Armchair Direction Research Articles

Strain-tunable electronic anisotropy of the AlSb double-layer honeycomb structure.

Two-dimensional (2D) materials show promising applications in nanoelectronic devices due to their excellent physical and chemical properties, large specific surface area, and good flexibility. 2D AlSb, a representative of a new class of two-dimensional materials with a double-layer honeycomb (DLHC) structure was recently obtained in experiments and was reported to be a direct band gap semiconductor. Strain engineering is an effective way of tuning the properties of 2D materials. Here, by first-principles calculations, the strain effects on the electronic structure of AlSb were investigated. An interesting inversion of band order near the bottom of the conduction band can be induced by applying uniaxial strain, which will introduce a large electronic anisotropy. In addition, under tensile strain along the armchair direction larger than 5%, band inversion occurs, indicating possible topological insulator properties of AlSb. Small carrier effective mass and strain tunable electronic anisotropy pave the way for the application of AlSb in future nanoelectronic devices.

Effect of point defects on the band alignment and transport properties of 1T-MoS2/2H-MoS2/1T-MoS2 heterojunctions.

Defects, which are an unavoidable component of the material preparation process, can have a significant impact on the properties of two-dimensional devices. In this work, we investigated theoretically the effects of different types and positions of point defects on band alignment and transport properties of metallic 1T-phase MoS2/semiconducting 2H-phase MoS2 junctions. We found that the Schottky barriers of junctions depend on the type of defects and their locations while showing anisotropic characteristics along the zigzag and armchair directions of 2H-phase MoS2. Moreover, defects in the central scattering region can generate local impurity states and introduce new transmission peaks, while defects at the interface do not generate impurity-state-related transmission peaks. Together, these defect-related peaks and Schottky barriers jointly affect the transport properties of the junctions. Understanding the complex behaviors of defects in devices can make the process of material preparation more efficient by avoiding harm.

A self-power photodetector based on Janus SnSSe–ZrSSe lateral heterojunction: A theoretical study

The photogalvanic effect (PGE) is attractive for self-powered photodetectors. We study the PGE of lateral Janus SnSSe–ZrSSe photodetectors using ab initio quantum transport simulations. Under the illumination of linearly polarized light, a globally enhanced PGE is found for the lateral SnSSe–ZrSSe photodetector compared to the ML SnSSe and ZrSSe photodetectors. The lateral SnSSe–ZrSSe photodetector exhibits better performance along the armchair direction than its counterpart along the zigzag direction, where a maximum PGE photocurrent of 34 a02/photon at Eph = 5.0 eV and a high extinction ratio of 194 at Eph = 6.0 eV are acquired. Our study suggests that lateral SnSSe–ZrSSe is a promising candidate for ultraviolet photodetectors.

Janus 2D B2P6: A promising anisotropic thermoelectric material with high power factor

A two-dimensional (2D) material with high thermoelectric (TE) performance is of great interest for the energy conversion device. In this study, by using first-principles calculations, we show that the Janus 2D B2P6 has an extremely high TE power factor of 0.10 and 0.07 W/mK2 for p-type and n-type at room temperature, respectively, which are higher than that of conventional 2D TE materials. We also investigate lattice thermal conductivity, κph, of B2P6 by using the Boltzmann transport theory with phonon–phonon interaction. With κph of 41.45 and 133.3 W/mK for the armchair and zigzag directions, respectively, the optimized values of the TE figure of merit are up to 0.7 at 900 K. Moreover, the Janus 2D B2P6 exhibits anisotropic mechanical properties, in which the ideal strength of B2P6 is 10.7 and 18.63 N/m for the armchair and zigzag directions, respectively. Thus, our results suggest that the Janus 2D B2P6 is the potentially promising flexible anisotropic TE material.

Conductive BSi4 monolayer with superior electrochemical performance for alkali metal ion batteries

Silicon has received great interest in the development of next-generation metal ion batteries (MIBs) due to its extremely high capacity; however, the large volume expansion during intercalation process and intrinsically low electrical conductivity has severely hindered its electrochemical performance. In this work, we theoretically report that the recently-developed BSi4 monolayer is an appealing candidate as a flexible anode for high-performance MIBs. The BSi4 anode exhibits high mechanical and thermal stability with inherent metallicity, which is very advantageous for the fast electronic transport during battery cycle. All metal atoms are stably deposited on the anode surface with high ion mobility. The lowest diffusion barrier is predicted to be 0.33 eV for Li, 0.22 eV for Na, and 0.17 eV for K. Remarkably, the BSi4 anode possesses high Li/Na/K storage capacities of 1087.97, 1087.97 and 870.37 mA h/g, and low averaged open circuit voltages of 0.80, 0.52 and 0.63 V. Moreover, the BSi4 anode could withstand a large ultimate strain of 16.72 % (20.27 %) along zigzag (armchair) direction, showing a good mechanical flexibility.

Directionally-Resolved Phononic Properties of Monolayer 2D Molybdenum Ditelluride (MoTe2) under Uniaxial Elastic Strain.

Understanding the phonon characteristics of two-dimensional (2D) molybdenum ditelluride (MoTe2) under strain is critical to manipulating its multiphysical properties. Although there have been numerous computational efforts to elucidate the strain-coupled phonon properties of monolayer MoTe2, empirical validation is still lacking. In this work, monolayer 1H-MoTe2 under uniaxial strain is studied via in situ micro-Raman spectroscopy. Directionally dependent monotonic softening of the doubly degenerate in-plane E2g1 phonon mode is observed with increasing uniaxial strain, where the E2g1 peak red-shifts -1.66 ± 0.04 cm-1/% along the armchair direction and -0.80 ± 0.07 cm-1/% along the zigzag direction. The corresponding Grüneisen parameters are calculated to be 1.09 and 0.52 along the armchair and zigzag directions, respectively. This work provides the first empirical quantification and validation of the orientation-dependent strain-coupled phonon response in monolayer 1H-MoTe2 and serves as a benchmark for other prototypical 2D transition-metal tellurides.

Effect of vacancy defects on anisotropic electronic transport behaviors of CoN4C2 based 2D devices: a first-principles study

The strong anisotropic electronic transport properties of the single-atom-thick material CoN4C2 monolayer hold immense importance for the advancement of the electronics industry. Using density functional theory combined with non-equilibrium Green’s function systematically studied the electronic structural properties and anisotropic electronic transport properties of the CoN4C2 monolayer. The results show that Co, N, and C single-atom vacancy defects do not change the electronic properties of the CoN4C2 monolayer, which remains metallic. The pristine device and the devices composed of Co, N single-atom vacancy defects exhibit stronger electronic transport along the armchair direction than the zigzag direction, which exhibit strong anisotropy, and a negative differential resistance (NDR) effect can be observed. In contrast to the results mentioned above, the device with C single-atom vacancy defects only exhibits the NDR effect. Among them, the device with the N single-atom vacancy defect regime exhibits the strongest anisotropy, with an I Z/I A of up to 7.95. Moreover, based on the strongest anisotropy exhibited by N single-atom vacancy defects, we further studied the influence of different sites of the N-atom vacancy on the electronic transport properties of the devices. The results indicate that N-1, N-2, N-3, N-12, N-23, N-123, N-1234, and N-12345 model devices did not change the high anisotropy and NDR effect of the device, and among them the N-1234 exhibits the strongest anisotropy, the I Z/I A reaches 6.12. A significant NDR effect is also observed for the electronic transport along the armchair direction in these devices. However, the current gradually decreases as an increase of the number of N defects. These findings showcase the considerable potential for integration of the CoN4C2 monolayer in switching devices and NDR-based multifunctional nanodevices.

Diffusion and Coalescence of Phosphorene Monovacancies Studied Using High-Dimensional Neural Network Potentials.

The properties of two-dimensional materials are strongly affected by defects that are often present in considerable numbers. In this study, we investigate the diffusion and coalescence of monovacancies in phosphorene using molecular dynamics (MD) simulations accelerated by high-dimensional neural network potentials. Trained and validated with reference data obtained with density functional theory (DFT), such surrogate models provide the accuracy of DFT at a much lower cost, enabling simulations on time scales that far exceed those of first-principles MD. Our microsecond long simulations reveal that monovacancies are highly mobile and move predominantly in the zigzag rather than armchair direction, consistent with the energy barriers of the underlying hopping mechanisms. In further simulations, we find that monovacancies merge into energetically more stable and less mobile divacancies following different routes that may involve metastable intermediates.

Electronic thermal conductivity of few-layer black phosphorus

The theoretical investigations of anisotropic electronic thermal conductivity of nanoscale black phosphorus along zigzag and armchair directions are presented. Electrons in black phosphorus assumed to be scattered by in-plane linear dispersion phonons and out-of-plane quadratic dispersion phonons via deformation potential couplings. The individual contributions from in-plane phonon and out-of-plane phonon processes to the electronic thermal conductivity show dominant role of in-plane phonon processes. The anisotropic behaviour of electronic thermal conductivity is noticed and found to be sensitive to the carrier density and temperature. Role and relative importance of anisotropic electronic thermal conductivity in the analysis of experimental thermal conductivity data of nanoscale black phosphorus samples is investigated.

Two-dimensional γ-PC3: A novel direct band gap semiconductor with ultrahigh carrier mobility for photovoltaics

Here, we report a novel two-dimensional phosphorus carbide, named γ-PC3, and determine its structural, electronic, transport, and photovoltaic characteristics utilizing density functional theory. γ-PC3 consists of P atom linked benzenes and has an anisotropic orthorhombic crystal structure. It exhibits excellent stability, as confirmed by the cohesive energy (6.38 eV/atom), phonon dispersion (the ultimate strain is as high as 13 %), molecular dynamics simulation, elastic modulus, and strain–stress relationship. The γ-PC3 monolayer has direct band gap (1.13 eV), ultrahigh electron mobility (up to 7.25 × 104 cm2 V−1 s−1 along armchair direction), and high optical absorption coefficient (up to 105 cm−1). By changing the stacking sequence and thickness to multilayer geometries or applying the tensile uniaxial strain to the monolayer geometry, its direct band gap and optical absorption coefficient can be effectively tuned. The γ-PC3/MoS2 heterojunction has a photoelectric conversion efficiency of 15.54 %, which demonstrates that γ-PC3 is a promising semiconductor for photovoltaics.

Mechanical Behaviors of the Single Crystal Two-Dimensional Silicon Carbide (SiC): A Molecular Dynamics Insight

This paper focuses on the two-dimensional silicon carbide (2D-SiC), which has an excellent opportunity to be used as an alternative to graphene in nanotechnologies such as in nanoelectronics, nanoelectromechanical systems (NEMS), nano-sensors, nano-energy harvesting devices, and nano-composites due to its unique structural, mechanical, electronic, and thermal properties. Their mechanical properties characterize the stability of the nanodevices. This study performs molecular dynamics (MD) simulation to examine the mechanical properties based on optimized Tersoff potential of the single crystal 2D-SiC at different temperatures, strain rates, point vacancies, and edge cracks. At room temperature (300 K), the obtained elastic modulus and fracture strength are 423 GPa and 68.89 GPa, respectively, along the armchair direction. As the temperature rises from 100 K to 800 K, the fracture stress falls by 21.96% and the fracture strain by 36.90%. An approximate linear reduction in fracture strength is noticed as the temperature rises from 100 K to 800 K. The elastic modulus also falls as the temperature rises but is not significant. Although the elastic modulus is unchanged, the fracture stress increases by 1.84% while the fracture strain increases by 5.84% for a change in strain rate from 0.0001 ps-1 to 0.005 ps-1. The fracture stress and strain are significantly reduced, primarily due to the edge crack, as the concentration of point vacancy grows from 0.1% to 0.6% and the edge crack size increases from 0.5 nm to 1.5 nm. Moreover, anisotropic behavior is also evaluated at 300 K temperature and 0.001 ps-1 strain rate. These findings would offer a deep understanding of the fracture mechanics of 2D-SiC and also help to address the mechanical instability issue with SiC-based nanodevices.

Isotropic Contact Properties in Monolayer GeAs Field-Effect Transistors

Owing to the tunable bandgap and high thermodynamic stability, anisotropic monolayer (ML) GeAs have arisen as an attractive candidate for electronic and optoelectronic applications. The contact properties of ML GeAs with 2D metal (graphene, Ti2CF2, V2CF2, and Ti3C2O2) and Cu electrodes are explored along two principal axes in field-effect transistors (FET) by employing ab initio electronic structure calculations and quantum transport simulations. Weak van der Waals interactions are found between ML GeAs and the 2D metal electrodes with the band structure of ML GeAs kept the same, while there is a strong interaction between ML GeAs and the Cu metal electrode, resulting in the obvious hybridization of the band structure. Isotropic contact properties are seen along the two principal directions. P-type lateral Schottky contacts are established in ML GeAs FETs with Ti3C2O2, graphene, and Ti2CF2 metals, with a hole Schottky barrier height (SBH) of 0.12 (0.20), 0.15 (0.11), and 0.29 (0.21) eV along the armchair (zigzag) direction, respectively, and an n-type lateral Schottky contact is established with the Cu electrode with an electron SBH of 0.64 (0.57) eV. Surprisingly, ML GeAs forms ideal p-type Ohmic contacts with the V2CF2 electrode. The results provide a theoretical foundation for comprehending the interactions between ML GeAs and metals, as well as for designing high-performance ML GeAs FETs.

Enhanced photogalvanic effect in MoSSe monolayer with grain boundaries

The photogalvanic effect (PGE) in two-dimensional materials has emerged as a fascinating mechanism for generating photocurrent in non-centrosymmetric crystals without semiconductor p–n junction or bias voltage. In this paper, the impact of grain boundaries (GBs) on the performance of photogalvanic device with Janus MoSSe monolayer is theoretically investigated by quantum transport simulations. Two 4|8 GBs along armchair direction are taken into consideration. Under the illumination of linearly polarized light, we observe a significant enhancement of the PGE photocurrent in the visible light region, which can be attributed to the reduction of device's symmetry. The averaged enhancement ratios reach around 20 and 13 for two 4|8 GBs, respectively.

Nonlinearity induced negative Poisson’s ratio of two-dimensional nanomaterials

Materials exhibiting a negative Poisson’s ratio have garnered considerable attention due to the improved toughness, shear resistance, and vibration absorption properties commonly found in auxetic materials. In this work, the nonlinear effect on the Poisson’s ratio was derived theoretically and verified by first-principle calculations and molecular dynamics simulations of two-dimensional nanomaterials including graphene and hexagonal boron nitride. The analytic formula explicitly shows that the Poisson’s ratio depends on the applied strain and can be negative for large applied strains, owing to the nonlinear interaction. Both first-principle calculations and molecular dynamics simulations show that the nonlinear effect is highly anisotropic for graphene, where the nonlinearity-induced negative Poisson’s ratio is much stronger for the strain applied along the armchair direction. These findings provide valuable insights into the behavior of materials with negative Poisson’s ratios and emphasize the importance of considering nonlinear effects in the study of the Poisson’s ratio of two-dimensional materials.

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.

Morphology of Bi(110) quantum islands on epitaxial graphene

Proximitized 2D materials present exciting prospects for exploring new quantum properties, enabled by precise control of structures and interfaces through epitaxial methods. In this study, we investigated the structure of ultrathin coverages formed by depositing high-Z element bismuth (Bi) on monolayer graphene (MLG)/SiC(0001). By utilizing electron diffraction and scanning tunneling microscopy, ultrathin Bi nanostructures epitaxially grown on MLG were studied. Deposition at 300 K resulted in formation of needle-like Bi(110)-terminated islands elongated in the zig-zag direction and aligned at an angle of approximately 1.75∘ with respect to the MLG armchair direction. By both strain and quantum size effects, the shape, the orientation and the thickness of the Bi(110) islands can be rationalized. Additionally, a minority phase of Bi(110) islands orthogonally aligned to the former ones were seen. The four sub-domains of this minority structure are attributed to the formation of mirror twin boundaries, resulting in two potential alignments of Bi(110) majority and minority domains with respect to each other, in addition to two possible alignments of the majority domain with respect to graphene. Notably, an annealing step at 410 K or lowering the deposition temperature, significantly increases the concentration of the Bi(110) minority domain. Our findings shed light on the structural control of proximitized 2D materials, showcasing the potential for manipulating 2D interfaces.

Plasmonic responses in Janus bAsP with elliptic-to-hyperbolic transition: an ab-initio study.

Plasmonic responses in materials with actively tunable elliptic-to-hyperbolic transition are rare in nature. Based on ab-initio calculations, we have theoretically predicted that Janus black arsenic phosphorus (bAsP) supports both elliptic and hyperbolic in-plane surface plasmon polaritons in the infrared after being doped with electrons. In the elliptic regime, anisotropic plasmonic responses have been observed, which can be explained by the anisotropic dispersions at the bottom of the conduction bands. In the hyperbolic regime, the total permittivity along the armchair/zigzag edge is negative/positive, which is the result of positive interband permittivities and largely different Drude plasma frequencies along two directions making the total permittivities change signs at different photon energies. In this material, changing the topology (elliptic or hyperbolic) of the plasmonic responses via doping is possible. Then, strains along the zigzag and armchair directions have been applied to modify the band structures as well as the plasmonic responses. Since plasmonic responses are mostly related to the bands near the Fermi energy, a relatively small strain along the zigzag direction can make bAsP become an indirect-bandgap material and change the Drude plasma frequencies under proper doping. With both strain and doping present in this material, we have even found a special case of hyperbolicity where the total permittivity in the zigzag/armchair direction is negative/positive, which is opposite to the normal case. In the end, we have extended our investigations to bAsP-graphene heterostructures. Since bAsP is a Janus material, such direct contact can change the Fermi energy through charge transfer making this heterostructure support strong plasmons without extra doping. Our investigations propose bAsP as a promising Janus material platform for plasmonic applications.

Nagaoka ferromagnetism in an array of phosphorene quantum dots.

We consider an array of four quantum dots defined in phosphorene containing three excess electrons, i.e., in the conditions of near half filling when itinerant Nagaoka ferromagnetism is expected to appear in a square array with isotropic interdot hopping. The interdot hopping in the array arranged in a square inherits the anisotropy from the form of the phosphorene conduction band. We apply the configuration interaction method for discussion of the appearance and stability of the spin-polarized ground state and discuss the compensation of the effective mass anisotropy by the geometry of the quantum dot array. Our study shows strong stability of Nagaoka ferromagnetism for optimized geometry of the array, with the Nagaoka gap as large as ∼230µeV. A phase diagram for the ground-state spin ordering versus the geometric parameters of the array is presented. We study the suppression of the ferromagnetism in a transition of the [Formula: see text] array to a quasi-1D chain and indicate that the shift of one of the quantum dots away from the array center is enough to transform the system to a quantum dot chain. A shift in the zigzag crystal direction induces the low-spin ground state more effectively than a shift along the armchair direction. We also discuss the robustness of the spin ordering against detuning one of the dots. The ferromagnetic ground-state survives as long as the detuning is not large enough to trap one of the electrons within a single quantum dot (for positive detuning) or remove one of the quantum dots of the accessible energy range (for negative detuning).

Investigating high harmonic yield from different alignments of WSe2 semiconductor

This paper theoretically investigates high harmonic generation from a monolayer WSe2 driven by a laser pulse with different alignments, in-plane and out-of-plane polarizations. Our simulations are performed in the framework of the time-dependent density functional theory. Analysis of the dependence of the cutoff energy on the laser field strength shows that the cutoff energy increases linearly/quadratically with the increase of the electric field strength for the out-of-plane/in-plane laser field. Also, the wavelength dependence of high harmonic yield exhibits a sharp decrease with a rate of λ−10 for the in-plane laser field, and a more gradual decline with a rate of λ−4.75 for the out-of-plane laser field. These wavelength dependencies are explained by the tunneling rate and the spatial spread of electron wave-packet due to propagation in time and space. Additionally, it is demonstrated that the maximum harmonic yield is obtained when the in-plane laser is aligned along the armchair direction, or when the out-of-plane laser is polarized at a 20° angle relative to the normal axis of the monolayer plane.

The electronic and mechanical properties of two dimensional multilayered GaN: A first-principles study

GaN is a wide-bandgap semiconductor extensively used in electronic and optoelectronic fields. When its thickness is reduced to a few monolayers (MLs), it can even be used for nano-electronic and nano-optoelectronic devices due to the extreme quantum confinement. Undoubtedly, thickness is one of the most important factors for the properties of two-dimensional (2D) materials. Up to now, however, the influence of thickness on the electronic and mechanical properties of 2D GaN has not been systematically studied in experiment or theory. As we know, it is difficult to synthesis perfect 2D GaN with controllable thickness in experiment. Thus, in this paper, we study the stability, electronic and mechanical properties of 1–5 ML 2D GaN employing the density functional theory. It is found that the 1 ML GaN reconstructs into honeycomb structure and 2–5 ML GaN reconstruct into 8|4 Haeckelite structure. All structures are dynamically stable. Additionally, as the layer numbers increase, the in-plane Young’s modulus and layer modulus along armchair and zigzag direction monotonically increase, but bandgap decrease. All results indicate that electronic and mechanical properties can be modulated by changing layer numbers, which can provide guidance in design, manufacture, and integration of 2D GaN devices in the future.