Molecular Precursor State Research Articles

Dynamic modeling of Si(100) thermal oxidation: Oxidation mechanisms and realistic amorphous interface generation

Silicon and its native oxide SiO2 have been utilized in semiconductor technology since the 1950s and are still crucial for the development of novel device technologies today. Recent theoretical and experimental studies show that the fabrication of high-quality interfacial layers becomes critical for reliable operation of modern nanoscale devices. This paper presents a first-principles based approach to theoretically assess the thermal oxidation process of the technologically relevant Si(100) surface in the ultra-thin layer regime below 2 nm. The oxidation process is dynamically modeled by means of ab-initio molecular dynamics and density functional based tight binding simulations. We qualitatively explain the experimentally well-known but poorly understood decrease of oxidation rate in the initial oxidation stage as a complex interplay between various oxidation mechanisms such as fast O2 dissociation at the surface, slower oxygen integration mediated by molecular precursor states and O2 diffusion through the oxide. Our model combines previously reported experimental insights into a comprehensive picture of Si oxide growth. Strong evidence for an immediate amorphization of the oxide surface layer was found and identified as a direct consequence of lattice vibrations. Furthermore, our modeling method is a novel approach for the generation of realistic, amorphous interface structures that is based on the stepwise oxidation of the crystalline Si surface and could be extended to other materials systems as well.

STM imagery and density functional calculations of C60fullerene adsorption on the 6H-SiC(0001)-3×3 surface

Scanning tunneling microscopy (STM) studies of the fullerene C${}_{60}$ molecule adsorbed on the silicon carbide SiC(0001)-$3\ifmmode\times\else\texttimes\fi{}3$ surface, combined with density functional theory (DFT) calculations, show that chemisorption of individual C${}_{60}$ molecules occurs through the formation of one bond to one silicon adatom only in contrast to multiple bond formation on other semiconducting surfaces. We observe three stable adsorption sites with respect to the Si adatoms of the surface unit cell. Comprehensive DFT calculations give different adsorption energies for the three most abundant sites showing that van der Waals forces between the C${}_{60}$ molecule and the neighboring surface atoms need to be considered. The C${}_{60}$ molecules are observed to form small clusters even at low coverage indicating the presence of a mobile molecular precursor state and nonnegligible intermolecular interactions.

Selectivity in the initial C–H bond cleavage of n-butane on PdO(101)

We used temperature programmed reaction spectroscopy (TPRS) and molecular beam reflectivity measurements to investigate the initial dissociation of n-butane isotopologues on PdO(101) and determine kinetic parameters governing the selectivity of initial C-H(D) bond cleavage. We observe differences in the reactivity of the n-butane isotopologues on PdO(101) due to kinetic isotope effects, and find that the initial dissociation probability decreases with increasing surface temperature for each isotopologue. We performed an analysis of the dissociation kinetics using a model that is based on a precursor-mediated mechanism for n-butane dissociation and enables quantification of kinetic parameters for selective C-H bond cleavage by considering differences in the reactivity among the n-butane isotopologues. From the analysis, we estimate that 49% of the n-butane molecules which react during TPRS do so through 1° C-H bond cleavage when the initial coverage of n-butane lies between ∼40% and 100% of the saturation coverage of the molecular precursor state. For dissociation in the limit of zero coverage, we estimate that the conditional probability for 1° C-H bond cleavage is equal to ∼87% and varies only weakly with surface temperature from 300 K to 400 K. Analysis of the temperature dependent rate data further predicts that the barrier for 1° C-H bond cleavage is 3.5 kJ mol(-1) lower than that for 2° C-H bond cleavage for n-butane dissociation on PdO(101) in the limit of zero coverage. Our results provide evidence that the selectivity for 1° C-H bond cleavage on PdO(101) increases as the n-butane coverage decreases below ∼40% of the saturation value. We speculate that intermolecular interactions among the n-butane species are responsible for the apparent coverage dependence of the C-H bond selectivity for n-butane dissociation on PdO(101).

Dissociations of O2 molecules on ultrathin Pb(111) films: first-principles plane wave calculations

Using first-principles calculations, we systematically study the dissociations of O2 molecules on different ultrathin Pb(111) films. According to our previous work revealing the molecular adsorption precursor states for O2, we further explore why there are two nearly degenerate adsorption states on Pb(111) ultrathin films, but no precursor adsorption states existing at all on Mg(0001) and Al(111) surfaces. The reason is concluded to be the different surface electronic structures. For the O2 dissociation, we consider both the reaction channels from gas-like and molecularly adsorbed O2 molecules. We find that the energy barrier for O2 dissociation from the molecular adsorption precursor states is always smaller than that from O2 gas. The most energetically favorable dissociation process is found to be the same on different Pb(111) films, and the energy barriers are found to be influenced by the quantum size effects of Pb(111) films.

Dissociation of O2 Molecules on Strained Pb(111) Surfaces

By performing first-principles molecular dynamics calculations, we systematically simulate the adsorption behavior of oxygen molecules on the clean and strained Pb(111) surfaces. The obtained molecular adsorption precursor state, and the activated dissociation process for oxygen molecules on the clean Pb surface are in good agreements with our previous static calculations, and perfectly explains previous experimental observations [Proc. Natl. Acad. Sci. U.S.A. 104, 9204 (2007)]. In addition, we also study the influences of surface strain on the dissociation behaviors of O2 molecules. It is found that on the compressed Pb(111) surfaces with a strain value of larger than 0.02, O2 molecules will not dissociate at all. And on the stretched Pb(111) surfaces, O2 molecules become easier to approach, and the adsorption energy of the dissociated oxygen atoms is larger than that on the clean Pb surface.

The dissociative adsorption of hydrogen on Pt(111): Actuation and acceleration by atomic defects

The dissociation of hydrogen at atomic surface defects is the strongly dominant, if not the decisive, step in the chain of events eventually leading to chemisorbed H-atoms on Pt(111). This holds for perpendicular kinetic energies of the gas phase molecules from 8 to 60 meV, i.e., covering the range relevant to hydrogenation reactions. This insight has been gained in the present study in which we reversibly varied the defect density on one and the same crystal in a controlled way. Information has been derived from measuring the adsorption kinetics as a function of coverage. Two distinct adsorption channels are distinguished. The first, indirect one, prevails at lower H-coverage and involves capture into a non-accommodated molecular precursor state followed by dissociation at step sites as described in our recent paper. The second one, dominant at higher coverage and non-negligible defect densities, obeys second order Langmuir kinetics. Here the dissociative adsorption takes place directly at step sites with a cross section of 0.24 unit cells (initial sticking probability 24% of the step density). These results are consistent with thermally programmed desorption data: the direct channel is responsible for the emergence of the low temperature peak in thermal desorption spectroscopy, usually denoted with β(1), while the indirect channel is represented by the β(2) state. The dependence on the perpendicular component of the hydrogen kinetic energy is distinctly different for the two channels: the indirect one shows power law behavior with an exponent 1.9 ± 0.1, while the direct one shows no perpendicular energy dependence at all.

Adsorption and dissociation ofO2at Be(0001): First-principles prediction of an energy barrier on the adiabatic potential energy surface

The adsorption and dissociation of O$_{2}$ molecules at the Be(0001) surface is studied by using density-functional theory within the generalized gradient approximation and a supercell approach. The physi- and chemisorbed molecular precursor states are identified to be along the parallel and vertical channels, respectively. It is shown that the HH-Z (see the text for definition) channel is the most stable channel for the molecular chemisorption of O$_{2}$. The electronic and magnetic properties of this most stable chemisorbed molecular state are studied, which shows that the electrons transfer forth and back between the spin-resolved antibonding $\pi^{\ast}$ molecular orbitals and the surface Be $sp$ states. A distinct covalent weight in the molecule-metal bond is also shown. The dissociation of O$_{2}$ is determined by calculating the adiabatic potential energy surfaces, wherein the T-Y channel is found to be the most stable and favorable for the dissociative adsorption of O$_{2}$. Remarkably, we predict that unlike the other simple $sp$ metal surfaces such as Al(111) and Mg(0001), the \textit{adiabatic} dissociation process of O$_{2}$ at Be(0001) is an activated type with a sizeable energy barrier.

Interaction of hydrogen with (2 1 0)-oriented metal surfaces: Molecular precursors, chemisorbed atoms and subsurface states

Some selected aspects of hydrogen interaction with metal surfaces are surveyed with emphasis on the formation of molecular precursor states on surfaces that are normally known to readily and spontaneously dissociate hydrogen molecules (Ni, Pd, Rh). We will demonstrate that the crystallographic (2 1 0) orientation of the face-centered cubic (“f.c.c”) lattice seems to provide a particular route for stabilizing a (weakly) chemisorbed H 2 species notwithstanding the ordinary H 2 dissociative chemisorption and – in the Pd case – subsurface H state formation. This molecular hydrogen species – although or just because it is bound with an adsorption energy of merely ∼20–25 kJ/mole may play an essential role both in heterogeneously catalysed surface and hydrogen uptake reactions that precede, e.g., H storage in solid materials.

CO-Coverage-Dependent Oxygen Dissociation on Pt(111) Surface

Oxygen dissociation is one of the most critical steps in the CO oxidation reaction on transition metal surfaces. It has been shown both experimentally and theoretically that oxygen dissociation on clean platinum (Pt) surface proceeds via a precursor-mediated reaction path, with negligible activation barrier. On the other hand, the oxygen dissociation pathway under diesel engine operating conditions, where the metal surface is packed with CO molecules, is understood less clearly. In this paper, we report density functional theory calculations for O2 dissociation on Pt(111) in the presence of varying CO coverage. Classical Monte Carlo simulations have been used to get an estimate of coadsorbed CO and O2 configurations. Oxygen molecular precursor states binding energies were found to shift up in energy with increasing CO coverage, with transition state energies and final product energies following the same trend. The dissociated product state becomes endothermic beyond a critical CO coverage of 0.44 monolaye...

Pathways of nitric oxide dissociation on Si(001) and subsequent atomistic processes: a first-principles molecular dynamics study

We present first-principles molecular dynamics (MD) simulations of the dissociation of a nitric oxide (NO) molecule on an Si(001) surface and subsequent incorporation of the nitrogen atom into the subsurface, which are important in the understanding of the synthesis of oxynitride thin films. The MD runs for different initial conditions revealed various dissociation pathways. In some, the dissociated N and O atoms bridge an Si dimer and substrate backbond. The dissociated nitrogen incorporates into the subsurface with the formation of . The NO dissociates across two Si dimers. All the processes finish within a couple of picoseconds at all of the temperatures considered, implying that the corresponding energy barriers are quite small (≲0.2). Inclusion of the spin polarization increases only the energy barrier for the initial dissociation process from a molecular state, suggesting the existence of a molecular precursor state. The facile dissociation of the NO and incorporation of N are consistent with photoemission measurements.

Dissociative Adsorption of O2 on Clean and CO-Precovered Pt Surfaces

ABSTRACTIt is generally accepted that CO oxidation on transition metals follows a Langmuir-Hinshelwood mechanism. The oxidation reaction takes place in two sequential steps where the oxygen molecule first dissociates into atomic oxygen and then reacts with an adsorbed CO to form CO2. One critical question concerning the reaction kinetics under high pressure is the probability of oxygen dissociation on a highly CO covered surface. On bare transition metal surfaces, molecularly adsorbed oxygen readily dissociates with little or no apparent activation barrier. In industrial diesel engine catalysis, the metal surface is initially packed with CO. Subsequent reactions such as oxygen dissociation must take place on a CO covered surface. In this paper, we performed density functional theory (DFT) calculations for O2 dissociation on Pt(111) in the presence of different CO adsorption environments. While several stable O2 molecular precursor states (top-bridge-top, top-fcc-bridge, and top-hcp-bridge) exist on a clean Pt(111) surface, these precursors become endothermic beyond a critical CO coverage of ∼0.44 ML. Furthermore, the reaction path for CO oxidation via dissociated atomic oxygen becomes less favorable at higher CO coverage, primarily due to competitive adsorption and lateral repulsion. It was found that the oxygen dissociation barrier and the binding energies of atomic oxygen are well correlated via the Evans-Polanyi relationship.

Molecular precursor-mediated methanol dissociation on Si(1 1 1)7 × 7: ab initio study

We present an ab initio study of methanol interaction with the Si(1 1 1)7 × 7 surface using a Si(1 1 1)4 × 2 model. The study of the methanol dissociation on Si(1 1 1)4 × 2 shows that pair dissociation on adatom–restatom dangling bonds is largely favoured, in agreement with the experimental observations. The “center” type adatom is slightly more reactive than the “corner” type one, although the difference is weak. Similar behaviour is observed in both adatom types. Our results for a direct CH 3OH dissociation favouring a basic cleavage (adsorption of OH and CH 3 fragments) rather than an acidic one (adsorption of H and OCH 3 fragments), we are finally led to take a kinetic effect into consideration to reconcile theory with experiment. We show that the presence of molecular precursor states is possible. Different orientations with respect to the silicon dangling bonds of these molecular precursors are investigated. However, the corresponding energies are very close and, considering their relative energies, it is finally difficult to discriminate between acidic and basic cleavages.

Oxygen adsorption on Cu(100): First-principles pseudopotential calculations

We have studied the adsorption characteristics of atomic and molecular oxygen, incident on the Cu(100) surface. Our pseudopotential first-principles calculations yield trajectories for the ${\mathrm{O}}_{2}$ molecule without dissociation barriers at the entrance channel. We discuss the energetics of the ${\mathrm{O}}_{2}$ adsorption and dissociation in terms of the elbow plots which are two-dimensional cuts of the full six-dimensional potential-energy surface. The top site is found to be the most reactive one while at the fcc site molecular adsorption takes place. The adsorption energies at horizontal configurations of the ${\mathrm{O}}_{2}$ molecule are found to be larger than those at the vertical configurations. The local densities of states reveal differences between the sites with direct dissociative adsorption and the ones with molecular precursor states. We also discuss the interactions between O and Cu atoms adsorbed at the hollow sites on Cu(100), and the corresponding diffusion barriers.

Adsorption of methylchloride on Si(100) from first principles

The chemisorption of methylchloride (CH3Cl) on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is one in which the methylchloride molecule is dissociated into CH3 and Cl fragments which are bound to the two Si atoms of the same surface dimer. Our calculations show that dissociative chemisorption of methylchloride on Si(100) may proceed along different reaction paths characterized by different energy barriers that the system must overcome: some dissociation processes are mediated by a molecular precursor state and, at least in one case, we find that the dissociation process is nonactivated, in agreement with recent experimental findings. We have also generated, for many possible adsorption structures, theoretical scanning tunneling microscopy images which could facilitate the interpretation of experimental measurements.

Unified picture of the molecular adsorption process: O 2/Pt(1 1 1)

Tight-binding molecular dynamics simulations of the adsorption of O 2/Pt(1 1 1) have been performed based on an ab initio potential energy surface. We demonstrate that, contrary to common belief, in this system the whole adsorption probability as a function of the kinetic energy can be understood in terms of trapping into chemisorbed molecular precursor states. This provides a novel unified picture of the trapping process which is relevant for the general understanding of adsorption. Furthermore, by simple steric arguments we are able to explain why O 2 does not dissociate on cold Pt(1 1 1) surfaces even at kinetic energies that are much greater than the dissociation barrier.

The potential energy surface for dissociation of N 2 on W( [formula omitted

Electronic total energy methods have been used to compute the potential energy surface for the dissociation of nitrogen molecules on the (1 0 0) face of tungsten. The dissociation is found to be activated, but there are also deep molecular chemisorption wells into which molecules may steer and trap. The molecular geometries in these precursor states agree well with experimental assignments. The dominant well is for the adsorption of a molecule oriented vertically on the atop site. An analysis of the bonding there indicates that the orbitals break into components largely located on either the inner or the outer N atom, rather than the standard Blyholder model. Dissociation paths are suggested based on several steps, passing through at least two molecular precursor states.

Structure sensitivity for NO dissociation on palladium and rhodium surfaces

We present a comparative density-functional theory study of the chemisorption and the dissociation of the NO molecule on the close-packed (111), the more open (100), and the stepped (511) surfaces of palladium and rhodium. The energetic and kinetic properties of the reaction pathways are reported. The structure sensitivity is correlated to the catalytic activity, which can be linked to the calculated dissociation rate constants at 300 K: Rh (100)⩾terrace Rh (511)>step Rh (511)>step Pd (511)>Rh (111)>Pd (100)⩾terrace Pd (511)>Pd (111). The effect of the steps on the activity is found to be clearly favorable for the Pd (511) surface and unfavorable for the Rh (511) surface. The reaction barriers are correlated to the stability of the final states and the geometries of the molecular precursor states.

Oxygen dissociation at close-packed Pt terraces, Pt steps, and Ag-covered Pt steps studied with density functional theory

Using density functional theory we have characterized O 2 dissociation on flat and stepped Pt(1 1 1) surfaces. The reactivity of the steps is significantly higher than that of the flat terraces. Inclusion of Ag monoatomic chains along the Pt steps modifies the reactivity of the Pt steps towards that of the flat Pt terraces. Our investigations reveal the reaction energetics and the geometries for the molecular precursor states (MPS), transition states (TS) and final states of the dissociating oxygen. Both the MPS and TS geometries on stepped Pt involve oxygen species at the top of the step with no oxygen atoms diffusing onto the lower terrace. We further find that the Ag chains mainly modify the adsorption and reaction bond strengths while they leave the MPS and TS geometries essentially unchanged. The high reactivity of the Pt steps is explained in terms of the coordinative unsaturation of the Pt step atoms which cause a highlying valence 5d-electron system.

Breaking the NO bond on Rh, Pd, and Pd3Mn alloy (100) surfaces: A quantum chemical comparison of reaction paths

Total energy calculations have been performed within the periodic density-functional theory framework to study the dissociation of molecularly adsorbed nitrogen monoxide NO over three different catalytic surfaces: palladium, rhodium, and palladium-manganese (100). The potential energy surfaces for NO dissociation on these metallic surfaces have been calculated in order to determine the minimal energy paths. The accurate optimizations of the transition states and their characterization with a complete vibrational analysis, including the degrees of freedom of the surface, have been presented. The order of increasing activation energy barrier is Rh, Pd3Mn, and Pd. Two types of reaction paths have been found: one involving a horizontal molecular precursor state and a low activation energy barrier (Rh and Pd3Mn) and the other involving a vertical molecular state and a high activation energy (Pd). Hence the improvement of the catalytic activity for dissociating NO by alloying manganese to palladium has been explained and interpreted. The simulation of the reaction rate constants is fully compatible with the observed catalytic behavior. The differences in catalytic activity have been analyzed with a bond breaking–bond forming energetic decomposition and a Mulliken population analysis.

Formation of atomically smooth, ultrathin oxides on Si([formula omitted

We report the first direct observation of dissociative chemisorption of oxygen molecules on a silicon surface at room temperature via a molecular precursor state. We link this to the fact that smooth oxide layers can be grown easily on Si(113). The process of initial oxidation is discussed in terms of surface diffusion paths and surface stress. First ab initio calculations elucidate the favored adsorption sites and the oxidation mechanism. Experimental evidence indicates bond geometries that lead to the quasi-epitaxial growth of an extremely thin SiO2 layer on the substrate at elevated temperatures (600°C). In contrast to Si(001) oxidation, neither defects nor the ejection of Si atoms plays a significant role during the initial oxidation of Si(113). Gate-controlled diodes prepared on Si(113) and Si(001) 4-in. wafers under optimized conditions for Si(001) show no significant differences in the density of rechargeable interface states, threshold voltages, and charge carrier generation and recombination.