Density functional theory study of one-dimensional growth of styrene on the hydrogen-terminated Si(001)-(3 x 1) surface

Recent experimental work has shown that the addition of styrene molecules to hydrogen-terminated Si(001) surfaces leads to the formation of one-dimensional molecular structures through a radical-initiated surface chain reaction mechanism. These nanometric structures are observed to be directed parallel to the dimer rows on the H-Si(001)-(2 x 1) surface and perpendicular to the same rows on H-Si(001)-(3 x 1). Using periodic density functional theory (DFT) calculations, we have studied the initial steps of the radical chain mechanism on the H-Si(001)-(3 x 1) surface and compared them to analogous results for H-Si(001)-(2 x 1). On the H-Si(001)-(3 x 1) surface, one of the crucial steps of the surface chain reaction, namely, the abstraction of a H atom from a nearby surface hydride unit, is found to have a somewhat smaller activation energy in the direction perpendicular to the dimer rows (H abstraction from the nearest dihydride site) than along the rows (H abstraction from a neighboring dimer). Additionally, due to the steric repulsion between the styrene molecules and the SiH2 subunits, growth along the dimer rows is not thermodynamically favorable on the (3 x 1) surface. On the other hand, due to the absence of the SiH2 subunits, growth parallel to the Si dimer rows becomes favored on the H-Si(001)-(2 x 1) surface.     

Optimizing sensor cover energy via DC programming

Wireless sensor coverage problem has been extensively studied in the last years, with growing attention to energy efficient configurations. In the paper we consider the problem of determining the radius of a given number of sensors, covering a set of targets, with the objective of minimizing the total coverage energy consumption. The problem has a non linear objective function and non convex constraints. To solve it we adopt a penalty function approach which allows us to state the problem in difference of convex functions form. Some numerical results are presented on a set of randomly generated test problems.     

Electronic states and electron-phonon coupling at the 2×1 reconstructed Si(111) surface

The electronic structure of Si(111)-2x1 is studied selfconsistently for several values of the reconstruction parameters within Haneman's model. The derivatives of the dangling-bond bands with respect to the surface atom displacements are calculated and the surface electron-phonon coupling is estimated.     

Model band structure of reconstructed (111) 2 × 1 surface of silicon

We have computed the electronic band structure for a model of the 2 × 1-reconstructed (111) surface of Si, based on 2 two-dimensional net of dangling bonds. A pairing of surface atoms is assumed, involving a displacement and a tilting of the dangling bonds towards each other. The energy gap that separates the two bands of surface states obtained increases with the degree of reconstruction, which is taken as a parameter. Experimental data, particularly surface optical absorption, find a satisfactory explanation in terms of this calculation, which indicates in Si (111) 2 × 1 a shift of the surface atoms by about 8% of their ideal distance.     

A combined computational and experimental study of polynuclear Ru-TPPZ complexes: insight into the electronic and optical properties of coordination polymers

We report a combined experimental and computational study of polynuclear [Ru(n)(TPPZ)(n)(+1)](2)(n)(+) complexes, of interest in the field of photoactive polymers. The complexes with n = 1, 2, 3 and n > 5 have been synthesized and spectroscopically characterized. A red-shift of the visible band maximum from 2.59 to 2.03 eV is observed going from the monomer to the longer oligomeric species (n > 5). To characterize the geometries, electronic structure, and excited states of these complexes, density functional theory (DFT) and time-dependent DFT calculations on the [Ru(n)(TPPZ)(n)(+1)](2)(n)(+) series with n = 1-4 in solution have been performed. The agreement between experimental and calculated spectra is good, both in terms of absorption maximum energies and relative intensities for different values of n. For all the investigated complexes, we assign the main band in the visible region as a metal-to-metal plus ligand charge transfer (MMLCT) transition. The resulting excited states are delocalized throughout the entire complexes, as they originate from a superposition of pi(TPPZ)-t(2g)(Ru) states. The low-energy shoulder of the main visible absorption band, present in the experimental spectra for n > 1, is proposed to arise from spin-forbidden singlet-triplet transitions of similar MMLCT character, consistent with the observed enhancement of this feature in the spectra of the corresponding Os oligomers.     

Characterization of paramagnetic species in N-doped TiO2 powders by EPR spectroscopy and DFT calculations

Electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined for the first time in an effort to characterize the paramagnetic species present in N-doped anatase TiO2 powders obtained by sol-gel synthesis. The experimental hyperfine coupling constants are well reproduced by two structurally different nitrogen impurities: substitutional and interstitial N atoms in the TiO2 anatase matrix. DFT calculations show that the nitrogen impurities induce the formation of localized states in the band gap. Substitutional nitrogen states lie just above the valence band, while interstitial nitrogen states lie higher in the gap. Excitations from these localized states to the conduction band may account for the absorption edge shift toward lower energies (visible region) observed in the case of N-doped TiO2 with respect to pure TiO2 (UV region). Calculations also show that nitrogen doping leads to a substantial reduction of the energy cost to form oxygen vacancies in bulk TiO2. This suggests that nitrogen doping is likely to be accompanied by oxygen vacancy formation. Finally, we propose that the relative abundance of the two observed nitrogen-doping species depends on the preparation conditions, such as the oxygen concentration in the atmosphere and the annealing temperature during synthesis.