We fabricated an optically transparent monopole antenna using graphene film and investigated the feasibility of the film as an electrode material for antennas. A low sheet resistance (80 Ω/sq) was attained by stacking the graphene films and carrier doping with an ionic liquid. The optical transmittance of the carrier-doped three-layer stacked graphene film was greater than 90%, enabling it to be embedded in highly transparent objects without altering their landscape. Using the monopole antenna structure with a metal ground plane, we measured the reflection and radiation characteristics of the graphene monopole antenna, excluding the contribution from the power feeding components. The radiation efficiency of the graphene monopole antenna, which was measured by the Wheeler cap method, was determined to be 52.5% at 9.8 GHz. Through the measurements of the graphene monopole antenna, we demonstrated that the carrier-doped three-layer stacked graphene film can be used as an electrode material for optically transparent antennas.

A new generation of wireless networks is expected to be utilized in various applications such as autonomous driving, remote healthcare, and the Internet of things. As these applications develop and expand, various objects will be connected by wireless networks, leading to an increase in the number of wireless devices. These developments require faster and more reliable wireless networks than the current ones. Fifth generation (5G) mobile communication has many advantages, including massive data processing with a higher data rate and lower delay. The frequency bands of 5G will be higher than those of the current bands. Specifically, 5G is expected to operate in millimeter-wave frequency ranges of 24.5–27.5 and 37.0–43.5 GHz.1 While the use of these higher frequency bands increases data transfer speeds, the propagation distance of electromagnetic waves decreases.2 Furthermore, the Fraunhofer diffraction effect of the electromagnetic waves is reduced as frequency increases, which leads to the suppression of the electromagnetic waves propagating to the backside of obstacles and decreases the coverage of the wireless networks.3 Recently, optically transparent antennas based on transparent conductive films have been gaining attention since they can be installed in places and on objects without disrupting the landscape. These antennas can be embedded in transparent objects that require high optical transparency such as building windows and car windshields. By using optically transparent antennas, the coverage area can be further expanded in 5G.

Optically transparent antennas have been fabricated using various transparent conductive films, e.g., indium tin oxide (ITO),4 gallium-doped zinc oxide (GZO),5 aluminum zinc oxide (AZO),6 indium zinc tin oxide (IZTO),7 conductive polymer,8 and metal mesh structures.9 We focus on graphene as an electrode material for optically transparent antennas. Many of graphene’s material properties are optimal, such as high carrier mobility,10 high optical transmittance,11 superior mechanical strength,12 and considerable flexibility.13 Additionally, the electrical conductivity of graphene can be increased by carrier doping,14–19 which does not cause a decrease in optical transmittance because graphene has a constant optical conductivity of 2.3% in the visible range.11,20,21 These ideal properties have motivated researchers to investigate graphene-based microwave devices such as antennas,22–24 transmission lines,25–27 and absorbers.28–30 We previously reported that chemical vapor deposition (CVD) graphene-based optically transparent dipole antennas radiated microwave power at around 20.7 GHz.31,32

In our previous study,31,32 the optically transparent antennas were based on a monolayer CVD graphene film and had a high optical transmittance of about 97.7%; however, the high sheet resistance of the monolayer CVD graphene film (about 750 Ω/sq) suppressed the performance of the antennas. Decreasing the sheet resistance of the graphene film is crucial for improving antenna performance.2 To date, Marco et al. have reported that the doped six-layer stacked graphene film, which has a sheet resistance of 18 Ω/sq and optical transmittance of about 85%, operates in the “quasi-metallic” region for an electrode material of antennas.33 In this study, we fabricated an optically transparent monopole antenna based on a doped three-layer stacked graphene film. Graphene films were stacked and doped to decrease the sheet resistance to 80 Ω/sq while maintaining a high optical transmittance of over 90%. As shown in Fig. 1(a), the graphene monopole antenna consisted of the monopole element based on the graphene film and an Al ground disk. By using a monopole antenna structure with a ground plane, the radiation pattern, gain, and radiation efficiency of only the antenna element could be precisely evaluated without the interference from other components, e.g., connectors and cables used for measurement. We designed the resonance frequency of the monopole antennas to be 6–8 GHz in consideration of the fabrication processes and measurement environments. We used the monopole antenna structure with a ground plane to investigate the feasibility of the carrier-doped three-layer stacked graphene film as an electrode material for practical optically transparent antennas.

FIG. 1.

(a) Overview of the monopole antenna with an Al disk. (b) Measured optical transmittance spectra of pristine (dashed black line) and doped (solid red line) three-layer graphene. The inset shows optical images of pristine and doped three-layer graphene. (c) Sectional view near the monopole element. (d) Optical image of the fabricated monopole element.

FIG. 1.

(a) Overview of the monopole antenna with an Al disk. (b) Measured optical transmittance spectra of pristine (dashed black line) and doped (solid red line) three-layer graphene. The inset shows optical images of pristine and doped three-layer graphene. (c) Sectional view near the monopole element. (d) Optical image of the fabricated monopole element.

Close modal

We stacked the CVD graphene films using a layer-by-layer transfer method34 and performed carrier doping with bis(trifluoromethanesulfonyl)amine (TFSA) to lower the sheet resistance of graphene. The fabrication procedure is shown in detail in Fig. 2. First, the graphene film was grown on a Cu foil by low-pressure CVD in a conventional quartz tube furnace using H2 and CH4 gases. The CVD growth was carried out for 30 min with H2 (20 sccm) and CH4 (2 sccm) flows at 1000 °C under a total pressure of 690 Pa. After the CVD growth, polymethylmethacrylate (PMMA) was spin-coated on one side of the Cu foil and baked at 180 °C for 1 min. The unwanted graphene grown on the backsides of the Cu foil was removed by oxygen plasma treatment. The Cu foil was etched away by an Fe(NO3)3 solution (0.5M). The PMMA-coated graphene film (first layer) was directly transferred onto the other CVD-grown graphene film on a Cu foil (second layer). It should be noted that PMMA was not spin-coated on the surface of the second layer graphene film. This transfer technique enables us to obtain a multi-layer stacked graphene film without PMMA residuals between graphene layers because PMMA was not coated on the second and third layer graphene films. After etching the Cu foil of the second layer, the two-layer stacked graphene film was transferred onto the third layer graphene film grown on the Cu foil. Finally, in the same manner, the three-layer graphene was transferred onto a quartz glass substrate. The transferred three-layer stacked graphene film was annealed under Ar flow at 200 °C for 3 h. After the deposition of Au (500 nm) on the graphene film by electron beam evaporation, the monopole antenna was patterned on the Au/graphene film by standard photolithography. Next, the Au layer on the outside of the monopole antenna was etched away at 60 °C by the KI solution, and the unwanted graphene on the outside of the monopole antenna was removed by oxygen plasma treatment. The Au layer on the antenna element was etched away at 60 °C in the KI solution. By using this technique, planar antennas with arbitrary shapes can be fabricated. The graphene monopole antenna was annealed under Ar flow at 200 °C for 12 h to remove impurities on the surface of graphene, followed by the spin-coating of 50 mM TFSA solution on the graphene surface.

FIG. 2.

Fabrication procedure of the graphene monopole antenna.

FIG. 2.

Fabrication procedure of the graphene monopole antenna.

Close modal

The electrical properties of the pristine and doped graphene films were verified by Hall measurements using van der Pauw geometry. The Hall measurements are described in detail in the supplementary material (Fig. S1 and Table SI). The measured sheet resistances of the pristine and doped three-layer stacked graphene films were 580 and 80 Ω/sq, corresponding to the 86.2% decrease in the sheet resistance induced by carrier doping. The Hall coefficients were positive (+138 and +9 m2/C), indicating that the charge carriers in the graphene films were holes. The carrier density of the doped graphene film was 6.6 × 1013 cm−2, which is much higher than that of the pristine one (4.5 × 1012 cm−2). In addition, the carrier doping led to blue shifts in the 2D and G peaks in the Raman spectra35 (Fig. S2). The carrier mobility decreased from 2380 to 1180 cm2/Vs by carrier doping since Coulomb scattering from the ions, bis(trifluoromethanesulfonyl)imide (TFSI), was likely enhanced. The optical transmittance spectra of the graphene films are shown in Fig. 1(b). The dashed black and solid red lines correspond to the measured data of the pristine and doped three-layer stacked graphene films, respectively. At 550 nm, the pristine graphene film had a transmittance of 90.4%, which coincided with the transmittance found in previous studies.16 On the other hand, for the doped three-layer graphene, the optical transmittance spectrum was almost the same as that of the pristine one, and the transmittance at 550 nm was 90.1%. This indicates the change in the optical transmittance caused by TFSA doping is negligible in the visible region. Consequently, we obtained the graphene film with low sheet resistance (80 Ω/sq) and high optical transmittance (over 90%) in the visible range.

The schematic structure of the fabricated monopole antenna with an Al ground disk is illustrated in Fig. 1(a). The monopole antenna on a quartz glass substrate was mounted in the center of the Al ground disk. The diameter and thickness of the Al disk were set to 400 and 4 mm. Figure 1(c) shows the sectional view of the monopole antenna. The monopole element on a quartz glass substrate consists of a rectangular graphene film and an inverted trapezoid-shaped Au feeding electrode. The wider the monopole element, the smaller the contact losses at the interface between graphene and metal.36 The length and width of the rectangular graphene film were set to 5 and 10 mm. The height, top side, and bottom of the inverted trapezoid-shaped Au feeding electrode were set to 1 mm, 10 mm, and 1.3 mm, respectively. The Au electrode was electrically connected to an SMA connector using a conductive paste through a via hole at the center of an Al disk. The origin of the coordinate was located at the interface between the Au electrode and an SMA connector. An optical image of the fabricated monopole antenna is shown in Fig. 1(d). The background can be clearly observed by the naked eye through the graphene film with high homogeneity and high transparency (over 90%). Additionally, as a reference, we fabricated a monopole antenna based on a 500-nm thick Au film with the same size as the graphene monopole antenna.

The characteristics of the fabricated graphene monopole antenna were measured. The detailed measurement setups and conditions are summarized in the supplementary material (Figs. S3–S5). In Fig. 3(a), the magnitude of the reflection coefficients (|Ṡ11|) of the graphene and Au monopole antennas is plotted as a function of frequency. The |Ṡ11| of the graphene and Au antennas show the reflection minima of −8.3 and −10.1 dB at 6.7 GHz and 11.9 GHz, respectively. Subsequently, the input impedance Żin of the antennas is calculated using the following equation: Ṡ11=Żin50Żin+50, and plotted in Fig. 3(b). The resonance frequencies fr, the frequency at which ImŻin=0, were 9.8 and 6.3 GHz for the graphene and Au monopole antennas. The ReŻin at fr was 20.2 Ω and 12.0 Ω, respectively. At fr, ReŻin is represented by the series radiation resistance Rr and series loss resistance RL, i.e., ReŻin=Rr+RL. RL can be experimentally determined by the Wheeler cap method,37–39 as follows. When the monopole antennas were covered with a metal cap (Wheeler cap), the radiated microwave power from the monopole antenna reflected at the inner wall of the metal cap and returned to itself, resulting in Rr = 0. Hence, the ReŻin at fr consists of only RL: ReŻin=RL. Subtracting the RL value from ReŻin at fr measured without the metal cap gives Rr. Thus, Rr and RL can be individually determined by the Wheeler cap method. The Rr and RL of the graphene monopole antenna were 10.6 and 9.6 Ω, whereas those of the Au monopole antenna were 11.5 and 0.5 Ω. The radiation efficiencies were calculated using the equation η(= Rr/(Rr + RL)), and the η of the graphene and the Au monopole antenna was determined to be 52.5% and 95.8%, corresponding to the difference of 2.6 dB. Katsounaros et al. reported an η of 20.7% for a patch antenna fabricated using a monolayer CVD graphene film with a sheet resistance of 985 Ω/sq.40 The η value in this study is higher than that reported by Katsounaros et al. because of the lower sheet resistance (80 Ω/sq) attained by the stacking and carrier-doping techniques. Approaches to further increase the radiation efficiency of the graphene antennas will be discussed later.

FIG. 3.

(a) Measured reflection coefficients of graphene (red line) and Au (blue line) monopole antennas as a function of frequency. (b) Measured input impedance of graphene (red line) and Au (blue line) monopole antennas. Real and imaginary parts of input impedance are represented by solid and dashed lines, respectively.

FIG. 3.

(a) Measured reflection coefficients of graphene (red line) and Au (blue line) monopole antennas as a function of frequency. (b) Measured input impedance of graphene (red line) and Au (blue line) monopole antennas. Real and imaginary parts of input impedance are represented by solid and dashed lines, respectively.

Close modal

In the near field, at fr, we measured the magnetic fields that approximately correspond to the current intensity on the monopole antenna. Figures 4(a)4(d) show the distributions of the magnetic fields on the rectangular graphene film and Au measured at fr (6.3 GHz and 9.8 GHz). The distributions were normalized with respect to their maximum values. The current flow on the rectangular graphene film was clearly observed. The rectangular graphene film had almost the same distributions at 6.3 [Fig. 4(a)] and 9.8 GHz [Fig. 4(c)]. The current on the rectangular Au was notably distributed along the edges, which is a typical feature of the skin effect in metal antennas. On the other hand, for the graphene monopole antenna, the current attenuation along the z-axis on the rectangular graphene film was stronger than that of the rectangular Au. These results indicate that the electrical length and width of the monopole antennas were different, which may be why the Żin of the monopole antennas had different values.

FIG. 4.

Measured distributions of the magnetic field of (a) and (c) graphene and (b) and (d) Au monopole antennas.

FIG. 4.

Measured distributions of the magnetic field of (a) and (c) graphene and (b) and (d) Au monopole antennas.

Close modal

The far-field radiation patterns were measured as a function of θ (E-plane, solid lines) and ϕ (H-plane, dashed lines). Figures 5(a)5(c) show the radiation patterns of the graphene (red) and Au (blue) monopole antennas at 6, 8, and 10 GHz. As the frequency increased in the E-plane, the number of lobes also increased. Furthermore, comparing the patterns at the same frequency, the number of lobes of the graphene and Au monopole antennas was the same. For the Au monopole antenna, the two wave sources were the direct waves from the monopole element and the reflected waves from the ground disk. Since the number of lobes depended on the number of wave sources,2 it was revealed that the graphene monopole antenna had an identical number of wave sources as the Au monopole antenna, indicating that the graphene film behaved similarly to metal. An operational frequency of the metal-based antenna depends on its physical size. Therefore, the scale-down of graphene film structures leads to transparent antennas that can operate in higher frequency ranges, such as 5G bands.

FIG. 5.

Measured radiation patterns in the E-plane (solid line) and H-plane (dashed line) of graphene (red) and Au (blue) monopole antennas at (a) 6, (b) 8, and (c) 10 GHz.

FIG. 5.

Measured radiation patterns in the E-plane (solid line) and H-plane (dashed line) of graphene (red) and Au (blue) monopole antennas at (a) 6, (b) 8, and (c) 10 GHz.

Close modal

Figure 6 shows the maximum gains of the graphene (solid red line) and Au (solid blue line) monopole antennas as a function of frequency. The measured gains were expressed with respect to the gain of a hypothetical isotropic antenna of 0 dBi in all directions. The maximum gains of the graphene and Au monopole antennas were measured as 0.3 dBi at 8.5 GHz and 5.5 dBi at 11.5 GHz. At fr, those of the graphene and Au monopole antennas were measured as −0.4 dBi at 9.8 GHz and 2.8 dBi at 6.3 GHz. The difference in the gains (3.2 dB) and radiation efficiencies (2.6 dB) for the antennas was almost consistent. The maximum gain of the Au feeding electrode without the graphene film is additionally shown in Fig. 6 as a dashed black line. The maximum gain of the graphene monopole antenna was considerably higher than that of the Au electrode, indicating that the microwave power radiated dominantly from graphene rather than the Au feeding electrode. Subsequently, we investigated the environmental stability of the graphene monopole antenna. The solid green line in Fig. 6 corresponds to the data of the measured maximum gain of the graphene monopole antenna left for 44 days in the ambient atmosphere. There was no significant change in the maximum gain from the initial value. Therefore, the carrier doping using an ionic liquid, TFSA, is suitable for enhancement of the environmental stability of graphene-based optically transparent antennas.

Through the fabrication and characterization of the graphene-based monopole antenna, we demonstrated that the graphene film with a sheet resistance of 80 Ω/sq can be utilized as an electrode material for optically transparent antennas. In practical antenna applications, the radiation efficiency and gain of the graphene antennas should be even higher than and comparable to those of metal antennas. Since the radiation efficiency and gain of the graphene antennas in this study degraded due to series loss resistance RL, a higher conductivity should be attained. The electrical conductivity σ is expressed as σ = qnμ, where q is the elementary charge, n is the carrier density, and μ is the carrier mobility, respectively. Thus, an increase in n and μ is desired for a higher σ. An increase in n can lead to an increase in σ and maintain high optical transparency. Figure 7 shows the optical transmittance for the monolayer (black line) and three-layer (red line) stacked graphene films from 400 to 2300 nm. Dashed and solid lines correspond to pristine and doped films, respectively. For the pristine graphene films, the universal absorption, about 2.3% for each layer, was observed in the wavelength region longer than 500 nm. On the other hand, for the doped graphene films, an increase in the optical transmittance due to carrier doping was observed in the wavelength region longer than around 800–1100 nm. Since the inter-band absorption of photons with energy less than twice the absolute value of the Fermi level (2|EF|) is suppressed due to Pauli blocking,20,21 the optical transparent window becomes wider in the short-wavelength region as the EF shifts farther away from the Dirac point. Therefore, further carrier doping can lead to higher σ as well as higher optical transparency, which is a feature of graphene not observed in other transparent conductive films such as ITO. The increase in μ is feasible in single crystalline graphene films, which can be grown by CVD on a single crystal metal catalyst substrate.41–44 Controlling the twist angle between bilayer graphene films45 may be an alternative way to attain a lower resistance.

FIG. 6.

Measured maximum gains of Au (blue line) and graphene (red line) monopole antennas as a function of frequency. The green line indicates the measured maximum gain of the graphene monopole antenna after 44 days.

FIG. 6.

Measured maximum gains of Au (blue line) and graphene (red line) monopole antennas as a function of frequency. The green line indicates the measured maximum gain of the graphene monopole antenna after 44 days.

Close modal
FIG. 7.

Measured optical transmittance from 400 to 2300 nm. The inset shows the schematic band structure of graphene for various Fermi levels and inter-band transitions associated with the electron–hole interaction.

FIG. 7.

Measured optical transmittance from 400 to 2300 nm. The inset shows the schematic band structure of graphene for various Fermi levels and inter-band transitions associated with the electron–hole interaction.

Close modal

In conclusion, we have demonstrated that a doped and three-layer stacked graphene film can be applied to the electrodes of optically transparent antennas. By stacking graphene films and carrier doping, we attained a sheet resistance of 80 Ω/sq and an optical transmittance of 90.1%. The reflection and radiation properties of the graphene monopole antenna were determined by using the monopole antenna structure with a ground plane. At the resonance frequency fr (9.8 GHz), the radiation efficiency η of the graphene monopole antenna was 52.5%, demonstrating the feasibility of the doped and three-layer stacked graphene film as the electrode for practical optically transparent antennas. The doped and three-layer stacked graphene film behaved similarly to metal, indicating that the graphene film can be used as an electrode material of transparent antennas for 5G. Furthermore, we found that a high conductivity and a high optical transmittance could be compatible in graphene-based transparent antennas.

See the supplementary material for the Raman spectra, Hall measurements, and antenna measurement setups.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

This work was supported by JSPS KAKENHI (Grant Nos. 19J14640, 19K05218, and 20H02209), the Aoyama Gakuin University Research Institute “Early Eagle” grant program for promotion of research by early career researchers, and the Nippon Sheet Glass Foundation for Materials Science and Engineering.

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