Jucheol Park

Less strained and more efficient GaN light-emitting diodes with embedded silica hollow nanospheres

Light-emitting diodes (LEDs) become an attractive alternative to conventional light sources due to high efficiency and long lifetime. However, different material properties between GaN and sapphire cause several problems such as high defect density in GaN, serious wafer bowing, particularly in large-area wafers, and poor light extraction of GaN-based LEDs. Here, we suggest a new growth strategy for high efficiency LEDs by incorporating silica hollow nanospheres (S-HNS). In this strategy, S-HNSs were introduced as a monolayer on a sapphire substrate and the subsequent growth of GaN by metalorganic chemical vapor deposition results in improved crystal quality due to nano-scale lateral epitaxial overgrowth. Moreover, well-defined voids embedded at the GaN/sapphire interface help scatter lights effectively for improved light extraction, and reduce wafer bowing due to partial alleviation of compressive stress in GaN. The incorporation of S-HNS into LEDs is thus quite advantageous in achieving high efficiency LEDs for solid-state lighting.

Exploiting metallic glasses for 19.6% efficient back contact solar cell

An interdigitated back contact silicon solar cell with conversion efficiency of 19.6% was fabricated by screen-printing the Ag paste. In the Ag paste, oxide glass frits were totally replaced by Al85Ni5Y8Co2, Al-based metallic glass (MG) ones. The thermoplastic forming of the MG in the super cooled liquid region led to large contact area at the interface between Ag electrodes and Si layers and thus to specific contact resistance (ρc) as low as 0.86 mΩ cm2. The specific contact resistance was a function of both contact area and thickness of the interlayer formed at the interface working as a tunneling barrier.

Nitrogen-Doping Effect on Ge2Sb2Te5 Chalcogenide Alloy Films during Annealing

The microstructural and electrical-property changes of undoped and 5.4% nitrogen-doped Ge2Sb2Te5 were investigated. The transition temperature of sheet resistance increased owing to nitrogen doping, which corresponded well with the observed phase-change states. The lattice parameters of the undoped and nitrogen-doped Ge2Sb2Te5 exhibited the same tendency of decrease with increasing annealing temperature. Considering the increase in the Ge2Sb2Te5 energy state owing to the presence of interstitial nitrogen, the increase in the crystallization temperature is contrary to the thermodynamic viewpoint. Nitrogen atoms and N2 gas can be located at the interstitial site without distorting the crystal structure.

Influence of bone morphogenetic protein and proportion of hydroxyapatite on new bone formation in biphasic calcium phosphate graft: Two pilot studies in animal bony defect model

PURPOSE The purpose of these two pilot studies using animal bony defect models was to evaluate the influence of bone morphogenetic protein (BMP) and proportion of hydroxyapatite (HA)/beta-tricalcium phosphate (β-TCP) in biphasic calcium phosphate (BCP) graft on new bone formation. METHODS In this study, four kinds of synthetic osteoconductive bone materials known for bone growth scaffold, OSTEON™II(HA:β-TCP 30:70), OSTEON™III (HA:β-TCP 20:80), OSTEON™II Collagen, and OSTEON™III Collagen, were prepared as BCP graft materials. In pilot study 1, three BCP materials (OSTEON™II, OSTEON™III, and OSTEON™II Collagen) were grafted in rabbit calvarial defects after impregnating in rhBMP-2. OSTEON™II without the rhBMP-2 impregnation was included in the study as the control. The amount of new bone was examined and measured histologically at 2, 4, and 8 weeks. In pilot study 2, four BCP materials (OSTEON™II, OSTEON™III, OSTEON™II Collagen, and OSTEON™III Collagen) were grafted in beagle dog mandibular defects after soaking in the rhBMP-2. The amount of total bone and new bone were measured three-dimensionally using microCT and healing process was examined histologically at 2, 4, and 8 weeks. RESULTS In pilot study 1, rhBMP-2 impregnated groups showed more new bone formation than the rhBMP-2 free group. In pilot study 2, increased new bone formation was observed in time-dependent manner after graft of BCP and BCP-collagen (OSTEON™II, OSTEON™III, OSTEON™II Collagen, and OSTEON™III Collagen) impregnated with rhBMP-2. Also, BCP with a higher proportion of HA (30% HA) showed more favorable result in new bone formation and space maintenance, especially at the 8 weeks. CONCLUSION From the results of the pilot studies, rhBMP-2 played positive roles in new bone formation and BCP could become a scaffold candidate for rhBMP-2 impregnation to induce new bone formation. Moreover, BCP with a higher proportion of HA (30% HA) could be considered more appropriate for rhBMP-2 carrier.

Analysis of multi-component pesticide residues in herbal medicines by GC-MS with electron impact ionization and with positive- and negative-ion chemical ionization

SummaryMulticomponent pesticide residues in herbal medicines have been analyzed by gas chromatography-mass spectrometry (GC-MS) with electron impact (EI) ionization and positive- and negative-ion chemical ionization (PCI and NCI). Herbal medicines (5 g) were extracted with 65∶35 (%,v/v) acetonitrile-water, and partitioned with hexane-diethyl ether (1∶1) and hexane-dichloromethane (1∶1). The organic phase of the extracted fraction was cleaned on a Florisil column and analyzed by GC-MS with selected-ion monitoring (SIM). Method detection limits for 27 pesticides were tens of picograms for ECD, NPD and EI-SIM MS, and a few picograms for NCI-SIM MS. The calibration curve for the pesticide standard solution was linear within the range 0.003–30 pg for EI-SIM MS, PCI-SIM MS, and NCI-SIM MS. Mean recoveries of pesticides from spiked herbal medicines (0.75, 1.5, 3 pg) were 61–125% (RSD1–32%) for NCI-SIM MS and 74–121% (RSD 4–12%) for EI-SIM MS. Detection sensitivity and specificity of NCI-SIM MS were better than for ECD and NPD. Parallel use of EI-SIM MS, PCI-SIM MS and NCI-SIM MS was an excellent complementary method for identification and confirmation of multi-component pesticide residues in variety of herbal medicines.

Direct Observation of Inherent Atomic-Scale Defect Disorders responsible for High-Performance Ti1−xHfxNiSn1−ySby Half-Heusler Thermoelectric Alloys

Structural defects often dominate the electronic- and thermal-transport properties of thermoelectric (TE) materials and are thus a central ingredient for improving their performance. However, understanding the relationship between TE performance and the disordered atomic defects that are generally inherent in nanostructured alloys remains a challenge. Herein, the use of scanning transmission electron microscopy to visualize atomic defects directly is described and disordered atomic-scale defects are demonstrated to be responsible for the enhancement of TE performance in nanostructured Ti1-x Hfx NiSn1-y Sby half-Heusler alloys. The disordered defects at all atomic sites induce a local composition fluctuation, effectively scattering phonons and improving the power factor. It is observed that the Ni interstitial and Ti,Hf/Sn antisite defects are collectively formed, leading to significant atomic disorder that causes the additional reduction of lattice thermal conductivity. The Ti1-x Hfx NiSn1-y Sby alloys containing inherent atomic-scale defect disorders are produced in one hour by a newly developed process of temperature-regulated rapid solidification followed by sintering. The collective atomic-scale defect disorder improves the zT to 1.09 ± 0.12 at 800 K for the Ti0.5 Hf0.5 NiSn0.98 Sb0.02 alloy. These results provide a promising avenue for improving the TE performance of state-of-the-art materials.

Kinetic roughening of a ZnO grain boundary

Using a model ZnO bicrystal, we examine grain boundary kinetics by high-resolution transmission electron microscopy. The grain boundary undergoes a transition from atomically stepped to undulating appearances with increasing driving force for migration at a given temperature, producing clear evidence that grain boundaries undergo kinetic roughening.

Sulfuric acid pretreatment for the simultaneous GC screening of organochlorine and organophosphorus pesticides in herbal essential oils

SummaryA rapid and new clean-up method utilizing sulfuric acid treatment is demonstrated to identify and quantify the level of organochlorine (OC) and organophosphorus (OP) pesticide residue concentrations in herbal essential oils by gas chromatography (GC). Essential oils fortified with pesticides, that are extracted from herbs by steam distillation are partitioned with 65% acetonitrile/water(v/v) and treated with sulfuric acid at different reaction times and sulfuric acid concentrations. Optimal conditions, to avoid interference from essential oils in gas chromatographic analysis, is 17% (v/v) of a mixed phase ratio of sulfuric acid to organic solvent (hexane/ethyl ether=9∶1, v/v) and a reaction time of 30s. The response of the flame ionization detector (FID) is linear for all pesticides determined. Recovery of pesticides from fortified herbs studied are in the range of 75% to 111% (RSD, 4% to 11%) for OC, and 72% to 116% (RSD, 2% to 11%) for OP. Although sulfuric acid treatment destroys carbamate and some organophosphorus pesticides, this method has efficiently reduced matrix interference and provides a rapid, economical clean-up method with excellent linear data having low coefficients of variation for the GC analyses of BHC isomers, DDD, DDE, DDT, chlorothalonil, chloropyrifos, tetradifon, fenitrothion, malathion, and parathion in matrices of herbal essential oils.

Electron Energy Loss Spectroscopy Characterization of TANOS (TaN/Al 2 O 3 /Si 3 N 4 /SiO 2 /Si) Stacks

The interfacial layer between the Al₂O₃ layer and the Si₃N₄ layer formed after postdeposition annealing (PDA) of TaN/Al₂O₃/Si₃N₄/SiO₂/Si (TANOS) stacks was investigated using transmission electron microscopy (TEM), scanning transmission electron microscopy, and electron energy loss spectroscopy (EELS). From the result of the TEM analysis, it was found that the 2-nm-thick interface layer between Al₂O₃ and Si₃N₄ layers was amorphous. The high-loss EELS analysis showed that the phases of the interfacial layer weakly bound together instead of the substoichiometric silicon oxide and amorphous Al₂O₃ near the bottom interface of the crystalline Al₂O₃. The low-loss EELS analysis showed that aluminum existed in metallic state at the interface. Therefore, we speculated that SiO(x)N(y) could be formed by oxidation of Si₃N₄ during PDA and that metallic aluminum could be formed by the decomposition of weakly bound amorphous Al₂O₃ during electron irradiation. These complicated reactions near the interface could induce oxygen deficiency in the Al₂O₃ layer and finally degrade the retention properties of TANOS stacks.

Forming-less and Non-Volatile Resistive Switching in WO X by Oxygen Vacancy Control at Interfaces

Resistive switching devices are recognized as candidates for next-generation memory devices in that they can replace conventional memory devices. In these devices, a WOX film deposited by RF magnetron sputtering with a significant number of oxygen vacancies exhibits a resistive switching property and does not involve the use of a forming process. The resistive switching mechanism involves the hopping of electrons through the sub-band states of the oxygen vacancies in E-field-driven electromigration. X-ray photoemission spectroscopy, ultra-violet photoemission spectroscopy, and transmission electron microscopy-electron energy loss spectroscopy were performed to analyze local variations in the O-vacancies and in the electronic band structure of a WOX thin film. The band structure is responsible for the correlation between the motion of the electrons under the interface effect at the electrodes with the change in the resistance and the bias-polarity dependence of the I-V property of the device. The optimized metal-insulator-metal structure (Pt/WOX/Au), which has an asymmetric electrode and many oxygen vacancies, gives rise to excellent resistive-switching properties with a high on/off ratio on the order of 105 times, a low set voltage of <0.34 V, and a uniform DC cyclic performance in the order of 1500 cycles at room temperature. These specifications can be further adopted for application to non-volatile memory-device applications.