Chieh Chang

Direct-Write Piezoelectric Polymeric Nanogenerator with High Energy Conversion Efficiency

Nanogenerators capable of converting energy from mechanical sources to electricity with high effective efficiency using low-cost, nonsemiconducting, organic nanomaterials are attractive for many applications, including energy harvesters. In this work, near-field electrospinning is used to direct-write poly(vinylidene fluoride) (PVDF) nanofibers with in situ mechanical stretch and electrical poling characteristics to produce piezoelectric properties. Under mechanical stretching, nanogenerators have shown repeatable and consistent electrical outputs with energy conversion efficiency 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.Nanogenerators capable of converting energy from mechanical sources to electricity with high effective efficiency using low-cost, nonsemiconducting, organic nanomaterials are attractive for many applications, including energy harvesters. In this work, near-field electrospinning is used to direct-write poly(vinylidene fluoride) (PVDF) nanofibers with in situ mechanical stretch and electrical poling characteristics to produce piezoelectric properties. Under mechanical stretching, nanogenerators have shown repeatable and consistent electrical outputs with energy conversion efficiency 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.

Continuous near-field electrospinning for large area deposition of orderly nanofiber patterns

A continuous near-field electrospinning (NFES) process has been developed to deposit solid nanofibers with orderly patterns over large areas. Before the onset of electrospinning, a bias voltage is applied to a semispherical shaped polymer droplet outside of a syringe needle, and a probe tip mechanically draws a single fiber from the droplet to initiate continuous NFES. Contrary to the conventional electrospinning process, we show that decreasing electrical field in continuous NFES results in smaller linewidth deposition, and nanofibers can be assembled into controlled complex patterns such as circular shapes and grid arrays on large and flat areas.

A direct-write piezoelectric PVDF nanogenerator

In this paper, we have successfully demonstrated the capability of a direct-write piezoelectric poly(vinylidene fluoride) (PVDF) nanogenerator using near-field electrospinning (NFES). Repeated and consistent output voltage up to 8.5 mV has been generated under an external strain of 0.092% from a single electrospun PVDF nanofiber, of which the output power is approximately 7.2 pW. We believe this technology can open up unprecedented possibilities in the field of NEMS/MEMS to convert mechanical strain into electricity for energy harvesting or strain sensing applications.

Piezoelectric actuation of a direct write electrospun PVDF fiber

Piezoelectric actuation of a doubly clamped suspended poly (vinylidene fluoride) (PVDF) fiber has been demonstrated. The PVDF fiber is directly deposited onto a collector using near-field electrospinning with in situ electrical poling process. The piezoelectric effect can be further enlarged by polarizing the fiber under a voltage of 2kV for two hours at a temperature of 80°C. A displacement of 2µm on the center of a suspended PVDF fiber, corresponding to a strain of 0.004%, as a result of piezoelectric effect has been detected under an applied electric field of 1.6V/µm. We believe this technology could enable new potential actuation applications for NEMS/MEMS.

Chip-to-chip fluidic connectors via near-field electrospinning

Site-specific, chip-to-chip fluidic connectors have been demonstrated via near-field electrospinning (NFES) in a fashion similar to the wire bonding technique in IC manufacturing. Electrospun polymer fibers function as the sacrificial material with deposition control better than 10 m in the planar direction to connect two separated chips. A coating process and sacrificial layer etching process are followed to make micro/nano fluidic channels of 50 nm~5 mum in inner diameter. Preliminary parameter and flow characterizations have been conducted. As such, this fabrication/packaging technology could enable on-chip and off-chip fluidic transportations and networks in MEMS applications, including bioMEMS and microfluidics.

Direct determination of flat-band voltage for metal/high κ oxide/ semiconductor heterointerfaces by electric-field-induced second-harmonic generation

We have employed electric-field-induced second-harmonic (EFISH) generation to determine the flat-band voltage (VFB) of Cr/ALD-Al2O3/MBE-HfO2/n-Si (001) MOS structure. Due to the phase sensitivity of EFISH signal to the electric field in the space charge region, the VFB of −1.20±0.07 V was determined by analyzing the relative phase change in the EFISH signal as a function of the applied gate voltage. The obtained value is in good agreement with that estimated by the capacitance-voltage measurement. This study demonstrated an all-optical technique to directly determine the flat-band voltage for the high κ oxide/Si heterointerfaces.