Structure Of Bilayer Graphene Research Articles

Restoring the electronic properties of epitaxial graphene on SiC substrate by Ar intercalation

Thermal decomposition of 6H-SiC(0001) under an argon atmosphere is a promising technique to fabricate large-scale epitaxial graphene monolayers for electronic applications. In this manuscript, we report on the intercalation of argon, hydrogen and oxygen atoms underneath a graphene buffer layer (BL) on SiC(0001), involving bond formation between the intercalated agent and Si atoms. Density Functional Theory calculations were performed to investigate the atomic structure, stability and electronic properties of quasi free-standing monolayer graphene and bilayer graphene, accomplished by intercalation. According to our results, as the Ar intercalated atoms saturate the silicon dangling bonds in the upper SiC surface, the BL is completely detached from the substrate and transformed into free-standing graphene. Ar intercalation would be hindered by the concurrent and thermodynamically more spontaneous adsorption of Ar atop the graphene layers, resulting in a pronounced decrease of the process efficiency. High Ar-coverage regimes and low kinetic barriers for Ar intercalation might shift the intercalation/adsorption balance. These factors as well as strategies such as ion implantation could be explored to identify more favorable conditions to experimentally test Ar intercalation as a simple method to achieve control of the electronic properties of the BL and help preserve the unique characteristics of graphene.

Magic-angle twisted bilayer graphene under orthogonal and in-plane magnetic fields

We investigate the effect of a magnetic field on the band structure of bilayer graphene with a magic twist angle of 1.08∘. The coupling of a tight-binding model and the Peierls phase allows the calculation of the energy bands of periodic two-dimensional systems. For an orthogonal magnetic field, the Landau levels are dispersive, particularly for magnetic lengths comparable to or larger than the twisted bilayer cell size. A high in-plane magnetic field modifies the low-energy bands and gap, which we demonstrate to be a direct consequence of the minimal coupling.

Strain cartography of bilayer graphene‐PDMS nanocomposites using Raman spectroscopy

AbstractEffective transmission of stress between fillers and the host matrix is instrumental in enhancing the properties of nanocomposites and optimizing filler reinforcement. This study delves into understanding stress transfer in materials, exploring the impact of synthesis methods and matrix materials through a comprehensive examination of a bilayer graphene (BLG)‐polydimethylsiloxane (PDMS) nanocomposite. BLG was synthesized via chemical exfoliation and subjected to various strain conditions within a PDMS matrix. A precisely controlled strain, ranging from 0% to 0.19%, was applied using a meticulous 4‐point bending method, monitored with a standard strain gauge. The study unveiled significant shifts in the Raman spectra's 2D band, indicating a direct linear relationship with applied strain. Utilizing contour mapping techniques on well‐defined Raman spectra and stress‐induced band shifts, we mapped strain distribution within the BLG layer. The results revealed a gradual increase in strain from the lower to the upper end of the BLG structure, resembling strain behaviors observed in differently shaped fibers. These compelling findings advance our understanding of strain dynamics within BLG‐PDMS nanocomposites, highlighting the crucial role of strain mapping methodologies in scrutinizing the material's structural response to deformation.Highlights The study reports successful synthesis of high‐quality bilayer graphene (BLG) using a chemical exfoliation method, as evidenced by the sharp 2D and G Raman peaks. Significant shifts in 2D band exhibited linear correlation between applied strain; observed within precise strain levels of 0% to 0.19%. Contour maps revealed a gradual increase in strain from the lower to the upper end of the BLG structure, akin to strain behaviors observed in differently shaped fibers.

Structural modulation of bilayer graphene under an external electric field and carrier doping

Density functional theory was used to investigate the geometric structure of bilayer graphene under an external electric field with carrier doping. Our calculations revealed the crucial impact of external electric fields and the hole injection on determining the geometric structure of bilayer graphene. The bond length of graphene monotonically increased when increasing the hole doping concentration, while it remained insensitive to electron doping. Additionally, there accumulated carriers predominantly distributed in the outermost layer located just below the gate electrode. These results enabled the construction of moiré superlattices in the bilayer graphene, possessing different moiré periodicity depending on the carrier concentration.

Short Versus Long Range Exchange Interactions in Twisted Bilayer Graphene

AbstractThis study discusses the effect of long‐range interactions within the self‐consistent Hartree‐Fock (HF) approximation in comparison to short‐range atomic Hubbard interactions on the band structure of twisted bilayer graphene (TBG) at charge neutrality for various twist angles. Starting from atomistic calculations, it determines the quasi‐particle band structure of TBG with Hubbard interactions for three magnetic orderings: modulated anti‐ferromagnetic (MAFM), (NAFM) and hexagonal anti‐ferromagnetic (HAFM). Then, it develops an approach to incorporate these magnetic orderings along with the HF potential in the continuum approximation. Away from the magic angle, it observes a drastic effect of the magnetic order on the band structure of TBG compared to the influence of the HF potential. Near the magic angle, the HF potential plays a major role in the band structure, with HAFM and MAFM being secondary effects, but NAFM appears to still significantly distort the electronic structure at the magic angle. These findings suggest that the spin‐valley degenerate broken symmetry state often found in HF calculations of charge neutral TBG near the magic angle should favor magnetic order, since the atomistic Hubbard interaction will break this symmetry in favor of spin polarization.

Vacancies and Stone–Wales defects in twisted bilayer graphene – A comparative theoretical study

The combination of miss-alignment and point defects on the electronic structure of bilayer graphene can lead to special properties, which deserve a theoretical investigation, since they are scarcely studied up to now. Although most graphene layers are nominally free of defects, they can be introduced in a simple way to achieve the desired properties. By performing density functional theory calculations implemented in two different computational approaches (SIESTA and QuantumEspresso) and a tight-binding hybrid method (DFTB+), we analyze the effects of single vacancies and Stone-Wale defects on the electronic structure of Twisted Bilayer Graphene (tBLG). Our results show that both kinds of defects can induce flat bands near the Fermi energy of tBLG, even for rotation angles quite larger than the well-known magic angles associated with the superconductivity of these systems. Additionally, with the aim to provide a theoretical background to identify defects on tBLG during experimental characterization, we simulate scanning tunneling microscopy (STM) images of these systems. As expected, such simulated STM images are dramatically influenced by the van Hove Singularities associated to the flat bands induced by defects. The results of our theoretical study may have an influence on the targeted introduction of defects in tBLG for the future design of tBLG with particular properties for some technological applications.

Growth and atomic structure of twisted bilayer graphene formed on Ni(111) by segregation

Angle-resolved photoelectron spectroscopy and intensity analysis of low-energy electron diffraction were used to explore the growth and atomic structure of twisted bilayer graphene (tBLG) formed on Ni(111) by segregation of dissolved carbon. tBLG was composed of an epitaxial graphene layer in contact with the substrate and an overlaying rotated one with rotation angles of ∼15°. The top rotated graphene layer has been shown to have little effect on the underlying epitaxial graphene/Ni(111) system, indicating that the adjacent graphene layers are effectively decoupled. The process in which the tBLG gradually became dominant on the surface with repeated heating is discussed.

Comparative study of the AA and AB bilayer graphene structures in the external electric and magnetic fields

Comparative study of the AA and AB bilayer graphene structures in the external electric and magnetic fields

Truncated atomic plane wave method for subband structure calculations of moiré systems

We propose a highly efficient and accurate numerical scheme named the truncated atomic plane wave method to determine the subband structure of twisted bilayer graphene (TBG) inspired by the Bistritzer-MacDonald model. Our method utilizes real-space information of carbon atoms in the moir\'e unit cell and projects the full tight-binding Hamiltonian into a much smaller subspace using atomic plane waves. Using our method, we are able to present accurate electronic band structures of TBG in a wide range of twist angles together with the detailed moir\'e potential and screened Coulomb interaction at the first magic angle. Furthermore, we generalize our formalism to solve the problem of low-frequency moir\'e phonons in TBG.

Band Structure of Bilayer Graphene Intercalated by Potassium Atoms. Ab Initio Calculations

Using the electron density functional theory, the electronic band structure of pure and potassium-intercalated bilayer graphene has been studied. It is shown that after the intercalation process, a band gap appears in the band structure of bilayer graphene. In addition, the energy gap changes nonlinearly depending on the intercalate concentration in the interlayer space of bilayer graphene. We also calculated the energy spectra of bilayer graphene containing vacancy defects, the presence of which leads to the appearance of mid-gap states.

Thermoelectricity in bilayer graphene superlattices

Low-dimensional thermoelectricity is based on the redistribution-accumulation of the electron density of states by reducing the dimension of thermoelectric structures. Superlattices are the archetype of these structures due to the formation of energy minibands and minigaps. Here, we study for the first time the thermoelectric response of gated bilayer graphene superlattices (GBGSLs). The study is based on the four-band effective Dirac Hamiltonian, the hybrid matrix method and the Landauer-Büttiker formalism. We analyze the Seebeck coefficient, the power factor, figure of merit, output power and efficiency for different temperatures and different superlattice structural parameters. We pay special attention to the impact of not only minibands and minigaps on the thermoelectric properties, but also to intrinsic resonances in bilayer graphene structures such as Breit-Wigner, Fano and hybrid resonances. In particular, we analyze the interplay between minibands and Fano resonances as a possible mechanism to improve the thermoelectric response of GBGSLs. We also compute the density of states to know if the redistribution-accumulation of electron states is implicated in the thermoelectric response of GBGSLs.

Theoretical characterization of codoped bilayer graphene

We studied the structure, stability and electronic propierties of bilayer graphene when both layers are doped with a pair of3p-2pelements. The interlayer interaction energy of codoped bilayer graphene can be fine-tuned by changing the dopants. Al/B codoped bilayer graphene presents the strongest stacking, almost twice the value determined for the undoped materials. In five cases, the interaction is weaker than in bilayer graphene. Interlayer covalent bonds form only in six cases; P/O codoped bilayer graphene presents the largest interlayer distance: 3.97 Å, about 0.72 Å longer than the value determined for undoped-bilayer graphene. In four cases, we observed that a semiconductor-to-metal transition occurs when two layers are stacked. For the remaining cases except P/N codoped bilayer graphene, the band gaps are reduced with respect to the values computed for the codoped monolayers. Controlling the number of layers is an exciting approach to fine-tune the band gap of codoped graphene.

Machine learning of the Γ-point gap and flat bands of twisted bilayer graphene at arbitrary angles

The novel electronic properties of bilayer graphene can be fine-tuned via twisting, which may induce flat bands around the Fermi level with nontrivial topology. In general, the band structure of such twisted bilayer graphene (TBG) can be theoretically obtained by using first-principles calculations, tight-binding method, or continuum model, which are either computationally demanding or parameters dependent. In this work, by using the sure independence screening sparsifying operator method, we propose a physically interpretable three-dimensional (3D) descriptor which can be utilized to readily obtain the Γ-point gap of TBG at arbitrary twist angles and different interlayer spacings. The strong predictive power of the descriptor is demonstrated by a high Pearson coefficient of 99% for both the training and testing data. To go further, we adopt the neural network algorithm to accurately probe the flat bands of TBG at various twist angles, which can accelerate the study of strong correlation physics associated with such a fundamental characteristic, especially for those systems with a larger number of atoms in the unit cell.

Band Structure and Quantum Transport of Bent Bilayer Graphene

We investigate the band structures and transport properties of a zigzag-edged bent bilayer graphene nanoribbon under a uniform perpendicular magnetic field. Due to its unique geometry, the edge and interface states can be controlled by an electric field or local potential, and the conductance exhibits interesting quantized behavior. When Zeeman splitting is considered, the edge states are spin-filtered, and a weak quantum spin Hall (WQSH) phase appears. In the presence of an electric field or local potential, a WQSH-QH junction or WQSH-spin-unbalanced QSH junction can be achieved, respectively, while fully spin-polarized currents appear in the interface region. Zeeman splitting lifts the spin degeneracy, leading to a WQSH around zero energy with a quantized two-terminal conductance of 4e2/h, which is robust against weak nonmagnetic disorder. These results provide a way to manipulate the band structures and transport properties of the system using an electric field, local potential, and Zeeman splitting.

Multiple plasmon-induced transparency based on black phosphorus and graphene for high-sensitivity refractive index sensing.

A hybrid bilayer black phosphorus (BP) and graphene structure with high sensitivity is proposed for obtaining plasmon-induced transparency (PIT). By means of surface plasmon resonance in the rectangular-ring BP structure and ribbon graphene structure, a PIT effect with high refractive index sensitivity is achieved, and the surface plasmon hybridization between graphene and anisotropic BP is analyzed theoretically. Meanwhile, the PIT effect is quantitatively described using the coupled oscillator model and the strong coherent coupling phenomena are analyzed by adjusting the coupling distance between BP and graphene, the Fermi level of graphene, and the crystal orientation of BP, respectively. The simulation results show that the refractive index sensitivity S = 7.343 THz/RIU has been achieved. More importantly, this is the first report of tunable PIT effects that can produce up to quintuple PIT windows by using the BP and graphene hybrid structure. The high refractive index sensitivity of the quintuple PIT system for each peak is 3.467 THz/RIU, 3.467 THz/RIU, 3.600 THz/RIU, 4.267 THz/RIU, 4.733 THz/RIU and 6.133 THz/RIU, respectively, which can be used for multiple refractive index sensing function.

Thermoelectric properties of bilayer graphene structures with bandgap opening

Bilayer graphene is very attractive for both the fundamental and technological standpoint due to the possible modulation of its physical properties by opening a bandgap. In this work, we show that the thermoelectric properties of bilayer graphene single and double barrier structures can be modulated by the bandgap opening. The effect of the bandgap on the thermoelectric properties was investigated by describing the charge carriers in bilayer graphene as massive Dirac electrons in combination with the hybrid matrix method and the Landauer–Büttiker formalism. Thermoelectric properties such as the Seebeck coefficient, power factor, figure of merit, output power and efficiency are analyzed. In the case of single barriers, we find that at low temperatures the thermoelectric properties enhance at a critical bandgap, while at T=200 K they reduce systematically as the bandgap increases. In particular, the figure of merit increases up to 0.65 for a bandgap of 30 meV and decreases up to 0.8 for a bandgap of 40 meV, representing an enhancement and a reduction of about 30% and 56% with respect to the gapless case. In the case of double barriers, the thermoelectric properties have a contrasting dynamic with the bandgap opening with respect to single barriers. At T=200 K the thermoelectric properties enhance at a critical bandgap, and at low temperatures they collapse in effective terms with the bandgap opening. Here, the figure of merit increases up to 5.2 for a bandgap of 20 meV and decreases up to 1.3 for a bandgap of 40 meV, constituting an enhancement and reduction of 11% and 73%, respectively. We also find that the charge neutrality point and Breit–Wigner resonances shape the thermoelectric response at low temperatures, while at T=200 K the Fano resonances determine the thermoelectric properties. In addition, we corroborate that the redistribution–accumulation of electronic states caused by the bandgap opening is behind the enhancement–reduction of the thermoelectric properties. Our results indicate that bandgap opening could be a versatile mechanism to modulate the thermoelectric performance of bilayer graphene barrier structures.

Relaxation effects in twisted bilayer graphene: A multiscale approach

We present a multiscale density functional theory (DFT) informed molecular dynamics and tight-binding approach to capture the interdependent atomic and electronic structures of twisted bilayer graphene. We calibrate the flat band magic angle to be at ${\ensuremath{\theta}}_{\mathrm{M}}=1.{08}^{\ensuremath{\circ}}$ by rescaling the interlayer tunneling for different atomic structure relaxation models as a way to resolve the indeterminacy of existing atomic and electronic structure models whose predicted magic angles vary widely between $0.{9}^{\ensuremath{\circ}}$ and $1.{3}^{\ensuremath{\circ}}$. The interatomic force fields are built using input from various stacking and interlayer distance-dependent DFT total energies including the exact exchange and random phase approximation ($\mathrm{EXX}+\mathrm{RPA}$). We use a Fermi velocity of ${\ensuremath{\upsilon}}_{\mathrm{F}}\ensuremath{\simeq}{10}^{6}$ m/s for graphene that is enhanced by $\ensuremath{\sim}15%$ over the local density approximation (LDA) values. Based on this atomic and electronic structure model we obtain high-resolution spectral functions comparable with experimental angle-resolved photoemission spectroscopy. Our analysis of the interdependence between the atomic and electronic structures indicates that the intralayer elastic parameters compatible with the DFT-LDA, which are stiffer by $\ensuremath{\sim}30%$ than widely used reactive empirical bond order force fields, can combine with $\mathrm{EXX}+\mathrm{RPA}$ interlayer potentials to yield the magic angle at $\ensuremath{\sim}1.{08}^{\ensuremath{\circ}}$ without further rescaling of the interlayer tunneling.

Perfect double-layer terahertz absorber based on graphene metamaterial

Using bilayer graphene structures (L-type and disk-type structures), we numerically demonstrate a tunable single-band terahertz metamaterial absorber with nearly perfect (100%) absorption by continuously adjusting the graphene Fermi energy parameters to determine the maximum absorption rate, the corresponding center frequency, and bandwidth. Graphene distinct dielectric–metal transition properties make it possible to achieve tunable terahertz absorption. The maximum absorption rate increases from 18% to 100% when the Fermi energy is increased from 0.1 to 1 eV. At 1 eV, the absorption rate exceeds 90% in the range of 0.57–1.03 THz. The incident angle is also tested. When the angle is in the range of 60°, excellent adsorption performance is maintained.

Probing the stochastic fracture behavior of twisted bilayer graphene: Efficient ANN based molecular dynamics simulations for complete probabilistic characterization

The present article outlines a probabilistic investigation of the uniaxial tensile behaviour of twisted bilayer graphene (tBLG) structures. In this regard, the twist angle (θ) and temperature (T) are considered as the control parameters and the ultimate tensile strength (UTS) and failure strain of the tBLG structures are considered as the responses. It is observed that with the increase in twist angle (θ) of tBLG; the fracture responses exhibit a declining trend deterministically. The tBLG twisted with the magic angle (θ = 1.08o) results in around 7% decrease in UTS and nearly 24% decrease in failure strain, when compared with normal BLG (θ = 0̊). The Monte Carlo simulation (MCS) based random sampling is performed for the considered control parameters, wherein θ is varied from 0o to 30o, and temperature is varied from 100 K to 900 K. Within such bounds of the input parameters, the training (64 samples) and validation (8 samples) sample spaces are constructed. In the next step, molecular dynamics (MD) simulation of uniaxial tensile deformation of the modelled tBLG structures is carried out for each instance of the sample space. The dataset is subsequently used to form and validate the artificial neural network (ANN). The computationally efficient machine learning (ML) model is further utilized to perform the detailed investigation of fracture behaviour of the tBLG structures in the probabilistic framework. Such analysis captures all the possible instances of variation in the input parameters and leads to deep insights in the material behaviour, which would have otherwise remained unnoticed due to the prohibitive nature of conducting a large number of MD simulations. The novelty of the present study lies in the probabilistic interpretation of the tensile behaviour of tBLG structures subjected to variation in twist angle (θ) and temperature (T). The preparation of nano-scale samples with the exact design specifications such as twist angle is often extremely difficult, which leads to inevitable stochastic system disorders. The current article essentially proposes a probabilistic avenue of quantifying the effect of such disorders on the failure properties of tBLG.

Chiral emission induced by the interaction between chiral phonons and localized plasmon

We demonstrate chiral photoluminescence and scattering induced by the interaction between chiral phonons and localized plasmon. In the experiment, we constructed a hybrid structure of single gold nanorods and bilayer graphene. The optical chirality was investigated with a helicity-resolved single-particle spectroscopy technique, including the dark-field scattering and photoluminescence spectra. The single-particle spectra can effectively indicate the chiral phonon in bilayer graphene. That is due to the interaction between achiral local surface plasmon resonance and chiral phonons, which influences the plasmon damping at the interface. We propose a plasmon–phonon coupled spectroscopy method for phonon chirality detection. This method provides an advantage for developing high spatial resolution detection of chiral phonon in low-dimensional materials due to the localization of plasmonic near-field.