Carrier-Tunable Magnetic Ordering in Vanadium−Naphthalene Sandwich Nanowires

First-principles calculations of inorganic metallocene nanowires

Inspired by the recently synthesized inorganic metallocene derivatives Fe(P4)22-, we have identified four stable inorganic metallocene nanowires, MP4 (M = Sc, Ti, Cr and Fe) in configurations of either regular quadrangular prism (Q-type) or anticube (A-type), and further investigated their magnetic and electronic characteristics utilizing the first-principles calculation. It shows that CrP4 is a ferromagnetic metal, while other nanowires are semiconducting antiferromagnets with bandgaps of 0.44, 1.88, and 2.29 eV within the HSE06 level. It also shows that both ScP4 and TiP4 can be stabilized in the Q-type and A-type, corresponding to the antiferromagnetic and ferromagnetic ground states, respectively, indicating a configuration-dependent magnetism. The thermodynamic and lattice stabilities are confirmed by the ab initio molecular dynamics and phonon spectra. This study has unmasked the structural and physical properties of novel inorganic metallocene nanowires, and revealed their potential application in spintronics.

Magnetic Order, Electrical Doping, and Charge-State Coupling at Amphoteric Defect Sites in Mn-Doped 2D Semiconductors

Two-dimensional (2D) dilute magnetic semiconductors (DMSs) are attractive material platforms for applications in multifunctional nanospintronics due to the prospect of embedding controllable magnetic order within nanoscale semiconductors. Identifying candidate host material and dopant systems requires consideration of doping formation energies, magnetic ordering, and the tendency for dopants to form clustered domains. In this work, we consider the defect thermodynamics and the dilute magnetic properties across charge states of 2D-MoS₂ and 2D-WS₂ with Mn magnetic dopants as candidate systems for 2D-DMSs. Using hybrid density functional calculations, we study the magnetic and electronic properties of these systems across configurations with thermodynamically favorable defects: 2D-MoS₂ doped with Mn atoms at sulfur site (Mn_S), at two Mo sites (2Mn_Mo), on top of a Mo atom (Mn-top), and at a Mo site (Mn_Mo). While the majority of the Mn-defect complexes provide trap states, Mn_Mo and Mn_W are amphoteric, although previously predicted to be donor defects. The impact of cluster formation of these amphoteric defects on magnetic ordering is also considered; both Mn_Mo-Mn_Mo (2Mn_2Mo) and Mn_W-Mn_W (2Mn_2W) clusters are found to be stable in ferromagnetic (FM) ordering. Interestingly, we observed the defect charge state dependent magnetic behavior of 2Mn_2Mo and 2Mn_2W clusters in 2D-TMDs. We investigate that the FM coupling of 2Mn_2Mo and 2Mn_2W clusters is stable in only a neutral charge state; however, the antiferromagnetic (AFM) coupling is stable in the +1 charge state. 2Mn_2Mo clusters provide shallow donor levels in AFM coupling and deep donor levels in FM coupling. 2Mn_2W clusters lead to trap states in the FM and AFM coupling. We demonstrate the AFM to FM phase transition at a critical electron density n^c_e = 3.5 × 10¹³ cm^-2 in 2D-MoS₂ and 2D-WS₂. At a 1.85% concentration of Mn, we calculate the Curie temperature of 580 K in the mean-field approximation.

Effect of a uniform random external magnetic field with spatiotemporal variation on compensation in Ising spin-1/2 trilayered square ferrimagnets

Trilayered spin-1/2 Ising ferrimagnets are interesting thin systems for compensation phenomenon. In this work, a Metropolis Monte Carlo study is performed on the magnetic and thermodynamic response of such a system on square Bravais lattice, driven by uniform random external magnetic field with spatiotemporal variations. In two distinct configurations, the surface layers are made up of A and the middle layer is made up of B atoms in an ABA-type stacking while in AAB-type stacking, the top layer and the middle layer is made up of A atoms while the bottom layer is made up of B atoms. The magnetic coupling between the like atoms (A-A and B-B) is ferromagnetic while between the unlike atoms (A-B), it is antiferromagnetic. For the time-dependent external uniform random field, the mean is always set to zero and the standard deviation is varied until spin-field energy is comparable to the dominant cooperative energy of the system. The findings show that the observed compensation and critical points shift and steady-state magnetic behaviors shift among N, L, P, and Q, etc., type of ferrimagnetic behaviors, depending on the strength of external uniform random field. The compensation phenomenon even vanishes after crossing a finite threshold of standard deviation of the magnetic field for particular choices of the other controlling parameters. Thus islands of ferrimagnetic phase without compensation appear within the phase area with compensation of field-free case, in the two-dimensional Hamiltonian parameter space. For both the configurations, the areas of such islands even grow with increasing standard deviation of the external field, σ, obeying a scaling relation of the form: f(σ,A(σ))=σ^{-b}A(σ) with b_{ABA}=1.958±0.122 and b_{AAB}=1.783±0.118.

Metal cyclopropenylidene sandwich cluster and nanowire: structural, electronic, and magnetic properties

Organometallic sandwich clusters and nanowires can offer prototypes for molecular ferromagnet and nanoscale spintronic devices due to the strong coupling of local magnetic moments in the nanowires direction and experimental feasibility. Here, on the basis of first-principles calculations, we reportTM_n(c-C₃H₂)_n+1(TM= Ti, Mn;n= 1-4) sandwich clusters and 1D [TM(c-C₃H₂)]_∞sandwich nanowires building from transitional metal and the smallest aromatic carbene of cyclopropenylidene (c-C₃H₂). Based on the results of lattice dynamic and thermodynamic studies, we show that the magnetic moment of Mn_n(c-C₃H₂)_n+1clusters increases linearly with the number ofn, and 1D [Mn(c-C₃H₂)]_∞nanowire is a stable ferromagnetic semiconductor, which can be converted into half metal with carrier doping. In contrary, both Ti_n(c-C₃H₂)_n+1and 1D [Ti(c-C₃H₂)]_∞nanowire are nonmagnetic materials. This study reveals the potential application of the [TM(c-C₃H₂)]_∞nanowire in spintronics.

Stabilities, Electronic Structures, and Bonding Properties of 20-Electron Transition Metal Complexes (Cp)2TMO and their One-Dimensional Sandwich Molecular Wires (Cp = C5H5, C5(CH3)H4, C5(CH3)5; TM = Cr, Mo, W)

First-principles calculations have been carried out for the 20-electron transition metal complexes (Cp)₂TMO and their molecular wires (Cp = C₅H₅, C₅(CH₃)H₄, C₅(CH₃)₅; TM = Cr, Mo, W). The calculation results at the BP86/def2-TZVPP level reveal that the ground state is singlet and the optimized geometries are in good agreement with the experimental values. The analysis of frontier molecular orbitals shows that two electrons in the highest occupied molecular orbital HOMO-1 are mainly localized on cyclopentadienyl and oxygen ligands. Furthermore, the nature of the TM-O bond was investigated with the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV). The attraction term in the intrinsic interaction energies ΔE_int is mainly composed of two important parts, including electrostatic interaction (about 52% of the total attractive interactions ΔE_elstat + ΔE_orb) and orbital interaction, which might be the major determinant of the stability of these (Cp)₂TMO complexes. All of the TM-O bonds should be described as electron-sharing σ single bonds [(Cp)₂TM]⁺-[O]^- with the contribution of 53-57% of ΔE_orb and two π backdonations from the occupied p orbitals of oxygen ligands into vacant π* MOs of the [(Cp)₂TM]⁺ fragments, which are 35-40% of ΔE_orb. The results of bond order and interaction energy from EDA-NOCV calculations suggest the influence of the radius of TM and methyl in the interactions between TM and O in (Cp)₂TMO. Additionally, the relativistic effects slightly amplify the strength of bonding with increasing ΔE_orb for the EDA-NOCV calculations on three metal complexes (C₅H₅)₂TMO. Finally, the geometries, electronic structures, and magnetics of infinitely extended systems, [(C₅H₅)TMO]_∞, have also been explored. The results of the density of states (DOS) and band structure revealed that [(C₅H₅)CrO]_∞ and [(C₅H₅)WO]_∞ are semiconductors with the narrow bands, whereas [(C₅H₅)MoO]_∞ behaves as metal.

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.

Theoretical Study on Sandwich-Like Transition-Metal-Cyclooctatetraene Clusters and One-Dimensional Infinite Molecular Wires

Using density functional theory calculations, we investigated the structure and electronic properties of cyclooctatetraene (C8H8, COT)-ligand mono- or bi-transition-metal (M) sandwich clusters, M n (COT) n+1 (M = Sc, Ti, Cr, Mn, n = 1, 2) or (COT)M1(COT)M2(COT), as well as their one-dimensional infinite molecular wires. All the sandwich M-COT clusters and molecular wires are rather stable with their binding energies ranging from 3.20 to 7.48 eV per transition-metal atom. Superior to M n Bz n+1 complexes, most sandwich M-COT complexes are in their high spin states with ultrahigh magnetic moments. Moreover, one-dimensional infinite molecular wires, [Cr(COT)]∞, [(COT)V(COT)Ti]∞ and [(COT)Sc(COT)Cr]∞, are predicted to be ferromagnetic half-metals. Our findings suggest that such M-COT sandwich complexes may be potential candidates for applications in spintronics.

Buckling behaviors of single-walled carbon nanotubes inserted with a linear carbon-atom chain

Buckling behaviors of single-walled carbon nanotubes (SWCNTs) inserted with a linear carbon-atom chain (CAC) (the composite structures are also called carbon nanowires (CNWs)) under torsion and bending as well as compression are studied using molecular dynamics (MD) simulations, respectively. Our MD results show that the critical buckling angles (or strains) of CNWs under the three presented kinds of loading patterns can be two times those of corresponding independent SWCNTs for long CNWs, while the buckling improvement is not obvious for short ones. The main reason is that the radial van der Waals force between the CAC and the SWCNT is very small for a short CNW, while it increases with increasing length and then tends to a constant for a long CNW. The obtained MD results agree well with those from available theoretical models. These findings will be a great help towards understanding the stability and reliability of the special CNT structures, and designing flexible CNT-based devices.

A novel class of one-dimensional Ta4TMTe4 (TM = Cr, Fe, Co and Ni) compounds with strain-switched magnetic states

The development of one-dimensional (1D) nanowires with controllable magnetic properties is important for spintronic applications. Herein, we systematically investigated the structural, electronic, and magnetic properties of 1D transition-metal (TM) compounds (Ta₄TMTe₄, TM = Cr, Fe, Co and Ni) through density functional theory (DFT) calculations. Ta₄CrTe₄ and Ta₄FeTe₄ are non-magnetic, while Ta₄CoTe₄ and Ta₄NiTe₄ are predicted to have antiferromagnetic ground states. Interestingly, uniaxial strain can induce nonmagnetism-ferromagnetism and nonmagnetism-antiferromagnetism transitions in Ta₄CrTe₄ and Ta₄FeTe₄ nanowires, respectively. Moreover, the antiferromagnetic ground states in Ta₄CoTe₄ and Ta₄NiTe₄ can be switched to ferromagnetic states through moderate strain. These strain-dependent magnetic moment and magnetic coupling transitions are related to the redistribution of d states in TM atoms. Our findings suggest a new route for facilitating the design of nanoelectronics and spintronics.

The degree of π electron delocalization and the formation of 3D-extensible sandwich structures

DFT B3LYP/6-31G(d) calculations were performed to examine the feasibility of graphene-like C42H18 and starbenzene C6(BeH)6 (SBz) polymers as ligands of 3D-extensible sandwich compounds (3D-ESCs) with uninterrupted sandwich arrays. The results revealed that sandwich compounds with three or more C42H18 ligands were not feasible. The possible reason may be the localization of π electrons on certain C6 hexagons due to π-metal interactions, which makes the whole ligand lose its electronic structure basis (higher degree of π electron delocalization) to maintain the planar structure. For comparison, with the aid of benzene (Bz) molecules, the SBz polymers can be feasible ligands for designing 3D-ESCs because the C-Be interactions in individual SBz are largely ionic, which will deter the π electrons on one C6 ring from connecting to those on neighbouring C6 rings. This means that high degree of π electron delocalization is not necessary for maintaining the planarity of SBz polymers. Such a locally delocalized π electron structure is desirable for the ligands of 3D-ESCs. Remarkably, the formation of a sandwich compound with SBz is thermodynamically more favourable than that found for bis(Bz)chromium. The assembly of 3D-ESCs is largely exothermic, which will facilitate future experimental synthesis. The different variation trends on the HOMO-LUMO gaps in different directions (relative to the sandwich axes) suggest that they can be developed to form directional conductors or semiconductors, which may be useful in the production of electronic devices.

Robust half-metallicity and topological aspects in two-dimensional Cu-TPyB

Half-metallicity due to the coexistence of metallic nature for one spin component and insulating nature for the other is a base of spintronics devices, but was only achieved in few materials. From first-principles calculations, we demonstrate that a recently-synthesized two-dimensional organometallic framework of 1,3,5-tris(pyridyl)benzene and Cu atoms (Cu-TPyB) has robust half-metallicity. High electron velocity in one spin channel at Dirac point and a relatively large band gap in the other make the material meeting the demand of filtering the current into a single spin component. Moreover, spin-orbit coupling induces topologically nontrivial band gaps in the vicinity of the Fermi level, which are implementable for achieving quantum anomalous Hall effect in a low temperature range (<8 K).

Performance modulation of α-MnO₂ nanowires by crystal facet engineering

Modulation of material physical and chemical properties through selective surface engineering is currently one of the most active research fields, aimed at optimizing functional performance for applications. The activity of exposed crystal planes determines the catalytic, sensory, photocatalytic, and electrochemical behavior of a material. In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Herein, we demonstrate controllable magnetic modulation of α-MnO₂ nanowires, which displayed surface ferromagnetism or antiferromagnetism, depending on the exposed plane. First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO₂ (1 1 0) surfaces exhibit ferromagnetic ordering. The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets. Moreover, we approved that the facet engineering pave the way on designing semiconductors possessing unique properties for novel energy applications, owing to that the bandgap and the electronic transport of the semiconductor can be tailored via exposed surface modulations.

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.

Carrier induced magnetic coupling transitions in phthalocyanine-based organometallic sheet

A two-dimensional sheet with long range ferromagnetic (FM) order has been hotly pursued currently. The recent success in synthesizing polymerized Fe-phthalocyanine (poly-FePc) porous sheets paves a possible way to achieve this goal. However, the poly-FePc and its analog poly-CrPc structure are intrinsically antiferromagnetic (AFM). Using first principles combined with Monte-Carlo simulations, we study systematically the carrier-induced magnetic coupling transitions in poly-CrPc and poly-FePc sheets. We show that electron doping can induce stable FM states with Curie temperatures of 130-140 K, while hole doping will enhance the stability of the AFM states. Such changes in magnetic couplings depend on the balance of AFM superexchange and FM p-d exchange.

First-principles study of one-dimensional sandwich wires [(P)₅TM]∞ (TM = Ti, V, Cr, Mn, Fe, Co)

Since the discovery of ferrocene, many one-dimensional metallic sandwich molecular wires have been identified. However, most of the known systems are assembled from organic molecules. Suffering from many drawbacks has, however, hampered their widespread applications. With the goal of breaking this logjam, we provide a blueprint for the designing of a variety of novel sandwich molecular wires ([(P)5TM]∞, TM = Ti, V, Cr, Mn, Fe, and Co) assembled from ferrocene-like inorganic molecules (P)5TM, offering evidence of the existence of inorganic molecular wires in this class. We present first-principles calculations to investigate systematically the electronic and magnetic properties of such novel inorganic sandwich molecular wires. Compared with the organic molecular wires, all the inorganic [(P)5TM]∞ wires are of large magnetic moment. Among them, we find that [(P)5V]∞, [(P)5Cr]∞ and [(P)5Mn]∞ display ferromagnetic character, while for [(P)5Ti]∞, [(P)5Fe]∞ and [(P)5Co]∞, the magnetic coupling is antiferromagnetic. More remarkably, the TM atoms distributed in these wires show regular docking and lead to structures with ordered spin signals, which is a long-term dream of spintronics. We propose that the difference in magnetic coupling for the studied systems is related to the competition between two exchange interactions of TM atoms. Specifically, we propound that the general mechanism for the formation of stable 1D [(P)5TM]∞ involves the transfer of one electron from the TM atom to the P5 ligand forming [Formula: see text] and TM(+) alternating structure.

Structures and magnetism of multinuclear vanadium-pentacene sandwich clusters and their 1D molecular wires

Two types of multinuclear sandwich clusters, (V(3))(n)Pen(n+1), (V(4))(n)Pen(n+1) (Pen = Pentacene; n = 1, 2), and their corresponding infinite one-dimensional (1D) molecular wires ([V(3)Pen](∞), [V(4)Pen](∞)) are investigated theoretically, especially on their magnetic coupling mechanism. These sandwich clusters and molecular wires are found to be of high stability and exhibit intriguing magnetic properties. The intra-layered V atoms in (V(3))(n)Pen(n+1) clusters prefer antiferromagnetic (AFM) coupling, while they can be either ferromagnetic (FM) or AFM coupling in (V(4))(n)Pen(n+1) depending on the intra-layered V-V distances via direct exchange or superexchange mechanism. The inter-layered V atoms favor FM coupling in (V(3))(2)Pen(3), whereas they are AFM coupled in (V(4))(2)Pen(3). Such magnetic behaviors are the consequence of the competition between direct exchange and superexchange interactions among inter-layered V atoms. In contrast, the 1D molecular wires, [V(3)Pen](∞) and [V(4)Pen](∞), appear to be FM metallic with ultra high magnetic moments of 6.8 and 4.0 μ(B) per unit cell respectively, suggesting that they can be served as good candidates for molecular magnets.

Tetracene-doped anthracene nanowire arrays: preparation and doping effects

Large-scale tetracene-doped anthracene nanowire arrays were prepared, and the doping effects were studied. The high doping concentration up to 10% (molar ratio) has been achieved, attributed to both the unique long-nanowire geometry and the excellent structural compatibility of anthracene and tetracene. The incorporation of long tetracene molecules into the matrix of short anthracene molecules induced an enlarged interlayer thickness, a decreased nanowire thickness, and an expanded nanowire width. The tetracene molecules were homogeneously embedded into the anthracene matrix at low doping concentrations (<1%). The doping became inhomogeneous at high doping concentrations (≥1%). The energy transfer efficiency between anthracene and tetracene is nearly 100% at doping concentrations ≥1%.

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.