LIA NEWS We also studied the distribution and alignment
of MWNTs inside the polymer matrix. As shown in figure 3, precise assembly of MWNTs was achieved by the combination of TPP fabrication and direct pyrolysis. Te length of MWNTs is longer than the laser focal volume, so the trapped MWNTs were forced to align with the laser scan direction. Volume shrinkage can cause tensile strength along the wires, which also contributes to the alignment. By incorporating MWNTs into the acrylic
polymer, the composite resin changed from an insulator to a conductor with greatly enhanced mechanical strength. With 0.2 wt per cent MWNT concentration, the electrical conductivity of the composite resin increased over 11 orders of magnitude and reached 46.8S/m, as shown in figure 4(a). Te superior conductivity of the MTA composite polymers originated from the high MWNT concentration and the uniform MWNT dispersion. Moreover, to utilise the alignment effect of MWNTs in composites, two bar-shaped channels were fabricated with two orthogonal laser scanning directions and showed a three-orders-of- magnitude difference in electrical conductance, which matched with the high anisotropy in electrical conductivity of MWNTs in directions parallel with or perpendicular to the MWNT axis. Two suspended micro-bridges in figure 4(c, d) with the same design were fabricated using the
Figure 4: Electrical and mechanical properties of the composite resins
acrylic and composite resins. Under the same fabrication condition, the bridge made by acrylic resin deformed, while the one fabricated by the composite resin remained straight without any obvious deformation, indicating the enhanced mechanical strength given by the MWNT loading. To demonstrate the potential of the composite
resins, we fabricated a series of microelectronic devices, including arrays of capacitors (figure 5(a)) and resistors (figure 5(c)). Figure 5(b) shows a typical hysteresis loop of a capacitor array containing 10 micro-capacitors in parallel. Figure 5(d) shows the frequency responses of a resistor array containing 20 zigzag micro-resistors in parallel. Te impedance performance of the composite polymer transmission enables its application at high frequency range, such as RF electronics.
Figure 5: MWNT-based functional structures for electronic applications
In summary, various functional 3D micro/
nanostructures using the composite resins have been successfully developed via TPP lithography. Precise MWNT assembly of approximately 100nm spatial resolution has been achieved by the combination of TPP lithography and thermal pyrolysis. Te composites demonstrated increased mechanical strength and enhanced electrical conductivity.l
The work was done at the University of Nebraska-Lincoln’s electrical and computer engineering department, where Ying Liu is a PhD student, Lijia Jiang is a postdoctoral researcher, Yunshen Zhou is a research associate professor, and Yongfeng Lu is Lott Distinguished University Professor of Engineering. Wei Xiong is a former UNL postdoctoral researcher, now a professor at Huazhong University of Science and Technology in China.
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