Electrospinning technology based on the electrostatic driving mechanism can construct long and continuous polymeric nanofibers with diameters less than 100 nm. However, conventional electrospinning process is chaotic by nature. The chaos is manifested in the difficulty to control the deposition position and amount of electrospun nanofibers. In this dissertation, a near-field electrospinning (NFES) process has been developed to deposit solid nanofibers in a direct and controllable manner. The process is further extended to continuous and large area deposition and applied to energy harvesting applications by constructing piezoelectric nanogenerators.
In near-field electrospinning process using polyethylene oxide (PEO) as the demonstration example, a tungsten electrode with tip diameter of 25 μm is used to construct nanofibers of 50~500 nm in line-width on silicon based collectors while the liquid polymer solution is supplied in a manner analogous to that of a dip pen. The minimum applied bias voltage is 600 V and minimum electrode-to-collector distance is 500 μm to achieve position controllable deposition. Charged nanofibers can be orderly collected, making NFES a potential tool in direct write nanofabrication for a variety of materials.
The discontinuity issue of the first-generation NFES process is resolved by a continuous NFES process. Before the onset of electrospinning, a bias voltage is applied to a semi-spherical shaped polymer droplet formed outside of a syringe needle. A probe tip is used to mechanically draw a single fiber from the droplet to initiate the continuous NFES process. It is observed that decreasing electrical field in continuous NFES results in smaller line-width deposition which is in contrast to the general understandings from the conventional electrospinning process. Nanofibers can be assembled into controlled, complex patterns such as circular shapes and grid arrays on large and flat areas.
Direct-write polyvinylidene fluoride (PVDF) nanofibers have been constructed as nanogenerators. These nanofibers attain both in-situ mechanical stretch and electrical poling to produce piezoelectric characteristics. Under mechanical stretching, nanogenerators have shown repeatable and consistent electrical outputs with energy conversion efficiency of 21.8%, which is an order of magnitude higher than those made of PVDF thin films. The early onset of the nonlinear domain wall motions behavior has been identified as one mechanism responsible for the apparent high piezoelectricity in nanofibers, rendering them potentially advantageous for sensing and actuation applications.
