We report on both the fabrication of hexagonally ordered, vertically aligned silicon nanowires (SiNW) and their characterization by means of analytical transmission electron microscopy. Combining colloidal lithography, plasma etching, and catalytic wet etching, SiNW with well defined diameter, length, and density can be obtained. For this purpose hexagonally closed packed monolayers of monodisperse polystyrene particles are transferred into non-closed packed particle arrays. These arrays are used as a mask for silver deposition by thermal evaporation. After the lift-off of the spheres, the silver layer serves as a catalyst for the wet-etching process in an H2O2/HF solution. In this way, arrays of SiNW of sub-50 nm diameter with an aspect ratio of up to 10 could be fabricated. Scanning electron microscopy as well as scanning transmission electron microscopy have been applied in order to investigate the morphology, the internal structure and the composition of the catalytically etched SiNW. The analysis yielded a single crystalline, porous structure composed of crystalline silicon and SiOx with x <= 2.