Woon Ih Choi

Hydrogen-Bond Dynamics of Water at the Interface with InP/GaP(001) and the Implications for Photoelectrochemistry

We investigate the structure, topology, and dynamics of liquid water at the interface with natively hydroxylated (001) surfaces of InP and GaP photoelectrodes. Using ab initio molecular dynamics simulations, we show that contact with the semiconductor surface enhances the water hydrogen-bond strength at the interface. This leads to the formation of an ice-like structure, within which dynamically driven water dissociation and local proton hopping are amplified. Nevertheless, the structurally similar and isovalent InP and GaP surfaces generate qualitatively different interfacial water dynamics. This can be traced to slightly more covalent-like character in the binding of surface adsorbates to GaP, which results in a more rigid hydrogen-bond network that limits the explored topological phase space. As a consequence, local proton hopping can give rise to long-range surface proton transport on InP, whereas the process is kinetically limited on GaP. This allows for spatial separation of individual stages of hydrogen-evolving, multistep reactions on InP(001). Possible implications for the mechanisms of cathodic water splitting and photocorrosion on the two surfaces are considered in light of available experimental evidence.

Electrical transport in small bundles of single-walled carbon nanotubes: Intertube interaction and effects of tube deformation

We report a combined electronic transport and structural characterization study of small carbon nanotube bundles in field-effect transistors (FETs). The atomic structures of the bundles are determined by electron diffraction using an observation window built in the FET. The electrical transport of single-walled nanotube bundles depends on the structure of individual tubes, deformation due to intertube interaction, and the orientation with respect to the electric field. Ab initio simulations show that tube deformation in the bundle induces a band gap opening in a metallic tube. These results show the importance of intertube interaction in electrical transport of bundled carbon nanotubes.

Enhanced dihydrogen adsorption in symmetry-lowered metal–porphyrin-containing frameworks

Porphyrin is a very important component of natural and artificial catalysis and oxygen delivery in blood. Here, we report that, based on first-principles density-functional calculations, a hydrogen molecule can be adsorbed non-dissociatively onto Ti-, V-, and Fe-porphyrins, similar to oxygen adsorption in heme-containing proteins, with a significant energy gain, greater than 0.3 eV per H(2). The dihydrogen-heme complex will be non-magnetic, as is oxyhemoglobin. In contrast to the backward electron donation of Fe(III)-O(2)(-) in oxyhemoglobin, the dihydrogen binding originates from electron donation from H(2) to the Fe(II). We have identified that the local symmetry of the transition metal center of porphyrins uniquely determines the binding strength, and, thus, one can even manipulate the strength by intentionally and systematically breaking symmetry.

Modification of the electronic structure in a carbon nanotube with the charge dopant encapsulation

We present the first-principles study of effects of the charge dopants such as cesium and iodine encapsulated on the electronic structure of carbon nanotubes (CNTs). An encapsulated cesium atom donates an electron to the nanotube and produces donorlike states below the conduction bands. In contrast, an iodine trimer (I3) accepts an electron from the nanotube and produces an acceptorlike state above the valence band maximum. We find that a Cs atom inside a metallic armchair CNT gives rise to spatial oscillations of the density of states near the Fermi level.

Reduction of activation energy barrier of Stone-Wales transformation in endohedral metallofullerenes

We examine effects of encapsulated metal atoms inside a C

Band-gap sensitive adsorption of fluorine molecules on sidewalls of carbon nanotubes: an ab initio study

_{60}

Thermal Conductance at Atomically Clean and Disordered Silicon/Aluminum Interfaces: A Molecular Dynamics Simulation Study

molecule on the activation energy barrier to the Stone-Wales transformation using {\it ab initio} calculations. The encapsulated metal atoms we study are K, Ca and La which nominally donate one, two and three electrons to the C

Combinatorial search for optimal hydrogen-storage nanomaterials based on polymers.

_{60}

Divacancy-nitrogen-assisted transition metal dispersion and hydrogen adsorption in defective graphene: A first-principles study

cage, respectively. We find that isomerization of the endohedral metallofullerene via the Stone-Wales transformation can occur more easily than that of the empty fullerene owing to the charge transfer. When K, Ca and La atoms are encapsulated inside the fullerene, the activation energy barriers are lowered by 0.30, 0.55 and 0.80 eV, respectively compared with that of the empty C

Ab initio study of dihydrogen binding in metal-decorated polyacetylene for hydrogen storage

_{60}