Zigzag Orientation Research Articles

A structural insight into mechanical strength of graphene-like carbon and carbon nitride networks

Graphene, one of the strongest materials ever discovered, triggered the exploration of many 2D materials in the last decade. However, the successful synthesis of a stable nanomaterial requires a rudimentary understanding of the relationship between its structure and strength. In the present study, we investigate the mechanical properties of eight different carbon-based 2D nanomaterials by performing extensive density functional theory calculations. The considered structures were just recently either experimentally synthesized or theoretically predicted. The corresponding stress–strain curves and elastic moduli are reported. They can be useful in training force field parameters for large scale simulations. A comparative analysis of these results revealed a direct relationship between atomic density per area and elastic modulus. Furthermore, for the networks that have an armchair and a zigzag orientation, we observed that they were more stretchable in the zigzag direction than the armchair direction. A critical analysis of the angular distributions and radial distribution functions suggested that it could be due to the higher ability of the networks to suppress the elongations of the bonds in the zigzag direction by deforming the bond angles. The structural interpretations provided in this work not only improve the general understanding of a 2D material’s strength but also enables us to rationally design them for higher qualities.

Perfect spin filtering effect in ultrasmall helical zigzag graphene nanoribbons

The spin-polarized transport properties of helical zigzag graphene nanoribbons (ZGNRs) are investigated by first-principles calculations. It is found that although all helical ZGNRs have similar density of states and edge states, they show obviously different transport characteristics depending on the curling manners. ZGNRs curled along zigzag orientation exhibit perfect spin filtering effect with a large spin-split gap near the Fermi level, while ZGNRs curled along armchair orientation behave as conventional conductors for both two spin channels. The spin filtering effect will be weakened with the increase of either ribbon width or curling diameter. The results suggest that ultrasmall helical ZGNRs have important potential applications in spintronics and flexible electronics.

In-Socket Sensory System for Transfemoral Amputees Using Piezoelectric Sensors: An Efficacy Study

This paper presents the design and evaluation of an in-socket sensory system for a transfemoral prosthetic leg using a set of piezoelectric sensors. The design process includes identifying the optimal mounting configuration of the sensors and determining their best placement. Two experiments were performed separately to address each objective. Results of the experiment suggested that cushionall with fit size cantilever with elastic foundation should be adopted to mount the sensors on the socket's wall. As for the placement of the sensors, the result suggested that the sensors should be positioned in zig-zag orientation from top to bottom, which can cover all of the most active area on the quadriceps and hamstring muscle groups. With the identified design, the socket was fabricated and instrumented for a transfemoral amputee and the performance of the in-socket sensory system was evaluated. The amputee performed level walking at normal speed on a 5-m straight pathway multiple times while the output voltage signal from each sensor was recorded. The output signals consistency was assessed by calculating the cross correlation, r, within five trials. In order to measure the strength of the correlation matrix of each sensor, the Frobenius norm, ||A|| <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> was calculated. The high norm value for all sensors pattern (||A|| <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> >4.5) for quadriceps implies that the mounting and placement of the sensors is most suitable for a consistent and reliable signal. For Hamstring, results inferred that except for the two sensors located on the top most position and the most bottom sensor, the other sensors showed good performance (||A|| <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> >4.4). Thus, the proposed design of the in-socket sensory system using piezo sensors was proven to be effective for a transfemoral prosthetic leg.

Luminescent Europium(III) Coordination Zippers Linked with Thiophene-Based Bridges.

Novel Eu(III) coordination polymers [Eu(hfa)3 (dpt)]n (dpt: 2,5-bis(diphenylphosphoryl)thiophene) and [Eu(hfa)3 (dpedot)]n (dpedot: 2,5-bis(diphenylphosphoryl)ethylenedioxythiophene) with hydrogen-bonded zipper structures are reported. The coordination polymers are composed of Eu(III) ions, hexafluoroacetylacetonato ligands, and thiophene-based phosphine oxide bridges. The zig-zag orientation of single polymer chains induced the formation of densely packed coordination structures with multiple intermolecular interactions, resulting in thermal stability above 300 °C. They exhibit a high intrinsic emission quantum yield (ca. 80 %) due to their asymmetrical and low-vibrational coordination structures around Eu(III) ions. Furthermore, the characteristic alternative orientation of substituents also contributes to the dramatically high ligand-to-metal energy transfer efficiencies of up to 80 % in the solid state.

Wafer-scale fabrication and growth dynamics of suspended graphene nanoribbon arrays.

Adding a mechanical degree of freedom to the electrical and optical properties of atomically thin materials can provide an excellent platform to investigate various optoelectrical physics and devices with mechanical motion interaction. The large scale fabrication of such atomically thin materials with suspended structures remains a challenge. Here we demonstrate the wafer-scale bottom–up synthesis of suspended graphene nanoribbon arrays (over 1,000,000 graphene nanoribbons in 2 × 2 cm2 substrate) with a very high yield (over 98%). Polarized Raman measurements reveal graphene nanoribbons in the array can have relatively uniform-edge structures with near zigzag orientation dominant. A promising growth model of suspended graphene nanoribbons is also established through a comprehensive study that combined experiments, molecular dynamics simulations and theoretical calculations with a phase-diagram analysis. We believe that our results can contribute to pushing the study of graphene nanoribbons into a new stage related to the optoelectrical physics and industrial applications.

A molecular dynamic simulation study of mechanical properties of graphene–polythiophene composite with Reax force field

In this paper, we performed molecular dynamic simulations by Reax force field to study the mechanical properties of graphene–polythiophene nanocomposite. By computing elastic constant, breaking stress, breaking strain and Young's modulus from the stress–strain curve for the nanocomposites, we investigated effects of tension orientation, graphene loading to the polymer, temperature of nanocomposite and defect of graphene on these mechanical characters. It is found that mechanical characters of tension along the zigzag orientation are higher than other directions. Also, by increasing the weight concentration of graphene in composite, the Young's modulus and breaking strain increase. Our results showed that the Young's modulus decreased with increasing temperature. Finally by applying defect on graphene structure, we found that one atom missing defect has lower Young's modulus. Also, by increasing the defects concentration, elastic modulus decreases gradually.

Structural and electronic properties of metalloporphyrin (MP, M = Fe, Co and Zn) adsorbed on single walled BNNT and SiCNT

In the present work, we report a detailed theoretical investigation of a series of metalloporphyrin, MP (M=FeP, CoP and ZnP), molecules interacting with silicon carbide nanotube (SiCNT) by means of density functional theory (DFT) calculations. In all calculations, we used the Perdew–Burke–Erzenhof (PBE) functional as employed in the SIESTA package. The detailed analysis of the structural and electronical properties of various optimized configurations is performed. The results show that among the MPs, adsorption of FeP molecule on the Si site with zigzag orientation is the most energetically preferable with a binding energy of −2.10eV. Compared to SiCNTs, boron nitride nanotubes (BNNTs) have weaker interaction strength with the FeP molecule with −0.34eV of binding energy. We have analyzed charge transfer between two interacting species trough well-known Mulliken, Hirshfeld and Voronoi charges analysis for aforementioned systems. The spin-polarized DFT calculations showed that the density of states (DOSs) are spin-polarized for the Fe-BNNT complex while the spin-polarization of the DOS spectra turn out to be less notable for MP-SiCNT complexes. Our results propose that FeP-SiCNT complex could be used for interesting applications in solar cell technology and nano-biosensors while FeP-BNNT complex might be considered for spintronic molecular devices.

Prediction of the elastic modulus of SWCNT/epoxy composite based on the micromechanics

Abstract Due to their superior mechanical and physical properties, carbon nanotubes seem to hold a great promise as an ideal reinforcing material for composites of high strength and low density. In most of the experimental results up to date, however, only modest improvements in the strength and stiffness have been achieved by incorporating carbon nanotubes in polymers. In the present paper, the stiffening effect of carbon nanotubes is quantitatively investigated by micromechanics methods. The Mori-Tanaka effective field method has been adopted to calculate the effective elastic moduli of composites with aligned or randomly oriented straight nanotubes. The rule-of-mixtures is used to calculate the modulus of elasticity for nanocomposite. The results of micromechanics methods indicated that the CNTs are highly anisotropic, with Young's modulus in the tube direction two orders of magnitude higher than that normal to the tube. The results of micromechanics methods were compared by those obtained from the rule-of-mixtures and good agreement was also achieved when the efficiency parameter Φ = 1 and typical results were achieved with Φ = 0.25. To predict the mechanical properties of the composite materials, it is worth considering the conventional rule-of-mixtures using exact value of the efficiency parameter Φ. To predict the elastic modulus of nanocomposite reinforced by SWCNT using the conventional rule-of-mixtures, the exact value of the efficiency parameter Φ is equal to 0.25 when using nanotubes with chirality (8,3) for determination the elastic modulus of SWCNT. However, for zigzag orientation and chirality (8,0) the efficiency parameter Φ is equal to 1. The conventional rule-of-mixtures is a powerful tool and easy method compared to the micromechanics methods.

Growth and characterization of sidewall graphene nanoribbons

We study the growth of epitaxial graphene nanoribbons on silicon carbide mesa sidewalls by means of scanning probe techniques, local transport, and Raman spectroscopy. The sidewall nanoribbons are demonstrated to consist of charge neutral monolayer graphene with a zig-zag type orientation. Two types of roughness, the step density of the substrate and the roughness of the sidewalls, were identified as being detrimental to the transport properties of these ribbons. By means of 4-point probe experiments, single channel ballistic transport was observed with a mean free path limited by the width of the underlying substrate terraces. Moreover, a transition from ballistic to one-dimensional diffusive transport can be obviously triggered by an increased roughness of the sidewall, e.g., by an enlarged depth of the mesa.

Room-temperature magnetic order on zigzag edges of narrow graphene nanoribbons.

The possibility that non-magnetic materials such as carbon could exhibit a novel type of s-p electron magnetism has attracted much attention over the years, not least because such magnetic order is predicted to be stable at high temperatures. It has been demonstrated that atomic-scale structural defects of graphene can host unpaired spins, but it remains unclear under what conditions long-range magnetic order can emerge from such defect-bound magnetic moments. Here we propose that, in contrast to random defect distributions, atomic-scale engineering of graphene edges with specific crystallographic orientation--comprising edge atoms from only one sub-lattice of the bipartite graphene lattice--can give rise to a robust magnetic order. We use a nanofabrication technique based on scanning tunnelling microscopy to define graphene nanoribbons with nanometre precision and well-defined crystallographic edge orientations. Although so-called 'armchair' ribbons display quantum confinement gaps, ribbons with the 'zigzag' edge structure that are narrower than 7 nanometres exhibit an electronic bandgap of about 0.2-0.3 electronvolts, which can be identified as a signature of interaction-induced spin ordering along their edges. Moreover, upon increasing the ribbon width, a semiconductor-to-metal transition is revealed, indicating the switching of the magnetic coupling between opposite ribbon edges from the antiferromagnetic to the ferromagnetic configuration. We found that the magnetic order on graphene edges of controlled zigzag orientation can be stable even at room temperature, raising hopes of graphene-based spintronic devices operating under ambient conditions.

Theoretical Investigation of Small Transition-Metal Clusters Supported on the CeO2(111) Surface

We have performed a systematic investigation of 4-atom transition-metal (TM) clusters (TM = Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au) supported on the unreduced CeO2(111) surface using density functional theory calculations within the Perdew–Burke–Ernzerhof functional and on-site Coulomb interactions for the Ce f-states. We found two structure TM4 patterns on CeO2(111), namely, two-dimensional (2D) arrays with zigzag orientation for Ru, Rh, Os, and Ir and tetrahedral three-dimensional (3D) configurations for Cu, Pd, Ag, Pt, and Au. Our analyses indicate that the occupation of the antibonding d-states and the hybridization of the TM d-states with O p-states play a crucial role in the magnitude of the TM–TM and TM–O interactions and determine the formation of the 2D and 3D configurations on CeO2(111). The interaction of TM4 with the CeO2(111) surface changes the nature of the occupied Ce f-states from itinerant (CeIV in the clean surface) to localized (CeIII) states; hence, it increases the atomic size of CeIII compared with CeIV by 4.4%, which plays a crucial role in building in a lateral tensile strain in the topmost Ce layer in the surface. Furthermore, we found an enhancement of the electron localization of the TM d-states upon the adsorption of TM4 on CeO2(111). We found that the number of Ce atoms in the CeIII oxidation state depends on the TM element and structure. For Ru, Rh, Os, and Ir on CeO2(111), all the Ce atoms in the topmost Ce layer change the oxidation state from IV to III (i.e., 100%), while for (Pd, Pt) and (Cu, Ag, Au) on CeO2(111), only 25% and 50% of Ce atoms, respectively, convert the oxidation state from IV to III.

Hydrothermal synthesis, structure, porosity, and luminescent properties of 3D coordination polymer of holmium with 1,4-phenylenediacetic acid [Ho2(PDA)3(H2O)] n ·2nH2O

Coordination polymer formulated as [Ho2(PDA)3(H2O)] n ·2nH2O having three-dimensional network was hydrothermally synthesized (H2PDA is 1,4-phenylenediacetic acid). The polymer crystallizes in the monoclinic space group P 21/c, unit cell parameters: a = 21.7025(2) Ǻ, b = 10.03560(10) Ǻ, c = 14.03430(10) Ǻ, β = 91.7110(10)° (Z = 4), and exhibits a three-dimensional porous framework. The polymer contains two and four crystallographically independent holmium(III) ions and 1,4-phenylenediacetate anions, respectively. The two types of Ho(III) ions adopt an extended zigzag orientation and act as nodes alternately, while three different carboxylate (COO−) groups bridge two adjacent Ho(III) ions. The coordination polymer chains are reinforced by well-directed hydrogen bonds involving guest water molecules. The polymer is mesoporous in nature with average pore size diameter of 5.6 nm. The polymer exhibits intense emission bands in visible region upon excitation at 390 nm. The polymer is thermally robust and undergoes thermal decomposition in well-defined steps.

Edge structures for nanoscale graphene islands on Co(0001) surfaces.

Low-temperature scanning tunneling microscopy measurements and first-principles calculations are employed to characterize edge structures observed for graphene nanoislands grown on the Co(0001) surface. Images of these nanostructures reveal straight well-ordered edges with zigzag orientation, which are characterized by a distinct peak at low bias in tunneling spectra. Density functional theory based calculations are used to discriminate between candidate edge structures. Several zigzag-oriented edge structures have lower formation energy than armchair-oriented edges. Of these, the lowest formation energy configurations are a zigzag and a Klein edge structure, each with the final carbon atom over the hollow site in the Co(0001) surface. In the absence of hydrogen, the interaction with the Co(0001) substrate plays a key role in stabilizing these edge structures and determines their local conformation and electronic properties. The calculated electronic properties for the low-energy edge structures are consistent with the measured scanning tunneling images.

Friction anisotropy dependence on lattice orientation of graphene

The observation of friction anisotropy on graphene by friction measurement at atomic scale has been reported in this paper. Atomic-scale friction measurement revealed friction anisotropy with a periodicity of 60°, which is consistent with the hexagonal periodicity of the graphene. Both experiments and theory show that the value of the friction force is related to the graphene lattice orientation, and the friction force along armchair orientation is also larger than the one along zigzag orientation. These results will play a critical role in the use of graphene to manufacture nanoscale devices.

Effects of loading mode and orientation on deformation mechanism of graphene nano-ribbons

Molecular dynamics simulation is performed to analyze the deformation mechanism of graphene nanoribbons. When the load is applied along the zigzag orientation, tensile stress yields brittle fracture and compressive stress results in lattice shearing and hexagonal-to-orthorhombic phase transformation. Along the armchair direction, tensile stress produces lattice shearing and phase transformation, but compressive stress leads to a large bonding force. The phase transformation induced by lattice shearing is reversible for 17% and 30% strain in compressive loading along the zigzag direction and tensile loading along the armchair direction. The energy dissipation is less than 10% and resulting pseudo-elasticity enhances the ductility.

Orientation dependence of the fracture behavior of graphene

Graphene has unique mechanical properties in that it is simultaneously very strong and stretchy, which severely hampers the prediction of its orientation-dependent fracture behavior based on conventional theories used for common brittle or ductile materials. For the first time, by exploring the entire range of available tensile orientations, this study reveals the unique anisotropic fracture response of graphene using molecular dynamics simulations. We found that, as the uniaxial tensile direction rotates from armchair (0°) to zigzag orientation (30°), both the tensile strength and strain remain almost constant up to an orientation angle of ∼12°, then they rapidly increase (exponential growth), resulting in a remarkable degradation of the tensile strength compared to brittle fracture counterpart (inverse-sinusoidal growth). This typical fracture pattern holds for 100–700K. We propose a model that can explain its physical origin in good agreement with the simulation results. We also found that the elastic behavior of graphene is quasi-isotropic for all tensile orientations, in contrast to its anisotropic fracture behavior. Using indentation simulations of graphene, we showed that the anisotropic/isotropic features of fracture/elasticity are also well-preserved in the two-dimensional tensile systems but its fracture anisotropicity is greatly attenuated due to the inherent sixfold symmetry of graphene.

Cutting forces related with lattice orientations of graphene using an atomic force microscopy based nanorobot

The relationship between cutting forces and lattice orientations of monolayer graphene is investigated by using an atomic force microscopy (AFM) based nanorobot. In the beginning, the atomic resolution image of the graphene lattice is obtained by using an AFM. Then, graphene cutting experiments are performed with sample rotation method, which gets rid of the tip effect completely. The experimental results show that the cutting force along the armchair orientation is larger than the force along the zigzag orientation, and the cutting forces are almost identical every 60°, which corresponds well with the 60° symmetry in graphene honeycomb lattice structure. By using Poisson analysis method, the single cutting force along zigzag orientation is 3.9 nN, and the force along armchair is 20.5 nN. This work lays the experimental foundation to build a close-loop fabrication strategy with real-time force as a feedback sensor to control the cutting direction.

The electronic properties of graphene and graphene ribbons under simple shear strain

A tight-binding model is used to study the energy band of graphene and graphene ribbon under simple shear strain. The ribbon consists of lines of carbon atoms in an armchair or zigzag orientation where a simple shear strain is applied in the x-direction keeping the atomic distances in the y-direction unchanged. Such modification in the lattice gives an energy band that differs in several aspects from the one without any shear and with pure shear. The changes in the spectrum depend on the line displacement of the ribbon, and also on the modified hopping parameter. It is also shown that this simple shear strain tunes the electronic properties of both graphene and graphene ribbon, opening and closing energy gaps for different displacements of the system. The modified density of states is also shown.

Wave packet simulations of phonon boundary scattering at graphene edges

Wave packet dynamics is used to investigate the scattering of longitudinal (LA), transverse (TA), and bending-mode (ZA) phonons at the zigzag and armchair edges of suspended graphene. The interatomic forces are calculated using a linearized Tersoff potential. The strength of a boundary scattering event at impeding energy flow is described by a forward scattering coefficient, similar in spirit to a specularity parameter. For armchair boundaries, this scattering coefficient is found to depend strongly on the magnitude, direction, and polarization of the incident wavevector, while for zigzag boundaries, the forward scattering coefficient is found to always be unity regardless of wavevector and polarization. Wave packet splitting is observed for ZA phonons incident on armchair boundaries, while both splitting and mode conversion are observed for LA and TA phonons incident on both zigzag and armchair boundaries. These simulation results show that armchair boundaries impede the forward propagation of acoustic phonon energy much more strongly than zigzag boundaries do, suggesting that graphene nanoribbons will have substantially lower thermal conductivity in armchair rather than zigzag orientation.

A comparison of stress distributions for different surgical procedures, screw dimensions and orientations for a Temporomandibular joint implant

Finite element analysis is a useful analytical tool for the design of biomedical implants. The aim of this study was to investigate the behavior of temporomandibular joint implants with multiple design variables of the screws used for fixation of the implant. A commercially available implant with full mandible was analyzed using a finite element software package. The effects of different design variables such as orientation, diameter and stem length of the screws on the stress distribution in bone for two different surgical procedures were investigated. Considering the microstrain in bone as a principal factor, the acceptable ranges for screw diameter and length were determined. Parallel orientation of the screws performed better from a stress point of view when compared to the zig-zag orientation. Sufficient contact between the implant collar and mandibular condyle was shown to reduce the peak stresses which may lead to long term success. The distance between screw holes in the parallel orientation was much closer when compared to the zig-zag orientation. However, the stresses in bone near the screw hole area for the parallel orientation were within acceptable limits.