Gil S. Lee

Synthesis and Electrochemical Properties of Spin-Capable Carbon Nanotube Sheet/MnOx Composites for High-Performance Energy Storage Devices

Inspired by the high specific capacitances found using ultrathin films or nanoparticles of manganese oxides (MnO(x)), we have electrodeposited MnO(x) nanoparticles onto sheets of carbon nanotubes (CNT sheets). The resulting composites have high specific capacitances (C(sp) ≤ 1250 F/g), high charge/discharge rate capabilities, and excellent cyclic stability. Both the C(sp) and rate capabilities are controlled by the average size of the MnO(x) nanoparticles on the CNTs. They are independent of the number of layers of CNT sheets used to form an electrode. The high-performance composites result from a synergistic combination of large surface area and good electron-transport capabilities of the MnO(x) nanoparticles with the good conductivity of the CNT sheets. Such composites can be used as electrodes for lithium batteries and supercapacitors.

Growth of InAs1−xSbx (0<x<1) and InSb‐InAsSb superlattices by molecular beam epitaxy

Thin films of InAs1−xSbx (0<x<1) have been deposited on GaAs and InSb substrates in the temperature range 300–400 °C using molecular beam epitaxy. The solid composition was found to be quite sensitive to the Sb flux and less sensitive to As flux. InSb‐InAsSb superlattice structures have also been grown and studied. Both the ternary alloy and the superlattice structures can be potential material systems for detectors covering the 8–12‐μ range.

Thickness modulated MoS2 grown by chemical vapor deposition for transparent and flexible electronic devices

Two-dimensional (2D) materials have been a great interest as high-performance transparent and flexible electronics due to their high crystallinity in atomic thickness and their potential for variety applications in electronics and optoelectronics. The present study explored the wafer scale production of MoS2 nanosheets with layer thickness modulation from single to multi-layer by using two-step method of metal deposition and CVD process. The formation of high-quality and layer thickness-modulated MoS2 film was confirmed by Raman spectroscopy, AFM, HRTEM, and photoluminescence analysis. The layer thickness was identified by employing a simple method of optical contrast value. The image contrast in green (G) channel shows the best fit as contrast increases with layer thickness, which can be utilized in identifying the layer thickness of MoS2. The presence of critical thickness of Mo for complete sulphurization, which is due to the diffusion limit of MoS2 transformation, changes the linearity of structural, ...

Highly Conductive Flexible Multi-Walled Carbon Nanotube Sheet Films for Transparent Touch Screen

Highly conductive and transparent thin films were prepared using highly purified multi-walled carbon nanotube (MWCNT) sheets. The electrical properties of the MWCNT sheet were remarkably improved by an acid treatment, resulting in densely packed MWCNTs. The morphology of the sheets reveals that continuous electrical pathways were formed by the acid treatment, greatly improving the sheet resistance all the while maintaining an excellent optical transmittance. These results encourage the use of these MWCNT sheets with low sheet resistance (450 Ω/sq) and high optical transmittance (90%) as a potential candidate for flexible display applications.

Fabrication of a Microneedle/CNT Hierarchical Micro/Nano Surface Electrochemical Sensor and Its In-Vitro Glucose Sensing Characterization

We report fabrication of a microneedle-based three-electrode integrated electrochemical sensor and in-vitro characterization of this sensor for glucose sensing applications. A piece of silicon was sequentially dry and wet etched to form a 15 × 15 array of tall (approximately 380 μm) sharp silicon microneedles. Iron catalyst was deposited through a SU-8 shadow mask to form the working electrode and counter electrode. A multi-walled carbon nanotube forest was grown directly on the silicon microneedle array and platinum nano-particles were electrodeposited. Silver was deposited on the Si microneedle array through another shadow mask and chlorinated to form a Ag/AgCl reference electrode. The 3-electrode electrochemical sensor was tested for various glucose concentrations in the range of 3∼20 mM in 0.01 M phosphate buffered saline (PBS) solution. The sensors amperometric response to the glucose concentration is linear and its sensitivity was found to be 17.73 ± 3 μA/mM-cm2. This microneedle-based sensor has a potential to be used for painless diabetes testing applications.

Mode selection and spatial hole burning suppression of a chirped grating distributed feedback laser

A new kind of distributed feedback (DFB) laser with a linearly chirped grating is proposed and investigated theoretically for the first time. The eigenmodes of the chirped grating DFB (CG‐DFB) and the field intensity distribution are calculated. With a suitable cavity length and chirp factor, the threshold gain difference to the next competence mode can be very large and will keep the single longitudinal mode stable. The chirp factor can be chosen to make the field intensity distribution quite uniform and at the same time keep the threshold gain difference large. This behavior effectively suppresses spatial hole burning and hence makes CG‐DFB lasers that are stable single mode at high power, and with a narrow linewidth.

Characterization of low dielectric constant plasma enhanced chemical vapor deposition fluorinated silicon oxide films as intermetal dielectric materials

The characteristics of fluorinated silicon oxide films prepared by plasma enhanced chemical vapor deposition are discussed. The deposition of these films was carried out in the temperature range of 80–220 °C in a conventional parallel plate plasma reactor with <±3% uniformity across 4 in. wafer by flowing 2 sccm of disilane, 100 sccm of nitrous oxide and 20 sccm of tetrafluoromethane. As the deposition temperature increased from 80 to 220 °C, the deposition rate of the films increased from 16.7 to 18.4 nm/min; meanwhile, the etch rate decreased from 2.69 to 1.48 nm/s. The refractive index was 1.46 regardless of the deposition temperature. The Fourier transform infrared spectra of the films showed a decreasing Si–O stretching wave number with an increasing full width at half maximum as the deposition temperature increased. The high frequency capacitance–voltage measurements of the metal–oxide–semiconductor (MOS) capacitors fabricated using the films showed increasing effective oxide charge density and decr...

Transverse junction stripe laser with a lateral heterobarrier by diffusion enhanced alloy disordering

We report the first cw, room‐temperature multiple quantum well transverse junction stripe laser (MQW TJS) with an alloy disordered heterobarrier. The addition of a lateral heterobarrier by zinc diffusion enhanced alloy disordering reduces the laser threshold by a factor of 3 compared to standard transverse junction stripe lasers processed simultaneously. The reduction in threshold and excellent single mode performance of the MQW TJS are attributed to the superior carrier and optical confinement of a single heterojunction compared to a homojunction. We demonstrate for the first time lateral injection in a multiple quantum well where the diffused junction plays an active (not passive) role in the injection process and that the transition region between complete disorder and no disorder forms the active region of the laser.

Phase‐shifted distributed feedback laser with linearly chirped grating for narrow linewidth and high‐power operation

Phase‐shifted distributed feedback laser with linearly chirped grating (PS‐CG‐DFB) was analyzed. The field distribution, threshold gain difference between the lasing mode and the next lowest threshold gain mode, linewidth of the lasing mode, and output power of PS‐CG‐DFB were self‐consistently calculated under different injection levels. It is found that the lasing mode pattern and the carrier concentration distribution are ‘‘locked’’ at high injection level due to the strong interaction between carriers and photons. The chirped grating compensates the spatial hole burning effect to the mode stability and makes the mode stable in the whole injection region.

Flexible transparent conductive heater using multiwalled carbon nanotube sheet

This paper reports highly flexible, transparent, conducting heaters based on multiwalled carbon nanotube (MWCNT) sheets. The MWCNT sheets were spun directly from a well-aligned MWCNT forest. The fabrication of the MWCNT sheet heater was quite simple and suitable for mass production, requiring only a one-step transferring process, in which the MWCNT sheet is drawn onto the target substrates. This study examined the parameters that affect the heat generation of the MWCNT sheet-based heater; input power, surface area, and thermal conductivity of the substrate. In particular, more effort was focused on how to increase the surface area and contact points between the individual MWCNTs; simple acid treatment and added metal nanoparticles increased the heat performance of the heater dramatically. Moreover, the heaters exhibited durability and flexibility against many bending cycles. Therefore, the MWCNT sheet-based heater can be used for versatile applications requiring transparency, conduction, and flexibility.