Jihye Bong

Metamaterial Absorber for Electromagnetic Waves in Periodic Water Droplets.

Perfect metamaterial absorber (PMA) can intercept electromagnetic wave harmful for body in Wi-Fi, cell phones and home appliances that we are daily using and provide stealth function that military fighter, tank and warship can avoid radar detection. We reported new concept of water droplet-based PMA absorbing perfectly electromagnetic wave with water, an eco-friendly material which is very plentiful on the earth. If arranging water droplets with particular height and diameter on material surface through the wettability of material surface, meta-properties absorbing electromagnetic wave perfectly in GHz wide-band were shown. It was possible to control absorption ratio and absorption wavelength band of electromagnetic wave according to the shape of water droplet–height and diameter– and apply to various flexible and/or transparent substrates such as plastic, glass and paper. In addition, this research examined how electromagnetic wave can be well absorbed in water droplets with low electrical conductivity unlike metal-based metamaterials inquiring highly electrical conductivity. Those results are judged to lead broad applications to variously civilian and military products in the future by providing perfect absorber of broadband in all products including transparent and bendable materials.

Dynamic graphene filters for selective gas-water-oil separation

Selective filtration of gas, water, and liquid or gaseous oil is essential to prevent possible environmental pollution and machine/facility malfunction in oil-based industries. Novel materials and structures able to selectively and efficiently filter liquid and vapor in various types of solutions are therefore in continuous demand. Here, we investigate selective gas-water-oil filtration using three-dimensional graphene structures. The proposed approach is based on the adjustable wettability of three-dimensional graphene foams. Three such structures are developed in this study; the first allows gas, oil, and water to pass, the second blocks water only, and the third is exclusively permeable to gas. In addition, the ability of three-dimensional graphene structures with a self-assembled monolayer to selectively filter oil is demonstrated. This methodology has numerous potential practical applications as gas, water, and/or oil filtration is an essential component of many industries.

Manipulation of graphene work function using a self-assembled monolayer

We report an effective and reliable method to increase the work function of graphene to as high as 5.50 eV by applying a self-assembled monolayer on its surface. The work function of pristine graphene (4.56 eV) was increased by approximately +0.94 eV following trichlorosilane (HDF-S) self-assembly. This increase in the work function was confirmed by ab initio calculations. HDF-S self-assembled graphene exhibited no significant changes in structural, optical, or electrical characteristics compared with pristine graphene. In addition, we verified that the modified work function of HDF-S self-assembled graphene was not affected by the underlying substrates.

Wettability of graphene-laminated micropillar structures

The wetting control of graphene is of great interest for electronic, mechanical, architectural, and bionic applications. In this study, the wettability of graphene-laminated micropillar structures was manipulated by changing the height of graphene-laminated structures and employing the trichlorosilane (HDF-S)-based self-assembly monolayer. Graphene-laminated micropillar structures with HDF-S exhibited higher hydrophobicity (contact angle of 129.5°) than pristine graphene thin film (78.8°), pristine graphene-laminated micropillar structures (97.5°), and HDF-S self-assembled graphene thin film (98.5°). Wetting states of the graphene-laminated micropillar structure with HDF-S was also examined by using a urea solution, which flowed across the surface without leaving any residues.

Controlled three-dimensional interconnected capillary structures for liquid repellency engineering

In this study, we investigated the wetting properties of solvents on highly periodic, porous substrates which have five different layer thicknesses (1, 3, 5, 9 and 13 layers) of three-dimensional (3D) nanoshell structured TiO2 with interconnected capillary spaces. After phosphonic acid (HDF-PA) self-assembly, contact angles of 3D nanoshell structured TiO2 enhanced with increasing number of layers but saturated from 9 layers and up. The omniphobic properties with different types of liquids—deionized water, dextrose, saline, and amino acid injection—were demonstrated by using the 3D-nanoshell-structured TiO2 with 9 layers.

Highly Stable Operation of Metal Oxide Nanowire Transistors in Ambient Humidity, Water, Blood, and Oxygen

The capability for robust operation of nanoscale transistors under harsh environments is equally important as their operating parameters such as high on-currents, high mobility, and high sensing selectivity. For electronic/biomedical applications, in particular, transistor operation must be stable under diverse conditions including ambient humidity, water, blood, and oxygen. Here we demonstrate the use of a self-assembled monolayer of octadecylphosphonic acid (OD-PA) to passivate a functionalized nanowire transistor, allowing the device to operate consistently in such environments. In contrast, without passivation, the characteristics (especially the threshold voltage) of identical nanowire transistors were dramatically altered under these conditions. Furthermore, the OD-PA-passivated transistor shows no signs of long-term stability deterioration and maintains equally high sensing selectivity to light under the harsh environments because of OD-PAs optical transparency. These results demonstrate the suitability of OD-PA passivation methods for fabricating commercial nanoelectronics.

Nanowire-based ternary transistor by threshold-voltage manipulation

We report on a ternary device consisting of two nanowire channels that have different threshold voltage (Vth) values and show that three current stages can be produced. A microscale laser-beam shot was utilized to selectively anneal the nanowire channel area to be processed, and the amount of Vth shift could be controlled by adjusting the laser wavelength. Microscale laser annealing process could control Vth of the individual nanowire transistors while maintaining the other parameters the constant, such as the subthreshold slope, on–off current ratio, and mobility. This result could provide a potential for highly integrated and high-speed ternary circuits.

Fabrication of Highly Transparent Nanowire Transistors with One-Step-Processed Graphene Gate-Source-Drain Electrodes

We report the fabrication of a highly transparent nanowire transistor using graphene as the gate and source–drain electrodes. Graphene gate–source–drain electrodes were simultaneously formed by a single-step transfer process. The graphene electrode and the nanowire channel exhibited near-ohmic contact characteristics. The threshold voltage, subthreshold slope, and mobility of the fabricated top-gate-structural In2O3 nanowire transistor with graphene gate–source–drain electrodes were -4.54 V, 0.43 V/dec, and 78 cm2/(Vs) respectively. The optical transmissions in the region that contained nanowire transistors on the quartz substrate were 88.5–90.3% in the 400–780 nm wavelength range.

Bacteria Repellent Properties of Trichlorosilane Self-Assembled Graphene

The bacteria repellent property and thermal stability of pristine graphene and graphene chemically modified with a trichlorosilane (HDF-S) self-assembled monolayer (SAM) were investigated. The contact angles of HDF-S self-assembled graphene (105.8±0.5°) improved by ~30% compared with those of pristine graphene (79.4±0.9°). In a bacterial atmosphere, while the bacteria were able to migrate to the pristine graphene surface, they were not able to migrate to the surface of the HDF-S self-assembled graphene. Moreover, the HDF-S SAM on graphene showed stable hydrophobic properties from -40 to 500 °C.

Development of omniphobic behavior in molecular self‐assembled monolayer‐coated nanowire forests

The wetting characteristics of self-assembled monolayers (SAMs) on three different surface structures of thin film, microcone array, and nanowire forest topologies, which were chemically modified using phosphonic acid (HDF-PA and OD-PA) and trichlorosilane (HDF-S), were investigated. The molecular SAM-coated nanowire forest structures exhibited superhydrophobic properties with contact angles of 150.6°-155.4°, compared with the other structures combined with OD-PA, HDF-PA, and HDF-S SAMs, which displayed contact angles of 99.5°-116.8°. Moreover, the HDF-PA and HDF-S SAM-coated nanowire forest structures showed omniphobic properties for both flat and curved surfaces, irrespective of the substrate form. Four liquid droplets of different viscosities and composition (water, urea solution, oil, and photoresist) slid on the HDF-PA and HDF-S SAM-coated nanowire forest surfaces without leaving any traces. The omniphobic properties of the molecular SAM-coated nanowire forest structures developed in this study could be used for various applications in which their slippery effect is desirable, such as in medical tubes and the interior of pipes.