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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2017)

Structurally two-dimensional materials such as graphene, transition metal dichalcogenide (TMDC), and transition metal carbide (TMC) have various unique properties for possible sensing and clean energy applications. The fabrication methods toward large scale, defect-free, single crystal manufacturing of these 2D materials have been difficult and challenge, include chemical vapor deposition (CVD), atomic layer deposition (ALD) and metal organic chemical vapor deposition (MOCVD). This work presents a few approaches for the synthesis and assembly of 2D materials by promoting the reaction dynamics with seeding catalysts or template structures for self-limiting kinetics to synthesize 2D materials, including graphene based on a droplet-CVD process, 2D-TMDC based on hydrogel scaffolds, and 2D-TMC based on hydrogel mixtures with direct-write laser processes.

A liquid metal droplet-based chemical vapor deposition process is developed for the synthesis of single-layer graphene flakes utilizing molten nickel droplets (nominal diameter from 0.5 to 1µm) as the catalysts. Experimentally, both single- and double-layer graphene flakes with low defects have been synthesized by using nickel thin films of either less or more than 100nm (up to 130nm) in thickness, respectively. When the original nickel thin film is 75 nm in thickness, the resulting nickel droplets are physically and electrically isolated, while after the CVD synthesis process, the graphene flakes are found to be electrically connected due to the outgrowth of graphene. These electrically-connected graphene sheets could be readily available for device applications without the need of transfer processes. We demonstrate the direct generation of photocurrents (up to 0.53 µA/mm2·W) due to the photo-thermal effect based on as-fabricated graphene sheets.

A two-step atomistic layer deposition process is developed by depositing TiN by ALD and then annealing TiN in sulfur vapor. Such method is used to coat TiS2 onto carbon nanotube (CNT) forest for highly conductive electrodes with high capacitance and cyclability in supercapacitors. Furthermore, a high concentration electrolyte (21m LiTFSI) is employed to intercalate with TiS2 and the results show high electrochemical window (3V). Such TiS2/CNT- LiTFSI system results in 195 F/g specific capacitance, 60.9 Wh/kg energy density and 1250 W/kg in power density. This material system also outperforms the most typical highest energy materials from oxide, nitride, transition metal carbide and chalcogenide families for unique advantage and promising application in high energy, high power, and high voltage supercapacitor applications.

A solution-based method is developed for the synthesis of 2D materials driven by the layer-by-layer self-assembly of gelatin to convert ions to 2D carbides and chalcogenides using either CVD or laser processes. The resulting material has typical thickness of 10~15nm with more than 20μm in domain size including MoCx, WCx and CoCx using CO2 laser under ambient condition. The conductive and highly porous structures are utilized for energy storage applications. Specifically, gravimetric capacitance of laser-induced Mo3C2 can produce up to 100 F/g supercapacitors in Mg2+ based electrolyte. Mo3C2 electrodes also show exceptional operation stability from -50 to 300 oC in conjunction with semi-solid LiTFSI-PVA electrolyte. Furthermore, 2D carbide materials can also be directed constructed on paper using laser for disposable and foldable electronics. Finally, with the control of precursor concentration, thin MoS2 sheets with thickness from few m to few tens of nm are developed with monolayer type photoluminescence characteristics at 1.8 eV.

    Cover page: Synthesis and Assembly of 2D Materials for Sensing and Clean Energy Applications
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2015)
    Electrospinning, a process that converts a solution or melt droplet into an ejected jet under a high electric field, is a well-established technique to produce one-dimensional (1D) fibers or two-dimensional (2D) randomly arranged fibrous meshes. Nevertheless, the direct electrospinning of fibers into controllable three-dimensional (3D) architectures is still a nascent technology. Here, we apply near-field electrospinning (NFES) to directly write arbitrarily shaped 3D structures through consistent and spatially controlled fiber-by-fiber stacking of polyvinylidene fluoride (PVDF) fibers. An element central to the success of this 3D electrospinning is the use of a printing paper placed on the grounded conductive plate and acting as a fiber collector. Once deposited on the paper, residual solvents from near-field electrospun fibers can infiltrate the paper substrate, enhancing the charge transfer between the deposited fibers and the ground plate via the fibrous network within the paper. Such charge transfer grounds the deposited fibers and turns them into locally fabricated electrical poles, which attract subsequent in-flight fibers to deposit in a self-aligned manner on top of each other. This process enables the design and controlled fabrication of electrospun 3D structures such as grids, walls, hollow cylinders, and other 3D logos. As such, this technique has the potential to advance the existing electrospinning technologies in constructing 3D structures for biomedical, microelectronics, and MEMS/NMES applications.
      Cover page: Direct-Write, Self-Aligned Electrospinning on Paper for Controllable Fabrication of Three-Dimensional Structures
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2018)
      While electrochemical supercapacitors often show high power density and long operation lifetimes, they are plagued by limited energy density. Pseudocapacitive materials, in contrast, operate by fast surface redox reactions and are shown to enhance energy storage of supercapacitors. Furthermore, several reported systems exhibit high capacitance but restricted electrochemical voltage windows, usually no more than 1 V in aqueous electrolytes. Here, it is demonstrated that vertically aligned carbon nanotubes (VACNTs) with uniformly coated, pseudocapacitive titanium disulfide (TiS2 ) composite electrodes can extend the stable working range to over 3 V to achieve a high capacitance of 195 F g-1 in an Li-rich electrolyte. A symmetric cell demonstrates an energy density of 60.9 Wh kg-1 -the highest among symmetric pseudocapacitors using metal oxides, conducting polymers, 2D transition metal carbides (MXene), and other transition metal dichalcogenides. Nanostructures prepared by an atomic layer deposition/sulfurization process facilitate ion transportation and surface reactions to result in a high power density of 1250 W kg-1 with stable operation over 10 000 cycles. A flexible solid-state supercapacitor prepared by transferring the TiS2 -VACNT composite film onto Kapton tape is demonstrated to power a 2.2 V light emitting diode (LED) for 1 min.
        Cover page: Titanium Disulfide Coated Carbon Nanotube Hybrid Electrodes Enable High Energy Density Symmetric Pseudocapacitors
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2018)
        The recent developments in material sciences and rational structural designs have advanced the field of compliant and deformable electronics systems. However, many of these systems are limited in either overall stretchability or areal coverage of functional components. Here, we design a construct inspired by Kirigami for highly deformable micro-supercapacitor patches with high areal coverages of electrode and electrolyte materials. These patches can be fabricated in simple and efficient steps by laser-assisted graphitic conversion and cutting. Because the Kirigami cuts significantly increase structural compliance, segments in the patches can buckle, rotate, bend and twist to accommodate large overall deformations with only a small strain (<3%) in active electrode areas. Electrochemical testing results have proved that electrical and electrochemical performances are preserved under large deformation, with less than 2% change in capacitance when the patch is elongated to 382.5% of its initial length. The high design flexibility can enable various types of electrical connections among an array of supercapacitors residing in one patch, by using different Kirigami designs.
          Cover page: Kirigami-inspired, highly stretchable micro-supercapacitor patches fabricated by laser conversion and cutting
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2019)
          Ultrathin transition metal carbides with high capacity, high surface area, and high conductivity are a promising family of materials for applications from energy storage to catalysis. However, large-scale, cost-effective, and precursor-free methods to prepare ultrathin carbides are lacking. Here, we demonstrate a direct pattern method to manufacture ultrathin carbides (MoCx, WCx, and CoCx) on versatile substrates using a CO2 laser. The laser-sculptured polycrystalline carbides (macroporous, ~10-20 nm wall thickness, ~10 nm crystallinity) show high energy storage capability, hierarchical porous structure, and higher thermal resilience than MXenes and other laser-ablated carbon materials. A flexible supercapacitor made of MoCx demonstrates a wide temperature range (-50 to 300 °C). Furthermore, the sculptured microstructures endow the carbide network with enhanced visible light absorption, providing high solar energy harvesting efficiency (~72 %) for steam generation. The laser-based, scalable, resilient, and low-cost manufacturing process presents an approach for construction of carbides and their subsequent applications.
            Cover page: Laser-sculptured ultrathin transition metal carbide layers for energy storage and energy harvesting applications
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