Armchair Direction Research Articles

Insights into electronic and transport properties of phosphorene nanorings in two perpendicular directions: Effects of circular and elliptical external potentials

In this work, we study the electronic and transport properties of phosphorene nanorings in two perpendicular directions (zigzag and armchair directions) in the presence of a zigzag metallic source and drain leads. Our results are based on the non-equilibrium Green’s function method and a five-parameter tight-binding approach. We investigate how system parameters affect the electronic transport. These parameters include the radius of the rings, the width of the leads, and the external potential. Our results show that for all configurations studied, a transport energy gap exists whose size can be tuned by the width of the leads and the radius of the nanoring. The transmission function of wider leads shows more sensitivity to the variation of the inner radius due to higher electronic states that can respond to smaller changes in the scattering region. In addition, the transport along the armchair direction is more susceptible to tuning than the transport along the zigzag direction. The effects of external potentials on the conductance are more pronounced than the geometrical parameters. In particular, the circular potential of the amplitude of 0.1 eV can widen the transport gap by about ∼0.35 eV.

Complex role of strain engineering of lattice thermal conductivity in hydrogenated graphene-like borophene induced by high-order phonon anharmonicity

Strain engineering has been used as a versatile tool for regulating the thermal transport in various materials as a result of the phonon frequency shift. On the other hand, the phononic bandgap can be simultaneously tuned by the strain, which can play a critical role in wide phononic bandgap materials due to the high-order phonon anharmonicity. In this work, we investigate the complex role of uniaxial tensile strain on the lattice thermal conductivity of hydrogenated graphene-like borophene, by using molecular dynamics simulations with a machine learning potential. Our findings highlight a novel and intriguing phenomenon that the thermal conductivity in the armchair direction is non-monotonically dependent on the uniaxial armchair strain. Specifically, we uncover that the increase of phonon group velocity and the decrease of three-phonon scattering compete with the enhancement of four-phonon scattering under armchair strain, leading to the non-monotonic dependence. The enhanced four-phonon scattering originates from the unique bridged B–H bond that can sensitively control the phononic bandgap under armchair strain. This anomalous non-monotonic strain-dependence highlights the complex interplay between different mechanisms governing thermal transport in 2D materials with large phononic bandgaps. Our study offers valuable insights for designing innovative thermal management strategies based on strain.

A study on mechanical properties of 1H-type crystalline nickel ditelluride

We used the molecular dynamic finite element method with Stillinger-Weber potential to study the mechanical behavior of monolayer 1H-type nickel ditelluride (1H-NiTe2) sheets under uniaxial tension. As the size of the pristine 1H-NiTe2 sheet increases, the Young’s modulus in the armchair direction increases by 6.2 %, while that in the zigzag one decreases by 6.0 %. Both tend to be size-independent in larger sheets and approach the same value at around 54 N/m. The ultimate stress in the armchair direction remains almost unchanged but that in the zigzag one reduces by about 9.5 % as the size inclines. Besides the influence of size, our results show that single vacancy defects strongly affect the ultimate stress and strain while having no effect on Young’s modulus and Poisson’s ratio. When a Te atom is missing at the sheet’s center, the ultimate stress in the armchair direction is reduced by 11.7 %, while a decrease of 16.0 % is caused by a Ni atom vacancy

Temperature and strain rate dependent tensile properties of titanium carbide/nitride MXenes

MXenes have rapidly gained prominence since their serendipitous discovery in 2011 in the field of 2D materials due to their exceptional properties, which leads to strong applications in energy storage, anti-wear coatings, nanocomposites and electronics sector. This study examined tensile characteristics of wide ranged titanium carbide and nitride pristine MXenes by molecular dynamics investigation where the directional, temperature and strain rate dependencies were explored. The uniaxial tension computations have been performed at temperature of 225–525K with a linear difference of 75K and strain rate of 5 × 10−5 to 1 × 10−2 ps−1 with an approximate regular logarithmic difference. Afterward a two steps graphical method has been proposed from temperature and strain rate dependencies to calculate the strain rate sensitivity and activation energy for both zigzag and armchair directions by using Arrhenius correlation. The temperature exerts a slight influence on the activation energy, wherein a decrease in temperature results in a reduction of the activation energy. After all, the Ti2X (X = N,C) expressed superior mechanical strength compared to their immediate pristine MXenes.

Anisotropic infrared absorption in monolayer black phosphorus metasurface with/without local oxidative effect

As one kind of anisotropic two-dimensional (2-D) layered material, the monolayer black phosphorus (BP) has ability to excite the surface plasmon resonance in the infrared spectrum, thus providing sufficient conditions for the strong light-matter interaction, so that some potential applications can be shown in the field of optoelectronics. Based on those physical properties, we design the patterned monolayer BP to achieve extremely high infrared absorption, and meanwhile use the finite element method to model the effects of factors such as n-type doping concentration, dielectric thickness, and angle of incidence light. Moreover, due to the strong anisotropic in-plane properties of monolayer BP, we analyze detailedly the absorption spectra in both the armchair and zigzag directions separately, and then understand that low-angle narrow-band absorption in the zigzag direction could be captured in that structure. The structure could achieve perfect absorption in the zigzag direction at an incidence angle of 89.3° with n-type doping concentration n = 3 × 1013 cm−2 and medium thickness d = 4.6 μm, and the maximum absorption can reach 99.98% with the full width at half peak (FWHM) of only 0.57 μm. Considering the case on surface oxidation, it may occur some possible effects for the perfect absorption, but with our structure, the proposed narrow-band absorption still have wide applications in the sensing and light body monitoring.

Directional dependence of band gap modulation via uniaxial strain in MoS2 and TiS3

Strain is widely employed to modulate the band structures of two-dimensional (2D) van der Waals (vdW) materials. Such band engineering with strain applied along different crystallographic directions, however, is less explored. Here, we investigate the band gap modulation of layered chalcogenides, MoS2 and TiS3, and the dependence of their band gaps on the directions of applied strain, using first-principles calculations. The band gap transition in MoS2 is found to reduce in energy linearly as a function of increasing tensile strain, with a weakly directional-dependent gradient, varying by 4.6 meV/% (from −52.7 ± 0.6 to −57.3 ± 0.1 meV/%) from the zigzag to armchair directions. Conversely, the band gap in TiS3 decreases with strain applied along the a lattice vector, but increases with strain applied in the perpendicular direction, with a non-linear strain-band gap relationship found between these limits. Analysis of the structure of the materials and character of the band edge states under strain helps explain the origins of the stark differences between MoS2 and TiS3. Our results provide new insights for strain engineering in 2D materials and the use of the direction of applied strain as another degree of freedom.

Layer-dependent excellent thermoelectric materials: from monolayer to trilayer tellurium based on DFT calculation.

Monoelemental two-dimensional (2D) materials, which are superior to binary and ternary 2D materials, currently attract remarkable interest due to their fascinating properties. Though the thermal and thermoelectric (TE) transport properties of tellurium have been studied in recent years, there is little research about the thermal and TE properties of multilayer tellurium with interlayer interaction force. Herein, the layer modulation of the phonon transport and TE performance of monolayer, bilayer, and trilayer tellurium is investigated by first-principles calcuations. First, it was found that thermal conductivity as a function of layer numbers possesses a robust, unusually non-monotonic behavior. Moreover, the anisotropy of the thermal transport properties of tellurium is weakened with the increase in the number of layers. By phonon-level systematic analysis, we found that the variation of phonon transport under the layer of increment was determined by increasing the phonon velocity in specific phonon modes. Then, the TE transport properties showed that the maximum figure of merit (ZT) reaches 6.3 (p-type) along the armchair direction at 700K for the monolayer and 6.6 (p-type) along the zigzag direction at 700K for the bilayer, suggesting that the TE properties of the monolayer are highly anisotropic. This study reveals that monolayer and bilayer tellurium have tremendous opportunities as candidates in TE applications. Moreover, further increasing the layer number to 3 hinders the improvement of TE performance for 2D tellurium.

Electronic transport in hexagonal zinc chalcogenide monolayers: 2D anisotropic nanodevices

The electronic transport properties of hexagonal ZnX (X = S, Se) monolayers have been studied by density functional theory (DFT) and non-equilibrium Green's function (NEGF) methods. Hexagonal ZnX monolayers have two distinct zinc-blende (ZB) and wurtzite (WZ) structures, each with two different directions (zigzag and armchair directions), thus giving rise to significant anisotropy. The ZnSe/ZB-based nanodevice displays the highest current along the zigzag direction, reaching a maximum value of 1.5 μA. This superiority over armchair direction demonstrates a significant anisotropy. At the same time, the current increases obviously under the control of gate voltage, and the maximum value can reach 3.4 μA. The photocurrent of the devices based on hexagonal ZnX monolayers also shows apparent anisotropy. The photocurrent of the ZnS/ZB-based device along the armchair direction reaches the maximum value of 15.1 a02/photon. The hexagonal ZnX monolayer-based devices achieve great extinction ratio, for instance 752.4 for ZnS/ZB-based device along the armchair direction. The excellent electronic and optoelectronic transport properties make hexagonal ZnX monolayers promising to be utilized as various electronic and optoelectronic devices.

Effects of sulfur vacancies on friction of two-dimensional MoS[formula omitted

Two-dimensional MoS2 is a promising material for solid lubrication, but sulfur vacancies are commonly observed during the manufacturing process. In this work, the frictional behaviors of MoS2 with sulfur vacancies are studied using molecular dynamics. The sliding process is simulated by changing the parameters such as sliding direction, vacancy concentration, and vacancy distribution. The results show that the armchair and zigzag directions exhibit different frictional behaviors. It is also observed that the friction force decreases with increasing vacancy concentration. At the same vacancy concentration, the vacancies in the line distribution have larger friction than those in the random distribution. The results are analyzed using the potential energy landscape and the interlayer distance. A model with two-vacancies and the resulting theoretical predictions explain the differences in friction force between the line and random distributions.

Strained Monolayer MoTe2 as a Photon Absorber in the Telecom Range.

To achieve the atomistic control of two-dimensional materials for emerging technological applications, such as valleytronics, spintronics, and single-photon emission, it is of paramount importance to gain an in-depth understanding of their structure-property relationships. In this work, we present a systematic analysis, carried out in the framework of density-functional theory, on the influence of uniaxial strain on the electronic and optical properties of monolayer MoTe2. By spanning a ±10% range of deformation along the armchair and zigzag direction of the two-dimensional sheet, we inspect how the fundamental gap, the dispersion of the bands, the frontier states, and the charge distribution are affected by strain. Under tensile strain, the system remains a semiconductor but a direct-to-indirect band gap transition occurs above 7%. Compressive strain, instead, is highly direction-selective. When it is applied along the armchair edge, the material remains a semiconductor, while along the zigzag direction a semiconductor-to-metal transition happens above 8%. The characteristics of the fundamental gap and wave function distribution are also largely dependent on the strain direction, as demonstrated by a thorough analysis of the band structure and of the charge density. Additional ab initio calculations based on many-body perturbation theory confirm the ability of strained MoTe2 to absorb radiation in the telecom range, thus suggesting the application of this material as a photon absorber upon suitable strain modulation.

Investigation of lattice thermal conductivity of [formula omitted] borophene and hydrogenated [formula omitted] borophene using reverse nonequilibrium molecular dynamics simulation

Recently, various two-dimensional boron structures have been predicted and some of them have been successfully synthesized. Borophene exhibits significant anisotropy in its electronic band structure, which has potential applications in electronic devices. In this study, a reactive molecular dynamics method was used to investigate the lattice thermal conductivity (LTC) and thermal anisotropy of α′ and α′−4H borophene structures at 300 K. The results show that the lattice thermal properties of α′ borophene monolayer are enhanced by hydrogenation along the armchair direction, and the lattice thermal conductivities of both α′ and α′−4H structures have high anisotropy. The results revealed that in the armchair direction, the thermal conductivity is increased by increasing the length of the samples while it is decreased for the zigzag direction. Owing to defects in the structure, interference of phonon waves, and phonon dispersion in the zigzag direction, the heat transfer decreases with increasing sample length.

Experimental Observation of Highly Anisotropic Elastic Properties of Two-Dimensional Black Arsenic.

Anisotropic two-dimensional layered materials with low-symmetry lattices have attracted increasing attention due to their unique orientation-dependent mechanical properties. Black arsenic (b-As), with the puckered structure, exhibits extreme in-plane anisotropy in optical, electrical, and thermal properties. However, experimental research on mechanical properties of b-As is very rare, although theoretical calculations predicted the exotic elastic properties of b-As, such as the anisotropic Young's modulus and negative Poisson's ratio. Herein, experimental observations on highly anisotropic elastic properties of b-As were demonstrated using our developed in situ tensile straining setup based on the effective microelectromechanical system. The cyclic and repeatable load-displacement curves proved that Young's modulus along the zigzag direction was ∼1.6 times greater than that along the armchair direction, while the anisotropic ratio of ultimate strain reached ∼2.5, attributed to the hinge structure in the armchair direction. This study could provide significant insights into the design of novel anisotropic materials and explore their potential applications in nanomechanics and nanodevices.

Near-Isotropic Local Attosecond Charge Transfer within the Anisotropic Puckered Layers of Black Phosphorus.

Black phosphorus possesses useful two-dimensional (2D) characteristics of van der Waals coupled materials but additionally features an in-plane anisotropic puckered layer structure that deviates from common 2D materials. Three distinct directions exist within the lattice of black phosphorus: the in-plane armchair and zigzag directions and the out-of-plane direction, with each distinct phosphorus 3p partial density of states. This structural anisotropy is imprinted onto various collective long-range properties, while the extent to which local electronic processes are governed by this directionality is unclear. At the P L1 edge, the directional selectivity of the core-hole clock method was used to probe the local charge transfer dynamics of electrons excited into the 3p-derived conduction band on an attosecond time scale. Here we show that the surprisingly small anisotropy of 3p electron transfer times reflects the similarly small differences in the 3p-derived unoccupied density of states caused by the underlying phosphorus bonding angles within the puckered layers.

Tailoring angle dependent ferroelectricity in nanoribbons of group-IV monochalcogenides

Ferroelectricity is significant in low dimensional structures due to the potential applications in multifunctional nanodevices. In this work, the tailoring angle dependent ferroelectricity is systematically investigated for the nanoribbons and nanowires of puckered group-IV monochalcogenides MX (M = ; X = ). Based on first-principles calculations, it is found that the ferroelectricity of nanoribbon and nanowire strongly depends on the tailoring angle. Firstly, the critical width for the bare nanoribbon of group-IV monochalcogenide is obtained and discussed. As the nanowires are concerned, the ferroelectricity will disappear when the tailoring angle becomes small. At last, H-passivation on the edge and the strain engineering are employed to improve the ferroelectricity of nanoribbon, and it is obtained that H-passivation is beneficial to the enhancement of polarization for nanoribbons tailored near the armchair direction, while the polarization of nanoribbons tailored along the diagonal direction will decrease when the edges are passivated with H atoms, and the tensile strain along the length direction always favors the improvement of ferroelectricity of the considered nanoribbons. Therefore, tailoring angle has great influence on the ferroelectricity of nanoribbons and nanowires, which may be used as an effective way to tune the ferroelectricity and further the electronic structures of nanostructures in the field of nanoelectronics.

Highly stable two-dimensional α1-MA2Z4 (M = Mg, Ca, Sr; A = Al; Z = S, Se) monolayers with promising photocatalysis and piezoresistive effect

The fundamental properties of two-dimensional α1-MA2Z4 (M = Mg, Ca, Sr; A = Al; Z = S, Se) monolayers have been systematically investigated based on the first principles calculations. Our results show that the α1-MA2Z4 monolayers have mixed ionic-covalent bonding character. The structural stability analyses reveal that all structures are dynamically stable and sustain stability below 800 K. All α1-MA2Z4 monolayers exhibit semiconducting property. The suitable bandgaps and the band edges alignment strides the redox potentials of water splitting, having potential as the candidates of the photocatalyst. All structures are predicted to possess isotropic electron-dominated mobility, which increases from 340.79 to 591.84 cm2 s−1 V−1 with the increase in atomic number of the alkaline-earth metal and chalcogen group. Further imposing the strain along the armchair direction, the electron mobility of α1-MA2Z4 can be enhanced to ∼103 cm2 V−1 s−1, especially the electron mobility of α1-MgA2S4 even increases to ∼2 × 103 cm2 V−1 s−1. The increased electron mobility indicates the reduced resistivity, which shows that the α1-MA2Z4 monolayers possess a remarked piezoresistive effect. The outstanding properties indicate that the α1-MA2Z4 is promising in photocatalysis and flexible electronics fields.

Regulation of Near-Field Radiative Heat Transfer between Multilayer BP/hBN Heterostructures.

As an allotrope of phosphorus and a promising 2D semiconductor, black phosphorus (BP) exhibits in-plane anisotropy along its armchair and zigzag crystal directions, allowing for efficient regulation of near-field radiative heat transfer (NFRHT). In this work, we investigate the NFRHT between two multilayer BP/hBN heterostructures and theoretically demonstrate that thermal regulation can be realized by tuning the electron density and rotation angle of BP. Results show that a larger electron density leads to the coupling of anisotropic surface plasmon polaritons (SPPs) of BP with hyperbolic modes of hBN, and rotation of BP changes the anisotropic characteristic of coupled SPPs on both sides, whereby a regulation ratio of 5.8 can be obtained. We also analyze the effects of period number, hBN layer thickness, and topmost-layer material on the NFRHT. This work may be beneficial for efficient nanoscale thermal management and physical understanding of radiative heat transfer based on anisotropic SPPs.

WS2 and WSSe bilayer with excellent carrier mobility and power conversion efficiency

The WS2/WSSe and WSSe/WS2 bilayers are constructed using single-layer WS2 and WSSe with a lattice mismatch rate of only 1.85 %, and their structural properties, carrier mobility, electrical properties, and photocatalytic performance are performed using density functional theory. The carrier mobility of WS2/WSSe and WSSe/WS2 bilayers is better than that of single-layer WS2. And the conductivity of the WSSe/WS2 bilayer in the Zigzag direction is superior to that in the Armchair direction. The results of band structure using the HSE06 method demonstrate that the most stable WS2/WSSe and WSSe/WS2 bilayers are type-II band alignment. WS2/WSSe and WSSe/WS2 bilayers can undergo redox reactions at pH = 0 and pH = 7 respectively. And they all emerge a built-in electric field to cause photo-generated carriers to recombine between the layers to upgrade the photocatalytic performance of the bilayer, which is a typical S-scheme heterojunction. And the power conversion efficiency (PCE) of the WS2/WSSe and WSSe/WS2 bilayers are 15.5 % and 15.7 % respectively, so the utilization rate of sunlight is relatively high. In general, WS2/WSSe and WSSe/WS2 bilayers are ideal materials in the field of photocatalytic water splitting.

Enhanced piezoelectricity of single layer Si–V (V[dbnd]N, P and As) compounds induced by uniaxial strain

Based on the first-principles calculations, we investigate the strain effect on the mechanical, dynamical, electronic and piezoelectric properties of single-layer Si–V (VN, P and As) compounds. Our results predict that the elastic regions for SiN, SiP and SiAs are below 18%, 18% and 19% along the armchair direction, corresponding to the ideal strengths of 12.7 GPa, 4.9 GPa and 3.8 GPa. In the elastic region, we further ascertain the structures do not distortion until the strain exceeding to 14%, 16% and 17% by performing the ab initio molecular dynamics simulations at room temperature. In the above strain ranges, the structures always keep semiconducting property. Due to the non-central symmetry, the Si–V compounds exhibit piezoelectricity along the armchair direction. As the strain increases, the piezoelectric coefficients e11 and d11 increase with parabolic character, of which the d11 increase to 13.49 p.m./V, 51.61 p.m./V and 77.91 p.m./V, implying the enhancement of electromechanical coupling ability. Our results provide a guidance for single-layer Si–V compounds applied in piezoelectric fields.

Molecular modelling of graphene nanoribbons on the effect of porosity and oxidation on the mechanical and thermal properties

Graphene is considered as the most promising nanomaterial of the recent decades given the huge amount of studies that have been performed to characterize its outstanding properties and in searching of novel applications. Following this tendency, this study covers the modelling of graphene nanoribbons (GNRs) with the aim of analyzing the effect of porosity and oxidation on the tensile mechanical properties and in-plane thermal conductivity through molecular dynamics (MD). Using quasi-static simulations the mechanical properties were evaluated in first place. A ‘hardening’ mechanism was observed for GNRs at porosities below 1%, i.e. perfect or near-perfect GNRs, by which the GNRs could withstand higher loading levels. This hardening effect was manifested in the carbon network by the generation of dislocation lines formed by pentagon-heptagon pairs (5–7 defects), which acted as a stress reliever. The failure of GNRs was produced as a tearing mechanism with cracks growing along the armchair or zigzag directions. The porosity affected all the analysed tensile mechanical properties (i.e., Young’s modulus, Poisson’s ratio, tensile strength and deformation at break), but with different tendency in the fracture properties due to the presence or absence of hardening behaviour in the GNRs. Nevertheless, the oxidation affected only the tensile modulus and Poisson’s ratio but not to the tensile strength and deformation at break. The thermal conductivity of the GNRs was affected either by the porosity and oxidation. Pores and oxidation groups acted as phonon scatterers since they disrupted the carbon network by the generation of vacancies or out-of-plane carbons, respectively, which decreased the phonon mean free path and thus the thermal conductivity. In conclusion, the porosity and oxidation of GNRs greatly determine the tensile mechanical properties and in-plane thermal conductivity of such materials and must be considered when tuning the synthetic pathways.Graphical abstractThe effect of porosity and oxidation on the tensile mechanical and thermal conductivity properties of graphene nanoribbons are evaluated through molecular dynamics simulations.

N‐Type Doping and Passivation of Black Phosphorus Using Pentacene for Infrared Polarization‐Sensitive Photodetection and Imaging

AbstractFew‐layer black phosphorus (FL‐BP) shows great potential in infrared polarization‐sensitive photodetection due to its narrow bandgap and strong in‐plane anisotropy. However, FL‐BP is easy to be oxidized and degraded, which severely restricts its applications. Herein, a simple and scalable in situ interfacial passivation method based on pentacene film is reported to enhance the ambient stability of FL‐BP as well as achieve n‐type doping for improved polarization‐sensitive photoimaging. High crystal quality FL‐BP flakes with intrinsic anisotropy are prepared by electrochemical exfoliation. The morphology and Raman spectra of FL‐BP/pentacene exhibit excellent stability 28 days after passivation. In addition, the hole mobility is significantly enhanced and dark current is substantially suppressed due to the interfacial passivation and n‐type doping effect. The FL‐BP/Pentacene based photodetector shows a broadband response from visible to near‐infrared with good stability. The photoresponsivity under 915 nm illumination is calculated as 2.2 mA W−1 with an anisotropy ratio of 1.29. Furthermore, the polarization imaging under the irradiation at zigzag and armchair directions directly reveals the strong infrared polarization‐sensitive imaging capability of FL‐BP/pentacene. This work also opens up new avenues to enhance the photoresponse and stability of various optoelectronics based on similarly unstable 2D materials.