Low Coverage Limit Research Articles

Insights into the interaction of nitrobenzene and the Ag(111) surface: A DFT study

This study explores the potential of nitrobenzene as an anolyte material for nonaqueous redox flow batteries (RFBs) by theoretically examining its low-coverage adsorption behavior on neutral and charged Ag(111) model electrode surfaces. At the low coverage limit, DFT calculations show a preference for nitrobenzene to adsorb parallel to the surface, with the benzene ring and nitro group centered over HCP sites. Interactions between nitrobenzene and the surface were analyzed using induced charge density analysis, Bader charge analysis, and projected density of states (PDOS). It was found that nitrobenzene adsorbs primarily through van der Waals interactions with the surface. As nitrobenzene accumulates negative charge, the strength of adsorption diminishes. Understanding the electrode-electrolyte interface is crucial for enhancing RFB electrochemical performance, and this study sheds light on nitrobenzene's interaction with a model Ag electrode.

Investigation of coverage dependence of the stretching frequency of CO adsorbed on Pd surfaces at low coverage limits

The stretching frequency of the CO bond is a sensitive probe of the local environment of a surface-bound CO molecule, including the adorption site and density, i.e. surface coverage. In this work, we extend our analysis beyond the frequency shift due to differences in adsorption configurations. Using density functional theory (DFT) calculations, we directly explore the correlations between surface coverage and the stretching frequency of adsorbed CO on Pd surfaces. We also perform constant pressure infrared reflection absorption measurements of CO on Pd(111) and use existing relations between pressure and coverage to derive coverage dependency. Both results are compared to previously reported experimental data. Our derived correlations of peak frequency and area with surface coverage can help interpret experimental IR spectra in real time and extract time-dependent concentration data from transient kinetic experiments.

Fingerprints of magnetoinduced charge density waves in monolayer graphene beyond half filling

A charge density wave is a condensate of fermions, whose charge density shows a long-range periodic modulation. Such charge density wave can be principally described as a macroscopic quantum state and is known to occur by various formation mechanisms. These are the lattice deforming Peierls transition, the directional, fermionic wave vector orientation prone Fermi surface nesting or the generic charge ordering, which in contrast is associated solely with the undirected effective Coulomb interaction between fermions. In two-dimensional Dirac/Weyl-like systems, the existence of charge density waves is only theoretically predicted within the ultralow energy regime at half filling. Taking graphene as host of two-dimensional fermions described by a Dirac/Weyl Hamiltonian, we tuned indirectly the effective mutual Coulomb interaction between fermions through adsorption of tetracyanoquinodimethane on top in the low coverage limit. We thereby achieved the development of a novel, low-dimensional dissipative charge density wave of Weyl-like fermions, even beyond half filling with additional magneto-induced localization and quantization. This charge density wave appears both, in the electron and the hole spectrum.

Effect of molecular impurities on properties of clean and cesiated Mo(001) surface: A DFT study of the low coverage limit

A random search of configuration space at the density functional theory level has been performed to explore the structures created upon chemisorption of small amounts of impurities O2 and H2 on cesiated Mo(001). Five structural models, namely clean Mo(001), surface with a low Cs coverage, surface with a saturated Cs monolayer, Mo(001) with a multilayer Cs deposition, and clean Cs(001) have been considered as substrates. The distributions of impurity atoms upon adsorption and the associated energy and the work function (ϕ) changes have been investigated. The adatoms are found to be adsorbed most strongly on sites located on the Mo surface or, in the case of thick Cs depositions, when the Mo sites are not accessible, under the uppermost Cs layer. The ϕ change due to adsorption exhibits a complex behaviour affected by both the location of the impurity particles and the Cs coverage. On average, Δϕ is positive in the case of Mo(001) with a low Cs coverage, and vanishes with increasing Cs amount. However, in cases with thick Cs depositions, when the impurities tend to reside the positions close the uppermost Cs layer, Δϕ becomes negative. Notable exceptions opposing these trends are also identified.

Hydrogen ion scattering from alkali/graphene surface: Alkali core states effects

We present a theoretical study of the frontal collision of protons with a graphene surface to which an alkali-adatom is added. Na and Cs adsorbates are studied in a low coverage limit. We analyze how the presence of adsorbates affects the charge exchange when the binary collision between the H+ protons and the adatoms or different carbon atoms close to the impurity occurs. The charge transfer process in the scattering of protons in the K and C atoms of a graphene surface to which a K adatom is added has already been analyzed and discussed in our previous works. We find that the inner states of the alkali atoms adsorbed on graphene introduce important differences in the charge transfer depending on their positions with respect to the valence band of graphene. For a better grasp of this, in this work we select Cs and Na as adatoms due to Cs has core levels that can resonate with the valence band of graphene, while Na does not. The interacting system is described by the Anderson Hamiltonian which takes into account the electronic repulsion at the projectile site; the ion charge fractions after the collision are calculated by using the non-equilibrium Green-Keldysh functions formalism. Also, we describe the adsorption of the alkali atom on the surface using the single impurity Anderson model and introduce the Green's functions required to calculate the adatom spectral density. In the scattering of H+ from Cs, the charge fractions as a function of the incident energy, behave similarly to the scattering from K. For the Na adatom, the dependence is different, showing a monotonous increase in the high energy regime. The results of this work allow us to infer the signals in the charge exchange, from the localized features of the density of states on the alkaline adsorbate during the scattering process, due to the peculiarities of the electronic band structure of graphene.

Random sequential adsorption of oriented rectangles with random aspect ratio

We studied random sequential adsorption (RSA) of parallel rectangles with random aspect ratio but fixed area using a newly developed algorithm that allows to generate strictly saturated packing of this kind. We determined saturated packing fraction for several different distributions of a random variable used for selecting side length ratio of deposited rectangles. It was also shown that the anisotropy of deposited rectangles changes during packing generation. Additionally, we examined the kinetics of packing growth, which near saturation obeys the power law with the exponent 1/d≈1/3, typical for the RSA of unoriented anisotropic shapes on a two-dimensional surface. Kinetics in the low coverage limit is determined using the concept of the available surface function. The microstructural properties of obtained random packings are evaluated in terms of two-point density correlation function.

One-dimensional structures of three quinone molecules on Au(111)

Quinone molecules form self-assembled structures in biological systems playing a key role in charge transport. Here, we report on the self-assembled chain structures of three quinone molecules, anthraquinone (AQ), naphthacenequinone (NQ), and pentacenequinone (PQ) on Au(111) studied using scanning tunneling microscopy. They all formed one-dimensional chain structures confined to herringbone structures of Au(111) at the low-coverage limit. The observed structures were explained with O•••H hydrogen bonds, as revealed by our density functional theory calculations.

CO adsorption on MnO(100): Experimental benchmarks compared to DFT

CO adsorption on the MnO(100) surface was studied using temperature programmed desorption (TPD) and density functional theory (DFT). TPD results show that CO is weakly-bound on MnO(100), with an experimental adsorption energy of -35.6 ± 2.1 kJ/mol at terrace sites in the low coverage limit. PBE simulations suggest that CO adsorption causes an implausible (2 × 2) surface reconstruction. PBE+U simulations show no signs of surface reconstruction, and provide an accurate estimate of the adsorption energy (-36.4 kJ/mol) when combined with the DFT-D3 method with Becke-Jonson damping to correct for van der Waals interactions. This simulation also shows that CO adsorbs C-down onto the Mn2+ terrace site in a tilted geometry, which is also observed experimentally and computationally on the similarly-structured NiO(100) transition metal oxide surface. TPD results for large doses show a plateauing of the coverage at about 0.4 monolayers of CO at 85 K, with a defect coverage equivalent to 0.08 monolayers. Adsorption associated with defect sites is indicated by a high-temperature desorption tail which is not satisfactorily explained by DFT simulations of simple step or oxygen vacancy defects.

Unveiling the adsorption properties of 3d, 4d, and 5d metal adatoms on the MoS2 monolayer: A DFT-D3 investigation

The adsorption of metal (M) adatoms on two-dimensional (2D) transition-metal dichalcogenides (TMDs) monolayers have been used to tune the physical-chemical properties of 2D TMDs such as MoS2, however, our atomistic understanding of the binding mechanism, site preference, charge transfer, diffusion barriers, etc., of metal adatoms on MoS2 is far from satisfactory. Thus, we report spin-polarized density functional theory calculations within the D3 van der Waals (vdW) correction to characterize the adsorption properties of 15 metal adatoms (3d, 4d, 5d) on MoS2 at low-coverage limit. All adatoms can be separated into two groups, namely, Fe, Co, Ni, Ru, Rh, Os, Ir, and Pt bind covalently to the MoS2 monolayer on the topMo site, where the binding mechanism can be explained by the combination of two factors, namely, (i) distortions within the MoS2 monolayers, which breaks the degeneracy of the electronic states and reduces their magnitude near the valence band maximum, and (ii) strong hybridization between the partially unoccupied d- and Mo dz2-states. In contrast, Pd has a strong preference for the hollow site compared with the topMo site, however, Cu, Zn, Ag, Cd, and Hg adatoms bind to the topMo and hollow sites with nearly the same energy (differences less than 0.10eV) due to the weak hybridization of the fully occupied metal d-states, while Au binds preferentially on the topS site. Thus, the M-MoS2 interactions are dominated by covalent and van der Waals interactions, which shows strong and weak dependence on the adsorption site preference, respectively.

Hydrogen ion scattering from a potassium impurity adsorbed on graphene

In this work we study the charge exchange process in the scattering of protons by potassium atoms adsorbed on a graphene surface in a low coverage limit. Both, the projected density of states on the alkaline atom site and the final charge states of the hydrogen projectile are calculated by considering the electronic Coulomb repulsion in the $s$-valence orbital. The inner $3p$ and $3s$ states of potassium are included and the local perturbations of the density matrix on the surrounding C atoms are also considered. The interacting systems are described by an Anderson Hamiltonian whose terms are calculated from the chemical properties of the atoms and the extended features of the graphene surface. The positive and negative ion fractions of hydrogen in the collision process are obtained from Keldysh-Green functions, which are calculated by employing the equation of motion method closed up to a second order in the atom-surface coupling term. It is found that the carbon atoms have no possibility of a direct charge exchange process in a frontal collision of the proton with the K adatom, and that the K-$3p$ band, broadened by the interaction with the graphene surface, provides an important source of electrons for the negative ionization of hydrogen, which is also promoted by the presence of a $\mathrm{K}\text{\ensuremath{-}}3s$ core state. The narrow $4s$ and $3p$ bands of the adsorbed potassium lead to an oscillatory dependence with the projectile incoming energy, of the probability for the three correlated charge states of hydrogen.

Nonadditivity of the Adsorption Energies of Linear Acenes on Au(111): Molecular Anisotropy and Many-Body Effects.

Adsorption energies of chemisorbed molecules on inorganic solids usually scale linearly with molecular size and are well described by additive scaling laws. However, much less is known about scaling laws for physisorbed molecules. Our temperature-programmed desorption experiments demonstrate that the adsorption energy of acenes (benzene to pentacene) on the Au(111) surface in the limit of low coverage is highly nonadditive with respect to the molecular size. For pentacene, the deviation from an additive scaling of the adsorption energy amounts to as much as 0.7 eV. Our first-principles calculations explain the observed nonadditive behavior in terms of anisotropy of molecular polarization stemming from many-body electronic correlations. The observed nonadditivity of the adsorption energy has implications for surface-mediated intermolecular interactions and the ensuing on-surface self-assembly. Thus, future coverage-dependent studies should aim to gain insights into the impact of these complex interactions on the self-assembly of π-conjugated organic molecules on metal surfaces.

Energetics of adsorbed benzene on Ni(111) and Pt(111) by calorimetry

The heat of adsorption and sticking probability of benzene were measured on Ni(111) and Pt(111) at 90 K using single crystal adsorption calorimetry (SCAC). Benzene adsorbs molecularly with a heat of 208 kJ/mol on terrace sites in the low-coverage limit on Ni(111) and has a standard enthalpy of formation (∆Hf0) of C6H6,ads of −251 kJ/mol at a coverage of 1/9 ML. These results are compared to calorimetric results on Pt(111), where the standard enthalpy of formation (∆Hf0) of C6H6,ads was found to be −244 kJ/mol at a coverage of 1/9 ML. The slightly stronger bonding to Ni is attributed to stronger CNi covalent bonding partially compensated by weaker van der Waals (vdW) attractions to Ni compared to Pt. The measured energetics for benzene are compared to Density Functional Theory (DFT) calculations from previous literature, showing that functionals that do not contain corrections for vdW interactions badly underestimate the bond energies of benzene to both Ni(111) and Pt(111), while many of those that correct for vdW interactions are much more accurate.

Theoretical study of the Pb adsorption on Ni, Cr, Fe surfaces and on Ni based alloys

Adsorption of Pb atoms on the Ni(111), Ni(100), Fe(110), and Cr(110) metallic surfaces was studied theoretically within an ab initio density functional theory approach (DFT). (√3×√3)R30° super structures for Ni(111), and (2×2) for the other surfaces, corresponding to the saturation state, were considered. The preferred adsorption sites are found to be ternary sites for Ni(111), Fe(110), Cr(110) and quaternary sites for Ni(100). Adsorption on Fe and Cr is less exothermic than on Ni, by 0.16 and 0.33eV/mol respectively. Adsorption on model surfaces of Ni based alloys was also investigated. It was found that the energy of adsorption depends mostly on the chemical composition of the ternary site, and can be described by a linear combination of the energies of adsorption on the pure metals. The nature of the second nearest neighbour of the adsorbed Pb atom has no significant influence on the adsorption energy. Average energies of adsorption were calculated in two cases: the limit of low coverage, and the saturation. The energies of adsorption of Pb at saturation on nickel base alloy surface representative of alloy 600 (Ni-15Cr-8Fe) and alloy 690 (Ni-30Cr-8Fe) were calculated to be 0.07 and 0.11eV lower than on pure Ni respectively.

Structural Characterization of Ordered, Non-Close-Packed Functionalized Silica Nanoparticles on Gold Surfaces.

Nanostructured surfaces play an important role in modern science and technology. In particular, ordered arrangements of nonclose-packed nanoparticles created by self-assembly offer a versatile route to prepare systems, which can be used in various applications such as sensing, plasmonic devices or antireflection coatings. Self-assembly based systems are particularly appealing as preparation is rather simple. The ability of nanoparticle systems to form nonclosed packed monolayers by self-assembly depends on the balance of various energetic contributions in particular the adsorption energy, the lateral barrier for diffusion and the repulsion between particles. Even for simple model systems such as the monodispersed silica particles adsorbed on a bare gold surface investigated here, none of these quantities is easy to determine experimentally. To this end, we will report on a detailed characterization of the adsorption in particular with respect to the structural properties of the above-mentioned model system. Based on experimental results obtained by using quartz crystal microbalance with dissipation monitoring (QCM-D) as well as scanning electron microscopy (SEM) it is possible to determine the electrostatic pair potential from the lateral arrangement of the nano particles in the limit of low coverage.

CO Chemisorption on Ultrathin MgO-Supported Palladium Nanoparticles

The adsorption of CO on Pd nanoparticles (NP) supported on MgO ultrathin films is studied by sum frequency generation (SFG) vibrational spectroscopy at room temperature as a function of CO coverage for NP lateral sizes ranging from 3 to 6 nm. While the spectroscopic signature of CO on (100) top facets dominates the spectra, other new bands are attributed to (111) facets, edges, and defects. CO remains strongly bonded for all sizes, but spectroscopic and kinetic parameters evolve as NP size decreases. It is only for the smallest NPs ∼3 nm in diameter that the deviation from Pd(100) single crystal is important. Modeling of adsorption kinetics, dipolar coupling and DFT calculations allow rationalizing the observations: the size dependent Pd–Pd bond extension by the substrate is the main parameter that controls CO frequency in the low coverage limit. SFG sensitivity is found to increase up to 10–4 ML as NP size decreases. This makes SFG particularly well suited to probe molecular adsorption in the submonolaye...

Energetics of Adsorbed Methyl and Methyl Iodide on Ni(111) by Calorimetry: Comparison to Pt(111) and Implications for Catalysis

The heat of adsorption and sticking probability of methyl iodide were measured on Ni(111) at 100 and 160 K using single-crystal adsorption calorimetry (SCAC). At 100 K, methyl iodide adsorbs molecularly with a heat of 102 kJ/mol on terrace sites in the low-coverage limit, giving a standard enthalpy of formation (ΔHf°) of CH3I(ad) of −87 kJ/mol. A heat of 122 kJ/mol is also measured on defect sites, probably step edges. Calorimetry of the dissociative adsorption of methyl iodide on Ni(111) at 160 K yielded an integral heat of adsorption of −270 kJ/mol at 0.04 ML, providing the energetics of adsorbed methyl, with ΔHf°[CH3(ad)] = −71 kJ/mol and a CH3–Ni(111) bond enthalpy of 218 kJ/mol. This is 22 kJ/mol stronger than the reported value for H3C–Pt(111) bonds, explaining the greater activity of Ni catalysts for hydrogenolysis in comparison to Pt. The measured energetics for methyl were compared to density functional theory (DFT) calculations from previous literature, showing that these methods systematically ...

BINDING ENERGY OF MOLECULES ON WATER ICE: LABORATORY MEASUREMENTS AND MODELING

ABSTRACT We measured the binding energy of N2, CO, O2, CH4, and CO2 on non-porous (compact) amorphous solid water (np-ASW), of N2 and CO on porous ASW, and of NH3 on crystalline water ice. We were able to measure binding energies down to a fraction of 1% of a layer, thus making these measurements more appropriate for astrochemistry than the existing values. We found that CO2 forms clusters on the np-ASW surface even at very low coverages. The binding energies of N2, CO, O2, and CH4 decrease with coverage in the submonolayer regime. Their values at the low coverage limit are much higher than what is commonly used in gas-grain models. An empirical formula was used to describe the coverage dependence of the binding energies. We used the newly determined binding energy distributions in a simulation of gas-grain chemistry for cold cloud and hot-core models. We found that owing to the higher value of binding energy in the submonolayer regime, a fraction of all these ices remains for much longer and up to higher temperatures on the grain surface compared to the single value energies currently used in the astrochemical models.

固体表面に存在する水素原子が示す量子効果

Quantum effect is manifest for hydrogen atoms on surfaces as well as for those in solution and solids. The excited states of a hydrogen atom directly adsorbed on metal surfaces are beyond the diffusion barrier, thus being delocalized to the next sites, and even spread over the entire surface in the low-coverage limit. In addition, water and hydroxyl group on the surfaces show significant quantum effect, due to tunneling and zero-point motion of the hydrogen atom. We briefly review some experimental and theoretical studies that revealed the quantum effect of the hydrogen atom on the surfaces.

Spontaneous Oxidation of Ni Nanoclusters on MgO Monolayers Induced by Segregation of Interfacial Oxygen.

We report the study of Ni nanoclusters deposited on MgO/Ag(100) ultrathin films (one monolayer) at T = 200 K. We show by STM analysis and DFT calculations that in the limit of low Ni coverage the formation of nanoclusters of four to six atoms occurs and that these aggregates are flat rather than 3D, as expected for Ni tetramers, pentamers, or hexamers. Both the shape of the clusters and the interatomic distance between neighboring Ni atoms are indicative that the nanoparticles do not consist of pure metal atoms but that a NiyOx structure has formed thanks to the availability of atomic oxygen accumulated at the MgO/Ag interface, with Ni clusters acting as oxygen pumps. Besides being of relevance in view of the use of metal nanoclusters in catalysis and other applications, this finding gives a further proof of the peculiar behavior of ultrathin oxide films.

CO desorption from a catalytic surface: elucidation of the role of steps by velocity-selected residence time measurements.

Directly measuring the rate of a surface chemical reaction remains a challenging problem. For example, even after more than 30 years of study, there is still no agreement on the kinetic parameters for one of the simplest surface reactions: desorption of CO from Pt(111). We present a new experimental technique for determining rates of surface reactions, the velocity-selected residence time method, and demonstrate it for thermal desorption of CO from Pt(111). We use UV−UV double resonance spectroscopy to record surface residence times at selected final velocities of the desorbing CO subsequent to dosing with a pulsed molecular beam. Velocity selection differentiates trapping-desorption from direct scattering and removes influences on the temporal profile arising from the velocity distribution of the desorbing CO. The kinetic data thus obtained are of such high quality that bi-exponential desorption kinetics of CO from Pt(111) can be clearly seen. We assign the faster of the two rate processes to desorption from (111) terraces, and the slower rate process to sequential diffusion from steps to terraces followed by desorption. The influence of steps, whose density may vary from crystal to crystal, accounts for the diversity of previously reported (single exponential) kinetics results. Using transition-state theory, we derive the binding energy of CO to Pt(111) terraces, D(0)(terr) (Pt−CO) = 34 ± 1 kcal/mol (1.47 ± 0.04 eV) for the low coverage limit (≤0.03 ML) where adsorbate−adsorbate interactions are negligible. This provides a useful benchmark for electronic structure theory of adsorbates on metal surfaces.