Jongwoo Shin

Effective Liquid-Phase Exfoliation and Sodium Ion Battery Application of MoS2 Nanosheets

Two-dimensional (2D) molybdenum disulfide (MoS2) has been taken much attention for various applications, such as catalyst, energy storage, and electronics. However, the lack of effective exfoliation methods for obtaining 2D materials in a large quantity has been one of the technical barriers for the real applications. We report a facile liquid-phase exfoliation method to improve the exfoliation efficiency for single-layer MoS2 sheets in 1-methyl-2-pyrrolidinone (NMP) with a sodium hydroxide (NaOH) assistant. The concentration of the exfoliated MoS2 nanosheets was greatly improved compared to that achieved with conventional liquid-phase exfoliation methods using NMP solvent. We demonstrate stable operation of sodium-ion battery by using the exfoliated MoS2 and MoS2-rGO composite as anode materials.

Large-Area Single-Layer MoSe2 and Its van der Waals Heterostructures

Layered structures of transition metal dichalcogenides stacked by van der Waals interactions are now attracting the attention of many researchers because they have fascinating electronic, optical, thermoelectric, and catalytic properties emerging at the monolayer limit. However, the commonly used methods for preparing monolayers have limitations of low yield and poor extendibility into large-area applications. Herein, we demonstrate the synthesis of large-area MoSe2 with high quality and uniformity by selenization of MoO3 via chemical vapor deposition on arbitrary substrates such as SiO2 and sapphire. The resultant monolayer was intrinsically doped, as evidenced by the formation of charged excitons under low-temperature photoluminescence analysis. A van der Waals heterostructure of MoSe2 on graphene was also demonstrated. Interestingly, the MoSe2/graphene heterostructures show strong quenching of the characteristic photoluminescence from MoSe2, indicating the rapid transfer of photogenerated charge carriers between MoSe2 and graphene. The development of highly controlled heterostructures of two-dimensional materials will further promote advances in the physics and chemistry of reduced dimensional systems and will provide novel applications in electronics and optoelectronics.

Metal‐Etching‐Free Direct Delamination and Transfer of Single‐Layer Graphene with a High Degree of Freedom

A method of graphene transfer without metal etching is developed to minimize the contamination of graphene in the transfer process and to endow the transfer process with a greater degree of freedom. The method involves direct delamination of single-layer graphene from a growth substrate, resulting in transferred graphene with nearly zero Dirac voltage due to the absence of residues that would originate from metal etching. Several demonstrations are also presented to show the high degree of freedom and the resulting versatility of this transfer method.

Growth mechanisms of thin-film columnar structures in zinc oxide on p-type silicon substrates

X-ray diffraction analysis reveals that the crystallinity of (0001)-oriented columnar grains in ZnO thin films grown on p-Si (100) substrates is enhanced with increasing growth temperature, and transmission electron microscopy confirms that the columnar structures become more stable at higher growth temperature. The morphological evolution of the columnar structure in ZnO thin films is described on the basis of experimental measurements.

Effects of thermal treatment on the formation of the columnar structures in ZnO thin films grown on p-Si (100) substrates

X-ray diffraction patterns showed that crystallinity of the annealed ZnO films was improved by thermal annealing. Transmission electron microscopy images showed that columnar structures were preferentially formed in ZnO thin films due to thermal annealing, and electron energy loss spectroscopy images showed that annealing caused O2 atoms to diffuse out from the upper region in the ZnO thin film. The effects of thermal treatment on the formation of the columnar structures in ZnO thin films grown on Si (100) substrates are described on the basis of the experimental results.

High performance graphene field effect transistors on an aluminum nitride substrate with high surface phonon energy

We demonstrate top-gate graphene field effect transistors (FETs) on an aluminum nitrite (AlN) substrate with high surface phonon energy. Electrical transport measurements reveal significant improvement of the carrier mobility of graphene FETs on AlN compared to those on SiO2. This is attributed to the suppression of surface phonon scattering due to the high surface phonon energy of the AlN substrate. The RF cut-off frequency of the graphene FET is also greatly increased when the AlN substrate is used. AlN can easily be formed on a Si or SiO2 substrate using a standard semiconductor process and thus provides a practical way to improve the performance of graphene FETs.

Irradiation-induced shrinkage and expansion mechanisms of SiO2 circle membrane nanopores.

20 nm diameter SiO(2) nanopore arrays on gradient-thickness membranes were formed by a focused electron beam with in situ transmission electron microscopy (TEM). Nanopore shrinkage was seen in nanopores on thicker membranes, with the rate of diameter change remaining constant during the shrinkage process. In contrast, pore expansion was observed in thinner membranes, with the expansion rate being constant at the initial stage but with a slight increase at the later stage. The geometry model of shrinkage and expansion of the nanopores in relation to the electron irradiation time was investigated by utilizing the TEM tilting method.

Correlation between the atomic structures and the misorientation angles of [0001]-tilt grain boundaries at triple junctions in ZnO thin films grown on Si substrates

The correlation between the atomic structures and the misorientation angles of [0001]-tilt grain boundaries at triple junctions in ZnO thin films grown on Si substrates was investigated by using high-resolution transmission electron microscopy (HRTEM) measurements. The HRTEM images showed three symmetric grain boundaries and one asymmetric grain boundary around the triple junction in the ZnO film. The correlation between the atomic structures and the misorientation angles of the grain boundaries at triple junctions in ZnO films is described on the basis of the HRTEM results.

Atomic arrangement variations of [0001]-tilt grain boundaries in ZnO thin films grown on p-Si substrates due to thermal treatment

The plane-view high-resolution transmission electron microscopy (HRTEM) images in ZnO thin films grown on p-Si substrates showed that (101¯0) asymmetric grain boundaries with a periodic array of strain contrast features existed in a sparse columnar structure for as-grown ZnO thin films and that (112¯0) asymmetric grain boundaries and (8513¯0) symmetric grain boundaries existed in a dense columnar structure for annealed ZnO thin films. The atomic arrangement variations of [0001]-tilt grain boundaries in ZnO thin films grown on Si substrates due to thermal treatment are described on the basis of the HRTEM results.