Electrospun Direct-write Multi-functional Nanofibers
Multi-functional fibers by means of direct-write near-field electrospinning process have been developed for versatile applications on a wide variety of substrates, including flexible ones. Several maskless lithographoy techniques have been established by using the direct-write fibers in dry etching, wet etching and lift-off processes. By selecting the proper functional materials, electrospun direct-write fibers have been demonstrated in prototype working devices, such as large array piezoelectric nanogenerators made of polymeric PVDF (Polyvinylidene fluoride) and direct-write micro heaters made of metallic copper nanoparticles.
In the first example, continuous yet uniform PVDF fibers have been electrospun on a flexible substrate. A post, electrical poling process has been introduced on electrodes with PDMS (Polydimethylsiloxane) as the filling media to achieve an electrical potential of 2×10^7 V/m. In the prototype device, 500 energy harvesting points formed by 50 pairs of fibers and 10 pairs of comb-shape electrodes have generated about 30nA of electrical current on a flexible substrate under an estimated strain of 0.1%. Both FTIR (Fourier Transform Infrared Spectroscopy) and XRD (X-Ray Diffraction) have been utilized to characterize the electrospun fibers and good beta-phase formation, an essential property for piezoelectricity, has been confirmed.
For the next example, electrospun direct-write fibers have been employed to show three maskless lithography techniques; lift-off, wet-etching and dry-etching. These include the demonstration of sub-micrometer wide gaps between a thin metallic gold film using the lift-off process; 20um-wide, 20mm-long lineshape micro heaters made of 30nm-thick copper film by a wet-etching process; and a 2um-wide, 10um-long graphene channel FET (Field Effect Transistor) via a dry-etching process. Electrospun PEO (Polyethylene oxide) fibers have been utilized in the aformentioned processes which has shown strong adhesion to the Kapton substrate, robust chemical resistance to wet copper etchant and good resistance to oxygen plasma for the related lift-off, wet etching and dry etching processes, respectively. Experimentally, a fabricated copper micro heater has reached 92°C under an input power of 37mWatt. The graphene-based channel FET on 285nm-thick gate dioxide has 30uA of current under 1V of applied bias voltage while the gate voltage sweep has shown Diract point at +21V.
In the final demonstration example, electropun direct-write conductive copper wire has been constructed on top of a glass substrate. Copper dinitrate and PVP have been tested as the best polymer mixture for the electrospinning process. A two-step post annealing process has been developed to decompose the polymer and reduce copper oxide to copper for good conductively. The prototype copper wires are 20~100um in length and 500nm in height with an average measrued resistivity of 5.6×10-6Ohm*m from tested 25 samples. As such, electrospun direct-write fibers could find potential applications in various fields, including low-cost electronics.