Device structure and operation principle. A typical F-P cavity consists of a thin film stack, which creates selectively coloured spectra through interference effects. By using a thin film with a tunable optical constant, adjustable interference effects can be achieved, resulting in a tunable colouration behavior. Figure 1a illustrates the schematic of the active colouration devices. These devices include an F-P nanocavity type working electrode based on MLG on nickel (Ni) foil, and a Li-doped nickel manganese cobalt oxide (LiNiMnCoO2, 5:3:2, Li-NMC, the Li source) counter electrode on aluminum (Al) foil with an optically transparent electrolyte solution (LiPF6 in EC/DMC = 50/50 (v/v)) as the ambience. Such an MLG-based EC device is a planar Li-ion battery simultaneously. To ensure reversible Li intercalation, this device is sealed between two optically transparent glass layers, protecting it from exposure to oxygen and moisture (see Materials and Methods). The F-P nanocavity type cathode on Ni foil, core elements of the EC devices, is shown in Fig. 1b and 1c. It is composed of an MLG film on Ni foil, a dielectronic SiO2 layers with different thickness, and an ultrathin semitransparent Cr layer. This device structure benefits from the high conductivity of MLG film and the tunability of the optical constant (n, k) of MLG through Li intercalation. MLG is a layered material (see Fig. 1d), where the layer is bonded by weak van der Waals forces. During the charging process, Li ions from Li-NMC move towards the van der Waals gaps of MLG (see Figure S1), controlled by external electric field or current stimulation. When fully charged, MLG forms LiC6, a stage I graphene intercalation compound. In this compound, graphene and Li layers alternate in a sequence of AαAαAα along the z-axis (see Fig. 1e). The layer space increases from 3.40 Å to 3.75 Å after intercalation (see Figure S5).
During the Li intercalation process, electrons are transferred from Li to graphene layers, resulting in an increased density of stages (see Figure S2-S4). This is further confirmed by the increase of conductivity (see Figure S6) and disappearance of Raman peaks (see Figure S7) after Li intercalation. The optical constants (n, k) of MLG can also be modified by Li intercalation, which are decreased after Li intercalation for the wavelength from 400 nm to 700 nm (see Fig. 1f), as calculated by density functional theory (DFT). For example, the refractive index (n) decreases from 2.93 to 2.07 at the wavelength of 700 nm after Li intercalation. Simulation results (see Figure S9) show that MLG with different thickness on Ni foil only exhibit inconspicuous structural colours before and after Li intercalation. The LiC6 films on Ni foil with different thickness display orange-yellow relevant colours, which can be easily mistaken for the gold colour of fully lithiated MLG (LiC6)14,17. Figure 1g illustrates the reflectance of MLG on Ni foil with the thickness of around 100 nm before and after Li intercalation, for the wavelength ranging from 400 nm to 700 nm. The pristine MLG appears dark grey due to its low reflectivity (~ 20%). However, the reflectivity is found to increase to ~ 70% after Li intercalation, appearing gold-yellow colour.
Regarding MLG-based F-P nanocavities, the colouration mechanism is induced by an interference effect, which is different from the band structures-based colouration mechanism in conventional graphene-based EC devices14,17,24. The interference effect for MLG-based F-P nanocavity could be adjusted by the degree of lithiation of MLG, allowing for tunable colour chromaticity. Figure 1h and 1j present the simulated reflectance spectra and colour points in CIE colour space of MLG (the thickness of the MLG film is 100 nm) based F-P nanocavities with different thickness of SiO2, respectively. These simulation results indicate that, before intercalation, the F-P nanocavities exhibit only small fluctuations in the reflectivity spectra and show a little perceptible structural colour for different thickness of SiO2 absorption layers. This can be attributes to the high absorption of pristine MLG, as shown in Fig. 1g. On the other hand, LiC6 exhibits high reflectivity and a golden colour. Therefore, multicolour can be expected in the Fabry-Perot nanocavities after Li intercalation by varying the thickness or materials of the dielectronic layer. The simulated reflectivity spectra (see Fig. 1h and 1i) shows that the peak-to-valley fluctuation increases from approximately 31.5% to approximately 58.1% after Li intercalation. The reflectivity spectra for F-P nanocavities after Li intercalation exhibit wavelength-selective reflections (see Fig. 1i). The colours points in CIE colour diagram (see Fig. 1k) are found to be closer to the edges, compared to those of MLG-based F-P nanocavities before Li intercalation (see Fig. 1j), indicating a more reflective and colourful state. Figure S10 presents the simulated reflectivity spectra for F-P nanocavities with the thickness of MLG of 300 nm before and after Li intercalation, highlighting the critical role of MLG thickness in these EC devices. It is important to note that the Li intercalation process is reversible and repeatable, which will be further discussed in the following sections. Consequently, these F-P nanocavities hold promise for the development of fully colour MLG-based EC devices.
F − P nanocavity enabled full-colour tunability. To examine the full-colour tunability, a F-P nanocavity type working electrode is fabricated by successively magnetron sputtering ultrathin SiO2 (thicknesses ranging from 50 nm to 250 nm) and Cr (8 nm) films on MLG, which is grown on Ni foil (~ 25 µm) by the chemical vapor deposition (CVD) method (see Figure S11). These F-P nanocavity type working electrodes (see Figure S12) show different structural colours depending on the thickness of the SiO2 layer, which is consistent with the simulation results. The thickness of each layer is measured using cross-sectional transmission electron microscopes (TEM), with the sample fabricated by focused ion beam (FIB) system (see Figure S13). The MLG thickness used in this work can be mainly classified into two groups: approximately 100 nm (Fig. 2 of 129 nm and Figure S14 of 121 nm) and approximately 300 nm (Figure S15 of 313 nm and S16 of 340 nm). Figure 2a presents a bright-field cross-sectional TEM image of the working electrode, with the corresponding high-angle annular dark-field (HAADF) TEM image in the inset. These images reveal a multilayer thin-film structure, consisting of an MLG layer of 129 nm, a SiO2 thin layer of 123 nm, and a Cr layer of 8 nm. The corresponding energy dispersive spectrometer (EDS) mapping in Fig. 2b demonstrates a uniform distribution of elements (C, O, Si, Cr, and Ni), indicating the high quality of the sputtered multilayer structure. It should be noted that controlling the MLG thickness accurately using the CVD method is difficult. In some area of these samples, MLG shows irregular morphology in Z direction (see Figure S17).
Figure 2c and S18 present scanning electron microscope (SEM) images of the top surface of the F–P nanocavity, where clearly show the presence of cracks. These cracks are caused by the rough surface of MLG (Figure S19) and facilitate the intercalation of Li. The rough surface also suggests that the light is diffusely reflected from the F–P nanocavity, indicating a wide visual angle25. Further discussion on this topic will be provided in the following section. Energy dispersive spectroscopy (EDS) and Raman spectra (see Figure S19 - S24) confirm the uniform elements distribution and the high quality of the MLG and F-P nanocavity in this work. To protect against the oxygen and moisture in air, the MLG-based EC devices are encapsulated between two transparent quartz slides (see Figure S25 - S29). The optical images for the MLG-based nanocavity EC devices are shown in Fig. 2d, with the thickness of MLG around 100 nm and the thickness of SiO2 ranging from 50 nm to 250 nm. Pristine MLG display a gray colour, which turns to bright yellow after Li intercalation, consistent with previous researches14,17. The colours of MLG-based F-P nanocavity devices are quite monotonous before intercalation, as seen in Fig. 1h and 1j. However, the colour becomes vibrant after Li intercalation, showing shades of yellow, green, red, violet, blue and so on.
The corresponding optical reflectivity spectra for F-P nanocavity type devices, dependent on the SiO2 thickness and wavelength, are shown in Fig. 2e and 2f, before and after Li intercalation respectively. Before intercalation, the reflectivity of MLG (see Figure S30, the MLG thickness ~ 100 nm) is almost flat with a reflectance of approximately 20% in the visible regime. For the electrode with a SiO2 thickness of 120 nm, there is a peak at approximately 415 nm in the reflectivity spectrum. As the thickness of SiO2 increases to 190 nm, the peak position gradually shifts to 775 nm. A new peak at approximately 405 nm appears for SiO2 thickness ~ 250 nm. When a constant electric current (~ 1 mA cm− 2) is applied, Li ions move from Li-NMC into the van der Waals gaps of MLG. After Li intercalation, the optical constant (n, k) of MLG changes, and the reflectivity increases to approximately 70%, as discussed in the previous parts (see Figure S30). The F-P nanocavity exhibits dynamical colouration capabilities. Figure 2f shows the reflectance spectra for F-P nanocavity type devices after Li intercalation, with various SiO2 thicknesses. The reflectance spectrum for the device with a SiO2 thickness of 110 nm shows a peak at 510 nm. This peak shifts to 800 nm, when the SiO2 thickness is increased to 185 nm. A new peak at 415 nm appears with the thickness of SiO2 film of 145 nm, which increases to 495 nm when the SiO2 thickness is increased to 250 nm. The peak-to-valley fluctuation increases from ~ 17.2% to ~ 41.0% after Li intercalation. In-situ measurements of the reflectance spectra of F-P nanocavity type devices during Li intercalation were carried out (see Figure S31) for a typical device with a SiO2 thickness of 110 nm. The peak position is found to increase from approximately 340 nm to 550 nm and then decrease to 510 nm during the in-situ Li intercalation process.
Figure 2| Colour tunability of graphene-based F − P nanocavity type electrochromic devices. (a) Bright field TEM image of the cross section of the F-P nanocavity. Inset: corresponding high-angle annular dark-field (HAADF) image. (b) EDS map of the cross section of F-P nanocavity. (c) SEM image of the top surface of the F-P nanocavity. Inset: zoomed-in SEM image. (d) Optical images for F − P nanocavity-type EC devices before and after Li intercalation. The thicknesses of SiO2 layers are 50 nm, 80 nm, 110 nm, 120 nm, 145 nm, 160 nm, 170 nm, 185 nm, 190 nm, 220 nm, and 250 nm. The scale bar is 1 mm. (e, f) Reflectance spectra as a function of SiO2 thickness and wavelength for F-P nanocavity before and after Li intercalation, respectively. (g) Corresponding colour points in CIE colour coordinates for MLG-based F-P nanocavities before and after Li intercalation.
To gain further insight into the dynamic colouration tunability of multiple-colour EC process, we characterized the optical memory effect, response time, cycle stability properties, and angle of view of MLG-based F-P nanocavity-type EC devices. Figure 3a presents the optical images of the representative EC device with a SiO2 thickness of 250 nm at different stages. It shows that the airtight sealed EC device can maintain its green colour stage for over 24 hours under open circuit conditions. The colour can also return to its original dark blue colour for the deintercalated device by reversing the current direction. This demonstrates that the MLG based EC device is bistable, which is further confirmed by the corresponding reflection spectra in Fig. 3b. The reflectance for intercalated device remains almost unchanged after keeping in air for 24 hours, and the visible reflection spectra can be recovered to the original stage after a complete cycle. The temporal response and cycling performance are crucial for real application. For a constant electric current of 1 mA cm− 2, Figure S39 suggests that the charging and discharging times are around 15 s and 30 s, respectively. Figure 3c presents 250 charging/discharging cycles through repeatable reversing current direction. The colouration behavior is repeatable during cycles with no obvious degradation (see Video S1). The incident angle-resolved reflection spectra shows that the reflection spectra (Figure S42) and the corresponding angle-resolved optical images (Figure S43) remain almost unchanged for the incident angle various from 0° to 40°. This indicates that MLG-based EC devices are suitable for display with a wide-viewing angle. These overall EC performances are superior to pervious EC alternatives14.
Low-powered dynamic colour display. To demonstrate the potential of MLG-based F-P nanocavity type EC device, we present a display capable of showing images. Figure 4a shows the optical images of the display devices before and after intercalation, with its working electrode patterned into the Arabic numerals of 1, 2, 3, and 4. This indicates that this EC device can be pattern into different geometries, making it promising for various applications of smart electronic devices. Unlike traditional display devices, the colouration is determined by the reflective spectra for the visual range. As shown in Fig. 4b, the patterned geometries are visible under the sun with the radiation of 1100 W/m2, which is different from that of the OLED display (such as the display of the smart phone Honor 50). The MLG-based EC device can be further pixelated, allowing for the generation of arbitrary colours. The reflectivity of such pixels/subpixels can be modulated by the intercalation/deintercalated process. Figure 4c presents the SEM and optical images for the EC devices with different sizes, respectively. Different colours in a circular shape can be observed under an optical microscope for different sizes, with the colour maintaining until 2 µm. This demonstrates that the pixel-scaling limit of the MLG-based EC device can be reduced to 2 µm, even in the electrolyte solution surroundings, which is half of the smallest size of micro LED device reported thus far26. This pixel size enables a dynamic colour display with high spatial resolution.
The energy consumption (the energy per unit area) of the EC device is around 3.702 mW cm− 2 for one charging process. The MLG-based EC device also functions as a Li-ion battery. Therefore, the charging energy could be stored in the coloured stage, which can lighten up an LED for around 600 seconds for a device with an F-P nanocavity area of 1 cm2, as shown in Fig. 4d and Video S2. It is conceivable that the MLG-based EC pixel could be used to charge another pixel. The energy consumption of our device and that of LCD, OLED and EPD are summarized in Fig. 4e. The energy efficiency for our device is ~ 57.06%, reducing the energy consumption to 1.590 mW cm− 2, which is only half of the commercial low-power display system - electronic paper display (EPD) system1. Given that graphitic materials can achieve energy efficiency above 90% in commercial Li-ion batteries19, it is possible to further reduce energy consumption below 0.370 mW cm− 2 by optimizing the device structure.