Jee-Gong Chang

Molecular dynamics analysis of temperature effects on nanoindentation measurement

A three-dimensional molecular dynamics (MD) model is carried out to study the effects of temperature on the atomic-scale nanoindentation process. The model utilizes the Morse potential function to simulate interatomic forces between the sample and tool. The results show that both Youngs modulus and hardness become smaller as temperature increases. The results also indicate that elastic recovery is smaller at higher temperatures. The softening behavior is similar to the prior experiment and the estimated elastic moduli are much higher than the prior experiment. The discrepancy may be due to simulations performed on defect-free single crystals. In addition, some defects of vacancies, atomic steps and plastic indent are observed on the surface region.

Molecular dynamics simulation of nano-lithography process using atomic force microscopy

A three-dimensional molecular dynamics (MD) model is utilized to study the effects of the scribing feed on the atomic-scale lithography process. The model utilizes the Morse potential function to simulate interatomic forces between the atoms of the workpiece and the tool, and also between the atoms of the workpiece themselves. MD simulation results are compared to atomic force microscopy (AFM) experimental results. Results show that both resultant force and surface roughness have a positive correlation with rate of feed when the feed is smaller than a critical value, after which they remain constant. Comparison of the feed effect behavior of the MD theoretical analysis and the AFM experiments shows good qualitative agreement.

First-principle calculations on CO oxidation catalyzed by a gold nanoparticle

We have elucidated the mechanism of CO oxidation catalyzed by gold nanoparticles through first‐principle density‐functional theory (DFT) calculations. Calculations on selected model show that the low‐coordinated Au atoms of the Au29 nanoparticle carry slightly negative charges, which enhance the O2 binding energy compared with the corresponding bulk surfaces. Two reaction pathways of the CO oxidation were considered: the Eley–Rideal (ER) and Langmuir–Hinshelwood (LH). The overall LH reaction O2(ads) + CO(gas) → O2(ads) + CO(ads) → OOCO(ads) → O(ads) + CO2(gas) is calculated to be exothermic by 3.72 eV; the potential energies of the two transition states (TSLH1 and TSLH2) are smaller than the reactants, indicating that no net activation energy is required for this process. The CO oxidation via ER reaction Au29 + O2(gas) + CO(gas) → Au29–O2(ads) + CO(gas) → Au29–CO3(ads) → Au29–O(ads) + CO2(gas) requires an overall activation barrier of 0.19 eV, and the formation of Au29–CO3(ads) intermediate possesses high exothermicity of 4.33 eV, indicating that this process may compete with the LH mechanism. Thereafter, a second CO molecule can react with the remaining O atom via the ER mechanism with a very small barrier (0.03 eV). Our calculations suggest that the CO oxidation catalyzed by the Au29 nanoparticle is likely to occur at or even below room temperature. To gain insights into high‐catalytic activity of the gold nanoparticles, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis.

Identifying the O2 diffusion and reduction mechanisms on CeO2 electrolyte in solid oxide fuel cells: A DFT + U study

The interactions and reduction mechanisms of O2 molecule on the fully oxidized and reduced CeO2 surface were studied using periodic density functional theory calculations implementing on‐site Coulomb interactions (DFT + U) consideration. The adsorbed O2 species on the oxidized CeO2 surface were characterized by physisorption. Their adsorption energies and vibrational frequencies are within −0.05 to 0.02 eV and 1530–1552 cm−1, respectively. For the reduced CeO2 surface, the adsorption of O2 on Ce4+, one‐electron defects (Ce3+ on the CeO2 surface) and two‐electron defects (neutral oxygen vacancy) can alter geometrical parameters and results in the formation of surface physisorbed O2, O2a− (0 < a < 1), superoxide (O2−), and peroxide (O22−) species. Their corresponding adsorption energies are −0.01 to −0.09, −0.20 to −0.37, −1.34 and −1.86 eV, respectively. The predicted vibrational frequencies of the peroxide, superoxide, O2a− (0 < a < 1) and physisorbed species are 897, 1234, 1323–1389, and 1462–1545 cm−1, respectively, which are in good agreement with experimental data. Potential energy profiles for the O2 reduction on the oxidized and reduced CeO2 (111) surface were constructed using the nudged elastic band method. Our calculations show that the reduced surface is energetically more favorable than the unreduced surface for oxygen reduction. In addition, we have studied the oxygen ion diffusion process on the surface and in bulk ceria. The small barrier for the oxygen ion diffusion through the subsurface and bulk implies that ceria‐based oxides are high ionic conductivity at relatively low temperatures which can be suitable for IT‐SOFC electrolyte materials.

Surface Morphological and Nanomechanical Properties of PLD-Derived ZnO Thin Films

This study reports the surface roughness and nanomechanical characteristics of ZnO thin films deposited on the various substrates, obtained by means of atomic force microscopy (AFM), nanoindentation and nanoscratch techniques. ZnO thin films are deposited on (a- and c-axis) sapphires and (0001) 6H-SiC substrates by using the pulsed-laser depositions (PLD) system. Continuous stiffness measurements (CSM) technique is used in the nanoindentation tests to determine the hardness and Young’s modulus of ZnO thin films. The importance of the ratio (H/Efilm) of elastic to plastic deformation during nanoindentation of ZnO thin films on their behaviors in contact-induced damage during fabrication of ZnO-based devices is considered. In addition, the friction coefficient of ZnO thin films is also presented here.

Modeling of polyethylene and poly (L-lactide) polymer blends and diblock copolymer: chain length and volume fraction effects on structural arrangement.

Dissipative particle dynamics (DPD), a mesoscopic simulation approach, has been used to investigate the chain length effect on the structural property of the immiscible polyethylene (PE)/poly(L-lactide) (PLLA) polymer in a polymer blend and in a system with their diblock copolymer. In this work, the interaction parameter in DPD simulation, related to the Flory-Huggins interaction parameter chi, is estimated by the calculation of mixing energy for each pair of components in molecular dynamics simulation. The immiscibility property of PE and PLLA polymers induces the phase separation and exhibits different architectures at different volume fractions. In order to observe the structural property, the radius of gyration is used to observe the detailed arrangement of the polymer chains. It shows that the structure arrangement of a polymer chain is dependent on the phase structure and has a significantly different structural arrangement character for the very short chains in the homopolymer and copolymers. The chain length effect on the degree of stretching or extension of polymers has also been observed. As the chain length increases, the chain exhibits more stretching behavior at lamellae, perforated lamellae, and cylindrical configurations, whereas the chain exhibits a similar degree of stretching or extension at the cluster configuration.

Hydrogen-bond structure at the interfaces between water/poly(methyl methacrylate), water/poly(methacrylic acid), and water/poly(2-aminoethylmethacrylamide).

The molecular dynamics approach was employed to study the structural characteristics at the interface of water/poly(methyl methacrylate) (PMMA), water/poly(methacrylic acid) (PMAA), and poly(2-aminoethylmethacrylamide) (PAEMA). It is found that the water on the PMAA surface shows a significant increase in the density at the interface, with a greater number of water molecules permeating into the bulk of the substrate region. The structure of hydrogen bonds of water and the radial distribution function for given polar atoms in the polymer substrate are calculated. We found that a network structure of hydrogen bonding between water and the polar atom of the polymer forms at the interface. PMAA exhibits a more hydrophilic property than PMMA and PAEMA because it generates a shell-like structure of water molecules around its functional group. Finally, the hydrogen bond numbers of PMMA, PMAA, and PAEMA are also analyzed. The results detail the hydrogen bond structure of each specific atom and find that, in all three cases, the carboxyl oxygen attracts the greatest number of water molecules compared with other atoms.

The influence of temperature and surface conditions on surface absorptivity in laser surface treatment

A one-dimensional, transient, inverse heat conduction problem is implemented to investigate the influence of temperature and surface conditions on surface absorptivity in the laser surface heating process. Analysis includes the utilization of the conjugate gradient method (CGM), with temperatures measured near the heated surface. To increase efficiency and accuracy of the calculation, the result retrieved from the least-square method is used as an initial guess for the CGM. Results show that absorptivity decreases when the temperature exceeds a certain value. This decrease is related to structural transformation during the temperature rise. As the surface temperature nears the melting point, the decreasing trend inverts and absorptivity increases. This final abrupt rise is caused by the phase transformation from order to disorder. Additionally, absorptivity is related to surface conditions such as absorption-enhancing coatings and surface roughness.

Investigation of cluster size and cluster incident energy effect on film surface roughness for ionized cluster beam deposition

Molecular dynamic simulation is used to investigate the influence of cluster size and cluster incident energy upon a Cu–Co magnetic film produced using the ionized cluster beam deposition process. The Co–Co, Cu–Cu, and Cu–Co atomic interactions are modeled using the many-body, tight-binding potential method, and the interface width is used to characterize the surface roughness properties at both transient and final state conditions. The results of this study indicate that the surface roughness of the deposited magnetic film is lower when a smaller incident cluster size is used. This observation is valid for all stages of the deposition process. Furthermore, it is determined that the nature of the relationship between cluster size and the produced film surface property is influenced by the cluster incident energy parameter. When the cluster incident energy is lower than an optimal value, it is observed that the produced film surface property is strongly dependent on the cluster size. However, when the valu...

Generating random and nonoverlapping dot patterns for liquid-crystal display backlight light guides using molecular-dynamics method

This paper employs the molecular-dynamics method to generate random-dot patterns for light guides designed for backlight systems. The proposed approach combines various numerical techniques and is designed to optimize the dot-density distribution in order to satisfy the uniform luminance requirements demanded by liquid-crystal displays. In the proposed algorithm, the total domain is divided into a prescribed number of cells whose dot densities can be individually adjusted in order to fine tune the luminance conditions in accordance with the light source position and type. In addition, a variable truncation distance is implemented in each cell according to the dot density of that cell. This variable r-cut technique localizes the repulsive force effects acting within each cell in order that a high-dot-density gradient can be achieved in the overall dot distribution. Finally, an average force control technique is developed to ensure the uniformity of the dot distribution as it passes across the cell boundari...