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NANO EXPRESS

Open Access

Ordered arrays of nanoporous silicon nanopillars and silicon nanopillars with nanoporous shells Dong Wang1*, Ran Ji2, Song Du1, Arne Albrecht3 and Peter Schaaf1*

Abstract The fabrication of ordered arrays of nanoporous Si nanopillars with and without nanoporous base and ordered arrays of Si nanopillars with nanoporous shells are presented. The fabrication route is using a combination of substrate conformal imprint lithography and metal-assisted chemical etching. The metal-assisted chemical etching is performed in solutions with different [HF]/[H2O2 + HF] ratios. Both pore formation and polishing (marked by the vertical etching of the nanopillars) are observed in highly doped and lightly doped Si during metal-assisted chemical etching. Pore formation is more active in the highly doped Si, while the transition from polishing to pore formation is more obvious in the lightly doped Si. The etching rate is clearly higher in the highly doped Si. Oxidation occurs on the sidewalls of the pillars by etching in solutions with small [HF]/[H2O2 + HF] ratios, leading to thinning, bending, and bonding of pillars. Keywords: Nanoporous Si, Pillars, Nanowires, Metal-assisted chemical etching, Nanoimprint lithography

Background Nanostructured Si is drawing a great deal of interest due to its potential applications in nanoscale electronics [1,2], optoelectronics [3], thermoelectrics [4], photovoltaics [5], biosensors [6], nanocapacitor arrays [7], and as electrodes in Li-ion batteries [8]. It is well known that porous Si can be produced by anodic (electrochemical) etching in HF aqueous solution or stain etching in HNO3/HF solution [9,10]. Recently, metal-assisted chemical etching (MaCE) as a simple and low-cost method to fabricate Si nanowires and nanoporous Si has attracted increasing attention [11-14]. In this process, Si wafer coated with a noble metal is etched in a solution consisting of HF and an oxidant (e.g., H2O2 or AgNO3) to form the nanostructures. Nanoparticles or thin films of noble metals (e.g., Au, Ag, or Pt) are used to catalyze the etching. Two-level nanoscaled porous Si nanowires were even synthesized with highly doped Si using MaCE, and Ag nanoparticles acted as catalyst [15-17]. Zigzag Si nanowires were fabricated with (111)-oriented Si by MaCE (with Ag nanoparticles as catalyst) [18]. These zigzag Si nanowires were even fabricated with (100)-oriented Si by a two-step * Correspondence: [email protected]; [email protected] 1 Materials for Electronics, Institute of Materials Engineering and Institute of Micro- and Nanotechnologies MacroNanoW, Ilmenau University of Technology, Gustav-Kirchhoff-Str. 5, Ilmenau 98693, Germany Full list of author information is available at the end of the article

MaCE (with Au film as catalyst) [19]. In general, the structural properties and morphologies of the nanostructured Si produced by MaCE are affected by three main factors: (1) the properties of the deposited noble metals, including the type and form of the metal, and its deposition met