Large Area Plasmonic Color Palettes with Expanded Gamut Using. Colloidal Self-Assembly

Transcription

1 Large Area Plasmonic Color Palettes with Expanded Gamut Using Colloidal Self-Assembly Liancheng Wang 1, Ray J. H. Ng 1, Saman Safari Dinachali 1, Mahsa Jalali 1, Ye Yu 1, and Joel K.W. Yang 1,2 1 Engineering Product Development Pillar (EPD), Singapore University of Technology & Design, 8 Somapah Road, Singapore Institute of Materials Research and Engineering (IMRE), A*STAR, 2 Fusionopolis Way. Innovis, Singapore KEYWORDS: Plasmonic color; Aluminium; Polystyrene; Colloidal Self-assembly; 1. PS shrinkage and HSQ etching Surface irregularities and roughness appeared when the particles were etched to around half of their original size, as shown in the bottom part in Fig. S1a). Even when deformed, the PS particles were still capable of serving as the DRIE etch mask, but the surface roughness was transferred to the underlying HSQ posts (Fig. S1b). With further size reduction, the integrity of the PS particles as an etch mask is severely reduced. In this paper, we investigated only colors arising from particles with diameter to pitch ratios greater than 1:5 (Fig. S1b).

2 Fig. S1 2. Long-range order of our proposed process Our proposed process shows high efficiency and cost-effectiveness, and yields structures with high uniformity over a large area. Figure S2 shows the structure of Al Disks (P=420nm, R=150nm) with long range order over 120μm by 100μm. Figure S2(b) is a magnified view of the dashed box in Fig. S2(a) while Fig. S2(c) is a magnification of the dashed box in Fig. S2(b).

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4 Fig. S2 3. Al disks with varying HSQ height Figure S3 shows the structures of Al disks (P=420nm, R=150nm) with varying HSQ heights of 60nm (a), 90nm (b) and 120nm (c). Fig. S3

5 4. Discussion and analysis of the resonances modes-al Disks To understand the underlying mechanism governing the reflectance response of Al Disks, we overlay the respective reflectance curves obtained from the corresponding FDTD simulations, as presented in Fig. S4.1. Figure 4.2 shows the spectra of the Al Disks with varying R, P and H. Figure. S4.1(a) shows the color contour plot of the reflectance spectra with varying R and fixed P,H=420nm, 100nm. Three bands of reflectance dip, A, B, C are clearly observed in Fig. S4.1(a). Band_A stays constant at λ~360nm with the narrowest FWHM of ~30nm, which is related to the lattice diffraction. Band_B is related to the high order mode, which overlaps with band_a when R is below 90nm, but further redshifts, intensifies and widens when R increases, to λ=~556nm when R=180nm. It was also found that, when R is small (i.e., R=60nm), the back reflector tends to increase the reflectance intensity. However, when R is large, the intensification effect of the back reflector Al weakens and the spectrum of the Al Disks evolves to quite resemble that of the structure without the back reflector. Band_C is caused by the fundamental mode, which is dominant when R is small. For the previous reported plasmonic structure, the fundamental mode is the dominant mode responsible for color generation in the visible range. The fundamental mode broadens and redshifts significantly, and moves out of the visible region when R is larger than 120nm. From Fig. S4.2, we noticed that, for the 420nm-pitch Al Disks, when R is large, band_a and band_b contribute together to filter the incidence, whereas the fundamental mode dominates when R is small. The diverse combination of these resonances when R varies produces color spanning from pink to yellow, green and blue. For the 628nm and 520nmpitch based Al Disks, the high order mode, i.e. band_b, which dominates the spectra, is

6 located at the yellow red region, producing a bluish color. We further examined the electric field intensity distribution through a vertical cross section of the Al Disks, as shown in Fig. S4.1 (b). Figure S4.1 (a2) shows the 2D color map of the reflectance intensity as a function of pitch, with fixed R=100 and H=100nm. The diffraction mode redshifts when P increases, as governed by the dispersion characteristics of SPR. Similarly, when R/P ratio is large, the spectra are dominant by the top disks and HSQ posts in Al Disks. Figure S4.1(a3) and (a4) are 2D color maps with varied H and fixed (a3): P, R=(420, 80nm) and (a4) P, R=(420, 150nm), respectively. In Fig. S4.1(a3), for small R (80nm), when H increases, the fundamental mode blueshifts from 500nm to 700nm since the mode energy increases. When R is large (150nm) (Fig. S4.1 (a4)), the high order mode dominates due to the large size of the disks. The high order mode redshifts from 448nm (H=60nm) to 527nm (H=150nm). Our theoretical assessment here provides a good understanding of the mechanism governing the plasmonic color generation, thus it could further facilitate the design of plasmonic coloration structure.

7 Fig. S 4.1 Fig. S 4.2

8 5. Discussion and analysis of the resonances modes-al Dome-Rings For the Al Dome-Rings, we confirmed the localized modes and various hybrid modes from the electric field distribution based on FDTD simulation. Various modes arise from the interaction of various localized modes supported by the dome and the plasmonic modes in the rings, as shown in Fig. S5. Fig. S 5 6. Discussion and analysis of the resonances modes-al Rings Figure S6 (a) shows the simulated spectra of Al Rings with varying outer and inner radii (R, r) with fixed pitch of 420nm, and Fig. S6(b) plots the electric field distribution in the cross section plane and transverse plane of the modes in Al Rings, denoted in Fig. S6 (a). The mode i shows a relatively wider FWHM (due to large radiative loss). Mode i redshifts when the inner radius r increases and the outer radius R is fixed due to increased circumferential length of the ring. The electric field of the mode ii is mostly confined at the rim of the outer radius and the gap between the neighbouring rings. Mode ii remains almost invariant when R is fixed and r is varied, and redshifts when R increases. This is because the circumferential length of the ring increases and the

9 gap between the neighbouring rings narrows (resulting in a lower energy stored in the rings). Mode iii blueshifts when R is fixed and r is decreased, due to a decreased width of the ring. The dip iv corresponds to the Bragg diffractive mode, which remains invariant at ~λ= 365nm for the 420nm-pitch Al Rings. Mode v corresponds to a high order modes. Figure S6 (c) further re-plots the experimental curves in Fig. 4 (a-iv) and the corresponding simulation results, which are similar to each other. Fig. S 6 (a) and (b)

10 Fig. S 6 (c)