Over the last two decades, scientists acquired more and more insights on the conversion of patterned polymer precursors into predictable 3D carbon shapes using pyrolysis (also carbonization). Our team, using photo lithography, electrospinning or a combination of both techniques, patterned polymer precursor structures so that after pyrolysis they yielded, for example, suspended carbon nanowires (see Figure 1a), carbon mats (see Figure 1b), carbon interdigitated arrays (C-IDEAS) (see Figure 1c) and even carbon origami (see Figure 1 d).More recently, gaining better control over the carbon microstructure in some of those carbon shapes also became possible. The key to the latter is a precise control of the polymer precursor chains and the exact atomic composition of the polymer before and during pyrolysis.Using simultaneous control of the linear speed of the spinneret and the rotational speed of a drum collector in near field electrospinning (NFE) enables one to organize polymer precursor chains, that after pyrolysis, lead to aligned ultra-thin (as low as 5 nm) highly crystalline carbon fibers freely suspended and in good ohmic contact with carbon scaffolds on a silicon substrate (Figure 1a) 1.The application of far field electrospinning (FFE) techniques allows one to utilize electrohydrodynamic forces to align polymer molecular chains within the spun fiber mats. This polymer chain alignment was then preserved by mechanical compression of the polymer fabrics during the formative stabilization step. Perhaps the most surprising outcome of this second C-MEMS approach illustrated in Figure 1 b was the demonstration of the conversion of these PAN fiber mats through pyrolysis into a uniformly graphitized carbon. The resulting carbon exhibits an oriented but fragmented lattice structure, as visualized by HRTEM imaging. Further characterization with XPS indicated that this type of stress activated pyrolytic carbon is innately rich in nitrogen heteroatoms. The electrochemical kinetics of these carbon mats reveal a heterogenous electron transfer rate 5 to 14 times higher than that of standard pyrolytic PAN carbons and 2 to 10 times higher than polished commercial glassy carbon in both the surface insensitive and sensitive redox probes 2.Using conventional UV photolithography to pattern SU8, followed by pyrolysis, we fabricated 3D carbon interdigitated electrode arrays (C-IDEAS) with redox amplification factors as high as 37 (Figure 1c)3. In ongoing follow-on work, we are now investigating means to further increase the amplification factor by reducing the electrode gap and increasing the electrode height. In this case the mechanical alignment of polymer chains (see FFE and NFE) is not applicable and we are converting the top carbon electrode material to graphene through the deposition of Ni and annealing instead.Two years ago (2019), we demonstrated for the first time an origami-based carbon microfabrication method for permanently folded three-dimensional structures by combining lithography, controlled thermal softening and hardening, and elastocapillarity4. Selective lithography for cross-linking of photopolymers was used to obtain localized control over the material properties (for folds and faces) to enable selective and programmed folding of the origami. In addition to achieving tailorable inhomogeneous material properties, selective exposure was also used to bond the origami shapes onto a substrate surface. The three-dimensional polymer structures were then converted through pyrolysis into carbon shapes thus enhancing the achievable structural, electrical, and electrochemical properties and to broaden the applications of this elastocapillary-based fabrication method (see Figure 1 d). Finally in 2020, we were able to create the same carbon origami by heat-assisted self-folding, doing away with the need for droplet induced folding 5. Pyrolysis leads to a glassy carbon microstructure in the carbon origami shapes obtained. As for C-IDEAS, Ni deposition and annealing can be employed to convert the microstructure from glassy carbon to graphene.References Jufeng Deng, Chong Liu, Marc Madou, "Ultra-thin carbon nanofibers based on graphitization of near-field electrospun polyacrylonitrile," Nanoscale, Royal Society of Chemistry, Issue 19, 2020.Holmberg, Sunshine, et al. "Stress-activated pyrolytic carbon nanofibers for electrochemical platforms." Electrochemical Acta290 (2018): 639-648.Kamath and M. J. Madou, "Three-Dimensional Carbon Interdigitated Electrode Arrays for Redox-Amplification," Anal. Chem., vol. 86, no. 6, pp. 2963-2971, Mar. 2014.Fabrication of polymer and carbon polyhedra through controlled cross-linking and capillary deformations, Derosh George, Edwin A. Peraza Hernandez, a Roger C. Lo and Marc Madou, Soft Matter, Volume 15 Issue 45 (2019): Pages 9171-9177.George, Derosh, Marc Madou, and Edwin A. Peraza Hernandez. "Programmable single-layer polymer films for millimeter and sub-millimeter self-folding origami." Active and Passive Smart Structures and Integrated Systems IX. Vol. 11376. International Society for Optics and Photonics, 2020. Figure 1
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Jan 1, 2022
Hyunseok Shim + 2
Open Access
