Zigzag Terminations Research Articles

Surface immobilization mechanisms of cobalt ions on hydroxyapatite catalyst supports

Cobalt deposition in an excess of solution was used to design Co-modified hydroxyapatite materials with various Co loadings and controlled dispersion. It is rationalized how the properties of hydroxyapatite supports (more or less stoichiometric compositions, nanorod or platelet morphologies and crystalline (100) zig-zag termination or non-apatitic hydrated external layer), influence the immobilization processes of Co by operating either with slightly acidic (natural) or basic pH of the suspension media. Four cobalt immobilization mechanisms impacting the final dispersion of Co on hydroxyapatites were identified by combining structural (XRD, 1H and 31P solid-state NMR, UV-Vis, Raman and X-ray fluorescence spectroscopies), surface (XPS) characterizations of Co-modified hydroxyapatites after drying and thermal treatment at 500 °C, and monitoring of the pH and the composition of the supernatant solutions during the Co deposition step. On dried Co-modified crystalline stoichiometric hydroxyapatite nanorods, cobalt is highly dispersed through cationic exchange or strong electrostatic adsorption (SEA) at slightly acidic or basic pH, respectively. After thermal treatment at 500 °C, only cation exchange preserved atomic dispersion of Co(II) ions since Co3O4 nanoparticles were observed on samples for which Co deposition occurred via SEA. On defective hydroxyapatite platelets, cobalt deposited at acidic natural pH could diffuse in an external non-apatitic layer, whereas under basic pH media, this surface layer was hydrolysed, resulting in the formation of a cobalt-substituted hydroxyapatite layer in which only a limited fraction of the surface cobalt species could be probed by XPS.

Insights into the Electronic, Optical, and Anti-Corrosion Properties of Two-Dimensional ZnO: First-Principles Study

The electronic, optical, and anticorrosion properties of planer ZnO crystal and quantum dots are explored using density functional theory calculations. The calculations for the finite ZnO quantum dots were performed in Gaussian 16 using the B3LYP/6-31g level of theory. The periodic calculations were carried out using VASP with the plane wave basis set and the PBE functional. The subsequent band structure calculations were performed using the hybrid B3LYP functional that shows accurate results and is also consistent with the finite calculations. The considered ZnO nanodots have planer hexagonal shapes with zigzag and armchair terminations. The binding energy calculations show that both structures are stable with negligible deformation at the edges. The ZnO nanodots are semiconductors with a moderate energy gap that decreases when increasing the size, making them potential materials for anticorrosion applications. The values of the electronic energy gaps of ZnO nanodots are confirmed by their UV-Vis spectra, with a wide optical energy gap for the small structures. Additionally, the calculated positive fraction of transferred electrons implies that electron transfer occurs from the inhibitor (ZnO) to the metal surface to passivate their vacant d-orbitals, and eventually prevent corrosion. The best anti-corrosion performance was observed in the periodic ZnO crystal with a suitable energy gap, electronegativity, and fraction of electron transfer. The effects of size and periodicity on the electronic and anticorrosion properties are also here investigated. The findings show that the anticorrosion properties were significantly enhanced by increasing the size of the quantum dot. Periodic ZnO crystals with an appropriate energy gap, electronegativity, and fraction of electron transfer exhibited the optimum anticorrosion performance. Thus, the preferable energy gap in addition to the most promising anticorrosion parameters imply that the monolayer ZnO is a potential candidate for coating and corrosion inhibitors.

Exploring Spin Distribution and Electronic Properties in FeN4-Graphene Catalysts with Edge Terminations.

Understanding the spin distribution in FeN4-doped graphene nanoribbons with zigzag and armchair terminations is crucial for tuning the electronic properties of graphene-supported non-platinum catalysts. Since the spin-polarized carbon and iron electronic states may act together to change the electronic properties of the doped graphene, we provide in this work a systematic evaluation using a periodic density-functional theory-based method of the variation of spin-moment distribution and electronic properties with the position and orientation of the FeN4 defects, and the edge terminations of the graphene nanoribbons. Antiferromagnetic and ferromagnetic spin ordering of the zigzag edges were considered. We reveal that the electronic structures in both zigzag and armchair geometries are very sensitive to the location of FeN4 defects, changing from semi-conducting (in-plane defect location) to half-metallic (at-edge defect location). The introduction of FeN4 defects at edge positions cancels the known dependence of the magnetic and electronic proper-ties of undoped graphene nanoribbons on their edge geometries. The implications of the reported results for catalysis are also discussed in view of the presented electronic and magnetic properties.

Nanoporous graphene quantum dots constructed from nanoribbon superlattices with controllable pore morphology and size for wastewater treatment

The pollution of water bodies with toxic components possesses a dangerous impact on human health and the environment. Therefore, it is vital to find efficient materials for water treatment. Herein we construct various nanoporous graphene quantum dots from nanoribbon superlattices with tunable pore size and abundant active sites. The investigations are performed using density functional theory calculations. Depending on the type of superlattice, the pore size can be tuned from 3 Å to 11 Å and the pore morphologies are also controllable. Some of the nanoporous graphenes have edge states at the zigzag terminations that decrease the band gap and act as active sites. While others do not have edge states due to the armchair terminations and form semiconductors with an energy gap of ∼ 2.7 eV Molecular electrostatic potential confirms that edge carbon atoms have high negative electronic densities and thus they are suitable for intermolecular coordination with the toxic metals. The considered 2D membranes show good capability to adsorb heavy metals, namely Cd, Pb, and Hg, with negligible deformation. Additionally, attaching chemical groups or elements, namely CN, COOH, CS2, N, O, and S, to the pores not only increases the adsorption energy but also helps control the pore size. Therefore, with the enhanced adsorption energy and tunable pore size, the constructed nanoporous graphene quantum dots are ideal candidates for 2D membranes.

Effect of the armchair and zigzag edge terminations on the properties of nanohelicenes: First-principles study

The work presents a comparison of the structural, mechanical, electronic and magnetic properties of armchair and zigzag terminated nanohelicenes, using the first principles density functional calculations. It was found that these nanohelicenes are metals in their most symmetric state, but undergo a spontaneous symmetry breaking which results in a metal-insulator transition (MIT). It was shown that the edge termination (armchair or zigzag) determines the type of the MIT in the global energy minima. The armchair terminated nanohelicene undergoes a Peierls MIT and it is a diamagnetic semiconductor. The zigzag terminated nanohelicenes is an antiferromagnetic semiconductor due to a Mott-Hubbard MIT. It was shown that the edge terminations and MIT type influence on helical electronic bands. Analysis of non-covalent interactions demonstrates that the van der Waals forces between coils are stronger in the case of zigzag termination.

Magnetism in diamond graphene nanoflakes

We numerically study in this work the magnetic properties induced by the on-site electron-electron interaction in graphene nanoflakes shaped diamond with a variety of sizes. By the mean-field Hubbard approximation, a phase transition in analogy to infinite graphene from non-magnetism to antiferromagnetism is observed. A very weak interaction U, approximately zero, is reported to be able to trigger magnetic ordering in a finite nanoflake compared to infinite structure. Furthermore, the investigation also indicates the edge and size dependence of magnetism. The antiferromagnetic ground state is of robust stability to larger zigzag nanoflake size and stronger interaction. The phase transition point, Uc, is found to be sensitive to the size denoted by means of the reduction of Uc as the size increases. The important role of edge effect causing the spin polarization along zigzag termination is confirmed for the diamond nanoflakes.

Dual-band valley topological refraction materials for bulk elastic waves manipulation

Valley topological refraction material (TRM) of elastic wave and its direction signal transmittance with robustness against structural disturbance is crucial for various applications such as communication, navigation and detection devices for medical and industrial purpose. However, the existing TRM is limited by the single topological band, which obviously hinders the exploration of the multi-band topological ultrasonic devices to guarantee its efficiency and stability. Here, we demonstrate the dual-band valley TRM for elastic waves, in which the valley edge states and their robustness are investigated. Furthermore, the topological refractions of the valley edge states from the interface into the free space at zigzag termination in two frequency bands are explored, where the mode conversion of the bulk elastic wave occurs at interface termination. Interestingly, the full mode conversion from longitudinal to transverse waves is revealed in the low frequency band while the non-full mode conversion between the longitudinal and transverse takes place in the high frequency band. The researched dual-band elastic TRM could facilitate the potential applications in multi-band and multi-directional devices.

Acoustic suppressed topological refraction in valley sonic crystals

We report both experimentally and numerically that an acoustic suppressed topological refraction is realized by two kagome-lattice valley sonic crystals (VSCs). By simply rotating triangle rods in the VSCs, acoustic valley Hall phase transitions can be obtained. In a designed topological waveguide composed of two VSCs with distinct valley topological phases, two types of valley edge states can be observed in the domain wall. Furthermore, the topological waveguide can support a suppressed topological refraction of sound, which arises from the excitation of an acoustic dipole mode at the exit of the domain wall. Such a phenomenon is experimentally demonstrated by scanning topological refractions of the edge states from a zigzag termination, in which the theoretical prediction of a negative refraction almost overlaps with the perpendicular bisector of the dipole mode, and thus it is suppressed totally. Finally, the robustness of the suppressed topological refraction is demonstrated experimentally. Our work can find potential applications in designing the devices of robust directional sound transports and communications.

Topological Refraction in Kagome Split-Ring Photonic Insulators.

A valley-Hall-like photonic insulator based on Kagome split-ring is proposed. Theoretical analysis and numerical calculations illustrate that symmetry can be broken not only by global rotation but also individual rotation of the split rings, providing topological phase transitions. Furthermore, refraction of the edge state from the interface into the background space at Zigzag termination is explored. It is shown that positive/negative refraction of the outgoing beam depends on the type of valley (K or ), from which the edge state is projected. These results provide a new way to manipulate terahertz wave propagation and facilitate the potential applications in directional collimation, beam splitting, negative refraction image, etc.

Atomically sharp jagged edges of chemical vapor deposition-grown WS2 for electrocatalysis

Chemical vapor deposition (CVD)-grown 2D transition metal dichalcogenides can adopt faceted edges. To investigate how sharp these sites can be, we utilize aberration-corrected annular dark-field scanning transmission electron microscopy (ADF-STEM) to resolve the atomic structure of two-dimensional (2D) WS2 domains that show jagged edges. Nanoscale triangular edge structures with S zigzag terminations are observed. Both the peak and valley regions exhibit near-atomic sharpness. The peaks are as sharp as two atoms in width. Highly ordered valley sites display a minimum width of three atoms, and prospective single-atom valleys appear as two-atom-wide sites in the ADF-STEM contrast. Regarding the kinetics, density-functional theory (DFT) calculations indicate that the 2-W-atom peak would not evolve into a single-W-atom peak even though the latter configuration is also stable with a high enough binding energy under the growth conditions. These results help deepen our understanding of the possible structuring at the nanoscale and the atomic-scale limits of peaks and valleys formed via intersection of two zigzag edges. The enriched edge sites lead to higher catalytic activities for the hydrogen evolution reaction (HER).

Band Structure and Spatial Localization of Electrons in Twisted Bilayer Graphene Nanoribbons

We have studied the band structure, density of states and spatial localization of electron in twisted bilayer nanoribbons by means of tight binding calculation. In Chiral geometries, edge states are also related to the presence of zig-zag edge atoms, although they presented remarkable size effects. Physical properties of chiral graphene nanoribbons and general edges can have a stong dependence on chirality. We have analysed the properties of edge states in twisted bilayaer ribbons, explaining their energy dispersion and spatial localization. The different splittings were found in edge bands due to the inhomogeneous interplay coupling. For stacked zig-zag atoms the coupling was larger than corresponding edge states splitted apart, but edge states stemming from regions with stacking, where the interlayer coupling was smaller and gave rise to zero energy bands. We have also been found that in the edge regions where top and bottom zig-zag terminations were stacked, inter-ribbon tunneling between the dispersions less zero energy states created bonding and antibonding combination with energies away from the energy of the Dirac point. The obtained results were found in good agreement with previously obtained results.

Zigzag nanoribbon of gated bilayer hexagonal crystals with spontaneous edge magnetism

Zigzag nanoribbons of monolayer graphene-like two-dimensional materials host spontaneous edge magnetism at the zigzag terminations, whose configuration controls the band gap. In this article, the edge magnetism of zigzag nanoribbons of bilayer hexagonal crystals are studied. The specific models of bilayer graphene and bilayer silicene are both studied. As the gated voltage increases, the total energy level, magnetic structure and band structures of the ground state and the first quasi-stable excited state are tuned. For certain region of the gated voltage, the band gaps of spin up and down are opened and closed, respectively, so that the systems are in the spin-polarized metallic phase. The spin-polarized conducting edge band of the bilayer nanoribbons could be applied for gate tunable spintronic devices.

A Scheme of Quantum Tunnel Field Effect Transistor Based on Armchair Graphene Nano-Ribbon

We proposed a scheme of armchair graphene nanoribbon (AGNR) based tunnel field-effect transistor (TFET). The simulated device consists of two (AGNR) electrodes with zigzag termination that are separated by a narrow gap. The Fermi level of two electrodes is controlled with a common back gate. The main idea is based on taking advantage of the electronic effects of smooth edge atoms of (AGNR) and investigating the effect of applied small uniaxial tensile strain and gate voltage on the output characteristics of simulated TFET. Our analysis shows that the simulated device will have a pronounced negative differential conductance high peak to valley ratio at room temperature. We see that by applying the uniaxial tensile strain, this ratio upgrades to 70 for (AGNR) with width N = 13.

Edge effects on the intrinsic magnetism in ferromagnetic triangular phosphorene quantum dots using fully spin-polarized calculations

This project involves discovering the electronic and magnetic properties of nanometer-sized phosphorene structures with triangular shapes in both zigzag and armchair termination types. The goal is to discuss the relationship between the electronic states belonging to the different conditions of these phosphorene quantum dots and their intrinsic magnetic properties. For this purpose, we consider electronic interactions utilizing the spin-polarized density functional theory calculations, and then the results compare with the data generated from tight-binding calculations. Both descriptions yield mid-gap states in the spectrum of ferromagnetic structures. Our results in non-spin computations without any geometry optimization were matched by tight-binding calculations which shows that the tight-binding method is an inefficient approximation in analyzing the optimized spin samples. Unlike graphene, in our spin-polarized calculations, we have obtained empty mid-gap states in the spectrum of ferromagnetic triangular phosphorene quantum dots. The edge atoms of these structures are known as the magnetic atoms with an unequal contribution of spin up and spin down. To prevent deforming the initial structures, the dangling bonds at the edge atoms were passivated in two types, fully hydrogenated and partial passivation with oxygen atoms. Oxygen doping was required for introducing magnetism to the non-spin edges of the fully hydrogenated case.

Very-few-layer graphene obtained from facile two-step shear exfoliation in aqueous solution

A defect-free graphene with 1–3 layers (very-few-layer graphene, VFLG) has been successfully produced using a facile and environmentally friendly two-step shear exfoliation method in rotating-blade mixer (kitchen blender) and in high shear rotor–stator mixer. An aqueous solution containing an inexpensive and safe anionic surfactant of sodium lauryl sulphate (SLS) was deployed as the working fluid. Raman spectroscopy (RS) and Raman imaging (RI), density functional theory (DFT) simulation, transmission electron microscopy and high-resolution transmission electron microscopy (TEM-HRTEM), as well as Fourier-transform infrared spectroscopy (FTIR) analyses were used to characterize the samples. Some configurations of mixer setting were applied, and the exfoliation mechanisms were discussed. RS analysis revealed that the exfoliation process in rotating-blade mixer continued by high shear in rotor–stator mixer for 3 h had produced VFLG dominated by a single layer of graphene. The Lorentzian peak fitting of 2D band was well-fitted by a single Lorentzian component at Raman shift of 2651.317 cm−1. The produced VFLG endured the lowest peak intensity ratio of D and G bands (ID/IG) of 0.146 which corresponded with the highest lateral size. RI analysis showed that produced VFLG had edge chirality of armchairs and zigzag terminations. TEM-HRTEM confirmed the presence of VFLG with the highest mean lateral size of ~375.4 nm. DFT calculations provided pertinent optical/electronic properties of graphene in terms of Raman Spectra, density of states (DOS) curve and nuclear magnetic resonance (NMR) shielding tensor. FTIR analysis showed that the exfoliation process did not cause any oxidation, sulphonation, or functional defects on the VFLG. The turbulence and hydrodynamic force in liquid were the critical factors in the exfoliation processes. High-quality VFLG, with a facile, inexpensive, and environmentally friendly synthesis process, rendered this process to be a promising route for large-scale high-quality graphene production.

Current distribution and group velocities for electronic states on lattice ribbons in a magnetic field

We study the group velocities of electronic states and distributions of currents in lattice ribbons under a uniform perpendicular magnetic field. Using the effective low-energy model we analyze all possible simple configurations of lattice termination with zigzag and armchair boundaries. We show that the edge current depends on the type of zigzag termination, and can be zero or finite near the edge. Also similar dependence is observed in the case of armchair termination and is related to the size of the ribbon. The nonzero current flowing along the edge can be used a signature of formation of propagating edge states. In addition, we show the qualitative difference in the distribution of the edge current between the case of α = 1 (dice model) and other values of model parameter α ≠ 1 for armchair-terminated ribbons.

Topological refraction in dual-band valley sonic crystals

Valley pseudospins, as quantum states of energy extrema in momentum space, have been introduced from condensed-matter systems into classical sound systems, and several valley sonic crystals (VSCs) have been realized experimentally. However, in the existing VSCs, topological kink states generally appear in a single band gap, which apparently has become an obstacle for multiband topological sound devices. To overcome this challenge, we here experimentally demonstrate dual-band VSCs, in which robust valley kink states exist in two separated bulk band gaps. More interestingly, two opposite valleys separately located in two band gaps are locked to a single propagation direction, which arises from the fact that the bands below two band gaps show opposite valley Chern numbers at the K/K\ensuremath{'} valley. This double valley-locking phenomenon has been demonstrated via measuring the topological refraction of the kink states into the ambient space at a zigzag termination. We observe positive refraction at the lower band gap, whereas the coexistence of positive and negative refraction at the higher band gap. Additionally, we observe the robust valley transport through the sharp corners at two band gaps. The designed VSCs with the dual-band topological refraction and robust valley transport could find potential applications in multiband and multidirectional devices.

Topological edge states of a graphene zigzag nanoribbon with spontaneous edge magnetism

The topological phases of graphene with spin-orbit coupling, an exchange field, and a staggered-sublattice potential determine the properties of the edge states of the zigzag nanoribbon. In the presence of the Hubbard interaction, the spontaneous magnetization at the zigzag terminations induces sizable magnetic moments at the lattice sites in the bulk region. Thus, the exchange field and staggered-sublattice potential in the bulk region are effectively changed, which in turn change the topological phase. Within a certain parameter regime, quasi-stable excited states of the zigzag nanoribbon exist, which have a different magnetism configuration at the zigzag terminations from the ground state. The quasi-stable excited states could effectively suppress the finite size effect of the topological edge states. The investigation of the topological edge states in the presence of interaction helps the engineering of spintronic nanodevices based on realistic materials.

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.

Topological valley, pseudospin, and pseudospin-valley protected edge states in symmetric pillared phononic crystals

We present a symmetric double-sided pillared phononic crystals (PPnCs) that can emulate both quantum spin Hall effect (QSHE) and quantum valley Hall effect (QVHE) by solely imposing different geometric perturbations. Indeed, the Dirac cones can occur in the low (deep subwavelength) and high frequency regime by judiciously turning the parameters of the attached pillars and even a double Dirac cone can be achieved. We realize the valley-protected, the pseudospin-protected or the pseudospin-valley coupled edge states with the proposed platform. Besides, we show a variety of refraction phenomena (positive, negative and evanescent) of the valley-polarized edge state at the zigzag termination when emulating QVHE. Further, we illustrate the valley-dependent feature of the pseudospin-valley coupled edge state and demonstrate the valley based splitting of the pseudospin-protected edge states in a Y-junction wave guide.