Hierarchical Porous Silicon and Porous Silicon Nanowires Produced with Regenerative Electroless Etching (ReEtching) and Metal Assisted Catalytic Etching (MACE)
Authors: Kolasinski Kurt W , Unger Bret A. , Yu Haibo , Ernst Alexis T. , Aindow Mark , Mäkilä Ermei , Salonen Jarno 
Published: February 2020

Regenerative electroless etching (ReEtching) [1] is a method of producing nanostructured semiconductors in which, e.g., dissolved V2O5 is used as a catalytic agent to facilitate etching between Si and H2O2. H2O2 regenerates dissolved V in a 5+ oxidation state, which is capable of initiating etching by injecting holes into the Si valence band. The enhanced control over the extent of reaction (controlled by the amount of H2O2 added) and the rate of reaction (controlled by the rate at which H2O2 is pumped into the etchant solution) allows us to porosify Si substrates of arbitrary size, shape and doping, including wafers, single-crystal powders, polycrystalline powders, metallurgical grade powder, Si nanowires (SiNW), Si pillars and Si powders that have been textured with metal assisted catalytic etching (MACE). The use of acetic acid as a surfactant during ReEtching greatly enhances the product yield, reduces foaming and improves homogeneity.
Hierarchical Si nanostructures containing pores within pores are produced by etching porous Si powder made by pulverization of an anodized wafer or Si nanowires. We call this material ReEtched anodized porous silicon or RaPSi. ReEtching of metallurgical grade powder represents an inexpensive method of producing porous silicon powders with tortuous ~3 nm pores that is scalable to large quantities for use in applications such as lithium ion batteries (LIB) and drug delivery. Mesoporous Si or SiNW that were nonluminescent after formation can be ReEtched to produce brilliantly photoluminescent powder with extremely high specific surface area. We measured specific surface areas over 400 m2 g–1 from metallurgical grade powder and as high as 888 m2 g–1 when ReEtching anodized porous powder. ReEtching allowed the etching of porous layers > 20 µm thick. Depending on the etching and drying conditions, such layers can result in the formation of amorphous silicon pillars that are > 1 µm in height.
PL bands from blue to red have been observed. PL in the red to near IR is extremely long lived, exhibiting multi-exponential decay with lifetime components in excess of 100 µs. Such long-lived PL from Si nanoparticles is promising for bioimaging applications in which image acquisition is delayed after the initial photoexcitation.
We have developed a model to describe the crystallographic dependence of MACE. This model allows us to estimate the temperature dependence of the MACE etch rate, formation energy of a pore and, by extension, the preferred structure of SiNW produced by MACE. Calculations of electrostatic and van der Waals forces reveal that a strong attractive force pins the catalytic nanoparticle to the Si surface throughout the etch process.
*E-mail: kkolasinski@wcupa.edu

‡Current address: Advanced Characterization Dept., Honeywell UOP, Des Plaines, IL 60017
[1] K. W. Kolasinski, N. J. Gimbar, H. Yu, M. Aindow, E. Mäkilä, J. Salonen, Regenerative Electroless Etching of Silicon, Angew. Chem., Int. Ed. Engl. 2017, 55, 624-627.
Figure 1 (a) Secondary electron (SE) scanning electron microscopy (SEM) image of focused ion beam (FIB)-sectioned ~4 μm powder etched in V2O5 + H2O2 + HF reveals that the particle is porosified to the core. Selected area diffraction pattern (inset) reveals the porosified region to be crystalline. (b) Bright field transmission electron microscopy (TEM) image of ReEtched anodized porous Si (RaPSi) powder reveals that highly tortuous ~3 nm pores have been introduced into the walls of 17 nm mesopores. (c) TEM image reveals that a SiNW cleaved by sonication from a MACE etched metallurgical grade Si powder particle is mesoporous with pore diameters up to ~15 nm prior to ReEtching. ReEtching transforms such mesoporous SiNW into a strongly visibly luminescent material.

Figure 1