A 20-nm-thick Au film was then deposited on the samples using e-beam evaporation. Subsequently, the samples Opaganib were submerged in a HF/H2O2 aqueous solution for MaCE. As an example, Figure 1c shows the formed Si nanopillars. The molar proportion of HF/H2O2/H2O is X:Y:Z, where (X + Y):Z is kept constant at 1:5, and the molar ratio λ is defined as λ = X / (X + Y). The solutions with different molar ratios, λ, used in this work are listed in

Table 1. During MaCE, only Au contact areas were etched, resulting in vertically aligned arrays of nanoporous Si nanopillars, arrays of nanoporous Si pillars with a nanoporous base, or Si nanopillars with nanoporous shells. The different structural properties are determined

by the molar ratio λ and the doping level of the Si wafer. After MaCE, the samples were investigated using an ultrahigh resolution scanning electron microscope (SEM; Hitachi S-4800). The resist and the Au film were not removed for SEM inspection. Figure 1 Fabrication of the ordered arrays of Si nanopillars. (a) Schematic sketch of the fabrication process for the ordered array of nanoporous Si nanopillars, ordered array of nanoporous Si nanopillars with nanoporous base layer, and ordered selleck chemicals llc array of Si nanopillars with nanporous shells. (b) SEM image of the pattern defined using SCIL. (c) SEM image of the formed Si nanopillars for the lightly doped Si wafer after MaCE (in λ 3 solution

for 10 min). Table 1 List of solutions with different molar ratios, λ , used for the etching solutions Molar ratio Value λ 1 0.5 λ 2 0.7 λ 3 0.85 λ 4 0.92 Results The nanopillars formed from highly doped Si after etching in λ 3 solution for 3, 6, and 10 min, respectively, are shown in Figure 2. After 3-min etching, the nanoporous Si nanopillars had a vertical length of 1.6 μm, accompanied by the formation of a nanoporous base with a homogenous thickness of 1.2 μm below the Au film and the nanopillars (Figure 2a). After 6-min etching, the length of the nanoporous Si nanopillars increased to 6.3 μm, while the thickness of the nanoporous base is clearly reduced to a few hundred nanometers and not being homogenous anymore. After Aldehyde dehydrogenase 10-min etching, the length of the nanoporous Si nanopillars increased to 15.1 μm, and the thickness of the nanoporous base was reduced even more. The nanoporosity of the nanopillars is more clearly shown in the cracked pillars (Figure 2b,d,f). It is also interesting to note that the nanoporous layer underneath the pillars is thicker than the nanoporous layer directly below the Au film after 6- and 10-min etching (Figure 2d,f). Figure 2 SEM images of nanopillars formed from the highly doped Si in λ 3 solution. After etching for 3 min (a, b), 6 min (c, d), and 10 min (e, f), respectively. Panels b, d, and f show the cracked nanopillars.