Taeyoung Kim

Parallel patterning of nanoparticles via electrodynamic focusing of charged aerosols

The development of nanodevices that exploit the unique properties of nanoparticles1,2 will require high-speed methods for patterning surfaces with nanoparticles over large areas and with high resolution3,4,5,6. Moreover, the technique will need to work with both conducting and non-conducting surfaces. Here we report an ion-induced parallel-focusing approach that satisfies all requirements. Charged monodisperse aerosol nanoparticles are deposited onto a surface patterned with a photoresist while ions of the same polarity are introduced into the deposition chamber in the presence of an applied electric field. The ions accumulate on the photoresist, modifying the applied field to produce nanoscopic electrostatic lenses that focus the nanoparticles onto the exposed parts of the surface. We have demonstrated that the technique could produce high-resolution patterns at high speed on both conducting (p-type silicon) and non-conducting (silica) surfaces. Moreover, the feature sizes in the nanoparticle patterns were significantly smaller than those in the original photoresist pattern.

CDI ragone plot as a functional tool to evaluate desalination performance in capacitive deionization

A novel concept to evaluate desalination performance in capacitive deionization (CDI) is proposed called the CDI Ragone plot. The plot can allow for intuitive acquisition of deionization capacity (mg g−1), rate (mg g−1 s−1), and time (s) and thus, will work as a functional tool to evaluate desalination performance in CDI.

Carbon nanotube growth enhanced by nitrogen incorporation

Abstract It is well known that the growth of carbon nanotubes (CNTs) by chemical vapor deposition using a transition metal catalyst is greatly enhanced in a nitrogen environment. We show here that the enhanced growth is closely related to nitrogen incorporation into the CNT wall and cap during growth. This behavior is consistent with theoretical calculations of CN x thin films, showing that nitrogen incorporation to the graphitic basal plane reduces the elastic strain energy for curving the graphitic layer. Enhanced CNT growth by nitrogen incorporation is thus due to a decrease in the activation energies required for nucleation and growth of the tubular graphitic layer.

Influence of attached bacteria and biofilm on double-layer capacitance during biofilm monitoring by electrochemical impedance spectroscopy.

Development of an effective strategy for biofilm control in water-related system has become a matter of significant concern nowadays. Electrochemical monitoring, especially electrochemical impedance spectroscopy (EIS), is one of the efficient approaches to dealing with biofilm-related issues. However, currently used EIS methods without a redox probe intend to detect all effects generated from media components, bacteria, and bacterial metabolites, which used to make the signals from the attached bacteria and biofilm weakened. In this study, we tried improved EIS measurement to monitor bacterial adhesion and biofilm maturation using a double-layer capacitance. In this improved method, we minimized background signal by subtracting the interference of electrolyte caused by bacterial metabolism. Pseudomonas aeruginosa PA14 wild type and wspF mutant that form the biofilm of distinct nature were used for the model strains to test our method. During bacterial adhesion and biofilm maturation, EIS data were collected and equivalent circuit analysis was carried out to obtain constant phase element (CPE) values representing double-layer capacitance. Since the influence by the bacterial growth-related culture media condition was eliminated by adopting fresh electrolyte at the measurement, the contribution of attached bacteria and biofilm was exclusively measured. As a result, the bacterial adhesion at the early stage of biofilm development was specifically monitored from reduction in double-layer capacitance. Particularly, the plateau in double-layer capacitance appeared upon biofilm maturation, indicating that biofilm maturation could be expected beyond this point. In conclusion, this study found that measurement of double-layer capacitance based on EIS could provide a monitoring parameter suggesting bacterial adhesion and the initiation point of biofilm maturation.

Irradiation Effects of High-Energy Proton Beams on MoS2 Field Effect Transistors

We investigated the effect of irradiation on molybdenum disulfide (MoS2) field effect transistors with 10 MeV high-energy proton beams. The electrical characteristics of the devices were measured before and after proton irradiation with fluence conditions of 10(12), 10(13), and 10(14) cm(-2). For a low proton beam fluence condition of 10(12) cm(-2), the electrical properties of the devices were nearly unchanged in response to proton irradiation. In contrast, for proton beam fluence conditions of 10(13) or 10(14) cm(-2), the current level and conductance of the devices significantly decreased following proton irradiation. The electrical changes originated from proton-irradiation-induced traps, including positive oxide-charge traps in the SiO2 layer and trap states at the interface between the MoS2 channel and the SiO2 layer. Our study will enhance the understanding of the influence of high-energy particles on MoS2-based nanoelectronic devices.

An efficient wavelet-based compression method for volume rendering

Since volume rendering needs a lot of computation time and memory space, many researches have been suggested for accelerating rendering or reducing data size using compression techniques. However, there is little progress in a research for accomplishing these goals. This paper presents an efficient wavelet-based compression method providing fast visualization of large volume data, which is divided into individual blocks with regular resolution. Wavelet transformed block is runlength encoded in accordance with the reconstruction order resulting in a fairly good compression ratio and fast reconstruction. A cache data structure is designed to speed up the reconstruction, and an adaptive compression scheme is proposed to produce a higher quality rendered image. The compression method proposed here is combined with several accelerated volume rendering algorithms, such as brute-force volume rendering with min-max table and Lacroutes shear-warp factorization. Experimental results have shown the space requirement to be about 1/27 and the rendering time to be about 3 seconds for 512/spl times/512/spl times/512 data sets while preserving the quality of an image much like using the original data.

Gate-bias stress-dependent photoconductive characteristics of multi-layer MoS2 field-effect transistors

We investigated the photoconductive characteristics of molybdenum disulfide (MoS2) field-effect transistors (FETs) that were fabricated with mechanically exfoliated multi-layer MoS2 flakes. Upon exposure to UV light, we observed an increase in the MoS2 FET current because of electron-hole pair generation. The MoS2 FET current decayed after the UV light was turned off. The current decay processes were fitted using exponential functions with different decay characteristics. Specifically, a fast decay was used at the early stages immediately after turning off the light to account for the exciton relaxation, and a slow decay was used at later stages long after turning off the light due to charge trapping at the oxygen-related defect sites on the MoS2 surface. This photocurrent decay phenomenon of the MoS2 FET was influenced by the measurement environment (i.e., vacuum or oxygen environment) and the electrical gate-bias stress conditions (positive or negative gate biases). The results of this study will enhance the understanding of the influence of environmental and measurement conditions on the optical and electrical properties of MoS2 FETs.

Nanoscale manipulation of tetrahedral amorphous carbon films

Abstract The outstanding mechanical, chemical and tribological properties of tetrahedral amorphous carbon (ta-C) films have attracted much attention. For a wide variety of applications, a great effort should be focused on the nanoscale structure control. In the present work, we have adopted a novel technique for nanoscale manipulation of ta-C films; incorporating nano Ni dots at the interface between the ta-C film and the substrate. For Ni dot pretreatment, the Ni thin film was deposited and annealed prior to the ta-C films deposition. TEM and Raman spectrum analysis shows that the nano Ni dots at the interface between the film and the substrate results in nanoscale graphitic phase embedded in hard ta-C matrix. Mechanical and electrical properties were strongly dependent on the changes of the Ni-induced second phase. The reduction of mechanical properties and electrical resistivity with increasing the size of Ni dots could be understood in view of the local increase of sp 2 bonds in hard ta-C matrix.

Enhancement of photodetection characteristics of MoS2 field effect transistors using surface treatment with copper phthalocyanine

Recently, two-dimensional materials such as molybdenum disulfide (MoS2) have been extensively studied as channel materials for field effect transistors (FETs) because MoS2 has outstanding electrical properties such as a low subthreshold swing value, a high on/off ratio, and good carrier mobility. In this study, we characterized the electrical and photo-responsive properties of MoS2 FET when stacking a p-type organic copper phthalocyanine (CuPc) layer on the MoS2 surface. We observed that the threshold voltage of MoS2 FET could be controlled by stacking the CuPc layers due to a charge transfer phenomenon at the interface. Particularly, we demonstrated that CuPc/MoS2 hybrid devices exhibited high performance as a photodetector compared with the pristine MoS2 FETs, caused by more electron-hole pairs separation at the p-n interface. Furthermore, we found the optimized CuPc thickness (∼2 nm) on the MoS2 surface for the best performance as a photodetector with a photoresponsivity of ∼1.98 A W(-1), a detectivity of ∼6.11 × 10(10) Jones, and an external quantum efficiency of ∼12.57%. Our study suggests that the MoS2 vertical hybrid structure with organic material can be promising as efficient photodetecting devices and optoelectronic circuits.

Effects of Mesh Density on Static Load Performance of Metal Mesh Gas Foil Bearings

Metal mesh materials have been used successfully in vibration isolators and bearing dampers due to their superior friction or hysteresis damping mechanism. These materials are formed to metal mesh (or wire mesh) structures in ring-shape by compressing a weave of metal wires, in general. Recently, oil-free rotating machinery implement metal mesh structures into hydrodynamic gas foil bearings by replacing bump strip layers with them, to increase its bearing structural damping. A metal mesh foil bearing (MMFB) consists of a top foil and support elastic metal mesh pads installed between a rotating shaft and a housing. The present research presents load capacity tests of a MMFB at rotor rest (0 rpm) and 30 krpm for three metal mesh densities of 13.1%, 23.2%, and 31.6%. The metal mesh pad of test MMFB is made using a stainless steel wire with a diameter of 0.15 mm. Test rig comprises a rigid rotor with a diameter of 60 mm supported on two ball bearings at both ends and test MMFB with an axial length of 50 mm floats on the rotor. Static loads is provided with a mechanical loading device on test MMFB and a strain gauge type load cell measures the applied static loads. A series of static load versus deflection tests were conducted for selected metal mesh densities at rest (0 rpm). Test data are compared to further test results of static load versus journal eccentricity recorded at the rotor speed of 30 krpm. Test data show a strong nonlinearity of bearing deflection (journal eccentricity) with static load, independent of rotor spinning. Observed hysteresis loops imply significant structural damping of test MMFB. Measured journal deflections at 0 rpm are in similar trend to recorded journal eccentricities at the finite rotor speed; thus implying that the MMFB performance depends mainly on the metal mesh structures. The paper also estimates linearlized stiffness coefficient and damping loss factor of test MMFB using the measured static load versus deflection test data at 0 rpm and 30 krpm. The results show that the highest mesh density of 31.6% produces highest linearlized stiffness coefficient and damping loss factor. With rotor spinning at 30 krpm, the linearlized stiffness coefficient and damping loss factor decrease slightly, independent of metal mesh densities. The present test data will serve as a database for benchmarking MMFB predictive models.