Fermi Energy Level Of Graphene Research Articles

Preparation and Electrical Testing of Double Top Gate Graphene Field-Effect Transistor

In this paper, we prepare and test a graphene field-effect transistor with two top gates. The Fermi energy level of graphene can be adjusted by applying positive and negative voltages to the two top gates, and N-type and P-type graphene are formed in the channel region, thus inducing a graphene p-n junction. The current model is established using the gradual channel approximation (GCA) method, and the current and p-n junction characteristics of the device were obtained by formula simulations. Based on the principle of p-n junction luminescence, this device with graphene p-n junction is expected to achieve terahertz wave radiation with an appropriate optical resonant cavity.

Graphene thermionic energy converter combined with an absorption heat transformer for electricity generation and thermal upgrading

To improve energy utilization by recycling waste heat, a novel hybrid system composed of a graphene-based thermionic energy converter (GTEC) and an absorption heat transformer (AHT) is presented, where the GTEC anode waste heat is absorbed by the AHT for thermal upgrading. Considering major irreversible losses, the mathematical expressions for the equivalent power density, energy efficiency and exergy efficiency of the hybrid system are derived using thermionic emission and nonequilibrium thermodynamic theory. The hybrid system's general performance characteristics are also discussed in detail. Additionally, according to numerical calculation, the hybrid system's maximum energy efficiency (MEE), maximum power density (MPOD), and maximum exergy efficiency are 54.53 %, 7.73 W/cm2 and 68.16 % respectively, at the high-temperature heat source temperature of 1500 K. The hybrid system MPOD and MEE have risen by 17.97 % and 17.96 %, respectively, and maximum exergy efficiency has increased by 17.97 % as compared to a single GTEC. Furthermore, the impacts of the high-temperature heat source temperature, the Fermi energy level of graphene, cathode work function, ambient temperature, heat transfer coefficients of AHT, and the temperature of heated space on the hybrid system are investigated in depth. This study provides a new avenue for waste heat utilization in GTEC.

Dual-band/ultra-broadband switchable terahertz metamaterial absorber based on vanadium dioxide and graphene

A dual-band/ultra-broadband switchable terahertz absorber based on vanadium dioxide (VO2) and graphene is proposed in this article. By controlling the properties of VO2 and graphene, the operation mode of the absorber can be switched between dual-band and ultra-wideband absorption. The simulation results show that the absorber works in the dual-band absorption mode when VO2 is in the insulating state and the Fermi energy level of graphene is 1.5 eV. The absorptivity are 99.4% and 99.8% at 1.08 and 3.52 THz, respectively. The frequency position of the absorption peaks can be switched by regulating the Fermi energy level of graphene. While the VO2 is transitioned to the metallic state and the Fermi energy level of graphene is 0.05 eV, the absorber operates in the ultra-broadband absorption mode with the absorptivity over 90% in the range 0.94–4.31 THz, and the relative absorption bandwidth reaches 128%. In addition, the working principle of the proposed absorber is analyzed by impedance matching theory and electric field distribution. Based on these characteristics, the proposed metamaterial absorber is expected to be used in the terahertz bio-detection and imaging fields.

Graphene‐based tunable terahertz electromagnetically induced transparency using metamaterial structure

AbstractA graphene‐based tunable electromagnetically induced transparency (EIT)‐like metamaterial structure operating at the terahertz regime is proposed and numerically analyzed. The unit cell of the metamaterial structure consists of a split‐ring resonator and twofolded‐line pair resonators, performing as the quasi‐dark mode and bright mode, respectively. When the incident waves vertically illuminate upon the metamaterial structure, a transmission peak can be observed. Moreover, the frequency of the transparency window can be flexibly adjusted by changing the Fermi energy level of graphene. A classical coupled two‐oscillator model is employed to theoretically analyze the physical mechanism of EIT‐like phenomenon, which is due to the near‐field coupling effect between the bright and the quasi‐dark modes. The proposed work will be a good candidate for the design of different graphene‐based tunable EIT devices at different frequency spectra with potential applications in optical sensors.

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.

Ultra-sensitive and selective 2D hybrid highly doped semiconductor-graphene biosensor based on SPR and SEIRA effects in the wide range of infrared spectral

Simultaneous amplification of surface plasmon resonance (SPR) and surface enhanced infrared absorption (SEIRA) spectroscopy can be very effective for the operation of biosensors in a wide range of mid-IR to terahertz wavelengths, with considering the position of the vibrational spectrum and the intensity of the absorption band to detect small biomolecules. In this paper, we introduce the hybrid high doped semiconductor-graphene plasmonic biosensor to achieve this goal. Our proposed structure has been formed of high doping InAsSb nanoribbons beside the plates or nanoribbons of graphene. By changing the geometrical characteristics of the proposed structure and selecting the appropriate fermi level for graphene nanostructures, the high doped semiconductor-graphene biosensor has been able to increase the sensitivity of all-semiconductor biosensors in the presence of small amounts of biomolecules to 156%. While its quality and Figure of merit (FOM) has also been greatly improved. The introduced type of biosensors, by combining the benefits of all-graphene and all-semiconductor biosensors and by controlling the fermi energy level of graphene and selecting the appropriate doping level for semiconductor can be the beginning for highly sensitive tunable biosensors over a wide range of wavelengths to identify fingerprints of specific biomolecules.

Terahertz mode switching of spin reflection and vortex beams based on graphene metasurfaces

Gradient metasurface provides a new way for controlling electromagnetic wave, and vortex beam can be widely used to improve the performance of wireless communication. Here, graphene-based metasurfaces are proposed for mode switching, which can dynamically adjust the wavefront of circularly polarized wave in terahertz band. The designed meta-atom is composed of quasi-I-shaped metal structure, graphene layer, spacer, and metal substrate. 360° phase cover is realized by rotating the top metal structure. Two examples are presented to demonstrate mode switching of metasurface, namely gradient metasurface and vortex beam generator. By setting Fermi energy level of graphene as 0.015 eV and 0.70 eV, the designed gradient metasurface realizes the switching between anomalous reflection and mirror reflection for left-handed/right-handed circular polarization based on Pancharatnam-Berry phase theory at 1.2 THz. Then, mode number of orbital angular momentum generated by vortex beam generator can be reconstructed by changing Fermi energy level. The proposed scheme could be applied in many fields, such as terahertz switch and wireless communication.

Realization of absorption, filtering, and sensing in a single metamaterial structure combined with functional materials.

In this paper, a hybrid vanadium dioxide (VO2)-graphene-based bifunctional metamaterial is proposed. The realization of the different functions of perfect transmission and high absorption is based on the insulator-metal phase transition of VO2 material. The Fermi energy level of graphene can be treated to dynamically tune the absorption and transmission rates of the metamaterial structure. As a result, when VO2 is in the insulating state, the designed metamaterial can be used as a filter providing three adjustable passbands with center frequencies of 1.892 THz, 1.124 THz, and 0.94 THz, and the corresponding transmittances reach 93.11%, 98.62%, and 90.01%, respectively. The filter also shows good stopband characteristics and exhibits good sensing performance at the resonant frequencies of 1.992 THz and 2.276 THz. When VO2 is in metal state, the metamaterial structure acts as a double-band absorber, with three absorption peaks (>90%) in the range of 0.684 THz to 0.924 THz, 2.86 THz to 3.04 THz, and 3.28 THz to 3.372 THz, respectively. The designed structure is insensitive to the polarization of vertically incident terahertz waves and still maintains good absorption performances over a large range of incidence angles. Finally, the effects of geometric parameters on the absorption and transmission properties of the hybrid bifunctional metamaterials have also been discussed. The switchable metamaterial structures proposed in this paper provide great potential in terahertz application fields, such as filtering, smart sensing, switching, tunable absorbers, and so on.

Graphene-based tunable linear and linear-to-circular polarization converters in the THz band

A switchable and tunable reflective terahertz (THz) polarization converter based on graphene metamaterial is designed and analyzed. By changing the Fermi energy level of graphene, the converter can achieve broadband linear polarization conversion (LPC) and linear-to-circular polarization conversion (LTC-PC). At relatively low Fermi energy level, the converter can achieve dual-band (0.1–0.3 eV) and single-band (0.4–0.6 eV) LPC, respectively, with a broadband frequency range of 2.34–3.54 THz, 3.77–4.22 THz, and 3.75–4.43 THz, and maximum polarization conversion rate (PCR) exceeding 99%. At high Fermi energy level, in addition to the LPC function, the converter can also achieve LTC-PC in the frequency range of 3.34–3.59 THz and 2.89–3.34 THz by adjusting the Fermi energy level. The physical mechanisms are elucidated from the point of the surface current distribution diagram. The structure shows great potential in switchable and tunable broadband THz converters and related applications.

Switchable efficiency terahertz anomalous refraction and focusing based on graphene metasurface

In this paper, a graphene metasurface based on split-ring resonators (SRRs) structure has been proposed and investigated numerically, which can achieve the highly efficiency switchability of anomalous refraction and planar focuing effects. Numerical simulation results indicated that the proposed graphene metasurface could not only realize polarization conversion with a transmission amplitude of about 0.4, but also achieve a full 2π phase shift by changing the geometrical parameters of SRRs structure. In addition, the efficiency of the cross-polarization transmission is highly depedent on the Fermi energy level of graphene. By elaborating design of a spatially phase profile and arrangment of unit-cell structure, anomalous refraction and planar focusing effect can be achieved, and the efficiencies are also switched by only changing the Fermi energy level rather than the physical size. It could be believed that the effective switchable graphene metasurface would lead a new path for controling the wavefront of THz waves.

Numerical Investigation of Graphene and STO Based Tunable Terahertz Absorber with Switchable Bifunctionality of Broadband and Narrowband Absorption.

A graphene metamaterial and strontium titanate (STO)-based terahertz absorber with tunable and switchable bifunctionality has been numerically investigated in this work. Through electrically tuning the Fermi energy level of the cross-shaped graphene, the bandwidth of the proposed absorber varies continuously from 0.12 THz to 0.38 THz with the absorptance exceeding 90%, which indicates the functionality of broadband absorption. When the Fermi energy level of the cross-shaped graphene is 0 eV, the proposed absorber exhibits the other functionality of narrowband absorption owing to the thermal control of the relative permittivity of STO, and the rate of change of the center frequency is 50% ranging from 0.56 THz to 0.84 THz. The peak intensity of the narrowband absorption approximates to nearly 100% through adjusting the Fermi energy level of the graphene strips. The calculated results indicate that it is not sensitive to the polarization for wide incidence angles. The proposed absorber can realize tunable bifunctionality of broadband absorption with a tunable bandwidth and narrowband absorption with a tunable center frequency, which provides an alternative design opinion of the tunable terahertz devices with high performance for high-density integrated systems.

Switchable bifunctional metamaterial for terahertz anomalous reflection and broadband absorption

A terahertz switchable metamaterial is proposed with bifunctional properties based on a mixed structure of graphene and vanadium dioxide (VO2). When VO2 is metal, this metamaterial can be used to perform the function of anomalous reflection. It is composed of VO2 blocks, topas spacer, and VO2 film. By adjusting the size of VO2 blocks, the gradient transformation is formed, so the normally incident beam is reflected to the left side at the angle of 38.4° at 3.0 THz. When VO2 is insulator, this design can be used to realize a broadband absorber. It has a broadband absorption with more than 90% absorptance in the range of 1.29–3.08 THz, and there are two absorption peaks with 99% absorptance at 1.545 THz and 2.55 THz. By adjusting Fermi energy level of graphene, intensity and bandwidth of broadband absorption can be dynamically changed. In addition, the designed metamaterial is insensitive to incident angle. Our design could be applied in many promising fields as intelligent absorption and terahertz switch.

VO2-Based Switchable Metasurface With Broadband Photonic Spin Hall Effect and Absorption

Arbitrary control and dynamic tuning of circularly polarized (CP) wave are of great significance to photonic research and application. Here, a terahertz switchable metasurface is designed with bifunctional properties based on a mixed structure of graphene and vanadium dioxide (VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). The design consists of VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> strips, topas spacer, VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> film, graphene patch, topas spacer, and metallic film. When VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is metal, this metasurface realizes photonic spin Hall effect (PSHE) for CP wave in a wide frequency band of 0.7-1.5 THz. When VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is insulator, the design behaves as an absorber. It has a broadband absorption with more than 90% absorptance in the range of 0.48-1.88 THz, and there are two resonant peaks with ~100% absorptance at 0.92 THz and 1.74 THz. Meanwhile, absorption bandwidth and intensity can be dynamically tuned by changing Fermi energy level of graphene. Besides, broadband absorption is robust against incident angle. Our design may promote the realization of terahertz switchable and multifunctional metasurfaces.

Bifunctional terahertz modulator for beam steering and broadband absorption based on a hybrid structure of graphene and vanadium dioxide.

A bifunctional metamaterial is proposed based on a hybrid graphene and vanadium dioxide (VO2) configuration, which can realize a dynamic switch between beam steering and broadband absorption. The structure consists of a VO2 square, graphene patch, topas spacer, VO2 film, topas spacer, and metal substrate. When VO2 is in the metallic state, the structure serves as a coding metamaterial. By engineering different sizes of the top VO2 square and adjusting the Fermi energy level of graphene, the incident wave is scattered in different patterns. When VO2 is in the dielectric state, the structure serves as a broadband absorber. By changing the Fermi energy level of graphene from 0.0 eV to 0.9 eV, absorptance can be gradually changed and working bandwidth widens. There is an absorption band with near 100% absorptance from 0.9 THz to 1.35 THz when the Fermi energy level is 0.73 eV. And the designed broadband absorber is polarization-insensitive within the incident angle of 50°. Our work may show great potential in applications such as terahertz switching and modulation.

Switchable terahertz metamaterial absorber with broadband absorption and multiband absorption.

Based on the phase-transition property of vanadium dioxide (VO2), a terahertz bifunctional absorber is proposed with switchable functionalities of broadband absorption and multiband absorption. When VO2 is metal, the system is regarded as a broadband absorber, which is composed of VO2 patch, topas spacer, and VO2 film with metallic disks inserted. The system obtains a broadband absorption with absorptance >90% from 3.25 THz to 7.08 THz. Moreover, the designed broadband absorber has a stable performance within the incident angle range of 50°. When VO2 is dielectric, multiband absorption with six peaks is realized in the designed system. Graphene and the metallic disk-shaped array play the dominant role in the mechanism of multiband absorption. Through changing the Fermi energy level of graphene, the performance of multiband absorption can be dynamically adjusted. Because of the switchable functionalities, the proposed design may have potential application in the fields of intelligent absorption and terahertz switch.

Terahertz absorber with dynamically switchable dual-broadband based on a hybrid metamaterial with vanadium dioxide and graphene.

An absorber based on hybrid metamaterial with vanadium dioxide and graphene has been proposed to achieve dynamically switchable dual-broadband absorption property in the terahertz regime. Due to the phase transition of vanadium dioxide and the electrical tunable property of graphene, the dynamically switchable dual-broadband absorption property is implemented. When the vanadium dioxide is in the metallic phase, the Fermi energy level of graphene is set as zero simultaneously, the high-frequency broadband from 2.05 THz to 4.30 THz can be achieved with the absorptance more than 90%. The tunable absorptance can be realized through thermal control on the conductivity of the vanadium dioxide. The proposed device acts as a low-frequency broadband absorber if the vanadium dioxide is in the insulating phase, for which the Fermi energy level of graphene varies from to 0.1 eV to 0.7 eV. The low-frequency broadband possesses high absorptance which is maintained above 90% from 1.10 THz to 2.30 THz. The absorption intensity can be continuously adjusted from 5.2% to 99.8% by electrically controlling the Fermi energy level of graphene. The absorption window can be further broadened by adjusting the geometrical parameters. Furthermore, the influence of incidence angle on the absorption spectra has been investigated. The proposed absorber has potential applications in the terahertz regime, such as filtering, sensing, cloaking objects, and switches.

Tunable Ultra-Broadband Terahertz Waveband Absorbers Based on Hybrid Gold-Graphene Metasurface

In this paper, we propose a tunable ultra-broadband terahertz absorber based on hybrid gold-graphene resonator to achieve an ultra-broadband, ultra-thin, and tunable metasurface, which has not only the high absorption rate of gold metamaterials but also the tunable properties of graphene metamaterials, compared with the traditional single gold-based and graphene-based methods. By simulating the response frequencies by the finite element method, we found that the absorber achieves a broadband 2.68-7.48 THz range (absorption>80%) absorbance with a relative bandwidth of 94.5%, where the center frequency is f c=5.08THz. The physical mechanism of the absorber was analyzed using the electric field of the surface plasmon resonance, due to the design structure presents periodic geometric symmetry, the polarization angle and large angle incidence both present better advantages. The dynamic tuning of the metamaterial absorber is achieved by changing the Fermi energy level of graphene by adjusting the bias voltage to effectively control the metamaterial absorption intensity and resonant frequency. Our proposed absorber has important application prospects in sensing, optical communication, detection, and optical devices.

Switchable and tunable terahertz metamaterial absorber with broadband and multi-band absorption.

In this paper, we propose and demonstrate a switchable terahertz metamaterial absorber with broadband and multi-band absorption based on a simple configuration of graphene and vanadium dioxide (VO2). The switchable functional characteristics of the absorber can be achieved by changing the phase transition property of VO2. When VO2 is insulating, the device acts as a broadband absorber with absorbance greater than 90% under normal incidence from 1.06 THz to 2.58 THz. The broadband absorber exhibits excellent absorption performance under a wide range of incident and polarization angles for TE and TM polarizations. Moreover, the absorption bandwidth and intensity of the absorber can be dynamically adjusted by changing the Fermi energy level of graphene. When VO2 is in the conducting state, the designed metamaterial device acts as a multi-band absorber with absorption frequencies at 1 THz, 2.45 THz, and 2.82 THz. The multi-band absorption is achieved owing to the fundamental resonant modes of the graphene ring sheet, VO2 hollow ring patch, and coupling interaction between them. Moreover, the multi-band absorber is insensitive to polarization and incident angles for TE and TM polarizations, and the three resonance frequencies can be reconfigured by changing the Fermi energy level of graphene. Our designed device exhibits the merits of bi-functionality and a simple configuration, which is very attractive for potential terahertz applications such as intelligent attenuators, reflectors, and spatial modulators.

Configurational Effects on Strain and Doping at Graphene-Silver Nanowire Interfaces

Graphene shows substrate-dependent physical and electronic properties. Here, we presented the interaction between single-layer graphene and silver nanowire (AgNW) in terms of physical straining and doping. We observed a snap-through event for single-layer graphene/AgNW at a separation of AgNWs of 55 nm, beyond the graphene suspended over the nanowires. The adhesion force between the Atomic Force Microscopy (AFM) tip apex and the suspended graphene was measured as higher than the conformed one by 1.8 nN. The presence of AgNW modulates the Fermi energy level of graphene and reduces the work function by 0.25 eV, which results in n-type doping. Consequently, a lateral p-n-p junction is formed with single AgNW. The correlation Raman plot between G-2D modes reveals the increment of strain in graphene of 0.05% due to the curvature around AgNW, and 0.01% when AgNW lies on the top of graphene. These results provide essential information in inspecting the physical and electronic influences from AgNW.

Terahertz bifunctional absorber based on a graphene-spacer-vanadium dioxide-spacer-metal configuration

A terahertz bifunctional absorber is presented with broadband and narrowband absorbing properties in a graphene-spacer-vanadium dioxide-spacer-metal configuration. Carrier relaxation time of graphene τ = 1.0ps (τ = 0.1ps) is chosen for narrowband (broadband) absorption. When vanadium dioxide is in the conducting state, the design behaves as a narrowband absorber, and it is composed of a square-shaped graphene, topas spacer, and metallic vanadium dioxide film. There is an absorption band with 100% absorptance at the frequency of 1.37 THz. Narrowband absorption is caused by the localized magnetic resonance. When vanadium dioxide is in the insulating state, the design behaves as a broadband absorber composed of a square-shaped graphene, topas layer, vanadium dioxide film, and metal film. It has a broadband absorption in the frequency range of 1.05-2.35 THz, and the corresponding absorptance is more than 90%. The merging of two resonances with overlapping region ensures broadband performance of the designed absorber. The working bandwidth and intensity of narrowband absorption and broadband absorption can be dynamically adjusted by changing the Fermi energy level of graphene. The influences of structure parameters are discussed on absorption performance. In addition, the designed absorber is not sensitive to incident angle. Because of the simple structure, our design can be applied to many promising fields in intelligent absorption and terahertz switch.