Tuning the Infrared Absorption of SiC Metasurfaces by Electrically

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Tuning the Infrared Absorption of SiC Metasurfaces by Electrically Gating Monolayer Graphene with Solid Polymer Electrolyte for Dynamic Radiative Thermal Management and Sensing Applications Linshuang Long, Xiaoyan Ying, Yue Yang, and Liping Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00735 • Publication Date (Web): 10 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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ACS Applied Nano Materials

Tuning the Infrared Absorption of SiC Metasurfaces by Electrically Gating Monolayer Graphene with Solid Polymer Electrolyte for Dynamic Radiative Thermal Management and Sensing Applications Linshuang Long,1 Xiaoyan Ying,1 Yue Yang,1,2 and Liping Wang1,* 1

School for Engineering of Matter, Transport & Energy, Arizona State University, Tempe, AZ

85287, USA 2

School of Mechanical Engineering and Automation, Harbin Institute of Technology, Shenzhen,

518055, PR China *Corresponding author: [email protected]

Abstract We experimentally demonstrate actively tunable infrared absorption based on graphene-covered SiC metasurfaces. A dry transfer method is employed to coat monolayer graphene on the metasurface characterized by scanning electron microscope, atomic force microscope, and Raman spectroscopy. Solid polymer electrolyte is introduced to tune graphene chemical potential upon electrical gating. In-situ optical measurement shows a shift in the absorption peak upon a change in gate voltage. Numerical simulations unveil that the tuning effect is attributed to the excitation of magnetic polariton, whose resonance frequency changes with graphene chemical potential upon electrical gating. The reported results realize the possibility of tuning thermal radiative property of

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graphene-covered metasurface through solid polymer electrolyte, providing a new approach to fabricating graphene-based tunable infrared devices for dynamic radiative thermal management and sensing applications.

Keywords: graphene; electrical gating; solid polymer electrolyte; phonon absorption; infrared absorber

Introduction As probably the most famous two-dimensional material, graphene has attracted extensive research interests due to its excellent thermal, mechanical, and electrical properties.1-7 These properties include high thermal conductivity,8-9 high Young’s modulus,10-11 and high mobility,1213

etc. Researchers found graphene promising for several applications like sensors,14-15 field-effect

transistors (FET),16-18 solar cells,19-21 etc. The broad range of applications raises the demand for reliable methods of fabricating and transferring graphene. Different types of methods have been developed, such as mechanical exfoliation,22-23 chemical vapor deposition (CVD) growth,24-25 and solution-based methods.26-27 These synthesis methods, in turn, facilitate the graphene applications in a variety of new fields. One of the widely studied fields is the graphene-based tunable optical devices.28-36 Yao et al. demonstrated a graphene-loaded plasmonic antennas, whose resonances were tunable within a wide range upon varying the gate voltage on graphene.28 Based on similar tuning principle, they experimentally showed other potential applications of graphene-based devices, such as tunable optical antennas,29 mid-infrared absorbers,30 and photocarrier detectors.31 Sherrott et al.32 and Brar et al.33 respectively proposed tunable graphene resonators for mid-infrared and terahertz ranges. 2 ACS Paragon Plus Environment

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For the aforementioned devices, the tunability was achieved by applying a gate voltage between graphene and electrode layer separated by a dielectric layer, which forms a gate capacitor with a capacitance of Cg. The charge q held by the capacitor can be tuned upon the gate voltage Vg, which results in a shift in the graphene chemical potential , following the relation of37 Vg  VDirac 

 e



q Cg

(1)

where VDirac is the measurable Dirac voltage of graphene and e is the elementary charge. The tuning range of graphene chemical potential highly depends on the capacitance of the structure, and a larger capacitance is desired to obtain a wider tuning range of . However, the capacitance is usually limited by the thickness of the dielectric layer (usually 150 ~ 300 nm for SiO2), while large gate voltage more than 100 V, which would possibly induce breakdown, is required to change graphene chemical potential to a few tenths of eV. On the other hand, solid polymer electrolyte has been proposed to replace the typical dielectric and electrode layers.38-41 The electrolyte, which is synthesized by dissolving salts in polymers, can be coated above or under graphene. By applying a gate voltage between the electrolyte and graphene, an ionic layer forms at the electrolyte-graphene interface, modifying charge density on graphene. The thickness of the ionic layer, which is approximately 1-5 nm,42 is much smaller than the dielectric layer, providing a higher capacitance and wider tuning range than the graphene-dielectric-electrode structure. Ju et al. used an ion gel as electrolyte on graphene metasurface to fabricate tunable terahertz metamaterials.43 Mehew et al. demonstrated a photodetector based on a transparent polymer electrolyte and graphene.44 Ohno et al. investigated an electrolyte-gated graphene transistor for detecting pH and protein absorptions.45



Recent studies demonstrated the theoretical possibility in tailoring infrared thermal radiation with graphene-covered metasurfaces by exciting magnetic polariton (MP), which is strong coupling between the external electromagnetic waves and magnetic resonance inside the structure. Theoretical studies showed that graphene could enhance the infrared absorption of metal gratings.46-47 In addition, graphene was found effective in improving the magnitude and coherence of infrared transmittance of metal gratings.48 The tunable infrared source was also proposed by coating graphene on silicon carbide grating.49 Several unique features of MP, such as tunable peak wavelength by graphene chemical potential and metasurface feature size as well as insensitivity to incident angle and polarization, are advantageous over other mechanisms like surface plasmon/phonon polaritons particularly for energy-related applications like radiative thermal management and coherent infrared thermal sources. However, few of these MP-based tunable graphene metasurface designs has been experimentally realized because of the challenge in the successful transfer of monolayer graphene onto metasurfaces and the difficulty in electrical tuning of graphene chemical potential. In the present study, we experimentally demonstrate actively tunable infrared absorption upon electrical gating from monolayer graphene covered SiC metasurface with the polymer electrolyte. A dry method is employed to transfer the graphene on SiC metasurface prepared by focus ion beam (FIB). Then the monolayer graphene is characterized by atomic force microscope (AFM) and Raman spectroscopy. By applying a gate voltage to the graphene via polymer electrolyte, the spectral absorption/emission peak of the metasurface is demonstrated to be tuned. The in-situ measurement of infrared absorptance is taken by Fourier transform infrared (FTIR) microscope at different gate voltages. Numerical simulation is conducted to elucidate the underlying physics.


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Results and Discussion

Figure 1. Tunable SiC metasurface covered with graphene and polymer electrolyte: (a) schematic of proposed tunable metasurface structure; (b) fabrication process of SiC metasurface by focus ion beam (FIB); (c, d) graphene transfer process via a dry method onto SiC metasurface followed by the coating of solid polymer electrolyte by a solution-based method.

The proposed metasurface, as depicted in Figure 1(a), has a stacked structure of a SiC metasurface, monolayer graphene, and solid polymer electrolyte with electrodes. The sample was fabricated through a multi-step procedure (see Figure 1b-d) including fabricating SiC metasurface by focus ion beam (FIB), transferring graphene via a dry method,50 and coating solid polymer electrolyte of lithium perchlorate (LiClO4) and poly(ethylene oxide) (PEO) by a solution-based method.42 The details of the fabrication process are elaborated in Methods section. The scanning electron microscope (SEM, Hitachi S4700 FESEM) and atomic force microscope (AFM, Nano3D100, Mad City Lab Inc.) images of the bare SiC metasurface after FIB are shown in Figure 2(a) and (b), respectively. The metasurface has a period of P = 5.2 m, a groove width of b = 0.41 m, and a depth of h = 1.15 m. For comparison, the AFM image of the graphene-covered SiC 5 ACS Paragon Plus Environment


metasurface is also provided in Figure 2(b), where the grooves are almost invisible. This is because the AFM probe tip could not go into the groove due to graphene coverage, thereby confirming that grooves were covered by graphene.

Figure 2. Materials characterization of bare and graphene-covered SiC metasurface: (a) SEM images of the bare SiC metasurface without graphene with period P = 5.2 m, groove width b = 0.41 m, and groove depth h = 1.15 m; (b) AFM image of bare SiC metasurface without graphene (left) and graphene-covered SiC metasurface (right); (c) Raman spectrum of the bare, graphenecovered, and electrolyte-coated graphene-covered SiC metasurfaces.

In addition, Raman spectroscopy (Renishaw InVia system) was also employed to identify graphene in this study.51-53 First, the bare SiC metasurface without graphene was characterized by a 488 nm laser through a 100x lens, and there is no feature of graphene from the Raman spectrum as shown in Figure 2(c). Then right after the graphene transfer, the metasurface was measured for which prominent Raman features of graphene were detected at ~1584 cm-1 (G band) and ~2700 6 ACS Paragon Plus Environment

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cm-1 (2D band) with the laser beam focused at the metasurface groove. Finally, after the cast of the solid polymer electrolyte, the structure was characterized again, and the Raman spectrum confirms the intact graphene monolayer below the thin layer of polymer electrolyte. AFM measurements and Raman characterizations clearly confirmed the successful transfer of graphene onto the SiC metasurfaces covered by the polymer electrolyte for electrical tuning. The change in chemical potential can be experimentally monitored using Raman spectroscopy.51

Figure 3. In-situ electrical, Raman and infrared spectroscopy measurements: (a) schematic of the capacitor formed between graphene and electrolyte; (b) IDS as a function of gate voltages (Vg) to determine the Dirac point of monolayer graphene; (c) Raman spectra of electrolyte-coated graphene-covered SiC metasurface at different gate voltages. (d) schematic of in-situ voltagedependent FTIR measurement setup; (e) measured and simulated spectral absorptance of SiC slab



and bare SiC metasurface; (f) measured spectral absorptance of the tunable metasurface coated with graphene and polymer electrolyte at gate voltages Vg = 0 V and 5 V.

In order to electrically tune the chemical potential of graphene, electrodes must be made to allow gate voltage to be applied. A drop of conductive silver paint (Structure Probe, Inc.) directly on the graphene layer (away from the SiC metasurface and not covered by the electrolyte) served as the bottom electrode, while a gold probe inserted into the electrolyte acted as the top electrode. The electrodes were connected to a source meter (Keithley 2400), which varies the gate voltage Vg. A capacitor is thus formed for electrically gating the graphene with the electrolyte as illustrated in Figure 3(a). Free cations in the electrolyte, i.e., Li+, accumulate near the interface between the electrolyte and graphene, which is connected to the bottom electrode. The layer of the charges around the electrode is called the Debye layer, which readily serves as a parallel-plate capacitor between the graphene and electrolyte51 with capacitance Cg = 0d/de. Note that d is the relative permittivity of the dielectric medium, which is taken as d = 5 as the medium is the PEO matrix constraining the movement of the cations.54 The Debye layer thickness de was reported to be ~1-5 nm,42 and de = 3 nm is assumed in this case. The Dirac voltage VDirac, which is related to the structure and materials,43 was measured based on a field effect transistor device, as shown in the inset of Figure 3(b). At a fixed drain-source voltage of VDS = 50 mV, the drain-source current IDS was measured upon different gate voltage Vg by a source meter (Keithley 2400). The Dirac point is the voltage at which IDS is minimal, and here we have VDirac = 0.3 V. With these parameters, the graphene chemical potential  was calculated to be approximately 0.2 eV at Vg = 0 V, and 0.8 eV at Vg = 5 V (see the detailed calculation in the Supporting Information). 8 ACS Paragon Plus Environment

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An in-situ Raman measurement has been taken at various applied voltages, which is shown in Figure 3(c). As the applied voltage increases, the Raman shift of the G peak increases and the fullwidth at half-maximum (FWHM) of the 2D peak decreases. These features are clear evidence of a change in graphene chemical potential caused by electron doping.51 The voltage-dependent Raman measurement confirmed that the chemical potential of graphene increased as the applied voltage increased. The schematic of the in-situ spectroscopic measurement is shown in Figure 3(d), where the tunable infrared absorption of graphene-covered SiC metasurface was measured by an FTIR microscope (Nicolet Continuμm, Thermo Scientific) by electrical gating with the polymer electrolyte. The modulated unpolarized infrared light from the FTIR bench (Nicolet iS50, Thermo Scientific) was focused on the metasurface sample through a 15x microscope objective (NA = 0.58) at a spot size of 45 m  45 m, and the reflected signal from the sample was collected by a liquidnitrogen-cooled detector (mercury cadmium telluride, MCT) inside the microscope. The spectral reflectance R of the sample was obtained from first dividing the sample signal by the reference signal from a gold mirror and then multiplying with theoretical normal reflectance of gold calculated with optical constants from Palik’s handbook.55 The reflectance was obtained as the average from three independent measurements by inter-changing the sample and gold mirror, while each measurement was averaged from 100 scans at a spectral resolution of 1 cm-1. The spectral absorptance of the sample was calculated from  = 1R since the SiC substrate is opaque within the wavelength range of 11-13 m due to its strong phonon absorption. It should be noted that the spectral-directional emittance equals spectral-directional absorptance according to Kirchhoff’s law, i.e.,   .



The spectral absorptance of planar SiC slab without metasurface was first measured. As shown in Figure 3(e), the SiC slab is highly reflective with close-to-zero absorption within the wavelength range of 11 to 12.5 m. In a broader spectral range of 8 to 13 m as shown in the inset, SiC is highly absorptive at  < 10 m with a sharp drop in absorptance around  = 10.5 m. The fabricated SiC metasurface, on the other hand, leads to an absorption peak between  = 11.5 and 12.5 m with maximum absorptance close to 0.8. Numerical simulations based on finite-difference time-domain (FDTD) method were performed to validate the FTIR measurement, where the dielectric function of SiC is described by a Lorentz oscillator model.56 The simulated spectralnormal absorptance of the SiC slab and bare SiC metasurface was also plotted in Figure 3(e), which agrees with the measurement quite well. The fabricated SiC metasurface sample exhibits slightly higher and broader absorption from measurement than that predicted by the simulation mainly due to the ion doping introduced into SiC during the FIB fabrication process.57 The SiC metasurface covered with monolayer graphene and polymer electrolyte was then characterized using the same FTIR microscope. Before electrical gating (i.e., Vg = 0 V), the absorptance of the metasurface covered by graphene and polymer electrolyte was measured first as shown in Figure 3(f), where the same spectral absorption peak from graphene-covered SiC metasurface was also observed but with slightly lower peak absorptance due to the slight absorption of the polymer electrolyte layer. When Vg = 5 V was applied, the absorptance peak shifts to a shorter wavelength by ~0.05 m, indicating the tunable absorption upon electrical gating. Multiple similar measurements (see Supporting Information) showed that the tunable absorptance spectrum is repeatable.


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Figure 4. Numerical simulations and analytical model: (a) FDTD simulated spectral absorptance of graphene-covered SiC metasurface at graphene chemical potential  = 0.2 eV (i.e., Vg = 0 V) and  = 0.8 eV (i.e., Vg = 5 V); (b) Magnetic field distribution and quiver of current within the groove at graphene chemical potential  = 0.2 eV; (c) equivalent LC circuit to predict MP resonance frequency.



To elucidate the electrical tuning mechanism of the spectral absorption, numerical simulation was conducted for graphene-covered SiC metasurfaces, where the electrical permittivity of monolayer graphene was used.49 Note that the permittivity of graphene is a strong function of chemical potential , which in turn can be varied by gate voltage Vg (see Supporting Information). The simulated absorptance of the graphene-covered SiC metasurface with graphene chemical potential  = 0.2 and 0.8 eV are presented in Figure 4(a). It should be noted that polymer electrolyte was not considered in the simulation because of unknown optical constants in the infrared. Similar to the experimental observation, the simulated absorption peak shifts to a shorter wavelength as the chemical potential  increases from 0.2 eV to 0.8 eV, or correspondingly with gate voltage Vg increased from 0 V to 5 V in the measurement. To further explain the mechanism of the tunable peaks, the magnetic field distributions and current quiver at the peak wavelength where the absorptance reaches maximum are plotted in Figure 4(b) for the case of  = 0.2 eV. The squared magnetic field normalized to the incident magnetic field is represented by contours, and the current quiver are indicated by arrows. The magnetic field between the SiC metasurface is about two orders of magnitude higher than the incidence one, indicating strong magnetic field confinement within the groove. Meanwhile, the electrical current around the groove forms a counterclockwise loop. These unique features signify the excitation of magnetic polariton (MP), and similar behaviors were observed for the case of  = 0.8 eV, revealing that the absorption peaks of the graphene-covered SiC metasurface are attributed to the excitation of MP. The tunable absorption peak of graphene-covered SiC metasurface with gate voltage is also explained here with the aid of an inductor-capacitor (LC) circuit model. As illustrated in Figure 12 ACS Paragon Plus Environment

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4(c), the SiC and graphene are treated as inductors, while the groove (air or vacuum) inside the SiC serves as a capacitor. The total impedance of the circuit is given by

Z  i LSiC 

i LGr 1   2 LGr Cair

The groove capacitance Cair is determined from Cair 

(2)

c1 0 h where c1 = 0.5 is the coefficient b

responsible for the non-uniform charge distribution at the SiC surfaces.49 The inductance of the SiC metasurface includes two parts, i.e., LSiC = Lm + Lk, where Lm and Lk are the mutual and kinetic inductances, respectively. The mutual inductance Lm based on the coil inductance is given by

Lm   0 h  b    where 0 is the permeability of vacuum, and  = /4k is the penetration depth of bulk SiC determined from the wavelength  and the extinction coefficient k. The kinetic inductance Lk  

Lk

stemming

from

oscillating

dipoles

by

lattice

vibrations

follows

 where s is the path length the induced current circulates in the circuit.  0       2  s

2

2

Here, s yields s = 2h + b + 2 with the consideration that the central plane of the current is /2 away from the SiC surface.  and  are the real and imaginary parts of permittivity, respectively. The graphene inductance LGr includes exclusively the kinetic part. On the other hand, the permittivity of graphene (see Supporting Information) can be simplified as G = iG/0tG due to the fact that the real part of the permittivity is much larger than 1 within the infrared range. The complex conductivity of graphene  G   G  i G is a function of chemical potential , which is provided in Supporting Information. The graphene thickness tG was set as 0.345 nm. As a result,



the inductance of graphene can be given by LGr 

 G where the path length yields s =   G   G 2  s

2

b + . In an LC circuit, the MP resonant frequency LC is the one that zeros the total impedance Z. As the graphene inductance LGr is a function of chemical potential  or applied gate voltage Vg, the resonant frequency LC is dependent of the applied voltage Vg, which explained why the absorption peak is tunable upon gating. Here the resonant wavelength, which is obtained from LC = 2c0/LC, is LC = 11.62 m for  = 0.2 eV (equivalently Vg = 0 V), or LC = 11.34 m for  = 0.8 eV (equivalently Vg = 5 V). The relative error between the resonant wavelengths calculated from the LC model and the peak wavelengths simulated from the FDTD method is only 3%. Clearly, LC shift to a shorter wavelength as the applied voltage increases, which is consistent with the measurement.

Figure 5. Simulated spectral absorptance (a) as a function of graphene collapse depth inside the SiC metasurface groove and (b) when the groove is filled with DI water with refractive index n


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around 1.1, to possibly explain the differences between measurement and simulation for tunable spectral absorption of graphene-covered SiC metasurface upon electrical gating.

While the trend on electrically tuning the infrared absorption is consistent between simulation and measurement, there are a couple of discrepancies that should be discussed. The peak magnitudes in the measurement are lower than the simulation, and the peak locations are also a bit different. In addition, the absorption peak wavelength range shifts by peak = 0.05 m from Vg = 0 V to 5 V observed by experiment, which is smaller than that predicted peak= 0.3 m from  = 0.2 eV to 0.8 eV by the simulation. These discrepancies can be possibly understood due to some imperfections of the sample during the measurement. As illustrated by the inset of Figure 5(a), the monolayer graphene may collapse after the electrolyte was dropped on, and the electrolyte may permeate into the groove through the monolayer graphene layer (see inset of Figure 5b). Although the graphene was carefully treated by the dry transfer method and flatten on the metasurface, the monolayer may still collapse into the groove as a result of bearing the electrolyte drop. To estimate the effect of a collapsed graphene, a series of cases were simulated by considering the graphene layer falling into the groove by a distance of  from the top surface of the metasurface.58 As shown in Figure 5(a), the fallen graphene has little effect on the peak location when  = 0.2 eV. However, the peak of  = 0.8 eV shifts to longer wavelength as  increases, which leads to a smaller peak shift peak than the flat graphene. On the other hand, the effect of electrolyte-filled grooves cannot be simulated directly as the optical properties of polymer electrolyte are unknown, which are highly dependent on the concentration of the solution and synthetic method. It is well known that the polymer is absorptive within the infrared wavelength range,59 which would affect the absorption spectra of the structure. An alternative case was simulated by considering the groove 15 ACS Paragon Plus Environment


was filled by water,60 and the resulting absorption peaks plotted in Figure 5(b) are less sharp compared to the case of empty grooves. More cautions should be taken in the future in order to prevent graphene from collapsing or polymer electrolyte from permeating into the grooves in order to achieve greater spectral shift with sharper absorption peaks. Alternative fabrication method, such as an upside-down process, may be a solution. Note that higher-order MP which could be excited by increasing grating depth and hybrid modes of various mechanisms may also be considered to enhance the peak sharpness and magnitude.34-36, 47, 61

Conclusions electrical tuning of infrared absorption has been experimentally demonstrated with a SiC metasurface covered by monolayer graphene and solid polymer electrolyte. The absorption peak of the metasurface can be shifted by varying the gate voltage applied to the graphene and electrolyte. It has been revealed that the shifting absorption peak is attributed to the tunable resonant frequency of MP, and the resonant frequency is a function of graphene chemical potential, which is related to the gate voltage. While the measured tunability was smaller than the predicted one, which might be caused by the collapsed graphene and permeated electrolyte, the results showed the possibility of electrically tuning graphene chemical potential to tailor MP based spectral absorption for infrared applications.

Methods Sample fabrication. The fabrication process is depicted in Figure 1(b-d). The fabrication started with a 340-m-thick SiC wafer (resistivity > 1105-cm, double side polished, Xiamen


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Powerway Advanced Material Co., Ltd.). Then the metasurface was patterned using focus ion beam (FIB, Nova 200 Nanolab-FEI. The as-fabricated metasurface was characterized using a scanning electron microscope (SEM, Hitachi S4700 FESEM) and has a period of P = 5.2 m, a groove width of b = 0.41 m, and a depth of h = 1.15 m.57 The second step was preparing the monolayer graphene (see Figure 1c). The easy-transfer monolayer graphene (Graphenea Inc.) sample, prepared by CVD growth on copper foil, was initially on a polymer substrate and covered by a protective PMMA layer. By immersing the PMMA/graphene/polymer in deionized water (DI water), the PMMA/graphene was released from the polymer substrate and fished by a SiO2/Si substrate (300 nm SiO2 on doped Si using plasma enhanced chemical vapor deposition). A dry-transfer method was employed then to transfer the graphene onto the as-fabricated SiC metasurface.50 A polydimethylsiloxane (PDMS) frame was deposited on top of PMMA layer to serve as a handle. The PDMS/PMMA/graphene/SiO2/Si composite was then immersed into the etchant of potassium hydroxide (KOH) solution to dissolve the SiO2 layer, leading to the separation of PDMS/PMMA/graphene from the substrate. Using the PDMS handle, the composite was rinsed and dried before being transferred onto the SiC metasurface, as shown in Figure 1(d). The composite was heated to 150C for 12 h before peeling off the PDMS handle. The heat treatment helped the PMMA/graphene make full contact with the metasurface top and flatten the wavy film.50 The PDMS can be peeled off because the adhesion of the graphene to the substrate is larger than that of PDMS to PMMA. The last step of the graphene transfer was to remove the PMMA layer by putting the PMMA/graphene/SiC metasurface into a furnace within nitrogen atmosphere at 450C for 2 h. The next step was to coat the structure with a layer of solid polymer electrolyte prepared by dissolving lithium perchlorate (LiClO4) and poly(ethylene oxide) (PEO) at the ratio of 0.12:1 in



methanol as a precursor. The precursor was drop-cast on top of the graphene/metasurface structure through a 0.22 m poly(tetrafluoroethylene) (PTFE) syringe filter to obtain a transparent drop. The whole structure was then baked by 90C to remove residual moisture and methanol. Numerical simulation. In FDTD simulation, a 3D model of one unit cell including the graphene and SiC metasurface was established. The simulation region had a size of 5.2 m  5.2 m  20 m (x  y  z) referring to the period of the SiC metasurface of P = 5.2 m. The boundary condition (BC) along the z direction was the perfectly matched layer (PML), and those for x and y directions were the Periodic BC. A mesh check was carried out to obtain converged results. A power monitor above the structure was placed to record the reflected energy, and a field monitor was placed in the center plane of the structure to record the electromagnetic field distribution. Based on the electric field E, the current density J was obtained by J = E, where  is the complex electrical conductivity of a material. Real part of J, which represents the actual current density, at different locations were then calculated to show the current quiver.

Supporting Information Relation between graphene chemical potential and gate voltage; additional measurements of tunable absorptance peaks; relation between graphene properties and applied gate voltage.

Author information Corresponding author e-mail: [email protected] Notes The authors declare no competing financial interest.


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Acknowledgments This work was mainly supported by the National Science Foundation under Grant No. CBET1454698 (L.L, Y.Y. and L.W.), and in part by AFOSR Young Investigator Program with Grant No. FA9550-17-1-0080 (X.Y. and L.W.). Y.Y would like to thank the support from the National Natural Science Foundation of China under project number of 51806045, and the Shenzhen Science and Technology Innovation Commission under project number of 201771343. Access to the NanoFab and Eyring Materials Center at Arizona State University for sample fabrication and materials characterizations was supported in part by NSF contract ECCS-1542160. We would like to thank Prof. Sefaattin Tongay for using their Raman facility.

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Tuning the Infrared Absorption of SiC Metasurfaces by Electrically

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