Jong-Myeong Jeon

Two-dimensional transition metal dichalcogenide nanomaterials for solar water splitting

Recently, 2-dimensional (2D) transition metal dichalcogenides (TMDs) have received great attention for solar water splitting and electrocatalysis. In addition to their wide variety of electronic and microstructural properties, their promising catalytic activities for hydrogen production make 2D TMDs as earth-abundant and inexpensive catalysts that can replace noble metals. This paper reviews the electronic, structural, and optical properties of 2D TMDs. We highlight the various synthetic methods for 2D TMDs and their applications in hydrogen evolution based on photoelectrochemical and electrocatalytic cells. We also discuss perspectives and challenges of 2D TMDs for hydrogen production and artificial photosynthesis.

Structural and optical characteristics of GaN/ZnO coaxial nanotube heterostructure arrays for light-emitting device applications

We report on the structural and optical characteristics of position-controlled GaN/ZnO coaxial nanotube heterostructure and GaN/InxGa1−xN coaxial nanotube quantum structure arrays for light-emitting diode (LED) applications. The GaN/ZnO nanotube heterostructures were fabricated by growing a GaN layer on the entire surface of position-controlled ZnO nanotube arrays using low-pressure metal-organic vapour-phase epitaxy. As determined by transmission-electron microscopy (TEM), an abrupt and coherent interface between the core ZnO and the GaN overlayer was observed. The optical characteristics of heteroepitaxial GaN/ZnO nanotube heterostructures were also investigated using cathodoluminescence (CL) spectroscopy. This position-controlled growth of high-quality single crystalline GaN/ZnO coaxial nanotube heterostructures allowed the fabrication of artificial arrays of high-quality GaN-based coaxial quantum structures by the heteroepitaxial growth of GaN/InxGa1−xN multiple quantum wells along the circumference of the GaN/ZnO nanotubes. The optical and structural characteristics of the position-controlled GaN/InxGa1−xN coaxial nanotube quantum structures were investigated by using CL spectroscopy and TEM analysis, respectively. The green LED microarrays were successfully fabricated by the controlled heteroepitaxial coaxial coatings of GaN/InxGa1−xN coaxial nanotube quantum structures and the outermost Mg-doped p-type GaN layer onto the GaN/ZnO coaxial nanotube heterostructures, presumably implying that the position-controlled growth of high-quality GaN/ZnO coaxial nanotube heterostructure arrays provides a general and rational route of integrating vertical nanodevices for nanoscale electronics and optoelectronics.

Synthesis of Atomically Thin Transition Metal Disulfides for Charge Transport Layers in Optoelectronic Devices

UNLABELLED Metal sulfides (MeS2) such as MoS2 and WS2 were used as charge transport layers in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells in order to enhance the stability in air comparing to poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT PSS). MeS2 layers with a polycrystalline structure were synthesized by a chemical deposition method using uniformly spin-coated (NH4)MoS4 and (NH4)WS4 precursor solutions. The ultraviolet-ozone (UV-O3) treatment on MeS2 leads to the removal of the surface contaminants produced by the transfer process, resulting in a uniform surface and an increase of the work function. The maximum luminance efficiencies of the OLEDs with UV-O3-treated MoS2 and WS2 were 9.44 and 10.82 cd/A, respectively. The power conversion efficiencies of OPV cells based on UV-O3-treated MoS2 and WS2 were 2.96 and 3.08%, respectively. These values correspond to over 95% of those obtained with ( PEDOT PSS) based devices. Furthermore, OLEDs and OPV cells based on MeS2 showed two to six times longer stability in air compared with PEDOT PSS based devices. These results suggest that UV-O3-surface-treated MeS2 could be a promising candidate for a charge transport layer in optoelectronic devices.

Vertically ordered SnO2 nanobamboos for substantially improved detection of volatile reducing gases

Vertically ordered SnO2 nanorods with Au nanoparticles deposited in multiple steps, namely, SnO2 nanobamboos are synthesized by a glancing-angle deposition technique. The highly ordered porous structures enable us to detect sub-ppb levels of volatile reducing gases with a fast response speed by maximizing the sensitization effects of the Au nanoparticles.

Selective formation of GaN-based nanorod heterostructures on soda-lime glass substrates by a local heating method

We report on the fabrication of high-quality GaN on soda-lime glass substrates, heretofore precluded by both the intolerance of soda-lime glass to the high temperatures required for III-nitride growth and the lack of an epitaxial relationship with amorphous glass. The difficulties were circumvented by heteroepitaxial coating of GaN on ZnO nanorods via a local microheating method. Metal-organic chemical vapor deposition of ZnO nanorods and GaN layers using the microheater arrays produced high-quality GaN/ZnO coaxial nanorod heterostructures at only the desired regions on the soda-lime glass substrates. High-resolution transmission electron microscopy examination of the coaxial nanorod heterostructures indicated the formation of an abrupt, semicoherent interface. Photoluminescence and cathodoluminescence spectroscopy was also applied to confirm the high optical quality of the coaxial nanorod heterostructures. Mg-doped GaN/ZnO coaxial nanorod heterostructure arrays, whose GaN shell layers were grown with various different magnesocene flow rates, were further investigated by using photoluminescence spectroscopy for the p-type doping characteristics. The suggested method for fabrication of III-nitrides on glass substrates signifies potentials for low-cost and large-size optoelectronic device applications.

A wafer-scale antireflective protection layer of solution-processed TiO2 nanorods for high performance silicon-based water splitting photocathodes

Sustainable and efficient conversion of solar energy to transportable green energy and storable fuels, hydrogen, represents a solution to the energy crisis and reduces the consumption of fossil fuels, which are mainly responsible for the rise in global temperature. Solar water splitting using semiconductors, such as silicon, is promising to satisfy the global energy demand by producing hydrogen molecules. However, the solar to hydrogen conversion efficiency of a silicon photoelectrode is suppressed by overpotential, high reflectance and/or instability in liquid electrolytes. Herein, we report the synthesis of multifunctional solution-processed TiO2 nanorods on a 4-inch p-silicon wafer with controllable heights and diameters for highly efficient water splitting photocathodes. The solution-processed passivation layer of TiO2 nanorods reduces the overpotential of the silicon photocathode due to its catalytic properties. The TiO2 NRs also dramatically improves the light absorption of silicon due to the antireflective ability of the nanorods. The reflectance of silicon is decreased from 37.5% to 1.4% and enhances the saturated photocurrent density. The Pt-decorated (1–2.5 nm diameter) TiO2 nanorods/p-Si photocathodes show a short circuit current density of up to 40 mA cm−2, an open circuit voltage ∼440 mV and incident photon to current conversion efficiency of >90% using 0.5 M H2SO4 electrolyte with simulated 1 sun irradiation. The heterostructure photocathodes are stable for more than 52 h without noticeable degradation and an ideal regenerative cell efficiency of 2.5% is achieved.

ZnO nanostructures with controlled morphologies on a glass substrate

We report morphology-controlled selective growth of ZnO nanostructures on glass substrates by using catalyst-free metal-organic chemical vapor deposition. For the morphology-controlled selective growth, a microheating method using a series of microheaters was developed, which provided well-controlled local heating based on the microheater geometry and spatial arrangement. ZnO nanostructure morphology depended on the local growth temperature, so various nanostructure morphologies were obtained selectively at specific positions on glass substrates by using local microheating. The monolithic integration of nanostructures with different morphologies will have great potential for applications in multifunctional devices.

Position-controlled AlN/ZnO coaxial nanotube heterostructure arrays for electron emitter applications

We studied the fabrication and field-emission characteristics of position-controlled AlN/ZnO nanotube heterostructure arrays. AlN layers with various thicknesses from 20 to 52 nm were deposited coaxially on the position-controlled ZnO nanotube arrays. The field-emission properties of the coaxial AlN/ZnO nanotube arrays were controlled using the AlN thickness and the nanotube interdistance. As compared to the bare ZnO nanotube arrays, the AlN-coated coaxial nanotube arrays exhibited enhanced electron emission, and the optimum AlN coating layer thickness on the nanotube tips was 26 nm. The improved field emission from the coaxial nanotube heterostructures is attributed to the low electron affinity and the thickness modulation effect of the AlN coating layer.

Microscopic Evidence for Strong Interaction between Pd and Graphene Oxide that Results in Metal‐Decoration‐Induced Reduction of Graphene Oxide

Graphene oxide (GO) is reduced spontaneously when palladium nanoparticles are decorated on the surface. The oxygen functional groups at the GO surface near the nanoparticles are absorbed to the palladium to produce a palladium oxide interlayer. Palladium therefore grows on the GO with preferred orientations, resulting in unique microstructural and electrical properties.

Local Crystallization of

Local microheating of amorphous LaB6 film could control the degree of crystallization as determined by spatially resolved Raman spectroscopy and transmission electron microscopy. With full crystallization of the LaB6, we achieved micrometer-sized thermionic electron emission source with the maximum current density of 1.2 A/cm2. The advantage of fabricating a micrometer-sized emitter with high current density enables versatile applications such as compact X-ray or vacuum type terahertz radiation sources. A new structure of micrometer-sized, lateral-type vacuum channel transistor is proposed based on the simulation. The calculated electron travelling time from emitter-to-collector was 8.3 ps at the traveling distance of ~10 μm, meaning that the maximum operation frequency is 120 GHz.