1,721,025 research outputs found
Kinetics of desorption from heterogeneous surfaces
No full textThe desorption kinetics from heterogeneous surfaces contain information on energy heterogeneity. Mathematical methods for extracting the energy distribution function from desorption kinetics are developed. The overall desorption kinetics are described as weighted averages of the local desorption kinetics from each energetically homogeneous zone forming the surface. Whatever the local kinetics, the energy distribution is determined from the overall kinetics by using the condensation approximation. The method is applied to two situations: the time-logarithm law observed in a lot of systems of practical interest in catalysis, and the time-power law observed in many relaxation kinetics of macromolecules
XPS detection of some redox phenomena in Cu-zeolites
No full textX-ray photoelectron spectroscopy analysis of Cu 2p peaks is able to discriminate between Cu(II) and Cu(I). This is particularly useful when copper is supported in a zeolite matrix, as in heterogeneous catalysts. The preparation route, the Si/Al ratio and the kind of zeolite all contribute to determine the final oxidation state of copper. In turn, the oxidation state determines the effectiveness of the catalyst. Samples prepared via the gas phase and solution are considered in this work after calcination. Both H-zeolite and Na-zeolite are found in different copper final states when considered just after preparation, depending on ion-exchange or impregnation routes. Copyright (C) 2002 John Wiley Sons, Ltd
Bonding configurations and observed XPS features at the hydrogen terminated (100) Si surface: what can we gain from computational chemistry.
No full textDensity functional (DFT) calculations for different size cluster models of the hydrogen-terminated HFaq-etched (100) Si surface have been performed to verify that the quantities of interest (namely, atomic net charges and interatomic distances) in assigning the lines observed by X-ray photoelectron spectroscopy (XPS) vary weakly with cluster size. Net charge analysis based on Voronoi Deformation Density (VDD) method and accurate DFT geometry optimization calculations involving the smallest clusters as local models of various surface silicon atoms are used to assign chemical species to the features observed in the XPS spectra through evaluation of the chemical shifts, which are controlled by both the net charge and the Madelung potential truncated to nearest neighbors of the considered atom
Using evidence from nanocavities to assess the vibrational properties of external surfaces
No full textInternal surfaces of nanocavities are an exceptionally useful laboratory wherein one can spotlight the factors ruling the intricate interplay between morphology and chemistry at silicon surfaces. At the same time, they offer unparalleled opportunities to validate the assignment of vibrational signals of silicon-terminating species under almost ideal experimental conditions. In the case of hydrogen, evidence will be provided of the detailed evolution of H-related species at surfaces depending on their orientation. Also, preliminary results concerning nitrogen at and around nanocavity surfaces will be reported
Hydrogen and helium bubbles in silicon
No full textHydrogen is a quite common impurity in semiconductor-silicon technology: it is unintentionally but unavoidably added to the silicon after crystal growth during wafer processing, and continues to be present during wet oxidation, film depositions, etching and annealing steps. The effects of hydrogen in single crystal silicon at low concentration have been the subject of many papers, books and conference proceedings. Much less considered is the case of hydrogen at massive concentration. One Anal effect of heavy hydrogen loading is the formation of cavities and bubbles, with size up to 100 nm. Cavities and bubbles are also observed after helium loading by high-fluence ion implantation. This article reviews the basic mechanisms responsible for the formation and growth of such structures in single-crystalline silicon. In particular, starting from the loading (ion implantation) and having in mind the formation of the cavities, this paper will cover: the effects of substrate temperature, the interaction of vacancies and self-interstitials with the impurity, the mechanisms of gas segregation inside the cavities, the pressure which arises because of the segregation and the subsequent displacement field in the crystal, the stability against heat treatments of the gas in the cavities and of the cavities themselves. The understanding of the physical processes should lead to gain more insight in the processes of cleavage of the Si-Si bond and vacancy agglomeration which can induce not only the formation of cavities and bubbles, but also planar cutting or explosion. (C) 2000 Published by Elsevier Science S.A. All rights reserved
Induced Surface Heterogeneity as an Autocatalytic way to Promote Acid-Base Heterogeneous Reactions
No full textGiant Neutron Trapping by a Molecular Species Produced During the Reaction of D+ with H- in Condensed Phase
No full text
Hydrogen injection and retention in nanocavities of single-crystalline silicon
No full textThe control of the chemical state of the inner surfaces of nanocavities (NCs) produced by the annealing of helium-implanted silicon has influence on lifetime control, gettering and wafer bonding. In this work it is demonstrated that the etching in HFaq of (1 0 0) silicon containing a buried array of NCs produces a giant injection of hydrogen with the consequent passivation of the inner surfaces, mainly via the formation of silicon monohydride at (1 1 1) faces and monohydride dimers at 2 × 1 reconstructed (1 0 0) faces. These terminations are very stable and survive heat treatments at 700 °C
Adsorption equilibria and kinetics of H2 at nearly ideal (2 x 1) Si(1 0 0) inner surfaces
No full textSilicon nanocavities can be terminated with hydrogen by wet chemical etching. Their infrared spectra can to a large extent be interpreted in terms of silicon monohydrides on H(7 x 7) Si(1 1 1), H(1 x 1) Si(1 1 1) and H(2 x 1) Si(1 0 0), and of silicondihydrides on H(1 x 1) Si(1 0 0). The time evolution under isothermal conditions (600 °C) of the (1 0 0) faces admits a description in terms of transformation from H(1 x 1) Si(1 0 0) into (2 x 1) Si(1 0 0) with simultaneous H2 adsorption onto (2 x 1) Si(1 0 0) neat dimers. In so doing the inner H2 pressure decreases by about one order of magnitude from the initial value of 3000 Torr. The unique properties of nanocavities allow their use as nanoreactors; this has led to the determination of the sticking coecient for H2 adsorption in conditions of pressure, temperature and adventitious contamination otherwise not achievable
- …
