Zhen-Guo Liu, Ming-Yang Geng, Hao Chen, An-Qi Zhang, Wei-Bing Lu
©SHUTTERSTOCK.COM/NEON_DUST
Over a decade ago, graphene became a hot topic in academic research, and its outstanding physical properties were measured and reported [1], [2], [3]. At that point, this novel two-dimensional material consisting of an atomic thick layer of carbon atoms had been widely applied in a variety of fields due to its unique characteristics, including its high electron mobility, light weight, mechanical strength, impermeability, planarization, and flexibility [4], [5]. In particular, its high carrier mobility makes graphene an outstanding electronics material for high-speed device applications such as field effect transistors, switches, and photodetectors [6], [7], [8], [9], [10]. However, the available graphene samples in the early years were limited to very small sizes by the growth and preparation process, which resulted in the frequency domain of initial research being mainly focused on the THz, infrared, and optic regimes [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Similarly, there was very little work using graphene in the microwave region, particularly in passive devices such as antennas, because the size of such devices generally is on the order of wavelengths. As a result, the outstanding tunable characteristics of graphene were mostly utilized to uncover the best in the higher THz region, while doubts remained regarding the possibilities of using graphene for microwave applications.
Beginning with graphene mechanical exfoliation with adhesive tape in 2004, the methods for growing graphene on metal foils, including copper and nickel or semiconductor substrates, have evolved to advanced chemical vapor deposition (CVD) techniques in only a few years, making the production of large-scale graphene possible [24]. Specifically, in [25], 30-in graphene films were successfully grown on flexible copper substrates using roll-to-roll production (Figure 1). Benefiting from this experience, since then, major components and devices based on graphene’s tunable properties (it is considered a lossy material) in the microwave range have been modeled, designed, and fabricated, and their functionalities have been actively tuned in situ. There is no doubt that graphene’s tunability enhances its functionality in application scenarios where dynamic electromagnetic (EM) wave manipulations are highly desired, for example, in dynamic wavefront control and dynamic beam steering. In this article, we present a review of the state of the art of graphene in microwave applications and RF devices, as well as current trends and intrinsic challenges, mainly focused on dynamically tunable EM absorbers, attenuators, beam steering, and antennas. Further, some interesting recent research topics are also surveyed in detail.
Figure 1. The roll-to-roll CVD graphene growth lab setup. (Source: Springer Nature, Scientific Reports [24]; used with permission.)
This perspective reviews applications of graphene in microwaves [26], as well as possible future development trends, which include multifunctional reconfigurable devices, especially those in practice-oriented applications.
The EM response of graphene is determined fundamentally by its dynamic surface conductivity, which is closely related to the chemical potential or Fermi level. According to the well-known Kubo formula [13], the surface conductivity of graphene, ${\sigma}_{\text{gra}}$, is highly dependent on the working frequency and chemical potential or Fermi energy. Figure 2(a) shows the interband and intraband transitions of electrons in graphene. At THz and lower frequencies, the energy of EM waves is insufficient to excite the interband transition of electrons bound by covalent bonds in graphene. Thus, when there is no external magnetic field, the interband part can be neglected, the local surface conductivity can be calculated directly by the simplified Kubo formula, and then the sheet impedance of monolayer graphene can be expressed by (1) [27]: \[{R}_{\text{gra}}\,{\simeq}\, \frac{1}{{\sigma}_{\text{gra}}} = \frac{{-}{\pi}{\hbar}^{2}{\left(\frac{{\omega}{-}{j}}{\tau}\right)}}{2{je}^{2}{k}_{B}T}{\left[\frac{{E}_{F}}{2{k}_{B}T} + {\ln} {\left({e}^{\frac{{-}{E}_{F}}{{k}_{B}T}} + {1}\right)}\right]}^{{-}{1}} \tag{1} \]
Figure 2. (a) The interband and intraband transitions of electrons in graphene. (b) The sheet resistance and (c) sheet reactance of graphene versus chemical potential in the microwave band from 2 to 100 GHz. (d) The surface resistance of graphene as a function of bias voltage [54].
where e is the electron charge, ${k}_{B}$ and ${\hbar}$ are the Boltzmann constant and the reduced Plank’s constant, ${E}_{F}$ is the Fermi energy of graphene, ${\tau}$ is the carrier relaxation lifetime, and n is the electron density. To better understand the impedance property of graphene in the microwave band, the sheet resistance and sheet reactance of graphene versus chemical potential are calculated from 2 to 100 GHz. Some of the parameters in (1) are chosen as follows: ${T} = {300}{\text{ K}}$ is the ambient temperature, ${\tau} = {0.06}{\text{ ps}}$, and the chemical potential increases from 0.01 to 0.3 eV [28]. Based on these specific parameter values, (1) can be visualized as in Figure 2. In Figure 2(b), from 2 to 26.5 GHz, the sheet resistance of graphene is almost frequency independent, but it drops from 3,858 to ${474}{\Omega} / {\text{sq}}$ as the chemical potential increases. Meanwhile, the sheet reactance of grapheme decreases from nearly ${38.5}{\Omega} / {\text{sq}}$ to lower than ${4.8}{\Omega} / {\text{sq}}$. Comparing Figure 2(b) and (c), when the frequency is within the range of 2–26.5 GHz and the chemical potential is fixed to any value from 0.01 to 0.3 eV, the value of the sheet resistance is at least one hundred times greater than that of the sheet reactance. However, when the frequency is greater than 26.5 GHz, the value of the sheet reactance is gradually growing as the frequency increases, which is 30 times smaller than the resistance at maximum. Hence, in the microwave frequency range from 2 to 26.5 GHz, the imaginary part of the sheet impedance of graphene can be ignored; thus, graphene can be modeled as a tunable resistive sheet almost without dispersion. As a result, the sheet impedance of graphene can be controlled by shifting the chemical potential or Fermi energy of graphene. When the Fermi energy enters the valence or conduction band, the hole or electron conduction begins to dominate the current transport, which results in the reduction of the resistance with negligible reactance. This variation in the Fermi energy can be obtained through an external electric field. As an example, Figure 2(d) shows the relation between the surface resistance of graphene and the bias voltage, which shows a variation range of monolayer graphene from 2,500 to ${600}{\Omega} / {\text{sq}}$ [54]. Compared with conventional metal materials, this significant feature of graphene has remarkable attraction, which provides a new theoretical method to acquire a different material performance from metal to dielectric material. Based on these features, tunable devices can be designed to manipulate the EM wave in free space or in circuit.
As shown in Figure 2, graphene behaves in the GHz band like a tunable resistive film by changing the electrostatic bias field via bias voltage. Specifically, graphene is an excellent conductor and has very high surface resistance compared with metal. Therefore, in the microwave range, graphene is more suitable for lossy devices such as attenuators or EM absorbers to manipulate the amplitude of guided EM waves in circuit or spatial EM waves in free space.
The graphene-based electronically tunable attenuator operating at a frequency of 5 GHz was first proposed in 2014 [29], [30], [31], [32], as shown in Figure 3, in which the graphene flakes were deposited on an air gap in a microstrip line and serially connected on it. Thus, this attenuator is in fact a reflective type; its return loss at two external ports will properly deteriorate, particularly when the conductivity of graphene is not high enough. The tuning range of this attenuator was not desirable, and the return loss was relatively high. After that, a broadband microwave attenuator, shown in Figure 4, operating at 1–20 GHz based on few layers of graphene flakes, was presented in [33], the structure of which integrated a micrometric layer of graphene flakes deposited on an air gap in a microstrip line. Depending on the applied voltage, which, in turn, modified the resistance of graphene, the measured insertion loss exhibited a tunability up to 5.5 dB, which was not a satisfactory tuning range. Subsequently, improved microstrip attenuators were proposed to obtain enhanced attenuation ranges [34], [35], [36]. In [34], [35], and [36], two (or four) graphene pads were located between the microstrip line and one (or two) pairs of grounded metal vias, as shown in Figure 5. Although these types of attenuators can achieve wide tuning ranges, the maximum return loss of the attenuator is still relatively large. Around the same time, a tunable grounded coplanar waveguide (GCPW) attenuator based on graphene nanoplates was proposed [37]. Three pairs of graphene nanoplate pads were periodically located in the gap between the signal line and the upper metal ground of the GCPW, as shown in Figure 6. The magnitude of the attenuation can be tuned by more than 10 dB, and reflection loss was slightly improved.
Figure 3. The graphene-based microstrip attenuator proposed in [29], [30], and [31].
Figure 4. The graphene-based microstrip attenuator proposed in [33]. VNA: vector network analyzer; SMA: Sub-Miniature A connector.
Figure 5. The graphene-based microstrip attenuator proposed in [34].
Figure 6. The graphene-based microstrip attenuator proposed in [37].
To reduce the return loss and conveniently integrate with planar circuits, a dynamically tunable substrate-integrated waveguide (SIW) attenuator using large-scale CVD graphene was first presented in 2018 [38], as shown in Figure 7(a). Two graphene sandwich structures (GSSs) [39] were placed inside an SIW, which served as a conductivity-tunable E-plane septum. By applying biased voltage to the graphene, the attenuation can be dynamically changed. Meanwhile, benefiting from this transmit type attenuator, the reflection can still maintain a relatively low level. Furthermore, a closed form of the attenuation based on a mode-matching method was also proposed. As shown in Figure 8, the fabricated attenuator presented a favorable attenuation tuning range from 2 dB to 15 dB with a stable 7.5-GHz wideband while changing the bias voltage from 0 to 4 V, and the return loss can always be smaller than −15 dB over the operating frequency. Based on this, a half-mode SIW (HMSIW) attenuator operating at 7.7 GHz to 19 GHz was presented [40], shown in Figure 7(b). Compared with the SIW attenuator [38], the fabrication of the HMSIW attenuator was relatively simple, and the return loss remained at a smaller level: lower than −20 dB.
Figure 7. An overview of CVD graphene-based attenuators. (a) The graphene-based SIW attenuator proposed in [38]. (b) The graphene-based half-mode SIW attenuator in [40]. (Source: AIP Publishing [40]; used with permission.) (c) The graphene-based microstrip line attenuator in [41]. (d) The graphene-based CPW attenuator in [42]. (e) The graphene-based filtering attenuator in [43]. (f) The graphene-based flexible spoof surface plasmon polariton attenuator in [44]. GSS: graphene sandwich structure; PVC: polyvinyl chloride; PET: Polyethylene terephthalate.
Figure 8. A comparison between the simulated and measured (a) |S21| and (b) |S11| of the graphene-based SIW attenuator in [38].
Since multimode EM fields exist in the SIW structure at the high-frequency stage, that can lead to unwanted flatness of attenuation in high-frequency bands. To overcome this limitation of the SIW structure, an attenuator based on the microstrip line, CPW, and slot line, as shown in Figure 7(c)–(e), which all have an ultrawide bandwidth, has been proposed in [41] and [42]. In these structures, the GSSs were placed near or directly on the signal strip along the direction of propagation to dissipate the EM fields. All of these kinds of attenuators realized favorable attenuation with a stable wideband range from 9 to 40 GHz, as well as relatively low return loss. A transverse equivalent network and a closed form of attenuation of each attenuator were also proposed, which can be utilized to analyze in detail the performance of attenuators with different critical parameters of graphene.
Unlike the abovementioned wideband tunable attenuators, [43] proposed a narrower graphene-based attenuator that may be more desirable for applications requiring damped high-selectivity filters; see Figure 7(e). The graphene was loaded on microstrip resonators to reduce the resonance intensity, which resulted in a valid attenuation in the passband. By varying the bias voltage on graphene, a controllable attenuation from 1.7 to 8.4 dB could be obtained over the operating bandwidth centering at 1.72 GHz while preserving good selectivity. The proposed filtering attenuator not only improved the degree of integration but also reduced the insertion loss compared with cascaded devices consisting of a filter and an attenuator, which could be useful in a multifunctional RF front end and in the feed network of an antenna array.
However, all of the graphene-based attenuators mentioned above were based on rigid substrates such as Rogers 5880, which reduced the flexibility of the attenuators and restricted their application in real-world curved settings. In 2019, a flexible and tunable spoof surface plasmon polariton (SSPP) attenuator was proposed for the first time [44]. It is shown in Figure 7(f). It was based on an SSPP waveguide combined with graphene. The prototype achieved an acceptable attenuation tuning range from 4 to 16 dB, as well as low return loss, even when it was bent to 90° with a curvature radius of 35 mm. In addition, compared with other tunable attenuators in the previous literature, the attenuator was structurally flexible, which holds great potential for future flexible wireless applications.
Microwave absorbers (MAs) have long been a major research topic, owing to their crucial importance in both academia and industry for absorbing EM radiation pollution. The main requirements for MAs are a light weight, a thin thickness, a wide band, and flexibility. In addition, MAs with tunability characteristics, mainly focused on amplitude and frequency, are greatly desired to deal with the vagaries of EM scattering. Graphene can range from being a discrete conductor to a highly resistive material, which makes it a prospective material for tunable MA designs.
Graphene-based MAs have been studied in several experimental works. Among them, a stacking solid transparent graphene-flake-based wideband absorber was first achieved in 2014 [45], as shown in Figure 9. After that, in 2015, a tunable graphene-based X-band radar-absorbing surface was proposed in [39], which applied different bias voltages on GSS, as shown in Figure 10. It works as a configurable Salisbury screen, so the substrate layer’s thickness must be set as a quarter of the operating wavelength, which does not satisfy the practical need for a thin absorber. To extend the bandwidth, an adaptive broadband MA based on tunable graphene was designed theoretically in [47], as shown in Figure 11(a). When applying a bias voltage below 15 V, a qualified absorption bandwidth of 15 GHz could be reached. However, the nanoscale electrostatic field bias and overall structural manufacture were difficult to realize in practice. A millimeter-wave domain from 30 to 300 GHz was realized in thin multilayered polymethyl methacrylate/graphene structures in [48]. To enhance the performance of a radar absorber (RA), metallic frequency-selective surfaces (FSSs) or metasurfaces have been suggested to construct absorbers integrated with graphene. In 2019, a wideband dynamically tunable MA combined with graphene and a random metasurface was proposed [49], as presented in Figure 11(b). Through the superimposition of a few layers of graphene, the tunable range of the graphene sheet resistance was reduced to 80–380 Ω/sq, which was more suitable for application as the resistance film of a broadband MA because it was easier to match the impedance of free space. In addition, by selecting 12 elements of the metasurface and distributing them randomly, more resonance frequencies and phase responses were achieved, thus improving the bandwidth from 5 to 31 GHz, while simultaneously obtaining a relatively low profile.
Figure 9. A comparison of the calculated and measured spectra of stacked graphene-quartz absorbers. (Source: Springer Nature, Scientific Reports [45]; used with permission.)
Figure 10. Photographs of (a) the front and (b) the back side of a fabricated MA. (Source: Springer Nature, Nature Communications [39]; used with permission.)
Figure 11. An overview of CVD graphene-based absorbers. (a) The theoretical proposal of a graphene-based active multilayer absorber in [47]. (b) An experimental demonstration of a graphene-based amplitude-tuning broadband absorber in [49]. (c) The theoretical proposal of a graphene-based multilayer frequency-tuning absorber in [50]. (d) An experimental demonstration of a graphene-based frequency selective surface (FSS) MA in [51]. (e) An experimental demonstration of a graphene-based frequency-tuning MA in the X-band in [53]. (f) An experimental demonstration of a graphene-based tunable MA in [56]. (Source: ACS Publications [56]; used with permission.) (g) An experimental demonstration of a graphene-based amplitude-tuning transmission/absorption frequency-selective surface in [57]. (h) An experimental demonstration of a graphene-based reconfigurable radar absorber (RA) in [58]. (i) An experimental demonstration of a graphene-based multifunctional and tunable RA in [59]. PEC: perfect electric conductor; MLG: multilayer graphene; RFSS: resistive frequency selective surfaces.
In addition, another method has been proposed to enhance the bandwidth of an MA by patterning the graphene to construct the resistive FSS. In 2017, Yi et al. proposed a tunable MA based on patterned graphene [50]. Although the working frequency of this MA can be directly changed by stacking different numbers of patterned graphene layers, as shown in Figure 11(c), its lack of a dynamically tunable property and limited sample size restrict its more practical applications. In 2018, an MA using a large-area multilayer graphene-based FSS with a size of 150 mm $\times$ 150 mm was designed and fabricated [36]. The multilayer graphene layers were produced using the CVD approach, the surface resistance of which depended on the growth temperature. The MA shown in Figure 11(d) expressed unprecedented absorption manipulation [51]. In particular, the absorption could realize a switch from dual band to broadband when the sheet resistance increased to ${70}{\Omega} / {\text{sq}}$. Afterward, an improved graphene-based optically transparent and flexible MA was proposed to obtain more versatile absorption control, such as tunable absorption with dual band, single band, and broadband, by applying bias voltage to graphene layers [52]. As presented in Figure 12, such an absorber with optical transparency and tunable microwave absorption might have potential applications in stealth technology and photovoltaic solar cells.
Figure 12. A schematic diagram of the designed graphene-based optically transparent and flexible MA. (Source: Elsevier [52]; used with permission.)
However, all of the MAs mentioned above only realized absorption amplitude tunability; they did not also achieve a frequency shift. Therefore, a graphene-based MA that was dynamically frequency tunable was still a big challenge. In [53] and [54], two dynamically frequency-tunable graphene-based absorbers in the X-band and Ku-band, respectively, were proposed for the first time. The center frequencies of these MAs could be continuously tuned while maintaining excellent absorption performance. The MA in [54] consisted of a periodically patterned GSS and periodic patches printed on the substrate backed with the ground, as displayed in Figure 13. The general design strategy could be applied to other wavelength-region applications such as mechanical deformation and bolometric detectors.
Figure 13. The schematics of the proposed absorber: (a) periodic structure, (b) unit cell, (c) top graphene sandwich layers, and (d) periodic copper patches [54].
A limitation of all of the aforementioned MAs was that they realized only a single EM modulation, either in frequency or in amplitude. Hence, it is an enormous challenge to realize both dual-tunable and flexible RAs. In [55], a flexible and dual-tunable RA with independent control of the frequency and amplitude was first designed and experimentally characterized, as illustrated in Figure 14. As a result of both simulations and experiments, by electrically tuning the Fermi energy of graphene according to the bias configuration loaded on the continuous GSS and the patterned GSS, the resonance frequency could be continuously tuned from 9.88 to 10.51 GHz, and the amplitude at each absorption peak could be tuned from −4.4 to at least −30 dB. In addition, all of the components of the proposed absorber were flexible. The experiment demonstrated the dual-tunability of the conformal absorber as a curved surface, which brings the application of this type of tunable absorber to real life one step closer. Most important is that achieving both amplitude and frequency control is fully based on the tunability of graphene, without any lumped devices or intricate feeding network and even without any metal involved. Hence, the dual-tunable RA demonstrated the priority characteristics of flexibility, light weight, and low environmental impact. Although the prototype in [55] is operating in the microwave spectrum, tunable functionalities at millimeter or terahertz spectral bands could also be obtained due to the intrinsic tunability of graphene.
Figure 14. The flexible and dual-tunable RA enabled by graphene. (Source: John Wiley and Sons [55]; used with permission.)
Over the past two years, reconfigurable MAs that combine graphene with an active metasurface have also been reported [56], [57], [58], [59]. A comparison of the size of the abovementioned graphene-based absorbers is included in Table 1. In these kinds of hybrid structures, graphene is used as an amplitude-tuning layer by changing its effective sheet resistance, while frequency or phase is tuned by controlling semiconductor devices including p-i-n or varactor diodes loaded in a periodic metallic metasurface. However, in addition to intricate feeding networks and complex fabrication processes, this approach still finds it very difficult to integrate conventional rigid components with a flexible substrate. Therefore, simultaneously achieving structural flexibility and independent control of the resonant frequency and amplitude in a single design is still very challenging.
Table 1. A comparison of the sizes of graphene-based absorbers.
Up to now, graphene-based antennas in the microwave band have been a rare approach; the reasons mainly focus on two points. One is that antennas in general require wavelengths on the order of electrical in size. Another is that the radiation efficiency of a graphene-based antenna is not high enough, as real graphene is very lossy, as well as relatively hard to tune in the microwave band. Actually, the crucial factor for graphene-based microwave applications is high-quality graphene with good conductivity. In the last few years, research has demonstrated that graphene is a suitable material for microwave antennas and other devices (Figure 15). In [60], [61], and [62], graphene-based antennas in the microwave band were proposed several years ago. In [62], a graphene-based patch antenna with resonating behavior at around 12 GHz with radiation characteristics similar to those of planar metallic patch antennas was demonstrated while employing a small impedance on the graphene’s surface (i.e., ${\leq}{10}{\Omega} / {\text{sq}}$). However, such a graphene patch antenna has yet to be experimentally verified. In [63], a microwave slot antenna in coplanar configuration based on graphene was reported both in an experiment and in a simulation. Finally, conductive multilayer graphene film (MGF), fabricated by high-temperature thermal treatment of graphene oxide film and subsequent compression rolling, with its high conductivity, may be a good candidate for antenna design [64], [65], [66]. The fabrication process of MGF is very similar to that of graphite film [67]. For example, a reconfigurable Vivaldi antenna centering at 30 GHz was proposed in [68], which realized different radiation beams at 90° and/or 270° with three states. In addition, integrating traditional metallic patches and graphene to design reconfigurable antennas is another alternative method, in which the radiator was formed by metallic patch and the graphene structure acted as a tunable device [69]. However, full-graphene-based microwave antennas with tunability are still a big challenge.
Figure 15. An overview of a graphene-based antenna and other devices. (a) An experimental demonstration of a graphene-based ultrawideband antenna. (Source: MDPI [64]; used with permission.) (b) An experimental demonstration of a graphene-based multibeam radiation antenna array in [65]. (Source: Elsevier [65]; used with permission.) (c) An experimental demonstration of a graphene-based wearable antenna in [66]. AMC: artificial magnetic conductor. (d) An experimental demonstration of a graphene-based reconfigurable Vivaldi antenna in [68]. FGF: flexible graphite film. (e) An experimental demonstration of a graphene-based microwave antenna with a reconfigurable pattern in [69]. (f) An experimental demonstration of a graphene-based nine-way power divider in [71]. (g) An experimental demonstration of a graphene-based integrated device for attenuation, amplification, and transmission of SSPP in [72]. (h) The graphene-based antenna in [73]. (Source: Springer Nature, Scientific Reports [73]; used with permission.) (i) The light scribe graphene (LSG) fabrication for microwave circuit applications in [75]. LNPA: low-noise power amplifier.
During recent years, there have also been various attempts to combine graphene with different microwave applications, such as power dividers [70], [71] enabled by graphene flakes and a CVD graphene-based integrated device for the attenuation, amplification, and transmission of SSPP [72]. A beam-scanning conformal antenna array with planar integrated phase shifter based on graphene was proposed in [73]. An RF signal detector in graphene was reported in [74]. In addition, a laser-scribed graphene fabrication method was used to create graphene to be used in a microwave application [75]. A comparison of types of graphene for different synthesis processes is included in Table 2. With the continued development of graphene research, there is reason to believe that more and more diverse applications in microwaves will be realized.
Table 2. A comparison of types of graphene for different synthesis processes.
The resistance characteristics of graphene intrinsically determine its possible application in microwave devices, including attenuators and absorbers. Each device deserves substantial effort and is attainable with current technologies. However, a single-function device may not be practical in the real world. For example, in many integrated circuits and systems [76], both attenuation and amplification are required to reduce the transmission loss and amplify the signal, respectively. Hence, integrating the attenuator and amplifier based on tunable graphene in one device is necessary in designing miniaturized and multifunctional circuits, as well as conferring reconfigurable characteristics on integrated circuits. However, the abovementioned absorbers enabled by graphene are manually controlled by human participation. Thus, self-tuning absorbers that can adaptively tune their resonance properties to match the incoming waves without any manual operation (which may be achieved automatically via closed-loop feedback microcontroller units and sensors [46]) will become significantly more important in the future.
Graphene’s ability to be tuned by changing its electric fields via bias voltage is the property that allows it to be configured for device applications. In the microwave range, graphene acts more like a tunable resistive film due to the dominant real part of graphene impedance. Benefiting from graphene’s unique characteristics such as high electron mobility, light weight, mechanical strength, impermeability, planarization, and flexibility compared with semiconductor elements, more and more researchers have made great progress in microwave applications in recent years. Thus, this perspective provides a summary of recent advances in microwave applications, including attenuators, absorbers, antennas, and other devices.
This progress in graphene-based devices in the microwave band further demonstrates its potential for use in the communication systems of the future. In addition, future directions of applications of graphene in microwave devices are predicted, which include multifunctional reconfigurable devices, especially in those practice-oriented applications.
This work was supported by the National Science Funds for Distinguished Young Scientists under Grant 61925103 and the Project for Jiangsu Specially-Appointed Professor, National Natural Science Foundation of China Grant 62101115, the Fundamental Research Funds for the Central Universities Grant 2242022k30008, and the Postdoctoral Research Start-up Fund in Southeast Unive rsity Grant 1104002106. The corresponding authors of this article are Zhen-Guo Liu and Wei-Bing Lu.
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Digital Object Identifier 10.1109/MMM.2023.3261679