Bok Ki Min

High-mobility ambipolar ZnO-graphene hybrid thin film transistors

In order to combine advantages of ZnO thin film transistors (TFTs) with a high on-off ratio and graphene TFTs with extremely high carrier mobility, we present a facile methodology for fabricating ZnO thin film/graphene hybrid two-dimensional TFTs. Hybrid TFTs exhibited ambipolar behavior, an outstanding electron mobility of 329.7 ± 16.9 cm2/V·s, and a high on-off ratio of 105. The ambipolar behavior of the ZnO/graphene hybrid TFT with high electron mobility could be due to the superimposed density of states involving the donor states in the bandgap of ZnO thin films and the linear dispersion of monolayer graphene. We further established an applicable circuit model for understanding the improvement in carrier mobility of ZnO/graphene hybrid TFTs.

Electrical Double Layer Capacitance in a Graphene-embedded Al2O3 Gate Dielectric.

Graphene heterostructures are of considerable interest as a new class of electronic devices with exceptional performance in a broad range of applications has been realized. Here, we propose a graphene-embedded Al2O3 gate dielectric with a relatively high dielectric constant of 15.5, which is about 2 times that of Al2O3, having a low leakage current with insertion of tri-layer graphene. In this system, the enhanced capacitance of the hybrid structure can be understood by the formation of a space charge layer at the graphene/Al2O3 interface. The electrical properties of the interface can be further explained by the electrical double layer (EDL) model dominated by the diffuse layer.

Direct Determination of Field Emission across the Heterojunctions in a ZnO/Graphene Thin-Film Barristor

Graphene barristors are a novel type of electronic switching device with excellent performance, which surpass the low on-off ratios that limit the operation of conventional graphene transistors. In barristors, a gate bias is used to vary graphenes Fermi level, which in turn controls the height and resistance of a Schottky barrier at a graphene/semiconductor heterojunction. Here we demonstrate that the switching characteristic of a thin-film ZnO/graphene device with simple geometry results from tunneling current across the Schottky barriers formed at the ZnO/graphene heterojunctions. Direct characterization of the current-voltage-temperature relationship of the heterojunctions by ac-impedance spectroscopy reveals that this relationship is controlled predominantly by field emission, unlike most graphene barristors in which thermionic emission is observed. This governing mechanism makes the device unique among graphene barristors, while also having the advantages of simple fabrication and outstanding performance.

Roll-to-Roll Production of Layer-Controlled Molybdenum Disulfide: A Platform for 2D Semiconductor-Based Industrial Applications

A facile methodology for the large-scale production of layer-controlled MoS2 layers on an inexpensive substrate involving a simple coating of single source precursor with subsequent roll-to-roll-based thermal decomposition is developed. The resulting 50 cm long MoS2 layers synthesized on Ni foils possess excellent long-range uniformity and optimum stoichiometry. Moreover, this methodology is promising because it enables simple control of the number of MoS2 layers by simply adjusting the concentration of (NH4 )2 MoS4 . Additionally, the capability of the MoS2 for practical applications in electronic/optoelectronic devices and catalysts for hydrogen evolution reaction is verified. The MoS2 -based field effect transistors exhibit unipolar n-channel transistor behavior with electron mobility of 0.6 cm2 V-1 s-1 and an on-off ratio of ≈10³. The MoS2 -based visible-light photodetectors are fabricated in order to evaluate their photoelectrical properties, obtaining an 100% yield for active devices with significant photocurrents and extracted photoresponsivity of ≈22 mA W-1 . Moreover, the MoS2 layers on Ni foils exhibit applicable catalytic activity with observed overpotential of ≈165 mV and a Tafel slope of 133 mV dec-1 . Based on these results, it is envisaged that the cost-effective methodology will trigger actual industrial applications, as well as novel research related to 2D semiconductor-based multifaceted applications.

Threshold voltage manipulation of ZnO-graphene oxide hybrid thin film transistors via Au nanoparticles doping

In order to fabricate a complementary inverter, precise control of the threshold voltages for n-type semiconductor based thin film transistors (TFTs) is highly required. Here we provided a facile methodology for controlling the threshold voltage of ZnO-based TFTs. Chemically-derived graphene oxide (GO) and Au-decorated GO (Au-GO) flakes were hybridized with solution-processed ZnO thin films to control electron injection determined by the workfunction difference between ZnO and GO or Au-GO. As a result, the threshold voltages for the ZnO, GO/ZnO, and Au-GO/ZnO TFTs were 24 ± 3 V, −11 ± 4 V, and 63 ± 5 V, respectively, which determine depletion or enhancement mode TFTs without any significant change in the field effect mobility and on/off ratio.

High Durability and Waterproofing rGO/SWCNT-Fabric-Based Multifunctional Sensors for Human-Motion Detection

Wearable strain-pressure sensors for detecting electrical signals generated by human activities are being widely investigated because of their diverse potential applications, from observing human motion to health monitoring. In this study, we fabricated reduced graphene oxide (rGO)/single-wall carbon nanotube (SWCNT) hybrid fabric-based strain-pressure sensors using a simple solution process. The structural and chemical properties of the rGO/SWCNT fabrics were characterized using scanning electron microscopy (SEM), Raman, and X-ray photoelectron spectroscopy (XPS). Complex networks containing rGO and SWCNTs were homogeneously formed on the cotton fabric. The sensing performance of the devices was evaluated by measuring the effects of bending strain and pressure. When the CNT content was increased, the change in relative resistance decreased, while durability was significantly improved. The rGO/SWCNT (0.04 wt %) fabric sensor showed particularly high mechanical stability and flexibility during 100 000 bending tests at the extremely small bending radius of 3.5 mm (11.6% bending strain). Moreover, the rGO/SWCNT fabric device exhibited excellent water resistant properties after 10 washing tests due to its hydrophobic nature. Finally, we demonstrated a fabric-sensor-based motion glove and confirmed its practical applicability.

AC-Impedance Spectroscopic Analysis on the Charge Transport in CVD-Grown Graphene Devices with Chemically Modified Substrates

A comprehensive study for the effect of interfacial buffer layers on the electrical transport behavior in CVD-grown graphene based devices has been performed by ac-impedance spectroscopy (IS) analysis. We examine the effects of the trap charges at graphene/SiO2 interface on the total capacitance by introducing self-assembled monolayers (SAMs). Furthermore, the charge transports in the polycrystalline graphene are characterized through the temperature-dependent IS measurement, which can be explained by the potential barrier model. The frequency-dependent conduction reveals that the conductivity of graphene is related with the mobility, which is limited by the scattering caused by charged adsorbates on SiO2 surface.

Fabrication of three-layered ZnO-CuO-SnO2 hybrid nanostructures n-p-n hetero-junctions

ABSTRACT Three-layered ZnO-CuO-SnO2 hybrid nanostructures n-p-n hetero-junctions were synthesized for the first time by 3-step chemical vapor deposition on SiO/Si substrates. Starting materials made of Cu, Zn and Sn powder mixed with carbon charcoal were heated at various temperatures and pressures under atmosphere of O2 and Ar. The prepared samples were investigated by field emission scanning electron microscope (FESEM), X-rays photoemission spectroscopy (XPS), and I-V characteristic curves. The results reveals the formation of CuO hollow microspheres with branching lines, ZnO nanowires, and SnO2 nanorods in which establish the three layers of the hetero-junctions. I-V characteristics exhibit rectifying behavior of hetero-layered semiconductors.

Highly Sensitive and Flexible Strain–Pressure Sensors with Cracked Paddy-Shaped MoS2/Graphene Foam/Ecoflex Hybrid Nanostructures

Three-dimensional graphene porous networks (GPNs) have received considerable attention as a nanomaterial for wearable touch sensor applications because of their outstanding electrical conductivity and mechanical stability. Herein, we demonstrate a strain-pressure sensor with high sensitivity and durability by combining molybdenum disulfide (MoS2) and Ecoflex with a GPN. The planar sheets of MoS2 bonded to the GPN were conformally arranged with a cracked paddy shape, and the MoS2 nanoflakes were formed on the planar sheet. The size and density of the MoS2 nanoflakes were gradually increased by raising the concentration of (NH4)2MoS4. We found that this conformal nanostructure of MoS2 on the GPN surface can produce improved resistance variation against external strain and pressure. Consequently, our MoS2/GPN/Ecoflex sensors exhibited noticeably improved sensitivity compared to previously reported GPN/polydimethylsiloxane sensors in a pressure test because of the existence of the conformal planar sheet of MoS2. In particular, the MoS2/GPN/Ecoflex sensor showed a high sensitivity of 6.06 kPa-1 at a (NH4)2MoS4 content of 1.25 wt %. At the same time, it displayed excellent durability even under repeated loading-unloading pressure and bending over 4000 cycles. When the sensor was attached on a human temple and neck, it worked correctly as a drowsiness detector in response to motion signals such as neck bending and eye blinking. Finally, a 3 × 3 tactile sensor array showed precise touch sensing capability with complete isolation of electrodes from each other for application to touch electronic applications.

Synthesis and Investigation of MoO3 Microfilms and Nanorods by Thermal Chemical Vapor Deposition

MoO3 microfilms and nanorods can be synthesized by using the powder of MoS2 as starting materials by thermal chemical vapor deposition. The prepared products on substrates were investigated by field emission scanning electron microscope (FESEM), X-ray photoemission spectroscopy (XPS) and Raman spectroscopy. FESEM images showed the uniformly microfilms and nanorods-like with diameter around 50-100 nm and length of about through 5 μm, respectively. XPS patterns and Raman shifts revealed the prepared products consisting of MoO3 structure phases.