Seunghun Jang

Selective nitrogen capture by porous hybrid materials containing accessible transition metal ion sites

Selective dinitrogen binding to transition metal ions mainly covers two strategic domains: biological nitrogen fixation catalysed by metalloenzyme nitrogenases, and adsorptive purification of natural gas and air. Many transition metal-dinitrogen complexes have been envisaged for biomimetic nitrogen fixation to produce ammonia. Inspired by this concept, here we report mesoporous metal-organic framework materials containing accessible Cr(III) sites, able to thermodynamically capture N2 over CH4 and O2. This fundamental study integrating advanced experimental and computational tools confirmed that the separation mechanism for both N2/CH4 and N2/O2 gas mixtures is driven by the presence of these unsaturated Cr(III) sites that allows a much stronger binding of N2 over the two other gases. Besides the potential breakthrough in adsorption-based technologies, this proof of concept could open new horizons to address several challenges in chemistry, including the design of heterogeneous biomimetic catalysts through nitrogen fixation.

Photoluminescence induced by thermal annealing in SrTiO3 thin film

We have grown SrTiO3 thin films by rf-sputtering and studied its photoluminescence (PL) property after postannealing treatments. While the as-grown film does not show any PL signal, visible frequency PL emissions are induced by high temperature (T>550 °C) annealing. When subsequent low-T (50 °C) and long term (>8 months) annealing was made, the PL-spectra evolved into another pattern in which four distinct luminescence peaks appear simultaneously at λ=1.8, 2.2, 2.7, and 3.1 eV. We propose that these remarkable room temperature PL effects are due to both metastable and energetically stabilized defect states formed inside the band gap.We have grown SrTiO3 thin films by rf-sputtering and studied its photoluminescence (PL) property after postannealing treatments. While the as-grown film does not show any PL signal, visible frequency PL emissions are induced by high temperature (T>550 °C) annealing. When subsequent low-T (50 °C) and long term (>8 months) annealing was made, the PL-spectra evolved into another pattern in which four distinct luminescence peaks appear simultaneously at λ=1.8, 2.2, 2.7, and 3.1 eV. We propose that these remarkable room temperature PL effects are due to both metastable and energetically stabilized defect states formed inside the band gap.

Effects of proton irradiation on Si-nanocrystal/SiO2 multilayers: study of photoluminescence and first-principles calculations

Si nanocrystal (NC)/SiO2 multilayers containing interfacial nitrogens are formed by radiofrequency magnetron-sputtering and post-annealing. By analyzing the photoluminescence (PL) of the multilayer structures after proton irradiation (PI), we found that the peak at ∼740 nm was shifted toward shorter wavelengths and that its intensity was considerably suppressed. Furthermore, a new peak simultaneously appeared at ∼500 nm. We interpret that PI not only modifies the shapes of the Si NCs and generate defects at the NC/SiO2 interface, but also induces simultaneously hydrogen passivation (HP) of interfacial nitrogens due to the attenuated protons. To investigate the relationship between the HP of N at the Si NC/SiO2 interface and the observed band gap reduction of Si NC structures within the SiO2 matrix, we modeled the atomic configurations at the interface and applied first-principles calculations. The results clearly indicate that the HP of composite structures containing Si–N bonds at the Si NC/SiO2 interface induces the blue-shift in PL. We expect that this investigation helps to pave the way for developing more efficient Si-based light emitting devices or solar cells.

First-principles calculation of metal-doped CaAlSiN3: material design for new phosphors

Eu-doped CaAlSiN3 (CASN) is widely utilized as an efficient red phosphor; however, the high price of rare-earth metals has driven efforts toward finding non-rare-earth metal dopants. This paper reports first-principles calculations based on density functional theory (DFT) and geared toward identifying new non-rare-earth metal dopants for use in the CASN-based phosphors. We calculated the formation energies, the electronic structures, and the optical absorption spectra of various metal dopants (Eu, Mn, Sn, and Bi) in CASN. The calculated density of states, band structures, and absorption spectra were consistent with previous experimental observations obtained from Eu- and Mn-doped CASN. The DFT calculations suggested that Sn and Bi are promising candidates as non-rare-earth metal dopants in CASN-based phosphors. Our calculations demonstrate that DFT-based first-principles calculations provide a viable tool for finding new phosphor materials.

Formation of silicon quantum dots by RF power driven defect control

We studied the structural and optical properties of silicon nitride (SiNx) films synthesized by changing the applied radio frequency (RF) power in plasma-enhanced chemical vapor deposition. By decreasing the RF power from 100 W to 40 W, the photoluminescence (PL) of SiNx becomes stronger and the central PL peak position shifts from 2.41 eV to 1.78 eV. The size of the silicon grains becomes bigger as the applied RF power decreases, and eventually nano-sized amorphous Si quantum dots (nsa-Si QDs) are formed in the sample fabricated using 40 W RF power. By analyzing the chemical states of the Si 2p X-ray photoelectron spectroscopy core-level spectra, we found that the formation of nsa-Si QDs is considerably enhanced due to the disappearance of N3Si–SiN3 defects when the applied RF power reaches a certain point between 60 W and 40 W. From these results, we conclude that the type and the number of defects in SiNx play crucial roles in the initial formation of nsa-Si QDs in SiNx. In addition, this investigation paves the way for controlling the formation of quantum structures from defects to nsa-Si QDs simply by tuning the RF power. We expect that this work will contribute to the realization of Si-based full-color LEDs or tandem solar cells in the near future.

Ultrathin Metal Crystals: Growth on Supported Graphene Surfaces and Applications

Controlled nucleation and growth of metal clusters in metal deposition processes is a long-standing issue for thin-film-based electronic devices. When metal atoms are deposited on solid surfaces, unintended defects sites always lead to a heterogeneous nucleation, resulting in a spatially nonuniform nucleation with irregular growth rates for individual nuclei, resulting in a rough film that requires a thicker film to be deposited to reach the percolation threshold. In the present study, it is shown that substrate-supported graphene promotes the lateral 2D growth of metal atoms on the graphene. Transmission electron microscopy reveals that 2D metallic single crystals are grown epitaxially on supported graphene surfaces while a pristine graphene layer hardly yields any metal nucleation. A surface energy barrier calculation based on density functional theory predicts a suppression of diffusion of metal atoms on electronically perturbed graphene (supported graphene). 2D single Au crystals grown on supported graphene surfaces exhibit unusual near-infrared plasmonic resonance, and the unique 2D growth of metal crystals and self-healing nature of graphene lead to the formation of ultrathin, semitransparent, and biodegradable metallic thin films that could be utilized in various biomedical applications.

Synthesis of silver sulfide nanoparticles and their photodetector applications

Silver sulfide nanoparticles (Ag2S NPs) are currently being explored as infrared active nanomaterials that can provide environmentally stable alternatives to heavy metals such as lead. In this paper, we describe the novel synthesis of Ag2S NPs by using a sonochemistry method and the fabrication of photodetector devices through the integration of Ag2S NPs atop a graphene sheet. We have also synthesized Li-doped Ag2S NPs that exhibited a significantly enhanced photodetector sensitivity via their enhanced absorption ability in the UV-NIR region. First-principles calculations based on a density functional theory formalism indicated that Li-doping produced a dramatic enhancement of NIR photoluminescence of the Ag2S NPs. Finally, high-performance photodetectors based on CVD graphene and Ag2S NPs were demonstrated and investigated; the hybrid photodetectors based on Ag2S NPs and Li-doped Ag2S NPs exhibited a photoresponse of 2723.2 and 4146.0 A W−1 respectively under a light exposure of 0.89 mW cm−2 at 550 nm. Our novel approach represents a promising and effective method for the synthesis of eco-friendly semiconducting NPs for photoelectric devices.

Graphene as a thin-film catalyst booster: graphene-catalyst interface plays a critical role

Due to its extreme thinness, graphene can transmit some surface properties of its underlying substrate, a phenomenon referred to as graphene transparency. Here we demonstrate the application of the transparency of graphene as a protector of thin-film catalysts and a booster of their catalytic efficiency. The photocatalytic degradation of dye molecules by ZnO thin films was chosen as a model system. A ZnO thin film coated with monolayer graphene showed greater catalytic efficiency and long-term stability than did bare ZnO. Interestingly, we found the catalytic efficiency of the graphene-coated ZnO thin film to depend critically on the nature of the bottom ZnO layer; graphene transferred to a relatively rough, sputter-coated ZnO thin film showed rather poor catalytic degradation of the dye molecules while a smooth sol-gel-synthesized ZnO covered with monolayer graphene showed enhanced catalytic degradation. Based on a systematic investigation of the interface between graphene and ZnO thin films, we concluded the transparency of graphene to be critically dependent on its interface with a supporting substrate. Graphene supported on an atomically flat substrate was found to efficiently transmit the properties of the substrate, but graphene suspended on a substrate with a rough nanoscale topography was completely opaque to the substrate properties. Our experimental observations revealed the morphology of the substrate to be a key factor affecting the transparency of graphene, and should be taken into account in order to optimally apply graphene as a protector of catalytic thin films and a booster of their catalysis.

The Charge Storage Characteristics of Si‐QDs Embedded in Silicon Nitride Films (abstract)

Silicon quantum dots (Si‐QDs) are being increasingly considered for memory applications. We investigated the effect of annealing on capacitance‐voltage (C‐V) characteristics of Si‐QDs embedded in silicon nitride films. The Si‐QDs dispersed in silicon nitride films were formed by plasma‐enhanced chemical‐vapor deposition and annealed at 1,000° C in ultra‐high‐vacuum (UHV) for 5 min. In order to measure the C‐V characteristics of the Si‐QDs, metal‐insulator‐semiconductor structures were fabricated. The C‐V characteristics were measured at room temperature using HP 4280A at a frequency of 1 MHz. In addition, the physical characteristics of Si‐QDs were measured by photoluminescence (PL) and Fourier transform infrared spectroscopy (FTIR). When the sweep voltage was increased from 5 to 11 V, the width of C‐V hysteresis in the annealed sample increased. These results were in good agreement with PL and FTIR results. As a result, we found that annealing in UHV is a significant factor affecting charge storage.Silicon quantum dots (Si‐QDs) are being increasingly considered for memory applications. We investigated the effect of annealing on capacitance‐voltage (C‐V) characteristics of Si‐QDs embedded in silicon nitride films. The Si‐QDs dispersed in silicon nitride films were formed by plasma‐enhanced chemical‐vapor deposition and annealed at 1,000° C in ultra‐high‐vacuum (UHV) for 5 min. In order to measure the C‐V characteristics of the Si‐QDs, metal‐insulator‐semiconductor structures were fabricated. The C‐V characteristics were measured at room temperature using HP 4280A at a frequency of 1 MHz. In addition, the physical characteristics of Si‐QDs were measured by photoluminescence (PL) and Fourier transform infrared spectroscopy (FTIR). When the sweep voltage was increased from 5 to 11 V, the width of C‐V hysteresis in the annealed sample increased. These results were in good agreement with PL and FTIR results. As a result, we found that annealing in UHV is a significant factor affecting charge storage.

Photoluminescence Properties in Ge Nanoclusters Formed by Nitridation of Ge Surface Modified by Dry Etching (abstract)

Nano‐size metals and semiconductors self‐assembled in the dielectric matrix have recently shown strong possibility in the development of memory and optical devices. We report the optical properties of the Ge nanoclusters in the GeNx matrix by analyzing photoluminescence (PL) spectroscopy data. Ge thin film (5–10 nm) was grown by the molecular beam epitaxy method on Si(100) wafer and dry‐etched by Ar+ plasma. The nitridation and rapid thermal annealing of Ge thin film were applied by the ion gun and e‐beam heating methods, respectively. All processes were performed in situ in the ultra‐high vacuum. Taking out the sample in air, we obtained the surface roughness of the film with about 1–2 nm from the atomic force microscopy data analysis. We measured the PL spectra at the room temperature and 150° K and determined the PL peak positions at 400, 500, and 600 nm. We suggest an annealing method for controlling the size and density of Ge nanoclusters and discuss the results of the conventional method using the s...