In a paper titled "Reconfigurable Engineered Motile Semiconductor Microparticles" published in Nature Communications, a team of researchers from Duke University demonstrated custom designed semiconductor microparticles that could be steered repeatedly into various configurations while suspended in water.
The researchers started with six types of thin film semiconductor silicon microparticles fabricated on silicon-on-insulator (SOI) wafers with n-type Si as the uppermost layer. All the microparticles were designed with the same 10x20μm footprint (3.5 microns thick) but then subjected to various dopant diffusion, metallization, dry etching, and wet-etching techniques so as to engineer different end results.
The microparticles types consisted of a p-n junction and without metal contacts (PN-0); diode microparticles with a p-n junction and with metal contacts on both the n-side and the p-side (PN-II); n-type silicon microparticles with a metal contact on one side (N-I); microparticles with a p-n junction and with one metal contact on either the n-side or on the p-side (PN-I), and also pure n-type microparticles (N-0, with uniform doping and without metallization).
According to the paper, adopting these different designs was akin to encoding each type of microparticle with a specific response and behaviour when stimulated by AC electric fields. Internal and external charge distributions, the polarizability and field rectification capability of the particles are all controllable design parameters, the authors noted, reporting three distinct locomotive effects, namely dielectrophoresis (DEP), induced charge electrophoresis (ICEP), and diode-based propulsion by AC field rectification all resulting from the varying distribution of counter-ions in the surrounding fluid, relative to the differently charged regions in the microparticles.