Wafer‐Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle‐Assisted Chemical Etching and Pseudomorphic Thermal Oxidation

• Published in: Small (Weinheim an der Bergstrasse, Germany) Vol. 19; no. 22; pp. e2206842 - n/a
• Main Authors: Gries, Stella, Brinker, Manuel, Zeller‐Plumhoff, Berit, Rings, Dagmar, Krekeler, Tobias, Longo, Elena, Greving, Imke, Huber, Patrick
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.06.2023
• Subjects: Amorphous materials; Biological materials; Capillaries; Chemical etching; Energy harvesting; Energy storage; hierarchical porosity; Macroporosity; Mass transport; metal‐assisted chemical etching; Nanoparticles; Nanotechnology; Oxidation; Photolithography; Porosity; Porous materials; Porous silicon; silica; Silicon; Silicon dioxide; Silver; silver nanoparticles; metal-assisted chemical etching; hierarchical porosity; porous silicon; silver nanoparticles; silica
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.202206842

Many biological materials exhibit a multiscale porosity with small, mostly nanoscale pores as well as large, macroscopic capillaries to simultaneously achieve optimized mass transport capabilities and lightweight structures with large inner surfaces. Realizing such a hierarchical porosity in artificial materials necessitates often sophisticated and expensive top‐down processing that limits scalability. Here, an approach that combines self‐organized porosity based on metal‐assisted chemical etching (MACE) with photolithographically induced macroporosity for the synthesis of single‐crystalline silicon with a bimodal pore‐size distribution is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 60 nm across. The MACE process is mainly guided by a metal‐catalyzed reduction–oxidation reaction, where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the AgNPs act as self‐propelled particles that are constantly removing silicon along their trajectories. High‐resolution X‐ray imaging and electron tomography reveal a resulting large open porosity and inner surface for potential applications in high‐performance energy storage, harvesting and conversion or for on‐chip sensorics and actuorics. Finally, the hierarchically porous silicon membranes can be transformed structure‐conserving by thermal oxidation into hierarchically porous amorphous silica, a material that could be of particular interest for opto‐fluidic and (bio‐)photonic applications due to its multiscale artificial vascularization. Synchrotron‐based X‐ray nanotomography and electron microscopy reveal that metal‐assisted chemical etching combined with photolithography allows one to fabricate single‐crystalline silicon with hierarchical, open porosity that can be transformed structure‐conserving by oxidation to silica glass. Due to their multiscale tailorable vascularization the resulting wafer‐scale monoliths are promising liquid‐infused hybrid materials, e.g., for energy storage and harvesting, on‐chip sensorics, actuorics and (bio‐)optofluidics.