Fermi Level Of Graphene Research Articles

Terahertz multi-band absorber and dual-bandwidth polarization converter based on VO2 and graphene

A terahertz metamaterial with multiple functions is proposed based on patterned graphene, concentric gold rings and gold columns, and vanadium dioxide (VO2). By adjusting the temperature of VO2, it can be in metal state or insulator state, and correspondingly the metamaterial can work as a dual-bandwidth polarization converter or a multi-bandwidth absorber. When the metamaterial acts as a polarization converter, by adjusting the Fermi level of graphene, the converter can realize simultaneously x-polarized wave to y-polarized wave polarization conversion (CPC) and line-to-circular (LTC) polarization conversion (PC). In addition, by adjusting the Fermi level of graphene, the operating frequency and operating efficiency of the absorber and polarization converter can be dynamically tuned. By studying the electric field distribution and the surface current of the metamaterial, the physical mechanism of efficient absorption and dual-bandwidth polarization is explored. Moreover, the proposed multifunctional metamaterial is highly robust to incidence angle. It has potential applications in communication, spectroscopy, sensors, and physical detection.

Design of a Penta-Band Graphene-Based Terahertz Metamaterial Absorber with Fine Sensing Performance.

This paper presents a new theoretical proposal for a surface plasmon resonance (SPR) terahertz metamaterial absorber with five narrow absorption peaks. The overall structure comprises a sandwich stack consisting of a gold bottom layer, a silica medium, and a single-layer patterned graphene array on top. COMSOL simulation represents that the five absorption peaks under TE polarization are at fI = 1.99 THz (95.82%), fⅡ = 6.00 THz (98.47%), fⅢ = 7.37 THz (98.72%), fⅣ = 8.47 THz (99.87%), and fV = 9.38 THz (97.20%), respectively, which is almost consistent with the absorption performance under TM polarization. In contrast to noble metal absorbers, its absorption rates and resonance frequencies can be dynamically regulated by controlling the Fermi level and relaxation time of graphene. In addition, the device can maintain high absorptivity at 0~50° in TE polarization and 0~40° in TM polarization. The maximum refractive index sensitivity can reach SV = 1.75 THz/RIU, and the maximum figure of merit (FOM) can reach FOMV = 12.774 RIU-1. In conclusion, our design has the properties of dynamic tunability, polarization independence, wide-incident-angle absorption, and fine refractive index sensitivity. We believe that the device has potential applications in photodetectors, active optoelectronic devices, sensors, and other related fields.

Development of a terahertz metamaterial polarization converter with multiple tuning modes based on graphene and vanadium dioxide

A dynamically tunable terahertz metamaterial polarization converter is proposed. By manipulating the Fermi level of graphene and the conductivity of vanadium dioxide (VO2) flexibly, multiple tuning modes for polarization conversion performance are realized. First, while graphene is in the metallic or insulating state, we can achieve an ideal broadband reflective or an ideal broadband transmissive polarization converter respectively. By manipulating the conductivity of VO2, a modulation depth of 80 % for intensity of the reflected polarization conversion rate (PCRr) and that of 70 % for bandwidth of the transmissive polarization conversion rate (PCRt) can be achieved. Then, while VO2 is in the insulating state, we obtain another broadband transmissive polarization converter. Similarly, we can achieve PCRt intensity modulation by manipulating the Fermi level of graphene. While VO2 is in the metallic state, the device can switch from a broadband polarization converter to a narrow one as the Fermi level of graphene increases. Finally, by manipulating graphene and VO2 simultaneously, the central frequency modulation of PCRt can be obtained with the bandwidth keeping unchanged. Compared with the ones reported recently, our device has some advantages on tuning mode, relative bandwidth, and polarization conversion performance.

A broadband absorber with multiple tunable functions for terahertz band based on graphene and vanadium dioxide

A broadband perfect absorber tunable in terahertz band based on vanadium dioxide (VO2) and graphene metamaterial is proposed. We have carried out numerical simulations using Finite-Difference Time-Domain (FDTD) method. This structure can tune the absorption rate of VO2 by changing its conductivity. Moreover, due to the addition of graphene, the absorption is improved and the bandwidth reaches 3.24 THz. The structure allows spectral shift by varying the Fermi level of graphene, which can achieve >90 % ultrawideband absorption in 3.74 to 6.96 THz range when the Fermi level is 0.8 eV. In the range of 4.338 to 6.332 THz, nearly 100 % absorption can be achieved. The proposed absorber also exhibits good polarization insensitivity and wide angle of incident stability. Our design has potential applications in terahertz spectroscopy and imaging, also can be applied to modulators, sensors, stealth and photoelectric switches.

Quadruple Plasmon-Induced Transparency and Dynamic Tuning Based on Bilayer Graphene Terahertz Metamaterial.

This study proposes a terahertz metamaterial structure composed of a silicon-graphene-silicon sandwich, aiming to achieve quadruple plasmon-induced transparency (PIT). This phenomenon arises from the interaction coupling of bright-dark modes within the structure. The results obtained from the coupled mode theory (CMT) calculations align with the simulations ones using the finite difference time domain (FDTD) method. Based on the electric field distributions at the resonant frequencies of the five bright modes, it is found that the energy localizations of the original five bright modes undergo diffusion and transfer under the influence of the dark mode. Additionally, the impact of the Fermi level of graphene on the transmission spectrum is discussed. The results reveal that the modulation depths (MDs) of 94.0%, 92.48%, 93.54%, 96.54%, 97.51%, 92.86%, 94.82%, and 88.20%, with corresponding insertion losses (ILs) of 0.52 dB, 0.98 dB, 1.37 dB, 0.70 dB, 0.43 dB, 0.63 dB, 0.16 dB, and 0.17 dB at the specific frequencies, are obtained, achieving multiple switching effects. This model holds significant potential for applications in versatile modulators and optical switches in the terahertz range.

Enhancing device efficiency using a reconfigurable terahertz wavefront modulator based on fermi level of graphene

This study investigates a Fermi-level-reconfigurable terahertz wavefront modulator composed of a hybrid material comprising graphene, silver metal, and silicon dioxide. Through theoretical simulation, analysis, and experimental investigations, it is found that graphene plays a crucial role in enhancing the performance of the modulator. Specifically, the presence of graphene increases the limiting factor of the modulator by 0.281, reduces the normalized modal field area by 0.15, decreases the transmission loss, and lowers the gain threshold by 30097 cm−1, thereby improving the efficiency, reliability, and stability of the device. The silicon dioxide and silver metal in the structure act as the primary reflective materials, achieving the effect of a metallic lens and validating the modulator's capability for efficient defocusing. Further investigations reveal that the carrier concentration of graphene can be easily controlled, allowing for manipulation of its physical properties through Fermi-level modulation. When incident light passes through the graphene array, it induces phase variations, altering the transmission path of light and thereby affecting the position of the defocused focal point, enabling dynamic adjustability. Under low Fermi-level control, the modulator achieves a 2πphase adjustment range with an average reflectivity of 75%. Thus, this provides potential for practical applications of more dynamic and tunable microdevices, particularly in the field of laser cutting technology and etching processes.

Dependence of Electrical Charge Transport on the Voltage Applied across Metal–Graphene–Metal Stack under Fixed Compressing Force

While charge transport in the horizontal plane of graphene has been widely studied, there is only limited understanding about the transport across a stack of films that include graphene sheets. In this report, a model of a metal–graphene–metal stack was produced and investigated via detailed analysis of experimental dependences of electrical current on applied external voltage. Scanning probe microscopy (SPM) was used to measure the dependences of the local tunneling current on the voltage under fixed compressing force. The SPM platinum probe produced the compressing force on gold-supported graphene in the metal–graphene–metal system. The experimental results were explained by a model that included the pinning of the Fermi level of graphene to platinum and the related changes in the parameters of the potential barrier for the electron flow. It was demonstrated that low-voltage and high-voltage intervals can be identified in the charge transport across the metal–graphene–metal stack. In the high-voltage interval (approximately > |±0.7| V in the tested stack), the history of the current measurement was detected due to the charge accumulation. In the low-voltage interval, the current was determined by the electronic states near the Fermi level. In this interval, the graphene layer can function as a blocking gate for the electron transport in the metal–graphene–metal system.

Tunable transparent terahertz absorber for sensing and radiation warming

The lack in efficient functional devices has hindered plenty practical applications in terahertz (THz) band. In this work, we developed transparent THz absorber by employing trilayer Salisbury screen structure. Monolayer graphene and indium tin oxide (ITO) glass were spaced by a thin dielectric film, forming a transparent THz absorber. Assisted by ion gel, the carrier density of graphene layer can be effectively tuned through small external electrical gating (−2.5 V-2.5 V), with a modulation depth reaching to 100%. Taking advantage of the strong π−π interaction between graphene and external molecules containing π electrons, our fabricated transparent THz absorber enabled diphenylamine (DPA) detection. The adsorbed DPA molecule moves Fermi level of graphene toward Dirac point, performing a limit of detection of 5.5 ng. Moreover, by transferring the absorbed radiation into heat, our fabricated absorber can serve as a radiation warming device. The surface temperature of transparent THz absorber is 0.7 °C higher than that of glass in IR radiation environment. Note that the absorber is highly transparent in visible band, it will not prevent photosynthesis process of plants. Thereby, the THz absorbers have potential practical applications in diverse fields including wave manipulation, molecule sensing, and radiation warming.

Tunable couplings between location-insensitive emitters mediated by an epsilon-near-zero plasmonic waveguide.

This work demonstrates the efficient tuning of incoherent and coherent coupling between emitters embedded in an epsilon-near-zero (ENZ) waveguide coated with a multilayer graphene. As a result, a tunable two-qubit quantum phase gate based on the ENZ waveguide is realized at the cutoff frequency. Furthermore, due to the vanishingly small permittivity of the ENZ waveguide, all incoherent coupling between any two identical emitters located in the central area of the slit approaches a maximum, enabling near-ideal bipartite and multipartite entanglement. The coherent coupling between emitters is much larger at an operating frequency far from the ENZ resonance frequency than at the cutoff frequency, and the coherent coupling and resulting energy transfer efficiency can also be effectively tuned by the Fermi level of graphene. These results demonstrate an efficiently tunable electro-optical platform for quantum devices.

Introduction of non-symmetric inserts into a symmetric split-ring resonator: design of a bifunctional metasurface for linear polarization conversion and circular dichroism

Metasurfaces have shown their versatile capabilities in light-field shaping. To further pursue dense integration and miniaturization in photonics, a combination of multiple diversified functionalities into a metasurface is a promising solution. Recent bifunctional metasurfaces have relied on meta-atom superposition and tunable material introduction. The former supports simultaneous multi-functions, while the latter provides flexible adjustment. To achieve simultaneous and tunable multi-functions using a simple structure, based on a split-ring resonator metasurface with the linear polarization modulation function, here, we additionally introduced resonance to induce anti-symmetric polarization absorption for circular polarization modulation. As a proof-of-concept, we propose a bifunctional THz metasurface that combines linear polarization conversion and circular dichroism for polarization control and detection applications. Moreover, by changing the Fermi levels of graphene, both the frequency ranges of linear polarization conversion and circular dichroism can be adjusted. This work provides a reference to photonics integration related to polarization engineering and other distinct functionalities.

High energy region formation with increased electron density located on the MoS2 layer within MoS2/Gr heterostructure induced by C60 decoration

MoS2/graphene (MoS2/Gr) heterostructure shows great potential in various applications due to its unique and complementary chemical and physical properties. However, high resistances of contact interfaces between the MoS2 and graphene lead to low performance of such heterostructure-based electronic devices. Doping strategy with C60 physically adsorbed on the surface of MoS2/Gr was explored theoretically here for the aim to tune the electron injection barrier of the MoS2/Gr. Calculated results indicate that the energy levels of the frontier orbitals of MoS2 and the Fermi level of graphene can be modulated effectively due to the increased interfacial interaction energy by doping with C60, which leads to a reduced electron injection barrier (by about 0.2 eV) from the Fermi level of graphene to the conduction band minimum of the MoS2. Therefore, it is predicted that C60 is an effective dopant to decrease the electron injection barrier and promote the photogenerated electron-hole separation in the MoS2/Gr. More importantly, a high energy region with increased electron density located on the MoS2 within in heterostructure was formed by C60 doping strategy. Our results indicate that C60 doping is an effective strategy to improve the electronic properties of MoS2/Gr, and then optimize the performance of such heterogeneous nanocomposites-based devices.

Multifunctional metasurface for broadband absorption and polarization conversion based on graphene-VO2

A multifunctional metasurface based on graphene and vanadium dioxide (VO2), which can function as a broadband absorber, half-wave plate (HWP), and quarter-wave plate (QWP) in the terahertz frequency band, is designed in this study. When VO2 is in the metal phase, the multifunctional metasurface operated as a broadband absorber in the range of 0.89–2.36 THz. The designed multifunctional metasurface can switch between HWP and QWP by adjusting the Fermi energy of graphene through voltage biasing when VO2 is in the insulated phase. When the Fermi level of graphene is 0 eV, the designed structure acts as an HWP to achieve linear-to-linear (LTL) polarization conversion and preserve the reflection chirality of circular polarization at 0.68–2.64 THz. When Ef = 0.3 eV, the designed structure functions as a QWP in 0.89–2.51 THz to realize the interconversion of linear polarization and circular polarization. Furthermore, the effects of the polarization and incidence angles on the operating performance are discussed. This study demonstrates that the multifunctional metasurface has promising applications in terahertz optical switches, electromagnetic stealth, modulators, and communication systems.

Optimized unidirectional and angle-insensitive ultra-broadband absorber based on layered graphene photonic structure

Using a chaotic gray wolf optimization algorithm to drive the transfer matrix method, a nonreciprocal and angle-insensitive absorber with ultra-wide bandwidth absorption is realized in a layered photonic structure comprised of graphene, which is spliced of a periodic sequence and a quasi-periodic sequence on a normalized scale. Compared to the periodic sequence, the proposed cascading construction has more preponderance in terms of bandwidth and angular stability. The surface conductivity of the graphene layer, which manipulates the absorption effect, can be modulated by the Fermi level, causing the characteristics of the proposed absorber tuned. The Fermi level of graphene, dielectric constants, and thicknesses of ordinary media, are optimized by the chaotic gray wolf algorithm, contributing to the decent improvement of absorption with a relative bandwidth of up to 127 %. Most importantly, in the 1.5–4.5 THz, the angular stability reaches 86° under the TM mode and 70° in TE one. The impacts of sequence number N of periodic sequence and M of quasi-periodic sequence on absorption in both angle and frequency domains are investigated. Additionally, the impedance match theory is introduced to express excellent absorption performance.

Tuning of graphene plasmons by ferroelectric superdomain for mid-infrared photodetector with high responsivity

A new strategy is demonstrated for confining graphene plasmons to resonantly enhance light–matter interactions for tunable mid-IR detection. Our devices consist of integrating monolayer graphene without patterning onto a nanoribbon-connected ring-shaped ferroelectric superdomain with alternately up- and down-polarization. The simulations show that our devices have a tunable spectral response from 11.7 to 19.5 μm by both reconfiguring the ferroelectric superdomain and varying the ferroelectric-gated graphene Fermi level. A highest photoresponsivity of 796–947 A W−1 has been achieved in 10–20 μm. The proof-of-concept photodetector offers the possibility to simplify the fabrication of plasmonic devices and helps the development of applications of tunable mid-IR detection.

Work function tuning of directly grown graphene via ultraviolet–ozone treatment for electrode application in organic photovoltaic devices

Graphene is an efficient electrode material for flexible optoelectronic devices because of its high transmittance and electrical conductivity. A graphene film directly grown using plasma-enhanced chemical vapor deposition is suitable for electrode applications because it can be prepared on a glass substrate at relatively low temperatures without a transfer process from a metal catalyst. However, to obtain efficient optoelectronic devices using graphene electrodes, surface modification is necessary. Herein, we tuned the work function of a graphene electrode using ultraviolet (UV)-O3 treatment. The power conversion efficiency of organic photovoltaic devices was significantly increased by the UV-O3 treatment of the graphene anode. To investigate the origin of the improved device performance, the interface between the graphene anode and tetraphenyldibenzoperiflanthene (DBP) donor layer was analyzed via in situ ultraviolet photoelectron spectroscopy. The energy offset between the Fermi level of graphene and the highest occupied molecular orbital level of DBP was significantly reduced because of the increased work function. The results indicate that the electronic structure modification of graphene is of significant importance for fabricating efficient optoelectronic devices.

Multifunctional and dynamically tunable coherent perfect absorber based on InSb and graphene metasurface

We propose a temperature-tunable and dual-broadband switchable coherent perfect absorber (CPA) composed of an annulus-disk-shaped InSb pattern and a complementary graphene layer. By controlling the Fermi level of graphene, the temperature of InSb, or the relative phase of incident beams, the working frequency and absorption of the CPA can be continuously adjusted. For a Fermi energy of 0 eV, the dual-band coherent perfect absorptivities exceed 99%. The designed structure achieves broadband absorptance of over 90% in the frequency range of 0.145 THz to 1.24 THz, with a relative bandwidth of about 158.12% for a Fermi energy of 0.5 eV. Moreover, the temperature-dependent permittivity of InSb enables thermal tuning of the dual-band CPA in the THz region. Furthermore, by altering the relative phase of input beams, the absorptivity can be continuously varied between less than 1.2% and more than 90% in the dual-band and broadband modes. Additionally, the designed CPA exhibits insensitivity to the polarization angle within the range of 0° to 90°. Therefore, this multifunctional CPA holds significant potential for applications in modulation, sensing, all-optical switches, and coherent photodetectors.

Super-broadband terahertz absorber: an optimized and magnetized graphene-embedded 1D disordered photonic system

In this study, a broadband terahertz absorber was designed and numerically demonstrated. The optical features were computed using 4×4 transfer matrix formalism. The broadband absorption is attained by optimizing the Fermi levels of graphene, the magnetic field, and the thickness of the dielectric layers using the differential evolution algorithm. The results demonstrated that the WMF-optimized scenario offered greater than 90% absorption over a bandwidth of 4.18 THz, and the WoMF&SL scenario provided the shortest bandwidth of 0.89 THz. These findings reveal the significance of the spacer layer to achieve broad absorption. Moreover, the absorption band is tailored to the required spectral range by careful choice of the structural and electrical parameters of the spacer layer; changing the refractive index to 1.2 offers a bandwidth of 4.42 THz and altering the thickness to 12 µm provides a bandwidth of 5.5 THz. The broadband absorption was attained due to the impedance matching provided by the optimized structure over a wide spectral range. Furthermore, the average absorption and bandwidth were enhanced, while fluctuations in the selected absorption band were minimized by engineering the magnetic biasing. The reported super-broadband absorber offers numerous applications in terahertz imaging, terahertz communications, and photodetectors.

Bifunctional metasurface for high-efficiency terahertz absorption and polarization conversion.

A reconfigurable metasurface with a switchable function, broad band, high efficiency, and ultra-compact size is crucial for the development of efficient and compact devices. We propose a bifunctional metasurface that utilizes vanadium dioxide (V O 2) and graphene to achieve high-efficiency absorption and polarization conversion (PC) in the terahertz (THz) range. In our design, an extra dielectric layer is added on the top of V O 2 and graphene. It is worth pointing out that the presence of the additional dielectric layer greatly enhances the coupling of the wave in the Fabry-Perot cavity, resulting in remarkable improvement in absorption and PC efficiency. Furthermore, by controlling the working state of V O 2 and graphene, the functionality of the metasurface can be flexibly switched among absorption, cross-polarized conversion, and linear-to-circular PC (LTC). Simulation results indicate that the metasurface works in the absorption mode when V O 2 is in a metal state, and it can efficiently absorb THz waves at 2.0-7.0THz with a remarkable relative bandwidth of 111.1%. Furthermore, the absorption is over 98.4% under a normal incident case and still maintains over 90% with an incident angle of 50° at 2.8-7.0THz. Importantly, by changing the conductivity of V O 2, the absorption can be flexibly adjusted, allowing for tuning the absorption between 10% and 98.4%. When V O 2 is in an insulator state, the function of the designed metasurface is altered to PC mode, and it can efficiently convert incident linearly polarized (LP) waves into cross-polarized waves with a PC ratio exceeding 95% at 1.8-3.4THz when the Fermi level of graphene is 1eV. When switched to the LTC mode, it can convert incident LP waves into right-circularly polarized waves with ellipticity less than -0.95 at 1.7-2.1THz and into left-circularly polarized waves with ellipticity greater than 0.90 at 2.7-3.0THz when the Fermi level of graphene is 0.55eV.

Potassium-doped nano graphene as an intermediate layer for graphene electronics

To suppress the intrinsic carrier density and increase the carrier mobility in graphene on a silicon dioxide (SiO2) substrate, potassium (K)-doped nano graphene was introduced as an intermediate layer between the graphene layer and SiO2 substrate. Back-gate type graphene field effect transistors with four terminal structures were fabricated, and their electrical properties were measured under vacuum. The results showed that the Dirac point shifted from +9.0 to −0.2 V after inserting the K-doped nano graphene. The results suggested that inserting the intermediate layer compensated for the intrinsic holes and achieved an electron doping of 2 × 1012 cm−2. The field-effect mobilities of electrons and holes also increased because the ionized K-atoms in the intermediate layer shielded the electric force from the negatively charged impurities in SiO2. The K density was estimated using x-ray photoelectron spectroscopy to be 1.49 × 1013 cm−2, and the C1s peak shifted by 0.2 eV, which confirms the upward modulation of the graphene Fermi level by the K-doped nano graphene intermediate layer. These results demonstrated the advantages of the intermediate layer on the carrier density and mobility in graphene.

Electronic structure of graphene thin films under a perpendicular electric field

Based on density functional theory (DFT), we investigated the electronic structure of multilayer graphene under the perpendicular electric field. DFT calculations revealed that the number of layers and their stacking arrangement play a crucial role in determining the electronic structure of multilayer graphene under the perpendicular electric field. When an electric field is applied, multilayer graphene with AB stacking exhibits metallic or semi-metallic properties depending on whether the number of layers is odd or even, respectively. Multilayer graphene with ABC stacking under the electric field is a semiconductor that possesses a flat band at the valence- and conduction-band edges, irrespective of the number of layers. The electronic structure near the Fermi level of multilayer graphene with AA stacking is insensitive to the external electric field.