Graphene-based Structure Research Articles

Wide-range electrically tunable photonic spin Hall effect in a quasi-PT-symmetric structure.

The photonic spin Hall effect (PSHE), manifesting itself as the spin-dependent shifts of left- and right-handed circularly polarized light beams, holds potential applications in nanophotonics and precision measurement. Thus, realizing effective enhancement and regulation of PSHE is highly desirable. It is known that by adjusting the Fermi energy of graphene, the spin shifts in a graphene-based optical structure can be actively modulated and amplified. However, this method generally works in a very narrow range of incident angles (near Brewster's angle) and the incident state is limited to the horizontal polarization. In this Letter, we address these issues by theoretically proposing a feasible way to amplify and control the PSHE in a wide range of incident angles by modulating the Fermi energy when the light beam is reflected at a quasi-PT-symmetric structure (gain-loss medium embedded with monolayer graphene). Interestingly, we reveal that the electrically tunable PSHE can be achieved for both horizontal and vertical polarizations near the quasi-exceptional points (quasi-EPs). Moreover, we can directly determine the tiny variation of the Fermi energy by observing the field distribution of a single circularly polarized component in this structure without using the weak measurements.

Numerical investigation and comparative analysis of tunable graphene-gold metasurface-based polarizer structures for infrared frequency

We present the comparative analysis of tunable graphene-based metasurface polarizer structure THz frequency range. These polarizer structures have been numerically investigated over the 10 THz to 25 THz frequency range. Reflectance co-efficient, phase variation, and phase difference parameters have been investigated to identify the behavior of polarization effect over the range of 10 THz to 25 THz of the frequency. The proposed polarizer has also been investigated for the different shapes of the top gold resonator structure. The proposed structure is tunable for the range of 0.1 eV–0.9 eV of the graphene Fermi energy. The proposed structure also works on the wide range of the input incident wave of the X and Y polarization. This structure has a small, compact, tunable design and can be used as a fundamental building block in the large THz circuits and structures.

An Analytical Model for Optimizing the Optical Absorption of Graphene-Based Two-Dimensional Multilayer Structure

Two-dimensional (2D) materials are promising but remain to be further investigated, with respect to their interesting usage in optoelectronic devices. These materials have far less than ideal absorption due to their thin thickness, limiting their deployment in practical optoelectronic applications. Graphene is a 2D material with honeycomb structure. Its unique and fantastic mechanical, physical electrical and optical properties make it to be an important industrial and economical material. In this work, a simple analysis is performed for the reflectance, transmittance, and absorption properties of multilayer thin film structures with graphene sandwiched in dielectric layers. Based on Maxwell’s electromagnetic wave theory and coupled Fresnel equations, we investigate how to get maximum absorption for a proper choice of media and graphene layers. Query ID="Q2" Text="Kindly check the corresponding author's affiliation is correctly identified. Numerical results show this absorption is controlled with matching thicknesses of layers, number of graphene layers, wavelength and angle of incident electromagnetic wave.

Perfect transmission of the ultrarelativistic quasielectrons in pristine graphene

Within the framework of the Landauer-Buttiker formalism, the conductivity of one- and two-barrier graphene structures is calculated. The conditions for formation of the maximum value of the conductivity depending on the energy of ultrarelativistic quasielectrons as well as on the parameters of these structures (the values of Fermi velocity in different regions and the potential barriers height) are analyzed. It is believed that there is an electrostatic barrier and also the Fermi velocity barrier due to the fact that this quantity may aсquire different values in the barrier and out-of-barrier regions of the considered structures. The obtained expression for the transmission rates accounts for the π phase change of the transmitted wave function (which was not taken into consideration in the known formulae for the transmission rates of some papers). Consideration of this phase change leads to some new important features which characterize the transmission process in the graphene structures. We are the first to show that the effect of the supertunnelling manifests itself in graphene and it is observed both in the dependences of the transmission rates and of the conductivity on the quasielectron energy. This effect consists in the fact that, under certain conditions, the transmission through the structure is perfect (transmission rates T = 1) for the arbitrary angle of incidence of quasielectrons on the barrier. The expression іs given that determines the specified conditions for the supertunneling. The analysis of the transmission dependence on the problem parameters is also provided. • All-angled perfect tunneling can be observed in the graphene-based single barrier resonant-tunneling structure. • This type of tunneling significantly affects the conductivity of the given structure. • The analogous effect manifests itself also in the graphene based double barrier resonant-tunneling structure.

Multi-scale electronics transport properties in non-ideal CVD graphene sheet

In this work, we benchmark non-idealities and variations in the two-dimensional graphene sheet. We have simulated more than two hundred graphene-based devices structure. We have simulated distorted graphene sheets and have included random, inhomogeneous, asymmetric out-of-plane surface corrugation and in-plane deformation corrugation in the sheet through autocorrelation function in the non-equilibrium Green’s function (NEGF) framework to introduce random distortion in flat graphene. These corrugation effects inevitably appear in the graphene sheet due to background substrate roughness or the passivation encapsulation material morphology in the transfer step. We have examined the variation in density of state, propagating density of transmission modes, electronic band structure, electronic density, and hole density in those device structures. We have observed that the surface corrugation increases the electronic and hole density distribution variation across the device and creates electron-hole charge puddles in the sheet. This redistribution of microscopic charge in the sheet is due to the lattice fields’ quantum fluctuation and symmetry breaking. Furthermore, to understand the impact of scattered charge distribution on the sheet, we simulated various impurity effects within the NEGF framework. The study’s objective is to numerically simulate and benchmark numerous device design morphology with different background materials compositions to elucidate the electrical property of the sheet device.

Terahertz graphene metasurfaces for cross-polarized deflection, focusing, and orbital angular momentum.

Polarization is an important characteristic of electromagnetic wave. Due to novel optical properties, graphene-based anisotropic structure is widely used to control polarization state of electromagnetic wave. In this work, four graphene-based meta-atoms are designed to regulate polarization state of terahertz wave by changing Fermi energy level of graphene. When Fermi energy level is 0.01 eV, cross-polarized wave is emitted by four meta-atoms with phase difference of 90° at 1.18 THz, and the corresponding polarization conversion ratio reaches ∼90%. When Fermi energy level is adjusted to 0.70 eV, linear phase gradient will disappear, and cross-polarized wave almost disappears. Using four selected elements, three dynamic metasurfaces are designed for controlling wavefront of reflected beam, and they are gradient metasurface, metalens, and vortex beam generator. The designed metasurfaces successfully combine wavefront control and polarization manipulation, and greatly improve the ability to control electromagnetic wave. Our designs may have many potential applications, such as terahertz switching, imaging, and polarization beam splitter.

Implementation of all two-input/one-output logical operations using a simple electro-optical graphene-based plasmonic structure

Implementation of all two-input/one-output logical operations using a simple electro-optical graphene-based plasmonic structure

Performance improvement of a graphene patch antenna using metasurface for THz applications

This paper presents the design and development of a miniaturized, efficient graphene-based reconfigurable antenna working in the terahertz (THz) domain. This design puts forward the analysis of a graphene-based microstrip patch antenna loaded with a graphene metasurface (MS) as a superstrate configuration. The MS is designed over a silicon dioxide (SiO2) substrate in which the square-shaped unit cells have been organized with a periodicity of λ/10.4, referred to as the lowest operating frequency. The proposed antenna operates over the lower THz frequency band of 1.44–2.84 THz; providing a bandwidth of 1.4 THz with a return loss dip of 35.54 dB at 2.02 THz. The maximally achieved realized gain and radiation efficiency at 2.74 THz are 8.91 dBi and 70.14%, respectively. The proposed antenna further exhibits the beam scanning feature over a range close to 50°. The proposed graphene-based MS structure is compact, directive, and highly efficient in the THz domain. It finds useful applications in biomedical, satellite communication, high-quality video imaging, etc.

Sensitivity Optimization of Biosensors Based on Two-Dimensional Materials for Breast Health Monitoring

In this article, we propose an efficient approach of 2-D materials and biological cells modeling, which is defined by redefining their electromagnetic boundary conditions in terms of waves. It is a 2-D approach, so it was carried out without involving volumetric discretization, which gives a short simulation time. Then, we propose and analyze a tunable THz sensor which consists of a patch and ring resonator, in which a sample carrier is located in the middle of it to put the biological cells. This resonator is composed by one or two layers of 2-D materials stacked in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> -direction. The sensing performance of this sensor can be readily tuned by controlling the black phosphorus (BP) electron doping or the graphene chemical potential. In the experiment, the performances of the proposed sensor are tested by analyzing different configurations based on 2-D materials. The final results indicated that when we combined the advantages of graphene and BP (G-BP), the hybrid structure (G-BP) gives more efficient results compared to pure BP or graphene-based structure. In our electromagnetic simulations, the sensitivity reaches 23.866 and 28.149 THz/RIU in healthy and cancer cells. The proposed G-BP sensor in this article is a promising candidate for the THz biomedical sensors due to its high sensitivity, tunability, and simple structure.

Tunable plasmonic optical responses and the sensing application in graphene-based metasurface

It is meaningful that the sensing performance is achieved in a simple graphene structure. In this paper, we proposed a simple graphene-based metasurface structure which contain a graphene ribbon and U-shaped graphene pattern, showing outstanding sensing performance in terahertz band. The transmission spectrum with three dips is excited in our proposed structure calculated by the finite-difference time domain (FDTD) simulation method. The results show that the transmission spectrum with three dips can be effectively tuned by the Fermi energy. Intriguingly, the tri-frequency detection of the sensing performance is focused on the proposed graphene-based metasurface. The maximum of sensitivity and figure of merit(FOM) for the sensing performance are 3.4269 THz/RIU and 21.92, respectively. These research results may pave the way for the development of optical terahertz sensing devices.

Graphene-based multifunctional surface and structure gradients engineered by atmospheric plasma

Surface and structure gradients are ubiquitous in nature, where gradual changes in physical, chemical, mechanical and other properties of biomaterial are generated aiming to achieve intricate biological functions. These bioinspired gradients have been explored and fabricated using different materials in small scales (nm or µm) and usually with single function. Leveraging an outstanding property of graphene and their multi functionality, this paper presents a demonstration of the surface and structure gradients with gradual change of multiple functionalities such as structure, chemistry, wettability, charge, electrical and thermal conductivity in all three dimensions (1D, 2D and 3D). These multifunctional and multidimensional gradients with a gradual transition from graphene oxide (GO) to partially reduced GO (prGO) and reduced GO (rGO) were created by atmospheric plasma treatment of GO substrates (films and foams) using non-uniform exposure of plasma beams. The plasma beams with controllable energy, asymmetric contact exposure and timing were used to generate a gradual chemical reduction of oxygen groups and GO to prGO and rGO and fabricate the sizable (cm) range surface and 3D gradients with gradual change of multiple functionalities. This simple and low-cost method is a powerful and straightforward strategy to produce multifunctional gradients with 1D, 2D and 3D structures in different forms (films, foams) on variety of substrates (metals, semiconductors, glass,plastics, textile, etc.). Practical application of fabricated gradients is successfully demonstrated as absorbers for solar-driven water evaporation showing excellent water evaporation rates results for 1 sun irradiation of 1.782 kg•m−2•h−1 and the net solar to steam efficiencies of 97%. These gradients are very useful for diverse fundamental and practical applications, and one elegant example of efficient solar-driven water evaporation was demonstrated.

Tunable plasmon-induced transparency with coupled L-shape graphene metamaterial

A graphene-based metamaterial structure, consisting of a graphene strip (GS) and a L-shaped graphene-rectangular block (GRB), is proposed to generate the plasmon-induced transparency (PIT) effect. The potential physical properties of the PIT effect are analyzed by using the coupled mode theory (CMT). The PIT has the unique characteristics of controlling light propagation through the static and dynamic regulations, resulting in a prospective switching application. The performance of the optical switch is evaluated through different parameters, including the geometric size, Fermi level and polarization angle. The modulation depth of the amplitude can reach 74.9% with a specific Fermi level, while the maximum polarization extinction ratio can reach 11.34 dB. Furthermore, dual and triple PIT effects are achieved by the designing and optimizing the structure. The maximum multiple switching effect is obtained with a modulation depth of 97.3%, which has a promising prospect in terahertz optical switches.

Optical amplification of surface plasmon polaritons in a graphene single layer integrated with a random grating

In this paper, we design and simulate a terahertz (THz) controllable active plasmonic waveguide structure based on a single graphene layer that is placed on a random SiC grating substrate whose grooves are filled with CaF2. Optical gain in the proposed THz active plasmonic waveguide structure is provided by the stimulated emission process in the photoexcited graphene monolayer that leads to the amplification of surface plasmon polariton (SPP) waves. We use a random grating substrate to introduce Anderson localization of the SPP waves propagating through the graphene monolayer to enhance their optical amplification at resonance frequencies in which more amplification occurs due to multi-scattering and interference effects. It is shown that the input SPP wave can be amplified by 130 times at resonance frequencies of the random system. Significant amplification occurs only for frequencies that are equal to the resonance frequencies corresponding to the passive structure and are located within the gain bandwidth of graphene. The effects of depth and width of filled grooves on the amplification characteristics are also investigated. Furthermore, we show that increasing the ambient temperature by 30 K can reduce the output intensity by a factor of 12. This property of the proposed graphene-based THz plasmonic waveguide structure makes it useful in temperature sensing applications and on/off switchable laser devices.

Design and modeling of a graphene-based composite structure optical pressure sensor.

In this paper, a novel graphene-based composite structure optical pressure sensor is designed and built with the aid of modeling. A PDMS force-sensitive structural mechanics model is established to optimize the size of the pyramid array distributed on the PDMS layer so that to support high levels of sensitivity and stability. Meanwhile, a graphene waveguide optical model is established to obtain the optimized interference length (L), arm spacing (H) and core width (W), with the objectives of advanced sensitivity, low propagation loss, high resolution. The experimental results show that the pressure sensitivity of the proposed sensor is 17.86 nm/kPa and the maximum pressure that can be detected is 3.40 kPa, which is consistent with the theoretical analysis and verifies the feasibility of the design, also the modeling methods of the graphene-based composite structure optical pressure sensor.

Tunable TE/TM mode converter with an orthogonal graphene-based grating structure

Tunable TE/TM mode converter with an orthogonal graphene-based grating structure

Impact of the crystal size of beta zeolite on the structural quality of zeolite templated carbons

The impact of the crystal size of the template zeolite on the textural properties of resulting Zeolite Templated Carbons was studied using nano- (20 nm) and micron-sized (5 μm) zeolite beta crystals. ZTCs were obtained for both nano- and micron-sized zeolite beta crystals, yet for the micron-sized sample the presence of supermicropores and small mesopores was detected, indicating a higher degree of structural defects during the templating process and hence in the final ZTC. The insights gained from nitrogen physisorption were confirmed by TEM, TGA and XPS. Despite, long range structural order, evidenced by X-ray powder diffraction (XRD) for both, nano- and micron-sized ZTCs was inferred. These results suggests that XRD patterns allow to evidence the well-ordered part of the sample but might not be the method of choice for characterizing the structural quality of ZTCs. Simulated XRD powder patterns allowed to propose a graphene-based nanoribbon structure grown in the zeolite pores as a satisfactory model for ZTCs in beta zeolite.

Tunable and enhanced Goos–Hänchen shifts in a monolayer graphene-based metallic grating structure

The Goos–Hänchen (GH) effect, a lateral displacement of the reflection beam, enables rich applications in sensors and detectors. Metallic grating structures have gained great attention due to their unique properties, such as enhanced optical transmission and surface enhanced Raman scattering. Here, instead of using the dielectric grating, we demonstrate an enhanced GH shift in the structure with the metallic grating layer, monolayer graphene, dielectric spacer, and metallic substrate. We find that the amplitude of the GH shift is as high as 3616 times of the incident wavelength at the specific working frequency. This enhancement can be understood from the generation of the coupled surface plasmon polaritons between two surfaces of the dielectric space layer. Furthermore, we show efficient control of the GH shift by modulating the geometric parameters of the proposed structure and chemical potential of the monolayer graphene. Our result establishes an alternative metallic grating structure for tunable and enhanced GH shifts in graphene-based nanostructures.

Graphene-Based Tunable Broadband Polarizer for Infrared Frequency

This paper proposes the tunable graphene-assisted polarizer structure which is working on the infrared frequency range. The tunable polarizer has been designed by a three-layered structure of silica, graphene, and gold. The polarizer behavior of the structure is analyzed for the frequency range of 3 to 12 THz. The tunability of the structure is analyzed for the different values of Fermi energy which is a tunable parameter of the single-layer graphene sheet. Polarizer response is derived in terms of different performance parameters such as reflectance, phase variation, phase difference, polarization conversion rate, and effective refractive indices. Graphene-based polarizer structure is investigated for the co-polarization and cross-polarization input incident conditions to check linear to circular polarization conversion. It also shows an effective refractive index response to check the metasurface behavior of the polarizer for the 3 to 12 THz range. We have observed that the polarization amplitude becomes stronger for the higher Fermi energy value of the graphene sheet. The reflection amplitude is achieved up to 90%. Results of the proposed polarizer structure can be used to design the various electro-optical structure which operates in the lower terahertz range.

Graphene-Based Bipolar Junction Transistor

Graphene was considered likely to revolutionize the electronics industry. This expectation has not yet been fulfilled, mainly due to the non-ideal characteristics of graphene-based transistors. Here, we propose a novel graphene-based structure as a graphene-based bipolar junction transistor (G-BJT), a nanoscale transistor which has the ideal characteristics of the common BJT transistor. In this device, N-P-N regions are formed in the graphene channel by applying voltages to the three gates. The carrier concentrations, energy band diagrams, and current-voltage curves are measured and presented. The base-emitter junction shows a rectifying behavior with the ideality factor in the range of (2.8–3.2), the built-in potential of 0.38V, and the saturation current of 10−12 A. The G-BJT provides a minimum current gain of 20 at the base-width of 10 nm, a feature that cannot be easily obtained in Si-based BJTs. Interestingly, the current gain(β) can be controlled by the gate voltages in G-BJT and changes by 26.5% compared to the maximum value, which leads to the controllability of this proposed transistor. Identical BJT behavior, scalability down to nanometer range, large carrier mobility, along the controllable current gain of G-BJT make this transistor a good candidate for the next generation of the nanoelectronics industry.

Nanoarchitectonics of the supercapacitor performance of LaNiO3 perovskite on the graphitic-C3N4 doped reduced graphene oxide hydrogel

Designing highly efficient, low-cost hybrid nanostructures with having favorable morphology and excellent conductivity is very promising for electrodes used in electrochemical storage devices. This study shows the synthesis of a new three-dimensional (3D) hybrid structure consisting of lanthanum nickel oxide (LaNiO3) perovskite, graphite carbon nitride (g-C3N4), and reduced graphene oxide hydrogel (RGOH) structures for the new hybrid supercapacitor (SC) application. The 3D hybrid structure, which is a new SC application, draws attention according to the specific capacitance (Cs) result for graphene-based hydrogel structure. For this purpose, while g-C3N4 is selected to plays an important role to increase the electrochemical activity of SC, the LaNiO3 perovskite structure is used to increase the discharge capacity of SC as well as to provide high discharge current and long cycle life. As a result of electrochemical studies, the superior capacitive performance of LaFeO3/g-C3N4@RGOH showed Cs and unique performance of 750.56 F g−1 at a current density of 1 A g–1. The 3D structure is used as anode material for fabricating symmetric two-electrode supercapacitor systems. Moreover, the LaFeO3/g-C3N4@RGOH materials display magnificent cyclic stability of 82% for 5000 cycles at a current density of 10 A g − 1, which claims the new material progress in the energy field.