Adatom Sites Research Articles

Mechanistic Insights into Nonadiabatic Interband Transitions on a Semiconductor Surface Induced by Hydrogen Atom Collisions.

To understand the recently observed enigmatic nonadiabatic energy transfer for hyperthermal H atom scattering from a semiconductor surface, Ge(111)c(2 × 8), we present a mixed quantum-classical nonadiabatic molecular dynamics model based on the time-dependent evolution of Kohn-Sham orbitals and a classical path approximation. Our results suggest that facile nonadiabatic electronic transitions from the valence band to the conduction band occur selectively at the rest atom site, where surface states are doubly occupied, but not at the adatom site, where empty surface states are localized. This drastic site specificity can be attributed to the changes of the local band structure upon energetic H collisions at different surface sites, leading to transient near degeneracies and significant couplings between occupied and unoccupied orbitals at the rest atom but not at the adatom. These insights shed valuable light on the collision-induced nonadiabatic dynamics at semiconductor surfaces.

Panoply of Ni-Doping-Induced Reconstructions, Electronic Phases, and Ferroelectricity in 1T-MoS2.

The distorted phases of monolayer 1T-MoS2 have distinct electronic properties, with potential applications in optoelectronics, catalysis, and batteries. We theoretically investigate the use of Ni-doping to generate distorted 1T phases and find not only the ones usually reported but also two further phases (3 × 3 and 4 × 4), depending on the concentration and the substitutional or adatom doping site. Corresponding pristine phases are stable after removal of dopants, which might offer a potential route to experimental synthesis. We find large ferroelectric polarizations, most notably in 3 × 3 which─compared to the recently measured 1T″─has 100 times greater ferroelectric polarization, a lower energy, and a larger band gap. Doped phases include exotic multiferroic semimetals, ferromagnetic polar metals, and improper ferroelectrics with only in-plane polarization switchable. The pristine phases have unusual multiple gaps in the conduction bands with possible applications for intermediate band solar cells, transparent conductors, and nonlinear optics.

Restructuring and Activation of Cu(111) under Electrocatalytic Reduction Conditions.

The dynamic restructuring of Cu surfaces in electroreduction conditions is of fundamental interest in electrocatalysis. We decode the structural dynamics of a Cu(111) electrode under reduction conditions by joint first-principles calculations and operando electrochemical scanning tunneling microscopy (ECSTM) experiments. Combining global optimization and grand canonical density functional theory, we unravel the potential- and pH-dependent restructuring of Cu(111) in acidic electrolyte. At reductive potential, Cu(111) is covered by a high density of H atoms and, below a threshold potential, Cu adatoms are formed on the surface in a (4×4) superstructure, a restructuring unfavorable in vacuum. The strong H adsorption is the driving force for the restructuring, itself induced by the electrode potential. On the restructured surface, barriers for hydrogen evolution reaction steps are low. Restructuring in electroreduction conditions creates highly active Cu adatom sites not present on Cu(111).

Restructuring and Activation of Cu(111) under Electrocatalytic Reduction Conditions

AbstractThe dynamic restructuring of Cu surfaces in electroreduction conditions is of fundamental interest in electrocatalysis. We decode the structural dynamics of a Cu(111) electrode under reduction conditions by joint first‐principles calculations and operando electrochemical scanning tunneling microscopy (ECSTM) experiments. Combining global optimization and grand canonical density functional theory, we unravel the potential‐ and pH‐dependent restructuring of Cu(111) in acidic electrolyte. At reductive potential, Cu(111) is covered by a high density of H atoms and, below a threshold potential, Cu adatoms are formed on the surface in a (4×4) superstructure, a restructuring unfavorable in vacuum. The strong H adsorption is the driving force for the restructuring, itself induced by the electrode potential. On the restructured surface, barriers for hydrogen evolution reaction steps are low. Restructuring in electroreduction conditions creates highly active Cu adatom sites not present on Cu(111).

Cu4@C2N for effective electrochemical CO2 reduction and intermediates dependent adsorption behaviours: A computational study

Electrochemical CO2 reduction (eCO2R) to valuable products is an effective solution for carbon emission and energy related issues but limited by the lack of high performance catalysts. Herein, Cu4 supported by C2N for CO2 reduction to various products was systematically investigated using first principles calculations. Our results indicate that Cu4@C2N is a promising catalyst for eCO2R with limiting potential of −0.46 and −0.81 V for HCOOH and C2 products, respectively. Cu on top site with higher d-band center is the major active site for eCO2R. Besides, due to the heterogeneity of the active sites of Cu and abundant intermediate, the complexity of the calculation increased significantly. Thus, possible intermediates dependent adsorption behaviours were proposed to determine the interaction types for searching the lowest energy adsorption configuration. More importantly, we found that electron donating ability of the active sites, the electronegativity difference between the adatom and active sites, and the coordinative unsaturation of the contact sites in the intermediates are the major physical origin governing the adsorption geometry. Our work not only provides guidance for developing high-performance small cluster electrocatalyst for CO2 reduction, but brings inspiration for the theoretical study of complex reaction systems to find the optimal adsorption model.

Structure and reaction condition dependent mechanism for ammonia synthesis on Ru-based catalyst

The ammonia synthesis by Haber–Bosch process is an important catalytic process for the production of fertilizers. Despite extensive research, the exact reaction mechanism remains unsettled. To date, only dissociative mechanism, in which the adsorbed N2 decomposes directly, is investigated on the archetypical Ru-based catalyst. By density functional theory calculation and micro-kinetic modeling, we show that the associative mechanism, which is initiated by N2 hydrogenation to NNH, is dominant on Ru terrace, vacancy and adatom sites regardless of temperatures (523.15–723.15 K) and sticking coefficients (or partial pressures) of N2 and H2 (SN2 = 1.0 × 10−12-1.0 and SH2 = 0.001–1.0). The associative mechanism also dominates on Ru step (A4) and kink sites at a high SN2 or SH2 (SN2 = 1.0 or SH2 = 1.0), which exhibit higher reaction rate than Ru B5 site. The preferable associative mechanism originates from the high θH⁎/θ⁎ ratio or the significant backward process of N2* dissociation induced by high θN⁎/θ⁎ ratio, and has markedly different rate-determining step from the traditional dissociative mechanism (N2 hydrogenation vs dissociation). We also show that the equilibrium approximation is more convenient and efficient than the micro-kinetic modeling, which can streamline the fundamental understanding and accelerate the high-throughput screening of ammonia synthesis materials.

Comparative Studies on Local Barrier Field Variations Above Field-Adsorbed Helium and Neon with a Micro-Probe Hole Field Ion Microscope.

The local field ion emission properties of helium and neon around a step edge atom of W(112) were examined at liquid nitrogen temperature using a micro-probe hole field ion microscope combined with a pulse-counting analysis. We have analyzed the mapped field ion densities obtained for both imaging gas atoms at their respective best local image voltages based on the formula for tunneling barrier strength and have evaluated the dipole moment of polarized adatom as well as the local field enhancement factor at the adatom site. We found that the dipole moments of helium and neon adatoms showed the same value, although the best local image field acting on the helium adatom is much higher than that on the neon adatom. We also found the same magnitude of local field enhancement factors for both noble gas field adsorptions. These results imply that the key to the best local image condition is the tunneling barrier field variations above the adatom. The vital role of the imaging gas atoms is to form an optimum dipole moment to create an ideal electric field distribution for the best local image appearance at each atom site depending on the different chemical nature of adatom species.

Atomistic thermodynamics and kinetics of dicalcium silicate dissolution

Low-temperature cement manufacturing has garnered academic and industrial attention for its low environmental footprints. However, the sluggish hydration kinetics of the resultant cement affects their early-age strength development. This motivates fundamental studies to unravel the mechanistic picture of the dissolution process and discover science-informed pathways to accelerate hydration. Standard atomistic simulations seldomly exceed a microsecond making them impractical to study slow dissolution processes. Here, using rare event sampling techniques, we provide the mechanistic picture of Ca2+ ion dissolution from a kink site on the dicalcium silicate surface. The Ca2+ ion dissolution is comprised of two sequential stages: breaking restraints from the kink sites to form a ledge adatom and detaching from the ledge/terrace adatom sites into the solution. The first and second stages feature free energy barriers of ~63 kJ/mol and ~ 29 kJ/mol respectively, making the first stage the rate-limiting step of the entire Ca2+ dissolution kinetics. Using the reactive flux method, the rate and equilibrium constants for each reaction step are calculated, which yield the Ca2+ ion activity of ~1.03 × 10−5. The diffusion calculations indicate that the surface effects lower the self-diffusion coefficient of Ca2+ ions at the solid-water interface.

New insight on the role of localisation in the electronic structure of the Si(111)(7\u2009\xd7\u20097) surfaces

New angle-resolved photoelectron spectroscopy (ARPES) data, recorded at several different photon energies from the Si(111)(7 × 7) surface, show that the well-known S1 and S2 surface states that lie in the bulk band gap are localised at specific (adatom and rest atom) sites on the reconstructed surface. The variations in the photoemission intensity from these states as a function of polar and azimuthal emission angle, and incident photon energy, are not consistent with Fermi surface mapping but are well-described by calculations of the multiple elastic scattering in the final state. This localisation of the most shallowly bound S1 state is consistent with the lack of significant dispersion, with no evidence of Fermi surface crossing, implying that the surface is not, as has been previously proposed, metallic in character. Our findings highlight the importance of final state scattering in interpreting ARPES data, an aspect that is routinely ignored and can lead to misleading conclusions.

Covalent and Hydrogen Bonding in Adsorption of Alanine Molecules on Si(111)7×7.

Molecular adsorption bonding configurations and specific interfacial chemistry of alanine on Si(111)7×7 have been determined by combining the results from scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) with ab initio calculations based on the density functional theory (DFT). XPS spectra of the N 1s region show that alanine molecules bind to the 7×7 surface by N-Si covalent bonding, while STM imaging reveals that such N-H dissociative adsorption of alanine occurs on an adjacent Si adatom-restatom pair, with the dehydrogenated alanine moiety and dissociated H atom occupying the Si adatom and restatom sites, respectively. At a sample bias above +2 V, the dehydrogenated alanine appears as a bright round protrusion, slightly off-center from a Si adatom site and leaning toward the opposite Si adatom across the dimer wall. The off-center character can be attributed to an electrostatic attraction between the electron-rich carbonyl O of the dehydrogenated alanine and electron-deficient nearest Si adatom across the dimer wall. Our DFT calculation also shows that the monodentate O-Si bonding configuration resulting from O-H dissociative adsorption is more thermodynamically favorable than the experimentally observed N-Si bonding configuration, suggesting that the interfacial dissociative adsorption reaction is a kinetically controlled rather than a thermodynamically driven process. Alanine molecules in the second adlayer (transitional layer) are found to attach to those in the first adlayer (interfacial layer) by N···HO hydrogen bonding, as supported by the presence of the N 1s feature at 401.0 eV. An alanine molecule H-bonded to a dehydrogenated alanine in the first adlayer has also been observed in STM as a brighter and larger protrusion close to the expected location of the free OH group in the dehydrogenated first-adlayer alanine. No thick zwitterionic alanine film can be obtained at room temperature possibly due to steric constraint caused by the methyl group.

Self-activation of copper electrodes during CO electro-oxidation in alkaline electrolyte

The development of low-temperature fuel cells for clean energy production is an appealing alternative to fossil-fuel technologies. CO is a key intermediate in the electro-oxidation of energy carrying fuels and, due to its strong interaction with state-of-the-art Pt electrodes, it is known to act as a poison. Here we demonstrate the ability of Earth-abundant Cu to electro-oxidize CO efficiently in alkaline media, reaching high current densities of ≥0.35 mA cm−2 on single-crystal Cu(111) model catalysts. Strong and continuous surface structural changes are observed under reaction conditions. Supported by first-principles microkinetic modelling, we show that the concomitant presence of high-energy undercoordinated Cu structures at the surface is a prerequisite for the high activity. Similar CO-induced self-activation has been reported for gas–surface reactions at coinage metals, demonstrating the strong parallels between heterogeneous thermal catalysis and heterogeneous electrocatalysis. CO is a key intermediate in the electro-oxidation of energy carrying fuels which typically acts as a poison. Here, the authors demonstrate that Cu is an efficient CO electro-oxidation catalyst in alkaline electrolyte due to the continuous formation of undercoordinated active Cu adatom sites in the presence of CO and OH.

DFT study of Pt sub-monolayer adsorption on the positive BiFeO3 (0001) surface

Density functional theory (DFT) calculations have been performed to determine the adsorption mechanism of Pt sub-monolayer (Θ ≤ 1 ML) on the positive BiFeO3 (0001) surface. We predicted that the adsorption behavior is different for Θ ≤ 1/2 ML and Θ > 1/2 ML based on detailed analysis of adsorption configuration, charge transfer effect, and orbital hybridization. For Θ ≤ 1/2 ML, there is only one type of Pt adatom site, which is binding directly with the low-lying O anion and shows electrons depletion/accumulation in the vertical/lateral 5d orbitals. For Θ > 1/2 ML, another type of Pt adatom site can be identified by not only the different adsorption configuration but also the distinct electronic states. Since the nearest neighbor of this type of Pt adatom site is the Bi cation, the charge transfer effect is characterized by significant electrons accumulation of the Pt-5d orbitals at the expense of the electron density of Bi cation. Another interesting feature occurs at Θ = 1 ML, i.e., the surface magnetic order displays ferromagnetic structure instead of antiferromagnetic arrangement. In addition, the existence of two types of Pt sites at Θ > 1/2 ML implies an intriguing possibility to enhance catalytic efficiency in the Pt/BFO (0001) heterocatalysis.

Structure dependent effect of silicon on the oxidation of Al(111) and Al(100)

The effect of sub-monolayer silicon on the oxidation of Al(111) and Al(100) surfaces was investigated using X-ray Photoelectron Spectroscopy (XPS) and density functional theory (DFT) calculations. On both surfaces the adatom site is preferred over substituting Si into the Al-lattice; on Al(100) the four fold hollow site is vastly favored whereas on Al(111) bridge and hollow sites are almost equal in energy.Upon O2 exposure, Si is not oxidized but buried at the metal/oxide interface under the growing aluminum oxide. On Al(111), Si has a catalytic effect on both the initial oxidation by aiding in creating a higher local oxygen coverage in the early stages of oxidation and, in particular, at higher oxide coverages by facilitating lifting Al from the metal into the oxide. The final oxide, as measured from the Al2p intensity, is 25–30% thicker with Si than without. This observation is valid for both 0.1 monolayer (ML) and 0.3 ML Si coverage. On Al(100), on the other hand, at 0.16 ML Si coverage, the initial oxidation is faster than for the bare surface due to Si island edges being active in the oxide growth. At 0.5 ML Si coverage the oxidation is slower, as the islands coalesce and he amount of edges reduces. Upon oxide formation the effect of Si vanishes as it is overgrown by Al2O3, and the oxide thickness is only 6% higher than on bare Al(100), for both Si coverages studied.Our findings indicate that, in addition to a vanishing oxygen adsorption energy and Mott potential, a detailed picture of atom exchange and transport at the metal/oxide interface has to be taken into account to explain the limiting oxide thickness.

Doping modulation of quasi-free-standing monolayer graphene formed on SiC(0001) through Sn1-xGex intercalation

In order to modulate the transfer doping of quasi-free-standing monolayer graphene (QFMLG) formed on SiC(0001), Ge atoms were intercalated additionally into QFMLG already formed by Sn intercalation between ZL and 6H-SiC(0001). By postannealing the Ge-deposited surface at 600 °C, the Sn1-xGex film with the 4×33 structure, composed of a bilayer and adatoms with dangling bonds under QFMLG, has been formed. It turns out that, in this Sn1-xGex film, Ge atoms preferentially occupy the bottom layer bound to the top Si atoms of the substrate, while Sn atoms occupy the top adatom sites. Strong correlation among the electrons localized at these adatom sites induces a semiconducting alloy film. As the postannealing temperature is increased up to 800 °C, the concentration of Ge in the intercalated film of the same 4×33 structure is gradually increased and the Dirac point also shifts gradually from −0.16 eV to +0.20 eV relative to the Fermi level. Such a result confirms that the transfer doping of QFMLG on SiC(0001) has been modulated by varying the alloy composition of the Sn1-xGex interfacial film.

Hydrogen adsorption induced nanomagnetism at the Si(111)- (7×7) surface

The creation of magnetism on non-magnetic semiconductor surfaces is of importance for the realization of spintronics devices. Especially, the coupling of electron spins within quantum nanostructures can be utilized for nanomagnetism applications. Here, we demonstrate, based on first-principles density-functional theory calculations, that the adsorption of H atoms on the Si(111)-(7$\times$7) surface induces the spin polarization of surrounding Si dangling bonds (DBs) and their spin orderings. It is revealed that the H adsorption on a rest-atom site exhibits a Jahn-Teller-like distortion that accompanies a charge transfer from the rest atom to the nearest neighboring adatoms. This charge transfer increases the local density of states of such three adatoms at the Fermi level, thereby inducing a Stoner-type instability to produce a ferrimagnetic order of adatom DBs around the adsorbed H atom. Meanwhile, the H adsorption on an adatom site cannot induce spin polarization, but, as adsorbed H atoms increase, the ferrimagnetic order of rest-atom DBs emerges through the charge transfer from rest atoms to adatoms. Our findings provide a microscopic mechanism of the H-induced spin orderings of Si DBs at the atomic scale, which paves a novel way to the design of nanoscale magnetism in the representative semiconductor surface.

Computational predictions of electronic properties of graphene with defects, adsorbed transition metal-oxides and water using density functional theory

The interfacial interactions between transition metal oxides (vanadium oxide VO2, vanadium pentoxide V2O5, cobalt oxide CoO and Co3O4, manganese oxide MnO2) and water adsorbates on graphene supports as solvated interfaces and influence of defects in graphene are studied using periodic density functional theory (DFT) calculations in view of their significance for applied electrochemistry. DFT complemented and synergized our experimental work. The optimized metal oxide adatom-graphene geometries identified the preferred adatom sites, whereas metal oxide-graphene strengths are correlated with the adatom distance from the graphene plane, the Metal-C overlap populations, and the adsorption energies. The presence of finite electronic density of states (DOS) near Fermi level and charge transfers between the adatom top layer and graphene supports reflect primarily covalent bonding nature. The presence of small orbital overlap integral of bonds between the s and p (and d) orbitals of the nearest carbon (graphene), carbon oxide (graphene oxide) and metal oxide atoms reveal localized orbital re-hybridization resulting in changes in DOS yielding high electrochemical activity. Moreover, for increased adatom coverage the extent of charge transfer reverses resulting in limited electroactivity. In fact, DFT calculations are corroborated with experimental findings, where graphene-based supports decorated with optimal mass loaded nanostructured Co3O4 and MnO2 (as well as V2O5) were capable of delivering maximum specific energy storage capacity (Cs) > 550 F·g−1 (Gupta et al. J. Mater. Res. 32, 301 (2017)) in contrast to higher or lower loading. The presence of defects in graphene materials results in new electronic states to endow unique functionalities that is not otherwise possible in the bulk and with adsorbed water molecules besides optimum C/O ratio in graphene oxide nanosheets that show redshift thus a decreasing bandgap and finite charge transfer from graphene to water molecules. The case examples studied in this work represent a first glimpse of what may become routine and integral step in materials design and discovery for alternative energy and sustainable environmental technologies.

Free-Energy Landscape of the Dissolution of Gibbsite at High pH.

The individual elementary reactions involved in the dissolution of a solid into solution remain mostly speculative due to a lack of direct experimental probes. In this regard, we have applied atomistic simulations to map the free-energy landscape of the dissolution of gibbsite from a step edge as a model of metal hydroxide dissolution. The overall reaction combines kink formation and kink propagation. Two individual reactions were found to be rate-limiting for kink formation, that is, the displacement of Al from a step site to a ledge adatom site and its detachment from ledge/terrace adatom sites into the solution. As a result, a pool of mobile and labile adsorbed species, or adatoms, exists before the release of Al into solution. Because of the quasi-hexagonal symmetry of gibbsite, kink site propagation can occur in multiple directions. Overall, our results will enable the development of microscopic mechanistic models of metal oxide dissolution.

Energy dissipation unveils atomic displacement in the noncontact atomic force microscopy imaging ofSi(111)−(7×7)

The kinetic energy of the oscillating cantilever of noncontact atomic force microscopy (nc-AFM) at room temperature was considerably dissipated over regions between a Si adatom and its neighboring rest atom for $\text{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ in close proximity to a Si tip on the cantilever. However, nc-AFM topographic images showed no atomic features over those regions, which were the hollow sites of the $(7\ifmmode\times\else\texttimes\fi{}7)$. This energy dissipation likely originated from displacement of Si adatoms with respect to the tip over the hollow sites, leading to a lateral shift of the adatoms toward the rest atom. This interaction led to hysteresis over each cantilever oscillation cycle; when the tip was retracted, the Si adatom likely returned to its original position. To confirm the atomic processes involved in the force interactions through Si dangling bonds, the $\text{Si}(111)\text{\ensuremath{-}}(7\ifmmode\times\else\texttimes\fi{}7)$ surface was partly terminated with atomic hydrogen (H) and examined by nc-AFM. When the Si adatoms and/or the rest atoms were terminated with H, the hollow sites were not bright (less dissipation) in images of the energy dissipation channels by nc-AFM. The hollow sites acted as metastable sites for Si adatoms in surface diffusion and atom manipulation; thus, the dissipation energy which is saturated on the tip likely corresponds to the difference in the potential energy between the hollow site and the Si adatom site. In this study, we demonstrated the ability of dissipation channels of nc-AFM to enable visualization of the dynamics of atoms and molecules on surfaces, which cannot be revealed by nc-AFM topographic images alone.

Atom manipulation method to substitute individual adsorbate atoms into a Si(111)-(7 × 7) substrate at room temperature

We demonstrate a method to substitute individual adsorbate atoms into a Si(111)-(7 × 7) substrate using the tip of an atomic force microscope (AFM) at room temperature. We show that single Sn atoms diffusing within the half-unit-cells (HUCs) of the Si(111)-(7 × 7) substrate can be substituted into Si adatom sites via a close approach of the tip, whereby the intrinsic Si adatoms are ejected onto the surface of the adjacent HUCs. The Sn atom substitution sites can be precisely controlled by the approach of the AFM tip toward the surface at certain positions near the boundary of the HUCs but slightly shifted away from the HUC with the diffusing Sn atom. This manipulation method is also demonstrated to replace Si adatoms in the Si(111)-(7 × 7) surface with Pb using scanning tunneling microscopy. This method can provide a way to induce single-atom substitutional doping at certain positions from an adsorbate atom diffusing within a confined space provided by a substrate, which would allow for control of the doping sites in nanostructural materials.

Density functional theory calculations on alkali and the alkaline Ca atoms adsorbed on graphene monolayers

The adsorption of the alkali Li, K, and Na and the alkaline Ca on graphene is studied using periodic density functional theory (DFT) under various adatom coverages. The charge transfers between the adatom and the graphene sheet and the almost unchanged densities-of-states spectra in the energy region near and below the Fermi level support an ionic bond pattern between the adatom and the graphene atoms. However, the presence of small orbital overlap between the metal and the nearest graphene atom is indicative of small covalent bonding. Van der Waals interactions are examined through a semiempirical correction in the DFT functional and by comparing adatom-graphene calculations between 3% and 1.4% adatom coverages. Optimized adatom-graphene geometries identify the preferred adatom sites, whereas the adatom-graphene strength is correlated with the adsorption energy and the adatom distance from the graphene plane. Calculated electronic properties and structural parameters are obtained using hybrid functionals and a generalized gradient approximation functional paired with basis sets of various sizes. We found that due to long range electrostatic forces between the alkali/alkaline adatoms and the graphene monolayer, the adatom-graphene structural and electronic properties could be well-described by specific DFT functionals paired with high-quality adatom basis sets. For Li, K, and Na adsorbed on graphene, increased adatom surface coverage weakens the adatom-graphene interaction. However, this statement does not apply for Ca adsorbed on graphene. In this case, the Ca adsorption strength, which is stronger at higher coverages, is opposite to increases in the Ca–4s orbital population.