Improved conductivity of suspended carbon fibers through integration of C-MEMS and Electro-Mechanical Spinning technologies

Nanogap fabrication by Joule heating of electromechanically spun suspended carbon nanofibers

“…In the case of the lower bound, the value also corresponds to the sublimation temperature of graphite in vacuum conditions [60]. Because the electrospun CNFs presented in this work are expected to be a mixture of glassy and graphitic carbons [26,45], they are likely less thermodynamically stable than multi-walled CNTs, therefore suggesting that their breaking point lies around 2400 K in a vacuum. Furthermore, we expect that for a given CNF with specific geometry, there will be a certain minimum threshold voltage that will cause the structure to heat up to maximum temperature ; in our experiments, this value was in the range between 2 and 6 V. Evidently, any higher bias would also break the wire due to excessive Joule heating.…”

“…First, UV photolithography was used to pattern SU-8 2025 (MicroChem Inc., MA, USA) supporting walls ranging in thickness from 2 to 30 μm and separation from 3 to 50 μm. Second, a polymer fiber was deposited over the walls created in the previous step by EMS, as described in earlier publications [44,45]. In order to obtain multiple fibers with similar composition, length ( ) and diameter ( ), six supporting walls were fabricated per device, and during the EMS step a single polymer fiber was deposited ( Fig.…”

“…1c). By effect of the carbonization step, the ultraviolet-cured resins were thermally decomposed at high temperatures, resulting in the loss of non-carbon atoms and volumetric shrinkage [45]. The upper portion of the support walls shrank considerably more than the lower part, which was firmly anchored to the substrate, causing an increase in the trench size at the upper part and resulting in elongation of the fiber.…”

“…The electrical properties of the CNFs were analyzed with a two-probe I-V method, rather than a four-probe technique. Contact resistance and spreading resistance can be neglected in the devices, as fibers are integrated to the carbon walls with ohmic contacts and their resistance is much greater compared to the carbon electrodes, as a result of their size and geometry difference [26,45,53]. Fig.…”

“…In EMS, by mechanically pulling on the fiber and optimizing the viscoelastic properties of the polymer solution, one is able obtain thinner polymer fibers. To fabricate polymer nanowires through EMS that can also be carbonized successfully, Canton et al [45] formulated a SU-8/polyethylene oxide (PEO)/tetrabutylammonium tetrafluoroborate polymer solution that was compatible with EMS, because of its sufficiently high viscoelasticity (through the high molecular weight of PEO) and its high carbon content (due to the carbon rich SU-8). This optimized ink formulation was used to deposit nanofibers on top of photolithographically patterned electrodes.…”