Self-Propagating Reaction Produces Near-Ideal Functionalization of Si(100) and Flat Surfaces

Chemical functionalization of the technologically important face of silicon, Si(100), to form a passivated semiconductor/organic interface would enable a wide variety of applications, including microelectronic devices with integrated chemical or biological functionality; however, this goal has been stymied by the sterically hindered structure of the (100) surface, which impedes uniform chemical reaction. Here we demonstrate production of near-atomically flat H-functionalized Si(100) surfaces from a self-propagating chemical reaction that targets a previously unrecognized reactive pair of silicon atoms. Scanning tunneling microscopy, infrared spectroscopy, and kinetic Monte Carlo simulations are used to measure the surface-site-specific rates of chemical reaction and to quantitatively explain the observed morphologies. The production of uniform H-terminated Si(100) surfaces is controlled primarily by two aspects of dihydride reactivity. First, row-end dihydrides are 1000 times more reactive than similar midrow dihydrides. Second, dihydride reactivity is not monotonically correlated with interadsorbate strain of the reacting site. Instead, dihydride reactivity is correlated with interadsorbate strain release by adjacent dihydrides during the chemical reaction. This unusual dependence on interadsorbate strain produces a characteristic alternating row morphology dominated by single-atom-wide rows. The proposed reaction mechanism, which involves a silanone intermediate, explains the etch morphology, the site-specific reactivities, the reaction kinetics, the production of H₂, and the hydrogen termination of the reacted surfaces. Strategies for the production of uniformly functionalized Si(100) surfaces based on this reaction are discussed

Lithium Diisopropylamide-Mediated Ortholithiation of 2‑Fluoropyridines: Rates, Mechanisms, and the Role of Autocatalysis

Lithium diisopropylamide (LDA)-mediated ortholithiations of 2-fluoropyridine and 2,6-difluoropyridine in tetrahydrofuran at −78 °C were studied using a combination of IR and NMR spectroscopic and computational methods. Rate studies show that a substrate-assisted deaggregation of LDA dimer occurs parallel to an unprecedented tetramer-based pathway. Standard and competitive isotope effects confirm post-rate-limiting proton transfer. Autocatalysis stems from ArLi-catalyzed deaggregation of LDA proceeding via 2:2 LDA–ArLi mixed tetramers. A hypersensitivity of the ortholithiation rates to traces of LiCl derives from LiCl-catalyzed LDA dimer–monomer exchange and a subsequent monomer-based ortholithiation. Fleeting 2:2 LDA–LiCl mixed tetramers are suggested to be key intermediates. The mechanisms of both the uncatalyzed and catalyzed deaggregations are discussed. A general mechanistic paradigm is delineated to explain a number of seemingly disparate LDA-mediated reactions, all of which occur in tetrahydrofuran at −78 °C