Adsorbate Substrate Research Articles

Ab initio density-functional study of NO on close-packed transition and noble metalsurfaces: I. Molecular adsorption

Ab initio density-functional calculations have been used to investigate the molecularadsorption of NO on the close-packed surfaces of late transition metals (Co, Ni, Ru,Rh, Pd, Ir, Pt) and noble metals (Cu, Ag, Au). The energetics, geometry, andvibrational properties of the adsorbate–substrate complex have been calculated. Withthe exception of Ir and Au, adsorption in a hollow-site is always preferred. OnIr(111) the potential-energy surface for NO adsorption is very flat, with a slightpreference for a linear on-top geometry. On Au(111), where NO adsorption is onlyvery weak, bridge-adsorption with a strong tilting of the NO molecule relative tothe surface normal is predicted. Among the different hollows on a (111) surfacepreference changes from hcp on Co, Ni, Ru, Rh to fcc on Cu, Pd, Pt, and Ag.However, not only on Ir, but also on Co, Ru, Rh and Pt are the site-dependentdifferences in the adsorption energies small enough to allow a coexistence of NOadsorbed on different sites. A careful comparison of the calculated vibrationaleigenmodes with the available experimental data leads to full agreement between thepredicted site preference and the observed NO stretching frequencies. This leadsto a redefinition of the characteristic frequency intervals to be used for the siteassignment. The trends in the adsorption energies and in the vibrational spectra arecompared to those derived from studies of CO adsorption on the same surfaces anddiscussed in terms of the filling of the d-band of the substrate. In a forthcomingpublication, these studies will be extended to NO dissociation on these substrates.

Diffusion of rhodium layers on the stepped surface of a tungsten micro-crystal

We have investigated the spreading of rhodium at coverages of 0.25, 0.5, 1, 2 and 3 ML over the curved surface of a field emitter tip using field electron microscopy. We have found that the activation energy of spreading as well as the prefactor for the diffusivity depend strongly on the thickness of the layer diffusing, due to a change in interactions in the adsorbate–substrate system. The derived average activation energy for spreading first decreases from E dif = 1.32 eV/atom at Θ = 0.25 ML to E dif = 0.71 eV at Θ ≈ 2 ML and than rises again to E dif = 1.20 eV at Θ ≈ 3 ML. The prefactor for the diffusivity D 0 also decreases with increasing coverage from 0.5 to 1 ML, and stays almost constant for multilayer diffusion in a range of few orders of magnitude lower than for single atom diffusion. We register typical spreading behavior with a sharp moving boundary in the (0 1 1)–(0 0 1) zone of the tip and an unusual picture of diffusion in the (0 1 1)–(1 1 2) region of the tip. In the second region diffusion proceeds without a sharp boundary, independent of the thickness of the moving layer. We think that such an unusual picture can be caused by the change in composition of the second and next layers of adsorbing material due to the early stage of faceting observed in this region of the tip at higher temperature. The results are compared with data for diffusion of individual Rh atoms and small clusters; to understand the observed diffusion we propose taking account of the atomic surface structure of the substrate, modified by strong interactions of the Rh adsorbate with the W micro-crystal surface.

Structural transitions of chemisorbed iodine on Au(1 0 0): A STM and LEED study

We have studied the chemisorption of molecular iodine on a reconstructed Au(1 0 0) surface by means of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) in an ultra high vacuum environment at 300 K and at temperatures down to 115 K. Several structural phases are observed during iodine adsorption. At 300 K, a low iodine coverage of θ I ∼ 0.1 ML is sufficient to lift the “hex” reconstruction, producing the substrate’s (1 × 1) diffraction pattern with a diffuse background. The first stable structure defined as c( p√2 × 2√2) R45°, is formed at coverages slightly higher than 0.4 ML that uniaxially compresses with increasing iodine coverage. For higher coverages θ I > 0.5 ML the iodine layer transforms to a rotated hexagonal structure with a maximum angle of 3.5° with respect to the [0 0 1] substrate direction. An iodine structure not previously reported for this system was observed to form at T ∼ 120 K when θ I ∼ 0.33 ML, and only observable under certain adsorption/cooling conditions. The findings are discussed in terms of the subtle competition between adsorbate–substrate and adsorbate–adsorbate interactions in determining the adsorbate structure.

Microscopic understanding of an ultrafast photochemical surface reaction: H ads + H ads → H 2,gas on Ru(0 0 1)

As a prototypical surface reaction initiated by excitation with an ultrafast light pulse, the associative desorption of hydrogen from a Ru(0 0 1) surface has been studied. Femtosecond laser techniques reveal detailed information on the underlying reaction mechanism, the pathway and time scales of energy flow and the partitioning of excess energy between different degrees of freedom within the reaction product. A response time of the desorption yield of only few hundred femtoseconds as manifested in two-pulse correlation measurements and a pronounced isotope effect in the H 2 versus D 2 yield unambiguously indicate a hot substrate electron-driven reaction mechanism. All experimental results can be well reproduced within the framework of electronic friction between the ruthenium substrate and the hydrogen adlayer. In addition, it is found that adsorbate–adsorbate interactions crucially influence the reaction as seen in promotion effects within isotopically substituted surroundings of the reactants. Finally, energy partitioning into various degrees of freedom is investigated by time-of-flight measurements and state-resolved detection of desorbing hydrogen molecules via resonance enhanced multiphoton ionization. The results show a pronounced preference for translational energy versus intramolecular excitations like vibrations and rotations. These studies provide an ideal benchmark for theoretical calculations of potential energy surfaces and photoinduced reaction dynamics of this adsorbate–substrate system.

Aniline on Ag(1 1 1): Adsorption configuration, adsorbate–substrate bond, and inter-adsorbate interactions

The adsorption of submonolayer aniline on the Ag(1 1 1) surface has been characterized using temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS). Analysis of the TPD curves yielded a desorption energy of 74 kJ/mol and revealed repulsive inter-adsorbate interactions resulting from a local dipole moment of 4.3 D at the adsorbate–substrate complex. This dipole moment arises from a weak charge transfer from aniline to silver in forming the adsorption bond. Analysis of the EELS spectrum revealed several vibrational modes that are consistent with weakly chemisorbed aniline with the amino group bound to the surface. Intensity analysis based on IR selection rules on metal shows that the molecule adsorbs nearly flat with a small tilt angle (−13 ± 8° between the CN bond and the surface) and a small orientation angle.

CO adsorption on Cu(1 1 1) and Cu(0 0 1) surfaces: Improving site preference in DFT calculations

CO adsorption on Cu(1 1 1) and Cu(0 0 1) surfaces has been studied within ab initio density functional theory (DFT). The structural, vibrational and thermodynamic properties of the adsorbate–substrate complex have been calculated. Calculations within the generalized gradient approximation (GGA) predict adsorption in the threefold hollow on Cu(1 1 1) and in the bridge-site on Cu(0 0 1), instead of on-top as found experimentally. It is demonstrated that the correct site preference is achieved if the underestimation of the HOMO–LUMO gap of CO characteristic for DFT is corrected by applying a molecular DFT + U approach. The DFT + U approach also produces good agreement with the experimentally measured adsorption energies, while introducing only small changes in the calculated geometrical and vibrational properties further improving agreement with experiment which is fair already at the GGA level.

In Situ Layer-by-Layer Film Formation Kinetics under an Applied Voltage Measured by Optical Waveguide Lightmode Spectroscopy

Layer-by-layer (LbL) thin film assembly occurs via the alternate adsorption of positively and negatively charged macromolecular species. We investigate here the control of LbL film growth through the electric potential of the underlying substrate. We employ optical waveguide lightmode spectroscopy (OWLS) to obtain in situ kinetic measurements of poly(allylamine hydrochloride)/poly(sodium 4-styrenesulfonate) (PAH/PSS) and poly(L-lysine)/dextran sulfate (PLL/DXS) multilayer film formation in the presence of an applied voltage difference (deltaV) between the adsorbing substrate, an indium tin oxide- (ITO-) coated waveguiding sensor chip, and a parallel platinum counterelectrode. We find initial layer adsorption to be significantly enhanced by an applied potential for both polyelectrolyte systems: the mass and thickness of (positively charged) PAH and PLL layers on ITO are about 60% and 500% larger, respectively, at deltaV = 2 V than at open circuit potential (OCP), in apparent violation of electrostatics. A kinetic analysis reveals the initial attachment rate constant to decrease with voltage, in agreement with electrostatics. To reconcile these results, we propose a more coiled and loosely bound adsorbed polymer conformation at higher applied potential. Following 10 adsorption steps, the mass and thickness of a PAH/PSS film grown under deltaV = 2 V are about 15% less than those of a comparable film grown under OCP, reflecting a lower degree of complexation between adsorbing polyanions and more highly coiled adsorbed polycations. Following 14 adsorption steps, the mass and thickness of a PLL/DXS film grown under deltaV = 2 V are about 70% greater than those of a comparable film grown under OCP, reflecting the increased charge overcompensation in the initial layer. We find the scaling of film mass () with the number of adsorption steps (n) to be linear in the PAH/PSS system and exponential (i.e., approximately eyn) in the PLL/DXS system, irrespective of applied voltage. We observe to decrease with applied voltage and to exhibit a crossover to a smaller value around n = 5. Extrapolation reveals PLL/DXS multilayer films to be suppressed by increased voltage in the limit of large n: the mass of films grown at OCP and deltaV = 1 V would surpass that of a film grown under deltaV = 2 V at about the 23rd and 18th adsorption steps, respectively. The formation kinetics of PLL/DXS, but not PAH/PSS, change qualitatively under voltage: PLL adsorption is slow to reach a plateau, possibly due to the formation of secondary structure, and a decrease in film mass occurs toward the end of each DXS adsorption step, suggesting spontaneous removal of some PLL/DXS complexes from the film.

Neutron and ion beams in biological research

Neutron reflectometry is an excellent technique to determine the structure of a protein layer adsorbed at a solid interface. The use of contrast-variation makes it possible to highlight the adsorbed protein layer. The neutron scattering lengths of D and H have opposite signs. By choosing the H 2 O/D 2 O ratio, the scattering length density can be made equal to that of the adsorbing substrate. To determine the scattering length density of protein, both its volume and its scattering length in solution is needed. To calculate the scattering length, the H-D exchange of the labile protons of the protein should be taken into account. For monitoring the H-D exchange, electrospray ionization mass spectroscopy (ES + I-MS) was applied. We show that ES + I-MS is the appropriate technique for quantitative analysis of the H-D exchange. We compare the experimental results for the exchange in lysozyme and b-casein with theoretical calculations.

Quantification of Ga surface coverages and their desorption kinetics on GaN (0001) and (000‐1) surfaces

We report on direct observations of the Ga surface coverages on both wurtzite (0001) and (000-1) GaN surface polarities during pure Ga adsorption and plasma-assisted molecular beam epitaxy (PAMBE). With the use of the novel line-of-sight quadrupole mass spectrometry technique the Ga coverages could be accurately quantified by monitoring the adsorbed Ga atoms at their subsequent desorption process. Consistent with theoretical predictions, the GaN surface was found to be terminated by an equilibrium Ga adlayer that differs substantially between the two surface polarities, being 1.1 monolayers (ML) on (000-1) GaN and 2.5 ML on (0001) GaN. In the case of (0001) GaN, the Ga adlayer increases continuously from 0 to 2.5 ML when the growth conditions are varied from N-rich to moderate Ga-rich conditions. For very Ga-rich conditions the regime of equilibrium Ga adlayer coverages changes abruptly to a regime where gradual surface accumulation of Ga droplets is favored. Further temperature-dependent measurements of the Ga desorption are used to derive typical Ga adatom lifetimes that yield a fundamental understanding of the adsorbate–substrate binding energetics for the Ga adlayer and for the Ga droplets. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dynamical scaling of single chains on adsorbing substrates: Diffusion processes

We study the dynamics of tethered chains of length N on adsorbing surfaces, considering the dilute case; for this we use the bond fluctuation model and scaling concepts. In particular, we focus on the mean-square displacement of single monomers and of the center of mass of the chains. The characteristic time tau of the fluctuations of a free chain in a good solvent grows as tau approximately N(a), where the coefficient a obeys a=2nu+1. We show that the same coefficient also holds at the critical point of adsorption. At intermediate time scales single monomers show subdiffusive behavior; this concurs with the behavior calculated from scaling arguments based on the dynamical exponent a. In the adsorbed state tau(perpendicular), the time scale for the relaxation in the direction perpendicular to the surface, becomes independent of N; tau(perpendicular) is then the relaxation time of an adsorption blob. In the direction parallel to the surface the motion is similar to that of a two-dimensional chain and is controlled by a time scale given by tau(parallel) approximately N(2nu(2)+1)L(-2Delta(nu/nu)), where nu(2) is the Flory exponent in two dimensions, nu is the Flory exponent in three dimensions, and Deltanu=nu(2)-nu. For the motion parallel to the surface we find dynamical scaling over a range of about four decades in time.

Triple-point wetting of molecular hydrogen isotopes

Triple-point wetting is a well-known phenomenon of simple adsorbates on solidsubstrates, which involves, in the liquid phase above the triple-point temperature,T3, complete wetting with the formation of arbitrary thick films being observed, whereas belowT3 only a few monolayers of the solid phase are adsorbed at saturated vapour pressure. Thiseffect is usually ascribed to the substrate-induced strain in the solid film, which occurs dueto the lattice mismatch and the strong van der Waals pressure in the first few monolayers.Molecular hydrogen is a suitable system in which to investigate this phenomenon, in particularby tailoring the adsorbate–substrate interaction by means of thin preplating layers of otheradsorbates, and by introducing disorder into the system by using not only the pure systemsH2 and D2, but also mixtures thereof. The experiments show that triple-point wetting is a ratherdominant effect which, in contrast to expectations, persists even if the system parametersare widely varied. This indicates that the present understanding of this effect isincomplete. We present an investigation of the influence of the roughness of thesubstrate which is expected to be responsible for the dewetting of the solid phase.

Surface roughening due to adsorbates

With a solid-on-solid model, we show how adsorbates can induce surface roughening. Roughened surfaces exhibit pits and regrowth structures that have characteristic patterns that depend directly on the adsorbate coverage and on the strength and type of involved interactions. Monte Carlo simulations were used to explore the consequences of adsorbate–adsorbate repulsion and two types of adsorbate–substrate interactions that lead to roughening.

Double photoionization of polyatomic molecules by arbitrarily polarized light: theory

This paper develops a theoretical framework suitable for studying simultaneous ejection of two electrons following the absorption of a single photon by a molecule belonging to one of the 32 point groups. The ionizing electromagnetic radiation is assumed to have any arbitrary polarization. Both photoelectrons are analysed in terms of their energies as well as directions of propagation. The transformation properties of the molecular point symmetry group are used to their full advantage to reduce the various expressions obtained herein to their simplest possible forms. The ionization amplitude is thus shown to decompose into a sum of transitions each involving a final state wavefunction belonging to the irreducible representations of the point group of the molecular target. Molecules with both a random or a fixed orientation in space have been considered. The results obtained herein can therefore be used to study double photoionization of any point symmetry molecule in either its gaseous phase or oriented on liquid and solid surfaces. The analysis is general in that it is independent of any particular dynamical models for the description of photoionization. The present results show that such double photoionization experiments are richer sources of information, specifically on interaction between adsorbed molecules and surfaces and on the geometry of the adsorbate–substrate system, in addition to the electron–electron correlation, than their counterpart experiments on single photoionization. Various possible cases and several observables are discussed.

Chemisorption of NTCDA on Ag(1 1 1): a NIXSW study including non-dipolar and electron-stimulated effects

The adsorption of one monolayer of 1,4,5,8-naphthalene-tetracarboxylicacid-dianhydride (NTCDA) on the Ag(1 1 1)-surface was studied using the normal incidence X-ray standing waves (XSW) technique. Results regarding two key-issues are presented: Most prominent, the precise adsorbate–substrate bonding distance could be evaluated to 3.02 ± 0.02 Å (for the “relaxed monolayer”-structure). This value is significantly smaller than a van der Waals bonding distance and clearly indicates the chemisorptive bonding character. Concordant results were obtained from both O 1s photo- and O KLL Auger electron emission. This was enabled by the development of a data analysis procedure––the second issue addressed––which takes into account non-dipolar contributions to the photoemission as well as electron-stimulated Auger excitations. The latter effect adds a fraction to the total Auger yield being as high as 50% and hence may be important for any XSW study using Auger signals.

Surface rearrangement at complex adsorbate–substrate interfaces

On the basis of the information theory approach we propose a novel statistical scheme for analyzing the evolution of coupled adsorbate-substrate systems, in which the substrate undergoes the adsorbate-induced transformations. A relation between the substrate morphology and the adsorbate thermodynamic state is established. This allows one to estimate the surface structure in terms of incomplete experimental information and the one concerning the adsorbate thermodynamic response to the structural modifications.

Understanding adsorbate bonding through quantitative surface structure determination

Quantitative structural studies of adsorption structures, especially using scanned-energy mode photoelectron diffraction (PhD) and low energy electron diffraction (LEED), are now capable of providing interatomic bondlengths with sufficient precision to provide chemically significant information regarding the nature of adsorbate–substrate bonding. Selected examples of such studies of molecular adsorbates on mainly metal surfaces are presented which provide insight into the relationship between adsorption energies, coordination site and both adsorbate–substrate and intramolecular bond lengths. Specific examples include simple hydrocarbons on transition and noble metal surfaces, and pure and coadsorbed CO on Ni and NiO surfaces.

The adsorption of Xe and Ar on quasicrystalline Al–Ni–Co

An interaction potential energy between an adsorbate (Xe and Ar) and the ten-foldAl–Ni–Co quasicrystal is computed by summing over all adsorbate–substrate interatomicinteractions. The quasicrystal atoms’ coordinates are obtained from LEED experiments,and the Lennard-Jones parameters of Xe–Al, Xe–Ni and Xe–Co are found usingsemiempirical combining rules. The resulting potential energy function of position is highlycorrugated.Monolayer adsorption of Xe and Ar on the quasicrystal surface is investigated in two cases:(1) in the limit of low coverage (Henry’s law regime), and (2) at somewhat larger coverage,when interactions between adatoms are considered through the second virial coefficient,CAAS. A comparison with adsorption on a flat surface indicates that the corrugation enhancesthe effect of the Xe–Xe (Ar–Ar) interactions. The theoretical results for the lowcoverage adsorption regime are compared to experimental (LEED isobar) data.

Single molecule vibrational spectroscopy of N2 on Cu(110)

Abstract Vibrational spectra of single N2 molecules on Cu(1 1 0) have been measured using inelastic electron tunnelling spectroscopy with a scanning tunnelling microscope (STM-IETS). At 5 K, N2 exhibits a molecular stretch vibrational loss at 265 meV. Two adsorbate–substrate modes are also observed, at 11 and 36 meV, which are assigned to a frustrated translation and to a mode involving the molecule vibrating against the substrate, respectively. These results point to chemisorbed N2, with the molecular axis aligned perpendicular to the substrate. On the basis of earlier studies in the monolayer regime, it is likely that the isolated molecules studied here are associated with Cu vacancy sites.

“In situ” STM studies of the ordering process of a nonplanar derivative of PTCDI on Ag(110)

Abstract The formation of ordered structures of large, nonplanar perylene derivative DPP-PTCDI/perylene-3,4,9,10-tetra-carboxylic-diimide-di(2,6-isopropylphenyl)/on Ag(1 1 0) has been studied “in situ” by means of variable temperature STM. The observations of the molecule diffusion and resulting growth behaviour were performed within a wide temperature interval, 100–190 K. The results indicate that below 160 K the ordering process occurs within existing adsorbate islands. Under these conditions the tilting angle of the nonplanar molecules versus the substrate plane is optimised. Above 160 K the ordering process occurs in a larger scale, leading to the reorganisation of the adsorbate islands. The newly formed islands exhibit an elongated shape which is a consequence of differences in the diffusion coefficient of molecules in perpendicular directions with respect to the Ag(1 1 0) surface structure: parallel and perpendicular to the rows. This difference results from the specific site of adsorbate–substrate interaction. Heating of the substrate with pre-deposited molecules leads to the substrate reorganisation caused by the adsorbate molecules. This phenomenon is explained by the simultaneous diffusion of adsorbate molecules and substrate atoms.

Substrate-induced supramolecular ordering of functional molecules: theoretical modelling and STM investigation of the PEBA/Ag(1 1 1) system

We model the geometry, binding site and absolute orientation of the self-assembled monolayer of 4-[pyrid-4-yl-ethynyl] benzoic acid (PEBA) adsorbed on the (1 1 1) surface of silver by means of molecular dynamics and first principles calculations, and compare these results with those we obtain from scanning tunneling microscopy experiments. Our results indicate that the adsorbate–substrate interaction controls the overall orientation of the self-assembled molecular superstructures. We investigate the nature of this interaction, which is found to induce some degree of readjustment of the molecular structure and substantial stretching of the inter-molecular hydrogen bonds. This is consistent with the nodal structure of the interaction-induced ab initio differential charge density, which can be interpreted in terms of a simple extended Hückel model.