Authors’ contributions SQH, JQW, HFW, and DSS performed the experiments and fabricating the hierarchical structure. JQW, ZHY, and CSF coordinated the project. RQX and YJ performed the SEM measurement. HWZ, KLW, and DHW discussed the results. SQH

and JQW drafted the paper. All authors read and approved the final manuscript.”
“Background Silicon has attracted attention as the most important material for the semiconductor industry. Various techniques such as reactive ion etching, electrochemical etching, and anisotropic chemical etching are used in fabricating silicon-based functional BIX 1294 devices [1]. Among them, metal-assisted chemical etching, which was proposed by Li and Bohn in 2000 see more [2], has also attracted attention as a key nanofabrication method owing to its Mocetinostat in vivo relative simplicity and low cost. In general, metal-assisted chemical etching proceeds by immersing a silicon substrate decorated with a noble metal in an etchant composed of HF and an oxidative agent such as H2O2. To form metal catalytic layers on a silicon substrate with or without pattern regularity, physical deposition techniques in vacuum such as focused ion beam deposition [3], sputtering

[2, 4, 5], conventional vacuum vapor deposition [6], and electron beam evaporation [7] are generally used. Because the morphology of the resultant silicon structures depends on the initial geometric pattern and dimensions of the noble metal catalyst, it is essential to use a patterned metal catalyst for the fabrication of ordered silicon nanostructures.

For example, if a metal catalytic layer with an ordered pore arrangement is applied, the silicon substrate is etched into an array of silicon nanowires. In 2007, Huang et al. demonstrated that silicon nanowires with an aspect ratio larger than 30 could be obtained using nanosphere lithography-based metal-assisted chemical etching [8]. For an overview of the fabrication of silicon by metal-assisted chemical etching, see review papers [9, 10]. Until now, we have focused on the direct patterning of metal catalysts using a mask without the use of conventional lithographic techniques and reported the fabrication of ordered silicon micro-hole arrays by metal-assisted chemical Farnesyltransferase etching using noble metal thin film arrays formed by sputtering through a polymer mask with micrometer openings [11–14]. In these cases, however, the periodicity and diameter of the obtained silicon hole arrays were limited to the micrometer order because the preparation of the polymer mask was based on colloidal crystal templating using microspheres. Although the fabrication of silicon hole arrays with a 200-nm periodicity was achieved using polystyrene nanospheres as an indirect mask in our other approach [15], further miniaturization of hole periodicity remains one of the significant projects.