Boris Stefanov

Accurate thermochemistry for larger molecules : gaussian-2 theory with bond separation energies.

Gaussian-2 (G2) theory is combined with isodesmic bond separation reaction energies to yield accurate thermochemistry for larger molecules. For a test set of 40 molecules composed of H, C, O, and N, our method yields enthalpies of formation, ΔHf0(298 K), with a mean absolute deviation from experiment of only 0.5 kcal/mol. This is an improvement of a factor of three over the deviation of 1.5 kcal/mol seen in standard G2 theory.

ASSESSMENT OF COMPLETE BASIS SET METHODS FOR CALCULATION OF ENTHALPIES OF FORMATION

Three complete basis set models of Petersson et al. [J. Chem. Phys. 104, 2598 (1996)], CBS-Q, CBS-q, and CBS-4, have been assessed on the G2 neutral test set of 148 molecules [J. Chem. Phys. 106, 1063 (1997)]. The average absolute deviations with experiment of the calculated enthalpies of formation from the three CBS methods are 1.57 kcal/mol (CBS-Q), 2.13 kcal/mol (CBS-q), and 3.06 kcal/mol (CBS-4). The maximum deviations of the methods are 11.2, 10.3, and 14.4 kcal/mol. respectively. The most accurate method, CBS-Q, has an average absolute deviation similar to that of G2 theory. The three CBS methods have also been assessed on a 40 molecule set using isodesmic bond separation reactions to calculate enthalpies of formation. There is a significant improvement in the accuracy of the enthalpies compared to those calculated using atomization energies, although not as much as for G2 theory. In a test on naphthalene, enthalpies calculated using the CBS methods have large deviations. The CBS-Q method has a devi...

Si–H bending modes as a probe of local chemical structure: Thermal and chemical routes to decomposition of H2O on Si(100)-(2×1)

Surface infrared spectroscopy and density functional cluster calculations are used to study the thermal and atomic hydrogen-induced decomposition of water molecules on the clean Si(100)-(2×1) surface. We report the first observation of the Si–H bending modes associated with the initial insertion of oxygen into the dimer and backbonds of a silicon dimer. We find that, while one and two oxygen-containing dimers are formed almost simultaneously during the thermal decomposition of water on this surface, atomic H can be used to drive the preferential formation of the singly oxidized dimer. This work highlights the sensitivity of Si–H bending modes to the details of local chemical structure in an inhomogeneous system, suggesting that the combined experimental and theoretical approach demonstrated herein may be extremely useful in studying even more complex systems such as the hydrogenation of defects in SiO2 films.

Spectroscopic studies of H-decorated interstitials and vacancies in thin-film silicon exfoliation

Abstract In this paper, we review the pivotal role that defects (in particular vacancy structures) play in driving the H-induced exfoliation of Si. We highlight the central role that infrared spectroscopy has played in delineating the microscopic details of the exfoliation process. We show that when the results of such spectroscopic studies are combined with those obtained using a variety of other experimental probes as well as ab initio quantum chemical cluster calculations, an unambiguous mechanistic picture emerges. Specifically we find that H-terminated vacancy structures drive the formation of internal surfaces into cracks where H2 is then evolved, resulting in the build-up of sufficient internal pressure to cause lift-off of the overlying Si. The role of coimplantation of He is also discussed.

Mechanistic studies of silicon oxidation

The microscopic mechanism of the formation of ultrathin oxides on Si(100) has been investigated using a combination of infrared spectroscopy and ab initio quantum chemical cluster calculations. The 0→2 monolayer oxide films are grown sequentially from the “bottom-up” using repeated water exposures and annealing cycles, with the partial pressure of water ranging from 10−10 to 10 Torr. The resultant films were then compared to the equivalent thicknesses of thermal and native oxide films. In this way, we obtain unprecedented insight into the essential chemical structures formed during the initial oxidation and subsequent layer growth of these technologically relevant films.

Oxidation of Si(100)2 × 1: thermodynamics of oxygen insertion and migration

Abstract Accurate quantum chemical calculations on model cluster molecules are used to identify and compare the most probable surface structures involved in the oxidation of the Si(100)2 × 1 surface. The energetics of various reaction channels for insertion of oxygen into Si-Si bonds are evaluated. The dimer bond is clearly shown to be the initial target of O entry and an oxygen-inserted dimer is proposed as the most likely structure at lower temperatures. At higher temperatures an asymmetrically oxidized dimer unit with three oxygen atoms inserted into Si-Si bonds at the same silicon is the dominant feature.

Pathways for initial water-induced oxidation of Si(100)

First-principles molecular orbital methods and gradient-corrected density functional calculations on silicon clusters are used to study possible pathways for the initial oxidation of Si (100)-2×1. In these reactions, the adsorbed hydroxyl oxygen inserts into the dimer Si–Si bond to form a suboxide (≡Si–O–Si≡) surface structure. The reaction typically follows a two-step pathway involving an intermediate energy minimum. In the case of an ideal surface with full water coverage, the reaction is exothermic by 1.3 eV and the overall reaction barrier is estimated at 2.4 eV. However, an alternative pathway involving a dangling bond site lowers the activation barrier to 2.1 eV. The implications for the oxidation reaction rates are discussed as well as possible alternative pathways.

Photoabsorption of the peroxide linkage defect in silicate glasses

First-principles quantum chemical techniques on cluster models have been used to investigate the photoabsorption of the peroxide linkage defect in silicate glasses. The effects of geometry, basis sets, and cluster size have been considered carefully to derive converged values for the low-lying excitation energies. The lowest singlet-to-singlet transition is a weak absorption at 5.5 eV. A stronger valence absorption occurs at 6.8 eV.

Cluster models for the photoabsorption of divalent defects in silicate glasses: Basis set and cluster size dependence

The two lowest singlet excited states of divalent Si defects in silicate glasses are studied by ab initio methods. Excitation energies of 5.2 and 6.8 eV are obtained. Steady convergence to these values is shown with the increase in size both of the model cluster and of the basis set employed in the calculations. The results clearly demonstrate the viability of the quantum-chemical cluster approach for the study of local excitations in glass defects.