Publications (18)18.72 Total impact
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Article: Protocol for classical molecular dynamics simulations of nano-junctions in solution
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ABSTRACT: Modeling of nanoscale electronic devices in water requires the evaluation of the transport properties averaged over the possible configurations of the solvent. They can be obtained from classical molecular dynamics for water confined in the device. A series of classical molecular dynamics simulations is performed to establish a methodology for estimating the average number of water molecules N confined between two static and semi-infinite gold electrodes. Variations in key parameters of the simulations, as well as simulations with non-static infinite gold surfaces of constant area and with anisotropically fluctuating cell dimensions lead to less than 1% discrepancies in the calculated N. Our approach is then applied to a carbon nanotube placed between the gold electrodes. The atomic density profile along the axis separating the slabs shows the typical pattern of confined liquids, irrespective of the presence of the nanotube, while parallel to the slabs the nanotube perturbs the obtained profile.Journal of Applied Physics. 10/2012; 112(8):083714. -
Article: First-principles study of high conductance DNA sequencing with carbon nanotube electrodes
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ABSTRACT: Rapid and cost-effective DNA sequencing at the single nucleotide level might be achieved by measuring a transverse electronic current as single-stranded DNA is pulled through a nano-sized pore. In order to enhance the electronic coupling between the nucleotides and the electrodes and hence the current signals, we employ a pair of single-walled close-ended (6,6) carbon nanotubes (CNTs) as electrodes. We then investigate the electron transport properties of nucleotides sandwiched between such electrodes by using first-principles quantum transport theory. In particular we consider the extreme case where the separation between the electrodes is the smallest possible that still allows the DNA translocation. The benzene-like ring at the end cap of the CNT can strongly couple with the nucleobases and therefore both reduce conformational fluctuations and significantly improve the conductance. The optimal molecular configurations, at which the nucleotides strongly couple to the CNTs, and which yield the largest transmission, are first identified. Then the electronic structures and the electron transport of these optimal configurations are analyzed. The typical tunneling currents are of the order of 50 nA for voltages up to 1 V. At higher bias, where resonant transport through the molecular states is possible, the current is of the order of several $\mu$A. Below 1 V the currents associated to the different nucleotides are consistently distinguishable, with adenine having the largest current, guanine the second-largest, cytosine the third and finally thymine the smallest. We further calculate the transmission coefficient profiles as the nucleotides are dragged along the DNA translocation path and investigate the effects of configurational variations. Based on these results we propose a DNA sequencing protocol combining three possible data analysis strategies.09/2011; -
Article: Spin transport in higher n-acene molecules
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ABSTRACT: We investigate the spin transport properties of molecules belonging to the acenes series by using density functional theory combined with the non-equilibrium Green's function approach to electronic transport. While short acenes are found to be non-magnetic, molecules comprising more than nine acene rings have a spin-polarized ground state. In their gas phase these have a singlet total spin configuration, since the two unpaired electrons occupying the doubly degenerate highest molecular orbital are antiferromagnetically coupled to each other. Such an orbital degeneracy is however lifted once the molecule is attached asymmetrically to Au electrodes via thiol linkers, leading to a fractional magnetic moment. In this situation the system Au/n-acene/Au can act as an efficient spin-filter with interesting applications in the emerging field of organic spintronics.01/2011; -
Article: Computational modeling of a carbon nanotube-based DNA nanosensor.
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ABSTRACT: During the last decade the design of biosensors, based on quantum transport in one-dimensional nanostructures, has developed as an active area of research. Here we investigate the sensing capabilities of a DNA nanosensor, designed as a semiconductor single walled carbon nanotube (SWCNT) connected to two gold electrodes and functionalized with a DNA strand acting as a bio-receptor probe. In particular, we have considered both covalent and non-covalent bonding between the DNA probe and the SWCNT. The optimized atomic structure of the sensor is computed both before and after the receptor attaches itself to the target, which consists of another DNA strand. The sensor's electrical conductance and transmission coefficients are calculated at the equilibrium geometries via the non-equilibrium Green's function scheme combined with the density functional theory in the linear response limit. We demonstrate a sensing efficiency of 70% for the covalently bonded bio-receptor probe, which drops to about 19% for the non-covalently bonded one. These results suggest that a SWCNT may be a promising candidate for a bio-molecular FET sensor.Nanotechnology 11/2010; 21(44):445501. · 3.98 Impact Factor -
Article: Quantum conductance of a single magnetic atom: An ab initio study
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ABSTRACT: Our ab initio study explains recent puzzling experiments on the conductance of a single Co adatom deposited either on Cu(111) or on ferromagnetic Co islands and contacted with both magnetic and nonmagnetic electrodes [ N. Néel, J. Kröger and R. Berndt Phys. Rev. Lett. 102 086805 (2009)]. We provide clear evidence that the conductance of a single atomic junction in the contact regime is close to G0/2 (G0 is the quantum of conductance) for ferromagnetic electrodes and to G0 for nonmagnetic ones. Spin-dependent calculations reveal that a conductance of G0/2 originates from a combination of partially open majority and minority channels. The bonding between the Co adatom and the Co island reduces significantly the contribution of the minority d electrons to the conductance, leading to the observation of half-integer conductance.Phys. Rev. B. 08/2010; 82(8). -
Article: Ab initio study of electron transport in dry poly(G)-poly(C) A-DNA strands
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ABSTRACT: The bias-dependent transport properties of short poly(G)-poly(C) A-DNA strands attached to Au electrodes are investigated with first principles electronic transport methods. By using the non- equilibrium Green's function approach combined with self-interaction corrected density functional theory, we calculate the fully self-consistent coherent I-V curve of various double-strand polymeric DNA fragments. We show that electronic wave-function localization, induced either by the native electrical dipole and/or by the electrostatic disorder originating from the first few water solvation layers, drastically suppresses the magnitude of the elastic conductance of A-DNA oligonucleotides. We then argue that electron transport through DNA is the result of sequence-specific short-range tunneling across a few bases combined with general diffusive/inelastic processes. Comment: 15 pages, 13 figures, 1 table06/2010; -
Article: Finite-bias electronic transport of molecules in water solution
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ABSTRACT: The effects of water wetting conditions on the transport properties of molecular nano-junctions are investigated theoretically by using a combination of classical molecular dynamics and first principles electronic transport calculations. These are at the level of the non-equilibrium Green's function method implemented for self-interaction corrected density functional theory. We find that water effectively produces electrostatic gating to the molecular junction, with a gating potential determined by the time-averaged water dipole field. Such a field is large for the polar benzene-dithiol molecule, resulting in a transmission spectrum shifted by about 0.6 eV with respect to that of the dry junction. The situation is drastically different for carbon nanotubes (CNTs). In fact, because of their hydro-phobic nature the gating is almost negligible, so that the average transmission spectrum of wet Au/CNT/Au junctions is essentially the same as that in dry conditions. This suggests that CNTs can be used as molecular interconnects also in water-wet situations, for instance as tips for scanning tunnel microscopy in solution or in biological sensors.02/2010; -
Article: Ab initio calculation of the bias-dependent transport properties of Mn_ {12} molecules
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ABSTRACT: The bias-dependent transport properties of a device constructed by sandwiching a Mn12 single molecule magnet between gold electrodes are investigated within an ab initio framework combining the nonequilibrium Green’s function approach with density-functional theory. The self-consistently calculated current-voltage, I-V, curves exhibit characteristic negative differential resistances. These originate from the interplay between electron localization and the rehybridization of the Mn12 molecular levels in an external electric field. Interestingly, such features in the transport are sensitive to the internal spin configuration of the molecule. This may therefore enable one to infer the internal spin state of the molecule from a detailed knowledge of the I-V.Phys. Rev. B. 09/2009; 80(10). -
Article: Switching a single spin on metal surfaces by a STM Tip: Ab Initio studies.
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ABSTRACT: The exchange coupling between single 3d magnetic adatoms (Cr, Mn, Fe, and Co) adsorbed on a Cu(001) surface and a Cr STM tip is studied with ab initio calculations. We demonstrate that the spin direction of single adatoms can be controlled by varying the tip-substrate distance, and the sign of the exchange energy is determined by the competition of the direct and the indirect interactions between the tip and the adatom. Based on the spin-dependent transport calculations, we find a magnetoresistance of about 70% at short tip-substrate distances.Physical Review Letters 07/2009; 103(5):057202. · 7.37 Impact Factor -
Article: Magnetic state electrical readout of Mn12 molecules
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ABSTRACT: We demonstrate that the different magnetic states of a Mn12 molecule can be distinguished in a two-probe transport experiment from a complete knowledge of the current-voltage curve. Our results, obtained with state-of-the-art non-equilibrium transport methods combined with density functional theory, demonstrate that spin configuration-specific negative differential resistances (NDRs) appear in the I-V curves. These originate from the interplay between electron localization and the re-hybridization of the molecular levels in an external electric field and allow the detection of the molecule's spin-state. Comment: 8 pages, 8 figures, 2 page supplementary material05/2009; -
Article: Structural relaxation effects on interface and transport properties of Fe/MgO(001) tunnel junctions
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ABSTRACT: The interface structure of Fe/MgO(100) magnetic tunnel junctions predicted by density functional theory (DFT) depends significantly on the choice of exchange and correlation functional. Bader analysis reveals that structures obtained by relaxing the cell with the local spin-density approximation (LSDA) display a different charge transfer than those relaxed with the generalized gradient approximation (GGA). As a consequence, the electronic transport is found to be extremely sensitive to the interface structure. In particular, the conductance for the LSDA-relaxed geometry is about one order of magnitude smaller than that of the GGA-relaxed one. The high sensitivity of the electronic current to the details of the interface might explain the discrepancy between the experimental and calculated values of magnetoresistance.12/2008; -
Article: I-V curves of Fe/MgO (001) single- and double-barrier tunnel junctions
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ABSTRACT: In this work, we calculate with ab initio methods the current-voltage characteristics for ideal single- and double-barrier Fe/MgO (001) magnetic tunnel junctions. The current is calculated in the phase-coherent limit by using the recently developed SMEAGOL code, combining the nonequilibrium Green function formalism with density-functional theory. In general we find that double-barrier junctions display a larger magnetoresistance, which decays with bias at a slower pace than their single-barrier counterparts. This is explained in terms of enhanced spin filtering from the middle Fe layer sandwiched in between the two MgO barriers. In addition, for double-barrier tunnel junctions, we find a well defined peak in the magnetoresistance at a voltage of V=0.1 V. This is the signature of resonant tunneling across a majority quantum well state. Our findings are discussed in relation to recent experiments.08/2008; -
Article: Lattice distortion effects on the magnetostructural phase transition of MnAs.
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ABSTRACT: We present a systematic experimental and theoretical study of the first-order phase transition of epitaxially grown MnAs thin films under biaxial tensile stress. Our results give direct information on the dependence of the phase-transition temperature of MnAs films on the lattice parameters. We demonstrate that an increase of the lattice constant in the hexagonal plane raises the phase-transition temperature (T(p)), while an increase of the perpendicular lattice constant lowers T(p). The results of calculations based on density functional theory are in good agreement with the experimental ones. Our findings open exciting prospects for magneto-mechanical devices, where the critical temperature for ferromagnetism can be engineered by external stress.Physical Review Letters 09/2005; 95(7):077203. · 7.37 Impact Factor -
Article: Switching a Single Spin on Metal Surfaces by a STM Tip: Ab Initio Studies
PHYSICAL REVIEW LETTERS. 103(5). -
Article: Interface and transport properties of Fe/V/MgO/Fe and Fe/V/Fe/MgO/Fe magnetic tunneling junctions
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ABSTRACT: The interface and transport properties of Fe/V/MgO/Fe and Fe/V/Fe/MgO/Fe magnetic tunneling junctions are investigated by using density-functional theory and nonequilibrium Green’s-function methods. Bader analysis reveals that the Fe layer at the interface with MgO loses a small amount of charge with respect to its bulk value, in contrast to a previous study [ W. H. Butler, X.-G. Zhang, T. C. Schulthess and J. M. MacLaren Phys Rev. B 63 054416 (2001)]. At the same time at the Fe/V interface a magnetic moment is induced on the V layer by proximity. Importantly, the direction of the magnetization of the first V monolayer immediately close to the MgO barrier oscillates with the total V-layer thickness and the relative stability of a particular magnetic configuration weakens as such a thickness is increased. These two aspects pose a challenge to the Fe/V/MgO/Fe device signal stability. A more intriguing situation is found for Fe/V/Fe/MgO/Fe junctions. Their transport properties depend sensitively on the thickness of the Fe layer intercalated between V and the MgO barrier. This is the result of resonances through quantum-well states of Δ1 symmetry localized in the intercalated Fe layer. In particular, for some geometries we find a massive magnetoresistance obtained by simply switching the direction of the magnetization of the Fe interlayer, while keeping the direction of the electrodes fixed. This effect may be employed in the design of new spin valves with extremely high spin polarization but still relatively large current densities.Phys. Rev. B. 79(21). -
Article: Spectroscopic characterization of a single dangling bond on a bare Si (100)-c (4× 2) surface for n-and p-type doping
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ABSTRACT: We investigate the charging state of an isolated single dangling bond formed on an unpassivated Si(100) surface with c(4×2) reconstruction, by comparing scanning tunneling microscopy and spectroscopy analysis with density functional theory calculations. The dangling bond is created by placing a single hydrogen atom on the bare surface with the tip of a scanning tunneling microscope. The H atom passivates one of the dimer dangling bonds responsible for the surface one-dimensional electronic structure. This leaves a second dangling at the reacted surface dimer which breaks the surface periodicity. We consider two possible H adsorption configurations for both the neutral and the doped situation (n- and p-type). In the case of n-doping we find that the single dangling bond state is doubly occupied and the most stable configuration is that with H bonded to the bottom Si atom of the surface dimer. In the case of p-doping the dangling bond is instead empty and the configuration with the H attached to the top atom of the dimer is the most stable. Importantly the two configurations have different scattering properties and phase shift fingerprints. This might open up interesting perspectives for fabricating a switching device by tuning the doping level or by locally charging the single dangling bond state.Phys. Rev. B. 86(3). -
Article: Charge transport through O-deficient Au-MgO-Au junctions
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ABSTRACT: Metal-oxide heterostructures have been attracting considerable attention in recent years due to various technological applications. We present results of electronic structure and transport calculations for the Au-MgO-Au (metal-insulator-metal) heterostructure based on density-functional theory and the nonequilibrium Green’s functions method. The dependence of the conductance of the heterostructure on the thickness of the MgO interlayer and the interface spacing is studied. In addition, we address the effects of O vacancies. We observe deviations from an exponentially suppressed conductance with growing interlayer thickness caused by Au-O chemical bonds. Electronic states tracing back to O vacancies can increase the conductance. Furthermore, this effect can be enhanced by enlarging the interface spacing as the vacancy induced Mg states are shifted toward the Fermi energy.Phys. Rev. B. 80(23). -
Article: Effects induced by single and multiple dopants on the transport properties in zigzag-edged graphene nanoribbons
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ABSTRACT: The effects of boron and nitrogen doping on the transport properties of zigzag-edged graphene nanoribbons ZGNRs with antiferromagnetically coupled edge states are investigated by first-principles electronic structure combined with a nonequilibrium Green's function technique. Specifically, the effects produced by single and multiple impurities as a function of their distance from the edges are analyzed. It is found that the introduction of single B or N atoms induces bound states and quasibound states in ZGNRs, which can be observed as dips or peaks in the electron transmission function. In particular, the transmission channel associated to the edge states is strongly suppressed when the impurities are close to the edges. Multiple impurities in general interfere and modify further the transmission function. However, if the impurities are placed at positions such that the associated bound and quasibound states appear at opposite sides of the Fermi level, then the transmission can be rationalized as a simple superposition of the transmission function of individually doped ribbons. Finally, an interesting situation appears for B and N codoping, since fully spin-polarized transmission peaks are generated at energies corresponding to the ribbon gap. This offers the hope of using such nanoribbons for low-bias spin-polarized tunneling in spintronics applications.8025.
Top co-authors
Top Journals
Institutions
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2008–2011
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Trinity College Dublin
- School of Physics
Dublin, L, Ireland (Republic of Ireland)
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2010
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Amirkabir University of Technology
- Department of Mechanical Engineering
Tehrān, Ostan-e Tehran, Iran
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2005
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Universidade Estadual de Campinas
- Instituto de Física "Gleb Wataghin" (IFGW)
Campinas, Estado de Sao Paulo, Brazil
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