Preserving surface area and porosity during fabrication of silicon aerocrystal particles from anodized wafers
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Preserving surface area and porosity during fabrication of silicon aerocrystal particles from anodized wafers C. J. Storey1 · E. Nekovic1 · A. Kaplan1 · W. Theis1 · L. T. Canham1 Accepted: 10 October 2020 © The Author(s) 2020
Abstract Porous silicon layers on wafers are commonly converted into particles by mechanical milling or ultrasonic fragmentation. The former technique can rapidly generate large batches of microparticles. The latter technique is commonly used for making nanoparticles but processing times are very long and yields, where reported, are often very low. With both processing techniques, the porosity and surface area of the particles generated are often assumed to be similar to those of the parent film. We demonstrate that this is rarely the case, using air-dried high porosity and supercritically dried aerocrystals as examples. We show that whereas ball milling can more quickly generate much higher yields of particles, it is much more damaging to the nanostructures than ultrasonic fragmentation. The latter technique is particularly promising for silicon aerocrystals since processing times are reduced whilst yields are simultaneously raised with ultrahigh porosity structures. Not only that, but very high surface areas (> 500 m2/g) can be completely preserved with ultrasonic fragmentation. Keywords Porous silicon · Ultrasonic fragmentation · Ball milling · Supercritical drying · Comminution · High porosity
1 Introduction Nanostructuring silicon can both tune its properties and endow it with novel properties. One very versatile technique for nanostructuring ultrapure semiconducting silicon is electrochemical etching of wafers [1]. This top-down technique generates crystalline silicon nanostructures with tunable porosity (20–95%) pore sizes (1.5–50 nm) and surface areas (100–1125 m2/g), initially in the form of layers [2]. Many chip-based applications can utilize this physical form. However, microparticles or nanoparticles are needed for a range of in-vivo applications such as biomedical therapy, which exploit the medical biodegradability and very low toxicity of mesoporous silicon [3]. Different methods are available for converting porous silicon (pSi) films to particles such as mechanical milling [4–6] and ultrasonic fragmentation (USF) [7–9]. Particle size reduction via milling can be conducted in a dry or wet environment [6], whereas ultrasonic fragmentation utilises liquid immersion of particles. * C. J. Storey [email protected] 1
Nanoscale Physics Research Laboratory, School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
However, there are outstanding challenges to rapidly generate nanoparticles with good yields and minimal reductions in porosity and surface area. Speed of processing and yields are critical parameters from an industrial perspective in determining the economic viability. Porosity and surface area are crucial technical parameters that can determine the performance. Examples are the porosity determining drug payload and the surfac
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