Daeil Kim

Fabrication of a Stretchable Solid-State Micro-Supercapacitor Array

We fabricated a stretchable micro-supercapacitor array with planar SWCNT electrodes and an ionic liquid-based triblock copolymer electrolyte. The mechanical stability of the entire supercapacitor array upon stretching was obtained by adopting strategic design concepts. First, the narrow and long serpentine metallic interconnections were encapsulated with polyimide thin film to ensure that they were within the mechanical neutral plane. Second, an array of two-dimensional planar micro-supercapacitor with SWCNT electrodes and an ion-gel-type electrolyte was made to achieve all-solid-state energy storage devices. The formed micro-supercapacitor array showed excellent performances which were stable over stretching up to 30% without any noticeable degradation. This work shows the strong potential of a stretchable micro-supercapacitor array in applications such as wearable computers, power dressing, electronic newspapers, paper-like mobile phones, and other easily collapsible gadgets.

Measurement of Secondary Electron Emission Coefficient (γ) of MgO Protective Layer with Various Crystallinities

The secondary electron emission coefficient γ of a MgO protective layer with various crystallinities has been successfully measured by the γ-focused ion beam system with complete elimination of the charge accumulation problem by scanning-area adjustment techniques. It is found that the (111) surface has the highest γ from 0.14 to 0.26 in comparison with the other films with (200) and (220) crystallinities for operating Ne+ ions, while ranged from 0.03 to 0.24 for Ar+ ions, under operating ion energies from 50 eV to 500 eV throughout this experiment. These observations explain why the (111) crystallinity of the MgO protective layer plays an important role in lowering the firing voltages in AC plasma display panel compared to the films with other crystallinities.

Biaxially stretchable, integrated array of high performance microsupercapacitors

We report on the fabrication of a biaxially stretchable array of high performance microsupercapacitors (MSCs) on a deformable substrate. The deformable substrate is designed to suppress local strain applied to active devices by locally implanting pieces of stiff polyethylene terephthalate (PET) films within the soft elastomer of Ecoflex. A strain suppressed region is formed on the top surface of the deformable substrate, below which PET films are implanted. Active devices placed within this region can be isolated from the strain. Analysis of strain distribution by finite element method confirms that the maximum strain applied to MSC in the strain suppressed region is smaller than 0.02%, while that on the Ecoflex film is larger than 250% under both uniaxial strain of 70% and biaxial strain of 50%. The all-solid-state planar MSCs, fabricated with layer-by-layer deposited multiwalled carbon nanotube electrodes and patterned ionogel electrolyte of poly(ethylene glycol) diacrylate and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide having high-potential windows, are dry-transferred onto the deformable substrate and electrically connected in series and parallel via embedded liquid metal interconnection and Ag nanowire contacts. Liquid metal interconnection, formed by injecting liquid metal into the microchannel embedded within the substrate, can endure severe strains and requires no additional encapsulation process. This formed MSC array exhibits high energy and power density of 25 mWh/cm(3) and 32 W/cm(3), and stable electrochemical performance up to 100% uniaxial and 50% biaxial stretching. The high output voltage of the MSC array is used to light micro-light-emitting diode (μ-LED) arrays, even under strain conditions. This work demonstrates the potential application of our stretchable MSC arrays to wearable and bioimplantable electronics with a self-powered system.

Fabrication of a stretchable and patchable array of high performance micro-supercapacitors using a non-aqueous solvent based gel electrolyte

In this study, we report the fabrication of a stretchable and patchable array of micro-supercapacitors (MSCs) using a gel-type electrolyte of poly(methyl methacrylate)–propylene carbonate–lithium perchlorate. As electrodes, a layer-by-layer-assembled thin film of multi-walled carbon nanotubes with a top layer of Mn3O4 nanoparticles was used. The fabricated MSC maintained over 85% of its performance for 2 weeks in ambient air without encapsulation owing to the use of a non-aqueous solvent based gel electrolyte. Dry-transferred MSC arrays on a specially designed stretchable polymer substrate exhibited stable electrochemical performance under various deformations, including bending, twisting, both uniaxial and biaxial stretching up to 50%, and winding around the curved substrate. Furthermore, the encapsulated MSC array with a thin polymer film directly attached to skin maintained its electrochemical performance under repeated body movement, cycles of attachment–detachment, and even in water. This study clearly demonstrates a stretchable and patchable MSC array for practical use as an energy storage device that can be attached to the body for electronic function, even under wet conditions.

Air-stable, high-performance, flexible microsupercapacitor with patterned ionogel electrolyte

We describe the fabrication of air-stable, high-performance, planar microsupercapacitors (MSCs) on a flexible poly(ethylene terephthalate) substrate with patterned ionogel electrolyte, i.e., poly(ethylene glycol) diacrylate/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, and electrodes of spray-coated multiwalled carbon nanotubes. The flexible MSC showed good cyclability, retaining ∼80% of initial capacitance after 30 000 cycles, and good mechanical stability down to a bending diameter of 3 mm under compressive stress; 95% of the initial capacitance was retained after 1000 bending cycles. The MSC had high electrochemical stability with retaining 90% of its initial capacitance for 8 weeks in air. Furthermore, vertical stacking of MSCs with patterned solid film of ionogel electrolyte could increase the areal capacitance dramatically. This flexible MSC has potential applications as an energy-storage device in micro/nanoelectronics, without encapsulation for air stability.

Photoconductance of aligned SnO2 nanowire field effect transistors

We report on the optoelectronic properties of the aligned SnO2 nanowire (NW) field effect transistors (FETs) fabricated via a sliding transfer of NWs grown by chemical vapor deposition. Photocurrent measurements with polarized UV light confirmed a well aligned NWs along the channels. UV photosensitivity of ∼107 at the gate voltage Vg=−40 V was obtained due to a small dark-current at the turn-off state of FET. The dynamic response of the photocurrent became faster for the higher mobility SnO2 NW FETs. We expect our aligned SnO2 NW FETs will be useful as polarized UV detectors with a high sensitivity.

Body-Attachable and Stretchable Multisensors Integrated with Wirelessly Rechargeable Energy Storage Devices.

A stretchable multisensor system is successfully demonstrated with an integrated energy-storage device, an array of microsupercapacitors that can be repeatedly charged via a wireless radio-frequency power receiver on the same stretchable polymer substrate. The integrated devices are interconnected by a liquid-metal interconnection and operate stably without noticeable performance degradation under strain due to the skin attachment, and a uniaxial strain up to 50%.

Encapsulated, High-Performance, Stretchable Array of Stacked Planar Micro-Supercapacitors as Waterproof Wearable Energy Storage Devices

We report the fabrication of an encapsulated, high-performance, stretchable array of stacked planar micro-supercapacitors (MSCs) as a wearable energy storage device for waterproof applications. A pair of planar all-solid-state MSCs with spray-coated multiwalled carbon nanotube electrodes and a drop-cast UV-patternable ion-gel electrolyte was fabricated on a polyethylene terephthalate film using serial connection to increase the operation voltage of the MSC. Additionally, multiple MSCs could be vertically stacked with parallel connections to increase both the total capacitance and the areal capacitance owing to the use of a solid-state patterned electrolyte. The overall device of five parallel-connected stacked MSCs, a microlight-emitting diode (μ-LED), and a switch was encapsulated in thin Ecoflex film so that the capacitance remained at 82% of its initial value even after 4 d in water; the μ-LED was lit without noticeable decrease in brightness under deformation including bending and stretching. Furthermore, an Ecoflex encapsulated oximeter wound around a finger was operated using the stored energy of the MSC array attached to the hand (even in water) to give information on arterial pulse rate and oxygen saturation in the blood. This study suggests potential applications of our encapsulated MSC array in wearable energy storage devices especially in water.

Ion-Induced Secondary Electron Emission Coefficient(γ)of Bulk-MgO Single Crystals

The ion induced secondary electron emission coefficient ? of bulk-MgO single crystal has been measured by the ?-focused ion beam system. It is found that the bulk-MgO single crystal with (111) orientation has the highest ? from 0.08 up to 0.21, and the ? values are in the order of the crystallinities, (111) > (200) > (220) for operating Ne+ ions ranging from 50 eV to 300 eV in this experiment. These results are consistent with our previous reports for MgO protective layers with respective orientations of (111), (200), and (220).

Influence of Sustaining Pulse-Width on Electro-Luminous Efficiency in AC Plasma Display Panels

The influence of sustaining pulse-width on electro-luminous efficiency has been experimentally investigated for the surface discharge of AC plasma display panels. A square driving voltage pulse with a variable duty ratio and a fixed rise time of 300 ns has been used in this experiment. It is found that the firing voltage decreases as the sustaining pulse-width increases from 2 µs to 8 µs under a given driving frequency, and also decreases as the frequency increases from 10 kHz to 50 kHz. The memory coefficient is found to be significantly increased from 0.16 to 0.71, as the sustaining pulse-width increases from 3 µs to 4 µs, beyond which it maintains a nearly constant value of approximately 0.8 for the driving frequency range from 30 kHz to 50 kHz. The optimal sustaining pulse-width is shown to be in the range of 3 µs and 4 µs for driving frequencies of 30 kHz and 50 kHz, based on the observations of memory coefficient, wall charge, wall voltage and luminous efficiency.