Browsing by Subject "first principles"

Now showing 1 - 5 of 5

Open Access
Adsorption and dissociation of hydrogen molecule on carbon nanotubes
(2004) Öztürk, Yavuz
Earlier, it has been suggested that carbon nanotubes can provide high storage capacity and other physical properties suitable for the fuel cell technologies. In this thesis we have investigated adsorption, desorption and dissociation of hydrogen molecule on the surface of the zigzag (8,0) single-wall carbon nanotube (SWNT) by carrying out extensive first-principles pseudopotential plane wave calculations within the Density Functional Theory (DFT). We found that while H2 molecule cannot be bound to the surface of bare SWNT, an elastic radial deformation leading to the elliptical deformation of the circular cross-section renders the physisorption of the molecule possible. Coadsorption of Li atom on the SWNT makes the similar effect, and hence enhances the physisorption. That an adsorbed H2 can be desorbed upon releasing the elastic radial strain is extremely convenient for the storage. In addition to that, we found that a Pt atom coadsorbed on the SWNT can form a strong chemisorption bond with a H2 molecule. If a single H2 molecule engages in interactions with more than one coadsorbed Pt atom at its close proximity it dissociates into single H atoms, which, in turn, make Pt-H bonds. The interaction between H2 and coadsorbed Pd atom is similar to Pt, but it is weaker. We believe that these findings clarify earlier controversial results related to the storage of H2 in carbon nanotubes, and makes important contributions to fuel cell technology.
Open Access
A first-principles study of defects and adatoms on silicon carbide honetcomb structures
(2009) Bekaroğlu, Erman
In this thesis a study of electronic and magnetic properties of two dimensional (2D), single layer of silicon carbide (SiC) in hexagonal structure and its quasi 1D armchair nanoribbons are presented by using first-principles plane wave method. In order to reveal dimensionality effects, a brief study of 3D bulk and 1D atomic chain of SiC are also included. The stability analysis based on the calculation of phonon mode frequencies are carried out for different dimensionalities. It is found that 2D single layer SiC in honeycomb structure and its bare and hydrogen passivated nanoribbons are ionic, non magnetic, wide band gap semiconductors. The band gap further increases upon self-energy corrections. Upon passivation of Si and C atoms at the edges of nanoribbon with hydrogen atoms, the edge states are discarded and the band gap increases. The effect of various vacancy defects, adatoms and substitutional impurities on electronic and magnetic properties in 2D single layer SiC and in its armchair nanoribbons are also investigated. Some of these vacancy defects and impurities, which are found to influence physical properties and attain magnetic moments, can be used to functionalize SiC honeycomb structures for novel applications.
Open Access
Magnetism of transition metal nanowires
(2008) Ataca, Can
In this thesis we investigated structural, electronic and magnetic properties of 3d (light) transition metal (TM) atomic chains and Cr nanowires using firstprinciples pseudopotential plane wave calculations. Infinite periodic linear, dimerized linear and planar zigzag chain structures, as well as their short segments consisting of finite number of atoms and chromium nanowires have been considered. For most of the infinite periodic chains, neither linear nor dimerized linear structures are favored; to lower their energy the chains undergo a structural transformation to form planar zigzag and dimerized zigzag geometries. Dimerization in both infinite and finite chains are much stronger than the usual Peierls distortion and appear to depend on the number of 3d-electrons. As a result of dimerization, a significant energy lowering occurs which, in turn, influences the stability and physical properties. Metallic linear chain of vanadium becomes half-metallic upon dimerization. Infinite linear chain of scandium also becomes half-metallic upon transformation to the zigzag structure. The end effects influence the geometry, energetics and the magnetic ground state of the finite chains. Structure optimization performed using noncollinear approximation indicates significant differences from the collinear approximation. Variation of the cohesive energy of infinite and finite-size chains with respect to the number of 3d-electrons are found to mimic the bulk behavior pointed out by Friedel. Furthermore, we considered Cr nanowires, which have cross section comprising a few (4,5 - 9,12) atoms. Chromium nanowires are found to be in a local minimum in the Born-Oppenheimer surface and are ferrimagnetic metals. The type of coupling, as for ferromagnetic or antiferromagnetic, between neighboring Cr atoms depends on their interatomic distances. The spin-orbit coupling of finite chains are found to be negligibly small for finite molecules and Cr nanowires.
Open Access
Silicon and carbon based nanowires
(2004) Tongay, Sefaattin
Nanowires have been an active field of study since last decade. The reduced dimensionality end size allowing electrons can propagate only in one direction has led to quantization which are rather different from the bulk structure. As a result, nanowires having cross section in the range of Broglie wavelength have shown stepwise electrical and thermal conductance, giant Young modulus, stepwise variation of the cross-section etc. Moreover, the atomic structure of nanowires have exhibited interesting regularities which are not known in two or three dimensions. These novel properties of nanowires have been actively explored since last decade in order to find an application in the rapidly developing field of nanotechnology. In the present thesis, we investigated the atomic and electronic structure of a variety of Si and C atom based very thin nanowires starting from linear chain including pentagonal, hexagonal and tubular structures. We found that the C and Si linear chains form double bonds and have high binding energy. Although bulk carbon in diamond structure is an insulator, carbon linear chain is metal and has twice conductance of the gold chain. We carried out an extensive analysis of stability and conductance of the other wires. Our study reveals that Si and C based nanowires generally show metallic properties in spite of the fact that they are insulator or semiconductor when they are in bulk crystal structure. Metallicity occurs due to change in the character and order of bonds.
Open Access
A study of adsorption of single atoms on carbon nanotubes
(2003) Durgun, Engin
The adsorption of individual atoms on the semiconducting and metallic singlewall carbon nanotubes (SWNT) have been investigated by using first-principles pseudopotential plane wave method within Density Functional Theory. The stable adsorption geometry and binding energy have been determined for a large number of foreign atoms ranging from alkali and simple metals to the transition metals and group IV elements. We have found that the character of the bonding and associated physical properties strongly depend on the type of adsorbed atoms, in particular on their valence electron structure. Our results indicate that the properties of SWNTs can be modified by the adsorbed foreign atoms. While the atoms of good conducting metals, such as Zn, Cu, Ag and Au, form very weak bonds, transition metal atoms, such as Ti, Sc, Nb and Ta, and group IV elements C and Si are adsorbed with relatively high binding energy. Owing to the curvature effect, these binding energies are larger than the binding energies of the same atoms on the graphite surface. We have showed that the adatom carbon can form strong and directional bonds between two SWNTs so that the tubes are connected. These connects can be used to produce nanotube networks or grids. Most of the adsorbed transition metal atoms excluding Ni, Pd and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as one-dimensional nanomagnets or nanoconductors and conducting connects etc.