Kirigami-inspired, highly stretchable micro-supercapacitor patches fabricated by laser conversion and cutting.

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

Synthesis and Assembly of 2D Materials for Sensing and Clean Energy Applications

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

Atomic Layer Deposition of TiO2 Nanocoatings on ZnO Nanowires for Improved Photocatalytic Stability

Photocatalytic water splitting represents an emerging technology well positioned to satisfy the growing need for low-energy, low CO2, economically viable hydrogen gas production. As such, stable, high-surface-area electrodes are increasingly being investigated as electrodes for the photochemical conversion of solar energy into hydrogen fuel. We present a titanium dioxide (TiO2)/zinc oxide (ZnO) nanowire array using a hybrid hydrothermal/atomic layer deposition (ALD) for use as a solar-powered photoelectrochemical device. The nanowire array consists of single crystalline, wurtzite ZnO nanowires with a 40 nm ALD TiO2 coating. By using a TiO2 nanocoating on the high surface area-ZnO array, three advancements have been accomplished in this work: (1) high aspect ratio nanowires with TiO2 for water splitting (over 8 μm), (2) improved stability over bare ZnO nanowires during photocatalysis, and (3) excellent onset voltage. As such, this process opens up new class of the micro/nanofabrication process for making efficient photocatalytic gas harvesting systems

Direct-Write, Self-Aligned Electrospinning on Paper for Controllable Fabrication of Three-Dimensional Structures.

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

Uniformly embedded metal oxide nanoparticles in vertically aligned carbon nanotube forests as pseudocapacitor electrodes for enhanced energy storage

Carbon nanotube (CNT) forests were grown directly on a silicon substrate using a Fe/Al/Mo stacking layer which functioned as both the catalyst material and subsequently a conductive current collecting layer in pseudocapacitor applications. A vacuum-assisted, in situ electrodeposition process has been used to achieve the three-dimensional functionalization of CNT forests with inserted nickel nanoparticles as pseudocapacitor electrodes. Experimental results have shown the measured specific capacitance of 1.26 F/cm3, which is 5.7 times higher than pure CNT forest samples, and the oxidized nickel nanoparticle/CNT supercapacitor retained 94.2% of its initial capacitance after 10 000 cyclic voltammetry tests. ? 2013 American Chemical Society

Flexible PET/EVA-Based Piezoelectret Generator for Energy Harvesting in Harsh Environments

Stable and repeatable operation is paramount for practical and extensive applications of all energy harvesters. Herein, we develop a new type of flexible piezoelectret generator, which converts mechanical energy into electricity consistently even under harsh environments. Specifically, the generator, with piezoelectric coefficient (d33) reaching ~ 6300 pC/N, had worked stably for continuous ~ 90000 cycles, and the generator pressed by a human hand produced load peak current and power up to ~ 29.6 μA and ~ 0.444 mW, respectively. Moreover, the capability to steadily produce electrical power under extreme moisture and temperature up to 70 oC had been achieved for possible applications in wearable devices and flexible electronics.Accepted versio

Laser-Induced Tar-Mediated Sintering of Metals and Refractory Carbides in Air

Refractory metals and their carbides possess extraordinary chemical and temperature resilience and exceptional mechanical strength. Yet, they are notoriously difficult to employ in additive manufacturing, due to the high temperatures needed for processing. State of the art approaches to manufacture these materials generally require either a high-energy laser or electron beam as well as ventilation to protect the metal powder from combustion. Here, we present a versatile manufacturing process that utilizes tar as both a light absorber and antioxidant binder to sinter thin films of aluminum, copper, nickel, molybdenum, and tungsten powder using a low power (<2W) CO2 laser in air. Films of sintered Al/Cu/Ni metals have sheet resistances of ∼10-1 ohm/sq, while laser-sintered Mo/W-tar thin films form carbide phases. Several devices are demonstrated, including laser-sintered porous copper with a stable response to large strain (3.0) after 150 cycles, and a laserprocessed Mo/MoC(1-x) filament that reaches T ∼1000 °C in open air at 12 V. These results show that tar-mediated laser sintering represents a possible low energy, cost-effective route for engineering refractory materials and one that can easily be extended to additive manufacturing processes