Electron Transport Calculations Research Articles

σ Interference: Through-Space and Through-Bond Dichotomy.

Dividing orbital interactions into through-space (TS) and through-bond (TB) modes is valuable for understanding various molecular properties. In this paper, we elucidate how the quantum interference phenomenon known as σ interference in electron transport through σ systems arises from TS and TB interactions. We performed electron transport calculations using a combination of density functional theory and nonequilibrium Green's function methods, focusing on ethylenediamine, a classical molecule that effectively highlights the contrast between TS and TB interactions. Our results confirm that destructive σ interference occurs in the syn and gauche conformers of this molecule. To further investigate both TS and TB interactions, we employed two analytical methods: the fragment molecular orbital (FMO) method, which captures the effects of both TS and TB interactions, and the chemical graph theory method, which specializes in TB interactions. The FMO analysis demonstrated that TB interactions lead to the characteristic distribution and energy level alignment of the frontier orbitals. Additionally, it was clarified that a change in TS interaction, due to a variation in the dihedral angle of the molecule, alters the energy gap between these orbitals, resulting in the manifestation of σ interference in the syn and gauche conformers, but not in the trans conformer. The chemical graph theory analysis based on the ladder C model, aimed at exploring the topological origin of σ interference from the network of TB interactions, revealed that σ interference is caused by the cancellation between the walk associated with geminal interactions (σ-conjugation) and the one related to vicinal interaction (σ-hyperconjugation). Notably, it was found that the vicinal interaction, which changes sign with the dihedral angle, has a decisive influence on whether this cancellation occurs. These findings clarify that σ interference arises from the interplay between TS and TB interactions. This insight will be valuable for designing molecular systems that utilize σ interference.

Electron-Surface Scattering from First-Principles.

Electron-surface scattering is important for many transport phenomena and practical applications. Particularly, the downscaling of microelectronics demands higher electrical conductivity for interconnects, which are currently based on Cu, which suffers from strong surface scattering. However, much is still unclear, such as which surface orientation causes stronger scattering. Existing theories require phenomenological parameters whose values are unknown unless fitted to experimental data or based on assumptions, thereby limiting their accuracy and predictive power. Here we present an accurate, parameter-free approach that enables an accurate calculation of electronic transport with surface scattering. Then we apply it to study the conductivities of Cu films with different surface orientations. Contrary to the common belief that a more compact surface should have higher conductivity, we find that (111) is less conductive than (001). This can be explained by the symmetry of the electronic structure. Furthermore, we propose a phenomenological model that has a better fit to the first-principles results than the conventional one. Our work offers insights into electronic transport and enables accurate calculation, understanding, and prediction for a broad range of systems where surface scattering matters.

Effects of Graphene Doping on the Electrical Conductivity of Copper

AbstractThere is great interest in developing advanced electrical conductors with higher conductivity, lighter weight, and higher mechanical strength than copper (Cu). One promising candidate is copper‐graphene (Cu‐Gr) composite, which is hypothesized to have a higher electrical conductivity than Cu. In this work, it is shown that this is not true, supported by state‐of‐the‐art first‐principles calculations of electron transport. Particularly, contrary to the belief that graphene in the composite is more conductive than pristine Cu, it is less conductive due to increased scattering despite increased carrier concentration. On the other hand, it is found that compressive strain along the (111) plane increases the conductivity, which is confirmed experimentally, while tensile strain has little effect. The work offers new insights into understanding and developing advanced conductors.

Average-atom approach for transport properties of shocked argon in the presence of a magnetic field.

We present electron transport calculations of shocked argon based on an average-atom modeling of the plasma and compare them with measurements, involving both incident and reflected shock waves. Since the corresponding experiments are subject to a 5T magnetic field, the impact of the latter on the Rankine-Hugoniot equationsis taken into account, starting from the magnetoresistive hydrodynamics, and the resistivity tensor is deduced from the Boltzmann equation. The resistivity tensor yields the electrical and Hall resistivities. Our average-atom code Paradisio provides the quantities required for the calculation of electrical resistivity within the Ziman-Evans formalism, as well as for the Hall resistivity. We obtain good agreement between calculated conductivities and experimental values, both for the incident and reflected shocks. Our values of the Hall constant are compared to experimental values derived from Hall voltage measurements, as well as to theoretical ones based on the quantum statistical linear-relaxation-time approach.

Impact of Single-Melamine Tautomerization on the Excitation of Molecular Vibrations in Inelastic Electron Tunneling Spectroscopy.

Vibrational quanta of melamine and its tautomer are analyzed at the single-molecule level on Cu(100) with inelastic electron tunneling spectroscopy. The on-surface tautomerization gives rise to markedly different low-energy vibrational spectra of the isomers, as evidenced by a shift in mode energies and a variation in inelastic cross sections. Spatially resolved spectroscopy reveals the maximum signal strength on an orbital nodal plane, excluding resonant inelastic tunneling as the mechanism underlying the quantum excitations. Decreasing the probe-molecule separation down to the formation of a chemical bond between the melamine amino group and the Cu apex atom of the tip leads to a quenched vibrational spectrum with different excitation energies. Density functional and electron transport calculations reproduce the experimental findings and show that the shift in the quantum energies applies to internal molecular bending modes. The simulations moreover suggest that the bond formation represents an efficient manner of tautomerizing the molecule.

Engineering Anomalously Large Electron Transport in Topological Semimetals.

Anomalous transport of topological semimetals has generated significant interest for applications in optoelectronics, nanoscale devices, and interconnects. Understanding the origin of novel transport is crucial to engineering the desired material properties, yet their orders of magnitude higher transport than single-particle mobilities remain unexplained. This work demonstrates the dramatic mobility enhancements result from phonons primarily returning momentum to electrons due to phonon-electron dominating over phonon-phonon scattering. Proving this idea, proposed by Peierls in 1932, requires tuning electron and phonon dispersions without changing symmetry, topology, or disorder. This is achieved by combining de Haas - van Alphen (dHvA), electron transport, Raman scattering, and first-principles calculations in the topological semimetals MX2 (M = Nb, Ta and X = Ge, Si). Replacing Ge with Si brings the transport mobilities from an order magnitude larger than single particle ones to nearly balanced. This occurs without changing the crystal structure or topology and with small differences in disorder or Fermi surface. Simultaneously, Raman scattering and first-principles calculations establish phonon-electron dominated scattering only in the MGe2 compounds. Thus, this study proves that phonon-drag is crucial to the transport properties of topological semimetals and provides insight to engineer these materialsfurther.

Quantifying the Electron Energy of Mars Aurorae Through the Oxygen Emission Brightness Ratio at 130.4 and 135.6 nm

AbstractMars discrete aurorae are caused by accelerated electrons precipitating into the atmosphere and interacting with species such as atomic oxygen. However, the energy of the electrons causing these aurorae remains currently unclear: no simultaneous and concurrent measurements of electron analyzers and spectrometers have been performed so far, preventing from assessing the exact energy of the downgoing auroral electrons. Several auroral emissions have been observed so far on Mars, among which are two oxygen emissions in the far ultraviolet at 130.4 and 135.6 nm. In this study, we simulate the vertical distribution of these auroral oxygen emissions with an electron transport calculation coupled with a radiative transfer model to account for the optical thickness of the atmosphere for the 130.4‐nm triplet. We show that the brightness ratio of these oxygen emissions is independent of the downward electron energy flux and only slightly depends on the atomic oxygen atmospheric composition. In contrast, the brightness ratio is strongly related to the initial energy of the auroral electrons. Measuring the brightness ratio is therefore a unique tool to remotely estimate the energy of the electrons causing the Mars discrete aurorae. We compare our model results with observations from the Emirates Mars Ultraviolet Spectrometer on board the Emirates Mars Mission and find that electrons with typical energies of 250–300 eV are compatible with the observed ratio of 5.

Reflections on use of Monte Carlo methods

Monte Carlo methods have become ubiquitous in our society. Although statistical estimation methods date to earlier times, Monte Carlo (MC) methods were born in 1949 with publication of the paper by Metropolis and Ulam (1949). We address some of the many ways that MC methods have evolved over the years. Specifically, we consider electron transport calculations by MC, improvements in sampling efficiency afforded by the discrete alias sampling method, and solution of inverse problems by the Symbolic Monte Carlo (SMC) method. As a graduate student, one of us (WLD) used MC to estimate the improvement of cylindrical Geiger-Müller tube efficiency at low energies due to use of a thin coating of high atomic number material on the inside tube surface. Our methods were approximate but the results were reasonably good. We will discuss how discrete alias sampling—introduced in 1974—can improve the sampling efficiency significantly over simple table look-up. Finally, we consider an inverse problem in which the form of the governing probability density function is unknown but desired. We also briefly address matters of perspective on use of MC methods.N. Metropolis, S. Ulam, The Monte Carlo Method, 1949, J. Am. Stat. Assoc. 44, 335–341.

Highly parallelizable electronic transport calculations in periodic rhodium and copper nanostructures

We extend the highly-parallelizable open-source electronic transport code TRANSEC (Feldman et al 2014 Phys. Rev. B 90 035445; https://gitlab.com/computational-physics2/transec/) to perform real-space atomic-scale electronic transport calculations with periodic boundary conditions in the lateral dimensions. We demonstrate the use of TRANSEC in periodic Cu and Rh bulk structures and in large periodic Rh point contacts, in preparation to perform calculations of reflection probability across Rh grain boundaries.

Boron-doped graphene topological defects: unveiling high sensitivity to NO molecule for gas sensing applications.

Global air quality has deteriorated significantly in recent years due to large emissions from the transformation industry and combustion vehicles. This issue requires the development of portable, highly sensitive, and selective gas sensors. Nanostructured materials, including defective graphene, have emerged as promising candidates for such applications. In this work, we investigated the B-doped topological line defect in graphene as a sensing material for various gas molecules (CO, CO2, NO, and NH3) based on a combination of density functional theory and the non-equilibrium Green's function method. The electronic transport calculations reveal that the electric current can be confined to the line defect region by gate voltage control, revealing highly reactive sites. The B-doped topological line defect is metallic, favoring the adsorption of NO and NH3 over CO and CO2 molecules. We notice changes in the conductance after gas molecule adsorption, producing a sensitivity of 50% (16%) for NO (NH3). In addition, the recovery time for nitride gases was calculated for different temperatures and radiation frequencies. At 300 K the ultraviolet (UV) has a fast recovery time compared to the visible (VIS) one by about two orders of magnitude. This study gives an understanding of how engineering transport properties at the microscopic level (by topological line defect and chemical B-doping) leads to promising nanosensors for detecting nitride gas.

Two-dimensional half-metals MSi2N4 (M = Al, Ga, In, Tl) with intrinsic p-type ferromagnetism and ultrawide bandgaps.

Intrinsic half-metallic nanomaterials with 100% spin polarization are highly demanded for next-generation spintronic devices. Here, by using first-principles calculations, we have designed a class of new two-dimensional (2D) p-type half-metals, MSi2N4 (M = Al, Ga, In and Tl), which show high mechanical, thermal and dynamic stabilities. MSi2N4 not only have ultrawide electronic bandgaps for spin-up channels in the range of 4.05 to 6.82 eV but also have large half-metallic gaps in the range of 0.75 to 1.47 eV, which are large enough to prevent the spin-flip transition. The calculated magnetic moment is 1 μB per cell, resulting from polarized N1-px/py orbitals. Moreover, MSi2N4 possess robust long-range ferromagnetic orderings with Curie temperatures in the range of 35-140 K, originating from the interplay of N1-M-N1 superexchange interactions. Furthermore, spin dependent electronic transport calculations reveal 100% spin polarization. Our results highlight new promising 2D ferromagnetic half-metals toward future spintronic applications.

Enhanced tunneling electroresistance through interfacial charge-modulated barrier in α-In2Se3-based ferroelectric tunnel junction

The manipulation of tunneling resistance is critical for ferroelectric tunnel junction (FTJ) devices. In this work, we propose a mechanism to manipulate tunneling resistance through interfacial charge-modulated barrier in two-dimensional (2D) n-type semiconductor/ferroelectric FTJs. Driven by ferroelectric reversal, different effective tunneling barriers are realized by the depletion or accumulation of electrons near the n-type semiconductor surface in such devices. Thus, the tunneling resistance in FTJs undergoes significant changes for different polarization orientations, resulting in a giant tunneling electroresistance (TER) effect. To illustrate this idea, we construct 2D FTJs based on n-InSe/α-In2Se3 van der Waals (vdW) heterostructures. Based on the electronic transport calculations, it is found that TER ratio can reach 4.20 × 103% in the designed FTJs. The physical origin of the giant TER effect is verified through analysis of the effective potential energy of the n-InSe/α-In2Se3 vdW heterostructures and the real-space transmission eigenstates of the designed FTJs. This work contributes to the knowledge of carrier tunneling mechanisms at the interface of semiconductor/In2Se3 vdW heterostructures, and providing a significant insight into the TER effect of this FTJ systems, also presenting an alternative approach for the design of FTJ-based devices.

Hybrid Fluid Model for Calculating Electron Transport in Air in Strong Electric Fields

The hybrid fluid model for calculating electron transport in air is presented. Within the model, the transport of electrons with energies less than 300 eV is calculated based on the drift-diffusion equation, and in the high-energy range, it is calculated using a set of multigroup equations for the first two moments of the electron distribution function: the balance equations for concentrations and electron flux densities. The hybrid model presented is intended to be used for numerical simulations of electric discharge processes in air proceeding with the participation of runaway electrons. The results of numerical simulations of electron transport in air in homogeneous and inhomogeneous electric fields, which were performed using the hybrid model, were compared with the corresponding results of simulations obtained using the Monte Carlo method.

Hybrid Fluid Model for Calculating Electron Transport in Air in Strong Electric Fields

Hybrid Fluid Model for Calculating Electron Transport in Air in Strong Electric Fields

Manipulating Quantum Interference between σ and π Orbitals in Single-Molecule Junctions via Chemical Substitution and Environmental Control.

Understanding and manipulating quantum interference (QI) effects in single molecule junction conductance can enable the design of molecular-scale devices. Here we demonstrate QI between σ and π molecular orbitals in an ∼4 Å molecule, pyrazine, bridging source and drain electrodes. Using single molecule conductance measurements, first-principles analysis, and electronic transport calculations, we show that this phenomenon leads to distinct patterns of electron transport in nanoscale junctions, such as destructive interference through the para position of a six-membered ring. These QI effects can be tuned to allow conductance switching using environmental pH control. Our work lays out a conceptual framework for engineering QI features in short molecular systems through synthetic and external manipulation that tunes the energies and symmetries of the σ and π channels.

The physiological response of an Antarctic key phytoplankton species to low iron and manganese concentrations

AbstractIron (Fe) and manganese (Mn) availability and the divergent requirements of phytoplankton species were recently shown to be potential important drivers of Southern Ocean community composition. Knowledge about Antarctic phytoplankton species requirements for Fe and Mn remains, however, scarce. By performing laboratory experiments and additional calculations of the photosynthetic electron transport, we investigated the response of the ecologically important species Phaeocystis antarctica under a combination of different Fe and Mn concentrations. Fe deprivation alone provoked typical physiological characteristics of Fe limitation in P. antarctica (e.g., lowered growth and photosynthetic efficiency). In comparison, under Mn deprivation alone, the growth and carbon production of P. antarctica were not impacted. Its tolerance to cope with low Mn concentrations resulted from an efficient photoacclimation strategy, including a higher number of active photosystems II through which fewer electrons were transported. This strategy allowed us to maintain similar high growth and carbon production rates as FeMn‐enriched cells. Due to its low Mn requirement, P. antarctica performed physiologically as Fe‐deprived cells under the combined depletion of Fe and Mn. Hence, our study reveals that different from other Southern Ocean phytoplankton species, P. antarctica possesses a high capacity to cope with natural low Mn concentrations, which can facilitate its dominance over others, potentially explaining its ecological success across the Southern Ocean.

Calculating electron transport properties scattered by inhomogeneous distributed Aluminium Galium Arsenide material in cylindrical nanowire

In this paper, the R-matrix method and Wigner–Eisenbud functions (WEFs) are used to compute the transport properties of an electron scattered in a cylindrical nanowire doped with Aluminium Galium Arsenide (AlGaAs) in the scattering region. In these calculations, a Gaussian potential was used to represent the AlGaAs dopants. The nanowire is considered a closed system. By assuming quantum transport, the electron flows in an elastic process from source to drain. An electron with energy E coming from the source can either be reflected or transmitted to the drain. One-dimensional (1D) and two-dimensional (2D) nanowire calculations were performed. The transmission coefficients show an inverse relation with the depth of the Gaussian potential and depend on the incident energy of the electron.

Electron Tunneling at Electrocatalytic Interfaces

It was recently proposed that tunneling current fluctuations in electrochemical scanning tunneling microscopy (EC-STM) can be used to map the electrocatalytic activity of surfaces with high spatial resolution. However, the relation between the increased noise in the electron tunneling signal and the local reactivity for such complex electrode/electrolyte interfaces is only explained qualitatively or hypothetically. Herein, we employ electron transport calculations to examine tunneling at Pt surfaces under the conditions of the oxygen reduction reaction as a case study. By computing current–voltage characteristics, we reveal that the tunneling barrier strongly depends on the chemical identity of the adsorbed reaction intermediate as well as on the orientation of the average dipole moment of water species mediating electron tunneling. Our theoretical results combined with EC-STM measurements suggest that detecting reaction intermediates at electrified interfaces in operando conditions is possible based on tunneling noise amplitudes. This study also aims to stimulate further explorations of tunneling-based electron-proton transfers to enable quantum electrocatalysis beyond conventional approaches.

Analysis of interaction between dislocation and interface of aluminum matrix/second phase from electronic behavior

• Difficulty in dislocation movement at interface is studied by electron transport • Change in valence electrons density at α-Al/second phase interface is calculated • Differential charge density at α-Al/second phase interfaces is analyzed • Inhibitory effect of α-Al/second phase interface on dislocation were studied The inhibitory effect of the second phase on dislocation movement has long been deemed as a great contribution to the strengthening of alloys. We investigate the electronic behavior at the α-Al matrix/second phase interface to explore its inhibitory effect on dislocation movement. This work focuses on the difficulty in dislocation movement on the interface of α-Al/Al 3 Sc, α-Al/θ'(Al 2 Cu), and α-Al/T 1 (Al 2 CuLi) of aluminum-lithium-scandium alloy based on detailed transmission electron microscopy investigation and electron transport calculation. The more drastic the electron transport between two atoms at the interface, the more intense the interaction between them, corresponding to the larger difficulty in breaking and forming bonds between them during the movement process of the extra half plane of dislocation on the interface. The calculated difference in density of valence electrons and differential charge density at α-Al/second phase interface reveals that Al 3 Sc is characterized by the largest resistance to dislocation movement compared to θ'(Al 2 Cu) and T 1 (Al 2 CuLi). The large differential charge density between the interface of (1 0 0) Al3Sc /(1 0 0) Al demonstrates the strong bonds between α-Al and Al 3 Sc and the large difficulty for the extra half plane of dislocation to form or break bonds during the movement process at α-Al/Al 3 Sc interface. The dislocation pile-up indicates a discernible hindering effect of the α-Al/Al 3 Sc interface on dislocation movement. The hindering effect presented by α-Al/Al 3 Sc interface is favorable for the tensile strength.

Cu-Doped MoSi2 N4 Monolayer as a Potential NH3 Sensor.

Cu doped MoSi2 N4 monolayer (Cu-MoSi2 N4 ) was firstly proposed to analyze adsorption performances of common gas molecules including O2 , N2 , CO, NO, NO2 , CO2 , SO2 , H2 O, NH3 and CH4 via density functional theory (DFT) combining with non-equilibrium Green's function (NEGF). The electronic transport calculations indicate that Cu-MoSi2 N4 monolayer has high sensitivity for CO, NO, NO2 and NH3 molecules. However, only NH3 molecule adsorbs on the Cu-MoSi2 N4 monolayer with moderate strength (-0.55 eV) and desorbs at room temperature (2.36×10-3 s). Thus, Cu-MoSi2 N4 monolayer is demonstrated as a potential NH3 sensor.