Ultra-thin polymer films are films with thicknesses below 100 nm. The thinnest freestanding films found in open literature are 20 nm thick and typically have diameters of less than 100 um. Polymer films of this thickness are typically made by spin coating or dip coating a dilute solution of the polymer onto a substrate. The substrate must be specially prepared to release the film: it can be freshly cleaved mica, or it can be a more typical substrate such as clean glass or silicon prepared with a release layer. This layer is typically a sacrificial layer that is dissolved when the film is removed from the substrate, such as sputtered salt or soap layers or similar release agents.
The films are removed from the substrate by immersing the substrate into water, which separates the film from the substrate, and the film will float on the water surface. From here, the films are transferred to the holder, commonly a grid with small (typically sub-micron to 100 um) openings that define the free-standing area of film.
The current method has several disadvantages, most of which are connected to the substrate. The substrate preparation can introduce roughness, especially in the case of sputtered liftoff layers, which can be on the order of several nanometers, which leads to film non-uniformity and becomes more severe as the film thickness is reduced below 20 nm. Sacrificial liftoff layers can contaminate the film and decrease the strength of the film. At thicknesses lower than 30 nm, the liftoff of the film from the substrate becomes impossible for some preparations.
A second drawback is the inability to produce large free-standing films. This inability is related to the shape of the holder, the liftoff technique itself, and the properties of the polymer that is used to produce the thin film. Large films will often tear when lifted out of the water, and in some cases they tear while drying.
LLNL’s Polyelectrolyte Enabled Liftoff (PEEL) process makes changes to the substrate preparation, the holder and liftoff technique, and suggests modifications to the material itself to enable the preparation of large ultrathin free-standing films.
PEEL enables ultrathin films by chemically modifying the deposition substrate and decreasing the interfacial energy so that even thin films with small strain energies will delaminate.
PEEL employs robust, water-based, and self-optimizing surface chemistry to fabricate ultrathin films up to 100 cm2 or more in area.
LLNL’s PEEL technology is used to fabricate free-standing polymer films as thin as 10 nm that are capable of bearing loads ranging from milligrams to grams and deformations of up to 40%.
PEEL produces free-standing films that are distinct from those that use the sacrificial
interlayer in three main ways: 1) stronger films, 2) larger areas, 3) and thinner films.
LLNL’s PEEL fabrication method overcomes the limitations of earlier methods and can produce films of less than 10 nm thickness and diameters of greater than 10 mm.
PEEL provides an alternative to membrane manufacturing processes like interfacial polymerization, accessing thicknesses and areas that are not accessible by current technologies.
The PEEL process is easily scalable in size and manufacturing quantity and applicable to a variety of polymeric materials.
The current primary use of PEEL is to produce ultrathin films for the assembly of inertial confinement fusion targets. These targets have a fuel capsule that must be supported with minimal mass to avoid perturbations to the implosion.
PEEL may also be used to fabricate free-standing polymer films for sensing, catalysis, filtration, and wound-healing applications.
Other potential uses involve the fabrication of separation membranes for carbon capture and for desalination.
Sample substrates for transmission electron microscopy and other microscopy or x-ray methods also frequently use very thin films to maximize contrast with the sample.
LLNL’s PEEL process is already in daily use at the National Ignition Facility (NIF), a national security and energy research facility, where it is used to fabricate compliant loadbearing polyvinyl formal (PVF) membranes that capture and center the hollow fuel spheres used as laser targets.
LLNL has filed for patent protection on the PEEL fabrication technology.PEEL was selected as a 2016 R&D 100 Award Finalist.