Jae Min Myoung

Assembled Monolayers of Hydrophilic Particles on Water Surfaces

A facile and quick approach to prepare self-assembled monolayers of water-dispersible particles on the water surface is presented. Particle suspensions in alcohols were dropped on a water reservoir to form long-range ordered monolayers of various particles, including spherical solid particles, soft hydrogel particles, metal nanoparticles, quantum dots, nanowires, single-wall carbon nanotubes (SWCNTs), nanoplates, and nanosheets. A systematic study was conducted on the variables affecting the monolayer assembly: the solubility parameter of spreading solvents, particle concentration, zeta potential of the particles in the suspension, surface tension of the water phase, hardness of the particles, and addition of a salt in the suspension. This method requires no hydrophobic surface treatment of the particles, which is useful to exploit these monolayer films without changing the native properties of the particles. The study highlights a quick 2D colloidal assembly without cracks in the wafer scale as well as transparent conductive thin films made of SWCNTs and graphenes.

ZnO single nanowire-based UV detectors

In this report, ZnO single nanowire (NW)-based devices were fabricated on the same nanowire by e-beam lithography so that both sides had Ohmic contact and one side had Schottky contact. Information about the mechanism for low-power UV detection by these devices was unambiguously provided by I-V measurements. Adsorption and desorption of oxygen molecules at the NW surface are responsible for the UV detection by the device with Ohmic contacts on both sides. Barrier height modulations and interface states are responsible for UV detection by the device with Schottky contact on one side.

Effects of the channel thickness on the structural and electrical characteristics of room-temperature fabricated ZnO thin-film transistors

We report the fabrication and characteristics of ZnO thin-film transistors (TFTs) having different channel thicknesses. The ZnO films were deposited as active channel layers on SiO2/p-Si substrates by rf magnetron sputtering at room temperature. Effects of the channel thickness on the structural and electrical properties of ZnO TFTs using a bottom-gate configuration were investigated. The crystalline quality and channel conductance of the ZnO films were enhanced as the channel thickness increased. The ZnO TFT with the optimized channel thickness exhibited enhancement mode characteristics with the threshold voltage of 9.9 V, the on-to-off current ratio of ~105 and the field-effect mobility of 0.1 cm2 V−1 s−1. This research implies that ZnO TFTs produced by a simple and low-cost technique could be applicable to electronic devices.

Biomimetic hierarchical ZnO structure with superhydrophobic and antireflective properties

A two step method, with a combination of top-down and bottom-up approaches, was developed for the fabrication of ZnO based hierarchical structures with nanorods on microcraters. A layer of well c-axis aligned, transparent, conductive ZnO thin film was deposited by pulsed DC sputtering on a Corning glass substrate. The microcraters were created with anisotropic etching on the as-deposited ZnO thin film. ZnO nanorods were then synthesized onto the etched film by means of metal organic chemical vapor deposition. The resulting hierarchical film exhibits a high water contact angle (>160 degrees) with a low contact angle hysteresis (2 degrees) and low reflection over a wide spectral range. This biomimetic material may find potential applications in many industrial fields, e.g., self-cleaning, solar cells, displays.

Junction properties of Au/ZnO single nanowire Schottky diode

In this study, we have analyzed the Au/ZnO single nanowire based Schottky diode by investigating temperature dependent current voltage and x-ray photoelectron spectroscopy (XPS) measurements. The calculated barrier height of the Schottky diodes by using the thermionic emission model is in good agreement with the value obtained from the XPS measurements but lower than the theoretically predicted value. The ionization of interface states has been considered for explaining this discrepancy.

Magnetic alignment of ZnO nanowires for optoelectronic device applications

Ni caps on ZnO nanowires were synthesized to control the direction and position of these nanowires using magnetism. Vertically ordered ZnO nanowires were grown 15μm in length by metal organic chemical vapor deposition and then ferromagnetic Ni was capped on top of the nanowires by sputtering. The synthesized nanowires were aligned at the edge of the magnetized Ni patterns by applying a magnetic field. The density of the aligned nanowires depended on the magnetic force of the Ni patterns. Nanowire-based ultraviolet sensors were demonstrated using the magnetic alignment method.

Binder‐Free and Full Electrical‐Addressing Free‐Standing Nanosheets with Carbon Nanotube Fabrics for Electrochemical Applications

Inorganic functional nanosheets (NSs), which are unilamellar nanoscale building blocks having a thickness on the order of nanometers with lateral dimensions of sub-micrometers, are characterized by their ultra-large surface area per volume, stoichiometrically well-defi ned chemical composition, single crystalline structure with high crystallinity, and unique physicochemical properties. [ 1 , 2 ] Therefore, they have been considered excellent nanomaterials that provide superior performance in various applications such as catalysts, [ 3 ] sensors, [ 4 ] die-sensitized solar cells, [ 5 , 6 ] super-capacitors, [ 7 ] batteries [ 8 ] and fuel cells. [ 9 ]

Programmable Direct-Printing Nanowire Electronic Components

In order for recently developed advanced nanowire (NW) devices(1-5) to be produced on a large scale, high integration of the separately fabricated nanoscale devices into intentionally organized systems is indispensible. We suggest a unique fabrication route for semiconductor NW electronics. This route provides a high yield and a large degree of freedom positioning the device on the substrate. Hence, we can achieve not only a uniform performance of Si NW devices with high fabrication yields, suppressing device-to-device variation, but also programmable integration of the NWs. Here, keeping pace with recent progress of direct-writing circuitry,(6-8) we show the flexibility of our approach through the individual integrating, along with the three predesigned N-shaped sites. On each predesigned site, nine bottom gate p-type Si NW field-effect transistors classified according to their on-current level are programmably integrated.

Dual-Gate InGaZnO Thin-Film Transistors with Organic Polymer as a Dielectric Layer

Dual-gated (DG) thin-film transistors (TFTs) with an amorphous InGaZnO (IGZO) channel are fabricated using a poly(4-vinyl phenol) polymer as a dielectric layer. Compared to single-gated (SG) devices, DG devices showed much stronger gate controllability and greatly enhanced device performance over conventional SG TFTs. Although all devices exhibited a positive V th shift under positive bias stress, the highly stable V th shift of 0.17 V was observed for the IGZO TFT with DG structure. It is demonstrated that DG operation is an appropriate gate configuration to produce high-performance TFTs, which is applicable to low-power devices.

Influence of surface morphology on the optical property of vertically aligned ZnO nanorods

In this letter we have studied an influence of surface morphology on the optical property of vertically aligned ZnO nanorods. At low temperature the near band edge excitonic emission shows a strong dependence on surface morphology. A prominent and well resolved near band edge photoluminescence (PL) peak was obtained for nanowires with decreasing diameter and thus assigned due to the contributions to the optical properties of individual nanorods. Depending on surface morphology, the difference in low temperature PL property is attributed to the tailing of the density of states due to the potential fluctuations in randomly distributed intrinsic defects.