Regenerative Electroless Etching of Silicon

Abstract

We introduce regenerative electroless etching (ReEtching). This is a method of producing nanostructured semiconductors in which an oxidant (Ox1) is used as a catalytic agent to facilitate an etching reaction between a semiconductor and a second oxidant (Ox2) that would be unreactive in the primary reaction. Ox2 is used to regenerate Ox1, which is capable of initiating etching by injecting holes into the semiconductor valence band. Thereby the extent of reaction is controlled by the amount of Ox2 added and the rate of reaction is controlled by the injection rate of Ox2. This general strategy is demonstrated specifically for the production of highly luminescent, nanocrystalline porous Si from the reaction of V2O5 in HF(aq) as Ox1 and H2O2(aq) as Ox2 with Si. The Si can be in the form of wafer, high-purity powder, metallurgical grade powder, or porous Si powder made by pulverization of an anodized wafer. ReEtching of metallurgical grade powder represents an inexpensive method of producing porous silicon powders that is scalable to large quantities for use in applications such as batteries and drug delivery. Depending on the etching conditions, the powder particles may be etched completely through (complete porosification), or be comprised of a porous layer surrounding a solid core. Pillared silicon particles can also be produced. Mesoporous Si that was nonluminescent after formation by anodization can be ReEtched to produce brilliantly photoluminescent powder with extremely high specific surface area. 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. Biological tissue autofluorescence exhibits lifetimes in the nanosecond range. Thus, the long-lived PL from Si nanoparticles is promising for bioimaging applications in which image acquisition is delayed after the initial photoexcitation.