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.