Graphene Strips Research Articles

Preferential graphitic-nitrogen formation in pyridine-extended graphene nanoribbons

Graphene nanoribbons (GNRs), nanometer-wide strips of graphene, have garnered significant attention due to their tunable electronic and magnetic properties arising from quantum confinement. A promising approach to manipulate their electronic characteristics involves substituting carbon with heteroatoms, such as nitrogen, with different effects predicted depending on their position. In this study, we present the extension of the edges of 7-atom-wide armchair graphene nanoribbons (7-AGNRs) with pyridine rings, achieved on a Au(111) surface via on-surface synthesis. High-resolution structural characterization confirms the targeted structure, showcasing the predominant formation of carbon-nitrogen (C-N) bonds (over 90% of the units) during growth. This favored bond formation pathway is elucidated and confirmed through density functional theory (DFT) simulations. Furthermore, an analysis of the electronic properties reveals metallic behavior due to charge transfer to the Au(111) substrate accompanied by the presence of nitrogen-localized states. Our results underscore the successful formation of C-N bonds on the metal surface, providing insights for designing new GNRs that incorporate substitutional nitrogen atoms to precisely control their electronic properties.

Tunable enhancement of the cylindrical Luneburg lens focusing ability with the aid of a conformal graphene strip.

We consider the plane wave focusing characteristics of the layered cylindrical Luneburg lens equipped with a conformal strip of graphene, in the H-polarization case. The angular width and location of the strip is arbitrary, and its surface impedance is characterized with the aid of the quantum-physics Kubo formalism. We use a mathematically accurate full-wave analytical regularization technique, which is based on the explicit inversion of the problem static part and yields a Fredholm second-kind matrix equation. This guarantees the convergence of the resulting meshless numerical algorithm. We compute the focusing ability of a microsize lens as a function of the frequency in the wide range up to 60 THz. This analysis shows that a graphene strip, placed into the focal area of the Luneburg lens, enhances its focusing ability at the resonance frequency of the strip plasmon mode proportionally to the quality factor. This frequency is defined by the strip width and is tunable with the aid of graphene's chemical potential.

Graphene modulator and 2-bit encoder based on plasma induced transparency effect

Within this study, a periodic metasurface structure made up of two graphene strips, two fan-shaped graphene and a square ring graphene with notches is introduced to achieve plasma-induced transparency (PIT). The PIT effect is analyzed utilizing the Lorentz oscillatory coupling model, and it has been discovered that the theoretical values closely match the simulated values. The role of the Fermi level adjustments in graphene on the PIT effect was examined, and the PIT window was dynamically modified as the Fermi level changed, which was used to realize a 2-bit graphene encoder at 5.78 THz and 7.18 THz. The encoder boasts a greatest modulation depth of 80.9 % and exhibits a minimal insertion loss of 0.17 dB. By controlling the polarization direction of the incident photoelectric field, a high performance dual-channel switching modulator is successfully realized. Moreover, by increasing carrier mobility, the depth of modulation for the modulator at 11.22 THz is increased from 97.5 % to 99.5 %, and the insertion loss has dropped from 0.075 dB to 0.06 dB. In addition, the structure has superior sensing properties and slow light properties, featuring a sensitivity reaching up to 1.07 THz/RIU and a maximum group index of 293. The outcomes of our study contribute novel insights for the advancement of modulator, encoder, sensor technology and slow-light devices.

Generation and tuning of notched bands in wideband THz antennas using graphene/metal strips

Abstract A technique is implemented for obtaining tunable band notch/filtering features in terahertz (THz) monopole wideband antennas. A parasitic metal strip is engraved in the antenna radiator, which creates the band notch characteristics in the wideband response. The tunability of the created notched band can be obtained by engraving a non-resonant U-shaped graphene strip in the radiator. The change in the material properties of the graphene strip alters the nature of field confinement in the antenna structure, enabling tunability in the filtering frequency band. Moreover, multiple notch bands can be obtained by carefully selecting the material of the parasitic element U-shaped graphene strip. Thus, the antenna response can be configured for dual- and triple- band filtering operation by varying the materials used for the strips to obtain band notch operation and a U-shaped graphene sheet.

Tunable and polarization-dependent electromagnetically induced transparency analogy in graphene-based terahertz metasurfaces

In this paper, we theoretically and numerically demonstrate a tunable and polarization-dependent electromagnetically induced transparency analogy based on graphene terahertz metasurfaces. The unit cell of the metasurface consists of three-layer graphene strips embedded in a silicon grating. The dynamic adjustment of the transparent window can be achieved by changing the coupling distance between the graphene layers and the polarization direction of the incident lights. The operation mechanism behind the phenomenon can be attributed to the near-field interaction and electromagnetic coupling of modes in graphene strips. Furthermore, the full wave electromagnetic simulations obtained by the finite-difference time-domain method agree well with the theoretical fitting results based on the three-harmonic oscillator model. In addition, by changing the Fermi levels, it can not only realize the outstanding slow-light effects with a maximum group index of 3750 but also obtain the four-frequency asynchronous optical switch function in terahertz regions. Therefore, our proposed metamaterial device may have potential applications in image switching, optical switches, slow-light device, optical communication, and optical storage.

The effect of functional group of Graphene-based woven filter on the desalination performance

Abstract Graphene-based braided filter membranes (GWFM) are becoming increasingly crucial in seawater desalination because of their excellent performance, moving from research to practical use and helping to tackle water scarcity. However, the mechanisms associated with GWFMs functionalized by functional groups are unclear. This paper demonstrates that graphene strips are intricately integrated into a filtration membrane characterized by X-functional groups (GWFM-X). Molecular dynamics (MD) simulations are effective for calculating water flux, assessing salt discharge effects, and understanding the physical mechanisms in seawater desalination, making them crucial for optimizing and advancing desalination technology and salt rejection rate of four types of GWFM-X, namely GWFM-1.2nm-COOH, GWFM-1.2nm-NH2, GWFM-1nm-COOH, and GWFM-1nm-NH2, which were designed with different functional groups and nanopore diameters. These results demonstrate the superior performance of GWFM-X in water filtration applications. Salt rejection efficiency of four distinct variants of GWFM-X, namely GWFM-1.2nm-COOH, GWFM-1.2nm-NH2, GWFM-1nm-COOH, and GWFM-1nm-NH2, is designed with different functional groups and nanopore diameters. The test membrane shows exceptional water flux, significantly outperforming current commercial reverse osmosis membranes, with measurements of 30.00±0.49 L cm−2 day−1 MPa−1, 35.41±0.71 L cm−2 day−1 MPa−1, 9.31±0.31 L cm−2 day−1 MPa−1, and 20.66±0.21 L cm−2 day−1 MPa−1, respectively. The free energy profiles of GWFM-X reveal a pronounced disparity in free energy barriers between water molecules and Na+/Cl− ions, with a difference of up to 14.26 kBT. The simulation results show that the seawater desalination system maintains over 75% efficiency at pressures between 10 and 500 MPa, highlighting its effectiveness and versatility. Moreover, advancements in nanofabrication technology are paving the way for new membrane materials that could improve desalination methods and help address water scarcity.

Optimized graphene metamaterial for dual plasmon-induced transparency in terahertz band with multifunctional applications

Abstract This paper proposes a patterned graphene periodic metamaterial structure, optimized using an improved genetic algorithm to adjust the position and size of each graphene strip, thereby achieving dual plasmon-induced transparency (PIT) effects in the terahertz band, resulting in extraordinary multifunctionality. The finite difference time domain method is employed to obtain the transmission spectrum, and coupled mode theory is used for theoretical analysis and verification of the dual-PIT effect. The structure exhibits multifunctionality: when used as a photoelectric switch, it achieves a modulation depth of up to 99.04% with an insertion loss as low as 0.16 dB by tuning the Fermi level. Additionally, the structure demonstrates excellent sensing performance, with a maximum sensitivity and figure of merit reaching 0.84 THz/RIU and 88.55, respectively. Furthermore, the slow light performance of the structure is investigated, showing a group delay of up to 0.5 picoseconds.

A triple plasmon-induced transparency terahertz sensor based on graphene metamaterials

A metamaterial-based terahertz sensor design is presented in this paper. The metamaterial unit structure comprises a graphene cross and three sets of graphene strips, which collectively achieve a triple plasmon-induced transparency (PIT) in both polarization directions. The theoretical results based on the Lorentz resonance theoretical model are in good agreement with finite difference time domain (FDTD) simulations. The sensor is polarization-independent and changes in the angle of incidence between 0° and 10° have a negligible impact on the sensing performance. Furthermore, the sensor exhibits a notable slow-light effect, with a maximum sensitivity of 1.65 THz/RIU and a FOM value of 4.74/RIU for the triple PIT, which is superior to that of comparable devices.

Triple plasmon induced transparency based on multilayer graphene metamaterials

In this study, a novel multilayer terahertz metamaterial with four graphene sub-structures is designed to achieve dynamically tunable triple plasmon-induced transparency (PIT) due to interference among bright-bright modes. Coupled mode theory (CMT) and finite-difference time-domain (FDTD) simulations show a high degree of consistency in their results. Interestingly, enhanced transmission dips are observed when various graphene structures couple with the two longitudinal graphene strips at the bottom layer. Based on the dynamic modulation characteristics of graphene, the metamaterial demonstrates seven-band optical switching capabilities, achieving modulation depths (MD) of 94.6%, 88.1%, 90.8%, 90.8%, 90.4%, 86.6%, and 87.5%. Additionally, the proposed graphene metamaterial also shows excellent slow-light effects, with a group index reaching up to 1034. This proposed terahertz metamaterial holds significant theoretical implications for the development of dynamically integrated terahertz optoelectronic devices.

Reconfigurable THz leaky-wave antennas based on innovative metal–graphene metasurfaces

Abstract Graphene ohmic losses notably hinder the efficiency of graphene-based terahertz (THz) devices. Hybrid metal–graphene structures have recently been proposed to mitigate this issue in a few passive devices, namely waveguide and Vivaldi antennas, as well as frequency selective surfaces. In this work, such a technique is extensively investigated to optimize the radiation performance of a THz Fabry–Perot cavity leaky-wave antenna based on a hybrid metal–graphene metasurface consisting of a lattice of square metallic patches interleaved with a complementary graphene strip grating. Theoretical, numerical, and full-wave results demonstrate that, by properly selecting the unit-cell features, a satisfactory trade-off among range of reconfigurability, antenna directivity, and losses can be achieved. The proposed antenna can find application in future wireless THz communications.

A tunable graphene dual mode absorber for efficient terahertz radiation absorption and sensing applications

This paper presents a novel plasmonic graphene absorber designed for the terahertz frequency range, utilizing a dual mode optical filter configuration. The absorber consists of a layered periodic array comprising gold, SiO₂ and graphene. Key components include a graphene disk and four strategically positioned graphene stripes on a SiO₂ substrate. An underlying gold layer enhances the absorption efficiency by serving as a reflector. The structure is numerically simulated using the 3D finite difference time domain (FDTD) method. A comprehensive parametric study has been conducted to optimize the absorber's performance. The simulation results demonstrate near perfect absorption at 4.16 THz and 5.88 THz. This innovative design enables efficient absorption of terahertz radiation due to the plasmonic resonance effects of the graphene components. Additionally, the absorption frequency can be dynamically adjusted by altering the chemical potential of graphene through applying an external bias voltage. The tailored geometry and material composition result in a compact absorber with high absorption values and tunability. Detailed performance analysis through numerical simulations demonstrates the absorber's potential for applications in sensing, imaging, and communication systems operating in the terahertz frequency range.

Adjustable slow light with high group index in a graphene metasurface based on plasmon-induced transparency

A relatively simple metasurface structure is proposed to achieve a dynamically tunable slow light effect. The metasurface consists of two horizontal graphene strips and two vertical continuous graphene strips. Strong interference between the bright and dark modes enables the metasurface to generate a significant triple plasmon-induced transparency (PIT). Varying the coupling distance between the horizontal strips, double-PIT and triple-PIT can be transformed into each other. During the reduction of the transparent window, electromagnetic waves undergo strong phase changes, resulting in a higher group index. After structural optimization, the maximum time delay and group index can be as high as 2.624 ps and 3934, surpassing comparable slow light devices. The proposed patterned graphene metasurface provides theoretical guidance for designing high-performance slow light devices for optical storage, nonlinear optics and quantum optics.

Analysis of the magnetic-thermal response of viscoelastic rotating nanobeams based on nonlocal theory and memory effect

Size-dependent effects in elastic deformation and thermal hysteresis effects in the heat transfer process are significant for small-size structures or devices. As an important component of micro- and nanoelectromechanical systems, the analysis of the thermodynamic properties of rotating nanobeams is crucial for the safe operation of the system. This work aims to construct a new nonlocal thermo-viscoelastic model and use this model to analyze the thermodynamic behavior of rotating nanobeams at the microscale. In this work, fractional order Kelvin-Voigt theory describes the viscoelasticity of materials. Moreover, the theory of continuous medium mechanics is improved by introducing nonlocal parameters to capture the size effect during deformation. Based on the generalized thermoelasticity theory, memory-dependent derivatives predict hysteresis effects during heat transfer. Nanobeams are placed in a longitudinal magnetic field. Graphene strips connected to a power supply are used as the nanobeam heat source. The effects of nonlocal parameters, damping coefficients, and fractional order parameters on the dynamic response of the system are analyzed. In addition, a new comparative study of the effects of voltage and resistance on the system is made in the context of the nonlocal generalized thermoelasticity theory.

Highly sensitive dual mode dual side cross polarization converter based biosensor

—This paper investigates a dual-side, dual-mode reflective cross-polarization converter for biosensing applications. The converter consists of a polyimide substrate with a modified dumbbell-shaped resonator on one surface and its complementary slot on another. The overall volume is 8.25μm × 8.25μm × 2.47 μm with dual-band operation of 6.39268–7.311692 THz and 9.783001–23.53473 THz for PCR≥80 % for front signal incidence and two full widths at half maximum (FWHM) bands i.e. 11.71594–12.26761 THz & 19.12042–19.87691 THz for back signal incidence. The periodicity is ∼λo/6, and substrate thickness is ∼λo/19 at 6.39268 THz. This device has performance stability for incident angle (θ)≤30o. The performance of this device can be tuned from dual-band operation to single-band by varying the chemical potential of the graphene strip for signal incidence from both sides. This device can classify cancer or tuberculosis-infected cells concerning healthy cells by using the variations in their corresponding refractive indices that lead to changes in the band edge frequencies. This device can differentiate between infected and healthy cells or different refractive indices with a high sensitivity of ∼5 THz/RIU. This device has merits of dual side sensing, wide operating bandwidth, multiple frequencies for sensing purposes, and high sensitivity over previously reported sensors.

Multi-frequency and multi-functional optical switch based on dual plasmon-induced transparency

Research into multi-frequency and multi-functional optical switches for complex applications is pioneering territory. By employing a single-layer structure comprising three distinct graphene strips, we successfully created a dual-PIT effect through the destructive interference among two bright modes and a dark mode. The numerical simulations were corroborated by coupled mode theory, reflecting a high degree of consistency between the theory and the simulations. Remarkably, the modulation of the Fermi level in graphene metamaterials through gate voltage enabled the realization of asynchronous optical switches capable of operating at six, five, four, and three frequencies. Notably, the six-frequency switch exhibited an impressive modulation depth of 88.54% and an insertion loss of just 0.15 dB, highlighting its superior performance. This study lays a solid foundation for future multi-frequency and multi-functional optical switch designs, offering significant implications and practical applications.

Beamforming of Electrically Tunable Plasmonic Graphene Strip Nanoantennas in the Terahertz, Far-Infrared, and Mid-Infrared Ranges

Beamforming of Electrically Tunable Plasmonic Graphene Strip Nanoantennas in the Terahertz, Far-Infrared, and Mid-Infrared Ranges

Tunable triple plasmon-induced transparency in E-type graphene metamaterials.

Enhancing light-matter interaction is crucial for boosting the performance of nanophotonic devices, which can be achieved via plasmon-induced transparency (PIT). This study introduces what we believe to be a novel E-type metamaterial structure crafted from a single graphene layer. The structure, comprising a longitudinal graphene ribbon and three horizontal graphene strips, leverages destructive interference at terahertz frequencies to manifest triple plasmon-induced transparency (triple-PIT). Through a comparison of simulations using the finite difference time domain (FDTD) method and theoretical coupled-mode calculations, we elucidate the physical mechanism behind triple-PIT. Our analysis shows that the PIT effect arises from the interplay between two single-PITs phenomena, further explored through field distribution studies. Additionally, we investigate the impact of varying Fermi levels and carrier mobility on the transmission spectrum, achieving amplitude modulation in photoelectric switches of 85.5%, 99.2%, and 93.8% at a carrier mobility of 2 m2/(V·s). Moreover, we explore the relationship between Fermi levels and carrier mobility concerning the slow light effect, discovering a potential group index of up to 1021 for the structure. These insights underscore the significant potential of this graphene-based metamaterial structure in enhancing optical switches, modulators, and slow light devices.

A four-port self-multiplexing tunable graphene based dielectric resonator antenna with defective ground structure for high isolation and MIMO capabilities

Objectives and designA novel self-multiplexing terahertz antenna with four tunable ports is proposed in this work. The configuration comprises four rectangular dielectric resonators (DRs) affixed to a substrate, which are excited through L-shaped microstrip-fed slots positioned beneath them. To guarantee efficient isolation among the tunable antenna components, periodic unit cells are introduced into the ground plane, forming a defective ground structure (DGS). Each L-shaped slot element is equipped with a graphene strip to regulate the tunability of each DR element. Results and applicationsThis antenna can operate in various modes with resonant frequencies at 4.01, 4.23, 4.37, and 4.57 THz. It holds potential for future terahertz wireless applications that require the utilization of adjacent channels with distinct communication frequencies. Furthermore, a four-port antenna design is implemented, offering tunable MIMO capabilities with self-quadruplexing ability. The MIMO parameters, specifically the envelope correlation coefficient (ECC) and diversity gain (DG), have been assessed and found to fall within acceptable limits across the operating frequency bands. Additionally, an equivalent circuit model (ECM) is analysed to shed light on the antenna's working principle and verify the obtained results.

Multifunctional terahertz device based on plasmon-induced transparency

Enhancing light-matter interaction is crucial in optics for boosting nanophotonic device performance, which can be achieved via plasmon-induced transparency (PIT). In this study, a polarization-insensitive PIT effect at terahertz frequencies is achieved using a novel metasurface composed of a cross-shaped graphene structure surrounded by four graphene strips. The high symmetry of this metasurface ensures its insensitivity to changes in the polarization angle of incident light. The PIT effect, stemming from the coupling of graphene bright modes, was explored through finite difference time domain (FDTD) simulations and coupled mode theory (CMT) analysis. By tuning the Fermi level in graphene, we effectively modulated the PIT transparent window, achieving high-performance optical switching with a modulation depth (88.9% < MD < 98.0%) and insertion losses (0.17 dB < IL < 0.51 dB) at a carrier mobility of 2 m2/(V·s). Furthermore, the impact of graphene carrier mobility on the slow-light effect was examined, revealing that increasing the carrier mobility from 0.5 m2/(V·s) to 3 m2/(V·s) boosts the group index from 126 to 781. These findings highlight the potential for developing versatile terahertz devices, such as optical switches and slow-light apparatus.

Tunable terahertz absorber based on quasi-BIC supported by a graphene metasurface

In this paper, a tunable terahertz (THz) absorber operating at a quasi-bound state in the continuum (quasi-BIC) mode supported by a graphene metasurface is proposed. There are two graphene strips and a fully covered graphene layer in one unit cell. By breaking the symmetrical arrangement of graphene stripes, the symmetry-protected BIC transforms into a quasi-BIC mode. The reflective configuration results in high-Q absorption of the metadevice at the quasi-BIC mode with the equivalent impedance matching the impedance in free space. The change in the Fermi level of graphene can cause a frequency shift in the position of the absorption peak at the quasi-BIC mode. Benefiting from the high Q-value and narrow linewidth of the quasi-BIC, the frequency shift of the absorption peak can easily exceed its linewidth. At this time, the designed THz absorber can be used as a switch, and the “on” and “off” states are achieved by tuning the Fermi level of graphene. Under normal incidence, the modulation depth of the absorption type THz switch can reach up to 99% with the insertion loss only 0.062 dB. Within the range of incident angle inclination approaching 10°, the absorption type THz switch can still achieve more than 90% modulation depth and insertion loss below 0.1 dB. Due to the characteristics of large modulation depth, low insertion loss, and wide angle incidence, the designed tunable THz absorber has great application prospects in fields such as THz communication and THz wavelength division multiplexing.