Ki Chang Kwon

Wafer-scale transferable molybdenum disulfide thin-film catalysts for photoelectrochemical hydrogen production

We demonstrate that wafer-scale, transferable, and transparent thin-film catalysts based on MoS2, which consists of cheap and earth abundant elements, can provide a low onset potential of 1 mA cm−2 at 0.17 V versus a reversible hydrogen electrode and the high photocurrent density of 24.6 mA cm−2 at 0 V for a p-type Si photocathode. c-Domains with vertically stacked (100) planes in the transferable 2H-MoS2 thin films, which are grown via a thermolysis method, act as active sites for the hydrogen evolution reaction, and photogenerated electrons are efficiently transported through the n-MoS2/p-Si heterojunction.

Effect of anions in Au complexes on doping and degradation of graphene

In this study, the role of anions in Au complexes was investigated as a dopant for graphene. Au(OH)3, Au2S, AuBr3, and AuCl3 were used as dopants. The average sheet resistance of the graphene decreased from 950 Ω sq−1 to 820, 600, 530, and 300 Ω sq−1 and the work function increased from 4.3 eV to 4.6, 4.8, 5.0 and 4.9 eV with Au(OH)3, Au2S, AuBr3, and AuCl3 dopants, respectively. However, thermal annealing of graphene increased the sheet resistance and decreased the work function. Furthermore, the G and 2D bands in the Raman spectra were recovered and the Au3+ peak in the Au 4f spectra decreased after thermal annealing, indicating the aggregation of metal particles. These results suggested that the degree of doping was related to the electronegativity of the anion in the Au complex and degradation of graphene was related to the bond strength between the Au cation and the counter anion. Therefore, anions with a high electronegativity and high bond strength are adequate for use as a p-type dopant in graphene.

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.

Role of ionic chlorine in the thermal degradation of metal chloride-doped graphene sheets

The degradation mechanism of graphene sheets doped with metal chloride was investigated as a function of the annealing process. The sheet resistance of doped graphene increased from 500–700 Ω sq−1 to 10 kΩ sq−1 and the transmittance at 550 nm decreased from 95% to 87–91% after annealing at 400 °C. Furthermore, the work function of doped graphene decreased from 4.7–5.1 eV to 4.2–4.5 eV after annealing. The G and 2D band peaks in the Raman spectra were shifted to lower wavenumbers by annealing at 400 °C, regardless of the kind of dopant, and reached nearly the same level as that of the pristine graphene. X-ray photoemission spectroscopy showed that the chlorine anions and chlorine atoms disappeared after thermal annealing. The scanning electron microscopy images revealed the capability of annealing to gather the unstable metal cations, thereby inducing the aggregation of metal particles. The degree of doping of the graphene sheets was strongly related to not only metal cations but also chlorine anions. Therefore, aggregation of metal particles and adsorption of chlorine ions degraded the properties of graphene as a function of annealing temperature.

Enhanced Endurance Organolead Halide Perovskite Resistive Switching Memories Operable under an Extremely Low Bending Radius

It was demonstrated that organolead halide perovskites (OHPs) show a resistive switching behavior with an ultralow electric field of a few kilovolts per centimeter. However, a slow switching time and relatively short endurance remain major obstacles for the realization of the next-generation memory. Here, we report a performance-enhanced OHP resistive switching device. To fabricate topologically and electronically improved OHP thin films, we added hydroiodic acid solution (for an additive) in the precursor solution of the OHP. With drastically improved morphology such as small grain size, low peak-to-valley depth, and precise thickness, the OHP thin films showed an excellent performance as insulating layers in Ag/CH3NH3PbI3/Pt cells, with an endurance of over 103 cycles, a high on/off ratio of 106, and an operation speed of 640 μs and without electroforming. We suggest plausible resistive switching and conduction mechanisms with current-voltage characteristics measured at various temperatures and with different top electrodes and device structures. Beyond the extended endurance, highly flexible resistive switching devices with a minimum bending radius of 5 mm create opportunities for use in flexible and wearable electronic devices.

Drastically enhanced hydrogen evolution activity by 2D to 3D structural transition in anion-engineered molybdenum disulfide thin films for efficient Si-based water splitting photocathodes

We synthesized transferrable and transparent anion-engineered molybdenum disulfide thin-film catalysts through a simple thermolysis method by using [(NH4)2MoS4] solution and powder precursors with different sulphur/phosphorus weight ratios. The synthesized sulphur-doped molybdenum phosphide (S:MoP) thin film changed from a two-dimensional van der Waals structure to a three-dimensional hexagonal structure by introduction of phosphorus atoms in the MoS2 thin film. The S:MoP thin film catalyst, which is composed of cheap and earth abundant elements, could provide the lowest onset potential and the highest photocurrent density for planar p-type Si photocathodes. The density functional theory calculations indicate that the surface of S:MoP thin films absorb hydrogen better than that of MoS2 thin films. The structurally engineered thin film catalyst facilitates the easy transfer of photogenerated electrons from the p-Si light absorber to the electrolyte. Anion-engineering of the MoS2 thin film catalyst would be an efficient way to enhance the catalytic activity for photoelectrochemical water splitting.

Eco-friendly graphene synthesis on Cu foil electroplated by reusing Cu etchants

Graphene film grown by chemical vapor deposition using Cu substrate is promising for industrial applications. After etching the Cu substrate, which is essential step in graphene transfer process, the etchant solution must be chemically treated to prevent water pollution. Here we investigated that a method of reusing Cu etchant used to synthesize graphene, the synthesis of graphene on the resulting reused Cu films (R-G), and the application of R-G to organic light-emitting diodes (OLEDs) and organic photovoltaic cells (OPVs). The turn-on voltage of OLEDs based on the R-G electrode was 4.2 V, and the efficiencies of OPVs based on the R-G electrode were 5.9–5.95%, that are similar to or better than those of the indium-tin-oxide-based devices. These results suggest that the reusing of Cu foil by the electroplating method could reduce the cost of graphene synthesis, thus opening a wide range of applications in graphene electronics.

Highly photoresponsive and wavelength-selective circularly-polarized-light detector based on metal-oxides hetero-chiral thin film

A highly efficient circularly-polarized-light detector with excellent wavelength selectivity is demonstrated with an elegant and simple microelectronics-compatible way. The circularly-polarized-light detector based on a proper combination of the geometry-controlled TiO2-SnO2 hetero-chiral thin film as an effective chiroptical filter and the Si active layer shows excellent chiroptical response with external quantum efficiency as high as 30% and high helicity selectivity of ~15.8% in an intended wavelength range. Furthermore, we demonstrated the ability of manipulating both bandwidth and responsivity of the detector simultaneously in whole visible wavelength range by a precise control over the geometry and materials constituting hetero-chiral thin film. The high efficiency, wavelength selectivity and compatibility with conventional microelectronics processes enabled by the proposed device can result in remarkable developments in highly integrated photonic platforms utilizing chiroptical responses.

Toward High-Performance Hematite Nanotube Photoanodes: Charge-Transfer Engineering at Heterointerfaces

Vertically ordered hematite nanotubes are considered to be promising photoactive materials for high-performance water-splitting photoanodes. However, the synthesis of hematite nanotubes directly on conducting substrates such as fluorine-doped tin oxide (FTO)/glass is difficult to be achieved because of the poor adhesion between hematite nanotubes and FTO/glass. Here, we report the synthesis of hematite nanotubes directly on FTO/glass substrate and high-performance photoelectrochemical properties of the nanotubes with NiFe cocatalysts. The hematite nanotubes are synthesized by a simple electrochemical anodization method. The adhesion of the hematite nanotubes to the FTO/glass substrate is drastically improved by dipping them in nonpolar cyclohexane prior to postannealing. Bare hematite nanotubes show a photocurrent density of 1.3 mA/cm(2) at 1.23 V vs a reversible hydrogen electrode, while hematite nanotubes with electrodeposited NiFe cocatalysts exhibit 2.1 mA/cm(2) at 1.23 V which is the highest photocurrent density reported for hematite nanotubes-based photoanodes for solar water splitting. Our work provides an efficient platform to obtain high-performance water-splitting photoanodes utilizing earth-abundant hematite and noble-metal-free cocatalysts.

Polarized Light‐Emitting Diodes Based on Patterned MoS2 Nanosheet Hole Transport Layer

Here, this study successfully fabricates few-layer MoS2 nanosheets from (NH4 )2 MoS4 and applies them as the hole transport layer as well as the template for highly polarized organic light-emitting diodes (OLEDs). The obtained material consists of polycrystalline MoS2 nanosheets with thicknesses of 2 nm. The MoS2 nanosheets are patterned by rubbing/ion-beam treatment. The Raman spectra shows that {poly(9,9-dioctylfluorene-alt-benzothiadiazole), poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)]} (F8BT) on patterned MoS2 exhibits distinctive polarization behavior. It is discovered that patterned MoS2 not only improves the device efficiency but also changes the polarization behavior of the devices owing to the alignment of F8BT. This work demonstrates a highly efficient polarized OLED with a polarization ratio of 62.5:1 in the emission spectrum (166.7:1 at the peak intensity of 540 nm), which meets the manufacturing requirement. In addition, the use of patterned MoS2 nanosheets not only tunes the polarization of the OLEDs but also dramatically improves the device performance as compared with that of devices using untreated MoS2 .