Armchair Direction Research Articles

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.

Thermal conductivity of the popgraphene monolayer tailored by strain and defect: A molecular dynamics study

As a new carbon allotrope, popgraphene comprising of 5-8-5 carbon rings has attracted great attention due to its extraordinary properties for its promising device applications, including the next-generation high-performance electronics, nano-electromechanical (NEMS) systems as well as nanocomposites. Herein, the thermal properties of popgraphene have been systematically studied via reverse non-equilibrium molecular dynamic and phonon spectrum analysis, focusing on size, strain, temperature, and defect effects. The in-plane thermal conductivity along the zigzag (x) direction is much higher than that along the armchair (y) direction at the same length, indicating a strong anisotropy. The thermal conductivity can be modulated by external strain, and it has a ∼T-0.37 dependence on temperature from 100 to 500 K. Additionally, the existence of defect can significantly decrease the thermal conductivity due to the suppressed in-plane phonon vibration in the high-frequency region. The findings provide helpful guidance in modulating the thermal conductivity of popgraphene, which is important for thermal management in NEMS applications.

First-principles design of an ultralow frictional interface of a black phosphorus and graphene heterostructure with oxide functionalization and high-pressure conditions

Using first-principles density functional theory (DFT) calculations, we demonstrate that the heterointerface of black phosphorus and graphene (BP/Gr) should be a promising lubricant, especially under high pressure conditions. The ultralow interfacial frictional sliding motion is enabled by graphene oxide (GO) functional groups, such as epoxy and hydroxyl groups. These functional groups significantly modulate the interfacial charge distribution to facilitate interfacial slip. The epoxy functional group enables a reduction in not only the bilayer adhesion but also the shear strength along the armchair direction by approximately 40% compared with that without functionalization. The potential energy surface (PES) calculation shows the possibility of an ultralow friction sliding process in BP/Gr and BP/GO due to the presence of almost frictionless paths. In addition, high-accuracy PES calculations predict an almost frictionless sliding path even under pressure. Interestingly, our calculation shows that the corrugation energy and shear strength of BP/Gr decrease under 10 GPa. We elucidated that the origin of the improved frictional properties under high pressure stems from the unique heterostructure of BP/Gr. These results consistently suggest that BP/Gr is promising lubricants for high-pressure applications, which can be further improved using a design principle of exploring optimal interfaces and functional groups.

High switching ratio and inorganic gas sensing performance in BeN4 based nanodevice: a first-principles study

Recently, Dirac material BeN4 has been synthesized by using laser-heated diamond anvil-groove technology (Bykov et al 2021 Phys. Rev. Lett. 126 175501). BeN4 layer, i.e. beryllonitrene, represents a qualitatively class of two-dimensional (2D) materials that can be built of a metal atom and polymeric nitrogen chains, and hosts anisotropic Dirac fermions. Enlighten by this discovered material, we study the electronic structure, anisotropic transport properties and gas sensitivity of 2D BeN4 using the density functional theory combined with non-equilibrium Green’s function method. The results manifest that the 2D BeN4 shows a typical semi-metallic property. The electronic transport properties of the intrinsic BeN4 devices show a strong anisotropic behavior since electrons transmitting along the armchair direction is much easier than that along the zigzag direction. It directly results in an obvious switching characteristic with the switching ratio up to 105. Then the adsorption characteristics indicate that H2S, CO, CO2 and H2 molecules are physisorption, while the NH3, NO, NO2, SO2 molecules are chemisorption. Among these chemisorption cases, the 2D gas sensor devices show an extremely high response for SO2 recognition, and the high anisotropy of the original 2D BeN4 device still maintains after adsorbing gas molecules. Finally, high switching ratio and inorganic gas sensing performance of BeN4 monolayer could be clearly understood with local density of states, bias-dependent spectra, scattered state distribution. In general, the results indicate that the designed BeN4 devices have potential practical application in high-ratio switching devices and high gas-sensing molecular devices.

Mechanical properties of 2D Zr n +1C n (n = 1, 2) MXenes with and without functionalization

Transition metal carbides and nitrides (MXenes) are considered the new generation of flexible electronic materials because of their superior mechanical strength and flexibility. Based on the density functional theory, the structures, electronic properties and mechanical properties of the 2D Zr-based MXenes with and without surface functional groups (O, F and OH) are investigated systematically to explore their elastic properties and tensile fracture mechanism. The results reveal the tensile strength and critical strain under biaxial tensile direction can reach 52 GPa, 12% for Zr2C and 55 GPa, 19% for Zr3C2, more outstanding than the mechanical behavior of the pristine Ti2C (47 GPa, 9.5%). The tensile behaviors of the functionalized Zr n +1CnT2 (n = 1, 2, T = O, F, OH) strongly depend on the crystallographic orientation and the surface functional group. The phonon spectrum under the critical strain indicates the tensile fracture of the pristine Zr-based MXenes was determined by phonon instability, except along the armchair direction of Zr2C and zigzag direction of Zr3C2. During tensile strain, the collapse of Zr n +1C n F2 and Zr n +1C n (OH)2 (n = 1, 2) are mainly caused by internal Zr–C bond rupture and transfer to the surface. While the O-functionalized Zr n +1C n O2 (n = 1, 2) presented the opposite collapse trend. Additionally, according to the research results of critical strain, elastic modulus and electrical conductivity, F/OH-terminated Zr2C MXene is relatively more suitable for flexible sensors of wearable devices than Zr3C2T2.

Mechanical properties of multi-walled beryllium-oxide nanotubes: a molecular dynamics simulation study.

Molecular dynamic (MD) simulation was employed to take the molecular fingerprint of mechanical properties of beryllium-oxide nanotubes (BeONTs). In this regard, the effect of the radius, the number of walls (single-, double-, and triple-walled), and the interlayer distance, as well as the temperature on the Young's modulus, failure stress, and failure strain, are visualized and discussed. It was unveiled that larger single-walled BeONTs have lower Young's modulus in zigzag and armchair direction, and the highest Young's modulus was obtained for the (8,0) zigzag and (4,4) armchair SWBeONTs as of 645.71 GPa and 624.81 GPa, respectively. Unlike Young's modulus, however, the failure properties of the armchair structures were higher than those of zigzag ones. Furthermore, similar to SWBEONTs, an increase in the interlayer distance of double-walled BeONTs (DWBeONTs) led to a slight reduction in Young's modulus value, while no meaningful trend was found among failure behavior. For double-walled BeONTs (TWBeONTs), the elastic modulus was obviously higher in both armchair and zigzag directions compared to DWBeONTs.

Ultra-Narrow Phosphorene Nanoribbons Produced by Facile Electrochemical Process.

Phosphorene nanoribbons (PNRs) have inspired strong research interests to explore their exciting properties that are associated with the unique two-dimensional (2D) structure of phosphorene as well as the additional quantum confinement of the nanoribbon morphology, providing new materials strategy for electronic and optoelectronic applications. Despite several important properties of PNRs, the production of these structures with narrow widths is still a great challenge. Here, a facile and straightforward approach to synthesize PNRs via an electrochemical process that utilize the anisotropic Na+ diffusion barrier in black phosphorus (BP) along the [001] zigzag direction against the [100] armchair direction, is reported. The produced PNRs display widths of good uniformity (10.3 ± 3.8nm) observed by high-resolution transmission electron microscopy, and the suppressed B2g vibrational mode from Raman spectroscopy results. More interestingly, when used in field-effect transistors, synthesized bundles exhibit the n-type behavior, which is dramatically different from bulk BP flakes which are p-type. This work provides insights into a new synthesis approach of PNRs with confined widths, paving the way toward the development of phosphorene and other highly anisotropic nanoribbon materials for high-quality electronic applications.

Structural lubricity of physisorbed gold clusters on graphite and its breakdown: Role of boundary conditions and contact lines.

The sliding motion of gold slabs adsorbed on a graphite substrate is simulated using molecular dynamics. The central quantity of interest is the mean lateral force, that is, the kinetic friction rather than the maximum lateral forces, which correlates with the static friction. For most setups, we find Stokesian damping to resist sliding. However, velocity-insensitive (Coulomb) friction is observed for finite-width slabs sliding parallel to the armchair direction if the bottom-most layer of the three graphite layers is kept at zero stress rather than at zero displacement. Although the resulting kinetic friction remains much below the noise produced by the erratic fluctuations of (conservative) forces typical for structurally lubric contacts, the nature of the instabilities leading to Coulomb friction could be characterized as quasi-discontinuous dynamics of the Moiré patterns formed by the normal displacements near a propagating contact line. It appears that the interaction of graphite with the second gold layer is responsible for the symmetry break occurring at the interface when a contact line moves parallel to the armchair rather than to the zigzag direction.

Ultratough Hydrogen-Bond-Bridged Phosphorene Films.

The rapid development of flexible electronic devices, especially based on 2D materials, has triggered the demand for high-strength materials. Mono- or few-layer phosphorene with excellent electronic properties has attracted extensive attention. However, phosphorene is affected by its low Young's modulus when applied to flexible electronic devices. Here, a strategy via ion intercalation to significantly improve the mechanical properties of black phosphorus to generate hydrogen-bond-bridged phosphorene films with Young's modulus as high as 316GPa is reported. This value is several times larger than the theoretical values of 166GPa in the zigzag direction, 44GPa in the armchair direction, and the averaged Young's modulus among all directions of 94GPa. The impact of intercalation on mechanical properties is also explored. Experimental nanoindentation results obtained by atomic force microscopy indicate that the relationship between the ratio of intercalated ions to phosphorus atoms and the corresponding Young's modulus satisfies the formula . Furthermore, a flexible NO2 gas sensor device based on this ultratough material presents excellent performance, even after 10000 bending cycles. The results provide new insight into the potential for practical applications of black phosphorus devices.

Graphene/biphenylene heterostructure: Interfacial thermal conduction and thermal rectification

The allotrope of carbon, biphenylene, was prepared experimentally recently [Fan et al., Science 372, 852–856 (2021)]. In this Letter, we perform first-principles simulation to understand the bonding nature and structure stability of the possible in-plane heterostructure built by graphene and biphenylene. We found that the graphene–biphenylene in-plane heterostructure only exhibits along the armchair direction, which is connected together by strong covalent bonds and energetically stable. Then, the non-equilibrium molecular dynamics calculations are used to explore the interfacial thermal properties of the graphene/biphenylene heterostructure. It is found that the graphene/biphenylene in-plane heterostructure possesses an excellent interfacial thermal conductance of 2.84 × 109 W·K−1·m−2 at room temperature. Importantly, the interfacial thermal conductance presents different temperature dependence under opposite heat flux direction. This anomalous temperature dependence results in increased thermal rectification ratio with temperature about 40% at 350 K. This work provides comprehensive insight into the graphene–biphenylene heterostructure and suggests a route for designing a thermal rectifier with high efficiency.

The High Photoresponse of Stress‐Tuned MoTe2 Optoelectronic Devices in the Telecommunication Band

There are attractive prospects to achieve large photocurrent and high optical response in telecommunication wavelengths for optoelectronic devices. Inspired by the recent experiment [Nat. Photonics 2020, 14, 578], the photocurrent of the devices composed of noncentral inversion symmetric monolayer MoTe2 along the armchair direction under different linearly polarized light by the quantum transport calculation is investigated. The results demonstrate that the maximum photocurrent will increase under the biaxial tensile stress, and importantly, the peak of photocurrent will move toward the direction of lower photon energy owing to the reduction of the bandgap of monolayer MoTe2, which is opposite to the case of applying the bias voltage. Ultimately, it is proved that monolayer MoTe2 devices with large photoresponse can be tuned to the interesting telecommunication band via the imposition of biaxial stress and bias voltage, which makes the devices have potential applications in the field of silicon photonics.

Molecular dynamics simulations for mechanical properties of the monolayer PtS2 with line defect

By using the classical molecular dynamics simulations, the mechanical properties of the monolayer (ML) PtS2 with the line defect of different tilting angles have been investigated. It has been found that mechanical properties including the fracture strain, fracture stress, toughness and Young's modulus in armchair and zigzag directions have been affected in very different levels. With the tilting angle being from 0° to45°, the mechanical properties in armchair direction is more superior compared to those in the zigzag direction. On the contrary, the mechanical properties in the zigzag direction are much stronger as tilting angle increasing from 45° to90°. Tilting angle of 45° has been received unrivalled attention, where the ML PtS2 with the line defect has presented isotropic mechanical properties in armchair and zigzag directions. The systematically parametric study on the line defect has been demonstrated that the tilting angle may change the isotropic property of the ML PtS2, resulting from its large length–width ratio and orientation of the line defect to the loading direction. The simulations in this paper shed light on a new possibility of fine-tuning mechanical properties and the realization of the isotropy-to-anisotropy of ML PtS2 and other two-dimensional materials via line defect, which may expand the potential application these materials in the future nanometer devices.

Monolayer Ge2S: An auxetic semiconductor with high carrier mobility for metal (Na, K, Mg)-ion battery anodes

Using first-principles calculations, we propose a new two-dimensional Ge2S (space group P21212) with unique mechanical and electronic properties. Monolayer Ge2S has excellent thermal, mechanical, and dynamic stabilities, exhibiting a semiconducting behavior with an indirect bandgap and anisotropic carrier mobility. The uniaxial strain along the zigzag direction can induce an indirect-to-direct bandgap transition. Remarkably, Ge2S possesses large in-plane negative Poisson's ratios, comparable with that of well-known penta-graphene. Moreover, we identify Ge2S as a high-performance anode material for metal-ion batteries. It shows metallic features after adsorbing Na, K, and Mg, providing good electrical conductivity during the charge/discharge process. The diffusion of metal ions on Ge2S is anisotropic with modest energy barriers in the armchair direction of 0.12, 0.39, and 0.76 eV for Na, K, and Mg, respectively. Ge2S can adsorb metal atoms up to a stoichiometric ratio of 1:1, which yields storage capacities of 151.17, 151.17, and 302.35 mA h g−1 for Na, K, and Mg, respectively. The volume of Ge2S shrinks slightly upon the adsorption of metal ions even at high concentrations, ensuring a good cyclic stability. Besides, the average open circuit voltage (0.30–0.70 V) falls within the acceptable range (0.1–1.0 V) of the anode materials. These results make Ge2S a promising anode material for the design of future metal-ion batteries.

Atomic-Scale Finite-Element Modeling of Elastic Mechanical Anisotropy in Finite-Sized Strained Phosphorene Nanoribbons

Nanoribbons are crucial nanostructures due to their superior mechanical and electrical properties. This paper is devoted to hybrid studies of the elastic mechanical anisotropy of phosphorene nanoribbons whose edges connect the terminals of devices such as bridges. Fundamental mechanical properties, including Young’s modulus, Poisson’s ratio, and density, were estimated from first-principles calculations for 1-layer, 3-layer, and 6-layer nanoribbons with widths of 10 Å. The data achieved from the ab initio simulations supplied the finite-element model (FEM) of the nanoribbons. The directional coefficients of strain pressure curves were estimated as Young’s effective modulus since the structure is one-dimensional (1D). The modulus values were equal to 85.8, 111.8, and 134 GPa for 6, 3 and 1 layers, respectively. Moreover, the variation in Poisson’s coefficient for the armchair direction was significantly smaller than for the zigzag direction. Monotonic changes in this twist were observed for structures with 3 and 6 layers within the plane along the zigzag axis. The phosphorene nanoribbons subjected to periodic excitation behaved similarly to those subjected to static loading, while their whippiness was inversely proportional to the length. Next, the deflection under static force, resonance frequencies, and response to a variable driving force were calculated.

Strain effect on transmission in graphene laser barrier

We investigate the strain effect along armchair and zigzag directions on the tunneling transport of Dirac fermions in graphene laser barrier through a time dependent potential along y-axis. Our system is composed of three regions and the central one is subjected to a deformation of strength S. Based on Dirac equation and the Floquet approach, we determine the eigenvalues and eigenspinors for each region. Using the boundary conditions at interfaces together with the transfer matrix method we identify the transmission in the different Floquet sideband states as a function of the physical parameters. In the strainless case, we show that the transmission of central band decreases for smaller values of the barrier width and rapidly oscillates with different amplitude for larger ones. Whereas the transmission for the first sidebands increases from zero and shows a damped oscillatory profile. It is found that the number of oscillations in all transmission channels reduces with increasing the strength of armchair strain, but becomes more important by switching the deformation to zigzag. Moreover, it is observed the appearance of Fano type resonance peaks by altering the amplitude and the frequency of the laser field.

Gate‐Tunable Photovoltaic Behavior and Polarized Image Sensor Based on All‐2D TaIrTe4/MoS2 Van Der Waals Schottky Diode

Abstract2D Weyl semimetal shows potential applications in photodetection, polarization‐sensitive imaging, and Schottky barrier diodes, due to its unique band structure and topological nature. However, its inherently large dark current hinders further improvements of the Weyl semimetal‐based photodetector's performance. Herein, a Td‐TaIrTe4/n‐type MoS2 van der Waals (vdWs) heterojunction photodetector is reported. Owing to the effective lateral build‐in electric field of 145.3 meV, the Schottky diode shows a high rectification ratio of 6 × 103. Benefiting from the photovoltaic effect, a maximum responsivity (R) of 750 mA W−1 and an Ion/Ioff ratio of 104 under 635 nm illumination are achieved. What's more, the photovoltaic R can be enhanced to 890 mA W−1 at the Vg of 40 V. Because of the highly anisotropic crystalline structure of TaIrTe4 component, a high self‐powered photocurrent anisotropic ratio up to 4.19 is realized under 635 nm light at Vg = 40 V. Under self‐powered mode, a high‐resolution letter of “T” within 150 × 120 pixels can be displayed when the polarization direction is parallel to the armchair direction, while it becomes weak along the zigzag direction. This work provides a valuable route to fabricate anisotropic Weyl semimetal/semiconductor vdWs junction for highly integrated optoelectronics.

Strain Benefits of Monolayer α‐GeTe and its Application in Low‐Power Metal–Oxide–Semiconductor Field‐Effect Transistors

2D semiconductors, which have been the candidate channel materials for next‐generation field‐effect transistors (FETs), are now reaching the fundamental limits to what they can offer for higher driving current and switching speed. Low carrier mobility and correspondingly high effective mass hinder such materials to become the ideal electron transport systems. Herein, the electronic and transport properties of strained α‐GeTe are studied for low‐power transistor applications. It is found that monolayer α‐GeTe exhibits a sensitive strain response and undergoes an indirect‐to‐direct bandgap transition at about 2% of tensile uniaxial strain in the zigzag direction, significantly decreasing the electron effective mass and improving electron mobility. In addition, applying tensile strain in the armchair direction can effectively reduce the band‐edge energy of the conduction band minimum (CBM), suggesting that strain can reduce the energy barrier of electrons. These favorable strain responses considerably impact the device performance where both steeper subthreshold swing (SS) and higher on‐state currents can be achieved in monolayer α‐GeTe‐based metal–oxide–semiconductor FETs (MOSFETs). It is hoped that such strain benefits can provide an effective way to modulate the properties of α‐GeTe and finally enhance the device performance.

One-Dimensional Strain Solitons Manipulated Superlubricity on Graphene Interface.

The frictional properties of a uniaxial tensile strained graphene interface are studied using molecular dynamics simulations. A misfit interval statistical method (MISM) is applied to characterize the atomistic misfits at the interface and strain soliton pattern. During sliding along both armchair and zigzag directions, the lateral force depends on the ratio of graphene flake length (L) to strain soliton spacing (Ls) and becomes nearly zero when L is an integer multiple of 3Ls. Furthermore, the strain solitons propagate along the armchair sliding direction dynamically, while fission and fusion are repeatedly evidenced along the zigzag sliding direction. The underlying superlubric mechanism is revealed by a single-atom quasi-static model. The cancellation of lateral force for the contacting atoms exhibits a dynamic balance when sliding along the armchair direction but a quasi-static balance along the zigzag direction. A diagram of flake length with respect to tensile strain (L-ε) is proposed to predict the critical condition for the transition from nonsuperlubricity to superlubricity. Our results provide insights on the design of superlubric devices.

Anisotropic interface characteristics of bilayer GeSe based field effect transistors

Moderate direct band gap and strong stability render bilayer gemanium selenide (GeSe) more suitable for the channel material in the electronic and optoelectronic devices. To improve carrier injection efficiency in an actual two dimensional device, the choice of the metal electrode is a key issue. Here using both electronic band calculations and quantum transport simulation, we comprehensively investigate the interface characteristics of bilayer GeSe contacted with six representative metals. The projected band structures of bilayer GeSe on these metals illustrate that all of these bilayer GeSe have undergone metallization, resulting in the contact natures are ohmic in the vertical interface. While，anisotropic lateral Schottky contacts are formed between the Ag, Au, Pd, Pt, Cu and bilayer GeSe along the armchair and the zigzag direction. The Ni electrode forms p-type quasi-Ohmic contact with bilayer GeSe with a small hole Schottky barrier height of 0.04 (0.08) eV along the armchair (zigzag) direction. Thus, the high performance of bilayer GeSe based FET would be realized by choosing the Ni as source/drain electrodes.