Session H19: Molecules on Surface: Structure

Competition between intermolecular and substrate interactions in a multicomponent lattice gas model

Recent experiments have shown that acridine-9-carboxylic acid (ACA) molecules form a dense phase consisting of chain-like structures on Ag(111) with alternating orientations along the chain direction that permit the formation of strong hydrogen bonds. Despite the anisotropy in intermolecular interactions that leads to chain formation, molecular boundaries along and normal to the chair direction have very similar thermodynamic properties and fluctuations. We introduce a multicomponent lattice gas model where molecules with different orientations are treated as different species with different intermolecular interactions as well as different interactions with the substrate. This a generalization of the familiar Blume-Emery-Griffiths model of a binary lattice gas but in a region of parameter space not usually explored. We argue that the novel domain shapes, boundary fluctuations and phase densities seen in experiment arise from a competition between favorable anisotropic pair interactions in the chain phase and less favorable substrate interactions due to the different orientations. Detailed results of Monte Carlo simulations of this model and analytic work using mean field and pair approximation theories will be presented.   

Van der Waals interactions at the molecule-metal interface: PTCDA on Ag(111)

A detailed understanding of the processes governing the adsorption of molecules on metallic surfaces is of major interest for the field of molecular electronics. In this context, the molecule 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) has been extensively studied on a variety of metallic surfaces and has so gained fundamental, academic importance. The theoretical description of the adsorption is, however, still controversial as standard density functional theory (DFT) does not include van der Waals interactions, which, for PTCDA and many other molecules, yields the dominant binding contribution. We present DFT calculations of PTCDA adsorbed on Ag(111) where a recently developed exchange-correlation functional was adopted to include van der Waals interactions. Adsorption energy and distances, molecular distortions, charge rearrangements and orbital occupancies are discussed in detail.   

Tilt Angle Determination in Thin Films with Anisotropic Molecules

Many electronic properties of organic semiconductors depend critically on their physical and chemical arrangement of atoms. Here, a method is described to extract information about the molecular tilt angle and to determine the center electron density of anisotropic molecular thin films by means of specular x-ray diffraction. Thin films of phthalocyanine (Pc), an anisotropic molecule with a metal ion in the center, have been deposited in an organic molecular beam deposition system and studied with high-resolution x-ray diffraction. In particular, two isomorphous molecules, H$_{2}$Pc and CuPc, are compared experimentally and then studied with numerical simulations. The results show that the intensity distribution of the diffraction peaks belonging to the same series of lattice planes provides important structural information such as the molecular tilt angle or the center electron density of the anisotropic molecule.   

The role of defects and phonons in O$_2$ adsorption on Cu(100)

We present the results of an extensive {\it ab initio} study for O$_2$ adsorption on the Cu(100) surface. The calculated potential energy surfaces and first principles molecular dynamics trajectories reveal that on the clean surface without defects the dissociation probablility is low at small translational energies of the incoming molecule wheras at higher energies the molecules dissociate directly without a barrier. Of the different defects, steps turn out to be less reactive than expected and adatoms rather lower than enhance the reactivity. In contrast, vacancies act as dissociative centers, which locally enhance the reactivity. This result is in agreement with the molecular beam surface scattering experiments which show an increase at the oxygen sticking probability at low energies. We also discuss the role of phonons in O$_2$ dissociation, showing that the open space involved at certain phonon modes lowers the dissociation barrier, an effect analogous to the vacancy induced dissociation. Our results clearly demonstrate the importance of including both defects and surface dynamics in a realistic description of the adsorption process.   

Vibrational spectroscopy and \textit{ab initio} dynamics of the O-induced \textit{added-row} reconstructed Cu(110) surface.

It is known that O$_{2}$ molecules dissociatively adsorb on Cu(110) and, upon subsequent annealing, Cu and O atoms catenate along the [100] direction arranging themselves in a striped periodic super-grating, depending on O coverage and annealing temperature. It has been proposed that stress along the [110] direction in the regions that locally hold a (2x1)O added-row structure causes the formation of the stripes. Our electron energy loss spectroscopy (HREELS) and density functional perturbation theory calculations show however that the Rayleigh wave softens along the [110] direction, providing no indication that stress relief drives the formation of the stripes. Nonetheless, our calculations show also the stiffening of an in-plane mode which is peeled off above the bulk band and signifies strong O-induced intralayer force constant stiffening. Furthermore, HREELS at the $\Gamma $-point of the striped phase shows different degrees of softening of the O vertical mode, suggesting a stress relief gradient from the center of the stripes to the edges. Support to this interpretation is provided by our calculated phonon dispersion along the [110] direction of the O(3x1) added-row structure in which the O vertical mode appears softened with respect to that in the O(2x1) structure.   

Insights from surface stress calculations on the structure of c(2$\times $2)-N overlayer on Cu(001)

We present results of calculated changes in surface stress for N overlayers on Cu(001), in an effort to discriminate between several proposed structural models. Our calculations of the surface geometry and electronic structure are based on the density functional theory in the generalized gradient approximation and the pseudopotential method. We find that a c(2$\times $2) N overlayer causes a large change in the surface stress ($\approx $ 5 N/m) on Cu(001) turning it from tensile to compressive. We also perform calculations for several stress relief models to find that the so-called ``rumpling'' and ``clock reconstruction'' models fail to relieve the N induced stress. On the other hand, formation of strips of clean Cu(001) areas, aligned along the $\langle $100$\rangle $ direction, and trench-like steps of Cu atoms, along the $\langle $110$\rangle $ direction on Cu(001), relieve the induced stress most effectively, in agreement with predictions from experiments. We consider the implications of these results on surface phonon dispersion curves for unreconstructed c(2x2)-N/Cu(001), which are in good agreement with experiment.   

First principles calculations of the vibrational dynamics of c(2x2)-CO on Ag(001).

The reaction pathway of CO oxidation on Ag surfaces is still a subject of debate because of the complicated chemistry of O and the possibility that contaminants stabilize CO. Indeed, at $\sim $150 K, the dissociative O$_{2}$ adsorption is scarcely triggered while the CO adsorption on clean Ag(001) is hardly stable. The nature of the CO adsorption is thus by itself a matter of discussion and the characteristic energy losses for exciting the phonon modes introduced by the adsorbed species have an uncertain assignment. We present an \textit{ab initio} study of the structure and phonon dispersion of a c(2x2), atop, CO overlayer on Ag(001). Comparison with a similar study of c(2x2) CO on Cu(001) indicates that CO chemisorbs on Ag(001) despite the low binding energy. The frequency of the C-O stretch mode at the $\Gamma $-point is in excellent agreement with HREELS measurements and is reduced on Ag(001) almost as much as on Cu(001). The weak Ag-CO bond is reflected in the low frequency of the rest of the CO modes. Yet, in the Ag-CO stretch, the Ag surface atoms are strongly coupled, as in the case of CO on Cu(001). Likewise, the CO frustrated translation mode couples to the substrate in the vicinity of the $\Gamma $-point but, unlike that on Cu(001), the CO frustrated rotation mode on Ag(001) couples to the substrate inside the surface Brillouin zone.   

Oxygen-Induced Reconstructions on the $\beta-$Si$_3$N$_4$ $(10\overline{1}0)$ Surfaces

Motivated by recent electron microscopy studies at the Si$_3 $N$_4$/rare-earth oxide (REO) interfaces, we present first principles calculations for the preferred bonding sites and configurations of oxygen on various terminations of the $\beta- $Si$_3$N$_4$ $(10\overline{1}0)$ surface as a function of coverage and surface stoichiometry. We predict that oxygen induces various surface reconstructions, and it has a strong tendency to replace N on the surface. The structural stability of most low-energy surfaces is driven by the tendency of Si to saturate its dangling bonds and of oxygen to bridge two Si atoms similar to the bonding in SiO$_2$. The present {\em ab initio} results resolve the discrepancy between the experimental observations at the Si$_3$N$_4$/REO interfaces and previous theoretical studies\footnote{Juan C. Idrobo {\em et al.}, Phys. Rev. B {\bf 72}, 241301(R) (2005).} for bare surfaces regarding the lowest energy surface termination.   

Insight into water molecules bonding on 4d metal surfaces

Water-metal interactions are of capital importance to a wide variety of phenomena in materials science, catalysis, corrosion, electrochemistry, etc. Here we address the nature of the bond between water molecules and metal surfaces through a careful systematic study. Specifically, the bonding of isolated water molecules to a series of close-packed transition metal surfaces - Ru(0001), Rh(111), Pd(111) and Ag(111) - has been examined in detail with density functional theory (DFT). Aiming to understand the origin behind energetic and structural trends along the 4d series we employ a range of analysis tools, such as decomposition of the density of states, electron density differences, electronic reactivity function and inspection of individual Kohn-Sham orbitals. The results obtained allow us to rationalize the bonding between water and transition metal surfaces as a balance of covalent and electrostatic interactions. A frontier orbital scheme based on so-called two-center four-electron interactions between molecular orbitals of water and d band states of the surface proves incisive in understanding these systems.   

Water uptake on thin film MgO using ambient pressure XPS

Understanding the molecular level interactions of water with metal oxide surfaces is important in both industrial processing and environmental chemistry. MgO(100) is one of the most widely studied metal oxide surfaces due to its simple rock salt cubic structure. However, whether water adsorbs dissociatively (hydroxylation) or molecularly (thin film wetting) remains unanswered. We have characterized the uptake of water on 7ML MgO(100) on Ag(100) at RT using ambient pressure XPS. Surface compositions were measured in-situ under water vapor pressures ranging up to 1 Torr. Our results indicate that initial hydroxylation occurs at low pressures mostly at Mg(2+) sites up to about 0.1 mTorr. At higher coverages both hydroxylation and thin film wetting continues up through 1 Torr. We will also discuss results from: 1. water uptake under higher/lower effective vapor pressures, 2. additional MgO(100) film thickness, and 3. water uptake on MgO(111)/Ag(111) surfaces.   

Surface-induced solid-liquid phase transitions in ultra-thin water films at T $>$ 0 $^{\circ}$C

We report here the measurements of both the adsorption isotherms and the dissipation in ultra-thin films of water adsorbed on the surfaces of SiO2 . The measurements were made in a small high vacuum chamber in which we have mounted a QCM. The chamber was evacuated to $\sim$10-8 Torr and then filled with water vapor at pressures ranging from 10-3 -- 40 Torr (the vapor pressure of water at room temperature is $\sim$22 Torr). In addition the temperature of the apparatus can be varied in the range 10 -- 60$^{\circ}$C. This is sufficient to measure the adsorption isotherm and to probe the phase of adsorbed water films over the range of conditions. Recently published work studying the adsorption of water on the SiO2 layer formed on Si single crystals has suggested that the phase of the water at temperatures well above 0$^{\circ}$C is actually that of a solid, ice-like structure rather than liquid water [1]. That work is based on the comparison of the vibrational spectrum of thin water films with those of liquid water and ice. In our study we are using the QCM to investigate the possibility of formation of Ice-like structures on SiO2. \newline [1] Asay, D. B. and Kim, S.H., Evolution of the Adsorbed Water Layer Structure on Silicon Oxide at Room Temperature. J. Phys. Chem. B. 2005, 109, 16760-16763   

Relaxation of the (111) Surface of $\delta $-Pu and Effects of Atomic Adsorption: An \textit{Ab Initio} Study

The full-potential all-electron linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method has been employed to study the relaxation of the $\delta $-Pu (111) surface and the consequent effects for atomic adsorption of C, N and O atoms on this surface. The surface was modeled by a 5-layer slab with a (2$\times $2) surface unit cell. Upon relaxation of the slab, the interlayer separation between the surface and the subsurface layers expanded by 7.1{\%} with respect to the bulk interlayer separation while the separation between the subsurface and central layers expanded by 0.4{\%}. The hollow fcc adsorption site was found to be the most stable site for C and N with chemisorption energies of 6.420 eV and 6.549 eV respectively, while the hollow hcp adsorption site was found to be the most stable site for O with a chemisorption energy of 7.858 eV. The adsorbate-induced changes in the surface properties, namely the Pu magnetic moments, work function, and electronic structure will be discussed.   

Probing Adsorption and Thin-Film Growth with Optical Reflectance.

Optical reflectance provides a simple, inexpensive, noninvasive and sensitive method of monitoring adsorption and thin-film growth on metals in real time. Conduction electron scattering from isolated surface impurities decreases the near-surface electrical conductivity and causes a drop in reflectance. For chemisorbed species, the reflectance decreases monotonically and then saturates with completion of the adsorbed monolayer. Our probe, based on a modified laser pointer, can follow adsorption of oxygen and carbon monoxide on Cu(100) in ultrahigh vacuum with submonolayer sensitivity. We also monitor the deposition of Fe and Cu thin films on the same surface, and for layer-by-layer growth expect to see an oscillatory variation of reflectance in which local maxima correspond to the completion of each atomic layer.   

A Phenomenological Study of Adsorption on fcc (335) Terraces

The thermodynamic equilibrium model of adsorption on fcc (112) terraces (Langmuir 22, 7646, 2006) is extended to adsorption on fcc (335). There is preferential adsorption on one of the steps, and the difference, $U$, between adsorbate-substrate interaction energy on this step,$V_s$, and that on the rest of the terrace, $V_b$, is not necessarily zero. First-neighbor adsorbate-adsorbate interaction energy $V$ is usually repulsive, and we consider $V<0$, account for arbitrary second-neighbor interaction energy $W$, and allow the chemical potential energy per particle in the gas $\mu'$ to vary. The relatively low temperature 3-D energy phase diagram is obtained in terms of $u = -U/V$, $v=-\mu/V$, and $w = -W/V$, where $\mu = \mu' + V_b$. Based on this diagram, the experimental observation of just one or more phases allows one to predict the ranges or, ultimately, all of the values of $u$, $v$, and $w$. As the model is phenomenological and does not require the knowledge of the chemical composition of the substrate and the adsorbates, it can be applied to adsorption on a wide variety of fcc (335) terraces.   

The Nature of the Dissociation Sites of Hydrogen

Previously, we have demonstrated [1] that pairs of neighboring monoatomic hydrogen vacancies (HVs) on Pd(111) are totally unreactive toward H$_{2}$ dissociative adsorption and that active sites for the catalytic reaction can only be constituted of ensembles of at least three aggregated HVs. Thanks to LT-STM atomic observations, we show that this new description of H$_{2}$ dissociative adsorption onto transition metal surfaces is not an exotic particular catalytic case relevant only to Pd(111) and close-packed faces of fcc metals, but that it also applies to close-packed faces of hcp metals such as Ru(001) [2, 3]. Close to saturation of 1 ML, HVs were observed either as single entities or forming transient aggregations. Vacancies diffuse and aggregate to form active sites for the dissociative adsorption of H$_{2}$. We have found that H$_{2}$ dissociation takes place only on Ru sites where the metal atom is not bound to any H atom [3]. Such active sites are formed when at least 3 HVs aggregate by thermal diffusion. Sites formed by single HV or pairs of adjoining HVs were found to be unreactive toward H$_{2}$. [1] T. Mitsui, et al., \textit{Nature} \textbf{422} 705 (2003) [2] M. Tatarkhanov, et al., \textit{Surf. Sci.,} \textit{in Press }(2007) [3] F. Rose, et al., \textit{J. Phys. Chem. C,} \textit{Accepted} (2007)