Layers Of Bilayer Graphene Research Articles

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.

Surface symmetry effect on the charge transfer at the black, blue, and green phosphorene/graphene interfaces

In the present study, a comparative prediction of atomic and electronic structure of black, blue, and green phosphorene/graphene heterostructures is presented using Density Functional Theory (DFT). The lowest total and interaction energies and highest charge transfer correspond to the blue phosphorene/graphene interface, followed by black and green phosphorene/graphene interfaces. This trend is the same for monolayer, AA-, and AB-stacked bilayer graphene. However, the charge transfer is more important to AB-stacked bilayer graphene at the interface with black and green phosphorene than to AA-stacked bilayer graphene. On another hand, for the charge transfer is more important from AA-stacked bilayer graphene to blue phosphorene, than from AB-stacked bilayer graphene. Besides, small bandgaps appear in phosphorene/bilayer graphene heterostructures, resulting from the symmetry breaking due to the charge difference between the two layers of bilayer graphene. These findings provide useful insights on energetic stability of graphene/phosphorene heterostructures with promising properties for future nanoelectronics devices.

The Effect of Relative In-Plane Twisting in Graphene Bilayer on Sensing Using Surface Plasmon Resonance

Surface plasmon resonance (SPR) is generally observed by the excitation of surface plasmon polaritons on the metal (Au/Ag) surface. In order to utilize the SPR phenomenon for sensing application, the metal surface is functionalized with suitable ligands. Although such functionalization can enhance the specific adsorption capability of the sensor however due to the large thickness of the ligands, the plasmonic field of the metal surface becomes less sensitive towards the adsorption of analytes. In the next generation SPR-based sensor, graphene can be utilized not only as plasmonic material but also as a suitable ligand for attracting analytes through π-π interaction. In this article, we present our theoretical simulation studies on the observation of the SPR phenomenon using graphene monolayer (MLG), bilayer graphene (BLG), and in-plane twisted layers of BLG (T-BLG) as plasmonic materials deposited over zinc-selenide substrate. The Kretschmann configuration under wavelength interrogation setup was simulated, and SPR wavelength for graphene systems/water interface was estimated. The bio-sensing simulation was performed, and the sensing parameters viz. sensitivity, figure-of-merit (FOM), and plasmonic field for different graphene systems were obtained. Interestingly, the excellent sensing parameters were found in T-BLG system with relative in-plane twist angle near to magic angle viz. 1°. The enhancement is due to strong coupling between the layers twisted at the magic angle. This study demonstrates that the MLG, BLG and T-BLG can be employed as a standalone layer system for not only the generation of plasmonic fields but also enhanced sensing due to its intrinsic interactions with bio-analytes.

Strong Second Harmonic Generation from Bilayer Graphene with Symmetry Breaking by Redox-Governed Charge Doping.

Missing second-order nonlinearity in centrosymmetric graphene overshadows its intriguing optical attribute. Here, we report redox-governed charge doping could effectively break the centrosymmetry of bilayer graphene (BLG), enabling a strong second harmonic generation (SHG) with a strength close to that of the well-known monolayer MoS2. Verified from control experiments with in situ electrical current annealing and electrically gate-controlled SHG, the required centrosymmetry breaking of the emerging SHG arises from the charge-doping on the bottom layer of BLG by the oxygen/water redox couple. Our results not only reveal that charge doping is an effective way to break the inversion symmetry of BLG despite its strong interlayer coupling but also indicate that SHG spectroscopy is a valid technique to probe molecular doping on two-dimensional materials.

Tunable multi-bands in twisted double bilayer graphene

The bandstructure of a material, playing an important role in its electron transport property, is usually governed by the lattice configuration. Materials with a field-effect tunable band, such as bilayer [1] and rhombohedral trilayer graphene [2, 3], are more flexible for electronic applications. Here, on dual-gated twisted double bilayer graphene (TDBG) samples with small twist angle around 1∘, we observe vertical electric-field-tunable bandstructures at multiple moiré fillings with bandgap values continuously varying from zero to tens of mili-electron volts. Moreover, within the first moiré filling on both electron and hole sides, the carrier transport deviates from Fermi liquid behavior, with measured resistivity exhibiting linear temperature dependence between 1.5 K and 50 K. Furthermore, under a vertical magnetic field, the coupling between the two bilayer graphene layers can also be turned on and off by a displacement field. Our results suggest TDBG with small twist angle is a platform for studying the evolution of multiple electric field tunable moiré bands and the resulting emergent correlated electronic phases.

Influence of correlated impurities on electrical conductivity in bilayer graphene double layer systems

We study the electrical conductivity σ of three bilayer graphene (BLG) double layer systems, taking into account the presence of correlations of charged impurities. Using the Boltzmann transport equation with relaxation time approximation and the continuum model for structure factor S(q⃗), we calculate the conductivity of the first BLG layer in presence of the second layer (BLG, monolayer graphene (MLG), two dimensional electric gas (2DEG)) due to screened Coulomb scattering at T ​= ​0K for different values of correlation length r0, interlayer distance d, dielectric constant ε2, ε3and 2DEG material parameters. We show thatσincreases with decreasing d for all values ofr0and increases strongly (remains almost unchanged) with increasingε2(ε3). It is found thatσis almost independent ofε3for BLG-2DEG system and the influence of correlation onσis negligible for low impurity concentration in layer I(ni1⪯1012cm−2).

Modeling the structure and interlayer interactions of twisted bilayer graphene

In this study, we have analyzed twist angles between layers at which the bilayer graphene (BLG) layer structure is periodic. In addition, the dependence of interlayer binding energies on the BLG layer’s twist angles was calculated using the atom-atom potential method. It was established that at small twist angles (below 4°) the interlayer binding energy changes monotonically, free of local energy minima and stable structural states. At large twist angles, the local energy minima positions weakly correlate with angles at which periodic Moiré superlattices arise.

General relation between stacking order and Chern index: A topological map of minimally twisted bilayer graphene

We derive a general relation between the stacking vector ${\bf u}$ describing the relative shift of two layers of bilayer graphene and the Chern index. We find $C = \nu - \text{sign}\left(|V_{AB}|-|V_{BA}|\right)$, where $\nu$ is a valley index and $|V_{\alpha\beta}|$ the absolute value of stacking potentials that depend on ${\bf u}$ and that uniquely determine the interlayer interaction; AA stacking plays no role in the topological character. With this expression we show that while ideal and relaxed minimally twisted bilayer graphene appear so distinct as to be almost different materials, their Chern index maps are, remarkably, identical. The topological physics of this material is thus strongly robust to lattice relaxations.

Optical absorption in bilayer graphene superlattices

A periodic statistical potential applied in each layer of bilayer graphene can change it to a superlattice, enabling its application as a flexible light absorber. In this work, the effect of structural parameters on the optical absorption and bandgap of such a bilayer graphene superlattice is studied theoretically in detail. The bilayer graphene superlattice consists of two single layers of graphene with different patterns of static periodic potential applied. The role of the unit cell length, different ratios between the width of each region, and the magnitude of the applied potential in the optical absorption is analyzed in detail. The results reveal that the optical absorption can be enhanced by increasing the magnitude of the square static periodic potential applied to each single layer of graphene. The effect of light polarization is also addressed. It is observed that different ratios of the widths of periodic regions do not change the absorption, while the configuration and value of the square static periodic potential applied plays a crucial role in the absorption spectrum.

Moiréless correlations in ABCA graphene

Atomically thin van der Waals materials stacked with an interlayer twist have proven to be an excellent platform toward achieving gate-tunable correlated phenomena linked to the formation of flat electronic bands. In this work we demonstrate the formation of emergent correlated phases in multilayer rhombohedral graphene--a simple material that also exhibits a flat electronic band edge but without the need of having a moiré superlattice induced by twisted van der Waals layers. We show that two layers of bilayer graphene that are twisted by an arbitrary tiny angle host large (micrometer-scale) regions of uniform rhombohedral four-layer (ABCA) graphene that can be independently studied. Scanning tunneling spectroscopy reveals that ABCA graphene hosts an unprecedentedly sharp van Hove singularity of 3-5-meV half-width. We demonstrate that when this van Hove singularity straddles the Fermi level, a correlated many-body gap emerges with peak-to-peak value of 9.5 meV at charge neutrality. Mean-field theoretical calculations for model with short-ranged interactions indicate that two primary candidates for the appearance of this broken symmetry state are a charge-transfer excitonic insulator and a ferrimagnet. Finally, we show that ABCA graphene hosts surface topological helical edge states at natural interfaces with ABAB graphene which can be turned on and off with gate voltage, implying that small-angle twisted double-bilayer graphene is an ideal programmable topological quantum material.

A Roadmap for the Chalcogenide-graphene Composites Formation Under a Glassy Regime

Background: Nano-composite is an innovative material having nano in which fillers dispersed in a matrix. Typ-ically, the structure is a matrix- filler combination, where the fillers like particles, fibers, or fragments are surrounded and bound together as discrete units by the matrix. The term nano-composite encompasses a wide range of materials right from three dimensional metal matrix composites to two dimensional lamellar composites. Therefore, the physical, chemical and biological properties of nano materials differ from the properties of individual atoms and molecules or bulk matter. The chalcogenide – graphene composites in glassy regime is the growing novel research topic in the area of composite material science. It is obvious to interpret such materials different physicochemical mechanism. Objective: The key objective of this research work to explore the internal physicochemical mechanism of the chalcogenide – graphene composites under the glassy regime. Including the prime chalcogen alloying element selenium amorphous atomic structure and their fullerene like bonding nature. By accommodating the essential properties of the stacked layers of bilayer graphene. The diffusion, compression and dispersion of the bilayer graphene in selenium rich ternary (X(1-x-y)-Y(x)- Z(y) + GF (bilayer graphene); X = Se, Y = Semimetal or metalloid, Z = None metal) alloys under the complex regime on and after thermal melting process are addressed. Materials and Methods: To synthesize the composite materials the well-known melt quenched method had adopted. More-over, to interpret the amorphous selenium (Se8) chains and rings molecular structures we had used vista software with an available CIF data file. While to show the armchair and zig-zag bonds with bilayer graphene structure the nanotube modeler simulation software has used. Results: Outcomes of this study reveals the chalcogenide -graphene nano composite formation under a glassy regime changes the individual materials structural and other physical properties that is reflecting in different experimental evi-dences, therefore, the modified theoretical concepts for the different properties of such composite materials are interpreted in this study. Discussion: The dispersion and diffusion of the high stiff graphene bonds in low dimension chalcogen rich alloys has been interpreted based on their quadric thermal expansion behaviour. In addition to this, a possible bond angle modification in the formation of X(1-x-y)-Y(x)- Z(y) + GF composites are also addressed. To interpret the distinct optical property behavior of the formed X(1-x-y)-Y(x)- Z(y) + GF composites and parent chalcogenide glassy alloys a schematic model of the energy levels is also addressed. Conclusion: To make a better understating on the formation mechanism such composites, the diffusion and deformation of high stiff graphene σ and π bonds in a low dimension chalcogenide alloy basic mechanism are discussed on basis of novel “thermonic energy tunneling effect” concept, which could result in quadratic thermal expansion of graphene. Moreover, the structural unit modifications of such composite materials are described in terms of their bond angle modifications and in-fluence of the coordination defects. The energy levels suppression and creation of addition sub energy levels in such com-posite materials are discussed by adopting the viewpoint impact of the foreign alloying elements and surface π-plasmonic resonance between the graphene layers in the honeycomb band structure. Thus, this study has described various basic aspects of the chalcogenide system – bilayer graphene composites formation under a glassy regime.

Influence of interlayer stacking arrangements on carrier accumulation in bilayer graphene field effect transistors

The electronic structure of bilayer graphene under an external electric field is studied in terms of the interlayer stacking arrangements using the density functional theory combined with the effective screening medium method. The calculations showed that the accumulated carrier distribution strongly depended on the interlayer stacking arrangement. The carriers were highly concentrated on the topmost layer of twisted bilayer graphene, while carriers were also found in the second layer of bilayer graphene with the AA and AB interlayer arrangements. In addition to the carrier accumulation, ab initio molecular dynamics calculations also revealed that the interlayer Seebeck coefficient and thermal diffusivity in the bilayer graphene depended on the interlayer stacking arrangements.

Tailoring electronic and optical parameters of bilayer graphene through boron and nitrogen atom co-substitution; an ab-initio study

Bilayer graphene (BLG) system substituted with boron (B) and nitrogen (N) atoms of different concentration is investigated in this study using first-principles study density functional theory (FPS-DFT). The effects of B/N impurity co-substitution on electronic and optical behaviors of BLG are systematically studied. The band gap of pure BLG obtained during this study agrees well with previous experimental studies. Formation energies for B/N co-doped BLG systems indicate that the, impurity substitution process in BLG is exothermic and stable B/N atom-doped BLG systems can be realized experimentally. Charge difference calculations suggest that, the dopant B and N atoms placed in different layers of BLG, try to deplete the electrons in the space between graphene layers and the charge density is localized at the dopant sites. It is discovered through electronic parameters that, individual B/N substitution in BLG, moves Dirac cone into conduction/valence bands, while providing a limited energy gap at the Dirac point. On the other hand, when B/N impurities are placed in BLG, lattice symmetry of BLG breaks, dispersion relation near the Fermi level (EF) is disturbed and a fixed energy gap emerges in its electronic structure. BLG optical parameters contain same profile as that of SLG, except that all the parameters show increased intensities. When B/N atoms are co-doped in BLG, the absorption spectra is improved in low electron energy range and higher static reflectivity is observed. From the results generated during this study, it can be generalized that B/N atoms can be considered as ideal doping impurities for BLG systems, as they not only produce stable and functional BLG systems, but also preserve the intrinsic properties of graphene material. Further experimental studies can be performed on the systems discussed in this work, in order to synthesize multilayer graphene systems; those are functional for nano/opto electronic device applications.

Insights into the effect of the interlayer spacings of bilayer graphene on the desolvation of H+, Li+, Na+, and K+ ions with water as a solvent: a first-principles study.

The desolvation effect of ions plays an important role in adjusting the capacity of supercapacitors and has attracted considerable attention after its discovery. Here, first-principles calculations were conducted to calculate the reaction energies of ions, water, and hydrated ions in bilayer graphene (BG) with different interlayer spacings (d) and to explore the desolvation behaviors of H+, Li+, Na+, and K+ ions. The calculated results showed that H+ can only exist in the state of H3O+ in AA-stacking BG, and desolvation exists only in the case of AB-stacking BG. The complete desolvation size for H+ ions in the AB-stacking system reached 5.6 Å, which was the largest desolvation size of the four ions studied. The critical desolvation sizes of Li+, Na+, and K+ in the BG layers of AA- and AB-stacking increased sharply as a consequence of the increasing ionic radius. However, the complete desolvation sizes of all three ions were in the range of 4-5 Å and with the increase in ionic radius, the complete desolvation sizes showed a reverse tendency. The complete desolvation size of Na+ in AB-stacking BG was slightly larger than that in AA-stacking BG. Further analysis presented that the ionic radii of H+, Li+, Na+, and K+ ions make a dominant contribution to the critical size of desolvation. Our present results provide useful information for improving the capacity of supercapacitors by precisely matching the pore structure and electrolyte through the adjustment of the pore structure of carbon materials.

Crystalline Bilayer Graphene with Preferential Stacking from Ni–Cu Gradient Alloy

We developed a high-yield synthesis of highly crystalline bilayer graphene (BLG) with two preferential stacking modes using a Ni-Cu gradient alloy growth substrate. Previously reported approaches for BLG growth include flat growth substrates of Cu or Ni-Cu uniform alloys and "copper pocket" structures. Use of flat substrates has the advantage of being scalable, but the growth mechanism is either "surface limited" (for Cu) or carbon precipitation (for uniform Ni-Cu), which results in multicrystalline BLG grains. For copper pockets, growth proceeds through a carbon back-diffusion mechanism, which leads to the formation of highly crystalline BLG, but scaling of the copper pocket structure is expected to be difficult. Here we demonstrate a Ni-Cu gradient alloy that combines the advantages of these earlier methods: the substrate is flat, so easy to scale, while growth proceeds by a carbon back-diffusion mechanism leading to high-yield growth of BLG with high crystallinity. The BLG layer stacking was almost exclusively Bernal or twisted with an angle of 30°, consistent with first-principles calculations we conducted. Furthermore, we demonstrated scalable production of transistor arrays based crystalline Bernal-stacked BLG with a band gap that was tunable at room temperature.

Layer Dependence of Graphene for Oxidation Resistance of Cu Surface

We studied the oxidation resistance of graphene-coated Cu surface and its layer dependence by directly growing monolayer graphene with different multilayer structures coexisted, diminishing the influence induced by residue and transfer technology. It is found that the Cu surface coated with the monolayer graphene demonstrate tremendous difference in oxidation pattern and oxidation rate, compared to that coated with the bilayer graphene, which is considered to be originated from the strain-induced linear oxidation channel in monolayer graphene and the intersection of easily-oxidized directions in each layer of bilayer graphene, respectively. We reveal that the defects on the graphene basal plane but not the boundaries are the main oxidation channel for Cu surface under graphene protection. Our finding indicates that compared to putting forth efforts to improve the quality of monolayer graphene by reducing defects, depositing multilayer graphene directly on metal is a simple and effective way to enhance the oxidation resistance of graphene-coated metals.

Cooling Growth of Millimeter-Size Single-Crystal Bilayer Graphene at Atmospheric Pressure

Single-crystal bilayer graphene is attractive for functional electronic and photonic devices because of its high carrier mobility as well as a tunable band gap induced by a perpendicular electric field. Chemical vapor deposition (CVD) has been increasingly reported for the production of single-crystal bilayer graphene, but it is limited with respect to grain size because of nonsynchronic growth between the second layer and the first layer of bilayer graphene. In this research, we established a nonisothermal atmosphere CVD (NI-APCVD) system to realize synchronization of nucleation and growth of single-crystal bilayer graphene that contains the surface adsorption for the decomposed carbons and the shallow bulk phase segregation for the dissolved carbons. Based on the above process engineering, we successfully produced large hexagonal single-crystal bilayer graphene (∼1.0 mm) on the polycrystalline Cu foils. This work paves a pathway to the synchronous growth of wafer-scale single-crystal bilayer graphene, w...

Strong second-harmonic generation from bilayer-graphene embedded in one-dimensional photonic crystals

We theoretically investigate second-harmonic generation (SHG) from bilayer-graphene (BLG) embedded in photonic crystals (PCs) using the transfer-matrix method. Two kinds of cases are considered. One is a single BLG layer embedded in the defective PC; the other is a stack of BLG layers in the dielectric multilayer structure. It is found that the improved field localization within the defective PC enhances SHG by three orders of magnitude compared with the bare BLG. Although the SHG efficiency from the stack of BLG layers in the dielectric multilayer structure is lower than that from the defective PC due to the effect of absorption, there are some advantages in controlling the polarization dependence of the SHG for such a case. For example, various intensity ratios of p-polarized and s-polarized second-harmonic outputs can be realized by adjusting the azimuthal rotational angles of the BLG layers for the incident wave with a certain polarization. We believe that these phenomena are very beneficial for the design of optical devices.

Comparative study on the nonlinear properties of bilayer graphene and silicene under tension

Atomic structures and nonlinear properties of single layer graphene (SLG), bilayer graphene (BLG), single layer silicene (SLS), and bilayer silicene (BLS) under equiaxial tension and uniaxial tensions along armchair and zigzag directions have been investigated comparatively using first-principles calculations. First, we have calculated the dependences of atomic structures (bond length, interlayer distance, and buckling height) of BLG and BLS on strain under three types of tensions. There exists the weak Van der Waals interaction between two layers of BLG and the interlayer distance is not variable with strain for three types of tensions. However, the interlayer of BLS is the covalent bond interaction, and the distance decreases with the increasing strain for three types of tensions. The continuum description of elastic response is formulated by expanding the elastic strain energy density in a Taylor series in strain truncated after the third-order term. The in-plane second- and third-order elastic constants of BLG and BLS have been obtained by fitting to the strain energy density versus Lagrangian strain relationships. The results show the in-plane stiffnesses of BLG and BLS become slightly larger than those of their single layer counterparts. In spite of the interlayer Si–Si covalent bond between two layers of BLS, its stiffness is still much less than BLG and SLG. Poisson’s ratios of BLG and BLS basically maintain unchanged compared to their single layer counterparts.

Magnetopolariton in bilayer graphene: A tunable ultrastrong light-matter coupling

Magnetopolariton in bilayer graphene (BLG) is theoretically investigated with the consideration of the influence of asymmetry between on-site energies in the two layers of BLG. The results show that an ultrastrong light-matter coupling regime can be achieved in a high filling factor and asymmetry has a strong effect on it. Although BLG in the low-energy regime and semiconductor have a similar quadratic dispersion of quasiparticles, a remarkably different cavity quantum electrodynamics (QED) effect occurs in BLG. In particular, a quantum phase transition, as predicted by the Dicke model, occurs in BLG in spite of the Schr\"odinger-like term ${p}^{2}$/2$m$ in the system Hamiltonian, while such quantum phase transition does not exist in semiconductors. Most noticeably, the ultrastrong light-matter coupling can be easily controlled by modulating the asymmetry in BLG, which provides an excellent platform to observe interesting QED effects and can lead to tunable polariton-based devices and cavity-controlled magnetotransport in BLG.