[Show abstract][Hide abstract] ABSTRACT: We developed a modified potential energy surface following three assumptions and verified that the most stable motion of a single particle with extreme speed in a modified potential energy surface is identical to the atomic model of hydrogen. Significant three equations are found regarding the stable motions of more than two particles. These equations are in good agreement with the motion of protons and neutrons, and one can arrange the periodic table successfully using these three equations.
[Show abstract][Hide abstract] ABSTRACT: Copper nanowires (Cu NWs) are promising materials for transparent electrode applications. However, their growth mechanism during water-based synthesis remains unclear. The steric hindrance of a surfactant, which has been considered in previous studies, is insufficient to explain the suppression of Cu oxide formation during water-based synthesis. In this paper, we suggest that ethylenediamine (EDA, C2H4(NH2)2), which is commonly used as a structure-directing agent (SDA), may play an important role as an oxidation inhibitor in water-based synthesis of Cu NWs. High-resolution transmission electron microscopy (HR-TEM) analysis indicated that the Cu NWs grow from icosahedral seeds. Additionally, energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) suggested that EDA affects the oxidation state of the as-prepared Cu NW surface. Finally, density functional theory (DFT) calculations revealed that the adsorption of EDA effectively hinders the adsorption of oxidizing species and prevents the oxidation of the Cu seeds. This effect is attributed to a filling of the antibonding states and an increase in the electrostatic repulsion between the Cu seeds and the oxidizing species.
No preview · Article · Feb 2016 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: In this work, we synthesized uniform Cu-Ag core-shell nanoparticles using a facile two-step process that consists of thermal decomposition and galvanic displacement methods. The core-shell structure of these nanoparticles was confirmed through characterization using transmission electron microscopy, energy-dispersive spectroscopy, and x-ray diffraction. Furthermore, we investigated the oxidation stability of the Cu-Ag core-shell nanoparticles in detail. Both qualitative and quantitative x-ray photoelectron spectroscopy analyses confirm that the Cu-Ag core-shell nanoparticles have considerably higher oxidation stability than Cu nanoparticles. Finally, we formulated a conductive ink using the synthesized nanoparticles and coated it onto glass substrates. Following the sintering process, we compared the resistivity of the Cu-Ag core-shell nanoparticles with that of the Cu nanoparticles. The results of this study clearly show that the Cu-Ag core-shell nanoparticles can potentially be used as an alternative to Ag nanoparticles because of their superior oxidation stability and electrical properties.
[Show abstract][Hide abstract] ABSTRACT: To solve the problems of deactivation of Au nanoparticle (NP) catalysts, we studied the catalytic activity of 10-atom Au-Ti bimetallic NPs on TiO2 (110) supports for CO oxidation by means of density function theory (DFT) calculation with DFT+U method. The calculations showed that Au-Ti NPs were more strongly adsorbed on TiO2 than Au monometallic NPs. The adsorption energy of O2 was higher on Au-Ti NPs than on Au NPs, leading to low CO poisoning. The reaction barrier for CO oxidation reaction at interfacial site was lower in the Au-Ti NP system. These results suggest that Au-Ti NPs are a promising catalyst for CO oxidation.
[Show abstract][Hide abstract] ABSTRACT: Sn whiskers are becoming a serious reliability issue in Pb-free electronic packaging applications. Sn whiskers are also observed in connector parts of electronics as well as on electroplated surface finishes. Sn whiskers found in connector parts are known to behave differently from the typical Sn whiskers reported on electroplated Sn surfaces. In this study, Sn whiskers on plastically deformed Sn-rich films were investigated to understand their growth behavior to establish mitigation strategies for Sn-rich films used in connectors. Therefore, a microhardness indentation technique was applied to plastically deform electroplated matte Sn samples, followed by temperature/humidity (T/H) testing (30°C, dry air). Each sample was examined by scanning electron microscopy at regular time intervals up to 4000 h. Various morphologies of Sn whiskers on plastically deformed matte Sn films were observed, and their growth statistics and kinetics are analyzed in terms of the plating conditions and plastic deformation by using transmission electron microscopy, x-ray diffraction, and the focused ion-beam technique. Sn whiskers were observed on plastically deformed regions of thin (2-μm) and thick (10-μm) matte Sn films, regardless of the current density applied. Plastic deformation was found to promote whisker formation on matte Sn films. A high density of dislocations and newly formed fine Sn subgrains were observed in deformed grains. In addition, the recrystallized grains and Cu6Sn5 intermetallic compound grew further with increasing time. Finally, a growth mechanism for deformation-induced Sn whiskers is proposed based on a recrystallization model combined with the formation of Cu6Sn5.
No preview · Article · Oct 2015 · Journal of Electronic Materials
[Show abstract][Hide abstract] ABSTRACT: It is important to develop Pb-free solder alloys suitable for automotive use instead of traditional Sn–Pb solder due to environmental regulations (e.g., Restriction of Hazardous Substances (RoHS)). Al addition has been spotlighted to enhance solder properties. In this study, we investigated the microstructural change of Sn–0.5Cu wt.% based Pb-free solder alloys with Al addition (0.01–0.05 wt.%). The small amount of Al addition caused a remarkable microstructural change. The Al was favored to form Cu–Al intermetallic compounds inside the solder matrix. We identified the Cu–Al intermetallic compound as Cu33Al17, which has a rhombohedral structure, using EPMA and TEM analyses. This resulted in refined Cu6Sn5 networks in the Sn–0.5Cu based solder alloy. In addition, we conducted thermal analysis to confirm its stability at a high temperature of approximately 230 °C, which is the necessary temperature range for automotive applications. The solidification results were substantiated thermodynamically using the Scheil solidification model. We can provide criteria for the minimum aluminum content to modify the microstructure of Pb-free solder alloys.
Full-text · Article · Aug 2015 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: A simple oleylamine-based thermal decomposition process using different time steps for precursor injection was used to obtain bimetallic Ag–Cu nanoparticles with a narrow size distribution. Experimental and theoretical studies were carried out to demonstrate that these bimetallic nanoparticles are less prone to oxidation. The calculated energy trends for O2 adsorption on the nanoparticles show that the adsorption energy declines rapidly when more than six O2 molecules are present, indicating that O2 is rarely adsorbed on Ag–Cu nanoparticles. Electron transfer from Cu to Ag within these bimetallic nanoparticles allows far better resistance to oxidation than monometallic Cu nanoparticles.
No preview · Article · Nov 2014 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Backside illumination complementary metal oxide semiconductor image sensors (BSI CISs) represent an advanced technology that produces high-quality image sensors. However, BSI CISs are limited by high dark signals and noise signals on the backside. To address these problems, backside junctions are commonly used. High-dose backside junctions effectively reduce dark signals and noise signals. The depth of the implantation profile is a key factor in determining the junction depth. A laser thermal annealing process is conducted only near the surface to the activation, and thus broader doping profiles are limitations to be activation of dopants. Changing the dopant from B to BF2 can decrease the implant projected range. However, there are abnormal activation rates for BF2 in applications involving laser thermal annealing processes for shallow junctions. Although the need for BF2 is increasing, a mechanism for its slow activation and low activation rates has not yet been confirmed. Here, we identify the mechanism by which BF2 undergoes low activation after a melting threshold temperature and explain why this phenomenon occurs. In addition, we confirm a condition that provides high activation rates of BF2 and show the reduction of dark signals and noise signals at the high density BSI CISs.
No preview · Article · Nov 2014 · Journal of Electronic Materials
[Show abstract][Hide abstract] ABSTRACT: Ammonia (NH3) nitridation on an Fe surface was studied by combining density functional theory (DFT) and kinetic Monte Carlo (kMC) calculations. A DFT calculation was performed to obtain the energy barriers (Eb) of the relevant elementary processes. The full mechanism of the exact reaction path was divided into five steps (adsorption, dissociation, surface migration, penetration, and diffusion) on an Fe (100) surface pre-covered with nitrogen. The energy barrier (Eb) depended on the N surface coverage. The DFT results were subsequently employed as a database for the kMC simulations. We then evaluated the NH3 nitridation rate on the N pre-covered Fe surface. To determine the conditions necessary for a rapid NH3 nitridation rate, the eight reaction events were considered in the kMC simulations: adsorption, desorption, dissociation, reverse dissociation, surface migration, penetration, reverse penetration, and diffusion. This study provides a real-time-scale simulation of NH3 nitridation influenced by nitrogen surface coverage that allowed us to theoretically determine a nitrogen coverage (0.56 ML) suitable for rapid NH3 nitridation. In this way, we were able to reveal the coverage dependence of the nitridation reaction using the combined DFT and kMC simulations.
Preview · Article · Oct 2014 · The Journal of Chemical Physics
[Show abstract][Hide abstract] ABSTRACT: The exemption clause for Pb-containing automobile parts in the RoHS (Restriction of Hazardous Substances) legislation will expire in a few years. Therefore, there is an urgent need to develop new Pb-free solder alloys that are suitable for automobiles. To improve the properties of the Pb-free solders used in automobiles that are consistently exposed to high temperatures and vibrations, we investigated the effects of minor alloying additions of chromium, calcium and palladium and examined the various properties of the resulting Sn–0.7Cu solders. We then investigated the new intermetallic compounds observed after the addition of chromium. In addition, we conducted thermal analysis to confirm the stability of the alloys at temperatures of approximately 230 °C. The wettability and interfacial reactions between the solder alloys and the Cu under bump metallurgy are discussed. Furthermore, to evaluate the mechanical properties of the developed solder alloys, tensile tests were conducted. We confirmed that our minor alloying strategy enhances the strength and hardness of solder alloys without decreasing their melting temperatures.
Full-text · Article · Sep 2014 · Journal of Alloys and Compounds
[Show abstract][Hide abstract] ABSTRACT: Although the reaction results of numerous iron-based Fischer–Tropsch synthesis catalysts containing various promoters have been reported, the research on their theoretical foundation is still insufficient. In the present work, highly activated K-doped χ-Fe5C2/charcoal nanocatalysts were designed using calculations based on density functional theory (DFT), and then prepared using a melt-infiltration process and a subsequent incipient-wetness method of K precursors. The catalyst at K/Fe = 0.075 in an atomic ratio that bears small iron carbide nanoparticles of 18 nm showed the highest activity (1.54 × 10−4 molCO gFe−1 s−1) and the best hydrocarbon yield (1.41 × 10−3 gHC gFe−1 s−1), as well as a good selectivity for gasoline-range (C5–C12) hydrocarbon products in the high-temperature Fischer–Tropsch reaction.
[Show abstract][Hide abstract] ABSTRACT: Catalytic decomposition of ammonia (NH3) is a promising chemical reaction in energy and environmental applications. Density functional theory (DFT) calculations were performed to clarify the detailed catalytic mechanism of NH3 decomposition on an Fe(100) surface. Specifically, the elementary steps of the mechanism were calculated for the general dehydrogenation pathway of NH3. The adsorption of two types of ammonia dimers (2NH3), locally adsorbed NH3 and hydrogen-bonded NH3, were then compared, revealing that locally adsorbed NH3 is more stable than hydrogen-bonded NH3. By contrast, the dehydrogenation of dimeric NH3 results in a high energy barrier. Moreover, the catalytic characteristics of NH3 decomposition on a nitrogen (N)-covered Fe surface must be considered because the recombination of nitrogen (N2) and desorption have an extremely high energy barrier. Our results indicate that the catalytic characteristics of the NH3 decomposition reaction are altered by N coverage of the Fe surface. This study primarily focused on energetic and electronic analysis. Finally, we conclude that Fe is an alternative catalyst for the decomposition of NH3 in COx-free hydrogen production.
No preview · Article · Mar 2014 · The Journal of Physical Chemistry C
[Show abstract][Hide abstract] ABSTRACT: Sn whiskers are becoming a serious reliability issue in Pb-free
electronic packaging applications. Among the numerous Sn whisker
mitigation strategies, minor alloying additions to Sn have been proven
effective. In this study, several commercial Sn and Sn-Ag baths of
low-whisker formulations are evaluated to develop optimum mitigation
strategies for electroplated Sn and Sn-Ag. The effects of plating
variables and storage conditions, including plating thickness and
current density, on Sn whisker growth are investigated for matte Sn,
matte Sn-Ag, and bright Sn-Ag electroplated on a Si substrate. Two
different storage conditions are applied: an ambient condition
(30°C, dry air) and a high-temperature/high-humidity condition
(55°C, 85% relative humidity). Scanning electron microscopy is
employed to record the Sn whisker growth history of each sample up to
4000 h. Transmission electron microscopy, x-ray diffraction, and focused
ion beam techniques are used to understand the microstructure, the
formation of intermetallic compounds (IMCs), oxidation, the Sn whisker
growth mechanism, and other features. In this study, it is found that
whiskers are observed only under ambient conditions for both thin and
thick samples regardless of the current density variations for matte Sn.
However, whiskers are not observed on Sn-Ag-plated surfaces due to the
equiaxed grains and fine Ag3Sn IMCs located at grain
boundaries. In addition, Sn whiskers can be suppressed under the
high-temperature/high-humidity conditions due to the random growth of
IMCs and the formation of thick oxide layers.
No preview · Article · Oct 2013 · Journal of Electronic Materials
[Show abstract][Hide abstract] ABSTRACT: To control the optical properties of Cu2O for a variety of application, we synthesized Cu2O in nanoscale without other treatments. Cu2O nanoparticles with an average size of 2.7 nm (sigma < or = 3.7%) were successfully synthesized in this study via a modified thermal decomposition process. Copper (II) acetylacetonate was used as a precursor, and oleylamine was used as a solvent, a surfactant and a reducing agent. The oleylamine-mediated synthesis allowed for the preparation of Cu2O nanoparticles with a narrower size distribution, and the nanoparticles were synthesized in the presence of a borane tert-butylamine (BTB) complex, where BTB was a strong co-reducing agent together with oleylamine. UV-vis spectroscopy analysis suggest that band gap energy of these Cu2O particles is enlarged from 2.1 eV in the bulk to 3.1 eV in the 2.7-nm nanoparticles, which is larger than most other reported value of Cu2O nanoparticles. Therefore, these nanoparticles could be used as a transparent material because of transformed optical property.
No preview · Article · Sep 2013 · Journal of Nanoscience and Nanotechnology
[Show abstract][Hide abstract] ABSTRACT: Intensive research on oxygen reduction reaction (ORR) catalysts has been undertaken to find a Pt substitute or reduce the amount of Pt. Ag nanoparticles are potential Pt substitutes; however, the weak oxygen adsorption energy of Ag prompted investigation of other catalysts. Herein, we prepared AgCu bimetallic nanoparticle (NP) systems to improve the catalytic performance and compared the catalytic performance of Ag, Cu, AgCu (core-shell), and AgCu (alloy) NP systems as new catalyst by investigating the adsorption energy of oxygen and the activation energy of oxygen dissociation, which is known to be the rate-determining step of ORR. By analyzing HOMO-level isosurfaces of metal NPs and oxygen, we found that the adsorption sites and the oxygen adsorption energies varied with different configurations of NPs. We then plotted the oxygen adsorption energies against the energy barrier of oxygen dissociation to determine the catalytic performance. AgCu (alloy) and Cu NPs exhibited strong adsorption energies and low activation-energy barriers. However, the overly strong oxygen adsorption energy of Cu NPs hindered the ORR.
[Show abstract][Hide abstract] ABSTRACT: Sn whiskers are becoming a serious reliability problem in microelectronics where Pb-free solder technology is being implemented with pure Sn or Sn-rich alloys. Numerous investigations have been performed to understand the whisker growth mechanisms and thereby to mitigate Sn whisker growth. Among many Sn whisker mitigation strategies, minor alloying additions to Sn have been found to be quite effective. One challenge in evaluating Sn whisker growth is a time-consuming aging test, such as 4000 h testing condition recommended by the JEDEC-JESD201A standard. In this study, several commercial Sn and Sn-Ag baths of low whisker formulations are evaluated. The effects of plating variables and aging conditions on Sn whisker growth are investigated with matte Sn, matte Sn-Ag, and bright Sn-Ag electroplated on a Cu/Ni/Si substrate. The layer thickness and current density are the major plating variables studied. Two different storage conditions are applied; an ambient condition (30°C/dry air), and a high temperature/humidity condition (55°C/85%RH). In addition, the effect of plastic deformation on Sn whisker growth is investigated as an acceleration method for Sn whisker testing. Microhardness indentation technique is applied to electroplated Sn and Sn-Ag samples to plastically deform them before T/H testing. Each sample is examined by SEM at a regular time interval up to 4000 h. Various morphologies of Sn whiskers are observed and their growth statistics are analyzed in terms of plating conditions, plastic deformation and T/H testing conditions. Plastic deformation is found to significantly accelerate Sn whisker growth both in pure Sn and Sn-Ag samples. The method of plastic deformation can be employed to shorten the time-consuming Sn whisker growth testing.
[Show abstract][Hide abstract] ABSTRACT: We report first-principles calculations of adsorption, dissociation, penetration, and diffusion for the complete nitridation mechanism of nitrogen molecules on a pure Fe surface (bcc, ferrite phase). The mechanism of the definite reaction path was calculated by dividing the process into four steps. We investigated various reaction paths for each step including the energy barrier based on the climb image nudged elastic band (CI-NEB) method, and the complete reaction pathway was computed as the minimum energy path (MEP). The adsorption characteristics of nitrogen (N) and molecular nitrogen (N2) indicate that nitrogen atoms and molecules are energetically favorable at the hollow sites on pure Fe(100) and (110). The dissociation of the nitrogen molecule (N2) was theoretically supported by electronic structure calculations. The penetration of nitrogen from the surface to the sub-surface has a large energy barrier compared with the other steps. The activation energy calculated for nitrogen diffusion in pure bcc Fe was in good agreement with the experimental results. Finally, we confirmed the rate-determining step for the full nitridation reaction pathway. This study provides fundamental insight into the nitridation mechanism for nitrogen molecules in pure bcc Fe.
No preview · Article · Mar 2013 · Physical Chemistry Chemical Physics