Jungmo Kim

Defect-free, size-tunable graphene for high-performance lithium ion battery.

The scalable preparation of graphene in control of its structure would significantly improve its commercial viability. Despite intense research in this area, the size control of defect-free graphene (df-G) without any trace of oxidation or structural damage remains a key challenge. Here, we propose a new scalable route for generating df-G with a controllable size of submicron to micron through sequential insertion of potassium and pyridine at low temperature. Structural and chemical analyses confirm that the df-G perfectly preserves the intrinsic properties of graphene. The Co3O4 (<50 nm) wrapped by ∼ 10.5 μm(2) df-G has unprecedented capacity, rate capability, and cycling stability with capacities as high as 1050 mAh g(-1) at 500 mA g(-1) and 900 mAh g(-1) at 1000 mA g(-1) even after 200 cycles, which suggests enticing potential for the use in high performance lithium ion batteries.

Three-Dimensional Continuous Conductive Nanostructure for Highly Sensitive and Stretchable Strain Sensor

The demand for wearable strain gauges that can detect dynamic human motions is growing in the area of healthcare technology. However, the realization of efficient sensing materials for effective detection of human motions in daily life is technically challenging due to the absence of the optimally designed electrode. Here, we propose a novel concept for overcoming the intrinsic limits of conventional strain sensors based on planar electrodes by developing highly periodic and three-dimensional (3D) bicontinuous nanoporous electrodes. We create a 3D bicontinuous nanoporous electrode by constructing conductive percolation networks along the surface of porous 3D nanostructured poly(dimethylsiloxane) with single-walled carbon nanotubes. The 3D structural platform allows fabrication of a strain sensor with robust properties such as a gauge factor of up to 134 at a tensile strain of 40%, a widened detection range of up to 160%, and a cyclic property of over 1000 cycles. Collectively, this study provides new design opportunities for a highly efficient sensing system that finely captures human motions, including phonations and joint movements.

Moisture Barrier Composites Made of Non-Oxidized Graphene Flakes

Graphene flakes (GFs) with minimized defects and oxidation ratios are incorporated into polyethylene (PE) to enhance the moisture barrier. GFs produced involving solvothermal intercalation show extremely low oxidation rates (3.17%), and are noncovalently functionalized in situ, inducing strong hydrophobicity. The fabricated composite possesses the best moisture barrier performance reported for a polymer-graphene composite.

Fast P3HT Exciton Dissociation and Absorption Enhancement of Organic Solar Cells by PEG-Functionalized Graphene Quantum Dots.

PEG-functionalized graphene quantum dots (GQDs) are shown to promote fast exciton dissociation in organic solar cells. Short-chain PEG promotes the most favorable interaction with other organic layers, and the overall efficiency is improved by 36% when compared to the reference devices. The mechanism of enhancement is shown to be increased absorption due to fewer charges remain-ing in the bound state.

Thermomechanical stress analysis of laminated thick-film multilayer substrates

As an increasing number of polymer dielectric layers were laminated, the maximum bow values were measured layer by layer using a laser profilometry during thermal cycling. In the lamination process, a polymeric film is overlaid on a silicon substrate using a polymeric adhesive. Since the lamination process uses relatively thick polymer films, the classical stress analyses assuming infinitesimally thin films, are no longer effective. In this letter, a simple model based on the composite beam theory is presented to analyze the experimental results, and compared with the well-known Stoney’s formula. The thermomechanical behavior of the laminated multilayer polymer films on a silicon substrate was better described by the proposed model, while an error as much as 30% was involved using Stoney’s formula. The model can be applied for the design and fabrication of multilayer multichip module substrates.

Development of three-dimensional memory die stack packages using polymer insulated sidewall technique

A newly designed three dimensional (3-D) memory die stack package has been established, and the prototype of the 3-D package using mechanical dies has been successfully demonstrated. Fabrication processes of the 3-D package consist of: (1) wafer cutting into die segments; (2) die passivation including sidewall insulation; (3) via opening on the original I/O pads; (4) I/O redistribution from center pads to sidewall; (5) bare die stacking using polymer adhesive; (6) sidewall interconnection; and (7) solder balls attachment. There are several significant improvements in this new 3-D package design compared with the current 3-D package concept. The unique feature of this newly developed package is the sidewall insulation of dies prior to the I/O redistribution of dies, which produces (1) better chip-to-wafer yields and (2) significant process simplification during subsequent fabrication steps. According to this design, 100% of die yields on a conventional wafer design can be obtained without any neighboring die losses which usually occur during the I/O redistribution processes of conventional 3-D package design. Furthermore, the new 3-D package design can simplify the following processes such as I/O redistribution, sidewall insulation, sidewall interconnection, and package formation. It is proven that the mechanical integrity of the prototype 3-D stacked package meets requirements of the JEDEC Level III and 85/spl deg/C/85% test.

Electron tunneling detected by electrostatic force

A method is introduced for measuring the tunneling of electrons between a specially fabricated scanning probe microscope tip and a surface. The technique is based upon electrostatic force detection of charge as it is transferred to and from a small (10−17 F) electrically isolated metallic dot on the scanning probe tip. The methods for dot fabrication, charging, and discharging are described and electron tunneling to a sample surface is demonstrated.

Atomic scale investigations of the Co/Pt(111) interface structure and magnetic properties

The interface structure of an ultrathin Co overlayer on a Pt(111) crystal was investigated with atomic-layer resolution medium-energy ion scattering spectroscopy and surface magneto-optical Kerr effect (SMOKE). For a 7 ML Co, interdiffusion begins at 673 K to form a heavily distorted Co–Pt surface alloy layer with little change in SMOKE intensity. However, annealing at 773 K formed a 30 atomic-layer-thick Co–Pt substitutional alloy with 3.7% maximum tensile strain, at which the SMOKE intensity increased more than 200%. The enhancement of the Kerr intensity is discussed with the interface alloy formation.

Low-Cost Black Phosphorus Nanofillers for Improved Thermoelectric Performance in PEDOT:PSS Composite Films

In recent years, two-dimensional black phosphorus (BP) has seen a surge of research because of its unique optical, electronic, and chemical properties. BP has also received interest as a potential thermoelectric material because of its high Seebeck coefficient and excellent charge mobility, but further development is limited by the high cost and poor scalability of traditional BP synthesis techniques. In this work, high-quality BP is synthesized using a low-cost method and utilized in a PEDOT:PSS film to create the first ever BP composite thermoelectric material. The thermoelectric properties are found to be greatly enhanced after the BP addition, with the power factor of the film, with 2 wt % BP (36.2 μW m-1 K-2) representing a 109% improvement over the pure PEDOT:PSS film (17.3 μW m-1 K-2). A simultaneous increase of mobility and decrease of the carrier concentration is found to occur with the increasing BP wt %, which allows for both Seebeck coefficient and electrical conductivity to be increased. These results show the potential of this low-cost BP for use in energy devices.

Two-Dimensional WO3 Nanosheets Chemically Converted from Layered WS2 for High-Performance Electrochromic Devices

Two-dimensional (2D) transitional metal oxides (TMOs) are an attractive class of materials due to the combined advantages of high active surface area, enhanced electrochemical properties, and stability. Among the 2D TMOs, 2D tungsten oxide (WO3) nanosheets possess great potential in electrochemical applications, particularly in electrochromic (EC) devices. However, feasible production of 2D WO3 nanosheets is challenging due to the innate 3D crystallographic structure of WO3. Here we report a novel solution-phase synthesis of 2D WO3 nanosheets through simple oxidation from 2D tungsten disulfide (WS2) nanosheets exfoliated from bulk WS2 powder. The complete conversion from WS2 into WO3 was confirmed through crystallographic and elemental analyses, followed by validation of the 2D WO3 nanosheets applied in the EC device. The EC device showed color modulation of 62.57% at 700 nm wavelength, which is 3.43 times higher than the value of the conventional device using bulk WO3 powder, while also showing enhancement of ∼46.62% and ∼62.71% in switching response-time (coloration and bleaching). The mechanism of enhancement was rationalized through comparative analysis based on the thickness of the WO3 components. In the future, 2D WO3 nanosheets could also be used for other promising applications such as sensors, catalysis, thermoelectric, and energy conversion.