Byung Deok Yu

Computational designing of graphitic silicon carbide and its tubular forms

By employing density-functional theory calculations we predict graphitic and tubular forms of silicon carbide (SiC) and proposes their synthesis. The present calculations suggest that the metastable SiC tubes can be synthesized by using a technique of extreme hole injection [J. M. Schon, Ch. Kloc, and B. Batlogg, Nature (London) 406, 702 (2000); 408, 549 (2000)]. This method is in contrast to the idea to synthesize new-tubular forms with a usage of a carbon nanotube as nucleation seed. Furthermore, the strain energies of SiC nanotubes are lower than those of the carbon nanotubes. The calculated band gaps of the investigated SiC tubes are either direct or indirect depending on the helicities. We expect that the SiC tubes with direct band gaps can be applied as nanoscale optoelectronics devices with strong oscillator strengths.

The presence of CH3NH2 neutral species in organometal halide perovskite films

We report the presence of CH3NH2 neutral species not only on the surface but also at grain boundaries in the interior of thin polycrystalline films of methylammonium lead iodide perovskite CH3NH3PbI3 (thickness ∼ 50 nm) that were prepared using a standard solution method. Different chemical states for C K-edge were observed at the surfaces and in the interiors of perovskite films. Salient features of σ*(CH3-NH3+: methylammonium cation) and σ*(CH3-NH2: methylamine neutral species) were observed at 290.3 and 292.8 eV in both partial (surface-sensitive) and total (bulk) electron yield modes by near-edge x-ray absorption fine structure measurements. Consistently, two chemical states originated from CH3NH3+ and CH3NH2 in C 1s and N 1s core-level spectra were observed using high-resolution x-ray photoelectron spectroscopy. Using density functional theory calculations, we show that CH3NH2 cannot reside stably in the MAPbI3 perovskite crystal structure. Therefore, we propose that these CH3NH2 neutral species are ...

Ab-initio Study of Transition-Metal Dimers on MgO(001) Surfaces: Comparison of Atomic and Electronic Properties

The adsorption properties of transition-metal (TM) dimers on defect-free MgO(001) surfaces were studied from a series of ab-initio electronic structure calculations for late TM atoms of Cu, Ag, Au, Ni, Pd, and Pt. Two dimer configurations were considered: an upright dimer structure and a flat lying dimer structure. For Cu, Ag, Au, and Pt, the upright dimer structure was found to be favored, and for Ni and Pt, the flat lying dimer structure. Also, for each dimer structure, the electronic structures of the TM dimers on MgO(001) were systematically examined. It was found that the electronic structures of the TM dimers depend on the adsorption structures and that their associated energy levels in the gap region of MgO significantly vary with the adsorption structures.

Structural and electronic properties of metal-silicide/silicon interfaces: A first-principles study

By employing first-principles total-energy calculations we studied the electronic and structural properties of the NiSi2/Si(001) and CoSi2/Si(001) interfaces. We found a new structural model that is energetically more stable than previously proposed models and well explains an experimentally observed 2×1 interfacial ordering. The new model is characterized by sevenfold-coordinated interface metals and interface Si dimers, and is achieved by compromising the bonding configuration of the interface metals and the reduction of the number of interface Si dangling bonds. The underlying mechanism of the model is in contrast to that of a semiconductor-surface reconstruction. Finally, the interfacial electronic properties are also presented to give a physical insight into the microscopic mechanism for the Schottky-barrier formation at metal/semiconductor junctions.

Ab-initio study of interactions of gold atoms with hydroxylated MgO(001) surfaces

By employing ab-initio electronic structure calculations based on the density functional theory, we investigated the adsorption, diffusion, bonding, and electronic properties associated with the enhanced bonding of Au on hydroxylated MgO(001) surfaces. The calculations in this study show that the binding of Au on hydroxylated MgO(001) is substantially enhanced by an ionic-like interaction as compared to the case of Au on regular MgO(001). Indeed, the adsorption of Au on hydroxylated MgO surfaces is stronger than that of Au and OH on MgO(001). AuOH complexes are formed on MgO(001) surfaces via the fast surface diffusion of OH and Au. It is found that the AuOH structure is very stable against not only dissociation (i.e., it is very difficult for it to decompose back into OH and Au) but also surface diffusion (i.e., it has low surface mobility). More detailed electronic structure analysis of the charge distribution of AuOH on MgO(001) reveals that the enhanced ionic-like bonding is achieved via the polarizat...

Mitigation of CO poisoning on functionalized Pt–TiN surfaces

It has been previously reported that the system of single Pt atoms embedded in N-vacancy (V(N)) sites on the TiN(100) surface (Pt-TiN) could be a promising catalyst for proton exchange membrane fuel cells (PEM FCs). The adsorption of molecules on Pt-TiN is an important step, when it is incorporated as the anode or cathode of PEM FCs. Utilizing first principles calculations based on density functional theory, systematic investigations are performed on the adsorption of several atomic and molecular species on the Pt-TiN system, as well as the co-adsorption of them. The favorable binding sites and adsorption energies of several molecular species, namely carbon dioxide (CO2), carbon monoxide (CO), oxygen (O2), hydrogen (H2), hydroxyl (OH), an oxygen atom (O), and a hydrogen atom (H), are explored. For each, the adsorption energy and preferred binding site are identified and the vibrational frequencies calculated. It is found that CO2, CO and H prefer the Pt top site while OH and O favorably adsorb on the Ti top site. When CO and OH are co-adsorbed on the Pt-TiN(100) surface, OH weakens the adsorption of CO. The weakening effect is enhanced by increasing the coverage of OH. A similar behavior occurs for H and OH co-adsorption on the Pt-TiN(100) surface. Because co-adsorption with OH and H species weakens the adsorption of CO on Pt-TiN, it is expected that the acid and base conditions in PEM FCs could mitigate CO poisoning on functionalized Pt-TiN surfaces.

Adsorption and Surface Diffusion of Pt Atoms on Hydroxylated MgO(001) Surfaces

Density-functional theory calculations have been used to investigate adsorption and surface dynamics of Pt atoms on MgO(001) surfaces with surface-functional hydroxyl groups. Our calculation results show that the adsorption of Pt on hydroxylated MgO(001) is considerably enhanced by interactions among Pt, OH, and MgO surface atoms. We also find that the formation of PtOH complexes instead of Pt dimers is clearly favorable. This behavior is very similar to that of Au deposition on hydroxylated MgO(001). With regard to the surface kinetics, however, the behavior is quite different. Indeed, after the formation of PtOH on MgO(001), the surface diffusion of PtOH is found to be enhanced, as compared to that of Pt on MgO(001). This behavior is in sharp contrast with the low surface mobility previously observed for AuOH on MgO(001). Finally, the reason for this difference is discussed, based on the calculated electronic structures and charge states of Pt on hydroxylated MgO(001).

Ab-initio Study of Transition-Metal Dimers on Defective MgO(001) Surfaces: Atomic and Electronic Structures

The adsorption and electronic structures of transition-metal (TM) dimers (Cu, Ag, Au, Ni, Pd, and Pt) on O-defective MgO(001) surfaces were investigated systematically using spin-dependent density-functional theory calculations. The F s centers formed from the removal of the surface oxygen atoms were considered surface defects of the O-defective MgO(001) surfaces. The preferential adsorption of the TM dimers on the F s site was found, and the adsorption energies were much lower than those of the TM dimers on the defect-free MgO(001) surfaces. The role of the F s center in the formation of the TM dimers on the O-defective MgO(001) was discussed. Further calculations of the electronic structures of the TM dimers on the F s site exhibited significant variation of the TM-induced gap states with the TM elements. The results were also discussed in connection with the adsorption of the TM dimers on metal-supported ultrathin oxide films.

Free energy approach to the formation of an icosahedral structure during the freezing of gold nanoclusters

The freezing of metal nanoclusters such as gold, silver, and copper exhibits a structural evolution. The formation of the icosahedral (Ih) structure is dominant despite its energetic metastability. The dynamical aspects of the structural transformations, which are eventually responsible for the kinetics, are studied by calculating free energies of gold nanoclusters. The transition barriers have been determined by using the umbrella sampling technique. Our calculations show that the formation of Ih gold nanoclusters is attributed to the lower free energy barrier from the liquid to the Ih phase compared to the barrier from the liquid to the face-centered-cubic crystal phase.

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.