Conformal CVD-Grown MoS2 on Three-Dimensional Woodpile Photonic Crystals for Photonic Bandgap Engineering

To achieve the modification of photonic band structures and realize the dispersion control toward functional photonic devices, composites of photonic crystal templates with high-refractive-index material are fabricated. A two-step process is used: 3D polymeric woodpile templates are fabricated by a direct laser writing method followed by chemical vapor deposition of MoS2. We observed red-shifts of partial bandgaps at the near-infrared region when the thickness of deposited MoS2 films increases. A ∼10 nm red-shift of fundamental and high-order bandgap is measured after each 1 nm MoS2 thin film deposition and confirmed by simulations and optical measurements using an angle-resolved Fourier imaging spectroscopy system.


Optical properties
The refractive index and extinction coefficient of the MoS 2 films are examined via ellipsometry using a 10 nm MoS 2 thin film on a 300 nm silica (SiO 2 ) coated silicon (Si) wafer substrate.
The results are shown in Figure S1. At 1500 nm, the measured refractive index is n ∼ 3.1 and the extinction coefficient k ∼ 0.62. Figure S2 shows measured transmission, reflection and calculated absorption for a 10 nm MoS 2 thin film deposited on a 170 µm silica coverslip (the same one used in the DLW system for fabricating woodpile templates). The absorption A is calculated as A=1-T-R, where T and R are the normalized transmission and reflection respectively. About 7% absorption can be observed across the wavelengths of interest from 0.9 µm to 1.7 µm.

Thickness on the substrate
In order to estimate the thickness of the coating, a thin vertical cross-section slice was taken near the coated woodpile. Figure S4(a) shows a Transmission electron microscopy (TEM) image of the slice. X-ray spectroscopy (EDX) analysis was then used to identify the elements in it and create the elemental maps for Pt, Ga, Mo (Kα and Lα transitions), S, Si and O shown in Figures S4(b-h).
Due to the diffusion of the MoS 2 coating into the Pt used to hold the TEM slice, it is difficult to determine the thickness from these elemental maps. The data was therefore projected along the layer direction to create the 1D plots of Figure S5.
The counts for the elements Mo and S show a ∼ 35 nm peak located between the Platinum layer (deposited on top of the substrate during the TEM sample preparation) and the SiO 2 substrate. Some Gallium contamination is visible in the Pt layer, due to the use of a Gallium focused ion beam in the TEM sample preparation. Based on this, the MoS 2 film thickness on the substrate is estimated to be ∼ 35 nm. However, the coating thickness on the woodpile is likely to be thinner than on the substrate, due to the 3D structure of it.
S-4 Figure S4: (a) TEM image of a cross-section of the sample surface next to the woodpile. (b-h) Corresponding elemental maps made using EDX analysis for Pt, Ga, Mo (Kα and Lα transitions), S, Si and O. The vertical white lines indicate a 35 nm wide layer of higher density counts for the elements Mo and S. Note that the X and Y axes do not represent the same length. This was done to maximize available information, while maintaining a small figure size.
S-5 Figure S5: Normalized element counts from the EDX analysis of the cross-section shown in Figure S4(a) as a function of depth. The vertical green dashed lines indicate a 35 nm wide layer of higher density counts for the elements Mo and S.
S-6 Figure S6 shows the measurement and simulation results for a BCC woodpile template similar to the one considered in this paper. While the parameters are different, they are similar enough and show a clear difference between the S and P reflectivities of woodpiles in general. Figure S6: Measured angle-resolved reflection spectra compared with FDTD simulations, for a BCC woodpile template of size 50µm×50µm and with N ′ layers = 24 stacking layers. The vertical period and lateral rod distance in this case are a ′ v = a ′ h = 1.14µm. The measured rod height h ′ = 640nm and rod width w ′ = 270nm. The black dashed lines in (a) and (c) are calculated photonic bands using the PWE method. (a) measured and (b) simulated reflection using S-polarized incident light, (c) measured and (d) simulated reflection using P-polarized incident light. S-7