Theoretically, a zero reflectance from the air-Si LDN-193189 interface can be achieved if an ideal nanopyramid array is fabricated on a Si surface [25]. Such an ideal nanopyramid array results in a constantly varying n without a sharp change at the interface (dotted line in Figure 5b); however, achieving an ideal nanopyramid array is very difficult in reality. Particularly, nanopyramids are generally separated

and some flat surface regions exist between the neighboring pyramids, as shown in Figure 6a. This non-compact nanopyramid structure prevents a smooth decline of n eff at the air-Si interface, creating a discontinuity of n eff (solid line in Figure 5b). The discontinuity of n eff at the interface can be alleviated using a buffer layer between the air and

Si nanostructures [26] (Figure 5c). If a buffer layer with n value between air and Si is deposited on the non-compact nanopyramids, the large difference in n between air and Si can be moderated by the buffer layer (Figure 5c). In our experiments, a Si-based polymer of PDMS was deposited on the fabricated Si nanostructures as a buffer layer because it has n of 1.4, which is an intermediate value between n Si = 3.4 and n air =1 [27]. After the PDMS layer deposition, the Si nanostructures (etched at 1,100°C) exhibited an average reflection of approximately 4.3% from 450 to 800 nm with a minimum reflectance of 2.5% at 760 nm (Figure 7c). This enhancement of the AR property could be clearly seen from the optical images of the Si substrates before and after the PDMS deposition. Ilomastat manufacturer The dark blue color of the Si nanostructure before the deposition (center image of the inset in Figure 7c)

transformed to a perfectly black color after the deposition (right image of the inset in Figure 7c). Consequently, the Si nanostructures coated with a PDMS buffer layer exhibited remarkably reduced reflectance at UV–Vis regions compared to a flat Si surface. Figure 6 Schematic of Si nanostructure, AFM image of the PDMS surface, and FDTD-simulated reflectance spectra. (a) The schematic of buffer layer deposition on the non-compact nanopyramids array. (b) AFM image of the PDMS surface after the deposition on the Si nanostructures. The width and height of the Si nanopyramid are 300 and 250 nm in the simulation, respectively. Vitamin B12 FDTD-simulated reflectance spectra from the air-Si interface (c) before and after the PDMS deposition with increase in the distance between neighboring nanopyramids and (d) with rough and flat surfaces of PDMS. Inset: schematic of the flat PDMS surface on Si nanostructures. Figure 7 Reflectance spectra before and after the PDMS deposition on the Si nanostructures. Etching done at (a) 1,350°C, (b) 1,200°C, and (c) 1,100°C. Inset: optical image of the pristine Si and the Si nanostructures (etched at 1,100°C) before and after the PDMS deposition. The AR properties of the non-compact nanopyramid structure and the effect of the buffer layer on the AR properties were analyzed with FDTD simulation.