Zigzag Orientation Research Articles

Evolutionary Search and Theoretical Study of Silicene Grain Boundaries' Mechanical Properties.

Defects such as grain boundaries (GBs) are almost inevitable during the synthesis process of 2D materials. To take advantage of the fascinating properties of 2D materials, understanding the nature and impact of various GB structures on pristine 2D sheets is crucial. In this work, using an evolutionary algorithm search, we predict a wide variety of silicene GB structures with very different atomic structures compared with those found in graphene or hexagonal boron-nitride. Twenty-one GBs with the lowest energy were validated by density functional theory (DFT), a majority of which were previously unreported to our best knowledge. Based on the diversity of the GB predictions, we found that the formation energy and mechanical properties can be dramatically altered by adatom positions within a GB and certain types of atomic structures, such as four-atom rings. To study the mechanical behavior of these GBs, we apply strain to the GB structures stepwise and use DFT calculations to investigate the mechanical properties of 9 representative structures. It is observed that GB structures based on pentagon-heptagon pairs are likely to have similar or higher in-plane stiffness and strength compared to the zigzag orientation of pristine silicene. However, an adatom located at the hollow site of a heptagon ring can significantly deteriorate the mechanical strength. For all of the structures, the in-plane stiffness and strength were found to decrease with increasing formation energy. For the failure behavior of GB structures, it was found that GB structures based on pentagon-heptagon pairs have failure behavior similar to that of graphene. We also found that the GB structures with atoms positioned outside of the 2D plane tend to experience phase transitions before failure. Utilizing the evolutionary algorithm, we locate diverse silicene GBs and obtain useful information about their mechanical properties.

Thermoelectric Properties of 2D Sn2SSe$Sn_{2}SSe$ Monolayer

Abstract2D Janus materials have drawn great interest in recent years due to their energy applications including thermoelectricity (TE) and photocatalysis. In this work, density functional theory and Boltzmann transport equations are coupled to look into the thermoelectric properties of monolayer. is an indirect bandgap semiconductor with a bandgap of 0.88 eV. This monolayer is chemically as well as dynamically stable. The thermoelectric parameters such as the Seebeck coefficient, electrical conductivity, and thermal conductivity due to electrons and phonons are explored along the zigzag and armchair orientation. The high value of electrical conductivity and low‐value lattice thermal conductivity along the zigzag orientation yields the high value of ZT = 1.93.

Sign-flipping intrinsic anomalous Hall conductivity with Berry curvature tunability in a half-metallic ferromagnet NbSe2-VSe2 lateral heterostructure.

Single-layer half-metal magnets offer exciting scope in spin electronic quantum applications owing to improved spin transport, reduced interfacial resistance and streamlined device fabrication. Herein, we report the emergence of sign-flipping intrinsic anomalous Hall conductivity (AHC) as a result of changes in Berry curvature under an external electric field and half metallicity in a lateral heterostructure composed of centrosymmetric metallic monolayers 1T-NbSe2 and 1T-VSe2. The metallic monolayers 1T-NbSe2 and 1T-VSe2 laterally interfaced along the zigzag orientation break inversion symmetry at the interface and result in distinctive Berry curvature features. Furthermore, the half-metallic character was prominent with gapped states in the spin-up channel, while the spin-down state remained conductive; we observed the unique manifestation of sign-flipping intrinsic AHC at the Fermi level in addition to the electron- and hole-doped regions. This sign-flipping aspect of AHC at the Fermi level is of fundamental importance from the prospect of real-time device applications as it eliminates the necessity of supplementary actions, such as doping and strain engineering, which are traditionally employed to achieve AHC sign reversal. Additionally, a phase transition from half metal to metal occurs at a field of 0.5 V Å-1 and beyond. Half metallicity with sign switching AHC via external electric field makes the lateral NbSe2-VSe2 heterostructure a potential candidate for real-time energy-efficient low-power spintronic devices.

Molecular dynamics simulation study on the friction performance of WS2 coated copper during the scratching process

Tungsten disulfide (WS2) has been assumed to have the excellent tribological property that has the potential to be used in the field of lubricating engineering. In this study, WS2 nanosheet was coated onto the surface of the copper to improve the wear resistance. The tribological properties of WS2 coated copper system were investigated by molecular dynamics (MD) simulation to understand the wear mechanisms from the nanoscale. The mechanical response of the bare and the WS2 coated copper in the indentation process were compared firstly. Then, scratching simulation tests were performed at the various indentation depths from 1.2 Å to 11.2 Å to understand the tribological mechanism from the elastic stage to the fracture stage. The effect of the nanoindentation depth and anisotropy orientation of WS2 on friction behavior were also analyzed during the scratching process. Results show that the load-bearing capacity and wear resistance of the copper substrate were improved by the WS2 coated layer before the fracture during the nanoindentation process, while the protection of the layer failed after rupture that the out-of-plane deformation induced the increase of the friction force. Additionally, the difference of the friction force could be induced by the lattice spacing of WS2 and the energy dissipation during ultralow friction stage. The crack propagation along the Zigzag direction dominated the fracture behavior in both Armchair and Zigzag orientation. This study provides important insights into the friction mechanism of the WS2 when conducting nanoscratching process.

A theoretical insight into the mechanical properties and phonon thermal conductivity of biphenylene network structure

The numerous applications of carbon-based nanomaterials clearly demonstrate the importance of polymorphism in carbon. Two-dimensional biphenylene is a recently-synthesized sp2-hybridized allotrope of carbon atoms that needs further investigation. Thermal conductivity and mechanical strength are two important features which determine the life span and the performance of two-dimentional biphenylene under operating conditions. Sevaral parametrs such as temeperature, temperature gradient, strain rate, length, would affect the mechanical characteristic and thermal conductivity. Therefore, it is imperative to look into how the aforementioned parameters affect thermo-mechanical performance in order to have a greater understanding of the two-dimensional biphenylene's competency under working conditions. This study aims to investigate the mechanical and phonon thermal conductivity of biphenylene nanosheet (BPNS) concerning some effective factors like strain rates, structure’s lengths, and temperatures by means of molecular dynamics (MD) simulation. Regarding the obtained results of mechanical properties of BPNS, it was revealed that the Young’s modulus, failure stress, and failure strain were decreased by elevating the temperature due to the rise in interatomic distances and decrement of binding energy. Moreover, better mechanical properties were generally observed along the zigzag direction. Mechanical features increased and decreased gradually by increasing strain rates and sample’ length, respectively. Infinite thermal conductivity of BPNS were obtained 146.41 W.m-1k−1 and 133.51 W.m-1k−1 in zigzag and armchair orientation, respectively. The variations of temperature difference between two hot and cold bathes had negligible effect on phonon thermal conductivity. However, the increment of temperature and strain caused the reduction and promotion in phonon thermal conductivity of BPNS, respectively. The relevance of this research lies in its capacity to analyze distinct BPNS behaviors, which has significant potential for use in sensors, reinforcements, and electrodes.

A stable ultrastructural pattern despite variable cell size in <i>Lithothamnion corallioides</i>

Abstract. Recent advances on the mechanism and pattern of calcification in coralline algae led to contradictory conclusions. The evidence of a biologically controlled calcification process, resulting in distinctive patterns at the scale of family, was observed. However, the coralline calcification process has been also interpreted as biologically induced because of the dependency of its elemental composition on environmental variables. To clarify the matter, five collections of Lithothamnion corallioides from the Atlantic Ocean and the Mediterranean Sea, across a wide depth range (12–66 m), have been analyzed for morphology, anatomy and cell wall crystal patterns in both perithallial and epithallial cells to detect possible ultrastructural changes. L. corallioides shows the alternation of tiers of short-squared and long-ovoid/rectangular cells along the perithallus, forming a typical banding. The perithallial cell length decreases according to water depth and growth rate, whereas the diameter remains constant. Our observations confirm that both epithallial and perithallial cells show primary (PW) and secondary (SW) calcite walls. Rectangular tiles, with the long axis parallel to the cell membrane forming a multi-layered structure, characterize the PW. Flattened squared bricks characterize the SW, with roundish outlines enveloping the cell and showing a zigzag and cross orientation. Long and short cells have different thicknesses of PW and SW, increasing in short cells. Epithallial cells are one to three flared cells with the same shape of the PW and SW crystals. Despite the diverse seafloor environments and the variable L. corallioides growth rate, the cell walls maintain a consistent ultrastructural pattern with unaffected crystal shape and arrangement. A comparison with two congeneric species, L. minervae and L. valens, showed similar ultrastructural patterns in the SW but evident differences in the PW crystal shape. Our observations point to a biologically control rather than an induction of the calcification process in coralline algae and suggest a possible new morphological diagnostic tool for species identification, with relevant importance for paleontological applications. Finally, secondary calcite, in the form of dogtooth crystals that fill the cell lumen, has been observed. It represents a form of early alteration in living collections which can have implications in the reliability of climate and paleoclimate studies based on geochemical techniques.

The Binding of Monoclonal and Polyclonal Anti-Z-DNA Antibodies to DNA of Various Species Origin

DNA is a polymeric macromolecule that can display a variety of backbone conformations. While the classical B-DNA is a right-handed double helix, Z-DNA is a left-handed helix with a zig-zag orientation. The Z conformation depends upon the base sequence, base modification and supercoiling and is considered to be transient. To determine whether the presence of Z-DNA can be detected immunochemically, the binding of monoclonal and polyclonal anti-Z-DNA antibodies to a panel of natural DNA antigens was assessed by an ELISA using brominated poly(dG-dC) as a control for Z-DNA. As these studies showed, among natural DNA tested (Micrococcus luteus, calf thymus, Escherichia coli, salmon sperm, lambda phage), micrococcal (MC) DNA showed the highest binding with both anti-Z-DNA preparations, and E. coli DNA showed binding with the monoclonal anti-DNA preparation. The specificity for Z-DNA conformation in MC DNA was demonstrated by an inhibition binding assay. An algorithm to identify propensity to form Z-DNA indicated that DNA from Mycobacterium tuberculosis could form Z-DNA, a prediction confirmed by immunoassay. Together, these findings indicate that anti-Z-DNA antibodies can serve as probes for the presence of Z-DNA in DNA of various species origin and that the content of Z-DNA varies significantly among DNA sources.

Consecutive charging and discharging of a PCM-based plate heat exchanger with zigzag configuration

Due to the remarkable energy savings, isothermal nature of the operation and low costs, energy storage with phase-change materials (PCMs) is a reliable technology for filling the gap between energy supply and demand. In this paper, an attempt has been made to modify the storage functionality of PCM in a plate type heat exchanger with zigzag configuration. A two-dimensional, time-dependent simulation model for the PCM phase transition during the charging and discharging modes has been developed and validated via earlier related findings. The effects of zigzag angle orientation, inlet flowrate and mean temperature of the heat transfer fluid (HTF) are thoroughly studied and revealed. Results show that increasing the angle of zigzag orientation has no noticeable impact on the development of phase transition during the early stages of operation. However, this effect becomes more noticeable and almost leads to faster storage/retrieval rates as time further elapses. It is found that the system with the zigzag angle of 60° augments the storage rate by 32.6% compared with the system of 30° zigzag angle. Also, higher HTF temperature and/or higher Reynold number result in faster phase-transition rates during both parts of the energy charging-discharging cycle.

Self-Powered 2D Material-Based pH Sensor and Photodetector Driven by Monolayer MoSe2 Piezoelectric Nanogenerator.

The large piezoelectricity of monolayer MoSe2, which is predicted to be stronger than that of all of the other group VIB transition-metal dichalcogenides (including MoS2), has only been theoretically investigated. Here, we report experimental evidence of in-plane piezoelectricity in MoSe2. Monolayer single-crystalline MoSe2 flake derived from chemical vapor deposition demonstrates a peak output voltage of 60 mV at 0.6% strain, which is ∼50% larger than that of MoS2. Piezoelectric signal along the armchair orientation of MoSe2 is ∼6 times larger than that along the zigzag orientation, indicative of strong anisotropic piezoelectricity. Piezoelectric nanogenerator based on a single MoSe2 flake illustrates remarkable electromechanical conversion ability, and thus is able to noninvasively monitor vital health signs, such as respiratory rate and heart rate. Despite the extremely small size, MoSe2 nanogenerator is able to drive pH sensor based on MoS2 and photodetector based on MoS2/WSe2 heterojunction due to the outstanding piezoelectricity of MoSe2 and the ultralow power consumption of two-dimensional (2D) material sensors. The self-powered, solely 2D-material-based sensor units demonstrate superb sensing performance. Therefore, the discovery of piezoelectricity in monolayer MoSe2 provides a route for achieving self-powered atomic-scale electromechanical systems that could stimulate further fundamental research and potential applications.

Long-Range LMCT Coupling in EuIII Coordination Polymers for an Effective Molecular Luminescent Thermometer*.

A design for an effective molecular luminescent thermometer based on long-range electronic coupling in lanthanide coordination polymers is proposed. The coordination polymers are composed of lanthanide ions EuIII and GdIII , three anionic ligands (hexafluoroacetylacetonate), and a chrysene-based phosphine oxide bridges (6,12-bis(diphenylphosphoryl)chrysene). The zig-zag orientation of the single polymer chains induces the formation of packed coordination structures containing multiple sites for CH-F intermolecular interactions, resulting in thermal stability above 350 °C. The electronic coupling is controlled by changing the concentration of the GdIII ion in the EuIII -GdIII polymer. The emission quantum yield and the maximum relative temperature sensitivity (Sm ) of emission lifetimes for the EuIII -GdIII polymer (Eu:Gd=1:1, Φtot =52 %, Sm =3.73 % K-1 ) were higher than those for the pure EuIII coordination polymer (Φtot =36 %, Sm =2.70 % K-1 ), respectively. Enhanced temperature sensing properties are caused by control of long-range electronic coupling based on phosphine oxide with chrysene framework.

Unconventional Peptide Presentation by Classical MHC Class I and Implications for T and NK Cell Activation.

T cell-mediated immune recognition of peptides is initiated upon binding of the antigen receptor on T cells (TCR) to the peptide-MHC complex. TCRs are typically restricted by a particular MHC allele, while polymorphism within the MHC molecule can affect the spectrum of peptides that are bound and presented to the TCR. Classical MHC Class I molecules have a confined binding groove that restricts the length of the presented peptides to typically 8–11 amino acids. Both N- and C-termini of the peptide are bound within binding pockets, allowing the TCR to dock in a diagonal orientation above the MHC-peptide complex. Longer peptides have been observed to bind either in a bulged or zig-zag orientation within the binding groove. More recently, unconventional peptide presentation has been reported for different MHC I molecules. Here, either N- or C-terminal amino acid additions to conventionally presented peptides induced a structural change either within the MHC I molecule that opened the confined binding groove or within the peptide itself, allowing the peptide ends to protrude into the solvent. Since both TCRs on T cells and killer immunoglobulin receptors on Natural Killer (NK) cells contact the MHC I molecule above or at the periphery of the peptide binding groove, unconventionally presented peptides could modulate both T cell and NK cell responses. We will highlight recent advances in our understanding of the functional consequences of unconventional peptide presentation in cellular immunity.

Wetting behavior of textured silicon surfaces- an experimental study

The behavior of a liquid on a solid surface has shown great interest in a variety of applications related to surfaces and its interfaces. In this paper, the wetting behavior of DI water on micropatterned silicon surfaces fabricated through photolithography and deep reactive ion etching (DRIE) is investigated. Micro pillars of both solid and hollow geometries at a varying pitch and its arrangement in an array has been examined with static contact angle measurement. However, the results concluded that the arrangement of pillars in an array plays an important role as hollow geometries in the case of chain type arrangement provide both hydrophilic and hydrophobic surface properties, while the same hollow geometries in case of zig-zag orientation experiences only hydrophobicity at a varying pitch. Decreased WCA with increased pitch has been observed in the case of a zig-zag arrangement, due to the effect of capillary and gravitation forces. Also the existence of air pockets at sharp corner in the case of hollow square assists in providing maximum contact angle (WCA = 144°) as compared to hollow circle and solid geometries; thus a non-sticky behavior would be possible between the droplet and the patterned surface, due to less adhesion force.

Constructing graphene nanostructures with zigzag edge terminations by controllable STM tearing and folding

Graphene nanostructures with specified edge terminations has attracted considerable attention owing to their unique electronic properties induced by the edges. However, an efficient way to controllably fabricate such nanostructures is still highly desired. Here, we show that graphene nanostructures with zigzag edge terminations can be easily and efficiently constructed at room-temperature by tip manipulation of a scanning tunneling microscope (STM). Though intentionally increasing the tip-graphene interaction, we controllably tear and fold graphene sheets against step edges and successfully fabricate nanoscale graphene islands such as quantum dots and nanoribbon-like structures on graphite. Interestingly, the tearing directions are found to mainly along the zigzag orientations of graphene hexagonal lattice, leading to the generated graphene nanostructures possessing the same specified edges with well zigzag terminations. Theoretical modelling demonstrates that the enhanced tip-graphene repulsive force can account for the tearing and folding processes, whereas the preferential zigzag-tearing is attributed to the predicted lowest bond-breaking energy of the zigzag orientation in graphene. Our experiment provides a simple and controllable method for fabrication of graphene nanostructures with precise zigzag edge terminations. The obtained zigzag-edge-terminated graphene nanostructures also provide a platform for engineering novel quantum properties.

Superlubricity in phosphorene identified by means of ab initio calculations

Phosphorene possesses a great potential for tribological applications due to its layered structure and for the capability of phosphorus to reduce friction and adhesion in steel–steel contacts. Here we present a comprehensive analysis of the static tribological properties of phosphorene based on first principles calculations. The most suitable exchange-correlation functional for describing the structural and electronic properties of multilayer phosphorene is carefully selected. The interlayer binding energy and shear strength are then calculated for two relative orientations of the layers. Layers stacked with the same orientation (armchair–armchair and zigzag–zigzag) are slippery as common solid lubricants, as MoS2 and graphite. While the armchair–zigzag orientation shows a remarkable superlubricity, with a reduction of one order of magnitude for the shear stress. We uncover the microscopic origin of such superlubric phase by analyzing the electronic charge at the layer interface.

Ab initio study of electronic and transport anisotropy of two square and rectangle phosphorene nanoflakes

Two-dimensional (2D) phosphorene nanoflakes (PNFs) revealed different properties along two different crystal directions of zigzag (ZZ) and armchair (AM). In this study, the electronic and transport properties of two different structures of 4 × 4 square and 4 × 6 rectangle phosphorene nanoflakes with zigzag and armchair orientation were investigated. The zigzag 4 × 6 rectangle phosphorene nanoflake with six unit cells along zigzag direction was revealed to be metal whereas the corresponding armchair one with 6 unit cells along armchair direction was semiconductor. Transport study of nanoflakes showed the anisotropic behaviors at two transport directions of zigzag and armchair. Transmission coefficient of the phosphorene nanoflakes was found to be higher in the ZZ crystal direction compared to the AM and the anisotropy ratio of 6% and 95% was estimated for the square and rectangle PNF at Fermi energy, respectively.

Unique mechanical responses of layered phosphorus-like group-IV monochalcogenides

As interesting analog of black phosphorus, group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se) are greatly expected to have potential applications in photodetectors, photovoltaics, thermoelectric, and piezoelectric devices. In this paper, we conduct a thorough investigation on the mechanical properties of four MX monochalcogenides, i.e., single- and multi-layered GeS, GeSe, SnS, and SnSe, including their ideal tensile strength, evolution of monolayer bulking height, and phase transition under uniaxial strain within the framework of density functional theory. The ultimate tensile strengths of the four monochalcogenides are found rather low, while the failure strains are remarkably high, showing ultrahigh mechanical flexibility. Meanwhile, the ideal strength demonstrates strong anisotropy, which may induce the exfoliated MX thin flakes showing an obvious in-plane cleavage tendency of much easier to crack along the zigzag orientation. In addition, it is found that all of them are potential auxetic materials with large negative Poisson's ratios, just like single-layer black phosphorus. Furthermore, we find all the monolayer monochalcogenides will demonstrate geometrical phase transition when uniaxial compressive strains are applied. It is expected that these newly found unique features of monolayer group-IV monochalcogenides and their derivatives make them attractive candidates for their incorporation in future mechanical, nanoelectronics, and thermal applications.

Theoretical study of scattering in graphene ribbons in the presence of structural and atomistic edge roughness

We investigate the diffusive electron-transport properties of charge-doped graphene ribbons and nanoribbons with imperfect edges. We consider different regimes of edge scattering, ranging from wide graphene ribbons with (partially) diffusive edge scattering to ribbons with large width variations and nanoribbons with atomistic edge roughness. For the latter, we introduce an approach based on pseudopotentials, allowing for an atomistic treatment of the band structure and the scattering potential, on the self-consistent solution of the Boltzmann transport equation within the relaxation-time approximation and taking into account the edge-roughness properties and statistics. The resulting resistivity depends strongly on the ribbon orientation, with zigzag (armchair) ribbons showing the smallest (largest) resistivity and intermediate ribbon orientations exhibiting intermediate resistivity values. The results also show clear resistivity peaks, corresponding to peaks in the density of states due to the confinement-induced subband quantization, except for armchair-edge ribbons that show a very strong width dependence because of their claromatic behavior. Furthermore, we identify a strong interplay between the relative position of the two valleys of graphene along the transport direction, the correlation profile of the atomistic edge roughness, and the chiral valley modes, leading to a peculiar strongly suppressed resistivity regime, most pronounced for the zigzag orientation.

A thermodynamic structural model of graphene oxide

Graphene oxide is an easy-to-make material that has a similar structure with graphene. However, the real structure of graphene oxide is still controversial, and an accurate structural model is crucial for understanding its various properties. In this study, by using molecular mechanics and density functional theory, we introduce a thermodynamically favorable structural model of graphene oxide with chemical composition variable from C1.5O to C2.5O. We also calculate their theoretical Raman spectra and electronic properties. It has been found that, in the proposed graphene oxide structure, the para-substituted epoxide groups stay in close proximity to the hydroxyl, but on the opposite sides of the carbon sheet. In addition, on the edge of graphene oxide sheet, the carboxyl prefers attachment in the armchair orientation, while the carbonyl prefers the zigzag orientation.

Dendrimeric triphenylbenzenes: helical versus zig-zag arrangement in columnar mesophases

ABSTRACTA small series of dendrimers consisting of a star-shaped triphenylbenzene core unit and bulky lateral groups was synthesised and their mesomorphic properties were investigated via differential scanning calorimetry, polarising optical microscopy and small- and wide-angle X-ray scattering (WAXS). The dendrimers showed two enantiotropic hexagonal columnar mesophases. The combination of temperature-dependent and fibre-extruded WAXS revealed a helical arrangement in the low-temperature and a zig-zag orientation in the high-temperature mesophase. Furthermore, at the mesophase-to-mesophase transition, the intracolumnar reorganisation and the intercolumnar lattice extension occurred at different time scales, with the latter process being the slower one.

On linear waveguides of zigzag honeycomb lattice

An analysis of the scalar linear waves in infinite honeycomb lattice strips with discrete Dirichlet and Neumann boundary conditions, as well as the periodic boundary condition, is presented for the zigzag orientation. The dispersion relations and the associated wave modes in these waveguides are provided in the paper; the former in terms of the Chebyshev polynomials. It is found that the localized propagating waves, addressed as surface wave modes, occur in case of certain boundary conditions only; for which the details are provided separately. The dispersion relations and wave modes for certain even and odd symmetry are also discussed in the paper, wherever applicable. Graphical illustrations of the dispersion curves for all waveguides are included. Applications include the transmission of phononic, electronic, magnetic, and photonic waves in nanostructures of carbon-like constituents. The analytical results are directly applicable for the out-of-plane phonons assuming only nearest-neighbor interactions in graphene nanoribbons on certain substrates, as well as the counterparts in the single orbital nearest neighbor tight-binding model for the electrons.