Distinct Roles of Tensile and Compressive Stresses in Graphitizing and Properties of Carbon Nanofibers

It is generally accepted that inducing molecular alignment in a polymer precursor via mechanical stresses influences its graphitization during pyrolysis. However, our understanding of how variations of the imposed mechanics can influence pyrolytic carbon microstructure and functionality is inadequate. Developing such insight is consequential for different aspects of carbon MEMS manufacturing and applicability, as pyrolytic carbons are the main building blocks of MEMS devices. Herein, we study the outcomes of contrasting routes of stress-induced graphitization by providing a comparative analysis of the effects of compressive stress versus standard tensile treatment of PAN-based carbon precursors. The results of different materials characterizations (including scanning electron microscopy, Raman and X-ray photoelectron spectroscopies, as well as high-resolution transmission electron microscopy) reveal that while subjecting precursor molecules to both types of mechanical stresses will induce graphitization in the resulting pyrolytic carbon, this effect is more pronounced in the case of compressive stress. We also evaluated the mechanical behavior of three carbon types, namely compression-induced (CIPC), tension-induced (TIPC), and untreated pyrolytic carbon (PC) by Dynamic Mechanical Analysis (DMA) of carbon samples in their as-synthesized mat format. Using DMA, the elastic modulus, ultimate tensile strength, and ductility of CIPC and TIPC films are determined and compared with untreated pyrolytic carbon. Both stress-induced carbons exhibit enhanced stiffness and strength properties over untreated carbons. The compression-induced films reveal remarkably larger mechanical enhancement with the elastic modulus 26 times higher and tensile strength 2.85 times higher for CIPC compared to untreated pyrolytic carbon. However, these improvements come at the expense of lowered ductility for compression-treated carbon, while tension-treated carbon does not show any loss of ductility. The results provided by this report point to the ways that the carbon MEMS industry can improve and revise the current standard strategies for manufacturing and implementing carbon-based micro-devices

High-throughput Synthesis and Metrology of Graphene Materials

Realistic implementation of graphene and carbon nanotubes (CNT) in energy devices and nano-electronics requires these carbon nanomaterials to have engineered architectures with sp2-hybridized carbon atoms as building blocks. Graphene-CNT hybrid structures will provide attractive material properties of both CNTs and graphene with the capability to develop into a variety of geometries. The first part of this dissertation presents a scalable approach for synchronous fabrication of multiple component carbon hybrids. Large and uniform graphene-CNT hybrid films are successfully synthesized via simultaneous CVD growth of graphene layers and CNTs on copper foil substrates. The graphene-CNT hybrids have tuneable nano-architectonics, which is essential for application-oriented design of hierarchical graphene structures. The unique mechanism of synchronized CVD growth of CNT and graphene contributed significantly to the composure of the final carbon structure. We also demonstrate the potential of Block Copolymer (BCP) self-assembly as a nano-fabrication and patterning tool. Block copolymers are employed as templates to fabricate desired arrays of catalyst particles for CVD growth. Thoughtful choices of constituent blocks, polymerization degree, and volume fractions in BCPs, enable us to modulate the size and the separation distance of micro/nano-domains in the coplymer templates. Such versatility provides us with a powerful tool to control the diameter and separation distance of grown CNTs in the pillared architectures, and eventually allows us to tune the surface area in the resulting carbon hybrids.The metrology part of this work focuses on the utilization of fluorescence quenching microscopy (FQM) for quick visualization of doping in large-area graphene layers. Reactive ion plasma etching allows us to generate patterns of p-type CVD-grown graphene layers doped with fluorine. We specifically employ the dye 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) as the fluorescent agent. The emission of DCM is quenched to different extents by p-doped and non-doped regions of graphene. This provides the fluorescence-imaging contrast, which is critical for visualization/mapping of the doped regions in high-throughput metrology techniques. To determine the method's resolution and scalability strengths, we increased the complexity of the doping pattern and successfully reproduced the mapping results. Upon that, we conducted a comparative investigation on steady-state and time-resolved fluorescence parameters of dye-coated pristine graphene and fluorinated graphene, using bare glass as the control sample. Fluorometry results support the increasing trend observed in the quenching level of DCM dye, from control sample to doped graphene to pristine graphene. The variation in fluorescence quenching by graphene after it is doped is also predicted by theory, as discussed in this work. Due to its simplicity, high speed, and small footprint, segmented FQM can be employed to address the chronic need for a microscopy based graphene metrology capable of "seeing" the features that are processed

Block Copolymer-assisted Tuning of Electron Transport in Graphene Field Effect Devices

-

Distinct Roles of Tensile and Compressive Stresses in Graphitizing and Properties of Carbon Nanofibers.

It is generally accepted that inducing molecular alignment in a polymer precursor via mechanical stresses influences its graphitization during pyrolysis. However, our understanding of how variations of the imposed mechanics can influence pyrolytic carbon microstructure and functionality is inadequate. Developing such insight is consequential for different aspects of carbon MEMS manufacturing and applicability, as pyrolytic carbons are the main building blocks of MEMS devices. Herein, we study the outcomes of contrasting routes of stress-induced graphitization by providing a comparative analysis of the effects of compressive stress versus standard tensile treatment of PAN-based carbon precursors. The results of different materials characterizations (including scanning electron microscopy, Raman and X-ray photoelectron spectroscopies, as well as high-resolution transmission electron microscopy) reveal that while subjecting precursor molecules to both types of mechanical stresses will induce graphitization in the resulting pyrolytic carbon, this effect is more pronounced in the case of compressive stress. We also evaluated the mechanical behavior of three carbon types, namely compression-induced (CIPC), tension-induced (TIPC), and untreated pyrolytic carbon (PC) by Dynamic Mechanical Analysis (DMA) of carbon samples in their as-synthesized mat format. Using DMA, the elastic modulus, ultimate tensile strength, and ductility of CIPC and TIPC films are determined and compared with untreated pyrolytic carbon. Both stress-induced carbons exhibit enhanced stiffness and strength properties over untreated carbons. The compression-induced films reveal remarkably larger mechanical enhancement with the elastic modulus 26 times higher and tensile strength 2.85 times higher for CIPC compared to untreated pyrolytic carbon. However, these improvements come at the expense of lowered ductility for compression-treated carbon, while tension-treated carbon does not show any loss of ductility. The results provided by this report point to the ways that the carbon MEMS industry can improve and revise the current standard strategies for manufacturing and implementing carbon-based micro-devices

Graphitizing Non-graphitizable Carbons by Stress-induced Routes.

Graphitic carbons' unique attributes have attracted worldwide interest towards their development and application. Carbon pyrolysis is a widespread method for synthesizing carbon materials. However, our understanding of the factors that cause differences in graphitization of various pyrolyzed carbon precursors is inadequate. We demonstrate how electro-mechanical aspects of the synthesis process influence molecular alignment in a polymer precursor to enhance its graphitization. Electrohydrodynamic forces are applied via electrospinning to unwind and orient the molecular chains of a non-graphitizing carbon precursor, polyacrylonitrile. Subsequently, exerting mechanical stresses further enhances the molecular alignment of the polymer chains during the formative crosslinking phase. The stabilized polymer precursor is then pyrolyzed at 1000 °C and characterized to evaluate its graphitization. The final carbon exhibits a uniformly graphitized structure, abundant in edge planes, which translates into its electrochemical kinetics. The results highlight the significance of physical synthesis conditions in defining the structure and properties of pyrolytic carbons

Rapid Iodine Sensing on Mechanically Treated Carbon Nanofibers.

In this work, we report on a rapid, efficient electrochemical iodine sensor based on mechanically treated carbon nanofiber (MCNF) electrodes. The electrode’s highly graphitic content, unique microstructure, and the presence of nitrogen heteroatoms in its atomic lattice contribute to increased heterogeneous electron transfer and improved kinetics compared to conventional pyrolytic carbons. The electrode demonstrates selectivity for iodide ions in the presence of both interfering agents and high salt concentrations. The sensor exhibits clinically relevant limits of detection of 0.59 µM and 1.41 µM, in 1X PBS and synthetic urine, respectively, and a wide dynamic range between 5 µM and 700 µM. These results illustrate the advantages of the material’s unique electrochemical properties for iodide sensing, in addition to its simple, inexpensive fabrication. The reported iodine sensor eliminates the need for specimen processing, revealing its aptitude for applications in point-of-care diagnostics

Electrospinning and characterization of polymer-graphene powder scaffolds

In this paper the morphological, mechanical and electrical characteristics of fibers electrospun from a of poly-ε-caprolactone polymer solution with different percentages of graphene nanoplatelets mixed in are reported. The morphology of the fibers was studied under optical and scanning electron microscopes to investigate the interaction of the two phases within the fibers. The scaffolds were characterized to identify the effects of the graphene on the intrinsic properties of the material. The preparation of an optimized suspension of the graphene in the solution was found to be a fundamental factor for enhancing the applicability of the resulting fibers

Nitrogen-Rich Polyacrylonitrile-Based Graphitic Carbons for Hydrogen Peroxide Sensing.

Catalytic substrate, which is devoid of expensive noble metals and enzymes for hydrogen peroxide (H₂O₂), reduction reactions can be obtained via nitrogen doping of graphite. Here, we report a facile fabrication method for obtaining such nitrogen doped graphitized carbon using polyacrylonitrile (PAN) mats and its use in H₂O₂ sensing. A high degree of graphitization was obtained with a mechanical treatment of the PAN fibers embedded with carbon nanotubes (CNT) prior to the pyrolysis step. The electrochemical testing showed a limit of detection (LOD) 0.609 µM and sensitivity of 2.54 µA cm-2 mM-1. The promising sensing performance of the developed carbon electrodes can be attributed to the presence of high content of pyridinic and graphitic nitrogens in the pyrolytic carbons, as confirmed by X-ray photoelectron spectroscopy. The reported results suggest that, despite their simple fabrication, the hydrogen peroxide sensors developed from pyrolytic carbon nanofibers are comparable with their sophisticated nitrogen-doped graphene counterparts