Organic nanostructures on hydrogen-terminated silicon report on electric field modulation of dangling bond charge state.

We pursue dynamic charge and occupancy modulation of silicon dangling bond sites on H-Si(100)-2 × 1 with a biased scanning tunneling microscope tip and demonstrate that the reactivity and mechanism of product formation of cyclobutylmethylketone (CBMK) on the surface at the active sites may be thus spatially regulated. Reactivity is observed to be dependent on the polarity between tip and surface while the area over which reactivity modulation is established scales according to the dopant concentration in the sample. We account for these observations with examination of the competition kinetics applicable to the CBMK/H-Si reaction and determine how said kinetics are affected by the charge state of DB sites associated with reaction initiation and propagation. Our experiments demonstrate a new paradigm in lithographic control of a self-assembly process on H-Si and reveal a variant to the well-known radical mediated chain reaction chemistry applicable to the H-Si surface where self-assembly is initiated with dative bond formation between the molecule and a DB site.

Stereoselective cycloaddition of 1,3-cyclohexadiene on Si(100): a simple algorithm for product identification based on secondary orbital interactions.

We consider the reaction of 1,3-cyclohexadiene (1,3-CHD) on Si(100) and show that the observed reactivity and stereoselectivity cannot be explained on the basis of thermodynamics. We postulate the existence of secondary orbital interactions (SOIs) and introduce a simple algorithm that examines all possible secondary interactions between the frontier orbitals of the molecule and the surface. We demonstrate using an orbital symmetry-based algorithm supported by DFT calculations that SOIs favor a particular molecular configuration, consistent with the experimental observations. The potential role of SOIs in controlling surface chemical reactions is discussed.

Frontier orbital description of the Si(100) surface: a route to symmetry-allowed and concerted [2 + 2] cycloadditions.

We present a conceptually simple frontier orbital description of an ideal Si(100) surface by extending the standard orbital description for a single Si dimer unit across the surface Brillouin zone. Density functional theory calculations are used to order the predicted frontier wave functions in terms of energy. When applied to the p(2 x 1) and c(4 x 2) reconstructions, this analysis provides a route for the controversial [2 + 2] cycloaddition reaction, which was previously thought to involve a violation of the Woodward-Hoffman rules. The calculated frontier states are shown to be a valuable aid in describing reactivity on Si(100) that is consistent with experiment and provides a rational means to predict allowed reaction products on Si(100).