2014 NTIS 입력매뉴얼

비매

Investigation of hydrogen bonding in ice using gradient corrected Ab Initio pseudopotential method

Thesis (master`s)--서울대학교 대학원 :물리학과,1995.Maste

Ab initio study on the carbon nanotube with various degrees of functionalization

We present our ab initio results on carbon nanotubes functionalized with dichlorocarbene (CCl2). We find that the electronic properties of carbon nanotubes are significantly modified depending on the site of [2 + 1] cycloaddition. The random addition of dichlorocarbene is found to reduce the band gap of semiconducting nanotubes while those of metallic nanotubes are increased. This is related to the quasi-bound states within the energy gap originated from dichlorocarbene, which are revealed at the large supercell calculation. © 2005 Elsevier B.V. All rights reserved

LDA+U study on fully relaxed LaTiO 3 and (SrTiO 3) m(LaTiO 3) n superlattice structures

Using LDA+U (where LDA stands for local-density-approximation and U for on-site coulomb energy) method, we study the structural and electronic properties of LaTiO 3 and (SrTiO 3)m(LaTiO 3) n superlattice structures. Lattice vectors, as well as ionic positions, are relaxed to minimize the LDA+U energy functional. We find that the inclusion of the U term increases the lattice parameters and leads to larger distortions of oxygen octahedra in LaTiO 3 and that the overall agreement with experiment is improved compared to LDA results. In the superlattice, we find that octahedral distortions around the La layer lower the total energy. The ionic relaxations, especially those of Ti atoms near the La layer, affect the spatial distribution of doped electrons, leading to a broader charge profile than the case without ionic relaxation. The corresponding Ti 3+ profile is in good agreement with the electron-energy-loss spectroscopy data

Pairing of cation vacancies and gap-state creation in TiO2 and HfO2

Based on the first-principles calculations, the authors study defect-defect interactions between cation vacancies in rutile TiO2 and monoclinic HfO2. It is found that vacancies are greatly stabilized at small separations because of a large reconstruction of nearby oxygen atoms that have two broken bonds. As a result, O-O bonds resembling O2 or O 3 molecules are formed near the divacancy site. The defect levels originated from antibonding states of O p orbitals are identified within the energy gap, which can affect leakage currents and the density of trapped charges of oxides substantially. © 2007 American Institute of Physics

Field emission of doped carbon nanotubes

We calculated field-emission currents from nitrogen- and boron-doped single-walled (5,5) carbon nanotubes by integrating time-dependent Schrödinger equations. Nitrogen doping increased the emission current owing to a shift in the energy level of a localized state to the Fermi level, and the creation of coupled states that have characteristics of both localized and extended states. On the other hand, boron doping had an opposite effect on the electronic structure by increasing the energy level of the localized state. The calculated emission currents of the boron-doped carbon nanotube fluctuated depending on the doping site and the external electric field. © 2006 American Institute of Physics

Ab initio study of the effect of nitrogen on carbon nanotube growth

The energetics and kinetics of carbon nanotube growth are studied using an ab initio method. Specifically, the role of the nitrogen atom is analysed in detail for various pathways to the growth of the nanotube edge. The energy barriers are estimated by identifying transition states and it is found that the growth rate of a zigzag-type edge is significantly enhanced. The underlying physical mechanism is explained based on the electronic structure of nitrogen atoms embedded in the carbon networks. © 2006 IOP Publishing Ltd

Band gap sensitivity of bromine adsorption at carbon nanotubes

We report results of our first-principles investigation on the energetics and electronic structures of bromine-adsorbed carbon nanotubes. While the bromine molecule binds preferentially to the outer wall of metallic nanotubes, the binding energy of adsorbed atomic bromines are found to depend on the radius as well as the energy gap. A recent experiment on the nanotube separation using bromines is discussed based on our computational data. The formation of strong C-Br chemical bonds at the zigzag edge of graphite demonstrates a close relationship between the density of states at the Fermi level and the binding strength. © 2005 Elsevier B.V. All rights reserved

Electronic structure tailoring and selective adsorption mechanism of metal-coated nanotubes

Effects of various metal coating (Co, Ti, Pd, W, and Ru) on electronic structures of carbon nanotubes are systematically studied by both ab initio calculations and field-emission experiments. The theoretical results indicate that the adsorption of metal atoms leads to substantial changes in the band structures and work functions of nanotubes. In particular, titanium is found to be the most effective coating material for the application of nanotubes to the field emission display, by lowering the work function and increasing the local density of states near the Fermi level. This is confirmed by the field-emission experiments using Ti-coated nanotubes, which shows enhanced emission performances. In addition, it is found that the Ti coating extends the lifetime of the nanotube substantially. Through the thermogravimetric analysis and theoretical modeling, we propose that this is related to the role of metal coating as a protection layer against residual gases such as oxygen, which cause the degradation of nanotubes. The applications of metal-coated nanotubes to other types of electronic devices are also discussed. © 2008 American Chemical Society

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

Using ab initio calculations, we examine effects of encapsulated metal atoms inside a C60 molecule on the activation energy barrier for the Stone-Wales transformation. The encapsulated metal atoms we study are K, Ca, and La which nominally donate one, two, and three electrons to the C60 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 empty C60 (7.16 eV). The lower activation energy barrier of the Stone-Wales transformation implies the higher probability of isomerization and coalescence of metallofullerenes, which require a series of Stone-Wales transformations. © 2006 The American Physical Society