Charge-Transfer in Silicon Governs the Pattern of Dissociative Attachment of Hydrogen Halides: HCl, HBr, and HI

Abstract

The dissociative attachment of hydrogen halides, HCl, HBr, and HI, on Si(100) was studied by scanning tunneling microscopy, and modeled by molecular dynamics computations based on density functional theory. The relative yields of on-dimer (OD), inter-dimer (ID), and inter-row (IR) products, reported here for the first time, were unaltered by temperature change, indicative of barrier-free reaction. Interdimer reaction was found experimentally to be overwhelmingly the favored reaction path at all temperatures, 175−300 K. This is a preference that theory accounts for by a combination of kinematics favoring the initial approach of H, and subsequent charge-displacement directing the halogen atom to an inter-dimer site. Molecular dynamics, with statistical sampling of the initial collision-geometry at the surface, showed that the light H-atom invariably was the first to engage with the surface, forming H−Si. This bond-formation resulted in directional charge-transfer to a neighboring Si dimer-pair of the same dimer-row. As a consequence this neighboring Si became attractive to the incoming electrophilic halogen-atom, accounting for the observed strong preference for the inter-dimer outcome.

Electron Attachment Leads to Unidirectional In-Plane Molecular Rotation of Para-Chlorostyrene on Si(100)

“…In state 2, the Cl is above an up-Si, as determined from examination of the adjacent surface buckling. This configuration is a short-lived intermediate since halogen atoms move to δ+ down-Si atoms. , The nearest available down-Si is one-lattice-spacing away along the dimer row (Figure ). At room temperature, the molecule can readily overcome the 0.91 eV energy barrier to convert to this position, thermally continuing the rotation.…”

How dissociated fragments of multiatomic molecules saturate all active surface sites—H2O adsorption on the Si(100) surface

“…The adsorption process for an H 2 O molecule on a pDB surrounded by monohydride dimers in figures 1(a) and (b) can be interpreted as that of a clean Si(100) surface through the reaction equation (1). It is also exothermic and spontaneous.…”

“…Dissociative chemisorption of diatomic or multiatomic molecules onto metal and semiconductor surfaces is one of the most fundamental surface chemical reactions in the practical applications, such as atomic layer epitaxy, catalyzed reactions, and dry etches, etc [1][2][3][4][5][6][7][8] The typical mechanism of the dissociative chemisorption of a gas molecule F 1 -F 2 describes that two new bonds are created simultaneously, or within tens of femtoseconds [1], between the two fragments, F 1 and F 2 , and the substrate surface while the molecular bond is cleaving [9,10]. In this scenario, two or more immediately neighboring active surface sites are present to break the molecular bond in favor of new bonds to the surface-S-F 1 and S-F 2 , where S denotes a surface atom.…”