Electronic and optical properties of graphene nanoribbons in external fields

Unique electronic and optical properties of stacking-modulated bilayer graphene under external magnetic fields

This study delves into the magneto-electronic and magneto-optical properties of stacking-modulated bilayer graphene. By manipulating domain walls (DWs) across AB-BA domains periodically, we unveil oscillatory Landau subbands and the associated optical excitations. The DWs act as periodic potentials, yielding fascinating 1D spectral features. Our exploration reveals 1D phenomena localized to Bernal stacking, DW regions, and stacking boundaries, highlighting the intriguing formation of Landau state quantization influenced by the commensuration between the magnetic length and the system. The stable quantized localization within different regions leads to the emergence of unconventional quantized subbands. This study provides valuable insights into the essential properties of stacking-modulated bilayer graphene.

Rich essential properties of silicon-substituted graphene nanoribbons: a comprehensive computational study

The diverse structural, electronic, and magnetic properties of silicon (Si)-substituted armchair and zigzag graphene nanoribbons (AGNRs and ZGNRs) were investigated using spin-polarized density functional theory (DFT) calculations. Pristine AGNRs belong to a nonmagnetic semiconductor with a direct bandgap of 1.63/1.92 eV determined by PBE/HSE06 functionals. Under various Si substitutions, nonmagnetic bandgaps were tuned at 1.49/1.87, 1.06/1.84, 0.81/1.45, 1.04/1.71, 0.89/1.05, and 2.38/3.0 eV (PBE/HSE06) in the single Si edge-, single Si non-edge-, double Si ortho-, double Si meta-, double Si para-, and 100% Si-substituted AGNR configurations, respectively. Meanwhile, pristine ZGNRs displayed antiferromagnetic semiconducting behavior with a spin degenerate bandgap of 0.52/0.81 eV (PBE/HSE06) and becomes a ferromagnetic semimetal in the single Si configurations or an unusual ferromagnetic semiconductor in the 100% Si configuration. Under the developed first-principles theoretical framework, the formation of quasi π (C-2pz and Si-3pz) and quasi σ (C-2s, -2pxy and Si-3s and -3pxy) bands was identified in the Si-substituted configurations. These quasi π and quasi σ bands showed weak separation, resulting in weak quasi sp2 hybridization in Si-C bonds, in which the identified hybridization mechanism was a strong evidence for the formation of stable planar 1D structures in the Si-substituted configurations. Our complete revelation of the essential properties of Si-substituted GNRs can provide a complete understanding of their chemically doped 1D materials for various practical applications.

Hybrid Edge Results in Narrowed Band Gap: Bottom-up Liquid-Phase Synthesis of Bent N = 6/8 Armchair Graphene Nanoribbons

Scalable fabrication of graphene nanoribbons with narrow band gaps has been a nontrivial challenge. Here, we have developed a simple approach to access narrow band gaps using hybrid edge structures. Bottom-up liquid-phase synthesis of bent N = 6/8 armchair graphene nanoribbons (AGNRs) has been achieved in high efficiency through copolymerization between an o-terphenyl monomer and a naphthalene-based monomer, followed by Scholl oxidation. An unexpected 1,2-aryl migration has been discovered, which is responsible for introducing kinked structures into the GNR backbones. The N = 6/8 AGNRs have been fully characterized to support the proposed structure and show a narrow band gap and a relatively high electrical conductivity. In addition, their application in efficient gas sensing has also been demonstrated.

Mechanistic Insights into Electronic Current Flow through Quinone Devices

Molecular switches based on functionalized graphene nanoribbons (GNRs) are of great interest in the development of nanoelectronics. In experiment, it was found that a significant difference in the conductance of an anthraquinone derivative can be achieved by altering the pH value of the environment. Building on this, in this work we investigate the underlying mechanism behind this effect and propose a general design principle for a pH based GNR-based switch. The electronic structure of the investigated systems is calculated using density functional theory and the transport properties at the quasi-stationary limit are described using nonequilibrium Green's function and the Landauer formalism. This approach enables the examination of the local and the global transport through the system. The electrons are shown to flow along the edges of the GNRs. The central carbonyl groups allow for tunable transport through control of the oxidation state via the pH environment. Finally, we also test different types of GNRs (zigzag vs. armchair) to determine which platform provides the best transport switchability.

A Theoretical Study of Armchair Antimonene Nanoribbons in the Presence of Uniaxial Strain Based on First-Principles Calculations

The optimized geometry and also the electronic and transport properties of passivated edge armchair antimonene nanoribbons (ASbNRs) are studied using ab initio calculations. Due to quantum confinement, the size of the bandgap can be modulated from 1.2 eV to 2.4 eV (indirect), when the width is reduced from 5 nm to 1 nm, respectively. This study focuses on nanoribbons with a width of 5 nm (5-ASbNR) due to its higher potential for fabrication and an acceptable bandgap for electronic applications. Applying uniaxial compressive and tensile strain results in a reduction of the bandgap of the 5-ASbNR film. The indirect to direct bandgap transition was observed, when introducing a tensile strain of more than +4%. Moreover, when a compressive strain above 9% is introduced, semi-metallic behavior can be observed. By applying compressive (tensile) strain, the hole (electron) effective mass is reduced, thereby increasing the mobility of charge carriers. The study demonstrates that the carrier mobility of ASbNR-based nanoelectronic devices can be modulated by applying tensile or compressive strain on the ribbons.

Size Limiting Elemental Ferroelectricity in Bi Nanoribbons: Observation, Mechanism, and Opportunity

Combined with the inherent spin-orbital coupling effect, the elemental ferroelectricity of monolayer Bi (bismuthene) is the critical property that renders this system a 2D ferroelectric topological insulator. Here, using first-principles calculations, we systematically investigate the ferroelectric polarization in bismuthene nanoribbons and discover the width size limiting effect arising from the edge effects. The decreasing width led to the spontaneous transformation of the zigzag (ZZ) and armchair (AC) paired Bi nanoribbons into newly discovered high-symmetric nonpolarized nanoribbons. For ZZ-paired nanoribbons, the driving force of the phase transition is attributed to the depolarization field, similar to the conventional perovskite ferroelectric thin films. Instead, edge stress as a novel mechanism played a major role in the phase transition of AC-paired nanoribbons. Inspired by such a revealed mechanism, the phase transition and related ultrahigh piezoelectricity can be achieved by strain engineering in Bi nanoribbons, which could enable new applications for 2D ferroelectric devices.

A two-dimensional tunable double Weyl fermion in BL-α borophene

Two-dimensional (2D) materials with nontrivial band crossings, namely linear or double Weyl points, have been attracting tremendous attention. However, it remains a challenge to find existing 2D materials that host such nontrivial states. Here, based on first-principles calculations and symmetry analysis, we discover that the recently synthesized BL-α borophene is a metal with a tunable double Weyl point. Remarkably, both bands forming the double Weyl point have upward band bending. In addition, it shows an anisotropic band dispersion when away from the double Weyl point. To characterize its anisotropy, we define a quantity G, which could be changed from 1 to infinity when going from the energy of the double Weyl point to the Fermi level. Furthermore, the outer band of the double Weyl point is sensitive to biaxial strain, and could be changed from upward bending to downward bending. During this process, it has a critical case, in which the outer-band becomes flat. The changes in outer-band induce a variation in the density of states around the double Weyl point, thus giving rise to changes in its macroscopic physical properties. Applying a uniaxial strain enables the double Weyl point to transform into a pair of Weyl points by breaking the threefold rotation of BL-α borophene. When breaking the inversion symmetry and in-plane twofold rotation symmetry by a vertical symmetry, the double Weyl point still persisted; meanwhile, an additional pair of linear Weyl points appears on the high-symmetry path, giving rise to a Weyl complex case. Overall, our work thus provides an existing 2D material, BL-α borophene, to study the nontrivial band crossings in 2D.

Environmental Performance of Road Asphalts Modified with End-of-Life Hard Plastics and Graphene: Strategies for Improving Sustainability

Road construction takes a heavy toll on the environment. Therefore, innovative strategies to improve the environmental performances of this sector are needed, and the use of recycled materials (e.g., plastic) has been recently pursued to achieve this goal. The present work aims to (i) assess the environmental benefits deriving from the use of recycled hard plastics in combination with graphene to generate a new bitumen modifier and related asphalt mixture (AM) formulations (ii) to compare the performance of the bitumen modified using this new modifier with the bitumen modified using a traditional polymer (Styrene-Butadiene-Styrene, SBS) and the non-modified bitumen. A detailed Life Cycle Assessment (LCA) study was performed according to a cradle-to-cradle approach. Different scenarios were compared, including the variability of the pavement’s layers thickness and the amount of reclaimed asphalt pavement during the road maintenance cycles. The results demonstrated that the addition of the innovative modifier enhanced the structural performance of AMs, which turns into pavement extended durability, reduced maintenance cycles as well as into reduction in raw material use. The innovative asphalt modifier also creates a synergistic effect by offering a valuable alternative to hard plastic incineration by using it as a secondary raw material. This analysis allowed us to indicate the new-modified AM as the solution with the least environmental burden in all impact categories, suggesting its significant role in implementing new strategies to improve the environmental sustainability of road pavements.

Electronic, transport, magnetic and optical properties of graphene nanoribbons review

Low dimensional materials have attracted great research interest from both theoretical and experimental point of view. These materials exhibit novel physical and chemical properties due to the confinement effect in low dimensions. The experimental observations of graphene open a new platform to study the physical properties of materials restricted to two dimensions. This featured article provides a review on the novel properties of quasi one-dimensional (1D) material known as graphene nanoribbon. Graphene nanoribbons can be obtained by unzipping carbon nanotubes (CNTs) or cutting the graphene sheet. Alternatively, it is also called the finite termination of graphene edges. It gives rise different edge geometries namely zigzag and armchair among others. There are various physical and chemical techniques to realize these materials. Depending on the edge type termination, these are called the zigzag and armchair graphene nanoribbons (ZGNR and AGNR). These edges play an important role in controlling the properties of graphene nanoribbons. The present review article provides an overview of the electronic, transport, optical and magnetic properties of graphene nanoribbons. However, there are different ways to tune these properties for device applications. Here, some of them are highlighted such as external perturbations and chemical modifications. Few applications of graphene nanoribbon have and chemical modifications. Few applications of graphene nanoribbon have also been briefly discussed.

Molecular Mapping of Antifungal Mechanisms Accessing Biomaterials and New Agents to Target Oral Candidiasis

Oral candidiasis has a high rate of development, especially in immunocompromised patients. Immunosuppressive and cytotoxic therapies in hospitalized HIV and cancer patients are known to induce the poor management of adverse reactions, where local and systemic candidiasis become highly resistant to conventional antifungal therapy. The development of oral candidiasis is triggered by several mechanisms that determine oral epithelium imbalances, resulting in poor local defense and a delayed immune system response. As a result, pathogenic fungi colonies disseminate and form resistant biofilms, promoting serious challenges in initiating a proper therapeutic protocol. Hence, this study of the literature aimed to discuss possibilities and new trends through antifungal therapy for buccal drug administration. A large number of studies explored the antifungal activity of new agents or synergic components that may enhance the effect of classic drugs. It was of significant interest to find connections between smart biomaterials and their activity, to find molecular responses and mechanisms that can conquer the multidrug resistance of fungi strains, and to transpose them into a molecular map. Overall, attention is focused on the nanocolloids domain, nanoparticles, nanocomposite synthesis, and the design of polymeric platforms to satisfy sustained antifungal activity and high biocompatibility with the oral mucosa.

Triaxially strained suspended graphene for large-area pseudo-magnetic fields

Strain-engineered graphene has garnered much attention recently owing to the possibilities of creating substantial energy gaps enabled by pseudo-magnetic fields (PMFs). While theoretical works proposed the possibility of creating large-area PMFs by straining monolayer graphene along three crystallographic directions, clear experimental demonstration of such promising devices remains elusive. Herein, we experimentally demonstrate a triaxially strained suspended graphene structure that has the potential to possess large-scale and quasi-uniform PMFs. Our structure employs uniquely designed metal electrodes that function both as stressors and metal contacts for current injection. Raman characterization and tight-binding simulations suggest the possibility of achieving PMFs over a micrometer-scale area. Current-voltage measurements confirm an efficient current injection into graphene, showing the potential of our devices for a new class of optoelectronic applications. We also theoretically propose a photonic crystal-based laser structure that obtains strongly localized optical fields overlapping with the spatial area under uniform PMFs, thus presenting a practical route toward the realization of graphene lasers.

Biperiodic superlattices and transparent states in graphene

The transmission and transport properties of biperiodic graphene superlattices are studied theoretically. Special attention is paid to the so-called transparent states of biperiodic superlattices. A Dirac-like Hamiltonian is used to describe the charge carriers in graphene. The transfer matrix method and the Landauer-Büttiker formalism are implemented to obtain the transmittance and conductance, respectively. Similar results to those reported for Schrödinger electrons are obtained. However, in the case of Dirac electrons the splitted bands and the transparent states associated to the biperiodicity depend strongly on the angle of incidence as well as the character of the charge carriers. In fact, the dynamic of the splitted bands and transparent states is inverted for holes. The origin of transparent states is unveiled by obtaining an analytic expression for the transmittance. It is found that resonant transmission through single and double barriers gives rise to transparent states. Regarding the transport properties, it is possible to identify the fundamental changes caused by the biperiodicity. In particular, it is found a splitting, shifting, and diminishment of the conductance peaks with respect to the case of regular periodicity. This opens the door to corroborate experimentally the fundamental characteristics of biperiodic gated graphene superlattices through transport measurements.

Evaluation of graphene/crosslinked polyethylene for potential high voltage direct current cable insulation applications

This paper evaluates the potential usage of graphene/crosslinked polyethylene (graphene/XLPE) as the insulating material for high voltage direct current (HVDC) cables. Thermal, mechanical and electrical properties of blends with/without graphene were evaluated by differential scanning calorimetry (DSC), tensile strength, DC conductivity, space charge measurements and water tree aging test. The results indicate that 0.007–0.008% weight amount of graphene can improve the mechanical and electrical insulation properties of XLPE blends, namely higher tensile/yield strength, improved space charge distribution, and shorter/fewer water tree branches. The improvements mainly attribute to the high stiffness of graphene, deep traps introduced by the interaction zones of graphene and XLPE, and the blockage effect of graphene within XLPE. For thermal performance of XLPE blends, graphene nano-fillers have but limited improvement. The crystallinity of the blends barely changes with the addition of graphene. However, the crosslinking degree increases as the additive-like amounts of graphene doped. The above findings provide a guide for tailoring lightweight XLPE materials with excellent mechanical and electrical performances by doping them with a small amount of graphene.

Spin current distribution in antiferromagnetic zigzag graphene nanoribbons under transverse electric fields

The spin current transmission properties of narrow zigzag graphene nanoribbons (zGNRs) have been the focus of much computational research, investigating the potential application of zGNRs in spintronic devices. Doping, fuctionalization, edge modification, and external electric fields have been studied as methods for spin current control, and the performance of zGNRs initialized in both ferromagnetic and antiferromagnetic spin states has been modeled. Recent work has shown that precise fabrication of narrow zGNRs is possible, and has addressed long debated questions on their magnetic order and stability. This work has revived interest in the application of antiferromagnetic zGNR configurations in spintronics. A general ab initio analysis of narrow antiferromagnetic zGNR performance under a combination of bias voltage and transverse electric field loading shows that their current transmission characteristics differ sharply from those of their ferromagnetic counterparts. At relatively modest field strengths, both majority and minority spin currents react strongly to the applied field. Analysis of band gaps and current transmission pathways explains the presence of negative differential resistance effects and the development of spatially periodic electron transport structures in these nanoribbons.

Feature-Rich Geometric and Electronic Properties of Carbon Nanoscrolls

How to form carbon nanoscrolls with non-uniform curvatures is worthy of a detailed investigation. The first-principles method is suitable for studying the combined effects due to the finite-size confinement, the edge-dependent interactions, the interlayer atomic interactions, the mechanical strains, and the magnetic configurations. The complex mechanisms can induce unusual essential properties, e.g., the optimal structures, magnetism, band gaps and energy dispersions. To reach a stable spiral profile, the requirements on the critical nanoribbon width and overlapping length will be thoroughly explored by evaluating the width-dependent scrolling energies. A comparison of formation energy between armchair and zigzag nanoscrolls is useful in understanding the experimental characterizations. The spin-up and spin-down distributions near the zigzag edges are examined for their magnetic environments. This accounts for the conservation or destruction of spin degeneracy. The various curved surfaces on a relaxed nanoscroll will create complicated multi-orbital hybridizations so that the low-lying energy dispersions and energy gaps are expected to be very sensitive to ribbon width, especially for those of armchair systems. Finally, the planar, curved, folded, and scrolled graphene nanoribbons are compared with one another to illustrate the geometry-induced diversity.

Crossover point of the field effect transistor and interconnect applications in turbostratic multilayer graphene nanoribbon channel

The electrical transport properties of a turbostratic multilayer graphene nanoribbon (GNR) with various number of layers (1-8 layers) were investigated using a field effect transistor with a single GNR channel. In the turbostratic multilayer GNR with 5 layers or less, the carrier mobility and Ion/Ioff ratio in the FETs were improved by slightly increasing the conductance with increasing the number of layers, meaning that the excellent semiconducting characteristic. The improvement of the carrier transport properties promotes by the turbostratic stacking structure. In the turbostratic multilayer GNR with 6 layers or more, although the Ion/Ioff ratio degraded, the conductance extremely improved with increasing the number of layers. This indicates that the turbostratic multilayer GNR with thicker number of layers becomes the significantly lower resistivity wire as a metallic characteristic. We revealed that the crossover point of the physical properties between the semiconducting and metallic characteristics is determined by the strength to screen the surrounding environment effects such as charged impurity on the substrate. Our comprehensive investigation provides a design guidance for the various electrical device applications of GNR materials.

Method of lines for analysis of plane wave scattering by periodic arrays of magnetically-biased graphene strips

In this paper, efficient analysis of the plane wave scattering by periodic arrays of magnetically-biased graphene strips (PAMGS) is performed using the semi-numerical, semi-analytical method of lines (MoL). In MoL, all but one independent variable is discretized to reduce a system of partial differential equations to a system of ordinary differential equations. Since the solution in one coordinate direction is obtained analytically, this method is time effective with a fast convergence rate. In the case of a multi-layered PAMGS, the governing equations of the problem are discretized concerning periodic boundary conditions (PBCs) in the transverse direction. The reflection coefficient transformation approach is then used to obtain an analytical solution in the longitudinal direction. Here, magnetically-biased graphene strips are modeled as conductive strips with a tensor surface conductivity which is electromagnetically characterized with tensor graphene boundary condition (TGBC). The reflectance and transmittance of different multi-layered PAMGS are carefully obtained and compared with those of other methods reported in the literature. Very good accordance between the results is observed which confirms the accuracy and efficiency of the proposed method.

Gap opening in graphene nanoribbons by application of simple shear strain and in-plane electric field

The effects of shear strain and applied in plane electric field on the electronic properties of monolayer graphene nanoribbons (GNRs) are theoretically investigated. Band structures and the probability densities are calculated within the tight-binding model and the mechanical stresses submitted to the GNRs are taken into account by using the theory of linear elasticity with joint modifications in the elongation of the nearest-neighbor vectors and the modification of the hopping parameters. The energy gaps for specific widths of (semiconducting) armchair nanoribbons are verified also in the presence of either strain or field, whereas zigzag nanoribbons are metallic for any value of strain and exhibit a small gap for any value of field. However, our results demonstrate that when both strain and electric field are combined, a significant energy gap is always observed in the band structure, for any width or edge type of the ribbon. Moreover, the obtained total wave function is asymmetric along the ribbon width due to the applied electric field that pushes the electrons to one side of the ribbon and, under shear strain, a peak at the center of the ribbon in the spatial distribution is also observed owing to the preferable localization around the almost undeformed carbon bonds at ribbon center.

Chirality effects on an electron transport in single-walled carbon nanotube

In our work, we investigate characteristics of conductivity for single-walled carbon nanotubes caused by spin–orbit interaction. In the case study of chirality indexes, we especially research on the three types of single-walled carbon nanotubes which are the zigzag, the chiral, and the armchair. The mathematical analysis employed for our works is the Green-Kubo Method. For the theoretical results of our work, we discover that the chirality of single-walled carbon nanotubes impacts the interaction leading to the spin polarization of conductivity. We acknowledge such asymmetry characteristics by calculating the longitudinal current–current correlation function difference between a positive and negative wave vector in which there is the typical chiral-dependent. We also find out that the temperature and the frequency of electrons affect the function producing the different characteristics of the conductivity. From particular simulations, we obtain that the correlation decrease when the temperature increase for a low frequency of electrons. For high frequency, the correlation is nonmonotonic temperature dependence. The results of the phenomena investigated from our study express different degrees of spin polarization in each chiral of single-walled carbon nanotube and significant effects on temperature-dependent charge transport according to carrier backscattering. By chiral-induced spin selectivity that produces different spin polarization, our work could be applied for intriguing optimization charge transport.

Stacking-configuration-enriched essential properties of bilayer graphenes and silicenes

First-principles calculations show that the geometric and electronic properties of silicene-related systems have diversified phenomena. Critical factors of group-IV monoelements, like buckled/planar structures, stacking configurations, layer numbers, and van der Waals interactions of bilayer composites, are considered simultaneously. The theoretical framework developed provides a concise physical and chemical picture. Delicate evaluations and analyses have been made on the optimal lattices, energy bands, and orbital-projected van Hove singularities. They provide decisive mechanisms, such as buckled/planar honeycomb lattices, multi-/single-orbital hybridizations, and significant/negligible spin-orbital couplings. We investigate the stacking-configuration-induced dramatic transformations of essential properties by relative shift in bilayer graphenes and silicenes. The lattice constant, interlayer distance, buckling height, and total energy essentially depend on the magnitude and direction of the relative shift: AA → AB → AA' → AA. Apparently, sliding bilayer systems are quite different between silicene and graphene in terms of geometric structures, electronic properties, orbital hybridizations, interlayer hopping integrals, and spin interactions.

Three-terminal spin/charge current router

Topological insulator materials have wide applications in electronic and spintronic devices by utilizing the protected edge states. In this paper, based on these topological edge states and energy gaps, we propose some types of spin/charge current router in a three-terminal system consisting of silicene-like nanoribbons (SiNRs). The current is well controlled by the helical edge states of zigzag SiNRs (ZSiNRs) and external fields. Using the tight-binding model and non-equilibrium Green's function theory, we investigate three types of such router. The first type is a spin current shunter which separates the spin-up and spin-down current into different leads. The second type is a spin filter which separates the spin-polarized electrons into one of those leads. The last type is a charge current switcher which switches the charge current from one lead to the other lead. The local current distribution is calculated for the specific electron path. We find that the small Rashba does not destroy the filtering properties of the system. Besides, as an example, we employ the Landauer-Büttiker formula to obtain the current-voltage curves of the first type router and investigate the transmittance reciprocity relations in such a three-terminal system. We believe these proposed spin/charge current routers, which can separate the specific current into the expected lead, have potential applications in the future spintronics designs.

Anomalous magneto-transport properties of bilayer phosphorene

The magneto-transport properties of phosphorene are investigated by employing the generalized tight-binding model to calculate the energy bands. For bilayer phosphorene, a composite magnetic and electric field is shown to induce a feature-rich Landau level (LL) spectrum which includes two subgroups of low-lying LLs. The two subgroups possess distinct features in level spacings, quantum numbers, as well as field dependencies. These together lead to anomalous quantum Hall (QH) conductivities which include a well-shape, staircase and composite quantum structures with steps having varying heights and widths. The Fermi energy-magnetic field-Hall conductivity (EF-Bz-σxy) and Fermi energy-electric field-Hall conductivity (EF-Ez-σxy) phase diagrams clearly exhibit oscillatory behaviors and cross-over from integer to half-integer QH effect. The predicted results should be verifiable by magneto-transport measurements in a dual-gated system.

Behavior of the Energy Spectrum and Electric Conduction of Doped Graphene

We consider the effect of atomic impurities on the energy spectrum and electrical conductance of graphene. As is known, the ordering of atomic impurities at the nodes of a crystal lattice modifies the graphene spectrum of energy, yielding a gap in it. Assuming a Fermi level within the gap domain, the electrical conductance diverges at the ordering of graphene. Hence, we can conclude about the presence of a metal–dielectric transition. On the other hand, for a Fermi level occurring outside of the gap, we see an increase in the electrical conductance as a function of the order parameter. The analytic formulas obtained in the Lifshitz one-electron strong-coupling model, describing the one-electron states of graphene doped with substitutional impurity atoms in the limiting case of weak scattering, are compared to the results of numerical calculations. To determine the dependence of the energy spectrum and electrical conductance on the order parameter, we consider both the limiting case of weak scattering and the case of finite scattering potential. The contributions of the scattering of electrons on a vapor of atoms to the density of states and the electrical conductance of graphene with an admixture of interstitial atoms are studied within numerical methods. It is shown that an increase in the electrical conductance with the order parameter is a result of both the growth of the density of states at the Fermi level and the time of relaxation of electron states. We have demonstrated the presence of a domain of localized extrinsic states on the edges of the energy gap arising at the ordering of atoms of the admixture. If the Fermi level falls in the indicated spectral regions, the electrical conductance of graphene is significantly affected by the scattering of electrons on clusters of two or more atoms, and the approximation of coherent potential fails in this case.

Featured properties of Li+-based battery anode: Li4Ti5O12

3D ternary Li4Ti5O12, a Li+-based battery anode, presents an unusual lattice symmetry (triclinic crystal), band structure, charge density, and density of states under first-principles calculations. It is a large direct-gap semiconductor with E d g ∼ 2.98 eV. The atom-dominated valence and conduction bands, the spatial charge distribution and the atom- and orbital-decomposed van Hove singularities are available for delicate identifications of multi-orbital hybridizations in Li-O and Ti-O bonds. The extremely non-uniform chemical environment, which induces very complicated hopping integrals, directly arises from the large bonding fluctuations and the highly anisotropic configurations. Also, the developed theoretical framework is very useful for fully understanding cathodes and electrolytes of oxide compounds.

2N+4-rule and an atlas of bulk optical resonances of zigzag graphene nanoribbons

Development of on-chip integrated carbon-based optoelectronic nanocircuits requires fast and non-invasive structural characterization of their building blocks. Recent advances in synthesis of single wall carbon nanotubes and graphene nanoribbons allow for their use as atomically precise building blocks. However, while cataloged experimental data are available for the structural characterization of carbon nanotubes, such an atlas is absent for graphene nanoribbons. Here we theoretically investigate the optical absorption resonances of armchair carbon nanotubes and zigzag graphene nanoribbons continuously spanning the tube (ribbon) transverse sizes from 0.5(0.4) nm to 8.1(12.8) nm. We show that the linear mapping is guaranteed between the tube and ribbon bulk resonance when the number of atoms in the tube unit cell is [Formula: see text], where [Formula: see text] is the number of atoms in the ribbon unit cell. Thus, an atlas of carbon nanotubes optical transitions can be mapped to an atlas of zigzag graphene nanoribbons.

A Narrow Dual-Band Monolayer Unpatterned Graphene-Based Perfect Absorber with Critical Coupling in the Near Infrared

The combination of critical coupling and coupled mode theory in this study elevated the absorption performance of a graphene-based absorber in the near-infrared band, achieving perfect absorption in the double bands (98.96% and 98.22%), owing to the guided mode resonance (the coupling of the leak mode and guided mode under the condition of phase matching, which revealed 100% transmission or reflection efficiency in the wavelet band), and a third high-efficiency absorption (91.34%) emerged. During the evaluation of the single-structure, cross-circle-shaped absorber via simulation and theoretical analysis, the cross-circle shaped absorber assumed a conspicuous preponderance through exploring the correlation between absorption and tunable parameters (period, geometric measure, and incident angle of the cross-circle absorber), and by briefly analyzing the quality factors and universal applicability. Hence, the cross-circle resonance structure provides novel potential for the design of a dual-band unpatterned graphene perfect absorber in the near-infrared band, and possesses practical application significance in photoelectric detectors, modulators, optical switching, and numerous other photoelectric devices.

Stacking stability of C2N bilayer nanosheet

In recent years, a 2D graphene-like sheet: monolayer C₂N was synthesized via a simple wet-chemical reaction. Here, we studied the stability and electronic properties of bilayer C₂N. According to a previous study, a bilayer may exist in one of three highly symmetric stacking configurations, namely as AA, AB and AB′-stacking. For the AA-stacking, the top layer is directly stacked on the bottom layer. Furthermore, AB- and AB′-stacking can be obtained by shifting the top layer of AA-stacking by a/3-b/3 along zigzag direction and by a/2 along armchair direction, respectively, where a and b are translation vectors of the unit cell. By using first-principles calculations, we calculated the stability of AA, AB and AB′-stacking C₂N and their electronic band structure. We found that the AB-stacking is the most favorable structure and has the highest band gap, which appeared to agree with previous study. Nevertheless, we furthermore examine the energy landscape and translation sliding barriers between stacking layers. From energy profiles, we interestingly found that the most stable positions are shifted from the high symmetry AB-stacking. In electronic band structure details, band characteristic can be modified according to the shift. The interlayer shear mode close to local minimum point was determined to be roughly 2.02 × 10¹² rad/s.

Spin-polarized magneto-electronic properties in buckled monolayer GaAs

We develop the generalized tight-binding model to fully explore the magneto-electronic properties of monolayer GaAs, where the buckled structure, multi-orbital chemical bondings, spin-orbit coupling, electric field, and magnetic field are considered simultaneously. The diverse magnetic quantization covers three groups of spin-polarized Landau levels (LLs) near the Fermi level, with the unique initial energies, LL degeneracy, energy spacings, magnetic-field-dependence, and spin splitting. Furthermore, the Landau state probabilities exhibit specific oscillation patterns, being composed of the localization centers, node regularities, and energy-dependent variations of the dominating orbitals. The density of states directly reflects the main features of the LL energy spectra in the form, height, number, and frequency of the spin-split delta-function-like prominent peaks. The electric field leads to the monotonous/nonmonotonous LL energy dispersions, LL crossing behavior, gap modulation, phase transition and enhancement of spin splitting. The complex gap modulations and even semiconductor-semimetal transitions are attributed to the strong competition among the intrinsic interactions, magnetic field, and electric field. Such predicted magneto-electronic properties could be verified by scanning tunneling spectroscopy and are helpful in designing the top-gated and phase-change electronic devices.

Magnetoplasmons in monolayer black phosphorus structures

Two-dimensional materials supporting deep-subwavelength plasmonic modes can also exhibit strong magneto-optical responses. Here, we theoretically investigate magnetoplasmons (MPs) in monolayer black phosphorus (BP) structures under moderate static magnetic fields. We consider three different structures, namely, a continuous BP monolayer, an edge formed by a semi-infinite sheet, and finally, a triangular wedge configuration. Each of these structures shows strongly anisotropic magneto-optical responses induced both by the external magnetic field and by the intrinsic anisotropy of the BP lattice. Starting from the magneto-optical conductivity of a single layer of BP, we derive the dispersion relation of the MPs in the considered geometries, using a combination of analytical, semi-analytical, and numerical methods. We fully characterize the MP dispersions and the properties of the corresponding field distributions, and we show that these structures sustain strongly anisotropic subwavelength modes that are highly tunable. Our results demonstrate that MPs in monolayer BP, with its inherent lattice anisotropy as well as magnetically induced anisotropy, hold potential for tunable anisotropic materials operating below the diffraction limit, thereby paving the way for tailored nanophotonic devices at the nanoscale.

Diverse Electronic and Magnetic Properties of Chlorination-Related Graphene Nanoribbons

The dramatic changes in electronic and magnetic properties are investigated using the first-principles calculations for halogen(X: Cl, Br, I, At)-adsorbed graphene nanoribbons. The rich and unique features are clearly revealed in the atoms-dominated electronic band structures, spin arrangement/magnetic moment, spatial charge distribution, and orbital- and spin-projected density of states. Halogen adsorptions can create the non-magnetic, ferromagnetic or anti-ferromagnetic metals, being mainly determined by concentrations and edge structures. The number of holes per unit cell increases with the adatom concentrations. Furthermore, magnetism becomes nonmagnetic when the adatom concentration is beyond 60% adsorption. There are many low-lying spin-dependent van Hove singularities. The diversified properties are attributed to the significant X-C bonds, the strong X-X bonds, and the adatom- and edge-carbon-induced spin states.

Electronic and optical responses of quasi-one-dimensional phosphorene nanoribbons to strain and electric field

Electronic and optical responses of zigzag- and armchair-edge quasi-one-dimensional phosphorene nanoribbons (Q1D-PNRs) to strain and external field are comparatively studied based on the tight-binding calculations. The results show that: (i) Zigzag-edge Q1D-PNR has the metallic ground state; applying global strains can not open the gap at the Fermi level but applying the electric field can achieve it; the direct/indirect character of the field-induced gap is determined by the electron-hole symmetry; an electric-field-enhanced optical absorption of low-energy photons is also predicted. (ii) Armchair-edge Q1D-PNR turns out an insulator with the large direct band gap; the inter-plane strain modulates this gap non monotonically while the in-plane one modulates it monotonically; in addition, the gap responses to electric fields also show strong direction dependence, i. e., increasing the inter-plane electric field will monotonically enlarge the gap but the electric field along the width direction modulates the gap non monotonically with three characteristic response regions.

Two-dimensional Dirac particles in a Pöschl-Teller waveguide

We obtain exact solutions to the two-dimensional (2D) Dirac equation for the one-dimensional Pöschl-Teller potential which contains an asymmetry term. The eigenfunctions are expressed in terms of Heun confluent functions, while the eigenvalues are determined via the solutions of a simple transcendental equation. For the symmetric case, the eigenfunctions of the supercritical states are expressed as spheroidal wave functions, and approximate analytical expressions are obtained for the corresponding eigenvalues. A universal condition for any square integrable symmetric potential is obtained for the minimum strength of the potential required to hold a bound state of zero energy. Applications for smooth electron waveguides in 2D Dirac-Weyl systems are discussed.

Alkali-created rich properties in grapheme nanoribbons: Chemical bondings

The alkali-adsorbed graphene nanoribbons exhibit the feature-rich electronic and magnetic properties. From the first-principles calculations, there are only few adatom-dominated conduction bands, and the other conduction and valence bands are caused by carbon atoms. A lot of free electrons are revealed in the occupied alkali- and carbon-dependent conduction bands. Energy bands are sensitive to the concentration, distribution and kind of adatom and the edge structure, while the total linear free carrier density only relies on the first one. These mainly arise from a single s − 2p_z orbital hybridization in the adatom-carbon bond. Specifically, zigzag systems can present the anti-ferromagnetic ordering across two edges, ferromagnetic ordering along one edge and non-magnetism, being reflected in the edge-localized energy bands with or without spin splitting. The diverse energy dispersions contribute many special peaks in density of states. The critical chemical bonding and the distinct spin configuration could be verified from the experimental measurements.

Optoelectronic Properties of Van Der Waals Hybrid Structures: Fullerenes on Graphene Nanoribbons

The search for new optical materials capable of absorbing light in the frequency range from visible to near infrared is of great importance for applications in optoelectronic devices. In this paper, we report a theoretical study of the electronic and optical properties of hybrid structures composed of fullerenes adsorbed on graphene and on graphene nanoribbons. The calculations are performed in the framework of the density functional theory including the van der Waals dispersive interactions. We found that the adsorption of the C 60 fullerenes on a graphene layer does not modify its low energy states, but it has strong consequences for its optical spectrum, introducing new absorption peaks in the visible energy region. The optical absorption of fullerenes and graphene nanoribbon composites shows a strong dependence on photon polarization and geometrical characteristics of the hybrid systems, covering a broad range of energies. We show that an external electric field across the nanoribbon edges can be used to tune different optical transitions coming from nanoribbon-fullerene hybridized states, which yields a very rich electro-absorption spectrum for longitudinally polarized photons. We have carried out a qualitative analysis on the potential of these hybrids as possible donor-acceptor systems in photovoltaic cells.

Tunable light trapping and absorption enhancement with graphene ring arrays

Graphene ring arrays are proposed to introduce tunable light trapping and enhance the absorption in the surrounding light-absorbing materials.

Geometric, magnetic and electronic properties of folded graphene nanoribbons

Geometric, magnetic and electronic properties of folded graphene nanoribbons (GNRs) are investigated by first-principles calculations.