Fermi Level Of Graphene Research Articles

Graphene‐Based Angle‐Insensitive and Tunable Single‐Band and Dual‐Band Metamaterial Terahertz Absorber

To solve the problem of incident and controllable characteristics of the multi‐band terahertz absorber, a kind of graphene‐based metamaterial angle‐insensitive and tunable terahertz absorber is proposed. The proposed absorber has single‐band and dual‐band absorption function, and profile parameters are presented by single‐groove and dual‐groove arrays, separately. The results show that the absorber has an absorption peak of 99.30% at 16.0 THz. In addition, the absorption of the absorber can reach 94.56% and 99.11% at 11.4 THz and 26.2 THz. Electromagnetic field energy distribution and the real and imaginary parts of the graphene surface conductivity are analyzed by the finite‐element method as a function of the Fermi level and the terahertz frequency. Moreover, compared with a reported terahertz absorber, the graphene‐based absorber is angle insensitive and tunable by adjusting the Fermi levels of graphene. It is believed that the proposed absorber has great potential in the fields of time‐domain spectroscopy technology and imaging technology.

Photoactive Control of Surface-Enhanced Raman Scattering with Reduced Graphene Oxide in Gas Atmosphere.

Surface-enhanced Raman scattering (SERS) is an ultrahigh sensitive detection technique for a variety of research fields. Both electromagnetic and chemical enhancement mechanisms are generally considered to contribute simultaneously to SERS signals. However, it is difficult to actively control the enhancement of SERS signals after the substrate is fabricated, since tuning one or both of the aforementioned enhancement mechanisms remains an experimental challenge. Here, we propose a method for actively implementing the photoinduced modulation of SERS signals, which is that under UV irradiation, the Fermi level of graphene can be dynamically modulated due to the adsorption and desorption of gas molecules. The method is validated in gas atmospheres of O2, CO2, N2, and air and also demonstrate its generality by different analytes. In addition, the method was successfully applied to the trace detection of pesticides on fruit peels in air environment, which show its practical implications in sensing.

Fano resonance of optical phonons in a multilayer graphene stack

We studied Fano interference between the Raman spectrum of G-band phonons and electron continuum in a multilayer graphene stack. The thickness and power dependencies of the Fano interference coefficient ∣1/q∣ in the G-mode, where q is the Fano asymmetry parameter, were spatially visualized and analyzed using the Gaussian-convoluted Breit–Wigner–Fano function. The estimated ∣1/q∣ decreases with an increase in the layer number and laser power in the low-power region at least for monolayer, bilayer, and trilayer graphene. In the higher-power region, ∣1/q∣ increases with power only for monolayer graphene. The observed behaviors of ∣1/q∣ reflect the phase difference of Raman signals from the electron continuum and G-band and possibly originate from changes in the electronic relaxation time and the Fermi level of graphene owing to the laser heating of the sample.

A tailored ultra-broadband electromagnetically induced transparency metamaterial based on graphene

Based on graphene, an ultra-broadband electromagnetically induced transparency (EIT) window with dynamic tunability is realized in theory. Through altering the Fermi level of graphene that can be regulated by the external voltage, the EIT window and the EIT effect, especially the slow-wave effect, can be easily adjusted. Moreover, the bandwidth of the EIT window can be changed by the incidence angle, achieving the transformation from broadband to narrowband. At the same time, by discussing the polarization state and loss index, the characteristics of polarization insensitivity and low loss are proved. Additionally, the influences of other parameters are discussed, such as the relaxation time of graphene and coupling distance. These unique features enable the designed EIT metamaterial to be masterly applied to optical switches, optical modulators, and slow-light devices.

Substrate-assisted Fermi level shifting of CVD graphene by swift heavy ions

The systematic modulation of surface electronic properties of graphene can be achieved through surface and interface engineering using swift heavy ion (SHI) irradiation. The effects of dense electronic excitation on the optical and surface electronic properties of chemical vapor deposited (CVD) graphene sheets are investigated in this work. The spectroscopic and microscopic results indicate that the sputtered substrate atoms are trapped in the graphene sheet during irradiation and alter the Fermi level of graphene. The change in the Fermi level calculated from the Raman spectra is investigated through the surface potential measurement by scanning Kelvin probe microscope (SKPM). The shifting of the Fermi level towards the valence band verifies the hole-doping of the graphene sheet by sputtered substrate atoms. The mechanism of ion interaction and its subsequent effects on the graphene and substrate material are further studied rigorously through Monte Carlo simulations using stopping and range of ions in matter (SRIM) and Surface Sputtering sub-routine of transport of ions in matter (TRIM) packages. This work offers a novel way of analyzing defect-induced modifications in the electronic properties of graphene by Raman spectroscopic features in combination with SKPM.

Graphene-based terahertz metamirror with wavefront reconfiguration

As an emerging material, graphene has been widely applied in the field of active metasurface. Recently, researchers employed graphene to achieve dynamical control of electromagnetic wavefront. In this work, graphene-based reflective metasurface is presented to realize dynamical wavefront reconfiguration for terahertz wave. Using a hybrid structure of cross-shaped graphene and metal patch, the designed metasurface has 360° phase modulation capability. Its wavefront is reconfigurable and can realize multiple functions. In order to verify this, three examples are designed to demonstrate the phenomenon of wavefront reconstruction. They are gradient metasurface, vortex beam generator, and focusing mirror, respectively. First of all, Fermi level of graphene is used to reconstruct the reflected wavefront of gradient metasurface, and then realize switching between positive and negative reflections. Secondly, a vortex beam generator is implemented, and it can reconstruct the mode number of orbital angular momentum through Fermi level. Finally, a reflective lens is proposed and verified, whose focus can appear or disappear with the tuning of Fermi level. The proposed functions have potential applications in the fields of terahertz switching, communication, and focusing.

Investigation of strontium titanate spherical shell supported terahertz all-dielectric metamaterials

Based on the strontium titanate ( S r T i O 3 , STO) spherical shell all-dielectric metamaterials (ADMs) structure, the tunable propagation characteristics have been systematically studied, taking into account the effects of structural geometrical parameter, temperature, and graphene Fermi level. The results indicate that STO-based ADMs exhibit excellent thermally and electrically tunable performances, e.g., when the temperature rises from 77 to 400 K, the modulation depths (MDs) of resonant frequency and amplitude of the reflection dip reach 62% and 93%, and the absorption is tunable in the range of 0.7995–0.9017. Additionally, when the Fermi level of graphene changes from 0.1 to 1.0 eV, the amplitude MD of the reflection dip reaches 99.89%, and the absorption of the STO-based ADM increases from 0.8352 to 0.9953. These results provide a guide for understanding the resonant mechanism of STO-based ADMs and designing multifunctional terahertz devices.

Temporally modulated metamaterial based on a multilayer graphene structure

The recently developed concept of time-varying metamaterials paves the way for advanced methods of electromagnetic property modulation, such as magnet-free isolators, frequency converters, and other nonreciprocal photonic components. Here, we analyze the metamaterial based on the periodically arranged temporally modulated graphene layers via a steplike graphene Fermi level. The material properties are described by the effective medium theory. The electromagnetic wave temporal scattering phenomenon caused by rapid steplike graphene Fermi level switching is considered. It is demonstrated that the frequency of scattered waves increases as the Fermi level grows rapidly and vice versa. The relation between the amplitudes of forward- and backward-scattered waves is found using the Laplace transform technique. The proposed concept allows us to efficiently generate frequency harmonics under instant switching of the graphene Fermi level at a few meV. Our investigation provides a basement for the realization of temporally modulated multilayer structures based on functional tunable materials as power-efficient frequency converters.

Polarization-Driven Asymmetric Electronic Response of Monolayer Graphene to Polymer Zwitterions Probed from Both Sides.

We investigated the nature of graphene surface doping by zwitterionic polymers and the implications of weak in-plane and strong through-plane screening using a novel sample geometry that allows direct access to either the graphene or the polymer side of a graphene/polymer interface. Using both Kelvin probe and electrostatic force microscopies, we observed a significant upshift in the Fermi level in graphene of ∼260 meV that was dominated by a change in polarizability rather than pure charge transfer with the organic overlayer. This physical picture is supported by density functional theory (DFT) calculations, which describe a redistribution of charge in graphene in response to the dipoles of the adsorbed zwitterionic moieties, analogous to a local DC Stark effect. Strong metallic-like screening of the adsorbed dipoles was observed by employing an inverted geometry, an effect identified by DFT to arise from a strongly asymmetric redistribution of charge confined to the side of graphene proximal to the zwitterion dipoles. Transport measurements confirm n-type doping with no significant impact on carrier mobility, thus demonstrating a route to desirable electronic properties in devices that combine graphene with lithographically patterned polymers.

Investigation of terahertz high Q-factor of all-dielectric metamaterials

We investigated the propagation properties of all-dielectric metamaterials (ADMs) based on a SiO2-Si asymmetric hybrid block, including the effects of structural parameters, asymmetrical degrees, carrier doping concentrations, and graphene Fermi levels. The Q-factor of Fano resonance reaches more than 270, and the amplitude modulation depth (MD) is about 75% if the asymmetric degree changes in the range of 2–10 μm. The carrier concentration of silicon significantly affects the intensity of excited Fano resonance. When the carrier concentration of Si is 1 × 1014 cm−3, the excited Fano resonance is the strongest, and the transmission peak is about 0.92. With the help of a uniform graphene layer, the Fano resonance can be effectively modulated, the frequency MD is about 20% if the Fermi level changes in the scope of 0.01–0.3 eV. These results can help us to design THz high Q-factor devices, such as sensors, filters, and modulators.

Synergy between Fermi Level of Graphene and Morphology of Polymer Film Allows Broadband or Wavelength‐Sensitive Photodetection

AbstractDeliberately tuning the photodetecting performances of polymer/graphene (G) hybrid films through a simple approach and understanding its underlying mechanisms are of significance for developing related devices. Herein, photodetecting features of polymer/G films are successfully tuned from broadband to IR/UV sensitive through a simple thermal treatment, providing optional working modes for varied demands. The morphologies and electronic/photodetecting properties of polymer/G hybrids are characterized in detail through atomic force microscopy, scanning electron microscopy, grazing‐incident wide angle X‐ray scattering, field effect transistor devices, and density functional theory calculations. The results suggest that the synergetic effect between Fermi level of G and morphologies of polymer film is responsible for the altered sensitivities to different radiations. The former factor shifts the energy barrier of charge carrier transferring for photocurrent generation, while the latter modifies the absorption capabilities of polymer films to the specific lights. Thus, in this work, besides proposing an in‐principle multimode photodetecting device for different applications, the intrinsic mechanisms behind the performance modifications and the robust capabilities of simple thermal treatments on tuning activities of polymer/G devices are revealed as well, which provide valuable supports for developing efficient photosensing devices in a facile and rational approach.

Direct modulation of microwave by visible light based on graphene microstrip patch antenna

A method for modulating microwave directly by visible light based on a graphene microstrip patch antenna is proposed. In the antenna, the microwave radiation is modulated with the variation of visible light intensity when both are interacting with the graphene layer. Simulation results indicate that the S11 coefficient of the microwave radiation can change by 20 dB if the Fermi level of graphene varies from 0.1 eV to 0.5 eV when it is radiated by the antenna. With normal incidence, the experimentally measured microwave modulation depth of the S11 coefficient is 1.3 dB when the intensity of the modulation light is 100 mW at the wavelength of 532 nm. The result indicates that the graphene microstrip patch antenna is capable of converting optical intensity signal to microwave intensity signal directly.

All-optical graphene-on-silicon slot waveguide modulator based on graphene’s Kerr effect

All-optical graphene-based optical modulators have recently attracted much attention because of their ultrafast and broadband response characteristics (bandwidth larger than 100 GHz) in comparison with the previous graphene-based optical modulators, which are electrically tuned via the graphene Fermi level. Silicon photonics has some benefits such as low cost and high compatibility with CMOS design and manufacturing technology. On the other hand, graphene has a unique large nonlinear Kerr coefficient, which we calculate using graphene's tight-binding model based on the semiconductor Bloch equations. Its real and imaginary parts are negative at the wavelength of 1.55 µm and EF=0.1eV. To simultaneously use the benefits mentioned above, we present an all-optical, CMOS-compatible, and graphene-on-silicon slot (GOSS) waveguide extinction and phase modulator that consists of two different geometries. The first one consists of a one-stage GOSS waveguide with a single layer of graphene. To increase the light-graphene interaction and consequently enhance the modulation efficiency (ME), another stage of the GOSS waveguide is placed over the first one. This two-stage configuration is called a graphene-on-silicon double-slot (GOSDS) waveguide. The ME, insertion loss (IL), and modulation depth (MD) for a 12.5 µm GOSDS waveguide modulator with a double layer of graphene can reach 0.241 dB/µm, 1.31 dB, and 77%, respectively, at optical pump intensities about 9MWcm-2. Our design has a smaller waveguide length (17.6 times) than the previous all-optical graphene-on-silicon ribbon waveguide extinction modulator and high MD (about 2 times) in comparison with a graphene-clad microfiber all-optical extinction modulator. Compared with an all-optical Mach-Zehnder interferometer phase modulator, our design has short graphene coated waveguide length (≈0.1 times) and low local optical intensities (≈0.043 times) needed for π phase shift. This study may promote the design and realization of high-performance, wideband, compact, and all-optical control on a single chip with a reasonable contrast level.

Simultaneous enhancement of specific capacitance and potential window of graphene-based electric double-layer capacitors using ferroelectric polymers

Despite their high power density, aqueous-based electric double-layer capacitors (EDLCs) possess relatively low energy density due to the limitation of potential window (~1.0 V) and low specific capacitance of active materials. To increase the energy density without sacrificing the power density, it is highly desired to achieve a simultaneous improvement of both specific capacitance and potential window of EDLCs. However, this remains a major challenge that is not been solved up to date. This work demonstrates that by inserting a polarized-polyvinylidene fluoride (PVDF) ferroelectric layer underneath the graphene, a simultaneous enhancement of both areal capacitance and potential window is achieved, in which the areal capacitance increases from 5.5 to 7.5 μF/cm 2 ( i.e. , 36%) (or 55 F/cm 3 to 75 F/cm 3 in terms of volumetric capacitance), and the potential window expands from 1.0 V to 1.5 V. This results in a threefold increase in the areal energy density of the capacitor. The enhancement in capacitance can be explained by the Gouy–Chapman–Stern model. The widening of potential window is due to the shift of the Fermi level of graphene caused by the doping effect of the polarized-PVDF layer. • Graphene capacitor is designed with polarized PVDF. • Increases the areal capacitance and potential window of graphene capacitors. • The areal capacitance increases 36% compared to without polarized PVDF. • The potential window increases from 1.0 V to 1.5 V.

Dynamically switchable dual-band absorber based on electromagnetically induced reflection in metal-graphene hybrid metamaterial

In this paper, a dynamically switchable dual-band absorber based on electromagnetically induced reflection (EIR) in metal-graphene hybrid metamaterial is proposed. The unit cell of metamaterial absorber has three layers. The top layer consists of two split-ring resonators (SRRs) and two orthogonal cut-wires (CWs) with different lengths. Due to the low reflection dips in EIR structure and good absorption of graphene, two absorption bands accompanied with more than 90% absorptions are obtained under y-polarized and x-polarized incident waves, respectively. The power loss densities at the absorption points clearly reveal the absorption positions of the metamaterial absorber. In addition, the electric field, magnetic field and surface current distributions explain the reason of high absorption. According to the surface currents at four resonant points, we draw the schematic diagrams of multipole moments, which are confirmed by multipole scattering theory. The tunable absorption performance of metamaterial absorber can be observed by tuning the Fermi level of graphene. The metamaterial absorber exhibits excellent wide polarization angle behavior and shows great prospects in multi-band metamaterial absorber under oblique incidence. The proposed metamaterial absorber expands the development of metamaterial and shows great potential for multifunctional metamaterial absorber.

Actively tunable multi-band terahertz perfect absorber due to the hybrid strong coupling in the multilayer structure.

In this work, we propose a multi-band terahertz perfect absorber employing the topological photonic crystal combined with VO2 and graphene. The hybrid strong coupling among the topological photonic state, the Tamm plasmon polaritons excited around the interfaces of VO2 and graphene results in the three perfect absorption bands. Benefiting from the reversible insulator-metal phase transition of VO2 and the tunable Fermi level of graphene, it can actively switch among no absorption, single-band, dual-band and multi-band absorptions around 1THz, with the absorption frequencies tunable as well. Besides, the absorption bands are sensitive to the incident angle in almost the same dispersion rate, with high absorptions in a large angle range. Moreover, the splitting frequencies between the adjacent absorption peaks strongly depend on the pair number of the alternating multilayers. Apart from the three absorption bands, there are still many absorption peaks in the large frequency range resulting from the standing waves, including other 7 peaks above 0.9 between 0.83THz and 1.55THz. Such a tunable multi-band absorber with multiple modulation methods may find extended applications in active integrated terahertz devices.

Dual dynamically tunable plasmon-induced transparency and absorption in I-type-graphene-based slow-light metamaterial with rectangular defect

A simple quasi-continuous I-type-graphene-based metamaterial with rectangular defect is proposed to realize a dual dynamically tunable plasmon-induced transparency (PIT) and plasmon-induced absorption (PIA) effect. The desired Fermi levels of the monolayer graphene can be dynamically tuned by applying the bias voltage, which can produce the spectral shift of the dual PIT and PIA. What’s interesting is that the dual PIT (PIA) can be evolved into the single PIT (PIA) by tuning the distance between two vertical graphene ribbons in the metamaterial. In addition, the distribution intensity and directions of electric field vectors are used to illustrate the distributions of electric field magnitude in the monolayer graphene metamaterial under the action of the polarized incident light. Both theoretical analysis and numerical simulation methods are introduced to study the mechanism of the PIT and PIA effect at terahertz frequency range, and they display very good consistency. Moreover, the surface plasmon polaritons (SPPs) of the monolayer graphene have a better dispersion property and a broad frequency range of the group delay, the maximum group delay of about 0.25 ps is obtained in the proposed metamaterial structure. The proposed metamaterial may have extensive applications in absorbers, tunable switches and slow light devices, et al.

Active Electromagnetically Induced Transparency Effect in Graphene-Dielectric Hybrid Metamaterial and Its High-Performance Sensor Application.

Electromagnetically induced transparency (EIT) based on dielectric metamaterials has attracted attentions in recent years because of its functional manipulation of electromagnetic waves and high refractive index sensitivity, such as high transmission, sharp phase change, and large group delay, etc. In this paper, an active controlled EIT effect based on a graphene-dielectric hybrid metamaterial is proposed in the near infrared region. By changing the Fermi level of the top-covered graphene, a dynamic EIT effect with a high quality factor (Q-factor) is realized, which exhibits a tunable, slow, light performance with a maximum group index of 2500. Another intriguing characteristic of the EIT effect is its high refractive index sensitivity. In the graphene-covered metamaterial, the refractive index sensitivity is simulated as high as 411 nm/RIU and the figure-of-merit (FOM) is up to 159, which outperforms the metastructure without graphene. Therefore, the proposed graphene-covered dielectric metamaterial presents an active EIT effect in the near infrared region, which highlights its great application potential in deep optical switching, tunable slow light devices, and sensitive refractive index sensors, etc.

An electrically switchable wideband metamaterial absorber based on graphene at P band

Abstract Graphene has the capability of dynamically tuning its conductivity through gate voltage. Based on this fact, an electrically switchable wideband metamaterial absorber at low frequencies is presented and investigated in this paper. Our calculated results show that its absorption is over 90% from 400 to 1,000 MHz with the Fermi level of graphene being at 0 eV and the absorption band can be switched by adjusting the Fermi level of graphene without changing its physical structure. Moreover, the surface current distribution enables us to reveal the switchable wideband absorption characteristics of our designed metamaterial absorber. At last, we prove that its absorption property is polarization-insensitive due to the rotational symmetry of the structural unit. This work may provide a further step in the development of switchable sensors and absorbers at low frequencies.

Correlation of the Graphene Fermi-Level Shift and the Enhanced Electrochemical Performance of Graphene-Manganese Phosphate for Hybrid Supercapacitors: Raman Spectroscopy Analysis.

A high structurally stable graphene-manganese phosphate (graphene-Mn3P2O8) composite with excellent cycling stability was prepared by the facile hydrothermal method. The correlation between the high electrochemical performance of graphene-Mn3P2O8 composite and the graphene Fermi-level shift was investigated using Raman spectroscopy by monitoring the disorder in the sp2 network of the composite graphene arising from the heterogeneous atoms doping during hydrothermal synthesis. The response of the Raman signatures of graphene to the chemical doping effect correlated to the electronic band structure in the vicinity of the Dirac point showed an upshift in the graphene Fermi level with an average value of about 190 meV, indicating that the composite graphene is n-type-doped. This was confirmed by X-ray photoelectron spectroscopy data, which showed a relatively high concentration of electron-donating heterogeneous atoms in the composite graphene. The electrochemical analysis confirmed that the n-type-doped composite graphene has enhanced the electrical conductivity of the Mn3P2O8 electrode and decreased the potential barriers between the electrode surface and electrolyte highest occupied molecular orbital (HOMO) for enhanced interfacial charge transfer between the electrode surface and the electrolyte; hence, the graphene-Mn3P2O8 composite electrode exhibited a high specific capacity of 38.4 mA h g-1 compared to the pristine Mn3P2O8 electrode (7.2 mA h g-1). Due to its excellent cycling stability (∼100% capacity retention over 5000 charge-discharge cycles at 5 A g-1), graphene-Mn3P2O8 composite is a promising electrode material for hybrid supercapacitors.