Browsing by Subject "Valence bands"

Now showing 1 - 4 of 4

Open Access
Energetics and Electronic Structures of Individual Atoms Adsorbed on Carbon Nanotubes
(American Chemical Society, 2004) Durgun, Engin; Dag, S.; Çıracı, Salim; Gülseren, O.
The adsorption of individual atoms on the semiconducting and metallic single-walled carbon nanotubes (SWNT) has been investigated by using the first principles pseudopotential plane wave method within density functional theory. The stable adsorption geometries and binding energies 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 depends 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. Although 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 a 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. 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 ID nanomagnets or nanoconductors, conducting connects, and so forth.
Open Access
Preparation, surface state and band structure studies of SrTi (1-X)Fe (x)O (3-δ) (x = 0-1) perovskite-type nano structure by X-ray and ultraviolet photoelectron spectroscopy
(2012) Ghaffari, M.; Shannon, M.; Hui H.; Tan O.K.; Irannejad, A.
In this report, SrTi (1 - x)Fe (x)O (3 - δ) photocatalyst powder was synthesized by a high temperature solid state reaction method. The morphology, crystalline structures of obtained samples, was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscopy (TEM), respectively. The electronic properties and local structure of the perovskite STF x (0 ≤ x ≤ 1) systems have been probed by extended X-ray absorption fine structure (EXAFS) spectroscopy. The effects of iron doping level x (x = 0-1) on the crystal structure and chemical state of the STF x have been investigated by X-ray photoelectron spectroscopy and the valence band edges for electronic band gaps were obtained for STF x by ultraviolet photoelectron spectroscopy (UPS). A single cubic perovskite phase of STF x oxide was successfully obtained at 1200 °C for 24 h by the solid state reaction method. The XPS results showed that the iron present in the STF x perovskite structure is composed of a mixture of Fe 3+ and Fe 4+ (SrTi (1 - x)[Fe 3+, Fe 4+] (x)O (3 - δ)). When the content x of iron doping was increased, the amount of Fe 3+ and Fe 4+ increased significantly and the oxygen lattice decreased on the surface of STF x oxide. The UPS data has confirmed that with more substitution of iron, the position of the valence band decreased. © 2011 Elsevier B.V. All rights reserved.
Open Access
Soft x-ray photoemission studies of the HfO2/SiO2/Si system
(American Institute of Physics, 2002) Sayan, S.; Garfunkel, E.; Süzer, Şefik
Soft x-ray photoelectron spectroscopy with synchrotron radiation was employed to study the valence-band offsets for the HfO2/SiO 2/Si and HfO2/SiOxNy/Si systems. We obtained a valence-band offset difference of -1.05±0.1eV between HfO 2 (in HfO2/15ÅSiO2/Si) and SiO 2 (in 15 Å SiO2/Si). There is no measurable difference between the HfO2 valence-band maximum positions of the HfO2/10ÅSiOxNy/Si and HfO 2/15ÅSiO2/Si systems. © 2002 American Institute of Physics.
Open Access
Theoretical investigation of excited states of oligothiophene anions
(2008) Alkan, F.; Salzner, U.
Electron-hole symmetry upon p- and n-doping of conducting organic polymers is rationalized with Hückel theory by the presence of symmetrically located intragap states. Since density functional theory (DFT) predicts very different geometries and energy level diagrams for conjugated π-systems than semiempirical methods, it is an interesting question whether DFT confirms the existence of electron-hole symmetry predicted at the Hückel level. To answer this question, geometries of oligothiophene anions with 5-19 rings were optimized and their UV/vis spectra were calculated with time-dependent DFT. Although DFT does not produce symmetrically placed sub-band energy levels, spectra of cations and anions are almost identical. The similarity in transition energies and oscillator strengths of anions and cations can be explained by the fact that the single sub-band energy level of cations lies above the valence band by the same amount of energy as the single sub-band level of anions lies below the conduction band. This and the resemblance of the energy level spacings in valence bands of cations to those in conduction bands of anions give rise to peaks with equal energies and oscillator strengths. © 2008 American Chemical Society.