Jaehyun Hur

Modulation of the Dirac point voltage of graphene by ion-gel dielectrics and its application to soft electronic devices.

We investigated systematic modulation of the Dirac point voltage of graphene transistors by changing the type of ionic liquid used as a main gate dielectric component. Ion gels were formed from ionic liquids and a non-triblock-copolymer-based binder involving UV irradiation. With a fixed cation (anion), the Dirac point voltage shifted to a higher voltage as the size of anion (cation) increased. Mechanisms for modulation of the Dirac point voltage of graphene transistors by designing ionic liquids were fully understood using molecular dynamics simulations, which excellently matched our experimental results. It was found that the ion sizes and molecular structures play an essential role in the modulation of the Dirac point voltage of the graphene. Through control of the position of their Dirac point voltages on the basis of our findings, complementary metal-oxide-semiconductor (CMOS)-like graphene-based inverters using two different ionic liquids worked perfectly even at a very low source voltage (V(DD) = 1 mV), which was not possible for previous works. These results can be broadly applied in the development of low-power-consumption, flexible/stretchable, CMOS-like graphene-based electronic devices in the future.

A recyclable, recoverable, and reformable hydrogel-based smart photocatalyst

We present a novel hydrogel-based photocatalyst that contains TiO2 nanoparticles (NPs) uniformly dispersed in an agarose hydrogel matrix. The preparation of the TiO2/agarose hybrid gel photocatalyst was based on the gelation of agarose in the presence of well-dispersed TiO2 NPs in hot water. TiO2 NPs were homogeneously distributed in the agarose gel as characterized by Fourier transform infrared spectroscopy (FT-IR) analysis. It was found that the size, uniformity, and concentration of the hybrid gel as well as the contents of constituent ingredients have significant effects on the photocatalytic activity. The smaller and uniform size of the hybrid gel at an appropriate concentration exhibited superior photocatalytic performance in both photodegradation of methylene blue (MB) under UV light and TiO2 leakage. In the moderate concentration range of TiO2 and agarose, the degradation rate of MB increased upon increasing the TiO2 content or decreasing the agarose concentration. Under the optimized conditions, our hybrid gel showed excellent recycling performance over repeated use. Furthermore, we demonstrate the additional excellent features of our hybrid gel, which are: i) regeneration of pure TiO2 NPs and ii) thermal reconstruction of the hybrid gel. During the recycling, the TiO2 NPs initially immobilized in the hydrogel could be recovered through the programmed heating and separation techniques. Also, our hybrid gel could be easily reshaped into a new hydrogel with a desired architecture in terms of its size and shape. These unprecedented properties make our hybrid gel a smart and cost-effective new promising material for use in practical waste/water treatment.

MoS2–TiC–C Nanocomposites as New Anode Materials for High-Performance Lithium-Ion Batteries

The MoS₂-TiC-C nanocomposite was prepared by high-energy ball milling (HEBM) for application as a new anode material for lithium-ion batteries. Pure molybdenum disulfide (MoS₂), Ti, and carbon black (C) were used as the starting materials for the synthesis process. Various analyses including X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) revealed that the nanosized MoS₂ active materials were uniformly dispersed in a TiC-C matrix formed via the HEBM process. We investigated the cyclic performance, rate capability, and electrochemical impedance spectra using the as-prepared composite as an anode material. The results showed that the electrochemical performances of the MoS₂-based nanocomposite were significantly improved compared to those of MoS₂-C and MoS₂ due to the presence of the TiC-C matrix in MoS₂. Furthermore, we determined the optimal milling time based on the cyclic performances of the materials.

Comparative Study of Mechanically Milled MoS2 and MoSe2 in Graphite Matrix as Anode Materials for High-Performance Lithium-Ion Batteries

Nanocomposites of MoS2/graphite and MoSe2/graphite were formed from two-dimensional materials (MoS2 and MoSe2) and graphite using a one-step ball-milling method (high energy mechanical milling, HEMM). As anode materials for lithium-ion batteries (LIBs), these nanocomposites showed higher specific capacity and greater stability during long cyclic operation compared to their pure counterparts (MoS2 and MoSe2). X-ray diffraction and transmission electron microscopy revealed that graphite nanoflakes were effectively exfoliated and covered MoS2 or MoSe2 layers to form homogeneous nanostructures via HEMM. As a result, the electrochemical performances of both MoS2/graphite and MoSe2/graphite were excellent; the specific capacities were as high as 684.8 (MoS2/graphite) and 787.3 mAh g-1 (MoSe2/graphite) after 100 cycles. Also, when compared with MoS2/graphite, the MoSe2/graphite nanocomposite showed higher specific capacity and better rate capability performance due to larger interlayer spacing, leading to fast and facile movement of Liions. Overall, we demonstrate that homogeneous nanocomposites between similar layered materials (MoS2, MoSe2 and graphite) can be easily synthesized via one-step HEMM, which can be used as excellent anode materials for LIBs.

In-Plane Crystallinity Effect on the Unipolar Resistance Switching Behavior of NiO Thin Film.

We report on the resistance switching behavior of high quality NiO thin films grown on Pt(111)/SiOx/Si and Pt(111)/Al2O3 crystals. Polarity independent resistance switching, i.e., unipolar resistance switching exhibited a substrate crystallinity dependence during the resistance switching. The unipolar resistance switching was observed commonly in NiO film grown on both substrates. High resistance state of NiO thin film without in-plane crystallinity showed higher resistance than that of NiO films with in-plane crystallinity. The NiO thin film without in-plane crystallinity also required high set voltages for the resistance switching from high resistance state to low resistance state and showed nonlinear I-V characteristics at high voltage region before the resistance switching.