Zigzag Direction Research Articles

Structural, mechanical, electronic and optical properties of biphenylene hydrogenation: a first-principles study.

Biphenylene networks typically exhibit a metallic electronic nature, while hydrogenation can open the band gap changing it to a semiconductor. This property makes hydrogenated biphenylene a promising candidate for use in semiconductor optoelectronic materials and devices. In this work, three representative configurations of hydrogenated biphenylene, denoted by α, β and γ, were investigated. The structural, mechanical, electronic, and optical properties of these hydrogenated biphenylene configurations were calculated by first-principles calculations. Band gaps with HSE correction were 4.69, 4.42 and 4.39 eV for α, β, and γ configurations, respectively. Among these three configurations, β presents the best electronic performance and special elastic properties (negative Poisson's ratio), while γ exhibits the best elastic properties. In addition, we comprehensively analyze the mechanical properties of these configurations and provide evidence that hydrogenated biphenylene possibly exhibits a negative-Poisson's-ratio along the zigzag and armchair directions when hydrogen atoms are added to biphenylene in certain ways. Furthermore, although the electronic properties of γ are weaker than those of β, they are also excellent. In addition, the binding energies of β and γ are relatively lower, which indicates that β and γ are more stable. Our findings demonstrate that the hydrogenated biphenylene is a promising material with significant application potential in optoelectronic devices.

Suppressed thermal conductivity of bilayer SnS: A comparative study among the monolayer, bilayer and bulk SnS

The layer-stacking of 2D materials have been proven to be an effective tool to modulate the low-dimensional electronic structures and transport properties. In this study, the lattice thermal transport properties of bilayer SnS are investigated systematically by solving phonon Boltzmann transport equation based on first-principles calculations. The stacking effect on the in-plane thermal conductivity is further revealed through comparing phonon transport properties of the monolayer, bilayer and bulk SnS. A dramatic suppression of the thermal conductivity, even below 0.5 Wm −1 K −1 per layer at 300 K, can be observed for the fully relaxed bilayer SnS, which implies the great potential for SnS-based thermoelectric applications. The underlying phonon transport mechanisms are also uncovered, and the drop of phonon relaxation time, resulted from the enhanced interlayer anharmonic phonon scattering, is responsible for the suppression of lattice thermal transport. • The stacking stability of bilayer SnS is revealed, and the AA′-stacking is the most dynamically stable configuration for the bilayer SnS. • A dramatically suppressed thermal conductivity for the special stacking configuration of the bilayer SnS, e. g. 0.294 (0.112) Wm −1 K −1 per layer at 300 K along the zigzag (armchair) direction, can be predicted. • The underlying phonon transport mechanisms, and the stacking effects, are also revealed through a comparative study among the monolayer, bilayer and bulk SnS.

Anisotropic charge transfer and gate tuning for p-SnS/n-MoS2 vertical van der Waals diodes.

2D-material-based van der Waals heterostructures (vdWhs) have shown great potential in next-generation multi-functional microelectronic devices. Thanks to their sharp interface and ultrathin thickness, 2D p-n junctions with high rectification properties have been established by combining p-type monochalcogenides with n-type transition metal dichalcogenides. However, the anisotropic rectification together with the charge transfer and gate effect has not been clarified. Herein, the electrical anisotropy of p-SnS/n-MoS2 diodes was studied. Optimum ideality factors within 1.08-1.18 have been achieved for the diode with 6.6 nm thick SnS on monolayer MoS2, and a high rectification ratio of 3.1 × 104 with strong in-plane anisotropy is observed along the zigzag direction of SnS. A strong gate effect on the anisotropic series resistance has been verified and an effective tuning over the transport length of the SnS channel can be established through adjustment of the current orientation and gate voltage. A thickness-dependent minority carrier transport mechanism has also been demonstrated for the reverse drain current, and Fowler-Nordheim tunneling and direct tunneling are proposed for the increase of the reverse current of the thicker and thinner diodes, respectively. This work will provide another strategy for high-performance diodes based on vdWhs via the control of the current orientation and the gate effect.

Unveiling adsorption characteristics of BC5 monolayer: High electronic anisotropy and gas sensing performance

The exploitation of highly sensitive, power-efficient, robust and recoverable gas sensing devices has become a key tool for the detection of toxic gases in the environment. In this work, we theoretically elucidate the electronic properties of the BC5 monolayer and the gas-sensitive performance of the BC5-based gas sensor for organic and inorganic gases using density functional theory and non-equilibrium Green’s function method. The results indicate that SO2, CH4, and CH3OH are physisorption, whereas CO, NH3, NO, NO2, C2H2, C2H4 and HCHO are chemisorption on the BC5 monolayer. Then, the transport properties of the BC5-based nanodevice are sensitive to the adsorption of CO, NH3, NO, NO2, C2H2, C2H4 and HCHO, hence the BC5 monolayer can be used to construct the gas sensor for detecting these gas molecules. In addition, the pristine BC5-based nanodevice shows a strong anisotropic transport properties under low bias voltages (Vb), and the ratio of the maximum anisotropy is 1.94 when Vb is 0.1 V. In the meantime, the BC5-based gas sensor is more sensitive to CO and HCHO with the gas sensitivity reaching 1.76 and 2.56 at the bias of 0.1 V along the zigzag direction, respectively. Finally, the microscopic mechanisms of inorganic and organic gas molecules adsorbed on the BC5-based gas sensor are systematically revealed by presenting the respective scattering states at Fermi level. All these theoretical predictions might trigger a new avenue to expand the application of fresh carbon based nano-materials in designing next generation gas sensing devices.

Van der Waals Interlayer Coupling Induces Distinct Linear Dichroism in WSe2 Photodetectors

AbstractDriven by the unique linear dichroism feature of anisotropic two‐dimensional (2D) materials, much attempt has been made to introduce artificial anisotropy into isotropic 2D materials to deliver polarization‐sensitive performance. However, those methods have not been widely promoted because of technical limitations. This paper reports on the successful modulation to the structural symmetry of WSe2 through van der Waals (vdW) interlayer coupling. Correspondingly, both the polarized Raman and photoluminescence (PL) spectra of WSe2 in heterostructure exhibit periodic variation tendency. Similarly, direction‐sensitive electronic transport has also been observed in WSe2/CrOCl heterostructure, and the conductance along armchair direction is ≈1.5 times of that along zigzag direction. As a functional illustration to the interlayer coupling induced linear dichroism, angle‐dependent photodetection is conducted to WSe2/CrOCl device. With light irradiation at 532 nm, the device photocurrent along armchair direction is 1.27 times higher than that along zigzag direction. The abnormal linear dichroism of WSe2 in the study clearly proves that few‐layer WSe2 on CrOCl flake has been entitled with artificial anisotropy. The interlayer coupling method is feasible and practical in 2D‐material range, offering the possibility to develop polarization‐dependent electronic, pyroelectric, and photoelectric devices from isotropic 2D materials.

Tunable absorptance by the magnetic field in multilayer black phosphorene dielectric structures

The magnetic field effect on the interband optical conductivity of black phosphorene (BP) is considered in the investigation of the absorptance of BP-based multilayer dielectric structures using the transfer matrix method. The BP interband optical conductivity is anisotropic and shows multi-peaks in the presence of the magnetic field because of the splitting of energy levels. The peak position of the absorptance is tuned via the magnetic field strength. In the energy region (0.5 eV - 1.7 eV), the maximum absorptance in the armchair direction is larger than that in the zigzag (ZZ) direction which depends on the angle of incidence and the dielectric structure. But the increased rate of the maximum absorptance in the presence of a magnetic field in the ZZ direction is quite obvious and increased about 10 times. An increased number of BP/Si layers makes the optical absorption in both directions much stronger.

Mechanical characterization of nanoporous two-dimensional Ti3C2 MXene membranes

• The Molecular Dynamics with ReaxFF potential were used to investigate the mechanical properties of nanoporous two-dimensional Ti 3 C 2 MXene. • Young's model decreases with the increase of the area of the nanoporous. • The fracture pressure decrease with the increase of separation distance in the zigzag direction due to anisotropy in the structure of MXene. • The nanoporous Ti 3 C 2 MXene membrane can maintain its mechanical integrity in the reverse osmosis desalination process. We performed a molecular dynamics simulation to study the mechanical properties of nanoporous two-dimensional Ti 3 C 2 MXene membranes. The aim is to find out whether these membranes are strong enough to maintain their mechanical integrity in the reverse osmosis desalination process. We modeled the atomic interaction potential of the Ti 3 C 2 system using a reactive force field (ReaxFF) potential. The obtained results indicate that the ultimate strength of nanoporous MXene membranes decreases with an increase in the nanoporous surface and the separation distance in the armchair direction. However, it increases with the decrease of separation distance following the zigzag direction. The Young's modulus of Ti 3 C 2 with a grating nanoporous (112 rectangle pores) was found to be equal to 475.699 GPa and 421.008 GPa, and the calculated fracture pressures at 300K were equal to 30.09 GPa and 28.25 GPa for porous with 28.666 Å 2 and 38.639 Å 2 respectively. The simulation findings are in good agreement with experimental ones for a sample without pores.

Anisotropy of the Optical Properties of Pentacene:Black Phosphorus Interfaces

Black phosphorus (BP) is a layered material with anisotropic properties. We study interfaces formed by a pentacene monolayer adsorbed on monolayer BP, a prototypical system for BP surface passivation. We place the pentacene monolayer along the zigzag and armchair directions of the BP substrate, respectively, to examine the anisotropy of the heterogeneous interfaces. We perform first-principles $GW$ plus Bethe-Salpeter equation ($GW$-BSE) calculations to determine the quasiparticle and optical properties. To quantitatively analyze the anisotropy of the optical properties, we develop a general computational scheme to decompose the interface excitons into different contributions. We find a distinct charge-transfer exciton formed when the monolayer pentacene is placed along the armchair direction, and discuss how the anisotropy of each component is modulated by the interface. Our results shine a light on the understanding of the BP surface passivation via molecular adsorption and provide a benchmark for future experimental and computational studies.

Variable thermal transport in black, blue, and violet phosphorene from extensive atomistic simulations with a neuroevolution potential

Phosphorus has diverse chemical bonds, and even in its two-dimensional form, there are three stable allotropes: black phosphorene (Black-P), blue phosphorene (Blue-P), and violet phosphorene (Violet-P). Due to the complexity of these structures, no efficient and accurate classical interatomic potential has been developed for them. In this paper, we develop an efficient machine-learned neuroevolution potential model for these allotropes and apply it to study thermal transport in them via extensive molecular dynamics (MD) simulations. Based on the homogeneous nonequilibrium MD method, the thermal conductivities are predicted to be 12.5±0.2 (Black-P in armchair direction), 78.4±0.4 (Black-P in zigzag direction), 128±3 (Blue-P), and 2.36±0.05 (Violet-P) Wm−1K−1. The underlying reasons for the significantly different thermal conductivity values in these allotropes are unraveled through spectral decomposition, phonon eigenmodes, and phonon participation ratio. Under external tensile strain, the thermal conductivity in black-P and violet-P are finite, while that in blue-P appears unbounded due to the linearization of the flexural phonon dispersion that increases the phonon mean free paths in the zero-frequency limit.

Negative Poisson's ratio effect of P2/m phosphine

Negative Poisson's ratio materials have the advantages of good shear resistance, dent resistance, and fracture resistance. Thus, they have great application potential in the manufacturing field with high requirements for material mechanical flexibility. However, negative Poisson's ratio materials are relatively rare. In this paper, the first principles calculation is used to study the single-layer P2/m-P phosphorene. It is found that when −5% to 5% strain is applied along the zigzag (Y) direction, there is a negative Poisson's ratio effect along the Z direction, and Poisson's ratio is −0.288, which is about ten times that of the single-layer black phosphorus (the negative Poisson's ratio of the single-layer black phosphorus is −0.027). Compared with black phosphorene, it has more potential applications in wearable and impact resistant equipment.

Giant bulk piezophotovoltaic effect in 3R-MoS2.

Given its innate coupling with wavefunction geometry in solids and its potential to boost the solar energy conversion efficiency, the bulk photovoltaic effect (BPVE) has been of considerable interest in the past decade1-14. Initially discovered and developed in ferroelectric oxide materials2, the BPVE has now been explored in a wide range of emerging materials, such as Weyl semimetals9,10, van der Waals nanomaterials11,12,14, oxide superlattices15, halide perovskites16, organics17, bulk Rashba semiconductors18 and others. However, a feasible experimental approach to optimize the photovoltaic performance is lacking. Here we show that strain-induced polarization can significantly enhance the BPVE in non-centrosymmetric rhombohedral-type MoS2 multilayer flakes (that is, 3R-MoS2). This polarization-enhanced BPVE, termed the piezophotovoltaic effect, exhibits distinctive crystallographic orientation dependence, in that the enhancement mainly manifests in the armchair direction of the 3R-MoS2 lattice while remaining largely intact in the zigzag direction. Moreover, the photocurrent increases by over two orders of magnitude when an in-plane tensile strain of ~0.2% is applied, rivalling that of state-of-the-art materials. This work unravels the potential of strain engineering in boosting the photovoltaic performance, which could potentially promote the exploration of novel photoelectric processes in strained two-dimensional layered materials and their van der Waals heterostructures.

Wigner molecules in phosphorene quantum dots

We study Wigner crystallization of electron systems in phosphorene quantum dots with confinement of an electrostatic origin with both circular and elongated geometry. The anisotropy of the effective mass allows for the formation of Wigner molecules in the laboratory frame with a confined charge density that has lower symmetry than the confinement potential. We find that in circular quantum dots separate single-electron islands are formed for two and four confined electrons but not for three trapped carriers. The spectral signatures of the Wigner crystallization to be resolved by transport spectroscopy are discussed. Systems with Wigner molecule states are characterized by a nearly degenerate ground state at $B=0$ and are easily spin-polarized by the external magnetic field. In electron systems for which the single-electron islands are not formed, a more even distribution of excited states at $B=0$ is observed, and the confined system undergoes ground state symmetry transitions at magnetic fields of the order of 1 Tesla. The system of five electrons in a circular quantum dot is indicated as a special case with two charge configurations that appear in the ground-state as the magnetic field is changed: one with the single electron islands formed in the laboratory frame and the other where only the pair-correlation function in the inner coordinates of the system has a molecular form as for three electrons. The formation of Wigner molecules of quasi-1D form is easier for the orientation of elongated quantum dots along the zigzag direction with heavier electron mass. The smaller electron effective mass along the armchair direction allows for freezing out the transverse degree of freedom in the electron motion.

Hydrogenated Boron Phosphide THz-Metamaterial-Based Biosensor for Diagnosing COVID-19: A DFT Coupled FEM Study.

Recent reports focus on the hydrogenation engineering of monolayer boron phosphide and simultaneously explore its promising applications in nanoelectronics. Coupling density functional theory and finite element method, we investigate the bowtie triangle ring microstructure composed of boron phosphide with hydrogenation based on structural and performance analysis. We determine the carrier mobility of hydrogenated boron phosphide, reveal the effect of structural and material parameters on resonance frequencies, and discuss the variation of the electric field at the two tips. The results suggest that the mobilities of electrons for hydrogenated BP monolayer in the armchair and zigzag directions are 0.51 and 94.4 cm2·V-1·s-1, whereas for holes, the values are 136.8 and 175.15 cm2·V-1·s-1. Meanwhile, the transmission spectra of the bowtie triangle ring microstructure can be controlled by adjusting the length of the bowtie triangle ring microstructure and carrier density of hydrogenated BP. With the increasing length, the transmission spectrum has a red-shift and the electric field at the tips of equilateral triangle rings is significantly weakened. Furthermore, the theoretical sensitivity of the BTR structure reaches 100 GHz/RIU, which is sufficient to determine healthy and COVID-19-infected individuals. Our findings may open up new avenues for promising applications in the rapid diagnosis of COVID-19.

First-principles determination of high thermal conductivity of PCF-graphene: A comparison with graphene

Poly-cyclooctatetraene framework (PCF)-graphene, an emerging all-sp2 hybridized two-dimensional (2D) carbon allotrope, possesses an intrinsic direct bandgap (0.77 eV) and excellent mechanical properties, indicating great potential in nanoelectronics. Understanding the thermal transport behavior of PCF-graphene is of vital importance for determining the reliability of related devices based on it. In this work, the thermal transport in PCF-graphene is systematically studied using the Boltzmann transport theory combined with first-principles calculations. The results show that the room-temperature thermal conductivity of PCF-graphene with only considering three-phonon scattering is as high as 1587.3 W/m K along the zigzag direction, and decreases by 27.1% (1157.4 W/m K) when including four-phonon scattering, indicating the four-phonon scattering plays a non-negligible role in in thermal transport. Although the thermal conductivity of PCF-graphene is not as large as that in graphene, it still exceeds most common 2D materials and makes it suitable for applications in the thermal management of microelectronics. Analyses of phonon group velocity and phonon scattering rates are conducted to reveal the high thermal conductivity of PCF. Moreover, as the temperature increases to 800 K, the reduction of thermal conductivity is close to 50% after including four-phonon scattering. The analysis of phonon group velocity and phonon scattering rates are conducted to reveal the underlying mechanism. Our results provide insights for constructing high-thermal-conductivity materials based on 2D carbon allotropes.

Enhanced Thermoelectric Performance in Black Phosphorene via Tunable Interlayer Twist.

Twist-angle two-dimensional (2D) systems are attractive in their exotic and tunable properties by the formation of the moiré superlattices, allowing easy access to manipulating intrinsic electrical and thermal properties. Here, the angle-dependent thermoelectric properties of twisted bilayer black phosphorene (tbBP) by first-principles calculations are reported. The simulations show that significantly enhanced Seebeck coefficient and power factor can be achieved in p-type tbBP due to merging of the multi-valley electronic states and flat moiré bands. Moreover, the twisted layers bring in a strong anharmonic phonon scattering and thus very low lattice thermal conductivity of 4.51Wm-1 K-1 at 300K. Consequently, a maximal ZT value can be achieved in p-type 10.11° tbBP along the armchair direction up to 0.57 and 1.06 at 300 and 500K, respectively. The room-temperature ZT value along the zigzag direction is also significantly increased by almost 40 times compared to pristine BP when the twist angle is close to 70.68°. This work demonstrates a platform to manipulate thermoelectric performance in 2D materials by creating moiré patterns, leading tbBP as a promising eco-friendly candidate for thermoelectric applications.

Zigzag direction nanoarchitectonics of monolayer GeSe for SO2 gas sensors with high sensitivity and selectivity: a first-principles study

Zigzag direction nanoarchitectonics of monolayer GeSe for SO2 gas sensors with high sensitivity and selectivity: a first-principles study

First-Principles Study of the Phonon Lifetime and Low Lattice Thermal Conductivity of Monolayer γ-GeSe: A Comparative Study

Germanium selenide (GeSe) is a unique two-dimensional (2D) material showing various polymorphs stable at ambient condition. Recently, a new phase with a layered hexagonal lattice ({\gamma}-GeSe) was synthesized with ambient stability and extraordinary electronic conductivity even higher than graphite while its monolayer is semiconducting. In this work, via using first-principles derived force constants and Boltzmann transport theory we explore the lattice thermal conductivity ({\kappa}_l) of the monolayer {\gamma}-GeSe, together with a comparison with monolayer {\alpha}-GeSe and {\beta}-GeSe. The {\kappa}_l of {\gamma}-phase is relatively low (5.50 W/mK), comparable with those of {\alpha}- and {\beta}- phases. The acoustic branches in {\alpha}-GeSe are well separated from the optical branches, limiting scattering channels in the phase space, while for \b{eta}-GeSe and {\gamma}-GeSe the acoustic branches are resonant with the low-frequency optical branches facilitating more phonon-phonon scattering. For {\gamma}-GeSe, the cumulative {\kappa}_l is isotropic and phononic representative mean free path (rMFP) is the shortest (17.07 nm) amongst the three polymorphs, indicating that the {\kappa}_l of the {\gamma} phase is less likely to be affected by the size of the sample, while for {\alpha}-GeSe the cumulative {\kappa}_l grows slowly with mean free path and the rMFP is longer (up to 20.56 and 35.94 nm along zigzag and armchair direction, respectively), showing a stronger size-dependence of {\kappa}_l. Our work suggests that GeSe polymorphs with overall low thermal conductivity are promising contenders for thermoelectric and thermal management applications.

First-principles investigation of aluminum intercalation in bilayer blue phosphorene for Al-ion battery

Here, we investigate the possibility of bilayer phosphorene as an anode material for aluminum-ion batteries using first-principles calculations. The characteristics of negative electrode materials are desirable with small volume expansion, large capacity and high mobility. We calculated the adsorption energies, optimal adsorption sites, diffusion barriers, theoretical capacities, open circuit voltages (OCVs), and structural stability of aluminum (Al) on the surface and interlayers of bilayer phosphorene. The intercalation of Al did not significantly change the volume of the bilayer phosphorene. The diffusion barrier of Al on the surface along the zigzag direction is 0.13 eV, while the diffusion barrier along the armchair direction is 0.49 eV. When Al is intercalated into the phosphorene interlayer, the diffusion barriers along the armchair and zigzag directions are 0.77 eV and 0.47 eV, respectively. It can be seen that the diffusion of Al between layers is also relatively easy. And with the increase of Al intercalation concentration, the theoretical capacity of bilayer phosphorene can reach 1047 mAhg−1. These results demonstrate that bilayer phosphorene is a potential candidate as anode material for aluminum-ion batteries.

Theoretical study of the line defect in χ3-borophene: structures, electronic properties, direct-current and alternating-current transport properties

Two-dimensional material χ3-borophene is a stable borophene allotrope and has been synthesized in experiment. Combining density functional theory with non-equilibrium Green’s function method, in the present study we theoretically investigated the structures, stabilities, electronic properties, and the direct-current (DC) and alternating-current (AC) transport properties of its line defects. The obtained results suggest that along the zigzag direction there exist some special line defects that can enhance the stability of χ3-borophene and the six-coordination B atoms play a key role for the enhanced stability. The line defects may change the atomic orbital components of the Dirac cone of χ3-borophene and have profound influences on the electronic properties. For the DC transport, under high bias voltages along a given transport direction the line defects may lead to constant current phenomena. For the AC transport, under different frequency ranges the line defects have different AC responses with respect to the external time-dependent field, exhibiting complicated AC transport behaviors. Furthermore, for the low-frequency range, the equivalent circuits of χ3-borophene and its line defects were suggested, which will be beneficial for designing borophene-based functional nanodevices.

Boosting the solar conversion efficiency of MoSe2/PtX2 (X = O, S) vdW heterostructure by strain and electric field engineering

Vertically stacking two-dimensional materials via weak van der Waals (vdW) forces is an effective strategy for modulating optoelectronic performance of materials. To accelerate more novel MoSe2-based heterostructure design, the interlayer coupling effect in MoSe2/PtX2 (X = O, S) heterostructure has been systematically studied, from the atomic structure to the electronic and optical properties, on the basis of first-principles calculations and BSE model with scissor inclusion. Density functional theory (DFT) calculations unveil a type-II indirect bandgap measuring between 0.85 and 0.91 eV at HSE06 level, with Bader and charge density difference analyses suggesting occurrence of charge redistributions at the interface and electrons diffusion from MoSe2 to PtX2 layers, driven by large band offsets. The thermodynamic and thermal stabilities of the heterostructures are demonstrated by the negative binding energy and AIMD simulation. The heterostructure interface is influenced by the weak vdW coupling with an equilibrium interlayer distance of 3.01 to 3.08 Å and binding energy of −5.5 to −11.2 meV Å−2, indicating an exothermic process and steady adhesion at the interface. Reasonable lattice mismatch that ranges from 1.5 to 4.7% between the vdW heterostructure and separate monolayers suggests good structure compatibility. The optical performance of the heterostructure was examined using the real and imaginary components of dielectric function, where enhanced light absorption of 104–105 cm−1 and prominent peaks are observed encompassing the infrared to ultraviolet domains. Record high spectroscopic limited maximum efficiency (SLME) of ∼33% was also predicted. The absorption strength of MoSe2/PtO2 and MoSe2/PtS2 enhances with increasing negative external electric field (Eext) and compressive strain, individually, inferring their optical properties modulation by Eext and biaxial strain. Both heterostructures present high carrier mobility up to 1322.98 cm2 V−1 s−1 in zigzag direction.