1. Introduction

The interaction of incident electromagnetic waves with free electrons inside metallic nanoparticles (NPs) results in plasmonic scattering. Varying shapes, materials, and the surrounding environment of metallic nanostructures can alter the behavior of the plasmonic resonance. Such diversity leads to a wide variety of innovative applications, including structural color [1–4]. When visible light of approximately 380–780 nm scattered by the NPs arrives into our human visual system, color is sensed by the retinas. Therefore, by tailoring the parameters as mentioned above, the corresponding spectra, either scattering, transmission, or reflection modified by NPs, can be manipulated to obtain multiple colors. By leveraging elaborately designed unit cells of nanostructures to form color pixels, vivid imagery can be generated by the controlled arrangement of these nanostructures. An emerging application that has recently attracted renewed interest to utilize metallic NPs for color generation is for an alternative to typical pigment-based color generation. For example, dye requires different components to produce multiple colors, while the same material is typically sufficient to produce a wide gamut for plasmonic color generation and also offering advantages of ultra-thinness, high image resolution beyond the optical diffraction limit, and long-term sustainability [5,6]. Nanostructure-based color pixels have also been pursued using high-refractive-index dielectrics, such as silicon [7–11], silicon nitride [12], and titanium oxide [13,14]. These pixel sizes are relatively large because they utilize Mie resonance to trap light, while plasmonic color pixels can be smaller because they trap light by surface plasmon resonance that originates from the negative dielectric constant.

However, most investigated noble-metal-based plasmonic pixels limit large-scale production for commercial purposes. Aluminum (Al) is one of the most promising plasmonic materials because of its low cost, abundance, environmental stability, broadband plasmonic resonance, and compatibility with the commercial complementary metal-oxide-semiconductor (CMOS) processes [15,16]. Therefore, Al is a suitable candidate for the commercial purposes of structural color, especially considering its cost control, environment-friendly management, and industrial production. To date, structural color generation employing various Al nanostructures has been reported [17–36], but limitations remain in realizing full-color variation. Specifically, by relying only on Al NPs and relatively simple designing strategies, difficulties lie in obtaining a wide color gamut and pure chromaticity, especially for the red color. This challenge is due to the nature of Al plasmonic resonance, which usually displays a broadband spectrum and an interband transition around 800 nm [15–36]. Thus, the large size Al NPs initially designed for the red color will cover the blue, green, and red ranges simultaneously, and subsequently bring a diminution on the purity of red color.

In this research, our investigation considers a robust method and principle for the mass production of commercial applications with Al plasmonic color generation. To avoid the problem of spectral purity in the red color and explore the advantages of Al broadband scatterers, we introduce a simple and robust approach to plasmonic color generation using Al NPs as the white pixel. A simple Al nanodisk (ND) with a diameter of 200 nm, 50 nm height, and 500 nm unit cell was designed and fabricated as a white pixel. The ND shape was chosen because of its insensitivity to the polarization of incident light and manufacturability. To obtain white pixels, a large diameter of the Al ND is required, so that fabrication errors during the industrial process can be tolerated relatively with a wider range and high reproducibility. At the same time, excellent visibility is expected under direct sunlight and various illumination conditions. Quick Response (QR) codes are frequently used over a vast range of commercial applications, including anti-counterfeit, product tracking, and labeling. To demonstrate the commercial applicability, one logo and one QR code were fabricated to represent the prospect of the Al white pixel.

2. Methods

First, finite difference time domain (FDTD) simulations (Lumerical FDTD solutions) were performed to obtain the forward scattering spectra of isolated Al NDs and the transmission spectra of periodic arrays under normal illumination (300–800 nm) from the quartz substrate that supports the Al ND. In the FDTD simulation, Al NDs were set with 50 nm thickness and modeled using a simplified 4 nm oxide layer [15,16]. For single Al ND, total-field scattered-field (TFSF) source was employed with perfect matched layer (PML) at x-, y- and z- directions. For periodic Al ND arrays, periodic boundary conditions at x- and y- directions, and PML at z direction were used for the unit cell with the plane wave source. 1 nm uniform mesh size was imposed on the Al ND and nearby 5 nm region, and an auto non-uniform mesh was used in other modeling region with mesh accuracy 4. The forward scattering spectra of single Al NDs were calculated with an increasing diameter (D), and the transmission spectra of the periodic array were obtained with increasing pitch (P). The designed patterns were fabricated onto a quartz substrate (or a TEM membrane for TEM measurement) using conventional electron beam lithography (EBL) followed by thermal evaporation of a 50 nm Al film and liftoff processes. The optical images were measured using an optical microscope under visible illumination by a halogen tungsten lamp. Additional experimental details can be found in Refs. [15,16]. Before transferring to the experiment, the artificial color was first characterized by the International Commission on Illumination (CIE) 1931 XYZ tristimulus values using a 2° Standard Observer under the D65 condition (Eqs. (1)–(3)). Next, the chromaticity coordinates values x, y and z were calculated using Eqs. (4)–(6)) based on the scattering or transmission spectra [37].

3. Results and discussion

The calculated chromaticity values from the simulated spectra are plotted in the CIE 1931xy diagram to illustrate the chromaticity, as shown in Fig. 1. For a single Al ND (black dotted line in Fig. 1), the color varies from blue to green to red with increasing diameter. Specifically, the chromaticity shows a V-shape turn near D = 80 nm in the blue region. When the diameter is increased to approximately 200 nm, the chromaticity value reaches the furthest in the red region with another V-shape turning towards the white point in the diagram. These V turns originate from the interference between the higher modes and the dipolar mode, which causes variation in the scattering spectra. Selected representative scattering spectra are plotted in Fig. 4(a) in the Appendix. With increasing diameter from D = 60 to 150 nm, the dipolar resonance redshifts and broadens. Then, the higher modes start to play an important role in determining the color performance. When D = 200 nm, a scattering from the higher mode appears at λ < 450 nm. The combination of the dipole and higher modes covers the entire visible spectrum, demonstrating the applicability of the ND as the white pixel. With further increasing the diameter, the dipolar mode is out of range of the visible part, and the higher mode starts to dominate the color performance (D400). Considering the color gamut and the size of the pixel, a diameter of 200 nm is selected for our demonstration of white pixels, although a wide range of diameters is feasible. Because only one type of ND exists during image generation, the pseudo periodic array containing a limited number of Al NDs supports narrow and strong resonances due to the coupling between the grating diffraction of the array and scattering of individual ND. Therefore, we also need to consider this effect. The transmission spectra of the periodic array of Al ND (D = 200 nm) were further calculated as a function of pitch (P), and the corresponding chromaticity values were plotted by the red square line in Fig. 1. Representative transmission spectra with varying pitch are plotted in Fig. 4(b) in the Appendix to demonstrate the influence of pitch. With increasing P from 400 nm, the chromaticity point shifts to the white region due to the redshift of the lattice resonance. The chromaticity values are aggregated near the white point with varying pitch and is closest to the white point in the diagram when P ∼ 500 nm. These distributions of chromaticity values indicate the tolerance of manufacturing error to obtain white pixels in the experiment. The chromaticity shift for P > 500 nm is complex because of the redshift of the first-order diffraction to near infrared and appearance of second order diffraction in the visible. Simultaneously, the dips in the transmission spectra elevate with the decrease of number density of NDs. So, we selected the parameters of D = 200 nm and P = 500 nm as the white pixel to demonstrate feasibility.

 

Fig. 1. The predicted chromaticity calculated from the simulated scattering spectra of a single Al ND on a quartz substrate with an increasing diameter (D) from 60 to 400 nm in steps of 10 nm (black dotted line), and from the simulated transmission spectra with a fixed D = 200 nm and increasing unit cell pitch (P) from 400 to 800 nm in a step of 20 nm (red square line). The corresponding starting points of the lines are indicated with arrows. The inset images respectively show the zoomed areas for increasing D and P. Some points are indicated with their coordinates (x, y) for clarity.

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Figure 2(a) presents the simulated scattering spectrum of a single Al ND with D = 200 nm and the transmission spectra from both the simulation and experiment of the periodic array with P = 500 nm. The simulated scattering spectrum of a single Al ND with D = 200 nm reveals a broad dipolar peak from 430 nm to 800 nm, and another peak below 430 nm belonging to the higher mode. To verify the concept of white pixels, a test pattern was fabricated for the Al ND array of D = 200 nm and P = 500 nm, and the desired white color was obtained as shown by the microscope reflection dark-field image in Fig. 2(b). Although the pixel was designed for forward scattering, the white color can be recognized with a standard microscopic readout through the objective lens (NA = 0.9). The transmission spectrum of the periodic NP array is governed by the lattice mode equation at normal incidence,$\lambda = {{P \times n} / {\sqrt {{i^2} + {j^2}} }}$, where n is the refractive index of the air side (n = 1) or from the quartz substrate side (n ∼ 1.46), and i and j are the grating orders [38,39]. The main signatures in the transmission spectra from the grating effect of the quartz side at 730 nm (1, 0) and 516 nm (1, 1), and of the air side at 500 nm (1, 0) are indicated by the dashed lines included in Fig. 2(a). The electric field (|E|²) distribution profile at 730 nm is displayed in Fig. 5 in the Appendix. The experimental transmission spectrum is consistent with the simulation, whereas the degradation and mismatch primarily arise from defects in the grain boundaries and fabrication error in the experiment. The typical quality of the experimental Al ND is shown by the TEM measurement in Fig. 2(c). Unambiguous grain boundaries inside the Al ND can be observed in the TEM images, which is responsible for the larger damping effect of the Al ND in the experimental optical measurement [16]. The grain boundaries and other defects inside the metallic NP act as scattering centers to the collective oscillation of electrons, thus the plasmonic resonance is weakened. We note that the post heat treatment has been proposed to reduce this plasmon damping effect [16].

 

Fig. 2. (a) The scattering spectrum normalized by its intensity maximum from simulations of a single Al ND with D = 200 nm (SingleD200, pink line) corresponding to the right axis, the simulated (D200P500sim, red line), and experimental (D200P500exp, black line) transmission spectra of the periodic array with P = 500 nm corresponding to the left axis. The colored dashed vertical lines show the grating orders from the quartz (blue) and air (green) sides. (b) A SEM image of the fabricated periodic ND array and the corresponding dark-field image of a 40 × 40 µm² array measured in the reflection geometry by the objective of magnification 100 × and numerical aperture (NA) = 0.9. (c) A typical TEM image of Al ND with a diameter of 200 nm fabricated on a TEM membrane.

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As a proof-of-concept, we designed and fabricated a group logo to represent a typical commercial trademark and a QR code encoded by the group website using the conventional EBL method, as shown in Fig. 3. To reduce the cost and time expenses, the available pixels (i.e., the black pixels in the digital images of Fig. 3(a)) were translated into our Al white pixels. To be clear, in the experiment, each black pixel is composed of an Al ND with D = 200 nm and height 50 nm in a square unit cell of 500 × 500 nm². The designed images (Fig. 3(a)) are well reproduced under the transmission (Fig. 3(b)) and reflection (Fig. 3(c)) configurations of a bright-field microscope under white-light illumination. In both cases, the images are perfectly repeated in the experiment by the pixels consisting of Al NDs (see the SEM images in Figs. 3(d) and 3(e)). The excellent contrast of the printed QR codes (Figs. 3(b)–(c)) in the experiment makes it readily interpreted by a scanning application on a mobile phone. Moreover, it is interesting to note the difference between Fig. 3(b) and Fig. 3(c). Under the transmission bright-field configuration (Fig. 3(b)), the white pixel will scatter the white light, while much more intense white light is transmitted and collected from the background (i.e., the area on the quartz substrate without ND) by the collecting objective. This difference in the intensity makes the white pixel black in the recorded image of Fig. 3(b). On the contrary, under the reflection configuration (Fig. 3(c)), white light scattered by the white pixel structure is collected more than the background area where light is transmitted more and not collected. Therefore, our white pixel appears as expected in white. The distinct pictures that emerged under both vision configurations in Figs. 3(b) and 3(c) illustrate the robustness of our white pixel under various illuminating environments and its promise for commercial markets.

 

Fig. 3. (a) Digital images of a designed group logo (1806 × 1207 pixels) and a QR code from the authors’ group website (800 × 800 pixels). In both images, the black line width is approximately 10 pixels. The bright-field photographs for two patterns under the (b) transmission and (c) reflection configuration of the optical microscope by the objective magnification of 10 × and NA 0.3. (d) Top-view SEM images of the selected areas of (d) the group logo and (e) the QR code.

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4. Conclusions

In summary, we demonstrated a robust and simple method using an Al nanodisk as a white pixel structure for large-scale commercial color printing. The digital images of a QR code and a logo were well reproduced in the experiment with high resolution using the designed white pixel comprised of an Al nanodisk with D = 200 nm and height 50 nm in a unit cell of 500 nm × 500 nm. The excellent color performance of the white pixel under the transmission and reflection configurations is robust for commercial applications. The design principle is straightforward and applicable to other substrates with a refractive index different than quartz. Moreover, a protective layer will be required in the commercial applications. Both changes of the substrate and covering layer will bring a different dielectric environment to the Al nanodisk from the situation discussed here, thus the spectra and color performance will be changed. Here, we aimed to demonstrate the strategy by simplifying the situation with air interface. However, depending on the different dielectric environment in reality, white pixel can be readily obtained by optimizing the parameter of Al nanodisk array with simulations. Meanwhile, it is easily transferred to other industrial nanofabrication methods, such as nanoimprinting and photolithography, via a one-step lithographic process. Considering the additional benefits from low-cost Al materials, numerous commercial perspectives can be anticipated in the future.

Appendix

 

Fig. 4. (a) Scattering spectra of single Al nanodisk normalized to each intensity maximum with diameter (D) 60, 80, 150, 200 and 400 nm. (b) Transmission spectra of Al nanodisk array with fixed diameter 200 nm and varying pitch (P) 400, 500, and 800 nm. The dashed vertical lines indicate the dominating grating orders from the quartz and air sides, and the colors correspond to the pitches.

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Fig. 5. Cross-sectional electric field (|E|²) profile at 730 nm for the periodic array. The white dashed lines sketch the material interfaces, and the area of Z below 0 nm stands for the quartz substrate. It is noted that the Al surface is covered with the oxide layer.

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Funding

Conseil Régional Grand Est; Ministère de l'Enseignement supérieur, de la Recherche et de l'Innovation; Ministry of Education, Culture, Sports, Science and Technology (18K19134, 19H02434).

Acknowledgments

The authors would like to thank Miss Shuman ZHANG for designing the group logo. This work was partly supported by the Nanotechnology Hub, Kyoto University (JPMXP09F19NMC042).

Disclosures

The authors declare no conflicts of interest.

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