Phillip Lee

Highly Stretchable and Highly Conductive Metal Electrode by Very Long Metal Nanowire Percolation Network

A highly stretchable metal electrode is developed via the solution-processing of very long (>100 μm) metallic nanowires and subsequent percolation network formation via low-temperature nanowelding. The stretchable metal electrode from very long metal nanowires demonstrated high electrical conductivity (~9 ohm sq(-1) ) and mechanical compliance (strain > 460%) at the same time. This method is expected to overcome the performance limitation of the current stretchable electronics such as graphene, carbon nanotubes, and buckled nanoribbons.

Nonvacuum, Maskless Fabrication of a Flexible Metal Grid Transparent Conductor by Low-Temperature Selective Laser Sintering of Nanoparticle Ink

We introduce a facile approach to fabricate a metallic grid transparent conductor on a flexible substrate using selective laser sintering of metal nanoparticle ink. The metallic grid transparent conductors with high transmittance (>85%) and low sheet resistance (30 Ω/sq) are readily produced on glass and polymer substrates at large scale without any vacuum or high-temperature environment. Being a maskless direct writing method, the shape and the parameters of the grid can be easily changed by CAD data. The resultant metallic grid also showed a superior stability in terms of adhesion and bending. This transparent conductor is further applied to the touch screen panel, and it is confirmed that the final device operates firmly under continuous mechanical stress.

Extremely Elastic Wearable Carbon Nanotube Fiber Strain Sensor for Monitoring of Human Motion

The increasing demand for wearable electronic devices has made the development of highly elastic strain sensors that can monitor various physical parameters an essential factor for realizing next generation electronics. Here, we report an ultrahigh stretchable and wearable device fabricated from dry-spun carbon nanotube (CNT) fibers. Stretching the highly oriented CNT fibers grown on a flexible substrate (Ecoflex) induces a constant decrease in the conductive pathways and contact areas between nanotubes depending on the stretching distance; this enables CNT fibers to behave as highly sensitive strain sensors. Owing to its unique structure and mechanism, this device can be stretched by over 900% while retaining high sensitivity, responsiveness, and durability. Furthermore, the device with biaxially oriented CNT fiber arrays shows independent cross-sensitivity, which facilitates simultaneous measurement of strains along multiple axes. We demonstrated potential applications of the proposed device, such as strain gauge, single and multiaxial detecting motion sensors. These devices can be incorporated into various motion detecting systems where their applications are limited to their strain.

Nanowire reinforced nanoparticle nanocomposite for highly flexible transparent electrodes: borrowing ideas from macrocomposites in steel-wire reinforced concrete

Inspired by steel-wire reinforced concrete, which is a strong building material, a novel nanocomposite of a nanowire reinforced nanoparticle matrix film was developed for flexible and transparent electrode applications. The transparent electrode made in this study exhibits superior mechanical characteristics under tensile as well as compressive bending, in comparison to conventional sintered metal nanoparticle based metallic grids.

Solution-Processed Ultrathin TiO2 Compact Layer Hybridized with Mesoporous TiO2 for High-Performance Perovskite Solar Cells

The electron transport layer (ETL) is a key component of perovskite solar cells (PSCs) and must provide efficient electron extraction and collection while minimizing the charge recombination at interfaces in order to ensure high performance. Conventional bilayered TiO2 ETLs fabricated by depositing compact TiO2 (c-TiO2) and mesoporous TiO2 (mp-TiO2) in sequence exhibit resistive losses due to the contact resistance at the c-TiO2/mp-TiO2 interface and the series resistance arising from the intrinsically low conductivity of TiO2. Herein, to minimize such resistive losses, we developed a novel ETL consisting of an ultrathin c-TiO2 layer hybridized with mp-TiO2, which is fabricated by performing one-step spin-coating of a mp-TiO2 solution containing a small amount of titanium diisopropoxide bis(acetylacetonate) (TAA). By using electron microscopies and elemental mapping analysis, we establish that the optimal concentration of TAA produces an ultrathin blocking layer with a thickness of ∼3 nm and ensures that the mp-TiO2 layer has a suitable porosity for efficient perovskite infiltration. We compare PSCs based on mesoscopic ETLs with and without compact layers to determine the role of the hole-blocking layer in their performances. The hybrid ETLs exhibit enhanced electron extraction and reduced charge recombination, resulting in better photovoltaic performances and reduced hysteresis of PSCs compared to those with conventional bilayered ETLs.

The Effect of Additives on the Early Stages of Growth of Calcite Single Crystals

Abstract As crystallization processes are often rapid, it can be difficult to monitor their growth mechanisms. In this study, we made use of the fact that crystallization proceeds more slowly in small volumes than in bulk solution to investigate the effects of the soluble additives Mg2+ and poly(styrene sulfonate) (PSS) on the early stages of growth of calcite crystals. Using a “Crystal Hotel” microfluidic device to provide well‐defined, nanoliter volumes, we observed that calcite crystals form via an amorphous precursor phase. Surprisingly, the first calcite crystals formed are perfect rhombohedra, and the soluble additives have no influence on the morphology until the crystals reach sizes of 0.1–0.5 μm for Mg2+ and 1–2 μm for PSS. The crystals then continue to grow to develop morphologies characteristic of these additives. These results can be rationalized by considering additive binding to kink sites, which is consistent with crystal growth by a classical mechanism.

Recent progress in silver nanowire based flexible/wearable optoelectronics

Among diverse nanomaterials, silver nanowire (AgNW) has reached a certain level of technological maturity, and numerous commercialized AgNW products are already on the market for research and prototype purposes. One of the potential applications for AgNW and its percolative form is in wearable electronics, owing to the superior electrical, optical and mechanical properties that arise from the material itself or the overall interconnected structure. For successful application towards wearable applications, constituent AgNWs should first have uniform and controllable properties. At the same time, it is preferential to develop relevant scalable fabrication processes, together with the verification of potential applications from a proof-of-concept standpoint. Based on these progresses, we summarize the recent developments in AgNW based flexible/wearable optoelectronic applications and foresee their future development.

Flash type MEMS-based Analog-to-Digital Converter of field emission

The current CMOS-based Analog-to-Digital Converter (ADC) is confronted with the performance limits. These circumstances propose the necessity for new approaches for the ADC. This paper suggests a MEMS-based ADC of field emission. On/off state is determined by the existence of electron emission between the tip electrodes pair, which is used to encode the thermometer code. The abrupt increase of the current on the turn-on threshold voltage functions as the border of on/off state. 8 resolutions of 3-bit flash type ADC is made with 7 tip electrodes pairs of different turn-on threshold voltage. Turn-on voltages are differentiated with the different electrodes separation gap. Suggested method has the potential of low power consumption and size reduction.

Enhanced Thermoelectric Conversion Efficiency of CVD Graphene with Reduced Grain Sizes

The grain size of CVD (Chemical Vapor Deposition) graphene was controlled by changing the precursor gas flow rates, operation temperature, and chamber pressure. Graphene of average grain sizes of 4.1 µm, 2.2 µm, and 0.5 µm was synthesized in high quality and full coverage. The possibility to tailor the thermoelectric conversion characteristics of graphene has been exhibited by examining the grain size effect on the three elementary thermal and electrical properties of σ, S, and k. Electrical conductivity (σ) and Seebeck coefficients (S) were measured in a vacuum for supported graphene on SiO2/Si FET (Field Effect Transistor) substrates so that the charge carrier density could be changed by applying a gate voltage (VG). Mobility (µ) values of 529, 459, and 314 cm2/V·s for holes and 1042, 745, and 490 cm2/V·s for electrons for the three grain sizes of 4.1 µm, 2.2 µm, and 0.5 µm, respectively, were obtained from the slopes of the measured σ vs. VG graphs. The power factor (PF), the electrical portion of the thermoelectric figure of merit (ZT), decreased by about one half as the grain size was decreased, while the thermal conductivity (k) decreased by one quarter for the same grain decrease. Finally, the resulting ZT increased more than two times when the grain size was reduced from 4.1 µm to 0.5 µm.

Efficient Wafer-Level Edge-Tracing Technique for 3-D Interconnection of Stacked Die

An efficient edge-tracing technique at the wafer-level is proposed and implemented in this paper. The proposed method can be applied to the fabrication of a stacked chip. Experiments were conducted by stacking four test chips each 100-μm-thick, and the configuration of the pad is based on a memory chip from an electronics company. The chips for stacking were fabricated by half-dicing the wafer and curing the adhesives in a trench. When the four chips were built up and metallized, the stacked chip was 430-μm high, which is comparable to that of a through-silicon via. The daisy chain resistance of the interconnection was measured to be 5 Ω, and further improvement is possible with modification. The interconnection quality of the stacked chip was examined through 3-D images obtained via computed tomography and X-ray imageries. The images proved the successful creation of the interconnections. The mechanical integrity of the stacked package meets the 85°C/85% relative humidity test, and the thermal stress analysis is implemented to investigate the reliability issues at the edge of the chip, and it is concluded that there are no critical reliability problems.