Electron Transport Calculations Research Articles

Conducting linear chains of sulphur inside carbon nanotubes

Despite extensive research for more than 200 years, the experimental isolation of monatomic sulphur chains, which are believed to exhibit a conducting character, has eluded scientists. Here we report the synthesis of a previously unobserved composite material of elemental sulphur, consisting of monatomic chains stabilized in the constraining volume of a carbon nanotube. This one-dimensional phase is confirmed by high-resolution transmission electron microscopy and synchrotron X-ray diffraction. Interestingly, these one-dimensional sulphur chains exhibit long domain sizes of up to 160 nm and high thermal stability (~800 K). Synchrotron X-ray diffraction shows a sharp structural transition of the one-dimensional sulphur occurring at ~450–650 K. Our observations, and corresponding electronic structure and quantum transport calculations, indicate the conducting character of the one-dimensional sulphur chains under ambient pressure. This is in stark contrast to bulk sulphur that needs ultrahigh pressures exceeding ~90 GPa to become metallic.

Effect of Anchoring Groups on Single Molecule Charge Transport through Porphyrins

Controlling charge transport through individual molecules and further understanding the effect of anchoring groups on charge transport are central themes in molecule-based devices. However, in most anchoring effect studies, only two, or at most three nonthiol anchoring groups were studied and compared for a specific system, i.e., using the same core structure. The scarcity of direct comparison data makes it difficult to draw unambiguous conclusions on the anchoring group effect. In this contribution, we focus on the single molecule conductance of porphyrins terminated with a range of anchoring groups: sulfonate (−SO3–), hydroxyl (−OH), nitrile (−CN), amine (−NH2), carboxylic acid (−COOH), benzyl (−C6H6), and pyridyl (−C6H5N). The present study represents a first attempt to investigate a broad series of anchoring groups in one specific molecule for a direct comparison. It also is the first attempt, to our knowledge, to explore single molecule conductivity with two novel anchoring groups sulfonate (−SO3–) and hydroxyl (−OH). Our experimental results reveal that the single molecule conductance values of the porphyrins follow the sequence of pyridyl > amine > sulfonate > nitrile > carboxylic acid. Electron transport calculations are in agreement that the pyridyl groups result in higher conductance values than the other groups, which is due to a stronger binding interaction of this group to the Au electrodes. The finding of a general trend in the effect of anchoring groups and the exploration of new anchoring groups reported in this paper may provide useful information for molecule-based devices, functional porphyrin design, and electron transfer/transport studies.

SU‐E‐T‐490: A Geant4 Implementation of the Generalized Boltzmann Fokker‐Plank Method

Purpose: A geant4 implementation of the Generalized Boltzmann Fokker‐Plank (GBFP) method, an advanced, computationally efficient Monte Carlo method for charged particle transport, is studied. Methods: The GBFP method is a systematic approach for extending the mean free path (MFP) of particles while exactly preserving moments of the true differential cross section (DCS) to some order, $N$, and approximating higher order moments by the exact moments of the approximate DCS. The approximate DCS preserves important physical quantities like mean scattering cosine, stopping power, and energy straggling. The GBFP method is a single event method, so transport mechanics are restored and step size and boundary crossing limitations inherent to condensed history methods are a non‐issue. A geant4 application with GBFP physics was developed for testing speed and accuracy of the GBFP method dose calculations. Results: Initial testing revealed that speed‐ups for the discrete GBFP model are consistent with previous work on the GBFP method. Computed dose profiles using the discrete GBFP method are in agreement with benchmarks within statistical uncertainty (in most cases < 1$\%) for sufficiently sampled regions. This indicates that the geant4 implementation was successful. A framework for further testing is now established. Conclusion: The GBFP method provides systematic control of calculation speed and accuracy for electron transport calculations. The GBFP method is single event Monte Carlo so pre‐existing production code transport algorithms are all that are needed for the implementation. Therefore, no major changes are necessary for incorporation of the GBFP method into production code systems.

Impact of finite temperatures and correlations on the anomalous Hall conductivity from ab initio theory

Finite-temperature effects in the first-principles calculations of electronic transport up to now include almost exclusively only electronic temperatures by means of the Fermi-distribution function neglecting the influence of lattice vibrations. Here, employing the linear response Kubo formalism as implemented in a fully relativistic multiple-scattering Korringa–Kohn–Rostoker Green function method a systematic first-principles study of the anomalous Hall conductivity (AHC) of the 3d-transition metals Fe, Co and Ni is presented. It is shown that the inclusion of both correlations and thermal lattice vibrations is needed to give a material-specific description of the AHC in transition metals. The employed general framework will allow a first-principles description of other transverse transport phenomena treating correlations, finite temperatures and disorder on the same footing, giving valuable insights for experiments.

Extended Hückel Theory for Carbon Nanotubes: Band Structure and Transport Properties

Extended Hückel theory (EHT) is a well established method for the description of the electronic structure of molecules and solids. In this article, we present a set of extended Hückel parameters for carbon nanotubes (CNTs), obtained by fitting the ab initio band structure of the (6,0) CNT. The new parameters are highly transferable to different types of CNTs. To demonstrate the versatility of the approach, we perform self-consistent EHT-based electron transport calculations for finite length CNTs with metal electrodes.

Dynamic and Electronic Transport Properties of DNA Translocation through Graphene Nanopores

Graphene layers have been targeted in the last years as excellent host materials for sensing a remarkable variety of gases and molecules. Such sensing abilities can also benefit other important scientific fields such as medicine and biology. This has automatically led scientists to probe graphene as a potential platform for sequencing DNA strands. In this work, we use robust numerical tools to model the dynamic and electronic properties of molecular sensor devices composed of a graphene nanopore through which DNA molecules are driven by external electric fields. We performed molecular dynamic simulations to determine the relation between the intensity of the electric field and the translocation time spent by the DNA to pass through the pore. Our results reveal that one can have extra control on the DNA passage when four additional graphene layers are deposited on the top of the main graphene platform containing the pore in a 2 × 2 grid arrangement. In addition to the dynamic analysis, we carried electronic transport calculations on realistic pore structures with diameters reaching nanometer scales. The transmission obtained along the graphene sensor at the Fermi level is affected by the presence of the DNA. However, it is rather hard to distinguish the respective nucleobases. This scenario can be significantly altered when the transport is conducted away from the Fermi level of the graphene platform. Under an energy shift, we observed that the graphene pore manifests selectiveness toward DNA nucleobases.

Negative differential resistance and spin polarization of metal-C6H6 multidecker complexes

We investigate the electronic properties of one-dimensional metal-C6H6 multidecker complexes: chromium(Cr)-benzene(Bz), benzene(Bz)-chromium(Cr)-benzene(Bz)-iron(Fe) wires and CrnBz2 (n = 1, 2, 3) and CrBz2Fem (m = 1, 2) clusters by applying nonequilibrium Green's functions combined with the density-functional theory. We predict that the ground state of the FeBzCrBzFe cluster is 100% spin-polarized. The density of states shows metallic for the spin-up electrons, while a semiconductor gap merges for the spin-down electrons (half-metallic). It is found that there is 92%–97% spin polarization of the electron transmission for the five sandwich molecular junctions constructed by the multilayer metal-benzene organometallic CrnBz2 (n = 1, 2, 3) and CrBz2Fem (m = 1, 2) clusters, which coupled to gold electrodes. In deed, these clusters will perform as nearly perfect spin-filters via the electron transport calculations. The current increases with the cluster length and depends on the bias and spin-up current-voltage characteristics for three kinds of sandwich molecular junctions (Bz2Cr, BzCrBzFe, and Bz2Cr3). It also shows robust negative differential resistance behavior, which is closely related with the evolution of the transmission spectrum under applied bias.

EfficientDFT+Ucalculations of ballistic electron transport: Application to Au monatomic chains with a CO impurity

An efficient method for computing the Landauer-Buettiker conductance of an open quantum system within DFT+U is presented. The Hubbard potential is included in electronic structure and transport calculations as a simple renormalization of the non-local pseudopotential coefficients by restricting the integration for the on-site occupations within the cutoff spheres of the pseudopotential. We apply the methodology to the case of an Au monatomic chain in presence of a CO molecule adsorbed on it. We show that the Hubbard U correction removes the spurious magnetization in the pristine Au chain at the equilibrium spacing, as well as the unphysical contribution of d electrons to the conductance, resulting in a single (spin-degenerate) transmission channel and a more realistic conductance of 1 G_0 . We find that the conductance reduction due to CO adsorption is much larger for the atop site than for the bridge site, so that the general picture of electron transport in stretched Au chains given by the local density approximation remains valid at the equilibrium Au-Au spacing within DFT+U.

Comparison of quantum mechanical methods for the simulation of electronic transport through carbon nanotubes

In the present work we study the electronic transport properties of finite length single-wall carbon nanotubes (CNTs) by comparing three different theoretical frameworks. A simple model is used to describe the electrodes and the way they are attached to both ends of the CNT. Electron transport calculations are carried out on three different levels of sophistication. That are the Landauer transport formalism in combination with single-orbital tight-binding, extended Hückel theory or density functional theory. The quantum mechanical transmission which plays a central role in Landauer theory is calculated by means of equilibrium and non-equilibrium Green’s function methods. Results of the three approaches are compared and discussed.

Electronic transport properties of atomic scale graphene/metal side contact

ABSTRACTThe transport properties of the atomic scale side contact between different metals (Au, Ag, Pt, Cu, Ni, Pd) and graphene with open zigzag ends have been studied from first-principles electron transport calculations. According to the contact configurations, we find the weakly interacting metals (Au, Ag, Pt and Cu) can form chemical bonds at the open graphene’s atomic edges, while the strongly interacting ones form chemical bonds in the whole contact region. Comparing with the case of end contact which could effectively decrease the contact resistance, the atomic scale side contact shows better transport properties than the end contact. And the graphene/metal side contact with hydrogen terminated graphene edge show obviously large resistance than the ones with open graphene edge, which signifies the importance of the termination of graphene edge in graphene/metal contact.

Current Rectification through π–π Stacking in Multilayered Donor–Acceptor Cyclophanes

Extended π-stacked molecules have attracted much attention since they play an essential role in both electronic devices and biological systems. In this article electron transport properties of a series of multilayered cyclophanes with the hydroquinone donor and quinone acceptor units in the external positions are theoretically studied with applications to molecular rectifiers in mind. Calculations of electron transport through the π–π stacked structures in the multilayered cyclophanes are performed by using nonequilibrium Green’s function method combined with density functional theory. Calculated transmission spectra show that the conductance decreases exponentially with the length of the molecule with a decay factor of 0.75 A–1, which lies for the values between π-conjugated molecules and σ-bonded molecules. Applied bias calculations provide current–voltage curves, which exhibit good rectifying behavior. The rectification mechanism in the coherent transport regime is qualitatively explained by the respon...

Coexistance of Giant Tunneling Electroresistance and Magnetoresistance in an All-Oxide Composite Magnetic Tunnel Junction

We propose, by performing advanced ab initio electron transport calculations, an all-oxide composite magnetic tunnel junction, within which both large tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) effects can coexist. The TMR originates from the symmetry-driven spin filtering provided by an insulating BaTiO(3) barrier to the electrons injected from the SrRuO(3) electrodes. Following recent theoretical suggestions, the TER effect is achieved by intercalating a thin insulating layer, here SrTiO(3), at one of the SrRuO(3)/BaTiO(3) interfaces. As the complex band structure of SrTiO(3) has the same symmetry as that of BaTiO(3), the inclusion of such an intercalated layer does not negatively alter the TMR and in fact increases it. Crucially, the magnitude of the TER also scales with the thickness of the SrTiO(3) layer. The SrTiO(3) thickness becomes then a single control parameter for both the TMR and the TER effect. This protocol offers a practical way to the fabrication of four-state memory cells.

Determination of Electron Collision of Cross-Sections for the H2Molecule for Plasma Discharge Simulation

Momentum transfer cross-section, rotational cross-sections, vibrational cross-sections, and electronic excitation cross-sections for H 2 molecule for plasma discharge simulation have been determined from measured and calculated electron transport coefficients in H 2 –Ar mixtures and in pure H 2 gas at 293 K using the electron swarm study and the two-term approximation Boltzmann solution for energy. The set of electron collisions cross-sections for H 2 molecule reproduced the experimental transport coefficients in pure H 2 gas and in 0.5% and 4% H 2 –Ar mixtures. The reproduced transport coefficients such as electron drift velocity ( W ), longitudinal diffusion coefficient per mobility ratio ( D L /µ), transverse diffusion coefficient per mobility ratio ( D T /µ) and ionization coefficient per density ratio (α/ N ) in pure H 2 gas and W, D T /µ in H 2 –Ar mixtures were within ±5% deviation from experimental results. The cross-section set also yielded drift velocity in 10.91% H 2 –He mixture within 0–2%.

Electron transport properties of carbon nanotube–graphene contacts

The properties of carbon nanotube-graphene junctions are investigated with first-principles electronic structure and electron transport calculations. Contact properties are found to be key factors in determining the performance of nanotube based electronic devices. In a typical single-walled carbon nanotube-metal junction, there is a p-type Schottky barrier of up to ∼0.4 eV which depends on the nanotube diameter. Calculations of the Schottky barrier height in carbon nanotube-graphene contacts indicate that low barriers of 0.09 eV and 0.04 eV are present in nanotube-graphene contacts ((8,0) and (10,0) nanotubes, respectively). Junctions with a finite contact region are investigated with simulations of the current-voltage characteristics. The results suggest the suitability of the junctions for applications and provide insight to explain recent experimental findings.

Birth of ball lightning

Many observations of ball lightning report a ball of light, about 10 cm in diameter, moving at about walking speed, lasting up to 20 s and frequently existing inside of houses and even aeroplanes. The present paper reports detailed observations of the initiation or birth of ball lightning. In two cases, navigation crew of aircraft saw ball lightning form at the windscreen inside the cockpit of their planes. In the first case, the ball lightning occurred during a thunderstorm, with much lightning activity outside of the plane. In the second case, large “horns” of electrical corona were seen outside of the plane at the surface of the radome, just prior to the formation of the ball lightning. A third case reports ball lightning formed inside of a house, during a thunderstorm, at a closed glass window. It is proposed, based on two‐dimensional calculations of electron and ion transport, that ball lightning in these cases is driven and formed by atmospheric ions impinging and collecting on the insulating surface of the glass or Perspex windows. This surface charge can produce electric fields inside of the cockpit or room sufficient to sustain an electric discharge. Charges of opposite sign to those outside of the window accumulate on the inside surface of the glass, leaving a ball of net charge moving inside of the cockpit or room to produce a pulsed discharge on a microsecond time scale.

Multi-band tight-binding calculation of electronic transport in Fe/trans-polyacetylene/Fe tunnel junctions

In this paper, the electronic transport characteristics of Fe/trans-polyacetylene/Fe magnetic tunnel junctions (MTJs) are investigated using multi-band tight-binding calculations within the framework of nonequilibrium Green function theory. A CH2 radical is added to different positions on the polymer chain and its effects on the tunnelling magnetoresistance of the MTJ are studied. The ferromagnetic electrodes are assumed to be single-band and their tight-binding parameters are chosen in such a way as to simulate the ab initio density functional calculations of the band structure of bcc-Fe along its [001] crystallographic direction. In building the Hamiltonian of the trans-polyacetylene (t-PA) chain, we have assumed an s orbital on the H atoms and one s and three p(px,py,pz) orbitals on the C atoms, and the dimerization effects are taken into account. It is found that moving the radical out of the centre of the polymer chain enhances the tunnelling magnetoresistance of the MTJ.

Effect of electron-electron interactions on the electronic structure and conductance of graphene nanoconstrictions

We present self-consistent calculations of electron transport in graphene nanoconstrictions within the Hartree approximation. We consider suspended armchair ribbons with V-shaped constrictions having perfect armchair or zigzag edges as well as mesoscopically smooth but atomically stepped constrictions with cosine profiles. Our calculations are based on a tight-binding model of the graphene and account for electron-electron interactions in both the constriction and the semi-infinite leads explicitly. We find that electron interactions result in the following. (i) Electrons accumulating along the edges of the uniform ribbon and along the zigzag and cosine constriction edges but not along armchair constriction edges. (ii) The first subband showing almost perfect transmittance due to localization at the uniform graphene boundary except at low energies for the cases of zigzag and cosine constrictions where Bloch stop-bands form in related periodic structures. (iii) The second subband being almost perfectly blocked by the constriction. (iv) Electron interactions favor intrasubband scattering while the noninteracting electron theory predicts the predominance of intersubband scattering. (v) Conductance quantization for the first few conductance steps being more pronounced for armchair constrictions but less so for zigzag constrictions. (vi) A much more prominent $2{e}^{2}/h$ conductance plateau for the cosine constriction than is found in the absence of electron interactions. Possible implications for recent experiments are briefly discussed.

DNA sequencing with nanopores from an ab initio perspective

Advances in materials research means that we find ourselves at the verge of constructing nano-scale devices capable of electrically addressing individual molecules in order to identify or utilize their electrical or electromechanical properties. An important application in life sciences would be electromechanical translocation of a DNA molecule through a nanopore, between nano-scale electrodes, allowing to electrically read out the base sequence (genome). This approach promises to drastically lower the cost per genome, allowing for extensive application in medical diagnostics. Owing to the involved extremely small dimensions which require nanometer-resolution in the fabrication, atomistic modeling plays a crucial role in testing hypothetical device architectures for their performance in nucleobase distinction. First-principles simulations are ideally suited to explore the interactions involved in such scenarios and lay the foundation for electronic transport calculations. This role of computations is even more important here, since it is experimentally not possible to observe directly the kinetics occurring during translocation of a DNA molecule through a nanopore. Here, we provide a brief review of the state of the field, focusing on ab initio studies of nanopore-based DNA sequencing, in particular on the promising recent development regarding graphene nanopores and nanogaps.

An Orbital Rule for Electron Transport in Molecules

The transfer of electrons in molecules and solids is an essential process both in biological systems and in electronic devices. Devices that take advantage of the unique electronic properties of a single molecule have attracted much attention, and applications of these devices include molecular wire, molecular memory, and molecular diodes. The so-called Landauer formula with Green's function techniques provides a basis for theoretical calculations of coherent electron transport in metal-molecule-metal junctions. We have developed a chemical way of thinking about electron transport in molecules in terms of frontier orbital theory. The phase and amplitude of the HOMO and LUMO of π-conjugated molecules determine the essential properties of their electron transport. By considering a close relationship between Green's function and the molecular orbital, we derived an orbital rule that would help our chemical understanding of the phenomenon. First, the sign of the product of the orbital coefficients at sites r and s in the HOMO should be different from the sign of the product of the orbital coefficients at sites r and s in the LUMO. Second, sites r and s in which the amplitude of the HOMO and LUMO is large should be connected. The derived rule allows us to predict essential electron transport properties, which significantly depend on the route of connection between a molecule and electrodes. Qualitative analyses of the site-dependent electron transport in naphthalene (as shown in the graphics) demonstrate that connections 1-4, 1-5, 2-3, and 2-6 are symmetry-allowed for electron transmission, while connections 1-8 and 2-7 are symmetry-forbidden. On the basis of orbital interaction analysis, we have extended this rule to metal-molecule-metal junctions of dithiol derivatives in which two gold electrodes have direct contacts with a molecule through two Au-S bonds. Recently we confirmed these theoretical predictions experimentally by using nanofabricated mechanically controllable break junctions to measure the single-molecule conductance of naphthalene dithiol derivatives. The measurement of the symmetry-allowed 1,4-naphthalene dithiol shows a single-molecule conductance that exceeds that of the symmetry-forbidden 2,7-naphthalene dithiol by 2 orders of magnitude. Because the HOMO and LUMO levels and the HOMO-LUMO gaps are similar in the derivatives, the difference in the measured molecular conductances arises from the difference in the phase relationship of the frontier orbitals. Thus, the phase, amplitude, and spatial distribution of the frontier orbitals provide a way to rationally control electron transport properties within and between molecules.

Electron Transport in SiGe Alloy Nanowires in the Ballistic Regime from First-Principles

Silicon-germanium alloying is emerging as one of the most promising strategies to engineer heat transport at the nanoscale. Here, we perform first-principles electron transport calculations to assess at what extent such approach can be followed without worsening the electrical conduction properties of the system, providing then a path toward high-efficiency thermoelectric materials.