High-quality, self-assembled benzoate monolayers were synthesized on rutile (110) using simple aqueous reactions. Sputtering and annealing cycles, which create surface and subsurface defects, were not needed. The monolayers were hydrophobic and remained largely contaminant free during exposures to laboratory air for tens of minutes. During this period, infrared spectroscopy showed that the monolayers did not spontaneously adsorb airborne hydrocarbons or other adventitious aliphatic species. Scanning tunneling microscopy (STM) images, infrared and X-ray photoemission spectra, Monte Carlo simulations, and ab initio calculations were all consistent with benzoate molecules adopting an edge-to-face ring geometry with their four nearest neighbors—a tetrameric bonding geometry. This bonding is further stabilized by a pairing interaction between adjacent benzoate molecules, a pairing that has previously been interpreted as dimerization. The coexistence of paired and unpaired regions of the monolayer is consistent with the relatively small additional energy gained by pairing and the cooperative nature of the pairing interaction. Monolayer stability is driven both by the strong bidentate bonding to unsaturated Ti atoms on the surface as well as by π–π interactions between adsorbates.

The chemical state of a catalyst in operando is particularly important for catalysts that target minority species, such as atmospheric CO2 which has a concentration of only 400 ppm. A reaction can be promoted by the selective binding of reactants or hindered by molecules that block active sites. We show that adsorbed CO2, a very weakly bonded species on TiO2, is unlikely to play the key role in CO2 photoreduction under ambient conditions, at least on rutile (110), as the vast majority of unsaturated Ti sites are terminated by a different, much more strongly bound carbonaceous species: adsorbed bicarbonate (HCO3). Using a combination of scanning tunneling microscopy (STM) and surface spectroscopies, we show that atmospheric CO2 readily and stably displaces adsorbed H2O on rutile (110), creating a self-assembled monolayer of HCO3 and H that is stable at room temperature even in vacuum. This reaction occurs on near-ideal, stoichiometric rutile (110) and does not require surface defects, such as O vacancies, Ti interstitials, or steps. This reaction is promoted both by the strong bidentate bonding of HCO3 as well as the nanoscale H2O film that spontaneously forms on TiO2 under ambient conditions. Density functional theory calculations show that the nanoscale water layer adsorbed to rutile (110) solvates the products and changes the reaction energetics significantly. The chemical state of the catalyst in operando will also be affected by the half-monolayer of adsorbed H produced by the reactive dissociation of H2O.

Cartesian polarization analysis transforms a set of surface infrared spectra obtained in different geometries into their Cartesian components using a mathematical transform, providing direct insight into the bonding geometry of adsorbed molecules. This technique was extended to uniaxial substrates and used to analyze solution-deposited, self-assembled benzoate and alkanoate monolayers on rutile (110). This analysis resolved a long-standing controversy regarding the existence of paired molecules in benzoate monolayers, showing that two distinct isomers exist within the monolayer: a tilted tetramer, which is paired, and a twisted monomer, which is not. The two isomers are nearly isoenergetic, as shown by analysis of STM images and complementary DFT simulations. Infrared and XPS spectra as well as STM images of heptanoate and octanoate monolayers showed the formation of complete monolayers (as opposed to sparse layers or multilayers); however, the alkyl chains in the monolayer are disordered and loosely packed with a significant density of conformational defects—a stark contrast to the near-crystalline, all-trans alkyl monolayers typically formed on Au and Si surfaces. The high disorder in the alkanoate monolayers was attributed to geometry, as the density of alkanoate binding sites on rutile (110) is 30% less than the density of alkyl monolayers on Si. The high density of gauche defects in alkanoate monolayers was attributed to the small energy difference between the all-trans and single-gauche-defect conformers in isolated alkyl chains. In contrast, strong intermolecular interactions in tight-packed alkyl monolayers on Au and Si surfaces suppress gauche defect formation.

Many chemical reactions—etching, growth, and catalytic—produce highly faceted surfaces. Examples range from the atomically flat silicon surfaces produced by anisotropic etchants to the wide variety of faceted nanoparticles, including cubes, wires, plates, tetrapods, and more. This faceting is a macroscopic manifestation of highly site-specific surface reactions. In this Account, we show that these site-specific reactions literally write a record of their chemical reactivity in the morphology of the surface—a record that can be quantified with scanning tunneling microscopy.Paradoxically, the sites targeted by these highly site-specific reactions are extremely rare. This paradox can be understood from a simple kinetic argument. An etchant that produces atomically flat surfaces must rapidly etch every surface site except the terrace atoms on the perfectly flat surface. As a result, the etch morphology is dominated by the least reactive species (here, the terrace sites), not the most reactive species. In contrast, the most interesting chemical species—the site where the reaction occurs most rapidly and most selectively—is the hardest one to find. This highly reactive site, the key to the reaction, is the needle in the haystack, often occurring in densities far below 1% of a monolayer and thus invisible to surface spectroscopies. This kinetic argument is quite general and applies to a wide variety of reactions, not just etching reactions. Understanding these highly site-specific reactions requires a combination of experimental and computational techniques with both exquisite defect sensitivity and high chemical sensitivity.In this Account, we present examples of highly site-specific chemistry on the technologically important face of silicon, Si(100). In one example, we show that the high reactivity of one particular surface site, a silicon dihydride bound to a silicon monohydride, or an “α-dihydride”, provides a fundamental explanation for anisotropic silicon etching, a technology widely used in micromachining to selectively produce flat Si{111} surfaces. Fast-etching surfaces, such as Si(100) and Si(110), have geometries that support autocatalytic etching of α-dihydrides. In contrast, α-dihydrides exist only at kink sites on Si(111) surfaces. As a result, the etch rate of surfaces vicinal to Si(111) scales with the step density, approaching zero on the atomically flat surface.In a second example, we explain the chemistry that underlies pyramidal texturing of silicon wafers, a technique that is sometimes used to decrease the reflectivity of silicon solar cells. We show that a subtle change in chemical reactivity transforms a near-perfect Si(100) etchant into one that spontaneously produces nanoscale pyramids. The pyramids are not static features; they are self-propagating structures that evolve in size and location as the etching proceeds. The key to this texturing is the production of a very rare defect at the apex of each pyramid, a site that also etches autocatalytically.These experiments show that simple chemical reactions can enable an exquisite degree of atomic-scale control if only we can learn to harness them.

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Chemical control of crystalline surfaces during etching or growth finds application in a wide variety of areas, including the low-cost texturing of silicon solar cells to reduce reflectivity losses. Nevertheless, the kinetic processes that govern these morphological transformations are poorly understood. Here, we study the spontaneous nanoscale faceting of Si(100) surfaces during reaction with deoxygenated H2O using a combination of scanning tunneling microscopy, infrared spectroscopy, and kinetic Monte Carlo simulation. We show that this reaction is inherently unstable to kinetic faceting and that the flat-to-faceted transition is driven by the reactivity of a single chemical site present in a concentration of less than 0.4% of a monolayer. In contrast to previously postulated mechanisms, pyramidal faceting does not require “micromasks”: chemical heterogeneities, such as impurities, insoluble etch products, or H2 (g), that collect on the surface during etching and act as transient nanoscale etch masks. Instead, the etching reaction drives the formation of self-propagating, {111}-faced nanoscale hillocks. This study shows that the chemical control of surface morphology can be driven by minority species at concentrations far below the detection limit of surface spectroscopy.

The peroxo ligand is one of the most promising structure-directing agents in TiO2 nanocatalyst synthesis; however, its role in nanocatalyst growth and polymorph control is not well understood. Using a combination of scanning tunneling microscopy, X-ray photoemission spectroscopy, and kinetic Monte Carlo simulation, we show that the base-catalyzed reaction of H2O2 targets very particular sites on the rutile (110) surface, sites that are present in concentrations of less than 1% of a monolayer, producing highly homogeneous surfaces characterized by flat terraces and straight atomic-height steps. This reaction produces surface steps with a different orientation, different structure, and different reactivity from those prepared in ultrahigh vacuum studies. The observed reactivity is explained by a simple, site-specific model that is based on metal oxo coordination chemistry. This study shows that one of the principle roles of peroxide in etching and growth reactions is destabilizing neighboring bonds and increasing their lability. As a result, the peroxo ligand adds a degree of reversibility to growth reactions, thereby promoting the formation of well ordered crystals.

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.

A dramatic, pH-dependent change in the steady-state chemical and morphological structure of Si(100) surfaces etched in aqueous fluoride solutions is observed with infrared spectroscopy and scanning tunneling microscopy. Low pH solutions (5 ≤ pH ≤ 7), such as the technologically important buffered oxide etchant (buffered HF), produce rough surfaces covered with nanoscale Si{110}-faceted hillocks. In contrast, higher pH solutions (7.8 ≤ pH ≤ 10), including 40% NH4F (aq.), produce atomically smooth surfaces. The etched surfaces are terminated by a monolayer of H atoms irrespective of pH. The pH-dependent transition is attributed to two competing multistep reaction pathways. At higher pH, the base-catalyzed formation of a surface silanone leads to the production of smooth surfaces. This reaction channel is suppressed at low pH, leading to the formation of {110}-faceted hillocks by a second reaction. The morphological transition is not affected by dissolved O2 in the etchant.

A simple, room temperature process—etching of Si(100) surfaces in 40% NH4F(aq) solutions—produces H-terminated surfaces of near-atomic smoothness over large areas (>1000 × 1000 Å2). The etched surface is primarily terminated by long alternating rows of strained and unstrained silicon dihydrides; no microfaceting or etching-induced surface roughness is observed. The Cartesian components of the infrared absorption spectrum of flat and vicinal etched surfaces show that the surface is almost entirely dihydride-terminated. This analysis disproves previous assignments of the infrared spectrum of NH4F-etched Si(100) which suggested that the etched surface was very rough and terminated by a variety of mono-, di-, and trihydride species. Although the steady-state etch morphology has lower interadsorbate strain than bulk-terminated H/Si(100), this morphology does not minimize interadsorbate strain as previously postulated. The relatively low reactivity of the strained dihydrides kinetically blocks this pathway.

Dissolved O2 is found to have a profound effect on the morphology of Si(1 1 1) surfaces etched in buffered hydrofluoric acid (BHF). Atomically smooth surfaces were exposed to BHF containing varying amounts of dissolved O2 and the resulting changes in etch morphology were measured with scanning tunneling microscopy. These morphological changes were correlated with spectral changes in the infrared absorption spectrum. Surfaces etched in BHF containing dissolved O2 are much rougher than those etched in O2-free BHF. This effect is explained by kinetic competition between O2-induced and etchant-induced oxidation reactions which are followed by rapid etching of the oxidized species. Dissolved O2 alters the net site-specific reactivity of low pH BHF and leads to the production of rougher surfaces. The previously observed pH dependence of Si(1 1 1) etch morphology is also explained by this kinetic competition.