Woong Choi

Bio: Woong Choi is an academic researcher from Kookmin University. The author has contributed to research in topics: Thin-film transistor & Thin film. The author has an hindex of 25, co-authored 80 publications receiving 4533 citations. Previous affiliations of Woong Choi include Applied Materials & Samsung.

High-mobility and low-power thin-film transistors based on multilayer MoS2 crystals

Abstract: Unlike graphene, the existence of bandgaps (1–2 eV) in the layered semiconductor molybdenum disulphide, combined with mobility enhancement by dielectric engineering, offers an attractive possibility of using single-layer molybdenum disulphide field-effect transistors in low-power switching devices. However, the complicated process of fabricating single-layer molybdenum disulphide with an additional high-k dielectric layer may significantly limit its compatibility with commercial fabrication. Here we show the first comprehensive investigation of process-friendly multilayer molybdenum disulphide field-effect transistors to demonstrate a compelling case for their applications in thin-film transistors. Our multilayer molybdenum disulphide field-effect transistors exhibited high mobilities (>100 cm2 V−1 s−1), near-ideal subthreshold swings (~70 mV per decade) and robust current saturation over a large voltage window. With simulations based on Shockley's long-channel transistor model and calculations of scattering mechanisms, these results provide potentially important implications in the fabrication of high-resolution large-area displays and further scientific investigation of various physical properties expected in other layered semiconductors. Molybdenum disulphide offers some tantalizing advantages over graphene as a material with which to fabricate field-effect transistors. Kimet al. present a comprehensive study of field-effect transistors made from multilayer samples of MoS2and find that they can achieve high carrier mobilities.

High-detectivity multilayer MoS(2) phototransistors with spectral response from ultraviolet to infrared.

Abstract: Phototransistors based on multilayer MoS(2) crystals are demonstrated with a wider spectral response and higher photoresponsivity than single-layer MoS(2) phototransistors. Multilayer MoS(2) phototransistors further exhibit high room temperature mobilities (>70 cm(2) V(-1) s(-1) ), near-ideal subthreshold swings (~70 mV decade(-1) ), low operating gate biases (<5 V), and negligible shifts in the threshold voltages during illumination.

Low-Power Flexible Organic Light-Emitting Diode Display Device

Abstract: Demands in extending fl at panel approaches to attain ultra-thin fl exible displays, which are lightweight, portable, and unbreakable for head-up displays, security identifi cation documents, conformable products, and electronic papers are ever increasing. [ 1‐3 ] A typical fl exible display comprises two major parts: i) driving circuitry to switch and address the display device, and ii) a fl exible display device to display an image and enhance outdoor readability. Signifi cant progress has been made in achieving stable rollable or bendable driving circuitry based on flthin fi lm transistors (TFTs), such as oxide transistors based on gallium indium zinc oxide (GIZO) [ 4 ] or hafnium indium zinc oxide (HIZO), [ 5 ] low temperature poly-Si (LTPS) on a plastic substrate (polyimide), [ 6 ] nanotube and nanowire-based transistors, [ 2 , 7 , 8 ] and organic thin fi lm transistors (OTFTs). [ 9 ] On the other hand, challenges to integrate a fl exible display device to realize full-color, low power, and outdoor readability have still not been addressed. Liquid crystal displays (LCDs) are widely used to fabricate commercial displays, but their optical system to switch a light source (backlight unit or light-emitting diode (LED) through a red/ green/blue (RGB) color fi lter) consists of a constant thick layer of liquid crystal molecules aligned between electrodes, and two polarization fi lms having the axes of transmission perpendicular to each other. Bending a LCD causes liquid crystal molecules to deform. The light that passes through the deformed liquid crystal molecules and two surrounding polarizing fi lms with perpendicular polarization axes is distorted causing display malfunction. In comparison, OLEDs do not suffer from such bending malfunctions, which makes OLEDs strong candidates for integration with fl exible electronics to achieve fl exible color displays. Current-generation OLEDs can afford a high performance and fl exibility, but this technology requires a polarization (POL) fi lm to enhance the contrast ratio for outdoor readability, and glass encapsulation to protect the OLED from oxygen and water. The fragile nature of these components limits their utility in fl exible OLED display devices. An advanced material to overcome the fragile components is required to allow the fl exible properties. In order to achieve a highly fl exible OLED display device, the following characteristics are needed: i) a low temperature process to prevent deformation in plastic substrates, ii) a new optical architecture providing both fl exibility and high outdoor readability, iii) a thinner and lighter platform than for current OLED technologies that allows bending and folding, iv) mechanical and electrical stability during repetitive folding, and v) optical reliability without malfunction from an ambient environment, especially water and oxygen.

Enhanced Performance in Polymer Solar Cells by Surface Energy Control

Abstract: Enhanced performance of an inverted-type polymer solar cell is reported by controlling the surface energy of a zinc oxide (ZnO) buffer layer, on which a photoactive layer composed of a polymer:fullerene-derivative bulk heterojunction is formed. With the approach based on a mixed self-assembled monolayer, the surface energy of the ZnO buffer layer can be controlled between 40 mN m − 1 and 70 mN m − 1 with negligible changes in its work function. For the given range of surface energy the power conversion effi ciency increases from 3.27% to 3.70% through enhanced photocurrents. The optimized morphology obtained by surface energy control results in the enhanced photocurrent and transmission electron microscopy analysis verifi es the correlation between the surface energy and the phase morphology of the bulk heterojunction. These results demonstrate that surface energy control is an effective method for further improving the performance of polymer solar cells, with potentially important implications for other organic devices containing an interface between a blended organic active layer and a buffer or an electrode layer.

Two-dimensional Layered MoS2 Biosensors Enable Highly Sensitive Detection of Biomolecules

Abstract: We present a MoS2 biosensor to electrically detect prostate specific antigen (PSA) in a highly sensitive and label-free manner. Unlike previous MoS2-FET-based biosensors, the device configuration of our biosensors does not require a dielectric layer such as HfO2 due to the hydrophobicity of MoS2. Such an oxide-free operation improves sensitivity and simplifies sensor design. For a quantitative and selective detection of PSA antigen, anti-PSA antibody was immobilized on the sensor surface. Then, introduction of PSA antigen, into the anti-PSA immobilized sensor surface resulted in a lable-free immunoassary format. Measured off-state current of the device showed a significant decrease as the applied PSA concentration was increased. The minimum detectable concentration of PSA is 1 pg/mL, which is several orders of magnitude below the clinical cut-off level of ~4 ng/mL. In addition, we also provide a systematic theoretical analysis of the sensor platform – including the charge state of protein at the specific pH level, and self-consistent channel transport. Taken together, the experimental demonstration and the theoretical framework provide a comprehensive description of the performance potential of dielectric-free MoS2-based biosensor technology.

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Abstract: Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

Ultrasensitive photodetectors based on monolayer MoS2.

Abstract: A very sensitive photodector based on molybdenum disulphide with potential for integrated optoelectronic circuits, light sensing, biomedical imaging, video recording or spectroscopy is now demonstrated.

Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

Abstract: Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Abstract: Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Photodetectors based on graphene, other two-dimensional materials and hybrid systems

Abstract: Graphene and other two-dimensional materials, such as transition metal dichalcogenides, have rapidly established themselves as intriguing building blocks for optoelectronic applications, with a strong focus on various photodetection platforms The versatility of these material systems enables their application in areas including ultrafast and ultrasensitive detection of light in the ultraviolet, visible, infrared and terahertz frequency ranges These detectors can be integrated with other photonic components based on the same material, as well as with silicon photonic and electronic technologies Here, we provide an overview and evaluation of state-of-the-art photodetectors based on graphene, other two-dimensional materials, and hybrid systems based on the combination of different two-dimensional crystals or of two-dimensional crystals and other (nano)materials, such as plasmonic nanoparticles, semiconductors, quantum dots, or their integration with (silicon) waveguides