Sheet Resistance Of Graphene Research Articles

One-step sustainable preparation of laser induced S-doped graphene for assembly of high-performance supercapacitors

Lignin, an industrial by-product that is abundant, renewable and has aromatic ring characteristics, is regarded as an ideal raw material for the synthesis of two-dimensional graphene materials. However, the sustainable development of graphene produced from lignin faces thorny challenges such as environmental pollution, low yield, cumbersome steps and harsh conditions. In this study, low-energy, green, large-scale one-step preparation of sulfur-doped graphene was achieved by co-blending lignin and sodium alginate into a fully biomass-based film, followed by easy-to-operate laser etching. Among them, alkaline lignin not only served as a carbon source but also acted as a sulfur doping source, avoiding the use of additional sulfur doping reagents. The results showed that the sheet resistance of graphene gradually decreased with the increase of laser power. The sheet resistance of graphene prepared when the laser power was 90% was as low as 8 Ω sq−1, which provided an inherent advantage for the subsequent assembly of flexible micro-supercapacitors. The specific capacitance and high energy density of the resulting supercapacitor were as high as 88.29 mF cm−2 and 12.26 μW h cm−2, respectively. This method successfully developed a sustainable and low-energy graphene preparation method, paving the way for the sustainable development of flexible energy storage devices.

Transfer-free rapid growth of 2-inch wafer-scale patterned graphene as transparent conductive electrodes and heat spreaders for GaN LEDs

A technique for the transfer-free growth of 2-inch wafer-scale patterned graphene directly on GaN LED epilayers is introduced. High-quality graphene as transparent electrodes and heat spreaders is synthesized directly on GaN by PECVD at only 600 °C deposition temperature and within 3 min growth time. Co acts as both the catalyst for graphene growth and the dry etching mask for GaN mesas, which greatly improves the efficiency of the semiconductor device process. Elegantly, the graphene growth is in accordance with the shape of Co, which offers a lithography-free patterning technique of the graphene. Afterward, using our penetration etching method through the PMMA and graphene layers, the Co is peacefully removed, and in-situ Ohmic contact is achieved between the graphene and p-GaN where the contact resistivity is only 0.421 Ω cm2. The graphene sheet resistance is as low as 631.2 Ω sq−1. The device is also superior to the counterpart graphene-free LED in terms of heat spreading behavior, as evidenced by the lower junction temperature and thermal resistance. Most importantly, the developed technique produces graphene with excellent performance and is intrinsically more scalable, controllable, and semiconductor industry compatible than traditionally transferred graphene.

Towards a graphene transparent conducting electrode for perovskite/silicon tandem solar cells

AbstractIndium‐based transparent conducting electrodes (TCEs) are a major limiting factor in perovskite/silicon tandem cell scalability, while also limiting maximum cell efficiencies. In this work, we propose a novel TCE based on electrostatically doped graphene monolayers to circumvent these challenges. The electrode is enabled by a thin film dielectric that is charged and interfaced to a graphene film, optimally exploiting electrostatic doping. The field effect mechanism allows the modulation of charge carriers in monolayer graphene as a function of charge concentration in the dielectric thin film. Electrostatic charge was deposited on SiO2 membranes, and graphene transferred onto them exhibited a reduction in sheet resistance because of the induced charge carriers. We show a reduction in sheet resistance of graphene by 60% in just 3 min of dielectric charging, without impacting the transmission of light through the film stack. Hall effect measurements indicated that the mobility of the films was not significantly degraded. The deposition of negative electrostatic charge reversed this effect, allowing for precise tunability of charge concentration from n‐ to p‐type. We develop a model to determine the required sheet resistance of a graphene TCE with 97% transmittance in a perovskite/silicon tandem cell. As the technique here reported does not impact transmittance, a graphene TCE with a sheet resistance below 50 Ω/□ could enable efficiencies up to 44%, presenting a promising alternative to indium‐based TCEs.

The Effect of C60 and Pentacene Adsorbates on the Electrical Properties of CVD Graphene on SiO2

Graphene is an excellent 2D material for vertical organic transistors electrodes due to its weak electrostatic screening and field-tunable work function, in addition to its high conductivity, flexibility and optical transparency. Nevertheless, the interaction between graphene and other carbon-based materials, including small organic molecules, can affect the graphene electrical properties and therefore, the device performances. This work investigates the effects of thermally evaporated C60 (n-type) and Pentacene (p-type) thin films on the in-plane charge transport properties of large area CVD graphene under vacuum. This study was performed on a population of 300 graphene field effect transistors. The output characteristic of the transistors revealed that a C60 thin film adsorbate increased the graphene hole density by (1.65 ± 0.36) × 1012 cm-2, whereas a Pentacene thin film increased the graphene electron density by (0.55 ± 0.54) × 1012 cm-2. Hence, C60 induced a graphene Fermi energy downshift of about 100 meV, while Pentacene induced a Fermi energy upshift of about 120 meV. In both cases, the increase in charge carriers was accompanied by a reduced charge mobility, which resulted in a larger graphene sheet resistance of about 3 kΩ at the Dirac point. Interestingly, the contact resistance, which varied in the range 200 Ω-1 kΩ, was not significantly affected by the deposition of the organic molecules.

Characteristics of Epitaxial Graphene on SiC/Si Substrates in the Radio Frequency Spectrum

Graphene is expected to bring substantial benefits for high-frequency applications, however, most of the studies in this area are based on theory. Here, the properties of epitaxial graphene grown on intrinsic silicon carbide on silicon substrates are investigated for potential radio frequency (RF) applications. Metal coplanar waveguides (CPWs) are fabricated that employ graphene as a shunt between the signal and ground planes. The CPWs are used for characterizing the frequency-dependent behavior of the sheet resistance of the graphene shunt from 10 MHz to 10 GHz. The process involves evaluating the CPW’s RLCG transmission line parameters and comparing them to a reference un-shunted CPW to extract the sheet resistance. We find that the quality of the metal contact with graphene is one key parameter to observe adequate current injection in the 2D material in the RF spectrum. A mild argon plasma treatment was applied to reach an adequate contact quality. Furthermore, we observe a monotonic decrease of the sheet resistance of the epitaxial graphene for frequencies roughly above 100 MHz. We attribute this behavior to the progressively smaller influence of small-scale discontinuities, such as grain sizes, at those higher frequencies.

RCS Reduction on Patterned Graphene-Based Transparent Flexible Metasurface Absorber

A metasurface absorber consisting of patterned graphene sandwich structure, transparent flexible polyvinyl chloride (PVC) dielectric layer, and indium tin oxide (ITO) bottom plate is proposed in microwave band. The graphene layer is patterned as periodic strip bands with the same rectangular holes. The radar cross section (RCS) characteristics are studied by simulation and experiment. The results indicate that the proposed transparent flexible metasurface shows more than 10 dB RCS reduction in dual band, wideband, and single band when the sheet resistance ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{s}$ </tex-math></inline-formula> ) of graphene is 70, 240, and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$350~\Omega $ </tex-math></inline-formula> /sq, respectively. For <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$R_{s} = 240\,\,\Omega $ </tex-math></inline-formula> /sq, RCS reduction in the conformal case is better than 10 dB in 8.52–16.98 GHz, and corresponding relative bandwidth is 66.35%. The regulation characteristics are attributed to the strong interference at the resonant frequency or in the broadband. It effectively captures the incident electromagnetic (EM) waves in the metasurface, and the incident wave energies are dissipated with high ohmic loss. Moreover, the RCS suppression ability of the conformal metasurface becomes better with the increase of curvature radius when the metasurface size is fixed. The proposed metasurface initiates a new way for research and development of multifunctional metasurface devices, which has important application in EM stealth in microwave band.

High current limits in chemical vapor deposited graphene spintronic devices

Understanding the stability and current-carrying capacity of graphene spintronic devices is key to their applications in graphene channel-based spin current sensors, spin-torque oscillators, and potential spin-integrated circuits. However, despite the demonstrated high current densities in exfoliated graphene, the current-carrying capacity of large-scale chemical vapor deposited (CVD) graphene is not established. Particularly, the grainy nature of chemical vapor deposited graphene and the presence of a tunnel barrier in CVD graphene spin devices pose questions about the stability of high current electrical spin injection. In this work, we observe that despite structural imperfections, CVD graphene sustains remarkably highest currents of 5.2 × 108 A/cm2, up to two orders higher than previously reported values in multilayer CVD graphene, with the capacity primarily dependent upon the sheet resistance of graphene. Furthermore, we notice a reversible regime, up to which CVD graphene can be operated without degradation with operating currents as high as 108 A/cm2, significantly high and durable over long time of operation with spin valve signals observed up to such high current densities. At the same time, the tunnel barrier resistance can be modified by the application of high currents. Our results demonstrate the robustness of large-scale CVD graphene and bring fresh insights for engineering and harnessing pure spin currents for innovative device applications.

Graphene-Based Optically Transparent Metasurface Capable of Dual-Polarized Modulation for Electromagnetic Stealth.

Microwave stealth technology with optical transparency is of great significance for solar-powered aircrafts (e.g., satellites or unmanned aerial vehicles) in increasingly complex electromagnetic environments. By coating them with optically transparent absorbing materials or devices, these large-sized solar panels could avoid detection by radar while maintaining highly efficient collection of solar energy. However, conventional microwave-absorbing materials/devices for solar panels suffer from bulky volume and fixed stealth performance that significantly hinders their practicality or multifunctionality. Particularly, dynamic modulation of microwave absorption for dual polarization remains a challenge. In this paper, we propose the design, fabrication, and characterization of an optically transparent and dynamically tunable microwave-absorbing metasurface that enables dual modulations (amplitude and frequency) independently for two orthogonal linearly polarized excitations. The tunability of the proposed metasurface is guaranteed by an elaborately designed anisotropic meta-atom composed of a patterned graphene structure whose electromagnetic responses for different polarizations can be dynamically and independently controlled via bias voltages. The dual tunability in such a graphene-based absorbing metasurface is experimentally measured, which agrees well with those numerical results. We further build an equivalent lumped circuit model to analyze the physical relation between the tunable sheet resistance of graphene and the polarization-independent modulations of the metasurface. Taking into account the advantages of optical transparency and flexibility, the proposed microwave-absorbing metasurface significantly enhances the multitasking stealth performance in complex scenarios and has the potential for advanced solar energy devices.

Varactor-graphene-based multi-reconfigurable triplet filter with independent tuning of frequency and amplitude

In this paper, a multi-reconfigurable filter which consists of three identical varactor-graphene-loaded coaxial substrate integrated waveguide (SIW) resonators is presented. The center frequency and transmission amplitude of the proposed filter is independently reconfigurable by means of electrically controlling the values of the varactor capacitance and the graphene resistance, respectively. Tuners and their bias circuits are mounted on the surfaces of the SIW resonator to adjust the tuning of resonant frequency and unloaded quality factor, which enable to realize the frequency and amplitude of the presented multi-reconfigurable filter. In addition, the proposed filter applies a cascaded triplet topology to generate a transmission zero at the upper side in order to enhance skirt selectivity and out-of-band rejection. The circular shape of the resonator provides great flexibility and produces an additional compactness of the filter. The proposed multi-reconfigurable coaxial SIW filter is designed and fabricated, with high-Q varactors and graphene thin sheets as frequency and amplitude tuning elements, respectively, exhibiting the following reconfigurable characteristics: (a) a wide tuning range of 0.58–1.01 GHz with a constant 1 dB fractional bandwidth of 2 ± 0.2%. (b) A continuous tuning range of amplitude at any center frequency in the achievable frequency band. For instance, the proposed filter can be tuned with an amplitude range of 3.02–19.8 dB by varying the value of the graphene sheet resistance at 1.01 GHz. (c) Free combination of the arbitrary center frequency and attenuation level. The proposed filter with tunable filtering and attenuating responses has widely potential in reconfigurable wireless communication systems and radar systems due to its high integration and versatility.

Optically Transparent Broadband Microwave Absorber by Graphene and Metallic Rings

The demand for optically transparent microwave absorbers has attracted increasing interest among researchers in recent years. However, integrating broadband microwave absorption and high optical transparency remains a challenge. This report demonstrates a scheme for broadband microwave absorbers, featuring a 90% absorption bandwidth of 10 GHz covering a frequency range of 25.2-35.2 GHz and high compatibility with good optical transparency in a wide band from the visible to infrared. The absorber is based on a Jaumann structure composed of two graphene sheets sandwiched by dielectric and backed by an arrayed-metallic-rings sheet. Guided by derived formulas, this absorber exhibits complete absorption if the sheet resistance of graphene is close to 500 Ω sq-1. The bandwidth and center frequency of the absorption spectra can be readily tuned simply via changes in the thickness of the dielectric between the graphene films and arrayed-metallic-rings sheet. Moreover, the absorber is insensitive to the incident angle of radiation and can achieve broadband and near-unity absorption even at oblique incidence. The graphene-based absorber proposed herein provides a viable solution for effectively integrating broadband and near-unity microwave absorption with high optical transparency, thereby enabling widespread applications in optics, communications, and solar cells.

Development of directly grown‐graphene–silicon Schottky barrier solar cell using co‐doping technique

Low-cost, highly efficient, and low-power devices are essential for energy harvesting applications. Despite numerous studies having been conducted on transferable graphene solar cells, large-area growth and long-term stability continue to remain elusive, which limits the practicability of graphene-based photovoltaic devices. Plasma-enhanced chemical vapor deposition is a promising, efficient, and facile method for synthesizing graphene over a large area without the use of a metal catalyst, but defects formed during graphene growth adversely affect the power conversion efficiency (PCE) of graphene/Si solar cells in which the graphene is used. In this work, we successfully employed acid-based graphene-Si macromolecular photovoltaic devices chemically doped with polymeric perfluorinated sulfonic acid (PFSA) macromolecules and nitric acid (HNO3). We achieved an improved PCE of about 9.27% with PFSA doping compared with the value (7.64%) without PFSA doping, and it increased substantially to 10.44% after co-doping with HNO3. PFSA macromolecular doping also led to a substantial increase in the carrier concentration, which helped to reduce the sheet resistance of graphene and improved the work function of the device. In particular, the synergistic effect of co-doping with HNO3 helped to improve the number of active sites and charge separation. Along with the increase in carrier concentration, doping with PFSA reduced the number of defects, resulting in the graphene having a smooth and uniform surface, which effectively increased the open-circuit voltage (Voc) from 0.500 to 0.521 V. We surmise that nonvolatile PFSA macromolecular and volatile HNO3 co-doping has high potential for use in the fabrication of low-cost directly grown-graphene-based photovoltaic devices. Highlights Plasma-enhanced chemical vapor deposition technique used for the direct growth of graphene without metal catalyst. Naturally grown oxide thickness optimized. Nonvolatile macromolecular dopant per-fluorinated polymeric sulfonic acid (PFSA) and volatile nitric acid (HNO3) were used for direct growth graphene doping. The power conversion efficiency of ~9.27% with PFSA doping from 7.64%; further, its substantial increase up to 10.44% after co-doping with HNO3 were achieved with long-term stability.

Graphene compared to fluorine-doped tin oxide as transparent conductor in ZnO dye-sensitized solar cells

Graphene could replace widely used transparent conductors (TC) like tin oxides due to its high transparency and electrical conductivity. This work focuses on the technical feasibility of graphene produced by chemical vapor deposition (CVD) to replace tin oxides in the photoanode of dye-sensitized solar cells (DSSCs). This is the first study in which graphene is used as photoanode TC in a tin oxide-free DSSC with ZnO as mesoporous semiconductor. The stability of graphene towards hybridization with ZnO was investigated using electrical measurements and Raman spectroscopy. A thorough comparison of performance between FTO- and graphene-DSSCs fabricated with otherwise identical methods was performed with linear voltammetry, electrochemical impedance spectroscopy and electron lifetime calculations. The higher optical transmittance of graphene resulted in larger open circuit voltage and short circuit current density than FTO devices. However, the larger sheet resistance of graphene caused a lower fill factor in the graphene-DSSCs. Graphene- and FTO-DSSCs yielded similar power conversion efficiencies of 0.4%, despite graphene being ∼250 times thinner than FTO. The findings have important cost and environmental implications for large scale production, not only for DSSCs, but for the photovoltaic technologies derived from them, such as perovskites and quantum dot-SSCs, and other photoelectrochemical devices. Tin oxides replacement with graphene is promising for development of lighter, and more flexible, sustainable and resilient PV technology.

Simultaneous Extraction of the Grain Size, Single-Crystalline Grain Sheet Resistance, and Grain Boundary Resistivity of Polycrystalline Monolayer Graphene.

The electrical properties of polycrystalline graphene grown by chemical vapor deposition (CVD) are determined by grain-related parameters—average grain size, single-crystalline grain sheet resistance, and grain boundary (GB) resistivity. However, extracting these parameters still remains challenging because of the difficulty in observing graphene GBs and decoupling the grain sheet resistance and GB resistivity. In this work, we developed an electrical characterization method that can extract the average grain size, single-crystalline grain sheet resistance, and GB resistivity simultaneously. We observed that the material property, graphene sheet resistance, could depend on the device dimension and developed an analytical resistance model based on the cumulative distribution function of the gamma distribution, explaining the effect of the GB density and distribution in the graphene channel. We applied this model to CVD-grown monolayer graphene by characterizing transmission-line model patterns and simultaneously extracted the average grain size (~5.95 μm), single-crystalline grain sheet resistance (~321 Ω/sq), and GB resistivity (~18.16 kΩ-μm) of the CVD-graphene layer. The extracted values agreed well with those obtained from scanning electron microscopy images of ultraviolet/ozone-treated GBs and the electrical characterization of graphene devices with sub-micrometer channel lengths.

Controlling Graphene Sheet Resistance for Broadband Printable and Flexible Artificial Magnetic Conductor-Based Microwave Radar Absorber Applications

Current artificial magnetic conductor (AMC) designs use metallic patterns on rigid substrates and focus on shapes and sizes of AMC structures, rather than on material performance, which has hindered operation bandwidth and design flexibility. Here, we introduce printed graphene AMC-based broadband and flexible microwave radar absorbers, which not only redirect but also absorb the incident wave so to broaden the operation bandwidth. Contrasting to other reported works, the phase characteristics of the AMCs are realized through the control of the surface resistance provided by printed graphene laminates. We produced a variety of AMC structures, composed of printed graphene circular ring arrays with exactly the same shape and size, but different sheet resistances. By carefully designing the sheet resistance of printed graphene laminates, the optimized anti-phase reflection cancellation between AMCs can be achieved. With printed graphene AMCs and flexible dielectric substrate, the absorber presented in this work has a broadband effective absorption (above 90% absorptivity) from 7.58 GHz to 18, is polarization insensitive under normal incident, and can work at relatively wide incident angles. Furthermore, this absorber is capable of bending easily with notable performance, which makes it ideal for applications with irregular and uneven shapes.

Integration of graphene with GZO as TCO layer and its impact on solar cell performance

In this study, we investigated the impact of incorporating graphene with Ga-doped ZnO (GZO) when employing them as a TCO layer on Si-based solar cell. GZO thin films with various thicknesses (50–450 nm) were fabricated by the sputtering method using a single target. The aim here was to determine the GZO film with the optimum thickness to incorporate it with single layer graphene as TCO. This thickness was found to be 350 nm as that was the best crystalline quality found in the XRD pattern. Further, this sample had the lowest sheet resistance and highest transmission values as confirmed by electrical (sheet resistance), and optical characterizations (transmission). Topographic (SEM and AFM), electrical (resistivity and carrier concentration) measurements were also conducted on the same sample. The graphene film grown on copper in a CVD system was then transferred on top of this sample to fabricate the hybrid TCO structure. We found that graphene integrated GZO hybrid TCO film showed higher sheet resistance due to high sheet resistance of graphene and similar optical properties thanks to high optical transmission of graphene. Employing graphene-based TCO layer in the solar cell resulted in higher open-circuit voltage, consequently improving the conversion efficiency from 10.0% to 11.2%.

Graphene-Based Transparent Conducting Substrates for GaN/AlGaN Nanocolumn Flip-Chip Ultraviolet Light-Emitting Diodes

Flip-chip ultraviolet light-emitting diodes based on self-assembled GaN/AlGaN nanocolumns have been fabricated, exploiting single-layer graphene not only as a growth substrate but also as a transparent conducting electrode. High crystalline quality of the nanocolumns is confirmed by detailed electron microscopy characterization, also showing the intrinsic GaN quantum disk in the active region of the nanocolumns. These features are further confirmed in the optical emission, where the absence of defect-related yellow emission and the presence of blue-shifted (from the usual 365 nm band gap emission of bulk wurtzite GaN) emission at ∼350 nm, ascribed to quantum confinement and strain effects, are observed. Despite a noticeable graphene damage after the nanocolumn growth that causes high sheet resistance of graphene and high turn-on voltage, the proof of concept of single-layer graphene used as the transparent conducting substrate for a nanocolumn device is demonstrated. This study offers an alternative platform for the fabrication of next-generation nano-optoelectronic and electronic devices.

Evaluation of the average grain size of polycrystalline graphene using an electrical characterization method

Graphene grown by chemical vapor deposition (CVD) is intrinsically polycrystalline. Since the electrical properties of graphene are degraded at grain boundaries, the performance of CVD graphene is significantly dependent upon its grain size. Thus, the evaluation of average grain size is of particular importance for the device applications of CVD graphene. However, conventional evaluation methods based on microscopic or spectroscopic measurements have limitations in accuracy due to limited inspection area and complicated processes. Here, we suggest an electrical characterization technique for precisely evaluating the average grain size of polycrystalline graphene. We found out that the sheet resistance of polycrystalline graphene is significantly dependent on device dimensions and this dependence is related to average grain size. For the evaluation of the average grain size, we synthesized CVD graphene layers with different growth conditions, fabricated transmission line model (TLM) patterns on the graphene layers, and measured the sheet resistance (RS) as a function of channel length (Lch). The average grain sizes of the CVD-graphene layers were extracted from a logistic function fitted to the measured RS–Lch curves. The results show that the average grain sizes of CVD graphene evaluated by the proposed electrical method are consistent with those evaluated from a large amount of microscopy images.

Active modulation of electromagnetically induced transparency analog in graphene-based microwave metamaterial

Metamaterial analogs of electromagnetically induced transparency (EIT) enable a unique route to enhance light-matter interactions without quantum approach, showing remarkable potentials for slow-light devices and highly sensitive sensors. In particular, dynamic control of EIT in metamaterials opens new avenues for optical networks and electromagnetic communications. Here, we experimentally present, for the first time, the active modulation of EIT analog by integrating graphene into a microwave metamaterial. Experimental results demonstrate that the EIT peak can be dynamically controlled under a relatively low bias voltage applied on graphene. A coupled harmonic oscillation model is exploited to analyze the physical mechanism which lies in the active tuning of the damping rate of the dark mode resonator through varying the graphene's sheet resistance. Via variably dampening the dark resonator using graphene, the coupling condition could be electrically modulated and the continuous tuning of the EIT resonance strength is achieved. In addition, the controllable slow-wave propagation is also performed theoretically based on the tunable phase delay of the transmitted waves. This work opens up promising possibilities for slow-wave manipulation with applications in smart and multifunctional devices.

Graphene‐Integrated Reconfigurable Metasurface for Independent Manipulation of Reflection Magnitude and Phase

AbstractReconfigurable metasurfaces are highly desirable for applications including dynamical camouflaging, intelligent electromagnetic (EM) skin and systems due to their tunable EM features or functions. So far most of the existing reconfigurable metasurfaces are developed by modulating their amplitude or phase. It is still very challenging to simultaneously achieve dynamical control of amplitude and phase in one design. This study reports a reconfigurable metasurface that can realize independent control of reflection magnitude and phase with the external bias voltages. In this metasurface, the graphene capacitor structure is used as an amplitude‐tuning layer for controlling the reflection magnitude by changing the effective sheet resistance of graphene, while the reflection phase is tuned by switching PIN diodes loaded in a periodic metallic structure. Such metasurface can control the scattering wave directions through phase coding, and the variation of graphene resistance can further modulate the intensity of the scattering beams. In addition, the metasurface can also function as a radar absorber where the absorbing magnitude and frequency can be dynamically tuned. Good agreements between simulations and experiments demonstrate its powerful ability to manipulate the EM waves by simultaneously tuning the reflection magnitude and phase, indicating a wide range of potential applications in EM fields.

Graphene-Driven Metadevice for Active Microwave Camouflage with High-Efficiency Transmission Window.

The general method to achieve electromagnetic (EM) invisibility is to use absorbing materials that can reduce the backscattering microwave signals. However, they will influence the transmission of useful signals, and their passive operation mode also blocks the realization of active camouflage systems. Here, the authors propose an electrically tunable metadevice which can dynamically manipulate the reflection magnitude and realize high-efficiency transmission at two distinctive frequency bands. Such a metadevice is driven by graphene sandwich structure integrated with multi-layer metasurface. By electrically controlling the sheet resistance of graphene, the authors have experimentally verified that the metadevice can tune the reflection magnitude from -5 to -20dB over a wide frequency band of 5-15GHz, and meanwhile realize a high transparent EM window with 3-dB transmission band covering from 23 to 25GHz. The operation mechanism is discussed by examining the electric field distribution and also employing an equivalent circuit model to analyze the input impedance. Furthermore, using the developed metadevice, the authors still demonstrate its dynamical camouflaging performance and application potential as an antenna radome by experiment. The finding in this study is expected to trigger great interest in adaptive camouflage, stealth radome, and multifunctional EM manipulation fields.