Taekyung Lim

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.

Fully transparent quantum dot light-emitting diode integrated with graphene anode and cathode.

A fully transparent quantum dot light-emitting diode (QD-LED) was fabricated by incorporating two types (anode and cathode) of graphene-based electrodes, which were controlled in their work functions and sheet resistances. Either gold nanoparticles or silver nanowires were inserted between layers of graphene to control the work function, whereas the sheet resistance was determined by the number of graphene layers. The inserted gold nanoparticles or silver nanowires in graphene films caused a charge transfer and changed the work function to 4.9 and 4.3 eV, respectively, from the original work function (4.5 eV) of pristine graphene. Moreover the sheet resistance values for the anode and cathode electrodes were improved from ∼63,000 to ∼110 Ω/sq and from ∼100,000 to ∼741 Ω/sq as the number of graphene layers increased from 1 to 12 and from 1 to 8, respectively. The main peak wavelength, luminance, current efficiency, and optical transmittance of the fully transparent QD-LED integrated with graphene anode and cathode were 535 nm, ∼358 cd/m2, ∼0.45 cd/A, and 70-80%, respectively. The findings of the study are expected to lay a foundation for the production of high-efficiency, fully transparent, and flexible displays using graphene-based electrodes.

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.

Tunable-white-light-emitting nanowire sources.

Tunable-white-light-emitting materials are developed by combining two single-crystal oxide nanowire materials-ZnO and SnO(2)-having different light emissions. The tuning of white-light emission from cool white to warm white is achieved for the first time by adjusting the growth sequence and growth time of the ZnO and SnO(2) nanowires. Combined ZnO:SnO(2) nanowire arrays yield a desired emission color from (0.30, 0.31) to (0.35, 0.37) and a white luminescence of approximately 100 cd m(-2), whose reproducibility can be controlled accurately. These results pave a new way to understand and generate a desired white-light emission, which is a key technology in large-area planar display devices, including flexible and/or transparent display devices.

Direct deposition of aluminum oxide gate dielectric on graphene channel using nitrogen plasma treatment

Deposition of high-quality dielectric on a graphene channel is an essential technology to overcome structural constraints for the development of nano-electronic devices. In this study, we investigated a method for directly depositing aluminum oxide (Al2O3) on a graphene channel through nitrogen plasma treatment. The deposited Al2O3 thin film on graphene demonstrated excellent dielectric properties with negligible charge trapping and de-trapping in the gate insulator. A top-gate-structural graphene transistor was fabricated using Al2O3 as the gate dielectric with nitrogen plasma treatment on graphene channel region, and exhibited p-type transistor characteristics.

Photostable Zn2SnO4 nanowire transistors for transparent displays.

Although oxide nanowires offer advantages for next-generation transparent display applications, they are also one of the most challenging materials for this purpose. Exposure of semiconducting channel areas of oxide nanowire transistors produces an undesirable increase in the photocurrent, which may result in unstable device operation. In this study, we have developed a Zn(2)SnO(4) nanowire transistor that operates stably regardless of changes in the external illumination. In particular, after exposure to a light source of 2100 lx, the threshold voltage (V(th)) showed a negative shift of less than 0.4 V, and the subthreshold slope (SS) changed by ∼0.1 V/dec. ZnO or SnO(2) nanowire transistors, in contrast, showed 1.5-2.0 V negative shift in V(th) and an SS change of ∼0.3 V/dec under the same conditions. Furthermore, the Zn(2)SnO(4) nanowire transistors returned to their initial state immediately after the light source was turned off, unlike those using the other two nanowires. Thus, Zn(2)SnO(4) nanowires achieve photostability without the application of a black material or additional processing, minimizing the photocurrent effect for display devices.

Homogeneous and stable p-type doping of graphene by MeV electron beam-stimulated hybridization with ZnO thin films

In this work, we demonstrate a unique and facile methodology for the homogenous and stable p-type doping of graphene by hybridization with ZnO thin films fabricated by MeV electron beam irradiation (MEBI) under ambient conditions. The formation of the ZnO/graphene hybrid nanostructure was attributed to MEBI-stimulated dissociation of zinc acetate dihydrate and a subsequent oxidation process. A ZnO thin film with an ultra-flat surface and uniform thickness was formed on graphene. We found that homogeneous and stable p-type doping was achieved by charge transfer from the graphene to the ZnO film.

Fabrication of reliable semiconductor nanowires by controlling crystalline structure.

One-dimensional SnO(2) nanomaterials with wide bandgap characteristics are attractive for flexible and/or transparent displays and high-performance nano-electronics. In this study, the crystallinity of SnO(2) nanowires was regulated by controlling their growth temperatures. Moreover, the correlation of the crystallinity of nanowires with optical and electrical characteristics was analyzed. When SnO(2) nanowires were grown at temperatures below 900 °C, they showed various growth directions and abnormal discontinuity in their crystal structures. On the other hand, most nanowires grown at 950 °C exhibited a regular growth trend in the direction of [100]. In addition, the low temperature photoluminescence measurement revealed that the higher growth temperatures of nanowires gradually decreased the 500 nm peak rather than the 620 nm peak. The former peak is derived from the surface defect related to the shallow energy level and affects nanowire surface states. Owing to crystallinity and defects, the threshold voltage range (maximum-minimum) of SnO(2) nanowire transistors was 1.5 V at 850 °C, 1.1 V at 900 °C, and 0.5 V at 950 °C, with dispersion characteristics dramatically decreased. This study successfully demonstrated the effects of nanowire crystallinity on optical and electrical characteristics. It also suggested that the optical and electrical characteristics of nanowire transistors could be regulated by controlling their growth temperatures in the course of producing SnO(2) nanowires.

Control of Semiconducting and Metallic Indium Oxide Nanowires

Oxide semiconductors are candidates for chemical sensors, transparent electrodes, and electronic devices. Here, we have investigated metal-to-semiconductor transitions during In(2)O(3) nanowire growth with variations in the O(2) gas rate. Photoluminescence and current-voltage characteristics of In(2)O(3) nanowire transistors have been used to understand the transition behavior. The proportion of metallic nanowires to semiconducting nanowires significantly changes from 80:20 to 25:75 when the O(2) fraction in argon increases from 0.005% to 0.2%. We believe that excessive oxygen vacancies at low O(2) gas rates increase the conductivity and thereby the number of nanowires with metallic characteristics. With an increase in oxygen flow, the oxygen vacancies in the nanowires are substituted with oxygen and the subsequent reduction in oxygen vacancies increases the number of semiconducting nanowires. The threshold voltage of transistors fabricated with semiconducting nanowires shifts in a positive direction by about +3.3 eV between nanowires grown with 0.005% and 0.2% oxygen. The results here indicate that electrical and optical characteristics of oxide nanowires can be controlled by the amount of oxygen during growth instead of relying on conventional postgrowth high-temperature annealing or other postprocessing techniques.

Threshold voltage control of oxide nanowire transistors using nitrogen plasma treatment

In developing complementary metal-oxide semiconductor logic circuits using N-type semiconducting nanowires, threshold voltage (Vth) control is crucial because the driving voltage should be established in relation to the Vth dispersion of transistors. In this study, using N2 plasma treatment, positive shifts of SnO2 nanowire transistor (NWT) devices were produced as desired without degrading the nanowire surfaces. The NWT devices exhibited positive Vth shifts of ∼0.9 and ∼1.5 V and decreases in on-currents of ∼20% and ∼40% at plasma source powers of 200 and 400 W, respectively, without any changes in subthreshold slope or off-current. The positive Vth shifts and decreases in on-current can be explained by assuming that nitrogen ions (N3−) filled in for the existing oxygen vacancies (Vo, Vo+, and Vo++) and that consequently, the amount of oxygen vacancies playing a role in electron-trapping decreased.