Bare Dimers Research Articles

Chemisorption-induced disruption of surface electronic structure: Hydrogen adsorption on theSi(100)−2×1surface

The influence of hydrogenated Si(100) dimers on the geometry and electronic structure of neighboring bare dimers is studied by scanning tunnel microscope (STM) and ab initio calculations. Two coverage regimes are of interest. First, the presence of isolated hydrogenated dimers is shown to perturb neighboring bare dimers such that the barrier-for-dimer tilting is reduced with respect to that on the clean surface. At low temperatures this manifests itself in STM images as regions of continuously tilting $p(2\ifmmode\times\else\texttimes\fi{}1)$ bare dimers that fail to condense into the $c(4\ifmmode\times\else\texttimes\fi{}2)$ or $p(2\ifmmode\times\else\texttimes\fi{}2)$ structures characteristic of the clean surface. In the second case we studied the structure of isolated bare dimers on an otherwise completely hydrogenated Si(100) surface. These isolated bare dimers are weakly buckled compared to those on the clean surface and exhibit an enhanced $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ energy splitting, in agreement with scanning tunneling spectroscopy studies. The spatial distribution of the \ensuremath{\pi} and ${\ensuremath{\pi}}^{*}$ density is significantly different from that found on the clean surface but corresponds closely to that seen in filled- and empty-state STM images.

Theoretical study of the chemical vapor deposition of (100) silicon from silane

We use density functional theory to investigate the chemical vapor deposition of (100) silicon from silane. The reaction proceeds through four sequential steps. The first step is activation of surface sites through hydrogen abstractions by atomic H or through ${\mathrm{H}}_{2}$ desorption. We find that hydrogen abstraction barriers by atomic H are less than 1 kcal/mol while ${\mathrm{H}}_{2}$ desorption proceeds through a two-step pathway with an overall barrier of 61.1 kcal/mol. Next, adsorption of ${\mathrm{SiH}}_{4}$ onto bare dimer sites occurs. We calculate the B3LYP barrier to ${\mathrm{SiH}}_{4}$ adsorption on a single dimer to be 7.4 kcal/mol while the barrier across two dimers is 14.3 kcal/mol. Then, adsorbed ${\mathrm{SiH}}_{3}$ transforms to bridged ${\mathrm{SiH}}_{2}(a)$ with a barrier of 5.7 kcal/mol relative to ${\mathrm{SiH}}_{3}(a)$ for the mechanism requiring $\mathrm{H}(g)$ while the barrier for the mechanism requiring no $\mathrm{H}(g)$ is 32.9 kcal/mol, where (g) and (a) represent gas and adsorbed species, respectively. Finally, the dihydride surface transforms to the monohydride surface through two-sequential steps with an overall barrier of 47.0 kcal/mol, which agrees well with the TPD barrier of 43 kcal/mol. The B3LYP ${\mathrm{H}}_{2}$ desorption barrier of 61.1 kcal/mol and ${\mathrm{SiH}}_{4}$ adsorption barrier of 7.4 kcal/mol are in good agreement with the TPD values of 57.2 to 58 kcal/mol and 3.3 to 4.0 kcal/mol, respectively.

Role of interdimer interactions in NH3 dissociation on si(100)-(2x1).

The dissociation of NH3 on Si(100)-(2x1) is investigated by a combination of infrared absorption spectroscopy and density functional cluster calculations, revealing that this reaction is governed by a complex set of interdimer interactions involving both bare and adsorbate-covered Si dimers. We propose that such adsorbate-induced changes in the electronic structure of neighboring dimers may have general implications for controlling the two-dimensional ordering of reactions on the dimerized Si(100) surface.

Adsorption of Water on the Si(100) Surface: An Ab Initio and QM/MM Cluster Study

The adsorption of water on the Si(100) surface is investigated using ab initio quantum chemical cluster calculations. A reaction profile is constructed using the multiconfigurational SCF method. The calculations demonstrate that the reactant should be described with a multireference wave function in order to obtain correct energetics, because it contains a bare dimer with significant diradical character. The system becomes almost single-configurational as water approaches the surface and forms a molecularly adsorbed intermediate. Therefore, except for the reactant, a single-configurational wave function seems to be sufficient for a correct description of the reaction. The adsorbed OH group in an isolated product can nearly freely rotate between the trans and gauche minima. Interactions between the OH groups and the dangling bonds are small and do not appear to change the OH orientation. However, the interdimer hydrogen bonding is stronger and forces the OH orientation to be perpendicular to the dimer bond. The free rotation of the OH group in an isolated dimer model and the hydrogen-bonding picture in an extended cluster model are consistent with the experimental finding for the OH orientation in the product. Si9H12, Si15H16, Si32H28, Si48H36, and Si64H44 cluster models for the Si(100) surface are used, and the SIMOMM (surface integrated molecular orbital molecular mechanics) method is used effectively for these large cluster calculations. The SIMOMM and full quantum results are compared.

Dimer bond geometry in D/Ge(100)-(2 x 1): A low-energy electron-diffraction structure analysis.

The asymmetry of the Ge dimer in the (2\ifmmode\times\else\texttimes\fi{}1) reconstruction of Ge(100) is removed upon adsorption of deuterium D. The $R$-factor analysis indicates a slight remaining asymmetry which is attributed to the coexistence of bare and D-covered dimers. The Ge-Ge bond length of 2.4(2) \AA{} in the dimer does not change within the error limits when compared to the clean surface. The D atoms bond on top of the Ge atoms, exhibiting a Ge-D bond length of 1.6(2) \AA{}.

Multiphoton Ionization Mass Spectrometric Studies of Transition Metal Carbonyl Complexes

The multiphoton photochemistries of a variety of gas phase transition metal carbonyl complexes are investigated by MPI time-of-flight mass spectrometry, and results indicate that molecular fragment ionization can compete with atomization in these systems. Molecular ions detected include a variety of homonuclear and heteronuclear bare metal clusters, metal-sulfur clusters and metal monocarbonyls. Thus, multiphoton excitation of stable metal carbonyl compounds provides a versatile synthetic route to highly unsaturated neutral and ionic gas phase clusters which offers unique product cluster specificity and does not require the use of high temperature sources. The detection under collision-free conditions of bare metal dimers and trimers following MPI of several dinuclear and trinuclear metal cluster carbonyl compounds, respectively, indicates that dissociative CO loss can compete with metal–metal bond cleavage as a primary excited state reaction upon UV photolysis of these systems.