New Method of Creating Porous Silicon Crystals

Schematic of porous silicon crystal.  Credit: Donghai Wang, Penn State University
Schematic of porous silicon crystal.
Credit: Donghai Wang, Penn State University

Porous silicon crystals are simply silicon crystals with gaps in the structure on the order of 2-50 nanometers. These holes give the crystal a very large surface area to volume ratio. Porous silicon is used in a variety of applications, such as optical sensors, rechargeable batteries and tissue engineering. Porous silicon in solar panels can be used to split water into hydrogen and oxygen using sunlight.

Typically, porous silicon is produced by etching silicon wafers with strong acids and water or applying a current while the silicon is floating in a bath of hydrofluoric acid. These processes waste a lot of silicon to produce porous silicon. Mechanical engineers at Penn State have discovered a process to reduce that waste and create crystals with holes ranging from 5 to 15 nanometers in diameter. Their method builds the crystals rather than destroying silicon.

They start with silicon tetrachloride and begin breaking down the silicon-chlorine bonds with a sodium potassium alloy. The chlorine begins to form new bonds as the material dries with the sodium and potassium ions making sodium chloride and potassium chloride salt crystals embedded in the silicon crystal matrix. The material is heated to set the silicon matrix and then washed with water to dissolve and wash away the salts. The spots where the salt crystals were are now the holes in porous silicon.

Hopefully this new manufacturing process will make porous silicon more readily available to researchers and engineers to find new uses for this amazing material.