Fermi Level Of Graphene Research Articles

A triple Fano resonance Si-graphene metasurface for multi-channel tunable ultra-narrow band sensing.

In this work, a dielectric metasurface composed of a silicon nanodisk etched with a square hole is proposed. By introducing C4v symmetry breaking, the symmetry-protected bound states in the continuum (SP-BIC) is transformed into a quasi-BIC (Q-BIC), simultaneously inducing triple Fano resonances in the near-infrared light band corresponding to one dipole and two Q-BIC resonances. The characteristics of Q-BIC resonances are elucidated through multipole decomposition and near-field distribution analysis. Subsequently, monolayer graphene is integrated into the Si metasurface. The light field in the composite metasurface can be flexibly modulated by changing the Fermi level of graphene. This modulation enables optimal transmission with an enhancement of up to 252%, while the confined electromagnetic energy experiences a remarkable increase of about 1020%. Simulation results demonstrate that the Si-graphene composite metasurface exhibits a high refractive index sensitivity of 162 nm RIU-1, accompanied by a figure of merit of 170.526 RIU-1. This composite metasurface holds promise as a high-performance sensor in the near-infrared band and has potential for application in the fields of active tunable optical devices and biochemical sensing.

Multifunctional metasurface for Terahertz wavefront manipulation based on the combination of vanadium dioxide and graphene

In this paper, we propose a multifunctional wavefront control metasurface based on vanadium dioxide (VO2) and graphene. The unit cell structure contains a VO2 substrate, three rectangular metal strips as the bottom layer, orthogonal graphene strips as the top layer, and an intermediate layer consisting of a split ring resonator and circular metal patches. The desired performances can be achieved by changing the parameters of the split ring resonator in the middle layer, as well as controlling the phase transition state of VO2 and the Fermi level of graphene. When VO2 is in the insulating phase and the Fermi level of graphene is 1 eV, a transmissive metasurface is established, which realizes cross-polarization conversion, anomalous refraction, focusing lens, and orbital angular momentum generation at the frequency of 0.63 THz by changing the geometric sizes of the unit cell. When VO2 is in the metal phase and the Fermi level of graphene is 0.01 eV, a reflective metasurface can be achieved, which realizes terahertz beam splitting, radar cross section, and anomalous reflection of a single beam at the frequency of 0.63 THz. This multifunctional wavefront control metasurface treating the combination of VO2 and graphene exhibits significant application prospects in terahertz transmission, imaging, and communication.

Graphene Architecture-Supported Porous Cobalt-Iron Fluoride Nanosheets for Promoting the Oxygen Evolution Reaction.

The design of efficient oxygen evolution reaction (OER) electrocatalysts is of great significance for improving the energy efficiency of water electrolysis for hydrogen production. In this work, low-temperature fluorination and the introduction of a conductive substrate strategy greatly improve the OER performance in alkaline solutions. Cobalt-iron fluoride nanosheets supported on reduced graphene architectures are constructed by a one-step solvothermal method and further low-temperature fluorination treatment. The conductive graphene architectures can increase the conductivity of catalysts, and the transition metal ions act as electron acceptors to reduce the Fermi level of graphene, resulting in a low OER overpotential. The surface of the catalyst becomes porous and rough after fluorination, which can expose more active sites and improve the OER performance. Finally, the catalyst exhibits excellent catalytic performance in 1 M KOH, and the overpotential is 245 mV with a Tafel slope of 90 mV dec-1, which is better than the commercially available IrO2 catalyst. The good stability of the catalyst is confirmed with a chronoamperometry (CA) test and the change in surface chemistry is elucidated by comparing the XPS before and after the CA test. This work provides a new strategy to construct transition metal fluoride-based materials for boosted OER catalysts.

Simultaneous measurement of relative humidity and temperature using EIT-like effect in photonic crystal cavity coupled system integrated with PVA and graphene

In this work, we propose a sensor based on a photonic crystal nanobeam cavity (PCNC) coupled system integrated with Polyvinyl alcohol (PVA) and graphene, which uses an electromagnetically induced transparency-like (EIT-like) effect for the simultaneous measurement of relative humidity (RH) and temperature (T). The structure consists of a bus waveguide and two types of one-dimensional PCNC supporting the dielectric and air mode resonance, respectively. The RH-sensitive material PVA is used in the dielectric mode cavity to increase the RH sensitivity and anti-interference ability. We demonstrate that the EIT-like transparency window can be actively and flexibly modulated by adjusting the Fermi level of graphene. In addition, the ability of the system to simultaneously detect RH and T is proved by using the three-dimensional finite-difference time-domain (3D-FDTD) method and the sensor matrix. The RH sensitives for the dielectric and air modes are −542.0 pm/%RH and −258.2 pm/%RH, respectively. The T sensitives for the dielectric and air modes are −14.6 pm/K and 35.4 pm/K, respectively. The sensor achieves a strong anti-interference ability and its footprint is only 12 μm × 2.02 μm. Therefore, our proposed sensor has potential applications in food storage, preservation of museum collections, human health, and biological and chemical experiments.

A tunable graphene-based phase-jump inversion plasmonic sensor for ultra-sensitive refractive index sensing over THz frequency

The surface plasmon resonance effect has been previously reported as an essential mechanism for terahertz wave sensing applications. However, due to its structural geometric parameters dependence, either the reconfiguration of the arrays or the realignment of the prism system is required, which always remains one of the challenges limiting practical applications. Here, a graphene-based phase-jump inversion plasmonic sensor (GPPS) operating at a fixed gap and incident angle is established. The optimal wave vector matching between specific analytes and the GPPS can be achieved by adjusting the Fermi level of graphene. On this basis, an ultrahigh sensitivity of 0.488 THz/RIU is achieved under the optimal Fermi level. Remarkably, the comparison between the phase spectra and the reflective amplitude spectra indicates that the phase-based FoM and Q-factor are two orders of magnitude larger than those calculated by amplitude spectra, which could reach 244RIU−1 and 218.5. The unique phase-jump responses on graphene surface plasmon resonance provide new ideas for ultra-sensitivity THz sensing methods.

Tunable nonreciprocal radiation for near vertical incident light based on graphene and Weyl semimetal

The tunable nonreciprocal radiation for near vertical incident light is investigated. The effect is realized by placing a cascaded one-dimensional graphene and Weyl semimetal (WS) nanowire array on a thick metallic layer. It is observed that perfect absorptivity and emissivity can be realized with different resonant wavelengths, resulting in the achievement of perfect nonreciprocity with an angle of merely 1o. This effect results from the cavity mode resonance (CMR) excited by the WS array, and it is disclosed through analyzing the amplitude distribution of electromagnetic field. In addition, the perfect nonreciprocal radiation could be maintained within a large geometrical dimension range. Furthermore, it is shown that the perfect spectral nonreciprocity could be dynamically tuned by controlling the axial vector of the WS as well as the Fermi level of graphene. The findings will provide new possibility for developing novel energy capture and conversion structure also with advanced thermal systems.

Terahertz dichroic device withy multi-functionality, multi-band operation and multi-dimensional tunability

Most existing dichroic metasurfaces have limitations such as single operating band, single functionality, fixed dichroic performance or only adjustable in a single dimension, as well as complex structure. Here, we propose a terahertz dichroic device based on patterned graphene layer and gold layer to address these limitations. By utilizing the anisotropy endowed by the patterned graphene layer and the gold layer, the proposed device can achieve both dual-band circular dichroism (CD) and linear dichroism (LD), with maximum CD values of 0.866 at 1.09 THz and −0.614 at 1.76 THz, and maximum LD values of 0.779 at 0.51 THz and −0.633 at 0.76 THz. Meanwhile, the dichroism of the device can be regulated through multiple dimensions, i.e., the rotation angle of the top patterned graphene layer, the Fermi level of graphene and the incident angle. The proposed dichroic device offers multi-functionality, multi-band operation and multi-dimensional adjustability with a relatively simple structure, and is expected to be widely used in the fields of terahertz imaging, polarization detection, optical sensing and biological spectroscopy.

Graphene-enabled chiral metasurface for terahertz wavefront manipulation and multiplexing holographic imaging

The ability of metasurfaces to simultaneously manipulate both the spin and phase of circularly polarized (CP) incident waves has attracted considerable attention from researchers. Here, we propose a reconfigurable double circular split rings (DCSRs) chiral metasurface. By introducing graphene and changing the Fermi level of graphene, dynamic circular dichroism (CD) can be achieved. The calculation results show that when the Fermi level of graphene is increased from 0.1 eV to 1 eV, the CD decreases from 0.72 to 0.09 at 0.839 THz, which provides a CD-tunable spin-controlled method. In addition, based on the graphene-enabled DCSRs, we design two metasurface arrays combining Pancharatnam-Berry (PB) phase and coding metasurface theory to realize terahertz CP wave switchable focusing and spin-controlled multi-pencil beam generation. Furthermore, we demonstrate the application of reflective holography for multiplexing circularly polarized waves. The results of this paper are expected to play a significant role in the future terahertz wireless communication system, multi-spectral imaging, and information encryption fields.

Tunable infrared surface phonon–plasmon coupling in graphene-integrated polar semiconductor heterostructure

Within Reststrahlen bands of polar semiconductors, surface phonon–plasmon coupling is of great interest in infrared nanophotonics. Here, we demonstrate an active long-wavelength infrared device of graphene integrated with an AlN/SiC polar heterostructure. As a low-loss dielectric design, the subwavelength structure device takes advantage of interfacial photogating effect on electrostatic doping of the graphene and the interfaced SiC, and the tunable spectral behavior is originated from the hybridization of the doping-dependent surface phonon–plasmon resonances. This finding provides a steady-state manipulating method to the surface modes for the low-loss nanophotonic devices on SiC platform, and the graphene Fermi level tunable to cross the Dirac point in a steady response even makes the intrinsic graphene photodetectors feasible.

A multi-functional tunable terahertz graphene metamaterial based on plasmon-induced transparency

A graphene terahertz metamaterial composed of multiple square split ring resonators is proposed to realize the excellent functions of sensing and switching based on plasmon-induced transparency (PIT). The switch, operating at multiple frequencies can be easily achieved by adjusting the Fermi level of graphene, corresponding to a maximum amplitude modulation of 89.5 %. When the device is employed as a sensing platform, the multi-PIT effect can improve the sensitivity due to greater interactions between the metamaterial and analytes, leading to a maximum sensitivity of 380 GHz/RIU. These findings suggest that the proposed metamaterial can serve as a multipurpose device with broad applicability.

Significant Enhancement of Near-Field Radiative Heat Transfer by Misaligned Bilayer Heterostructure of Graphene-Covered Gratings

Abstract The near-field radiative heat transfer of heterostructure consisting of SiC gratings and graphene is investigated in this work. The rigorous coupled-wave analysis is employed to calculate the spectral heat flux. Nevertheless, monolayer heterostructure and nonmisaligned bilayer heterostructure consistently suffer from a lack of spectral heat flux. In this work, we investigate the prominent effect of misaligned bilayer heterostructure in enhancing near-field radiative heat transfer by plotting energy transmission coefficients and electromagnetic fields. The results show that when the misalignment reaches half a period, the bilayer heterostructure exhibits optimal performance with a total heat flux of 3.5 × 104 W/m2. Besides the well-known coupled surface phonon polaritons supported by SiC gratings, the surface plasmon polaritons supported by graphene dominate the enhancement of heat flux from 0.01 × 1014 rad/s to 1.5 × 1014 rad/s. Due to the spatial misalignment of the upper and lower gratings, the lower layer graphene surface plasmon polaritons are intensified, compensating for the lack of spectral heat flux. Meanwhile, the graphene surface plasmon polaritons and SiC surface phonon polaritons can be hybridized to form surface plasmon-phonon polaritons. In addition, the dynamic modulation of near-field radiative heat transfer in the misalignment state is achieved by manipulating the Fermi level of graphene. We finally show that the superiority of misaligned heterostructure is robust with respect to the frequency shift in the phonon band, providing an effective way to improve the near-field radiative heat transfer in different configuration.

Nonreciprocal reflection based on asymmetric graphene metasurfaces.

We propose a scheme to achieve controllable nonreciprocal behavior in asymmetric graphene metasurfaces composed of a continuous graphene sheet and a poly crystalline silicon slab with periodic grooves of varying depths on each side. The proposed structure exhibits completely asymmetric reflection in opposite directions in the near-infrared range, which is attributed to the pronounced structural asymmetry and its accompanying nonlinear effects. The obtained nonreciprocal reflection ratio, reaching an impressive value of 21.27 dB, combined with a minimal insertion loss of just -0.76 dB, highlights the remarkable level of nonreciprocal efficiency achieved by this design compared to others in its category. More importantly, the proposed design can achieve dynamic tunability by controlling the incident field intensity and the graphene Fermi level. Our design highlights a potential means for creating miniaturized and integratable nonreciprocal optical components in reflection mode, which can promote the development of the integrated isolators, optical logic circuits, and bias-free nonreciprocal photonics.

Elucidating the effect of spin crossover materials on graphene sensing devices

Graphene films are used to detect the presence and transition of spin crossover nanoparticle aggregates. Experiments performed far from the graphene neutrality point, combining impedance spectroscopy and Hall measurements, provide better insight into the mechanism for the change of impedance of the graphene layer in proximity with different states of the molecular structure. We observe that the change of spin state shifts the graphene Fermi level and its intrinsic resistance, with resulting positive insight into using this type of hybrid device for fast molecular electronics purposes.

2D Heterostructures Induced Charge Transfer and Trapping for Hybrid Optically and Electrically Controllable Nonvolatile Memory

AbstractNonvolatile memory is an indispensable component of electronic devices. However, the current technology makes it difficult to satisfy the emerging big data demand. To circumvent the existing problems, herein, a first attempt is made to achieve multifunctional nonvolatile memory based on all 2D heterostructures, consisting of histidine‐doped molybdenum disulfide quantum disks mixed with graphene oxide and a graphene macroscopic heterojunction. The designed device possesses intriguing hybrid electrically and optically controllable nonvolatile memory functionalities. By harnessing the unique properties of these materials, memory devices demonstrate long‐term stability and nonvolatile characteristics under both optical and electrical control signals. These devices possess outstanding features, such as multiple read‐write cycles, multi levels, and fast switching speeds, overcoming the limitations of traditional components. To explore the underlying physical mechanism, the Fermi level of graphene is measured and it is confirmed that the charge transfer and trapping across the heterojunctions are the major factors responsible for the observed behavior. This study demonstrates that 2D heterostructures for hybrid optically and electrically controllable nonvolatile memory pave an alternative route for the next‐generation information technology.

Switchable polarization-independent THz reflector/absorber

This paper introduces a novel metamaterial switchable terahertz (THz) reflector/absorber, which incorporates a composite graphene and vanadium dioxide (VO2) structure. The structure comprises gold and graphene layers separated by VO2 dielectric layers. By leveraging the electrical adjustability of graphene and the temperature adjustability of VO2, the operating frequency and absorption amplitude of the absorption spectrum can be controlled by adjusting the Fermi levels of graphene and the conductivity of VO2. The designed dual-function switch can be utilized as a reflector when VO2 exhibits a metallic state and as a perfect multiband absorber when VO2 exhibits dielectric properties. Numerical simulation results demonstrate that the proposed device exhibits exceptional performance for TE and TM waves, achieving a more than 99% absorption rate at 2.2, 6.5, and 10.5 THz. This structure holds considerable potential for a wide range of applications, including temperature monitoring, stealth technology, wireless communication, and biomedicine.

Broadband terahertz metasurface with three switchable functions based on complementary structure

In this work, we propose a switchable multifunctional metasurface based on complementary VO2-graphene structure. Tuning the state of VO2 and the Fermi level of graphene, the metasurface can act as broadband half-wave plate (HWP), quarter-wave plate (QWP) or absorber in the same frequency range of 0.65–1.27THz, respectively. In the V1G0 state, the metasurface behaves as a broadband HWP to realize linear-to-linear polarization conversion with the polarization conversion ratio >0.9 in the range of 0.64–1.37THz. In the V0G1 state, the metasurface acts as a broadband QWP achieving linear-to-circular polarization conversion with the ellipticity below −0.9 in the range of 0.65–1.27THz. In the V1G1 state, the metasurface is a broadband absorber with the absorption over 0.9 in the 0.63–1.32THz range. In addition, the physical mechanisms of the metasurface in different states are analyzed through the field and current distributions. Such a switchable multi-functional metasurface can simultaneously achieve high efficiency polarization conversion and absorption, which has potential applications in the integrated terahertz devices.

Role of defects in the photoluminescence and photoresponse of WS2–graphene heterodevices

Two-dimensional (2D) transition metal dichalcogenides are promising materials for next-generation photodetectors. Therefore, controlling point chalcogen vacancies in chemical vapor deposition (CVD) synthesis is inevitable. In this work, the number of sulfur vacancies in monolayer WS2 flakes is well controlled in CVD synthesis, which resulted in a photoluminescence (PL) intensity difference. In addition, the relationship between the PL intensity and photoresponse of monolayer WS2 on graphene is discussed. The sulfur vacancies introduce defect trap states that cause carrier recombination and reduce carrier drift to graphene, thus decreasing the photocurrent. Furthermore, the gate-tunable Fermi level of graphene allows tunable responsivity of the WS2–graphene photodetector of up to 5 A/W with metal hard-mask fabrication. Our findings on the PL intensity and responsivity provide a simple and efficient strategy for choosing high-performance CVD-synthesized 2D TMD photodetectors.

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.