SUPPLEMENTARY INFORMATION - media.nature.com · 3 . I. Preparation and characterization of graphene...

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Switching terahertz waves with gate-controlled active graphene metamaterials

Seung Hoon Lee, Muhan Choi, Teun-Teun Kim, Seungwoo Lee, Ming Liu, Xiaobo Yin,

Hong Kyw Choi, Seung S. Lee, Choon-Gi Choi, Sung-Yool Choi, Xiang Zhang & Bumki

Min

I. Preparation and characterization of graphene A. CVD growth of SLG and MLG and their transfer method B. Characterization of CVD-grown graphene

II. Transparent EOT gate electrode A. Design of EOT electrode B. Polarization dependency

III. Study on MLG metamaterials A. Amplitude and phase modulation

IV. Study on SLG metamaterials A. Simulation method B. Modulation speed C. Mitigation of large voltage requirement D. Humidity effect on memory performance E. Reproducibility

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT3433

NATURE MATERIALS | www.nature.com/naturematerials 1

© 2012 Macmillan Publishers Limited. All rights reserved.

http://www.nature.com/doifinder/10.1038/nmat3XXX

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Figure S1 | Schematics of the gate-controlled active graphene metamaterials and their optical images a, Schematic view on fabrication of the active graphene metamaterial (PI: polyimide, td: total device thickness). b, Optical micrographs of the fabricated hexagonal meta-atoms c, Asymmetric double split ring (aDSR) meta-atoms.

2 NATURE MATERIALS | www.nature.com/naturematerials

SUPPLEMENTARY INFORMATION DOI: 10.1038/NMAT3433



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I. Preparation and characterization of graphene

A. CVD growth of SLG and MLG and their transfer method

For the fabrication of samples, we used commercially available single-layer graphene

(SLG) from GRAPHENE SQUARE Inc., and multilayer graphene (MLG) grown by in-

house thermal chemical vapour deposition (CVD). For MLG growth, 300-nm-thick

nickel was deposited as a catalyst layer on a SiO2/Si substrate by an electron-beam

evaporator. Prepared Ni/SiO2/Si substrate was loaded in a quartz tube furnace, then

heated up to 1,000 °C under vacuum condition (30 mTorr) with a constant H2 flow (10

sccm). MLG was grown by flowing gas mixtures (CH4:H2 / 30:10 sccm) for 3 min and

MLG on the substrate was dismounted from the furnace and cooled down to room

temperature (25 °C) at a cooling rate of 5 °C/min under Ar gas flow of 1,000 sccm.

In order to transfer the CVD-grown SLG and MLG to a polyimide substrate, a

polydimethylsiloxane (PDMS) film was used to minimize undesired cracks and solvent-

induced effects. The thermally grown SLG (MLG) on the Cu foil (Ni/SiO2/Si substrate)

was peeled off from the substrate after 8-hour etching in a 0.1 mol% ammonium

persulfate (3-hour etching in a 12.5 mol% iron chloride) solution. After a two-step

rinsing process with DI water, the graphene layer was successfully transferred and

conformally attached to the prepared polyimide layer, on which the meta-atoms were

patterned.

B. Characterization of CVD-grown graphene

Confirmation of Single-Layer Graphene Raman spectroscopy was used with a 532

nm excitation laser for the characterization of graphene layers. The SLG, grown in the

same batch for samples used in the main manuscript, was transferred onto a 300-nm-thick

SiO2 substrate. The representative features of the SLG can be summarized as follows [1];

(1) The 2D-peak over G-peak intensity ratio (I2D/IG) is generally in the range between 2

and 4, (2) The full width at half maximum (FWHM) of 2D-peak is about ~30 cm-1. As




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shown in Fig. S2a, the location and FWHM of 2D peak of CVD-grown is 2,686 cm-1 and

30 cm-1, respectively. To further investigate the graphene’s quality, we have also

conducted Raman spectroscopic mapping of I2D/IG for CVD-grown graphene over an

arbitrarily chosen area of 20 × 20 μm2, and the data are presented in Fig. S2c. For

comparison, the mapping of I2D/IG is also added for a mechanically exfoliated graphene

in Figs. S2b and d.

Figure S2 | Comparison between CVD-grown graphene and mechanically exfoliated graphene a, Raman intensity plot of CVD-grown graphene shows that 2D peak is located at 2,686 cm-1 with the FWHM of about 30 cm-1. b, Raman intensity plot for mechanically exfoliated graphene, c, Mapping of Raman intensity ratio of G-peak to 2D-peak (I2D/IG) for arbitrarily chosen area of 20 × 20 μm2 of CVD-grown graphene and d, mechanically exfoliated graphene on a 300-nm-thick SiO2 substrate.





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Amount of Doping In the main manuscript, we have estimated the doping level

(carrier density as a function of gate voltage) of the graphene integrated into the device in

Fig. 2f. It is generally known that during the synthesis and additional fabrication

processes CVD-grown graphene is in a p-doped state. For our graphene in the device, the

hole density under the condition at Vg = 0 V was estimated to be ~2.99 × 1012 cm-2 from

the gate capacitance in the electron charge (α = 8.54 × 109 cm-2V-1) and the measured

charge neutral gate voltage VCNP. Therefore, we have concluded that the corresponding

Fermi level of graphene inside the device was about -203 meV at Vg = 0 V.

For direct estimation of the amount of doping in CVD-grown graphene on a SiO2

substrate, we conducted further measurements using Raman spectroscopy as with the

work by Basko et al [2]. In their method, the doping level can be estimated non-

destructively by the ratio of the integrated area under 2D to that of G-peak (denoted as

A2D/AG) in the Raman spectra. Figures S3a and b show the measured spectrally-

integrated area ratio of 2D-peak to G-peak (A2D/AG) of the CVD-grown graphene on a

SiO2 substrate with the value of mechanically exfoliated graphene. The value of A2D/AG

for CVD-grown graphene is measured at about 2.8 while the value (A2D/AG) for

exfoliated graphene is slightly above 5. From the A2D/AG versus doping level plot from

the reference [2], we could estimate the doping level of the CVD-grown as -250 meV,

while that of mechanically exfoliated graphene as -100 meV. The doping level of CVD-

grown graphene on the SiO2 substrate (EF = -250 meV) is estimated to be slightly higher

than that of the graphene imbedded in the polyimide-based metamaterial (EF = -203

meV). This difference can be attributed to the following factors; (1) device (graphene)

variation as can be seen in the reference [2], (2) dependence on the substrate (i.e., SiO2

versus polyimide), (3) single-side versus double-side coverage of dielectric materials, and

(4) different method of graphene preparation.

We have also estimated the doping level using the other method established by Das et al.

[3]. The method is based on the measurement of the ratio of 2D- to G-peak intensity

(I2D/IG). In our graphene metamaterial, the doping level was comparable with the value

in the reference.





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Figure S3 | Raman mapping images Area ratio A2D/AG of a, CVD-grown graphene and b, mechanically exfoliated graphene as a reference sample. Intensity ratio ID/IG for c, the CVD-grown and d, exfoliated graphene.

Amount of Defects The D-peak is a crucial measure for the quantitative estimation of

the amount of defects, because the breathing modes of hexagonal carbon ring,

represented by D-peak, cannot be excited without defects. The relation between the

amount of defect and the intensity ratio of D-peak to G-peak (denoted as ID/IG) was

thoroughly discussed in the reference [4]. In Figs. S3c and d, the ID/IG mapping data for

the arbitrarily chosen area of 20 × 20 μm2 of CVD-grown graphene (mechanically

exfoliated graphene) shows that the average values of ID/IG is given by 0.287 (0.0441).

From that result, we have estimated the distance between point-like defects in graphene

of LD = 19.0 ~ 25.3 nm (48.6 ~ 64.6 nm) and the defect density of nD = 4.96 ~ 8.77 × 1010

cm-2 (0.76 ~ 1.35 × 1010 cm-2), respectively. Since ID/IG value corresponds to two

different defect densities, it is therefore necessary to use additional information such as





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the FWHM of G-peak. The FWHM of G peak in our CVD-grown graphene (single layer

part) has a value < 15 cm-1, which allow the distance LD to be determined unambiguously

[4]. To conclude, from the evaluation of the intensity ratio of D- to G-peak, our CVD-

grown graphene turns out to be more defective and its defect density (single layer part) is

at least 6.5 times larger than that of the reference sample (exfoliated graphene).

Multilayer graphene Raman spectroscopy of MLG shows that the number of layer

varies spatially and is estimated to be approximately 1 to 10, as confirmed by

transmission electron micrographs (TEM) as shown in Fig. S4. A cross-sectional TEM

image of the fabricated MLG metamaterial shows that there are several discontinuities

with defects sites, such as gaps and wrinkles, which can be attributed to the one of the

dominant causes of electrically controlled photonic memory effects.





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Figure S4 | Characterization of MLG by Raman spectroscopy and transmission electron microscopy (TEM) a, OM image and b, Raman spectrum of MLG shows that the FWMH of 2D peak is larger than 45 cm-1. c, A cross-sectional TEM image of the MLG metamaterial. Between the polyimide layer and metallic meta-atoms, layered graphene is clearly identified. d, Average intensity profile across the graphene layer marked by red arrows.





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II. Transparent EOT gate electrode

A. Design of EOT electrode

The gate electrode for the active graphene metamaterial was designed to apply a uniform

electric field in the graphene layer while maintaining a large transmission of terahertz

waves. In order to satisfy this requirement, an array of deep subwavelength-scale metallic

wires, referred to as an EOT electrode, was optimized. Figure S5a shows the schematics

(inset) of the EOT electrode dimension with numerically calculated transmission at

terahertz frequencies (0 – 2 THz). The EOT electrodes for the sample were fabricated

with the periodicity (a) of 6 μm and the gap width (a - wm) of 2 μm. Although the gap

width of the wire array is in the deep subwavelength-scale (~λ/150 at 1 THz), the

simulated transmission is found to be over 89% in the frequency band of interest. This

large transmission can be explained on the basis of well-known phenomena of

extraordinary optical transmission [5].

In addition, we have conducted finite element analysis using COMSOL Multiphysics and

calculated the gate-induced electric potential distribution inside the graphene

metamaterial. As illustrated by the flattened light blue area in the vicinity of graphene

(marked by the deep blue colour, Figs. S5b-d), the EOT gate electrode (even with low

width, 2 μm) can clearly induce a uniform static electric field in the vicinity of the

graphene layer.





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Figure S5 | EOT electrode design for the gate-controlled graphene metamaterials a, Schematic rendering of EOT electrodes along with simulated transmission spectra for metallic wire widths (wm) of 3, 4, and 5 m. b-d, Electric potential distribution in the periodic EOT electrode as function of wm when applying Vg = 100 V (periodicity a = 6 μm, device thickness t = 4 μm, and spacing s = 2 μm). Nearly uniform electric potential is distributed in the graphene layer which is conformally deposited onto the hexagonal meta-atoms.





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B. Polarization dependency

Figure S6 | Polarization dependent transmission characteristics of the gate-controlled active SLG metamaterials The polarization dependency of graphene metamaterials is attributed to the polarization dependent transmission through the EOT electrode a, Schematic rendering of an active graphene metamaterial with only a EOT electrode. b-d, Polarization dependent transmission through the active graphene metamaterials as a function of gate voltage.





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III. Study on MLG metamaterials

A. Amplitude and phase modulation

Figure S7 | Amplitude and phase modulation of terahertz waves with the gate-controlled active MLG metamaterial a, Measured transmission spectra and their phase change as a function of ∆V (= VCNP - Vg) from 0 to 120 V. With increasing gate voltage, the resonant frequency was red-shifted and broadened by Joule losses in the graphene layer. Relative change in transmission (-∆T/TCNP) and phase change (∆ϕ) exceeded -138% and /3 radians at 120 V, respectively. b, For comparison, the simulated results of the active graphene metamaterials are plotted as a function of the Fermi level in a graphene layer (Here, we assumed the intraband scattering time of 25 fs and zero carrier density at conductivity minimum).





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IV. Study on SLG metamaterials

A. Simulation method

For the modelling of gate-controlled active graphene metamaterials, we employed finite

element analysis provided by the commercially-available CST MICROWAVE STUDIO.

Frequency-dependent material parameters (complex permittivity) of gold at terahertz

frequencies were taken from the literature [6] and those of polyimide were taken from a

previously reported paper [7]. The complex dielectric constants of gold for the frequency

range of interest (0.1 ~ 2.5 THz) can be fitted by using the Drude model with a plasma

frequency of ωp = 1.37 × 1016 rad/s and collision frequency of γ = 4.07 × 1013 rad/s. For

the simulation of a unit cell, the periodic boundary condition is applied. After running

the finite element simulations with the appropriate boundary conditions, S-parameters

(transmission and reflection) were carefully extracted from the simulation results.

For graphene, Kubo formula was used to model the gate-variable complex optical

conductivity of graphene, σ(ω) = σintra(ω) + σinter(ω), where intraband transitions are the

dominant factors in the terahertz regime. The formulae for complex conductivity of

graphene are given as follows [8],

σ��ω� � � �� ∆��

∆ ,

σ��ω� � ��

�� ∆�

�� ∆ .

Here, �� is the Fermi distribution function with Fermi energy ��, � describes the

broadening of the interband transitions, � is the momentum relaxation time due to carrier

intraband scattering, and ∆ is a half bandgap energy from the tight-binding Hamiltonian

near K points of the Brillouin zone [9].

The effective complex permittivity of graphene can be directly calculated once the

complex conductivity is obtained by the Kubo formula given above. The conversion

between these two parameters is discussed in detail in the reference [10] and the final





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expressions are given here; ��eff � � �� and ��eff � ��

�� , where �� and �� are the real and

imaginary part of complex conductivity of graphene, � is the angular frequency, and � is

the thickness of graphene. Here, � is not the actual thickness of graphene and it is

sufficient to choose the value of � much smaller than the wavelength of interest for the

simulation. The complex permittivity of graphene is shown in Fig. S8. The plots are

generated by assuming the thickness of graphene � = 1 nm and the intraband scattering

time of � = 16 fs.

Figure S8 | Complex permittivity of graphene used in the simulation of the active graphene metamaterials a, Real and b, imaginary part of the effective permittivity





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B. Modulation speed

In order to estimate the modulation speed of our devices, we have applied ac rectangular

voltage (0 – 10 V) simultaneously to the gate of the active metamaterial and the terahertz

emitter while sweeping the frequency of modulation from 1 to 100 kHz, as with the

method used in the previous studies of THz modulators [11, 12] (For our THz-TDS setup,

the highest frequency response that can be measured is limited to ~100 kHz, which is due

to the limited frequency response of the constituting components, such as the terahertz

emitter and the lock-in amplifier). Figure S9a shows the time-domain waveforms with

(bottom) and without (top) the active graphene metamaterials for the modulating

frequencies of 1, 50, and 100 kHz, respectively. As can be clearly seen from the plots,

the shape of the waveforms was not significantly altered with a variation in modulation

frequency. The peak-to-peak THz wave signal transmitted through the sample (i.e.,

active graphene metamaterial) is normalized to the reference signal (measured without

the sample) at each modulation frequency, and these normalized values are recorded as a

function of the modulation frequency (black circles in Fig. S9b). Up to 100 kHz, the

frequency response roll-off was not obviously observed in the frequency response. Thus,

we can conclude that the modulation speed of the proposed active graphene metamaterial

exceeds at least 100 kHz.

In order to make up for the limited response range that can be measured with THz-TDS

setup, we have measured the dynamic response (S21) of the device using a radio

frequency (r.f.) network analyser. Figure S9c shows a frequency dependent response of

the graphene metamaterial. With the RC time constant fitting, the 3-dB roll-off frequency

of the device was estimated to be 12.6 MHz, the value of which is comparable to those of

2DEG-based high speed terahertz modulators with metamaterials (~10 MHz) [13]. From

these results, we established that the operating bandwidth is not limited by the carrier

transit time in graphene layer, but by the parasitic response of the device.





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Figure S9 | Transmitted terahertz waves with and without the gate-controlled active graphene metamaterials and its frequency response measured by THz-TDS and network analyser a, The transmitted terahertz wave signals with and without the gate-controlled active graphene metamaterials for modulation frequencies of 1, 50, and 100 kHz. b, The frequency response of the graphene metamaterials as a function of modulation frequency (black circle). c, Frequency response of the graphene metamaterials measured by r.f. network analyser (black square). The modulation speed is limited by RC time constant of the device. The RC time constant fitting (red curve) shows that 3-dB roll-off frequency for the graphene metamaterial (spacing ~2 μm) is 12.6 MHz.





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C. Mitigation of large voltage requirement

The maximum gate voltage applied to our graphene metamaterials (presented in the main

manuscript) was on the order of a few hundred volts (for devices with dielectric spacing

of ~2 μm). However, the required gate voltage can be reduced simply by decreasing the

dielectric (here, polyimide layer) spacing (d) between the EOT gate electrode and the

graphene layer. As mentioned in the main manuscript, the EOT gate capacitance in the

electron charge is estimated to be α = 8.54 × 109 cm-2V-1. This value is approximately an

order of magnitude smaller than previously reported value [14], for which the dielectric

spacing d between graphene and back-gate was ~300 nm.

A simple electrostatic parallel capacitor model can be adapted to provide the required

voltage dependence on the spacing of dielectric layer. The total number of charge

carriers accumulated in the proposed EOT gate (or graphene layer) can be expressed as:

�� |��|��, where q is the elementary charge, �� is

the relative permittivity of dielectric (polyimide), �� is the permittivity of free space, and

A is the area of graphene layer. By defining the EOT gate capacitance in the electron

charge � � ��, we can obtain the gate-voltage dependent charge (hole) density in

the graphene layer as follows,

��

��

where VCNP is the charge neutral gate voltage in our device. As seen from above equation,

the rate of change in charge density with respect to the gate voltage (�� ⁄ � � �) is the

EOT gate capacitance in the electron charge. Thus, one of the easiest ways to obtain a

high proportional constant � could be to reduce the dielectric spacing d.

As a proof of concept, we have fabricated active graphene metamaterials with two

different dielectric spacing and measured the rate of change of transmission with a

variation in gate voltage for these two sets of samples. The thickness of dielectric

spacing could be precisely controlled both by the spin-coating speed and by the

polyimide solution. Two sets of newly fabricated samples were designed to have





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dielectric spacing distance d of 1 and 2 μm, respectively. Figure S10 shows the gate

voltage dependent transmission spectra and its relative change as a function of dielectric

spacing for these two samples. For comparison, the relative changes in transmission at

resonance frequency for two samples are plotted as a function of Vg in Fig. S10e. As

seen from the plot, the sample with 2-μm-thick spacing needs approximately two times

larger gate voltage than that with 1-μm-thick spacing to achieve a comparable change of

transmission. This result confirms that the required voltage can be effectively reduced by

decreasing the dielectric spacing between the EOT electrodes and the graphene layer.





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Figure S10 | Gate voltage dependent transmission spectra and its relative change as a function of dielectric spacing a and b for 2 μm spacing; c and d for 1 μm spacing. e, Relative change in transmission at resonance frequency plotted as a function of Vg for samples with two different dielectric spacing. The sample with 2-μm-thick spacing needs approximately two times larger gate voltage than that with 1-μm-thick spacing sample.





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D. Humidity effect on memory performance

In the main manuscript, we attributed one of the possible origins of hysteretic behaviour

in transmission to the adsorption/desorption process of H2O molecules. Although the

graphene layer in our device is partially protected from the air by the encapsulation of PI

(see schematic of device architecture), H2O molecules can still reach and adsorb onto the

graphene layer. This is because H2O molecules are able to penetrate into the PI layer

covering the graphene [15]. In order to clarify the effect of H2O adsorption, we have

carried out measurement under a controlled humidity environment. First, we recorded

the binary memory operation of active graphene metamaterial held in dry air (humidity of

~0.5%) and the results are shown in Fig. S11a. Prior to the measurement, the sample

under test was put in dry air chamber (humidity of ~0.5%) for 24 hours. As mentioned in

the main manuscript, the measured retention time is a function of applied gate voltage

and doping time. Here, for this measurement, the doping time and peak voltage are set to

60 seconds and 300 V gate voltages, respectively. The measured memory retention time

was found to be ~30 minutes and its decay time was estimated to be ~23 minutes. For

comparison, the identical sample was exposed to a highly humid environment (humidity

of ~70%) for 3 hours and the measurement was performed to estimate the retention time

and decay constant. Interestingly, the measured retention time of the sample held in

highly humid condition was approximately two times larger than that of the sample held

in dry air. The decay time was around 46 minutes as can be confirmed in Fig. S11b.

From these controlled experiments, we can conclude that the water adsorption plays a

pivotal role in memory retention time and decay time.





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Figure S11 | Effect of humidity on the memory operation Binary memory operation in transmission near resonance (top panel) with gating pulse signal of 300 V (low panel) for the same sample measured a, in a dry air condition (humidity of ~0.5%) and b, after putting the samples in ~70% humidity for 3 hours. The measured memory retention time before and after the water adsorption is about ~30 and ~60 minutes, respectively. Owing to considerable water adsorption, decay constant of 70%-humidity-sample is also increased to be 46 minutes from sample in dry air of 23 minutes.





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E. Reproducibility

To check the reproducibility of hysteretic behaviour in gate-controlled transmission and

the photonic memory operation, we have fabricated three devices with the same

architecture (Fig. 1a in main manuscript). Three devices were constructed simultaneously

onto a single 4-inch silicon wafer. Arbitrary chosen parts of CVD-grown graphene (on

80 80 mm2 Cu foil) were transferred to three lithographically identical devices (parts

being shown in Fig. S12a). Finally, all samples were soldered to drilled PBC substrates

(Fig. S12b). We summarized in this response the results for three devices labelled as #1,

#2, and #3. First, the resonance frequencies for three devices are nearly the same, as can

be seen in Fig. S12c. The observation of nearly identical resonance frequencies indicates

that the structural fidelity of metamaterials (e.g., sizes, widths, gaps of meta-atoms) was

reproducible across three devices on a single wafer. However, for the hysteretic

behaviour in gate-controlled transmission (Fig. S12d) and the photonic memory operation

(Fig. S12e), while quite a similar trend is observed, they are not perfectly identical across

the three devices. This sample-to-sample variation in a single batch can be attributed to

the spatial variations (of defects and doping) of CVD-grown graphene. Even with the

state-of-the-art preparation of CVD-grown graphene, such variables seem to be

unavoidable at the current stage [16], and further improvements in the production of

CVD-graphene would be required to guarantee the complete reproducibility of the gate-

controlled active graphene metamaterials.





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Figure S12 | Reproducibility test for the gate-controlled active graphene metamaterial a, large area CVD-grown graphene on Cu foil (80 × 80 mm2) purchased from GRAPHENE SQUARE. The parts designated with numbers represent the graphene used in the fabrication. b, All samples were soldered to drilled PBC substrates. c, Measured relative change in transmission with respect to the gate voltage of 80 V, d, hysteretic on-resonance transmission, e, binary memory operation of the three samples.





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