The distance mark in (a) indicates the range of the nanoporous base layer underneath the Au film and the nanopillars. Figure 3 SEM images of nanopillars formed from the highly doped Si after 10-min etching. In (a) λ 1, (b) λ 2, (c) λ 3, and (d) λ 4 solutions. The distance mark

in (d) indicates the range of the nanoporous base layer under the Au film and nanopillars. The highly doped Si was etched for 10 min in solutions with different values of the molar ratio λ, and the formed nanopillars are shown in Figure 3. Relatively long nanopillars and a thin nanoporous base layer were observed after etching in the λ 1, λ 2, and λ 3 solutions, while shorter nanopillars and a thick homogenous nanoporous base layer with a thickness of 4.3 μm below the pillars were observed after etching in the λ 4 solution. The nanoporosity of the nanopillars etched in the λ 1, λ 2, and λ 4 solutions becomes obvious in the cracked pillars (Additional Decitabine cell line AZD6244 manufacturer file 1: Figure S2). After 10-min etching in the λ 1 and λ2 solutions (Additional file 1: Figure S2a,b), it was also observed that the nanoporous base layer below the pillars is thicker than that directly below the Au film. The nanopillars are strongly bent and bonded together at the top after etching in the λ 1 solution (Figure 3a).

The bonded nanopillars at the top can be clearly seen in the magnified SEM image (Additional file 1: Figure S3). In addition, the thickness of these nanopillars is about 50% smaller at the top compared to the bottom of the pillars. The bonded and bent nanopillars were also observed after etching in the λ 2 solution (Figure 3b), but they are less bent than those after etching in the λ 1 solution. The nanopillars etched in the λ 1 solution were bonded as bundles, while the nanopillars etched in the λ 2 solution were

bonded in rows (Additional file 1: Figure S4a,b). The same thickness is seen both at the top and bottom of the nanopillars etched in the λ 2 solution. Long isolated nanopillars without bending were observed after etching in the λ3 solution (Figure 3c). The dependence of the bonding and bending phenomena on the STK38 value λ is more clearly seen in the tilted SEM images (Additional file 1: Figure S4). The lightly doped Si was etched for 10 min in solutions with different values of λ, and the formed nanopillars are shown in Figure 4. The etching in the λ 1 solution was not homogenous, and at some places, only a nanoporous base was etched underneath the Au film, while at other places, nanopillars with a nanoporous base were observed, and somewhere else, nanopillars without a nanoporous base layer were observed (Additional file 1: Figures S5 and S6). The nanopillars were strongly bonded together at the top and strongly bent after etching in the λ 1 and λ 2 solutions (Figure 4a,c). The thickness on top of the nanopillars is reduced to about 40% and 55% after etching in the λ 1 and λ 2 solutions for 10 min.