Light modulation is an indispensable operation in the photic and photonics fields. Research on on-chip broadband optical modification is particularly appealing and yet challenging. Most traditional optical bulk crystals are applied to manipulate light, suffering from issues of bandwidth, loss, and integration^[1]. Therefore, it has great significance for developing optical materials with compact, efficient, broadband response and other characteristics. Two-dimensional (2D) layered materials with atomic thickness, high carrier mobility, and large surface area may create a unique combination to control on-chip light. Recently, many strategies, such as gate tuning^[2,3], defect design^[4,5], mechanical strain^[6,7], heterojunction construction^[8], and anomalous refraction and planar focusing effect^[9,10] have been proposed to engineer the band structure of 2D materials. Compared to these methods, intercalation reaction is generated in host interlayers, with charge transfer between host materials and guest species, making a more obvious Fermi level (Ef) shift than any others^[11,12]. Many excellent works have demonstrated that intercalation is a powerful tool to manipulate light–matter interaction^[11,13,14]. For example, Zeng et al. found color changes and second-harmonic generation in multilayer graphene (about 150 layers) with Li or H2SO4 intercalation^[15,16], and nonlinear absorption effect transformation in few-layer graphene (about 6–8 layers) with Li intercalation^[17]. Omer et al. produced a graphene optical surface and controlled the infrared emission via ionic liquid intercalation^[18]. Wan et al. applied alkali-ion intercalation in a composite film of carbon nanotubes and reduced graphene, and the transmittance had a ∼20%–30% increase from visible (450 nm) to infrared (5 µm)^[19].

Graphene has a unique linear energy and momentum dispersion relationship near the Dirac point with zero gaps, having intraband and interband transition modes, which allows a broad-spectrum response range from ultraviolet, visible, and infrared to terahertz (THz) and microwave^[20]. Geim et al. found that the optical absorption rate of single-layer graphene was 2.3% and calculated that the optical absorption of few-layer graphene (FLG) was proportional to the number of layers^[21]. Therefore, an appropriate increase in the thickness of graphene can enhance the optical response. FLG presents a stacking of graphene layers with an interlayer spacing of 3.3 Å^[22]. Owing to the strong covalent bonding (347 kJ/mol)^[23] in the layers but with weak van der Waals interaction (16.7 kJ/mol)^[24] between layers, FLG is considered an excellent host material that enables abundant ions to dwell in graphene interlayers. Metallic Li is a promising anode candidate for Li batteries. Due to the minimum radius in all of the metal elements^[25] and electrons easily lost, Li-ions can rapidly shuttle between graphene layers. In addition, its high theoretical capacity of 3860mAhg−1 can donate many electrons to host materials^[26]. During the lithium (Li)-ion battery reaction process, a large charge transfer occurs between the Li anode and graphene cathode, shifting Ef from 0 to 1.5^[27] and even 2 eV^[28] for LiC6. Therefore, Li intercalation should be able to operate optical properties in FLG over a wide spectral bandwidth and with high modulation depth.

In this study, a Li few-layer graphene intercalation compound (LiGIC) is prepared by galvanic reaction. Raman spectra, transmittance spectra, and optical images are used to investigate and understand the optical properties changes in FLG before and after Li intercalation. After Li intercalation, the formed LiGIC is proved to be LiC12. We observe obvious optical transmittance changes in visible, infrared, and THz waves. The striking increase in sheet conductivity implies charge transfer between Li atoms and graphene layers, which will cause a move of the Ef of graphene. Hence, the behavior of the electron transition both in intraband and interband will be changed, leading to modifying properties in a broad range. Our result provides possibilities for optoelectronic applications based on 2D intercalated compounds in the broad-spectrum stealth regime.

FLG (6–8 layers) was grown by chemical vapor deposition (CVD) on 50 µm thick copper (Cu) foils, which were purchased from SixCarbon Technology Shenzhen. The surface of FLG was spin-coated with 300 nm polymethyl methacrylate (PMMA) film (2000 r/min, 45 s; PMMA A4, Microchem). Then, the PMMA/FLF/Cu was floated on the FeCl3 saturated solution, and the Cu substrate was etched after about 6 h. Afterward, the FLG film supported by PMMA was transferred to deionized water and cleaned thoroughly 3 times. A JGS1 quartz substrate was used to pick up the FLG/PMMA in the deionized water and the PMMA/FLG/JGS1 was dried in an oven at 80°C for 6 h. Next, it was immersed in acetone for 10 h to remove the PMMA completely. After the FLG/JGS1 was fetched out of the acetone, it was immediately immersed in anhydrous ethanol for 30 s. Finally, we baked it in an oven at 80°C to dry the remaining anhydrous ethanol and acetone.

The FLG on the JGS1 quartz substrate was transferred inside a glovebox [Super (1220/750/900), MIKROUNA, China] filled with an argon atmosphere. Then small Li metal was pressed on the surface of FLG and placed in a corner area of FLG. Then, a few drops of electrolyte (LiPF6 in ethylene carbonate/diethyl carbonate, w/w = 1∶1) were added and covered on the whole FLG. The lithiation reaction is based on the galvanic cell. Due to the cathode FLG and anode Li contact with each other, their interface acts as the lithiation reaction channel. The Li reaction can be started with the electrolyte added and finished for several minutes. After that, the electrolyte is cleaned by the diethyl carbonate solvent. Next, a double-sided square frame tape is stuck to the substrate with the sample in the center of the frame, and another JGS1 quartz is placed on it.

The Raman spectra were acquired by a commercial MStarter 100 micro spectral scanning test system armed with an excitation laser of 532 nm. The collection and analysis of emitted signals is achieved by a spectrometer (ANDOR-rock). During measurement, we used a microscope objective with 50× magnification to focus the laser on the surface of the FLG film through the top glass. The laser power was set to 2.8 mW. The sheet resistance of FLG and LiGIC was measured using a Keithley 2636B source meter (Tektronix, USA) and with the van der Pauw method. Visible optical absorption and transmittance spectra were performed by an angle-resolved spectrum system (Ideaoptics, China). Infrared absorption and transmittance spectra were recorded on a Fourier transform infrared spectrometer (FTIR, Thermo Scientific Nicolet IS50, USA). Large-scale thermal mapping could be measured with a FLIR T560 thermal camera, which allows broad wavelengths of 7.5–14 µm and has a high resolution of 640 pixel × 480 pixel. THz transmission was measured with an optical pump THz-probe (OPTP) setup. The mode-locked Ti:sapphire regenerative amplifier (Spectra-Physics) was employed as a laser source, with 100 fs pulse duration, 800 nm central wavelength, and 1 kHz repletion rate. The laser beam shows each pulse with a certain energy of 6 mJ in the entire OPTP setup. The output pulses were divided into three parts. Two of them were applied to THz wave generation and detection through ZnTe crystal. Another part of the femtosecond beam was radiating on the sample as an optical pump light. By employing a standard Fourier transformation and normalization, the frequency-dependent THz amplitude T(ω)=Es(ω)/ER(ω) was gained.

Figure 1(a) demonstrates the device fabrication process. A sandwich-structured galvanic cell is designed for optical measurements. The CVD-grown FLG film is contacted with a Li slice and covered by a few drops of electrolyte (LiPF6 in ethylene carbonate/diethyl carbonate, w/w = 1∶1). The sample is sealed with two thin transparent glass and a double-sided square frame tape to prevent oxidation. Details of the device fabrication are provided in Sec. 2. Different from the conventional charging mode, galvanic reaction makes a faster Li intercalation because of the direct contact of the cathode (FLG) and anode (Li). During the Li intercalation process, the crystal structure of FLG remains unchanged because of Li-ions accommodated in the graphene interlays, as shown in Fig. 1(b). According to previous research, intercalation behavior often leads to a strong charge transfer between host and guest^[29,30], and the direction of interfacial charge transfer defers to interfacial level alignment regulation^[30-32]. For Li intercalation on FLG, the charge transfer occurs from Li atoms to graphene layers, as illustrated in Fig. 1(b). Large numbers of electrons fill the π orbitals in graphene with Ef increased, as shown in the graphene band structure before and after Li intercalation in Fig. 1(c).

Raman spectroscopy is employed to characterize the structures’ change of FLG with Li intercalation, as depicted in Fig. 2(a). Before Li intercalation, FLG shows a typical Raman spectrum (black line). The G peak is located at 1583.9cm−1, which is a normal first-order Raman scattering process and is associated with the in-plane vibrations of sp2 carbon atoms^[33]. The 2D peak appears at 2704.2cm−1, originating from a second-order process and involving two phonons near the Dirac point^[33]. The D peak is observed at 1354.5cm−1, being a double resonance scattering process due to the presence of structural defects^[34]. After Li intercalation, the Raman spectrum is shown in the red line. The G peak shows a blue shift to 1591.4cm−1 due to Li doping^[12]. The 2D peak completely disappears. This is because of the strong electron transfer from Li atoms to graphene layers, leading to the inter-band optical transition Pauli-blocked^[12]. Thus, there is no resonant Raman process. According to previous research, the result corresponds to the formation of LiC12 with a graphene layer flanked on one side by Li-ions^[12,35]. The position and intensity of the D peak remain almost unchanged, implying no obvious structural deformation during the Li intercalation process.

To investigate the carrier transport behavior of FLG film before and after Li intercalation, we transfer the sample onto pre-deposited electrical contacts with a Hall bar arrangement. The van der Pauw technique is applied to measure the sheet resistance, which is not prone to errors due to the existence of contact resistance, as described by the I−V curves in Fig. 2(b). Before Li intercalation (black line), the FLG presents a high resistance characteristic with a sheet resistance of 1224.5 Ω/sq. After Li intercalation (red line), sheet resistance drops by 2 orders of magnitudes to 66.1 Ω/sq. According to previous research, intercalation can increase the concentration of carriers (electrons or holes)^[27,36], which can effectively enhance the material’s electrical conductivity^[37].

The demonstration of visible modulation of FLG with Li intercalation is shown in Fig. 3. The devices are encapsulated with JGS1 quartz for visible optical measurements. Figures 3(a) and 3(b) show the optical images of FLG and LiGIC. Apparently, the LiGIC has a higher transparency compared with FLG. To further examine the light modulation effect of Li intercalation, we measured the optical spectra of FLG and LiGIC in the wavelength range of 480 to 1040 nm. The transmissivity spectra are shown in Fig. 3(d). For FLG (black line), the transmissivity increases gradually with the wavelength, from 84.9% at 480 nm to 87.6% at 1040 nm. After Li intercalation, LiGIC (red line) has a higher transmissivity of over 90% from 480 to 1040 nm, with a maximum increase of 7.7% from 86.4% to 94.1% at 600 nm. The reflectivity spectra of FLG before (black line) and after (red line) Li intercalation are displayed in Fig. 3(e). From the wavelength of 500 to 600 nm, FLG presents almost the same reflectivity as LiGIC. From 600 to 970 nm, the reflectivity of LiGIC is higher than FLG, with an increase of 0.33% at 702 nm. From 970 to 1040 nm, the reflectivity of LiGIC is lower than FLG. The absorptivity spectra can be computed based on the transmissivity and reflectivity spectra, as illustrated in Fig. 3(f). The FLG has a remarkable absorptivity change after Li intercalation over the wavelength from 480 to 1000 nm, with a maximum decrease of 7.89% from 10.99% to 3.1% at 650 nm.

The transmittance increase and absorptivity decrease indicate that graphene has a shift of Ef after Li intercalation. Li is an active alkali metal with strong electron donation capability, and Li-2s electrons tend to transfer into carbon π-bands^[27]. Therefore, it results in the Ef moving into the region of the high density of states. The diagram of the doped graphene band structure and interband transition is illustrated in Fig. 3(c). The optical response of graphene at visible frequency depends on interband transition^[38,39]. When photon energy is lower than 2Ef, the optical interband transitions are locked due to the Pauli exclusion principle. Hence, the uplift of Ef causes reduced absorption and increased transmission in the visible waveband. The transparency change in the macro-scale sample enables us to perceive the Li intercalation behavior on the micro-scale.

In the infrared frequency band, the optical modulation of FLG by Li intercalation is displayed in Fig. 4. Figure 4(a) illustrates a cross-section of the optical device on a hot plate for qualitative demonstration of infrared transmission. Two BaF2 with a high infrared transmittance are applied to seal the device, as shown in Fig. 4(e) (blue line). To increase the contrast between the sample and the background, an aluminum (Al) foil with low infrared emissivity is placed under the device. The surface temperature distribution of the devices is detected by an infrared camera with a wavelength operation range from 7.5 to 14 µm, as shown in Fig. 4(b). As the hot plate is fixed to 30°C, the Al foil shows a black color because its low infrared emissivity and low infrared transmittance result in a low apparent temperature. In the sample area marked with a blue dotted box, LiGIC shows a higher 1°C apparent temperature than FLG. When the hot plate is fixed to 40°C, the Al foil remains only the whole covered by the device. The LiGIC shows a higher 1.9°C apparent temperature than FLG. For LiGIC and in the corner far away from the Li metal, we can observe a lower apparent temperature compared to other regions. This is attributed to the Li deintercalation and recovery of its optical properties.

To better comprehend the thermal imaging temperature difference of FLG before and after Li intercalation, we additionally measured the infrared absorption and transmission spectra, as illustrated in Figs. 4(d) and 4(e). FLG shows almost the same absorption spectra as LiGIC, in which the absorptivity has a small increase with wavelength, from ∼0.1% at 2.5 µm to ∼0.5% at 14 µm. As for the transmission, the FLG reduces from ∼80% to ∼68% over the wavelength range from 2.5 to 11 µm after Li intercalation. From 11 to 14 µm, the FLG and LiGIC present a transmittance reduction with wavelength. The decreased transmittance may come from the BaF2, as shown in the blue line in Fig. 4(e). These results are consistent with their thermal images. Compared to FLG, LiGIC has the same optical absorption (∼0%) but a lower transmittance. Therefore, LiGIC shows a higher apparent temperature than FLG with Al foil as the background. The schematic of the increased Ef of the Li-intercalated graphene and the electronic transitions is depicted in Fig. 4(c). The optical absorption of graphene is mainly due to the interband and intraband electron transitions^[39,40]. Graphene with low doping (Ef of 200 meV) can block infrared absorption due to interband transition. On the other hand, increased carrier concentration leads to a small enhancement of infrared absorption due to intraband transition. According to relevant studies, the mid-infrared optical absorption of graphene is very small and modulated no more than 0.25% at 14 µm by ionic liquid intercalation^[37]. Therefore, when the total contributions of the two transition processes in the mid-infrared waveband are almost equivalent, FLG and lithiated graphene may illustrate the same infrared absorption.

The THz response is measured by conventional THz time-domain spectroscopy. Two Z-cut quartz without FLG are regarded as a reference. Figure 5(a) shows the time-domain signals of the Z-cut (blue line), FLG (black line), and LiGIC (red line). It can be observed that Li intercalation makes a remarkable change in the peak amplitude of the THz pulse. With the Fourier transform of time-domain signals and normalization with the reference signal, we obtained transmitted THz amplitude spectra, as shown in Fig. 5(b). For FLG (black line), the average of the transmitted signal is ∼90% from 0.4 to 1.2 THz. After Li intercalation (red line), the amplitude of the transmitted signal shows about 20% modulation over broad spectra from 0.4 to 1.2 THz and decreases from 97.2% to 68.2% at the wavelength of 1.2 THz. The inset of Fig. 5(b) shows the electronic transitions of the Li-intercalated graphene. For the long wavelengths of THz frequencies, the interband transitions are Pauli-blocked for even an unintentional doping. Therefore, the long-wavelength optical absorption is mainly due to the low-energy intraband transition^[41]. In the THz regime, graphene is regarded as tunable Drude metal, and its optical conductivity depends on frequency expressed as^[42,43]σ(ω)intra=σDC1+jωτ,(1)where σDC, τ, and ω represent the conductivity of graphene, scattering time of graphene, and frequency, respectively. Therefore, the direct current conductance is beneficial to the increase of THz optical absorption. As shown in Fig. 2(b), Li intercalation leads to striking resistance reduction of FLG. Thus, it will make an enhancement of THz optical absorption, which implies the decrease in transmission [Fig. 5(b)].

In conclusion, we have developed Li-intercalated FLG with a galvanic cell structure, realizing the broadband optical modulation of FLG, with a visible transparency increase higher than 90% from the wavelength of 480 to 1040 nm, with an infrared transmittance decrease from ∼80% to ∼68% in the wavelength range from 2.5 to 11 µm, and with ∼20% reduction in the wavelength range from 0.4 to 1.2 THz. This is attributed to the strong electron doping resulting in the uplift Ef of graphene, which makes significant changes in the intraband and interband optical transitions. The visible, mid-infrared, and THz optical responses are attributed to interband transitions, intraband and interband transitions, and intraband transitions, respectively. Our research may provide a new strategy for the design of optical modulation devices in a broad spectral region.