Fabrication of Large-area Free-standing Ultrathin Polymer Films.

This procedure describes a method for the fabrication of large-area and ultrathin free-standing polymer films. Typically, ultrathin films are prepared using either sacrificial layers, which may damage the film or affect its mechanical properties, or they are made on freshly cleaved mica, a substrate that is difficult to scale. Further, the size of ultrathin film is typically limited to a few square millimeters. In this method, we modify a surface with a polyelectrolyte that alters the strength of adhesion between polymer and deposition substrate. The polyelectrolyte can be shown to remain on the wafer using spectroscopy, and a treated wafer can be used to produce multiple films, indicating that at best minimal amounts of the polyelectrolyte are added to the film. The process has thus far been shown to be limited in scalability only by the size of the coating equipment, and is expected to be readily scalable to industrial processes. In this study, the protocol for making the solutions, preparing the deposition surface, and producing the films is described.

Strain-induced modification of optical selection rules in lanthanide-based upconverting nanoparticles.

NaYF4:Yb(3+),Er(3+) nanoparticle upconverters are hindered by low quantum efficiencies arising in large part from the parity-forbidden nature of their optical transitions and the nonoptimal spatial separations between lanthanide ions. Here, we use pressure-induced lattice distortion to systematically modify both parameters. Although hexagonal-phase nanoparticles exhibit a monotonic decrease in upconversion emission, cubic-phase particles experience a nearly 2-fold increase in efficiency. In-situ X-ray diffraction indicates that these emission changes require only a 1% reduction in lattice constant. Our work highlights the intricate relationship between upconversion efficiency and lattice geometry and provides a promising approach to modifying the quantum efficiency of any lanthanide upconverter.

Enhanced delamination of ultrathin free-standing polymer films via self-limiting surface modification.

Free-standing polymer thin films are typically fabricated using a sacrificial underlayer (between the film and its deposition substrate) or overlayer (on top of the film to assist peeling) in order to facilitate removal of the thin film from its deposition substrate. We show the direct delamination of extraordinarily thin (as thin as 8 nm) films of poly(vinyl formal) (PVF), polystyrene, and poly(methyl methacrylate). Large (up to 13 cm diameter) films of PVF could be captured on wire supports to produce free-standing films. By modifying the substrate to lower the interfacial energy resisting film-substrate separation, the conditions for spontaneous delamination are satisfied even for very thin films. The substrate modification is based on the electrostatic adsorption of a cationic polyelectrolyte. Eliminating the use of sacrificial materials and instead relying on naturally self-limiting adsorption makes this method suitable for large areas. We have observed delamination of films with aspect ratios (ratio of lateral dimension between supports to thickness) of 10(7) and have captured dry, free-standing films with aspect ratios >10(6). Films with an aspect ratio of 10(5) can bear loads up to 10(6) times the mass of the film itself. The presence of the adsorbed layer can be observed using X-ray photoelectron spectroscopy, and this layer is persistent through multiple uses. In the system studied, elimination of sacrificial materials leads to an enhancement in the failure strength of the free-standing thin film. The robustness, persistence, and the self-optimizing nature distinguish this method from various fabrication methods utilizing sacrificial materials and make it a potentially scalable process for the fabrication of ultrathin free-standing or transferrable films for filtration, MEMS, or tissue engineering applications.