Tunable electronic and optoelectronic characteristics of two-dimensional g-GeC monolayer: a first-principles study.

Two-dimensional (2D) semiconductor materials have emerged as one of the hotspots in recent years due to their potential applications in beyond-Moore technologies. In this work, we systematically investigate the electronic and optoelectronic properties of the g-GeC monolayer combined with strain engineering using first-principles calculations. The results show that g-GeC monolayer possesses a suitable direct bandgap and a strain-tunable electronic structure.

Tunable electronic and optoelectronic characteristics of two-dimensional β-AsP monolayer: a first-principles study.

Two-dimensional (2D) semiconductors have attracted a great deal of interest from the electrical engineering community due to their intriguing electronic properties. In this study, we have systematically investigated the electronic and optoelectronic properties of β-AsP monolayers by first-principles calculations combined with strain engineering. The results show that the β-AsP monolayer has a suitable indirect bandgap and a strain-tunable electronic structure.

Electronic and Transport Engineering of A-Type Antiferromagnets with Ferroelectric Sandwich Structure: Toward Multistate Nonvolatile Memory Applications.

Achieving higher-order multistates with mutual interstate switching at the nanoscale is essential for high-density storage devices; yet, it remains a significant challenge. Here, we demonstrate that integrating A-type antiferromagnetic semiconductors sandwiched between ferroelectric layers is an effective strategy to achieve high-performance multistate data storage. Taking the ScCO/VSiP bilayer (bi-VSiP)/ScCO van der Waals multiferroic heterostructure as an example, our first-principles calculations show that by switching the polarization direction of the upper and bottom ferroelectric ScCO layers, antiferromagnetic bi-VSiP can exhibit four distinct states with different band structures.

Substitutional doping of MoTe/ZrS heterostructures for sustainable energy related applications.

Stacking and/or substitutional doping are effective strategies to tune two-dimensional materials with desired properties, greatly extending the applications of the pristine materials. Here, by employing first-principles calculations, we propose that a pristine MoTe/ZrS heterostructure is a distinguished lithium-ion battery anode material with a low Li diffusion barrier (∼0.26 eV), and it has a high maximum Li storage capacity (476.

Enhanced thermoelectric performance in Sb-Br codoped BiSe with complex electronic structure and chemical bond softening.

Prior experimental work showed that BiSe, as a sister compound of the best room-temperature thermoelectric material BiTe, has remarkably improved thermoelectric performance by Sb-Br codoping. But the relationship between its crystalline structure and thermoelectric properties is still unclear to date. Here, we use first-principles calculations to explore the possible reasons for such improvement.

First-principles investigation of the ferroelectric, piezoelectric and nonlinear optical properties of LiNbO-type ZnTiO.

The newly synthesized LN-type ZnTiO (J. Am. Chem.

Strain-Tunable Electronic Properties and Band Alignments in GaTe/CN Heterostructure: a First-Principles Calculation.

Recently, GaTe and CN monolayers have been successfully synthesized and show fascinating electronic and optical properties. Such hybrid of GaTe with CN may induce new novel physical properties. In this work, we perform ab initio simulations on the structural, electronic, and optical properties of the GaTe/CN van der Waals (vdW) heterostructure.

Electric field control of magnetization direction across the antiferromagnetic to ferromagnetic transition.

Electric-field-induced magnetic switching can lead to a new paradigm of ultra-low power nonvolatile magnetoelectric random access memory (MeRAM). To date the realization of MeRAM relies primarily on ferromagnetic (FM) based heterostructures which exhibit low voltage-controlled magnetic anisotropy (VCMA) efficiency. On the other hand, manipulation of magnetism in antiferromagnetic (AFM) based nanojunctions by purely electric field means (rather than E-field induced strain) remains unexplored thus far.

First-principles prediction of a rising star of solar energy material: SrTcO.

SrTcO as a new star of solar energy material is investigated in terms of its band gap evolution with biaxial strain from first-principles calculations. Compared to the theoretical equilibrium lattice constant a(b) of bulk SrTcO, a set of lattice constants with a deviation of -8.75% to +3.

Highly tunable spin-dependent electron transport through carbon atomic chains connecting two zigzag graphene nanoribbons.

Motivated by recent experiments of successfully carving out stable carbon atomic chains from graphene, we investigate a device structure of a carbon chain connecting two zigzag graphene nanoribbons with highly tunable spin-dependent transport properties. Our calculation based on the non-equilibrium Green's function approach combined with the density functional theory shows that the transport behavior is sensitive to the spin configuration of the leads and the bridge position in the gap. A bridge in the middle gives an overall good coupling except for around the Fermi energy where the leads with anti-parallel spins create a small transport gap, while the leads with parallel spins give a finite density of states and induce an even-odd oscillation in conductance in terms of the number of atoms in the carbon chain.

Plasmon excitations in sodium atomic planes: a time-dependent density functional theory study.

The collective electronic excitation in planar sodium clusters is studied by time-dependent density functional theory calculations. The formation and development of the resonances in photoabsorption spectra are investigated in terms of the shape and size of the two-dimensional (2D) systems. The nature of these resonances is revealed by the frequency-resolved induced charge densities present on a real-space grid.

Bandgap engineering of zigzag graphene nanoribbons by manipulating edge states via defective boundaries.

One of the most severe limits of graphene nanoribbons (GNRs) in future applications is that zigzag GNRs (ZGNRs) are gapless, so cannot be used in field effect transistors (FETs), and armchair GNR (AGNR) based FETs require atomically precise control of edges and width. Using the tight-binding approach and first principles method, we derived and proved a general boundary condition for the opening of a significant bandgap in ZGNRs with defective edge structures. The proposed semiconducting ZGNRs have some interesting properties one of which is that they can be embedded and integrated in a large piece of graphene without the need to completely cut them out.

Time-dependent transport through molecular junctions.

We investigate transport properties of molecular junctions under two types of bias--a short time pulse or an ac bias--by combining a solution for Green's functions in the time domain with electronic structure information coming from ab initio density functional calculations. We find that the short time response depends on lead structure, bias voltage, and barrier heights both at the molecule-lead contacts and within molecules. Under a low frequency ac bias, the electron flow either tracks or leads the bias signal (resistive or capacitive response) depending on whether the junction is perfectly conducting or not.

Conductive junctions with parallel graphene sheets.

The establishment of conductive graphene-molecule-graphene junction is investigated through first-principles electronic structure calculations and quantum transport calculations. The junction consists of a conjugated molecule connecting two parallel graphene sheets. The effects of molecular electronic states, structure relaxation, and molecule-graphene contact on the conductance of the junction are explored.

Thermopower of molecular junctions: an ab initio study.

Molecular nanojunctions may support efficient thermoelectric conversion through enhanced thermopower. Recently, this quantity has been measured for several conjugated molecular nanojunctions with gold electrodes. Considering the wide variety of possible metal/molecule systems-almost none of which have been studied-it seems highly desirable to be able to calculate the thermopower of junctions with reasonable accuracy and high efficiency.

Quantum-interference-controlled molecular electronics.

Quantum interference in coherent transport through single molecular rings may provide a mechanism to control the current in molecular electronics. We investigate its applicability, using a single-particle Green function method combined with ab initio electronic structure calculations. We find that the quantum interference effect (QIE) is strongly dependent on the interaction between molecular pi-states and contact sigma-states.

Electron transport through single conjugated organic molecules: basis set effects in ab initio calculations.

We investigate electron transport through single conjugated molecules--including benzenedithiol, oligophenylene ethynylenes of different lengths, and a ferrocene-containing molecule sandwiched between two gold electrodes with different contact structures--by using a single-particle Green function method combined with density functional theory calculation. We focus on the effect of the basis set in the ab initio calculation. It is shown that the position of the Fermi energy in the transport gap is sensitive to the molecule-lead charge transfer which is affected by the size of basis set.

Cobaltocene as a spin filter.

In the context of investigating organic molecules for molecular electronics, doping molecular wires with transition metal atoms provides additional means of controlling their transport behavior. The incorporation of transition metal atoms may generate spin dependence because the conduction channels of only one spin component align with the chemical potential of the leads, resulting in a spin polarized electric current. The possibility to create such a spin polarized current is investigated here with the organometallic moiety cobaltocene.

Contact transparency of nanotube-molecule-nanotube junctions.

The transparency of contacts between conjugated molecules and metallic single-walled carbon nanotubes is investigated using a single-particle Green's function method which combines a Landauer approach with ab initio density functional theory. We find that the overall conjugation required for good contact transparency is broken by connecting through a six-member ring on the tube. Full conjugation achieved by an all-carbon contact through a five-member ring leads to near perfect contact transparency for different conjugated molecular bridges.

Role of the exchange-correlation potential in ab initio electron transport calculations.

The effect of the exchange-correlation potential in ab initio electron transport calculations is investigated by constructing optimized effective potentials using different energy functionals or the electron density from second-order perturbation theory. The authors calculate electron transmission through two atomic chain systems, one with charge transfer and one without. Dramatic effects are caused by two factors: changes in the energy gap and the self-interaction error.

Nanotube-metal junctions: 2- and 3-terminal electrical transport.

We address the quality of electrical contact between carbon nanotubes and metallic electrodes by performing first-principles calculations for the electron transmission through ideal 2- and 3-terminal junctions, thus revealing the physical limit of tube-metal conduction. The structural model constructed involves surrounding the tube by the metal atoms of the electrode as in most experiments; we consider metallic (5,5) and n-doped semiconducting (10,0) tubes surrounded by Au or Pd. In the case of metallic tubes, the contact conductance is shown to approach the ideal 4e2/h in the limit of large contact area.

Negative differential resistance and hysteresis through an organometallic molecule from molecular-level crossing.

Analogous to a quantum double-dot system, diblock structured molecules could also show negative differential resistance (NDR). Using combined density functional theory and nonequilibrium Green function technique, we show that molecular-level crossing in a molecular double-dot system containing cobaltocene and ferrocene leads to NDR and hysteresis.

Organometallic molecular rectification.

We study the rectification of current through a single molecule with an intrinsic spatial asymmetry. The molecule contains a cobaltocene moiety in order to take advantage of its relatively localized and high-energy d states. A rectifier with large voltage range, high current, and low threshold can be realized.

Models of electrodes and contacts in molecular electronics.

Bridging the difference in atomic structure between experiments and theoretical calculations and exploring quantum confinement effects in thin electrodes (leads) are both important issues in molecular electronics. To address these issues, we report here, by using Au-benzenedithiol-Au as a model system, systematic investigations of different models for the leads and the lead-molecule contacts: leads with different cross sections, leads consisting of infinite surfaces, and surface leads with a local nanowire or atomic chain of different lengths. The method adopted is a nonequilibrium Green's-function approach combined with density-functional theory calculations for the electronic structure and transport, in which the leads and molecule are treated on the same footing.

Organometallic spintronics: dicobaltocene switch.

A single-molecule spintronic switch and spin valve using two cobaltocene moieties is proposed. Spin-dependent transport through a lead-molecule-lead junction has been calculated using first-principles density functional and nonequilibrium Green function methods. We find that the antiparallel (singlet) configuration of the cobaltocene spins blocks electron transport near the Fermi energy, while the spin parallel (triplet) configuration enables much higher current.