Armchair Interface Research Articles

Mechanical and Lattice Thermal Properties of Si-Ge Lateral Heterostructures.

Two-dimensional (2D) materials have drawn extensive attention due to their exceptional characteristics and potential uses in electronics and energy storage. This investigation employs simulations using molecular dynamics to examine the mechanical and thermal transport attributes of the 2D silicene-germanene (Si-Ge) lateral heterostructure. The pre-existing cracks of the Si-Ge lateral heterostructure are addressed with external strain. Then, the effect of vacancy defects and temperature on the mechanical attributes is also investigated. By manipulating temperature and incorporating vacancy defects and pre-fabricated cracks, the mechanical behaviors of the Si-Ge heterostructure can be significantly modulated. In order to investigate the heat transport performance of the Si-Ge lateral heterostructure, a non-equilibrium molecular dynamics approach is employed. The efficient phonon average free path is obtained as 136.09 nm and 194.34 nm, respectively, in the Si-Ge heterostructure with a zigzag and armchair interface. Our results present the design and application of thermal management devices based on the Si-Ge lateral heterostructure.

Ultrabroadband valley transmission and corner states in valley photonic crystals with dendritic structure

In photonic crystal systems, topologically protected edge states and corner states can be achieved by breaking spatial inversion symmetry, which is expected to be applied to topologically protected lasers, optical communication and integrated photonics. However, designing ultrabroadband topological photonic crystals is still a challenge. In this work, we propose a valley photonic crystal composed of dendritic structures, which can realize valley transmission with a relative bandwidth up to 59.65%. Compared with the previously reported two-dimensional broadband photonic crystals with 32.02% bandwidth, the relative bandwidth of the proposed valley transmission is increased by almost 100%. Theoretical analysis, numerical simulation and experimental measurement all confirm flexible manipulation of electromagnetic wave propagation paths. Ultrabroadband topological waveguides with the zigzag and armchair interface are demonstrated, which can achieve experimentally 58.71% and 36.78% relative bandwidth, respectively. In addition, several topological channel intersections are designed. Finally, two types of corner states with valley switchability and selectivity are demonstrated.

Investigation of topological valley kink states along zigzag and armchair domain wall in a two-dimensional photonic crystal

This paper provides a numerical analysis of the topological valley kink states along both zigzag and armchair domain walls of a dielectric two-dimensional photonic crystal (PC), considering the photonic energy band folding mechanism. By engineering the side length of triangular holes in a honeycomb PC, we created inequivalent valleys in the momentum space. We utilized two adjacent valley PCs with inverted structures to induce a topological transition of the TE mode energy band as it crossed the interface. Further research into the projected energy bands along both zigzag and armchair directions revealed that topologically protected valley kink states can be supported by both configurations. The zigzag interface enabled valley waveguides to transport chiral optical fields at the 0°, 60°, and 120° bending angles, while maintaining their backscattering immune properties. The armchair interface, on the other hand, supported the straight propagation. By combining both armchair and zigzag interfaces, the valley waveguide can facilitate bending propagation at 90° and 180°, while also enabling the equal splitting of chiral fields at the intersection between these two interfaces. Our analyzation can be helpful to improve the applications of valley waveguides in integrated photonics.

First‐Principles Investigation of NO Molecule Adsorption on As6/Sb6 and Sb6/Bi6 Lateral Heterostructures

Predicting and designing highly gas‐sensitive semiconductors is crucial for solving growing environmental problems. Herein, four lateral heterostructures (LHSs), As6/Sb6 AC‐/ZZ‐LHSs and Sb6/Bi6 AC‐/ZZ‐LHSs, are constructed. The adsorption of NO molecule on these LHSs is investigated using first‐principle calculations. The results indicate that the adsorption of NO molecule on As6/Sb6 LHS with armchair (AC) interface is physisorption, whereas NO molecule is chemically adsorbed on As6/Sb6 LHS with zigzag (ZZ) interface and Sb6/Bi6 AC‐/ZZ‐LHSs, with strong adsorption energy and large charge transfer. All LHSs act as charge donors for the NO molecule. Meanwhile, the adsorption of NO molecule causes a significant change in the electronic properties of As6/Sb6 ZZ‐LHS and Sb6/Bi6 AC‐/ZZ‐LHSs, which shows that these LHSs have great potential for application in NO gas sensors.

First principles studies on infrared band structure and absorption of As/Sb lateral heterostructures

Two-dimensional materials have been extensively investigated for fabricating high-performance visible optoelectronic devices. Considering the significance of mid-infrared band, narrow-band two-dimensional semiconductor materials have become the key point. In this work, we bring out two kinds of monolayer lateral heterostructures (LHSs) based on arsenic (As)/antimony (Sb) to realize the narrow band structure. The bandgap of LHS with an armchair interface is calculated to be 1.1 eV with an indirect band through the first principle, and the bandgap of LHS with a zigzag interface is 0.57 eV with a direct band. Their bandgaps are all shrunk by applying tensile or compressive strains. Furthermore, indirect-to-direct transitions appear in the armchair LHS when tensile strains are applied. Partial density-of-states and charge density distributions indicate that electron transmission from Sb atoms to As atoms may be the main factor for the reduction of the bandgap. In addition, the tensile strain extends the optical absorption to the infrared region. The As/Sb lateral heterostructures proposed in this paper are of great significance for infrared optoelectronic devices.

Interface dark excitons at sharp lateral two-dimensional heterostructures

We study the dark excitons at the interface of a sharp lateral heterostructure of two-dimensional transition metal dichalcogenides (TMDs). By introducing a low-energy effective Hamiltonian model, we find the energy dispersion relation of exciton and show how it depends on the onsite energy of composed materials and their spin–orbit coupling strengths. It is shown that the effect of the geometrical structure of the interface, as a deformation gauge field (pseudo-spin–orbit coupling), should be considered in calculating the binding energy of exciton. By discretization of the real-space version of the dispersion relation on a triangular lattice, we show that the binding energy of exciton depends on its distance from the interface line. For exciton near the interface, the binding energy is equal to 0.36 eV, while for the exciton far enough from the interface, it is equal to 0.26 eV. Also, it has been shown that for a zigzag interface the binding energy increases by 0.34 meV compared to an armchair interface due to the pseudo-spin-orbit interaction (gauge filed). The results can be used for designing 2D-dimensional-lateral-heterostructure- based optoelectronic devices to improve their characteristics.

Multichannel Topological Transport in an Acoustic Valley Hall Insulator

Valley interface states, resulting in acoustic valley Hall topological insulators, have recently become a hot topic in the study of acoustic systems. On the basis of structural diversity and potential applications, we construct a two-dimensional triangular-lattice phononic crystal with ${C}_{3v}$-symmetric scatterers, and obtain two distinct valley Hall phases with nonvanishing valley Chern indices by rotating the scatterers. We numerically calculate the dispersion relations of these valley Hall phases including two kinds of interfaces, and find that valley interface states exist at not only the zigzag interface but also the armchair interface. We demonstrate the acoustic splitting and merging of valley interface states in the cross-waveguides, and numerically achieve the xor and or logic functions. We also design three complicated waveguides by assembling phononic crystals with distinct valley Hall phases. By experimental measurements in these waveguides, we successfully implement one-, two-, and three-channel topological transport. This research possibly provides a design route exploiting valley interface states to fabricate multichannel acoustic communication devices.

Theoretical design of SnTe/GeS lateral heterostructures: A first-principles study

Using first-principles calculations, we theoretically designed new (SnTe)m/(GeS)n LHSs with the armchair interface connected by covalent bonds. We verified (SnTe)m/(GeS)n LHSs can be synthesized experimentally due to their small heat of formation. In contrast to the indirect bandgaps of pristine SnTe and GeS monolayers, the (SnTe)2/(GeS)2 LHS is a direct gap semiconductor with type-II alignment. The (SnTe)m/(GeS)n LHSs can achieve a transition from type-II to type-I alignment and from direct to indirect bandgap by increasing component units of (SnTe)m/(GeS)n LHSs. Meanwhile, the bandgaps of (SnTe)m/(GeS)n LHSs decrease monotonously as the component units increases. The conduction band minimum (CBM) and the valence band maximum (VBM) of (SnTe)2/(GeS)2 LHS are mainly distributed on SnTe and GeS, respectively. However, the CBM and VBM of (SnTe)4/(GeS)4 and (SnTe)6/(GeS)6 LHSs are dominated by SnTe. Furthermore, we found that the bandgap of (SnTe)2/(GeS)2 LHS can be effectively tuned by applying external strain.

Electronic and Spin‐Dependent Optical Properties of Fe‐Adsorbed Armchair Silicene/Silicane Superlattices

The electronic and optical properties of Fe‐adsorbed silicene/silicane superlattices (Fe‐SSSLs) with armchair interface are studied by first‐principles calculations. It is demonstrated that the SSSLs show spin polarization after adsorption of Fe atoms, and the bandgaps of different Fe‐SSSLs exhibit oscillatory behaviors with the width of silicene region. Except for (6,4)‐SSSL with a small total magnetic moment (about 0.06 μB cell−1), all other Fe‐SSSLs have a total magnetization moment of 2.00 μB cell−1, which comes from the 3d orbitals of Fe atom. Furthermore, there is a great enhancement of optical absorption in the infrared‐light region for Fe‐SSSLs, which is caused by the transition in a single spin channel due to Fe adsorption. The results show that Fe adsorption can tune the electronic and optical properties of armchair SSSLs effectively, which makes armchair Fe‐SSSLs attractive candidates for spin‐polarized photoelectric device applications.

Electronic transmission in the lateral heterostructure of semiconducting and metallic transition-metal dichalcogenide monolayers

We investigate the electronic transport property of lateral heterojunctions of semiconducting and metallic transition-metal dichalcogenide monolayers, MoSe2 and NbSe2, respectively. We calculate the electronic transmission probability by using a multiorbital tight-binding model based on the first-principles band structure. The transmission probability depends on the spin and valley degrees of freedom. This dependence qualitatively changes by the interface structure. The heterostructure with a zigzag interface preserves the spin and the valley of electrons in the transmission process. On the other hand, the armchair interface enables conduction electrons to transmit with changing the valley and increases the conductance in the hole-doped junctions due to the valley-flip transmission. We also discuss the spin and valley polarizations of electronic current in the heterojunctions.

Tuning of adsorption energies of CO2 and CH4 in borocarbonitrides BxCyNz: A first-principles study

We present density functional theory based calculations with semiempirical dispersion (DFT-D) analysis of CO2, CH4, N2 and H2 adsorption at various sites of 2-dimensional BCN with various chemical ordering of B, C and N and also on graphene and C-BN arm-chair interface of BC2N having graphene and BN domains. We find BCN and BC2N shows greater ability to absorb these gases as compared to graphene. CO2 binds strongly as compared to other gases with optimum storage of 44 wt% for a monolayer of BCN. A significant new result is that when BC2N sheets are doped with boron they show significant increase in adsorption energies for CO2 and CH4. Through the present work, we propose a way to tune adsorption energies of CO2 and CH4 by changing the doping concentration of boron.

Aharonov–Bohm effect in bilayer graphene

We investigate Aharonov–Bohm effect in bilayer graphene. We consider a setup of n–p(n′)–n junction with Aharonov–Bohm loop connected in the transmission region. In the presence of trigonal warping we show that, due to the anisotropic dispersion of eigenspectrum, the Aharonov–Bohm interference depends on the geometry of junction: it exists for armchair interface but vanishes for zigzag interface. For the armchair interface, it is demonstrated that the period of Aharonov–Bohm oscillation is Φ0=h/e and the amplitude of oscillation can be varied with incident energy and the barrier height of the junction.

A theoretical study of the electrical contact between metallic and semiconducting phases in monolayer MoS2

We present a theoretical study of the electrical contact between the two most common crystallographic phases of MoS2 monolayer crystals: the stable semiconducting 2H phase and the metastable metallic 1T phase. A density functional theory (DFT) study of the electronic structure of interface between the two phases shows a higher Schottky barrier for electrons than for holes for the undoped 2H phase. Charge transfer from the 1T to the 2H phase occurs, but, as expected for a one-dimensional contact, the generated dipole potential decays away from the interface and the naive Schottky-Mott band-alignment picture is recovered away from the interface. The decay length of the dipole potential turns out to be larger for the zigzag interface than for the armchair interface due to the different penetration of the edge states into the bulk. Tight-binding quantum transport calculations aided by the DFT results generically confirm a low contact resistance in the range of ≈200–400 Ωμm, as experimentally reported. Furthermore, the contact resistance is predicted to be smaller at the armchair interface for electron injection and, on the contrary, smaller for hole injection at the zigzag interface.

Tuning Magnetic States of Planar Graphene/h-BN Monolayer Heterostructures via Interface Transition Metal-Vacancy Complexes

Planar graphene/h-BN (GPBN) heterostructures promise low-dimensional magnetic semiconductor materials of tunable bandgap. In the present study, interplay between 3d transition metal (TM) atoms and single vacancies (SVs) at the armchair interface in a planar GPBN monolayer was investigated through first principle density functional theory calculations. The TM-SV complexes were found to give rise to a rich set of magnetic states, originated from the interactions between valence electrons of the TM atom with dangling orbitals at the SV. The magnetic state at a TM-SV complex was further shown to be tunable upon the application of strain and electric field. The present study suggests a route to enrich and engineer the magnetic states of planar GPBN heterostructures, providing new insights for the design of tunable low-dimensional spintronic devices.

Structures and electronic properties of vacancies at the interface of hybrid graphene/hexagonal boron nitride sheet

As a novel two-dimensional material, hybrid graphene/h-BN has attracted a great deal of attention. Recently, vacancies were observed as main defects at the interface of such hybrids. In this work, the structural, electronic and magnetic properties of vacancies at the interface of hybrid graphene/h-BN sheet were studied using density functional theory. Calculated results showed that existence of vacancies at the interface of such hybrids slightly reduce the stability of the systems. The cohesive energy of the hybrid graphene/h-BN with a zigzag edge was comparable to that of the hybrid graphene/h-BN with an armchair edge. Graphene/h-BN hybrid systems with vacancies at a zigzag interface exhibit obvious metal properties. Some of the graphene/h-BN systems with vacancies at an armchair interface changes from an insulator to a metal property due to the effect of vacancies. These findings will contribute to the understanding of characteristics of the interface growth for the graphene-based composite material.

Efficient approach for modeling phonon transmission probability in nanoscale interfacial thermal transport

The Kapitza or interfacial thermal resistance at the boundary of two different insulating solids depends on the transmission of phonons across the interface and the phonon dispersion of either material. We extend the existing atomistic Green's function (AGF) method to compute the probability for individual phonon modes to be transmitted across the interface. The extended method is based on the concept of the Bloch matrix and allows us to determine the wavelength and polarization dependence of the phonon transmission as well as to analyze efficiently the contribution of individual acoustic and optical phonon modes to interfacial thermal transport. The relationship between the phonon transmission probability and dispersion is explicitly established. A detailed description of the method is given and key formulas are provided. To illustrate the role of the phonon dispersion in interfacial thermal conduction, we apply the method to study phonon transmission and thermal transport at the armchair interface between monolayer graphene and hexagonal boron nitride. We find that the phonon transmission probability is high for longitudinal (LA) and flexural (ZA) acoustic phonons at normal and oblique incidence to the interface. At room temperature, the dominant contribution to interfacial thermal transport comes from the transverse-polarized phonons in graphene (45.5%) and longitudinal-polarized phonons in boron nitride (47.4%).

Tuning Carrier Confinement in the MoS2/WS2 Lateral Heterostructure

To determine and control the spatial confinement of charge carriers is of importance for nanoscale optoelectronic device applications. Using first-principles calculations, we investigate the tunability of band alignment and charge localization in lateral and combined lateral–vertical heterostructures of MoS2 and WS2. First, we show that a type-II to type-I band alignment transition takes place when tensile strain is applied on the WS2 region. This band alignment transition is a result of the different response of the band edge states with strain and is caused by their different wave function characters. Then we show that the presence of the grain boundary introduces localized in-gap states. The boundary at the armchair interface significantly modifies the charge distribution of the valence band maximum (VBM) state, whereas in a heterostructure with tilt grain domains both conducation band maximum (CBM) and VBM are found to be localized around the grain boundary. We also found that the thickness of the con...

Stacking-dependent superstructures at stepped armchair interfaces of bilayer/trilayer graphene

We present the first study of quantum interference patterns at a bilayer-trilayer armchair interface, for different stacking sequences. Visualization using scanning tunneling microscopy and theoretical calculations provides direct evidence that near armchair edges electron behavior is dominated by the “hard” edge, where the layer is abruptly truncated, as opposed to the “soft” edges, where layers continue across the boundary. Intervalley reflection causes universal quenching of the wavefunction with a periodicity of three C atoms, while the exact interference patterns depend on the stacking sequence and appear to be robust to disorder and chemical terminations.

Adsorption of hydrogen on the interface of a graphene/boron nitride hybrid atomic membrane

The synthesis of a graphene/boron nitride hybrid atomic membrane offers a novel class of line interface that is chemically coherent at the atomic scale. We employ first-principles calculations to study the adsorption of atomic hydrogen at the interface of the graphene/boron nitride monolayer. We demonstrate that hydrogen shows a remarkable selective affinity toward the zigzag-type interface in the hybrid material, with notably higher binding energy compared to the bulk graphene, boron nitride, or the armchair interface. We also show that hydrogen adsorption can be a potential route for tuning the electronic property of the hybrid atomic membrane. Our calculation indicates that adding hydrogen to the zigzag interface can cause a semiconductor-to-metal transition in the material.