The spin transport properties of a series of 3d transition metal(ii) phthalocyanines (MPc, M = Mn, Fe, Co, Ni, Cu and Zn) sandwiched between two semi-infinite armchair single-walled carbon nanotube electrodes are investigated by using a self-consistent ab initio approach that combines the non-equilibrium Green's function formalism with spin density functional theory. Our calculations show that among the six molecules only MnPc and FePc can act as nearly perfect spin filters and at the same time have a large transmission around the Fermi level. This is dominated by the highest occupied molecular orbital (HOMO) of the corresponding MPc molecule. In contrast to the other four MPc molecules, whose HOMO is the a(1u) orbital located over the Pc ring, the HOMO of MnPc and FePc is a doubly degenerate pi-type orbital composed of the 3d(xz) and 3d(yz) atomic orbitals of the metal center. The spin polarization of MnPc and FePc is independent of the size of the SWCNT electrodes and can be tuned by chemisorption at the metal center, demonstrating that MPc and carbon nanotubes are a promising materials platform for applications in molecular spintronics.
We present an efficient method for evaluating current-induced forces in nanoscale junctions, which naturally integrates into the non-equilibrium Green's function formalism implemented within density functional theory. This allows us to perform dynamical atomic relaxation in the presence of an electric current while also evaluating the current-voltage characteristics. The central idea consists in expressing the system energy density matrix in terms of Green's functions. In order to validate our implementation we perform a series of benchmark calculations, both at zero and finite bias. Firstly we evaluate the current-induced forces acting over an Al nanowire and compare them with previously published results for fixed geometries. Then we perform structural relaxation of the same wires under bias and determine the critical voltage at which they break. We find that, while a perfectly straight wire does not break at any of the voltages considered, a zigzag wire is more fragile and snaps at 1.4 V, with the Al atoms moving against the electron flow. The critical current density for the rupture is estimated to be 9.6×10 10 A/cm 2 , in good agreement with the experimentally measured value of 5×10 10 A/cm 2 . Finally we demonstrate the capability of our scheme to tackle the electromigration problem by studying the current-induced motion of a single Si atom covalently attached to the sidewall of a (4,4) armchair single-walled carbon nanotube. Our calculations indicate that if Si is attached along the current path, then current-induced forces can induce migration. In contrast, if the bonding site is away from the current path, then the adatom will remain stable regardless of the voltage. An analysis based on decomposing the total force into a wind and an electrostatic component, as well as on a detailed evaluation of the bond currents, shows that this remarkable electromigration phenomenon is due solely to the position-dependent wind force.
The electronic transport properties of a single benzene molecule connected to gold and platinum electrodes through the direct Au-C or Pt-C bond are investigated by using a self-consistent ab initio approach that combines the non-equilibrium Green's function (NEGF) formalism with density functional theory (DFT). Our calculations show that the benzene molecule can bind to the Au(111) surface via direct Au-C bond at the adatom, atop and bridge sites. The largest zero-bias conductance is calculated for the bridge site but it is only G = 0.37G(0) (G(0) = 2e(2)/h). In contrast benzene binds to the Pt(111) surface via direct Pt-C bond only at the adatom and atop sites. When the binding site is the adatom a stable molecular junction forms with a zero-bias conductance as large as 1.15G(0). This originates from the efficient coupling between the extended π-type highest occupied molecular orbital of benzene and the conducting states of the Pt electrodes via the 5d(xz) atomic orbital of the adatoms. The calculated transmission is robust to the choice of DFT functionals, illustrating the potential of the Pt-C bond for constructing future molecular electronic devices.
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