Graphene-like Systems Research Articles

Molecular picture of electric double layers with weakly adsorbed water.

Water adsorption energy, Eads, is a key physical quantity in sustainable chemical technologies such as (photo)electrocatalytic water splitting, water desalination, and water harvesting. In many of these applications, the electrode surface is operated outside the point (potential) of zero charge, which attracts counter-ions to form the electric double layer and controls the surface properties. Here, by applying density functional theory-based finite-field molecular dynamics simulations, we have studied the effect of water adsorption energy Eads on surface acidity and the Helmholtz capacitance of BiVO4 as an example of metal oxide electrodes with weakly chemisorbed water. This allows us to establish the effect of Eads on the coordination number, the H-bond network, and the orientation of chemisorbed water by comparing an oxide series composed of BiVO4, TiO2, and SnO2. In particular, it is found that a positive correlation exists between the degree of asymmetry ΔCH in the Helmholtz capacitance and the strength of Eads. This correlation is verified and extended further to graphene-like systems with physisorbed water, where the electric double layers (EDLs) are controlled by electronic charge rather than proton charge as in the oxide series. Therefore, this work reveals a general relationship between water adsorption energy Eads and EDLs, which is relevant to both electrochemical reactivity and the electrowetting of aqueous interfaces.

Tunable wave localization at the Dirac frequency in a metallic photonic crystal cavity

In this study, the two-dimensional (2D) triangular lattice metallic photonic crystals (PCs) in visible and infrared bands have been utilized to achieve light confinement at the Dirac frequency. Distinct from the traditional bandgap or total internal reflection cavity modes, the unique photonic localization mechanism leads to an unusual algebraic decay of state and a unique frequency located beyond any bandgaps. This investigation delves into the band structure analysis of 2D metallic PCs, specifically focusing on their distinctive features, such as photonic bandgaps and Dirac cones. The plane wave expansion (PWE) method, enhanced with a linearization technique, is employed for band structure calculations, considering both the frequency-dependent dielectric properties and the intrinsic lossy nature of metallic materials described by the Drude model. The study provides a comprehensive derivation of the PWE equations for metallic PCs and investigates their band characteristics under both TM and TE polarizations. Focusing on TM modes in triangular lattice metallic PCs, it reveals zero density of states (DOS) at K points of the Brillouin corner and the existence of Dirac cones with linearly dispersion and linearly vanishing DOS. The study extends to exploring localized modes at Dirac frequencies, employing a relativistic quantum mechanics approach analogous to graphene's charge carriers. Theoretical predictions are corroborated by numerical simulations, and the potential for tunable Dirac localized modes is highlighted. This research not only deepens the understanding of Dirac properties in graphene-like systems but also lays the groundwork for further exploration of the practical quasi-2D devices, which will provide assistance in the integration of micro- and nano- devices, especially in applications requiring long-range coupling, given the critical importance of optical cavities in contemporary optical technologies.

A water structure indicator suitable for generic contexts: Two-liquid behavior at hydration and nanoconfinement conditions and a molecular approach to hydrophobicity and wetting.

In a recent work, we have briefly introduced a new structural index for water that, unlike previous indicators, was devised specifically for generic contexts beyond bulk conditions, making it suitable for hydration and nanoconfinement settings. In this work, we shall study this metric in detail, demonstrating its ability to reveal the existence of a fine-tuned interplay between the local structure and energetics in liquid water. This molecular principle enables the establishment of an extended hydrogen bond network, while simultaneously allowing for the existence of network defects by compensating for uncoordinated sites. By studying different water models and different temperatures encompassing both the normal liquid and the supercooled regime, this molecular mechanism will be shown to underlie the two-state behavior of bulk water. In addition, by studying functionalized self-assembled monolayers and diverse graphene-like surfaces, we shall show that this principle is also operative at hydration and nanoconfinement conditions, thus generalizing the validity of the two-liquid scenario of water to these contexts. This approach will allow us to define conditions for wettability, providing an accurate measure of hydrophobicity and a reliable predictor of filling and drying transitions. Hence, it might open the possibility of elucidating the active role of water in the broad fields of biophysics and materials science. As a preliminary step, we shall study the hydration structure and hydrophilicity of graphene-like systems (parallel graphene sheets and carbon nanotubes) as a function of the confinement dimensionality.

Robust photo-induced pure spin current in graphene–graphane superlattices

The photogalvanic effect has been demonstrated to be an effective method to generate pure spin current. However, obtaining robust pure spin current remains a big challenge as most of the photo-induced pure spin current is dependent on the light polarization/phase angle, photon energies, or the spin alignment of electrodes. In this paper, we present a scheme for obtaining robust pure spin current in zigzag graphene–graphane superlattices (ZGSLs). Through systemic first-principles calculations, we demonstrate that robust generation of pure spin current can occur in ZGSLs with varying widths due to the centrosymmetry of the system's geometric structure and the antiferromagnetic magnetic features. Moreover, the generation of pure spin current does not depend on the photon energy or the polarization/phase angles for both linearly and elliptically polarized light, exhibiting very strong robustness. Our study suggests that ZGSLs could be highly promising candidates for practically realizing pure spin current in spintronics experiments, which presents an avenue for using graphene and its derivatives in advanced electronic devices. Furthermore, considering the experimental advancements in graphene and graphene-like materials, our investigation presents a universally applicable methodology for the generation of robust pure spin currents within low-dimensional graphene-like systems.

On valley asymmetry in a topological interaction for quasi-particles

This paper is focused on investigating the effects of a statistical interaction for graphene-like systems, providing Haldane-like properties for topologically trivial lattices. The associated self-energy correction yields an effective next-nearest hopping, inducing the topological phase, whose specific solutions are scrutinized. In the case of an external magnetic field, it leads to a renormalized quasi-particle structure with generalized Landau levels and explicit valley asymmetry. A suitable tool for implementing such achievements is a judicious indefinite metric quantization, leading to advances in field theory foundations. Since the topological behavior is encoded in the radiative corrections, an unequivocal treatment using an integral representation is carefully developed.

Dependence of the tunneling properties on an anisotropic factor in a graphene-like system

In analogy with the Klein tunneling in an isotropic linear energy graphene system, we reveal an anisotropic Klein tunneling in an anisotropic two dimensional (2D) linear energy system. The chirality and the absence of the back scattering leads to the Klein tunneling. The tunneling characteristics can be tuned by the barrier structure, the incident energy and angle, and more importantly the anisotropic parameter and different potential direction. The controllable Klein and side tunneling peaks can be understood by the carrier states and the phases in barrier/well regions. This interesting tunneling behaviors enrich the 2D tunneling properties and broaden the studies on the three-dimension tunneling features which will enlarge its electronic applications.

Ultra-Fast Preparation of Large-Area Graphdiyne-Based Membranes via Alkynylated Surface-Modification for Nanofiltration.

Graphdiynes (GDYs), two-dimensional graphene-like carbon systems, are considered as potential advanced membrane material due to their unique physicochemical features. Nevertheless, the scale-up of integrated GDY membranes is technologically challenging, and most studies remain at the theoretical stage. Herein, we report a simple and efficient alkynylated surface-mediated strategy to prepare hydrogen-substituted graphdiyne (HsGDY) membranes on commercial alumina tubes. Surface alkynylation initiates an accelerated surface-confined coupling reaction in the presence of a copper catalyst and facilitates the nanoscale epitaxial lateral growth of HsGDY. A continuous and ultra-thin HsGDY membrane (∼100 nm) can be produced within 15 min. The resulting membranes exhibit outstanding molecular sieving together with excellent water permeances (ca. 1100 L m-2 h-1 MPa-1 ), and show a long-term durability in cross-flow nanofiltration, owing to the superhydrophilic surface and hydrophobic pore walls.

Superconducting phase transition in planar fermionic models with Dirac cone tilting

The chiral and superconducting gaps are studied in the context of a planar fermion model with four-fermion interactions. The effect of the tilt of the Dirac cone on both gaps is shown and discussed. Our results point to two different behaviors exhibited by planar fermionic systems. We show that there is a threshold value $\tilde{t}^*$ for the effective tilt parameter such that when $|{\bf \tilde{t}}| < \tilde{t}^*$, the superconducting phase persists for negative values of the superconducting coupling constant. For positive values of the superconducting coupling constant, the induction of a superconducting gap by a chemical potential exists and which is similar to the one seen in graphene-like systems. For $|{\bf \tilde{t}}| > \tilde{t}^*$ and a negative superconducting coupling constant, the superconducting phase can be present, but it is restricted to a smaller area in the phase portrait. Our analysis also shows that when $|{\bf \tilde{t}}| > \tilde{t}^*$ and for positive values for the superconducting coupling constant, the induction of a superconducting gap in the presence of a chemical potential is ruled out. In this case, the increase of the chemical potential works in favor of the manifestation of a metallic phase.

Emerging D massive graviton in graphene-like systems

Unlike the fundamental forces of the Standard Model the quantum effects of gravity are still experimentally inaccessible. Rather surprisingly quantum aspects of gravity, such as massive gravitons, can emerge in experiments with fractional quantum Hall liquids. These liquids are analytically intractable and thus offer limited insight into the mechanism that gives rise to quantum gravity effects. To thoroughly understand this mechanism we employ a graphene-like system and we modify it appropriately in order to realise simple -dimensional massive gravity model. More concretely, we employ -dimensional Dirac fermions, emerging in the continuous limit of a fermionic honeycomb lattice, coupled to massive gravitons, simulated by bosonic modes positioned at the links of the lattice. The quantum character of gravity can be determined directly by measuring the correlations on the bosonic atoms or by the interactions they effectively induce on the fermions. The similarity of our approach to current optical lattice configurations suggests that quantum signatures of gravity can be simulated in the laboratory in the near future, thus providing a platform to address question on the unification theories, cosmology or the physics of black holes.

Nonlinear oscillations, chaotic dynamics, and stability analysis of bilayer graphene-like structures.

In this work, we have investigated the nonlinear oscillations and chaotic dynamics of perturbed bilayer graphene-like structures. The potential energy surface (PES) of bilayer graphene-like geometries is obtained by considering interactions of a co-aligned and counter-aligned arrangement of atoms. We studied the dynamics using the Poincaré surface of section for co-aligned hydrofluorinated graphene (HFG) and counter-aligned hexagonal boron nitride (h-BN) and generalized it for other systems using various choices of interaction parameters. The nature of the oscillations is understood via power spectra and the Lyapunov exponents. We found that the PES is very sensitive to the perturbation for all bilayer graphene-like systems. It is seen that the bilayer HFG system displays chaotic oscillations for strong perturbation, while for the h-BN system, the signature of chaos is found for weak perturbation. We have also generalized the work for perturbed bilayer graphene-like geometries, considering different interlayer interactions and the strength of perturbation. We found a signature of transition from regular to quasiperiodic and finally chaotic oscillations tuned via the strength of the perturbation for these geometries. The nature of the equilibrium points for bilayer graphene-like systems is analyzed via Jacobian stability conditions. We found three stable nodes for co-aligned HFG and counter-aligned h-BN systems for all interaction strengths. Though all other nodes are unstable saddle nodes, the signature of a local bifurcation is also found for weak perturbation.

Silicene: an excellent material for flexible electronics

The outstanding properties of graphene have laid the foundation for exploring graphene-like 2D systems, commonly referred to as 2D-Xenes. Among them, silicene is a front-runner due to its compatibility with current silicon fabrication technologies. Recent works on silicene have unveiled its useful electronic and mechanical properties. The rapid miniaturization of silicon devices and the useful electro-mechanical properties of silicene necessitate the exploration of potential applications of silicene flexible electronics in nano electro-mechanical systems. Using a theoretical model derived from the integration of ab initio density-functional theory and quantum transport theory, we investigate the piezoresistance effect of silicene in the nanoscale regime. As with graphene, we obtain a small value of the piezoresistance gauge factor (GF) of silicene, which is sinusoidally dependent on the transport angle. The small GF of silicene is attributed to its robust Dirac cone and strain-independent valley degeneracy. Based on the obtained results, we propose to use silicene as an interconnect in flexible electronic devices and as a reference piezoresistor in strain sensors. This work will hence pave the way for exploring flexible electronics applications in other 2D-Xene materials.

Ritus functions for graphene-like systems with magnetic fields generated by first-order intertwining operators

In this work, we construct the exact propagator for Dirac fermions in graphene-like systems immersed in external static magnetic fields with non-trivial spatial dependence. Such field profiles are generated within a first-order supersymmetric framework departing from much simpler (seed) magnetic field examples. The propagator is spanned on the basis of the Ritus eigenfunctions, corresponding to the Dirac fermion asymptotic states in the non-trivial magnetic field background which nevertheless admits a simple diagonal form in momentum space. This strategy enlarges the number of magnetic field profiles in which the fermion propagator can be expressed in a closed-form. Electric charge and current densities are found directly from the corresponding propagator and compared against similar findings derived from other methods.

Spin-wave resonance frequency and low-temperature properties of an antiferromagnetic graphene-like bilayer system

We studied the spin-wave resonance (SWR) frequency and low-temperature magnetization, internal energy, and specific heat of an antiferromagnetic graphene-like bilayer system with two sublattices by applying the retarded Green's function method and the linear spin-wave approximation. The numerical results show that two SWR frequencies in the system can be adjusted by changing the longitudinal magnetic field, interlayer (intralayer) exchange coupling, and intralayer anisotropy. These parameters greatly influence the magnetization, internal energy, and specific heat at low temperatures. Crossovers between the two sublattice magnetizations were observed for the different intralayer exchange couplings, which are affected by the quantum and thermal fluctuations.

Different types of magnetic edge configurations in selected graphene-like nanoribbons

Theoretical studies of electronic and magnetic properties of selected graphene-like nanoribbons are presented. The method is based on a tight-binding type Hamiltonian with Hubbard correlations. The main attention is focused on the zigzag nanoribbons and their magnetic configurations. It is shown that the latter determine energy band structures and may lead to a half-metallic behaviour of some graphene-like systems. Interestingly, apart from the well-known parallel and antiparallel arrangements of the ferromagnetic edges, it is also possible that only one edge is ferromagnetic and the other is paramagnetic. Such a one-edge magnetic configuration may appear in graphene, stanene and phosphorene for properly chosen gating and edge-doping.

Structural stability and electronic characteristics of cellulose nanowires on graphene-like systems

The adsorption of two kinds of cellulose chains on graphene and boron-nitride (insulating and semi-conducting) sheets were studied using first principles density-functional theory calculations. The effect of adsorption was analyzed in terms of the binding energy, band-structure, density of states, charge density differences and current behavior. The stable conformations demonstrate the possibilities of creating bio-conducting materials important for various applications utilizing cellulose, one of the most abundant biopolymers in nature. The charge-density differences reveal the charge accumulation on the atoms of the substrates/cellulose depending on the type of atom. Moreover, the increase in conductivity of the combined systems compared to the biopolymer as is evident from the current vs bias voltage study indicates the conducting nature of the systems.

DFT-1/2 method applied to 2D topological insulators: fluorinated and hydrogenated group-IV honeycomb systems

We present a computationally efficient and accurate methodology to compute Z 2 topological invariants for systems without inversion symmetry including quasiparticle (QP) effects within the density functional theory (DFT)-1/2 method. The Wannier charge center evolution is applied to compute the Z 2 topological invariant and investigate the topological properties of group-IV graphene-like systems, graphene, silicene, germanene and stanene, whose inversion symmetry is broken by simultaneous functionalization with hydrogen and fluorine atoms. Different atomic arrangements are studied. The systems are stable with cohesive energy decreasing along the row from carbon to tin. A similar trend is observed for band gaps. The resulting topological invariants are compared with values obtained within conventional DFT and using a hybrid exchange–correlation functional. The variation of the results with the treatment of exchange and correlation demonstrates the importance of QP corrections for the prediction of the topological or trivial character.

Excited States Symmetry Breaking and In-Plane Polarization Cause Chiral Reversal in Diastereomers.

In this work, we investigate the electronic transitions and chirality of three isomers of huge conjugated systems: asymmetric diastereomers (MMMM) and two symmetrical diastereomers (PMPM and PPMM). The physical mechanism of flipping has been studied theoretically. The new ribbon-shaped polycyclic aromatic hydrocarbons (PAHs) molecule is formed by connecting three graphene-like systems with large conjugated π orbitals. By calculating and analyzing electromagnetic interaction decomposition over distance, it can be found that the chirality reversal of different energies is caused by the symmetrical fracture of TMDM in the Z direction. The chirality reversal at the same energy is caused by the in-plane polarization of the TMDM along the Y direction.

One edge magnetic configurations in graphene, stanene and phosphorene zigzag nanoribbons

Graphene is well known for its extraordinary electronic and mechanical properties. Moreover, it has been shown that graphene’s magnetic properties are also interesting due to its long spin diffusion length and, theoretically expected and experimentally demonstrated, spontaneous spin-polarization of nanoribbon zigzag edges. However, it is also quite clear now that there exist many other quasi two-dimensional (2D) graphenelike systems which do deserve attention, too. So, here the attention is also directed to the edge magnetism problem in stanene (2D Sn) and phosphorene (2D P). It is shown that these materials can exhibit a (so far hardly explored) phase with one zigzag edge being magnetic and the other - paramagnetic. This new phase may prove to be attractive for spintronics.

Bandgap evolution in nanographene assemblies.

Recently cycloarene has been experimentally obtained in a self-assembled structure, forming graphene-like monoatomic layered systems. Here, we established bandgap engineering/prediction in cycloarene assemblies within a combination of density functional theory and tight-binding Hamiltonians. Our results show that the inter-molecule bond density rules the bandgap. The increase in such bond density increases the valence/conduction bandwidth decreasing the energy gap linearly. We derived an effective model that allows the interpretation of the arising energy gap for general particle-hole symmetric molecular arrangements based on inter-molecular bond strength.

Type-II Dirac points and edge transports in phononic crystal plates

The accidentally degenerate type-II Dirac points in sonic crystal has been realized recently. However, elastic phononic crystals with type-II Dirac points have not yet been explored. In this work, we design a two-dimensional phononic crystal plate in square lattice with type-II Dirac points for elastic waves. The type-II Dirac points, different from the type-I counterparts, have the tiled dispersions and thus the iso-frequency contours become crossed lines. By tuning structures to break the mirror symmetry, the degeneracies of the type-II Dirac points are lifted, leading to a band inversion. In order to have a further explanation, we also calculate the Berry curvatures of phononic crystals with opposite structure parameters, and it turns out that these two crystals hold opposite signs around the valley. The phononic crystal plates before and after the band inversion belong to different topological valley phases, whose direct consequence is that the topologically protected gapless interface states exist between two distinct topological phases. Topologically protected interface states are found by calculating the projected band structures of a supercell that contains two kinds of interfaces between two topological phases. Robustness of the interface transport is verified by comparing the transmission rate for perfect interface with that for defective interface. Moreover, owing to the special stress field distributions of the elastic plate waves, the boundaries of a single phononic crystal phase can similarly host the gapless boundary states, which is found by calculating the projected band structures of a supercell with a single phase, thus having two free boundaries on the edges. This paper extends the two-dimensional Dirac points and valley states in graphene-like systems to the type-II cases, and obtains in the same structure the gapless interface and boundary propagations. Owing to the simple design scheme of the structure, the phononic crystal plates can be fabricated and scaled to a small size. Our system provides a feasible way of constructing high-frequency elastic wave devices.