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Abstract
Graphene is an attractive material for terahertz (THz) absorbers because of its tunable Fermi-Level (EF). It has become a research hotspot to modulate the EF of graphene and THz absorption of graphene. Here, a sandwich-structured single layer graphene (SLG)/ Polyimide (PI)/Au THz absorber was proposed, and top-layer graphene was doped by HAuCl4 solutions. The EF of graphene was shifted by HAuCl4 doping, which was characterized by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and Raman tests. The results showed that the EF is shifted about 0.42 eV under 100 mM HAuCl4 doping, the sheet resistance is reduced from 1065 Ω/sq (undoped) to 375 Ω/sq (100 mM). The corresponding absorbance was increased from 40% to 80% at 0.65 THz and increased from 50% to 90% at 2.0 THz under 100 mM HAuCl4 doping. Detailed studies showed that the absorption came from a sandwich structure that meets the impedance matching requirements and provided a thin resonant cavity to capture the incident THz waves. In addition, not only the absorber can be prepared simply, but its results in experiments and simulations agree as well. The proposed device can be applied to electromagnetic shielding and imaging, and the proposed method can be applied to prepare other graphene-based devices.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
THz technology has been honored as one of the “top ten technologies that will change the world of tomorrow”. In recent years, the rapid development of THz technology has provided the basis for its application [1–6]. As an essential part of THz technology, THz absorbers have a widespread application. There is an urgent need for efficient THz absorbers in communications [7], imaging [8,9], radar systems [10], and electromagnetic shielding [11,12]. Typically, the conventional absorber [13–16] is designed as a “sandwich” structure, consisting of two layers of metal separated by an insulating dielectric layer. It can obtain high absorbance at a specific frequency by adjusting the subwavelength structure of the top layer metal. However, the absorbed frequency is usually to be fixed when pre-design patten is fabricated, which limits its application to a large extent.
Recently, graphene has attracted much attention due to its excellent optoelectronic properties [17], such as single-atom thickness, fast response and tunable EF. Especially, graphene can interact strongly with THz waves in light-matter interaction. [18,19]. This makes graphene a suitable material for THz absorbers. Andryieuski et al. [20] proposed a metamaterial absorber using two vertically stacked layers of separated graphene, and theoretical calculations and simulations confirmed that the wide band tunability of THz wave absorption could be achieved by changing the EF of graphene. Xiao et al. [21] provided a metamaterial structure of stacked double-layer crossover graphene arrays, simulations demonstrated that it could achieve broadband tunability of THz wave absorption. Huang et al. [22] revealed a broadband tunable THz metamaterial absorber consisting of complementary crossed elliptical graphene structures and a dielectric substrate affixed on a continuous metal film. The simulation results show that the absorber can achieve effective absorption in the frequency range of 1.2-1.8 THz. Ye et al. [23] proposed a metamaterial absorber consisting of periodically arranged graphene pattern, dielectric and a metal layer, and that the absorbance of the absorber could be adjusted from more than 90% to less than 20% by changing the graphene EF from 0.7 eV to 0 eV. However, most studies have been focused on simulations and theoretical analysis. Graphene is a semi-metallic material, and its EF cannot be tuned spontaneously. Therefore, it is essential to precisely tune the EF of graphene. Electrical tuning and chemical doping are the typical means for the EF of graphene tuning. The operation of electrical tuning of EF is more complicated, and the devices are usually need to be configured with complex external power. Jiang et al. [24] reported an electrical tuning experimental study of graphene-based THz absorber, which presents the fabrication and characterization of graphene THz absorber regulated by electricity with an effective bandwidth from 1.54 to 2.23 THz at ≥ 90% absorption. However, this method is limited to construct external source and patterned graphene arrays, which has high fabricating cost and low-yield. Chemical doping is divided into two main categories, one by replacing carbon atoms in the graphene lattice by impurity atoms, and the other by adsorbing metal ions or organic matter onto the graphene surface [25]. Lin et al. [26] demonstrated gas-phase doped graphene by incorporating nitrogen atoms into the graphene lattice by NH3 plasma exposure. Li et al. [27] reported in electrical and magneto transport measurements of the high-quality graphitic nitrogen-doped graphene. Although atomic substitution doping has good structural stability, it destroys the original lattice structure of graphene and leads to a decrease in carrier mobility. As a result, the optoelectronic properties of graphene are greatly affected and difficult to be utilized efficiently. In contrast, the adsorption of dopant molecules on graphene leads to interfacial events between two components. For example, electronic coupling forms new electronic structures and subsequent charge transfer [25]. Yu et al. [28] report a general approach to regulating EF of graphene precisely by designing azobenzene molecules with different dipole moments and dipole orientations. The Raman spectra results demonstrated the π-π interaction between the azobenzene molecules and graphene, which resulted in the modulation of the EF of graphene. HAuCl4 is a more widely used dopant than azobenzene molecules. Kim et. al. [29] reported that by doping with Au3+, the sheet resistance of graphene was reduced up to 77%, also controlled by doping concentration change. A graphene film prepared with the optimized Au doping concentration has a sheet resistance of 150 Ω/sq at a light transmission of 87%. The surface potential of graphene films is affected by the doping concentration shifts up to 0.5 eV. Krajewska et. al. [30] studied the doping mechanism of graphene doped with HAuCl4 and analyzed the effect of doping on the electrical properties of graphene. The results indicated that the method can effectively regulate the carrier concentration to achieve a sheet resistance of 79 Ω/sq.
In this paper, we have proposed a “sandwich” THz absorber based on SLG, using HAuCl4 doping of SLG to shift the EF, and have characterized the doping by SEM, Raman, and XPS. In addition, the absorbance of the proposed absorber was simulated and tested by COMSOL Multiphysics software and terahertz time-domain spectroscopy system (THz-TDS). It is demonstrated that the EF is shifted about 0.42 eV after doping with 100 mM HAuCl4. The corresponding absorbance was increased from 40% to 80% at 0.65 THz and increased from 50% to 90% at 2.0 THz under 100 mM HAuCl4 doping. Our absorbers can be used for communication, imaging, electromagnetic shielding, and radar detection. Furthermore, our method can be used to design and fabricate similar graphene-based devices.
2. Experimental
2.1 Simulation
The schematic diagram of the proposed absorber is shown in Fig. 1(a), which consists of a SLG film at the top and an Au reflection layer at the bottom, separated by PI. Due to the presence of Au at the bottom, it is ensured that no transmission of THz waves occurs. The basic unit period of the absorber structure is set to 40 µm, the thickness of the PI is 50 µm, the thickness of the Au is 200 nm, and the thickness of SLG is set to 0.35 nm. The conductivity of Au is taken as 4.56 × 107 S/m, and the refractive index of PI is 2.48. Since the thickness of the SLG is several orders of magnitude smaller compared to other structures, transition boundary condition are used for graphene using a 2D planar approximation instead of a 3D solid. Periodic boundary conditions are set in the x and y directions, and two Floquet ports are set in the z direction, The electromagnetic wave vertical incidence excitation condition is set at port 1. The absorbance of the proposed absorber can be defined as A(ω) = 1-R(ω)-T(ω), where the reflectance R(ω)=|S11|2 and the transmittance T(ω)=|S21|2. The S-parameters at different frequencies can be extracted directly from the simulations results. Since the thickness of Au is greater than the skinning depth of THz waves, the transmission S21 = 0. The absorbance can be simplified to A(ω) = 1-R(ω).
Fig. 1. (a) The basic structure of the proposed absorber schematic diagram. (b) HAuCl4 doping SLG schematic diagram. (c) HAuCl4 doping SLG mechanism schematic diagram.
2.2 Device preparation
During fabrication, the silicon wafer is used to support the designed absorber. Cr and Au films are deposited on the wafer by magnetron sputtering, and Cr is coated on the wafer to increase the adhesion between the Au and the wafer. The PI tape is applied to the wafer so that the Si/Au/PI wafer is ready for subsequent use. The chemical vapour deposition (CVD) copper-based SLG (SixCarbon Technology Shenzhen) was transferred to the Si/Au/PI substrate surface as follows: (1) Cut the SLG into suitable sizes, and it is fixed face-up on the center of the Si wafer with PI tape. Then, spin-coat polymethyl methacrylate (PMMA) (950 K A4) onto the SLG surface with a spin-coating speed of 2500 r/s and a spin-coating time of 20 s, and heat it at 120°C for 5 minutes to cure the PMMA. (2) The sample is held backside up in the center of a new silicon wafer and bombarded with oxygen plasma at 500 W for 10 minutes to remove SLG from the backside. (3) Cut off the part of the sample edge that is not spin-coated with PMMA, and place the sample face-up in a solution of copper sulfate, water and hydrochloric acid in the mass ratio of 1:5:5 for about 30 minutes, after which the copper is completely corroded, leaving the PMMA-supported SLG floating above the solution. (4) The PMMA-supported SLG was fished into deionized water by taking clean silicon oxide wafers, and after about ten minutes the PMMA-supported SLG was fished up with the prepared substrate and placed vertically to dry. PMMA was removed with acetone. In this way, undoped absorbers were obtained.
The SLG was transferred to silicon oxide wafers by the above method used for Raman spectra and sheet resistance measurements.
Different concentrations (20 mM and 100 mM) of HAuCl4 solutions were used to dope SLG. The solutions of corresponding concentrations were spin-coated on the sample surface for 2 minutes each time, as shown in Fig. 1(b).
2.3 Characterization
Raman tests were performed by confocal micro Raman spectroscopy at room temperature using an Alpha 300R (WETEC) Raman spectrometer set up with a 532 nm He-Ne laser and a 50× objective. The presence of graphene on silicon dioxide/silicon substrates was confirmed by G-peaks, D-peaks and 2D-peaks. The number of layers, defects and the doping of graphene can also be characterized. Imaging was performed using an SEM microscope with a SUPRA-55 (ZEISS). SEM images were acquired to determine the surface morphology of graphene before and after HAuCl4 doping. The change in surface elemental composition of graphene before and after doping with HAuCl4 was shown using an EDS energy spectrum analyzer based on this SEM itself. An X-ray photoelectron spectrometer (XPS) from Nexsa (Thermo Scientific) was used to characterize the elemental composition and valence changes of the samples before and after doping. For different samples, high-resolution core level peaks of C-1s and Au-4f were measured. A four-probe test system was used to characterize the change in sheet resistance before and after graphene doping. Absorbance tests were performed using THz-TDS on samples with different doping concentrations.
3. Results and discussions
The mechanism of HAuCl4 doping of SLG is given in Fig. 1(c). HAuCl4 doping of SLG is based on the charge transfer between SLG and Au3+, Au3+ is reduced to Au nanoparticles (AuNPs) upon contact with graphene. Au3+ has a positive reduction potential and the reduction process from Au3+ to AuNPs is a spontaneous reaction. The electron transfer from SLG to Au3+ depletes the electrons near the Dirac point of SLG, which increases the EF of SLG. Through electron transfer of SLG in HAuCl4 solution, HAuCl4 is reduced to AuNPs, and graphene exhibits p-type doping behavior. The reduced AuNPs particles are simultaneously adsorbed on the SLG surface, leading to significant changes in the electrical properties of SLG. The HAuCl4 can be directly reduced on graphene in an aqueous solution [25]:
Figures 2(a)-(c) show the SEM images of samples before (undoped) and after (20 mM and 100 mM) doping with different concentrations of HAuCl4 solutions. The photos show that the SLG is homogeneous, but the SLG transfer process causes a few surface folds. For the samples doped with HAuCl4 solutions, there are some bright small spots on the surface, the number of small spots increases with the doping concentration and forms an agglomeration phenomenon at the doping concentration of 100 mM. Figures 2(e) and (f) are the energy dispersive spectroscopy (EDS) characterization of graphene dopped by 100 mM HAuCl4 solution, and the corresponding SEM images are shown in Fig. 2(d). The presence of Au elements can be clearly seen from the results, and the distribution of the mapping map of Au is approximately the same as the distribution of the small spots on the surface of the sample in the SEM image. It shown that the small spots on the surface of SLG are AuNPs.
Fig. 2. SEM images of SLG at different HAuCl4 doping concentrations. (a) Undoped, (b) 20 mM, (c) 100 mM, (d) SEM image of SLG dopped by 100 mM HAuCl4 solution, (e),(f) are corresponding EDS mapping and spectrum of SLG sample exhibited in (d), respectively.
The XPS spectra of SLG at different doping concentrations of HAuCl4 are shown in Fig. 3. Figure 3(a) is the C-1s XPS spectra of the original SLG. Four sub-spectra are required to properly fit the emission signal of C-1s. The main signal is the sp2 hybridization of the C−C bond with binding energy (BE) of 284.5 eV. The functional groups on SLG such as C−OH and C−O lead to peaks with BE located at 285.8 eV and 286.5 eV, and the signal located at BE of 288.4 eV originated from the O = C−O and C−C = O groups [30]. We can observe a low intensity of the C−OH, C−O, O = C−O and C−C = O components, proving the high quality of the SLG. In Fig. 3(b), the spectra of SLG doping with a concentration of 100 mM HAuCl4 show a similar shape, which exhibit a predominant C−C sp2 component and low intensity of other carbonaceous species. Compared to the original SLG (284.5 eV), the maximum shift in BE (284.1 eV) of the 100 mM doped SLG corresponds to an increase of 0.4 eV of the EF.
Fig. 3. Original (a) and 100 mM Doping (b) of SLG high resolution XPS C-1s core level spectrum. (c) XPS peaks of Au-4f spectrum after SLG doping with 20 mM HAuCl4. (d) XPS peaks intensity of Au-4f as a function of HAuCl4 concentration.
The XPS spectrum (Fig. 3(c)) shows the Au-4f signal after doping of SLG with 20 mM HAuCl4 solution. For a better fit of this spectrum, two double peaks corresponding to the 4f7/2 and 4f5/2 final states, are required, including 83.6 eV corresponding to AuNPs and 85.9 eV corresponding to Au3+. As can be observed, the fitted spectra show a predominant metallic Au signal, while the Au3+ emission is less significant. The XPS data suggest that the Au3+ in contact with the graphene surface undergoes a reduction, leading to the metallic state. These results demonstrate the existence of a reduction mechanism for the Au3+ formation of AuNPs on the graphene surface. In addition, Fig. 3(d) demonstrates the effect of HAuCl4 concentration on the efficiency of the reduction process. With the increase of the HAuCl4 concentration, enhanced Au emission was observed, while the Au3+ emission was also enhanced at a doping concentration of 100 mM. This phenomenon is probably due to the existence of saturation of the reduction mechanism, which saturates the electronic interaction between Au3+ and graphene with the increase of the doping concentration and cannot continue to reduce Au3+ to AuNPs. The behavior of the XPS emission indicates the diffusion of Au3+ on the SLG, which facilitates their binding to form discrete AuNPs.
The Raman spectra of SLG before and after doping are illustrated in Fig. 4(a), where the standard D, G, and 2D peaks of the SLG samples were normalized and fitted with the Lorentz function. A small D peak which indicates the presence of defects or disorder is introduced in the Raman spectra of the pristine SLG and its intensity is slightly changed after the doping treatment. The pristine G peak is located at ∼1583 cm−1 and involves the E2g phonon mode at the Γ point in the Brillouin zone; the 2D peak is located at ∼2670 cm−1 and involves the transverse phonon emission near the K point in the Brillouin zone [31]. The shift of the G and 2D peaks to higher frequencies is due to the p-type doping of SLG. In the present experiments, the G peak positions were shifted from the original ∼1583 cm−1 to ∼1591 cm−1 (20 mM), ∼1598 cm−1 (100 mM). The shift of the G and 2D peaks to higher frequencies confirms the hole doping of SLG, and the change of the EF of SLG is closely related to the shift of the Raman peak. The change in EF is determined based on the G-peak shift of 35.4 cm−1/eV [32–34], as shown in Fig. 4(b), where we analyze the EF of graphene doped with 100 mM HAuCl4 to be elevated by 0.42 eV compared to the original SLG. However, SLG prepared by CVD usually tends to cause p-type doping due to adsorption of water molecules in the air or residual PMMA. It is reported that the EF of original SLG is about 0.08 eV [33,35]. Therefore, the EF of graphene doped with 100 mM HAuCl4 is about 0.50 eV. Similar to the shift of the Raman peak, the intensity ratio between the 2D and G peaks (I2D/IG) is an important parameter for estimating the variation of SLG doping level. The I2D/IG value of the original SLG exceeds a factor of 2, confirming that our original sample is SLG. The decrease in 2D peak intensity and I2D/IG values due to p-type doping is attributed to the increase in charge carrier scattering in graphene. The ID/IG of SLG is essentially unchanged after the doping treatment, which confirms that chloroauric acid doping hardly disrupts the SLG lattice structure. Figure 4(c) demonstrates the sheet resistance of SLG on P-type silicon oxide wafers as a function of doping concentration. According to the Drude model [36], The surface conductivity of graphene is:
where $\tau = \frac{{\mu EF}}{{e\mathop v\nolimits_F^2 }}$ is the relaxation time, $\mu$ is the carrier mobility, ${v_F}$ = 106 m/s is the Fermi velocity, $\hbar$ is the reduced Planck constant. The sheet resistance of SLG as the reciprocal of $\sigma (\omega )$ is closely related to the EF. The sheet resistance depends on the doping concentration, after doping with 100 mM HAuCl4 the sheet resistance of SLG is reduced to about 375 Ω/sq, while the original SLG sheet resistance is about 1065 Ω/sq. The reduction of sheet resistance after HAuCl4 doping confirms the enhancement of EF. The doping concentration of HAuCl4 as a function of absorbance in the simulation is shown in Fig. 4(d). The results suggest that the proposed absorber has two absorption peaks. With the increase of the doping concentration of HAuCl4, the absorbance of the absorber increases from 50% to more than 95%, and the absorption peak undergoes a slight blue shift.Fig. 4. (a) Raman spectra of original and different HAuCl4 doping concentrations of SLG. (b) Shifts of G peaks of original and different HAuCl4 doping concentrations of SLG. (c) The sheet resistance of SLG as a function of HAuCl4 concentration. (d) In the simulation, the doping concentration of HAuCl4 as a function of absorbance.
Figures 5(a) and (b) are the THz-TDS and the prepared absorber, respectively. The doping concentration of HAuCl4 as a function of tested absorbance is shown in Fig. 5(c). In the test, a significant increase in the absorption rate was observed with the increase in the doping concentration of HAuCl4. Despite the differences between the test and simulation results, the two showed remarkable agreement at resonant frequencies of 0.65 THz and 2.0 THz. The validity of the simulation is also confirmed.
Fig. 5. (a) The THz-TDS setup. (b) The prepared absorber. (c) The tested absorbance as a function of the doping concentration of HAuCl4.
4. Conclusion
We have presented an easily-fabricated THz absorber that avoids complex preparation processes such as photolithography. The absorber consists of a sandwich structure, from bottom to top are the Au reflector layer, PI dielectric layer and SLG which doped with HAuCl4. Simulations and experiments showed that after doping with 100 mM HAuCl4, the EF of SLG is shifted about 0.42 eV, the sheet resistance is reduced from 1065 Ω/sq to 375 Ω/sq. Due to the EF shifted, The absorbance is increased from 40% to 80% at 0.65 THz and is increased from 50% to 90% at 2.0 THz due to the EF shifted. In addition, we analyzed the reason for the change in the EF of SLG, and the XPS data showed the dominance of the AuNPs signal, thus proving the existence of a reduction mechanism of Au3+ forming AuNPs on the SLG surface. The shift of the EF of graphene was confirmed by Raman and XPS analysis. The absorber shows promise in the application areas of sensing, imaging, THz communications and other applications. Further more, our method can be used to design, fabricate, and apply similar graphene-based devices.
Funding
Key Research and Development Project Key Program of Shanxi Province, China (202102040201007); General project of Natural Science Foundation of Shanxi Province (20210302123056); Research Project Supported by ShanXi Scholarship Council of China (2020-109); National Natural Science Foundation of China (62171414, 51975541).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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