atoms, display unique physical and chemical properties. In this work, the electronic and spin transport properties of one-dimensional (1D) VBz2 ligand bimetallic sandwich complexes, VBz2-TM (TM = Cr, Mn, and Fe), are systematically investigated using density functional theory and nonequilibrium Green's function method. The results show that all the 1D infinite molecular wires [(VBz2)TM]∞ (TM = Cr-Fe) are found to be thermodynamically stable with high binding energies (∼1.0-3.45 eV). In particular, they are predicted to be ferromagnetic half metals. Moreover, the I-V curves exhibit negative differential resistance for one, two, and three VBz2-TM wires at TM = Cr, Mn, and Fe, respectively, which is of great significance for certain electronic applications. Our findings strongly suggest that the benzene ligand bimetallic sandwich molecular wires are good candidates for potential electronics and spintronics.

Electronics without bridging components

We propose a new paradigm of electronic devices based only on two electrodes separated by a gap, i.e. without any functional element bridging them. We use a tight-binding model to show that, depending on the type of material of the electrodes and its structure, several electronic functionalities can be achieved: ohmic behaviour, rectification, negative differential resistance, spin-filtering and magnetoresistance. In particular, we show that it is possible to deliver a given functionality by changing the coupling between the surface and bulk states and between the surface states across the gap, which dramatically changes the current-voltage characteristics. These results prove that it is possible to have functional electronic and spintronic elements on the nanoscale without having physical components bridging the electrodes.

Large Tunneling Magnetoresistance in VSe2/MoS2 Magnetic Tunnel Junction

Two-dimensional (2D) van der Waals (vdW) materials provide the possibility of realizing heterostructures with coveted properties. Here, we report a theoretical investigation of the vdW magnetic tunnel junction (MTJ) based on VSe₂/MoS₂ heterojunction, where the VSe₂ monolayer acts as a ferromagnet with room-temperature ferromagnetism. We propose the concept of spin-orbit torque (SOT) vdW MTJ with reliable reading and efficient writing operations. The nonequilibrium study reveals a large tunneling magnetoresistance of 846% at 300 K, identifying significantly its parallel and antiparallel states. Thanks to the strong spin Hall conductivity of MoS₂, SOT is promising for the magnetization switching of VSe₂ free layer. Quantum-well states come into being and resonances appear in MTJ, suggesting that the voltage control can adjust transport properties effectively. The SOT vdW MTJ based on VSe₂/MoS₂ provides desirable performance and experimental feasibility, offering new opportunities for 2D spintronics.

Spin-filtering and tunneling magnetoresistance effects in 6,6,12-graphyne-based molecular magnetic tunnel junctions

In the present study, by cutting 6,6,12-graphyne along vertical and horizontal directions, two kinds of 6,6,12-graphyne nanodots (6,6,12-GYNDs) with different sizes are obtained. Using these 6,6,12-GYNDs, we theoretically designed two kinds of 6,6,12-graphyne-based molecular magnetic tunnel junctions (MMTJs) and investigated their spin-dependent transport properties. Depending on the orientation of the 6,6,12-GYNDs and the connection of the 6,6,12-GYNDs to electrodes, our results show that the two MMTJs have novel transport behaviors. Two different net spin currents can be obtained by tuning the spin configurations and the maximal order of magnitudes of tunneling magnetoresistance values of the two MMTJs reaches 106%. The high spin-filtering ratio and large tunneling magnetoresistance value provide high sensitivity for practical applications. Therefore, the spin-filtering and tunneling magnetoresistance effects enable 6,6,12-graphyne-based MMTJs to be used as spintronic devices.

Efficient terahertz transmission modulation in plasmonic metallic slits by a graphene ribbon array

Extraordinary optical transmission is the widely known phenomenon of enhanced transmission of waves through subwavelength periodic metallic apertures. Here, we propose efficient terahertz transmission modulation in subwavelength metallic slits by a graphene plasmonic ribbon array. The extraordinary optical transmission through the subwavelength metallic slits can be tuned by coupling with the plasmonic resonance of graphene ribbon array, resulting in a deep transmittance depression. Numerical simulations show that maximal transmission modulation depth of 98.99% can be obtained at 1.03 THz via this mechanism.

Electronic and transport properties of the (VBz)n@MoS2NT nanocable

The electronic structure of a novel inorganic (8, 8) MoS2 nanotube nanocable, (VBz)n@MoS2NT, (where Bz refers as C6H6), is investigated using density functional theory. Transport property calculations are further performed employing non-equilibrium Green's function methods by modeling a two-probe device with a finite-sized nanocable sandwiched between two electrodes of its own. It is found that the transport properties of the nanocable agree well with its electronic structure. The core (VBz)n nanowire in the (VBz)n@MoS2NT plays a significant role in electron transportation, meanwhile, the sheath MoS2NT also participates in electron transportation. This phenomenon is different from those of (VBz)n@CNT and (VBz)n@BNNT nanocables. For the (VBz)n@CNT, the transport properties are majorly dominated by the metallic CNT sheath, while for the (VBz)n@BNNT, it is merely decided by the core (VBz)n. The conductivity of the (VBz)n@MoS2NT is slightly better in comparison with pure (VBz)n. Similar to pure (VBz)n, the (VBz)n@MoS2NT shows spin-polarized transport properties: the spin-down state gives a higher conductivity than the spin-up state. The values of the spin filter efficiency of the (VBz)n@MoS2NT can be up to >80%, indicating it to be a good candidate for spin filters. In addition, it is also found that encapsulating (VBz)n into the MoS2NT could introduce magnetism. More importantly, the ferromagnetic (VBz)n@MoS2NT is thermally rather stable. Therefore, encapsulating (VBz)n into the MoS2NT can effectively tune the electronic and transport properties for exploring novel functional nanodevices.

Spin transport properties in lower n-acene–graphene nanojunctions

A series of n-acene–graphene (n = 3, 4, 5, 6) devices, in which n-acene molecules are sandwiched between two zigzag graphene nanoribbon (ZGNR) electrodes, are modeled through the spin polarized density functional theory combined with the non-equilibrium Green's function technique.

Experimental and theoretical studies of the structural and electronic properties of vanadium-benzene sandwich clusters and their anions: V(n)Bz(n)(0/-) (n = 1-5) and V(n)Bz(n-1)(0/-) (n = 2-5)

One end open V(n)Bz(n)(-) (n = 1-5; Bz = benzene) and both ends open V(n)Bz(n-1)(-) (n = 2-5) vanadium-benzene cluster anions were studied using anion photoelectron spectroscopy and density functional calculations. The smaller (n ≤ 3) V(n)Bz(n) and V(n)Bz(n-1) clusters and corresponding anions were found to have structural isomers, whereas full-sandwiched V(n)Bz(n+1) clusters preferred to form multiple-decker sandwich structures. Several isomeric V2Bz2 structures were identified theoretically and the anion photoelectron spectra of V2Bz2(0/-) were explained well by the coexistence of two isomeric structures: (1) a V2-core structure sandwiched between benzene molecules and (2) an alternating sandwich structure with the spin state strongly dependent on the structure. The adiabatic electron affinity of both V(n)Bz(n) and V(n)Bz(n-1) was found to increase with the cluster size at larger sizes (n = 4 or 5) and approaches to that of V(n)Bz(n+1). The evolution of the structural and electronic properties of V(n)Bz(m) and V(n)Bz(m)(-) (m = n and n - 1) with size is discussed in comparison with V(n)Bz(n+1) and V(n)Bz(n+1)(-).

Difficulties in the ab initio description of electron transport through spin filters

Spin-transport calculations present certain difficulties which are sometimes overlooked when using density-functional theory (DFT) to analyze and predict the behavior of molecular-based devices. We analyze and give examples of some caveats of spintronic calculations using DFT. We first describe how the broken-symmetry problem of DFT can cause serious problems in the evaluation of the spin polarization of electron currents. Next, we signal the low-energy scale of magnetic excitations, which makes them ubiquitous at already rather small biases. The existence of excitations in spin transport has catastrophic consequences in the reliability of the usual transport calculations. Finally, we compare DFT and configuration-interaction calculations of a ferrocene-based double decker that has been heralded as a possible spin-filter, and we cast a word of caution when we show that DFT is qualitatively wrong in the description of both the ground state and the excited states of ferrocene double deckers.

Size-dependent magnetic order and giant magnetoresistance in organic titanium-benzene multidecker cluster

Using density functional theory and non-equilibrium Green's function method, we investigated the magnetic and transport properties of small organic titanium-benzene sandwich clusters TinBzn+1 (n = 1-3). The results show that TiBz2 is nonmagnetic while Ti2Bz3 and Ti3Bz4 are ferromagnetic, and our prediction is in agreement with experimental observation. The double exchange mechanism plays a key role in the ferromagnetism of larger clusters. With Ni as the two electrodes, significant spin-filter efficiency (SFE) and giant magnetoresistance (GMR) were found in the TinBzn+1 molecular junction. These transport properties could be controlled by cluster size, bias voltage or gate voltage. Specially, a sign-reversible GMR effect was observed in the Ti2Bz3 molecular junction. Finally, the microscopic mechanisms of SFE and GMR were suggested.

A first principles study of novel one-dimensional organic half-metal vanadium-cyclooctatetraene wire

The structural, electronic, and magnetic properties of one-dimensional vanadium-cyclooctatetraene[(V-COT)]∞ wire and sandwich clusters are investigated by means of density functional theory. It is found that the (V-COT)∞ SMW is half-metallic. Through the spin transportation calculations, the system for V-COT clusters coupled to gold electrodes performs nearly perfect spin filters. In addition, the I-V curve shows obviously negative differential resistance effects. These results suggest the potential applications of (V-COT)∞ in spintronics.

The magnetic and quantum transport properties of benzene-vanadium-borazine mixed sandwich clusters: a new kind of spin filter

Using density functional theory and the non-equilibrium Green's function technique, we performed theoretical investigations on the magnetic and quantum transport properties of benzene-vanadium-borazine mixed organic/inorganic ligand sandwich clusters. The calculated results show that these finite sandwich clusters coupled to Ni electrodes exhibit novel quantum transport properties such as half-metallicity, negative differential resistance and spin-reversal effect, and can be viewed as a new kind of spin filter. However, for the infinite molecular wire, the ground state was identified as a ferromagnetic semiconductor with high stability. These findings suggest that the mixed organic/inorganic ligand sandwich clusters and molecular wires are promising materials for application in molecular electronics and spintronics.

Dual conductance, negative differential resistance, and rectifying behavior in a molecular device modulated by side groups

We investigate the electronic transport properties for a molecular device model constructed by a phenylene ethynylene oligomer molecular with different side groups embedding in a carbon chain between two graphene electrodes. Using the first-principles method, the unusual dual conductance, negative differential resistance (NDR) behavior with large peak to valley ratio, and obvious rectifying performance are numerically observed in such proposed molecular device. The analysis of the molecular projected self-consistent Hamiltonian and the evolution of the frontier molecular orbitals (MOs) as well as transmission coefficients under various external voltage biases gives an inside view of the observed results, which suggests that the dual conductance behavior and rectifying performance are due to the asymmetry distribution of the frontier MOs as well as the corresponding coupling between the molecule and electrodes. But the NDR behavior comes from the conduction orbital being suppressed at certain bias. Interestingly, the conduction properties can be tuned by introducing side groups to the molecule and the rectification as well as the NDR behavior (peak to valley ratio) can be improved by adding different side groups in the device model.

Strong spin-filtering and spin-valve effects in a molecular V-C(60)-V contact

Motivated by the recent achievements in the manipulation of C(60) molecules in STM experiments, we study theoretically the structure and electronic properties of a C(60) molecule in an STM tunneljunction with a magnetic tip and magnetic adatom on a Cu(111) surface using first-principles calculations. For the case of a vanadium tip/adatom, we demonstrate how spin coupling between the magnetic V atoms, mediated by the C(60), can be observed in the electronic transport, which display a strong spin-filtering effect, allowing mainly majority-spin electrons to pass (>95%). Moreover, we find a significant change in the conductance between parallel and anti-parallel spin polarizations in the junction (86%) which suggests that STM experiments should be able to characterize the magnetism and spin coupling for these systems.

Efficient spin filter based on FeN4 complexes between carbon nanotube electrodes

We present a theoretical study to explore the spin transport properties of FeN(4) complexes sandwiched between two armchair (5, 5) carbon nanotube (CNT) electrodes. The ab initio modeling is performed by combining the spin-dependent density functional theory with nonequilibrium Green's function formalism. The calculated results clearly demonstrate that the transport properties of FeN(4) complexes are sensitive to the contact configuration. Near the Fermi level the conductance through three examined junctions is mainly governed by the spin-up electrons. The FeN(4) complex coupled to CNT electrodes with the π-type contact conjugation can act as a nearly perfect spin filter, and its spin filter efficiency is up to 98.0%. Our theoretical results demonstrate that FeN(4) complexes are promising for future molecular spintronics devices.

Electronic and magnetic properties of early transition-metal substituted iron-cyclopentadienyl sandwich molecular wires: parity-dependent half-metallicity

Electronic and magnetic properties of early transition metals (V, Ti, Sc)-Fe(k)Cp(k + 1) sandwich molecular wires (SMWs) are investigated by means of ab initio calculations. It is found that all SMWs favor a ferromagnetic ground state. Significantly, V-Fe(k)Cp(k + 1) SMWs are either half-metallic or semiconducting, dependent upon the parity (even or odd) of the number (k) of Fe atoms in the unit cell of SMWs. This parity oscillation of conductive properties results from the combined effects of the band-folding and gap-opening at the Brillouin-zone boundary of one-dimensional materials. In contrast, Sc-Fe(k)Cp(k + 1) and Ti-Fe(k)Cp(k + 1) SMWs are always semiconducting. Our work may open up the way toward half metal/semiconductor heterostructures with perfect atomic interface.

Electronic and magnetic properties of metal-(4,4'-bipyridine) sandwich complexes and their nanowires

Using calculations based on density functional theory, we have investigated the geometric, energetic, and magnetic properties of complexes of 4,4'-bipyridine (BPY) with metal atoms M (= Li, V, and Ti). The systems of interest include BPY-M(2) and BPY(2)-M(2) complexes and (BPY-M(2))(x) nanowires in which the sandwich structure is stacked infinitely along one direction. For each of these systems, a detailed analysis was performed on the electronic structure. First, we found that BPY-M (M = V and Ti) binding is stronger than BPY-Li binding because of the covalent nature of the former interaction. The difference in the magnetic properties of BPY-M(2) and BPY(2)-M(2) (M = V and Ti) complexes can be understood in terms of the different strengths of the M-M interactions mediated by d(M) or sd(M)-hybridized orbitals. Second, we found that the (BPY-Li(2))(x) nanowire is a semiconductor, whereas (BPY-V(2))(x) and (BPY-Ti(2))(x) nanowires are magnetic metals due to the spin-polarization in d(z)2(M)-derived bands.

Electronic structure and transport properties of early transition metal tripledeckers

The electronic structure and transport properties of the Cp(2)BzM(2) (M = Sc, Ti, and V) tripledeckers are studied by spin polarized density functional theory and nonequilibrium Green's function method considering high-spin and low-spin states. Total energy calculations show that the sandwich structured Cp(2)BzSc(2) exists in a singlet state with no local magnetic moment on the Sc atoms. Cp(2)BzTi(2) in triplet state exists as a distorted tripledecker and is more stable than singlet and quintet states. Cp(2)BzV(2) stabilizes in the quintet state with a spin density of 2.4 on each vanadium atom. Hund's coupling plays a vital role in stabilizing the higher multiplets in case of titanium and vanadium clusters. In bigger clusters like Cp(3)Bz(2)M(4), Sc multidecker has one unpaired spin, Ti multidecker has five unpaired spins, and V multidecker has seven unpaired spins in total. Spin polarized electronic transport is found for all states of vanadium tripledecker and one state of the titanium tripledecker when connected to a gold two probe junction. Moderate to high-spin filter efficiencies are calculated for these states. Cp(2)BzSc(2) shows spin-independent electronic transport for all electronic states when introduced in the gold two probe junction. Current versus voltage curves are reported for selected clusters in the two probe setup.

Electron transport properties of atomic carbon nanowires between graphene electrodes

Long, stable, and free-standing linear atomic carbon wires (carbon chains) have been carved out from graphene recently [Meyer et al. Nature (London) 2008, 454, 319; Jin et al. Phys. Rev. Lett. 2009, 102, 205501]. They can be considered as extremely narrow graphene nanoribbons or extremely thin carbon nanotubes. It might even be possible to make use of high-strength and identical (without chirality) carbon wires as a transport channel or on-chip interconnects for field-effect transistors. Here we investigate electron transport properties of linear atomic carbon wire-graphene junctions by combining nonequilibrium Green's function with density functional theory. For short wires, linear ballistic transport is observed in wires consisting of odd numbers of carbon atoms but not in those consisting of even numbers of carbon atoms. For wires longer than 2.1 nm as fabricated above, however, the ballistic conductance of carbon wire-graphene junctions is independent of the structural distortion, structural imperfections, and hydrogen impurity adsorbed on the linear carbon wires, except for oxygen impurity adsorption under a low bias. As such, the epoxy groups might be the origin of experimentally observed low conductance in the carbon chain. Moreover, double-atomic carbon chains exhibit a negative differential resistance effect.