Center for Functional Nanomaterials Seminar

Commercially available dialysis and ultrafiltration membranes are commonplace in biotech research, but for many applications, these materials have unacceptably low transport efficiency and are difficult to integrate into micro/nano fabricated systems. We have developed a new class of membrane [Striemer et al., Nature 445, 749, 2007.] that overcomes these limitations with 1) nanoscale thickness, and 2) fabrication on silicon wafer substrates. Nanoscale thickness virtually eliminates the loss of filtrate associated with standard >10 micron thick polymeric membranes and greatly reduces frictional losses, leading to unprecedented transport rates. Their extreme thinness also enables their use in high resolution electron and optical microscopy. Because these membranes are fabricated on planar silicon wafers, system integration is greatly simplified, and scaleable manufacture can be easily achieved.

We fabricate porous and non-porous membranes that are typically 5 nm - 50 nm thick and free-standing over rectangular apertures measuring up to several millimeters across, depending on the application. The porous nanocrystalline silicon (pnc-Si) membranes are formed by thermal crystallization of thin amorphous silicon films. Pore formation, as a bottom-up process, requires no direct patterning, with voids forming naturally through strain and volume contraction during a rapid crystallization anneal. We have demonstrated the tunability of the average pore size from 5 nm to 25 nm, by controlling the crystallization temperature. We also routinely fabricate non-porous membranes from a variety of materials including amorphous silicon, silicon dioxide, silicon nitride, etc. These ultrathin membranes are suspended by carefully removing portions of the underlying substrate, and remarkably, these structures are mechanically stable to differential pressures in excess of 1 atmosphere, sufficient for a variety of applications.