Session Q12: Structure, Dynamics, and Diffusion at Surfaces

Water Adsorption on Wurtzite GaN Surfaces

A solid solution of wurtzite GaN/ZnO absorbs light in the visible and can photosplit water.\footnote{K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, and K. Domen, Nature 440, 295 (2006)} The water is oxidized by the photo-holes at the surface of the semiconductor alloy. However, microscopic details of the oxidation process are unknown. We present a first-principles study of water adsorption on wurtzite GaN. We study the structures and energetics of water adsorption, calculate the energy barrier for water dissociation, analyze the water-water interactions, and study the electronic structure. The results are compared with water adsorption on ZnO surface. We also study the behavior of the holes near the water-semiconductor interface.   

Does Water Adsorb Molecularly or Dissociatively on a Plutonium Surface?

DFT-GGA has been used to study adsorption of water in molecular and dissociative configurations on $\delta $-Pu (111) surface. In molecular state, water is physisorbed in an almost flat-lying orientation at a one-fold coordinated on-top site. The interaction of the water 1$b_{1}$ orbital and the Pu-6$d$ orbital provides the stability of water on the surface, implying that the Pu-5$f$ electrons remain chemically inert. The coadsorption cases of partially dissociated and fully dissociated products at the three-fold hollow sites yield chemisorption, coupled with rumpling of the surface layer and delocalization of the Pu-5$f$ electrons and formation of strong ionic bonds.   

Reaction path for dissociative water adsorption on vicinal anatase (101) surface

Using density functional calculations and a periodical slab model, we investigate water adsorption on edges formed by intersection of two anatase TiO$_2$ (101) surfaces. We find that after adsorption of a water molecule on a low-coordinated Ti atom on the ridge, decomposition may happen over a moderatly high barrier. In this process, a proton gets abstracted by bonding to a low-coodinated ridge oxygen atom. The hydroxyl anion remains bonded to the acidic Ti site on the edge. The methods used to find the saddle points and to map out the reaction paths are briefly discussed. We also give a discussion of the reaction rates that may be expected based on these calculations. The presence of hydroxyl groups and protons is favoring nucleation of Ca phoshate bonding by allowing exchange of the adsorbed proton against Ca$^{2+}$ ions. Such processes are thought to be essential for the biocompatibility of titanium and its use in dental and orthopedic applications.   

First Principles Study of Bulk and Surface Ordering Phenomena in Pt-X Binary Alloys in the Presence of Oxygen

Alloying in metal catalyst particles (such as platinum) may change surface structure due to segregation and ordering, which may in turn significantly impact catalytic activity. Using first principles density functional theory (DFT) calculations in conjunction with a cluster expansion (CE) technique, we have studied the ordering/phase-separation phenomena of bulk and surface Pt-X binary alloys, with a specific focus on Pt-Au. The surface DFT+CE calculations are performed both in the presence and absence of oxygen. For Pt-Au, the calculated results reveal a phase separating tendency in bulk Pt-Au and a small coherency energy between these two elements. The Pt-Au phase diagram calculated by combining DFT+CE with Monte Carlo simulation shows a slightly asymmetric miscibility gap is in good agreement with experimental results. In contrast to the bulk tendency, the surface does not show a pronounced phase-separation tendency, with several low-energy ``striped'' ordered structures (with a very small, negative formation energy). The presence of oxygen qualitatively changes the surface segregation tendency of Au, and our DFT+CE results show that the equilibrium structure of the Pt-Au (111) surface varies with oxygen coverage.   

A first-principles study of the electropotential dependent shape stability of metal nanoparticles

Understanding the catalytic activity of transition metal nanoparticles is a central issue in the development of novel fuel cell materials. Observed trends are often interpreted in terms of the size dependent shape of nanoparticles, in particular the relative density of low coordination sites. However, no consensus exists regarding the direction or even the existence of such an effect. In this context, ab-initio methods can be useful to extract relevant parameters. Here, we calculate surface energies under realistic electrochemical conditions and use the Wulff construction to infer stable nanoparticle contours. The electropotential is adjusted through its conjugate variable, the charge, and density countercharge periodic-image corrections are applied. The surrounding solvent is treated as a combination of a continuum dielectric and a classical ionic distribution at equilibrium.   

Lattice Dynamics of Cu$_2$O: Bulk and (110) Surface

A number of theoretical studies have been carried out in the past to investigate the stability of various surface oxides for the O/Cu system however despite the fact that catalytic processes usually proceed at elevated temperatures stability at finite temperatures has never been studied for these systems. Modern ab-initio methods however allow for the determination of the lattice dynamics and the phononic contribution to the free energy. Using density functional perturbation theory we have studied the lattice dynamics of Cu$_2$O-bulk as well as Cu$_2$O(110). In calculating the free energy as function of lattice constant we obtained for the bulk a negative thermal expansion up to roughly 300 K in excellent agreement with experiments. This is due to anomalous mode Gr\"uneisen parameters for vibrational modes in the low energy regime. Due to the anomalous behavior of the mode Gr\"uneisen parameter the bulk system is highly unstable against variations of the lattice constant by more than 2\%. To investigate the stability of the O/Cu surfaces we have investigated the lattice dynamics of Cu$_2$O(110) as a prototype. Despite a large number of low lying modes no instability has been found. These calculations allow also for the O/Cu system for the first time for a realistic estimation of the surface-free energy which is important for the determination of surface thermodynamic properties.   

Ab-initio Study of the Erlich-Schoewebel Barriers for fcc(100)

We present the results of density functional theory based calculations for the activation energies for the diffusion of adatoms (Cu or Ag) on Cu(100) and Ag(100) surfaces with and without steps\textbf{.} For Ag adatom diffusion via hopping on Cu(100), we find the energy barrier to be 0.37 eV, which is less than that (0.60 eV) of Cu adatom diffusion on Ag(100). In the presence of a step edge, we find the Erlich-Schoewebel (ES) barriers (via hopping process) for both Ag and Cu atoms on Cu(100) to be 170 meV\textbf{.} For Ag and Cu adatoms on Ag(100), the corresponding barriers are 50 meV and 60 meV, respectively\textbf{.} The ES barrier (via exchange process) for Ag on Ag(100) is found to be 20 meV and for Cu on Cu(100) it is 60 meV\textbf{. }The barriers for diffusion along the step edges at the lower terraces are 0.36 eV and 0.20 eV for Cu on Ag(100) and Ag on Cu(100), respectively. We trace the differences in the diffusion barriers of the homo-and hetero-epitaxial systems to the differences in the corrugation of the potential energy surface, and discuss the implications for homo-and heteroepitaxial growth on these surfaces.   

On the $\textrm{Si(111)}5\times2\textrm{-Au}$ surface, Si adatom diffusion is defect-mediated

The $\mathrm{Si(111)}5\times2\textrm{-Au}$ surface is a member of a family of metal-induced chain reconstructions of Si. Studies of these reconstructions have led to new understanding of the physics of one-dimensional electronic states. The $5\times2\textrm{-Au}$ surface is speckled with Si adatoms, which are intimately linked with the surface electronic properties. At temperatures $>423\textrm{ K}$, the adatoms diffuse along the chains between adjacent $5\times2$ cells. We have measured scanning tunneling microscopy movies of the diffusing adatoms. Distinctive diffusion statistics, e. g. correlations between displacements, imply that the displacements are triggered by an interaction with a defect. By a statistical characterization of the diffusion, we show that the adatoms move by a defect-mediated mechanism similar to the vacancy-mediated diffusion observed on some metal surfaces. We use a Monte Carlo simulation to model the diffusion process, accurately reproducing the unique diffusion statistics over the temperature range ($145-215^o$C) of our experiments. We have also determined the diffusion activation barrier=$1.24\pm0.08$ eV. Sandia National Labs is operated by Sandia Corp, a Lockheed-Martin Company, for the DOE under Contract No. DE-AC04-94AL85000. FJH and SB acknowledge NSF support under DMR-0705145.   

The diffusion pathways of phosphorus atoms in the silicon (001) surface

Effective use of surface chemical reactions to control the placement of dopants in semiconductors requires a detailed understanding of the reaction pathways involved. The most highly developed approach to accurate placement of phosphorus in silicon involves reacting phosphine gas with selected areas of the silicon (001) surface, incorporating the phosphorus atom into the surface and silicon overgrowth. In this computational study we investigate the least understood chemical reactions involved in this process, namely the phosphorus incorporation and associated diffusion reactions. We use density functional theory, combined with an efficient method for locating transition states, to identify the reaction pathways involved in three processes: (1) the diffusion of a phosphorus adatom along the silicon (001) surface, (2) the incorporation of a phosphorus atom into the surface, and (3) the migration of the incorporated phosphorus atom within the surface. The calculated pathways and corresponding reaction barriers provide insight into the conditions required for accurate incorporation and encapsulation of phosphorus atoms into the silicon surface.   

Kinetic Monte Carlo studies of the behavior of CO on sulfur-covered Pd(100) surface

Investigations of the behavior of CO on the surface of Pd modified with sulfur were carried out using first-principles Kinetic Monte Carlo method. In particular, the influence of adsorbed sulfur on the adsorption, diffusion and desorption of CO on the Pd(100) surface was studied. A kinetic Monte Carlo code was developed which enables the simulation of hosts of competing elementary steps with lateral interaction between the adspecies taken into account. The barriers and energetics of the relevant elementary processes were determined by density-functional theory. The rates entering the simulation were derived using transition state theory. The adsorbates were assumed to interact via pairwise additive interactions. We demonstrate that adsorbed sulfur has an adverse effect on the behavior of CO on Pd(100).   

Charge-State Dependent Hydrogen Diffusion on Silicon (001)

The diffusion of hydrogen atoms is relevant to a number of chemical and technological processes of the silicon (001) surface. These include the dissociative adsorption of molecules, the growth of overlayers by chemical vapor deposition (CVD), and the directed atomic-scale functionalization of the surface by scanning tunnelling microscopy (STM) lithography. The basic inter- and intradimer shift reactions of hydrogen are well studied, and activation energies of respectively 1.7 eV and between 1.0 and 1.4 eV are commonly cited. In this presentation, I will pose and discuss two questions: (1) Are single energy barriers adequate to describe H-shift reactions on silicon, and (2) are STM measurements of H-diffusion truly representative for hydrogen desorption in the absence an STM tip? These questions warrant examination because hydrogen adatoms on Si(001) are known to adopt a variety of charge states depending on factors such as the doping level of the silicon substrate, the defect density on the surface, or the presence of an STM tip. High-level density functional calculations are reported to shed some clarity on these questions.   

Unidirectional Linear Diffusion on an Isotropic Cu(111) Surface in a Periodic and Asymmetric Potential

We performed an STM study of the diffusion of 1,4-benzenedithiol, 9-thioanthracene, 9,10-dithioanthracene (DTA) and 2,3-dimethyl-9,10-dithioacethylanthracene (DMDTA) as well as napthaquinone, anthraquinone and pentacenetetrone on Cu(111). Inherently uniaxial motion of all species with two thiol groups and at least three aromatic rings are observed. Sequential placement of the substrate linkers prevents DTA and DMDTA from rotating or veering off course. Asymmetric methylation impacts DTA's diffusive behavior by about 100-fold decrease in surface mobility caused by a about 2-fold increase of the diffusion barrier, with the overall symmetry of DTA diffusion not affected: A forward/backward ratio of 1.009$\pm $0.01, were found i.e. less than 1{\%} deviation from unity. This is in stark contrast to the classical behavior but in perfect agreement with Tolman's ``Principle of Microscopic Reversibility?. Density functional theory (DFT) calculations are performed and results are discussed   

Surface Morphological Response of Stressed Elastic Solids under Electromigration Conditions

We present a theoretical analysis of the surface morphological response of electrically conducting, stressed elastic crystalline solids under the simultaneous action of an electric field that drives surface electromigration. The analysis is based on a fully nonlinear model of driven surface morphological evolution and combines linear stability theory with self-consistent dynamical numerical simulations. We report results of the surface morphological response of a uniaxially stressed solid as a function of electric field strength, surface crystallographic orientation, and temperature. We find that a properly directed and sufficiently strong electric field can stabilize the surface morphology of the stressed solid against crack-like surface instabilities, as well as surface rippling instabilities, and determine the required critical electric-field strength over a broad temperature range and for various surface crystallographic orientations. We also demonstrate the superior morphological response of $<$111$>$-oriented surfaces of face-centered cubic metals.   

Theory of the lifetime of adsorbate vibrations on semiconductor surfaces

On semiconductor surfaces, the vibrational lifetime of covalently bonded adsorbates is rather long (nanoseconds or more) since the band gap precludes electronic dissipation. Due to the quantum nature of vibrational states, such slow relaxation is expected, as the large quantum of the bond stretching must be converted into several smaller (phononic or vibronic) quanta by a high-order process. We use density functional theory calculations to map out the high-dimensional potential energy surface governing the anharmonic coupling of the stretching to the local bending and shift modes, while the coupling to the substrate phonons is treated perturbatively. Applying our method to the vibrational lifetime of CO on Si(100), we find that the CO stretching relaxes predominantly via an intermediate state consisting of four shift and/or bending quanta and one phonon. Good agreement with the measured lifetime of 2.3 ns at $T=100$~K is achieved. As a second application, we elucidate the role of intermediate vibrational states in the relaxation of the stretching vibration at Ge(100):H surfaces. For Ge surface dimers saturated by one H and one D atom, the lifetime of the Ge--H stretching vibration is up to five times shorter and less temperature-dependent than in Ge dimers homogeneously saturated by H. Our analysis shows that the symmetry breaking associated with the isotope mixture opens up additional relaxation channels.   

Modeling H$_2$-Surface Interactions on Interstellar Dust Grains: A Classical Molecular Dynamics Study

A classical Molecular Dynamics (MD) method is employed to model hydrogen molecules interacting with the surface of amorphous ice and to calculate the sticking coefficient as a function of various system parameters. This study, combining molecular physics and surface science, is part of a larger program of research to provide theoretical input for models of dust-grain-mediated physico-chemical processes in the interstellar medium. Many dust-grain species are thought to be clad in amorphous ice, which motivates the choice of substrate for this study. Our method simulates the various dynamical processes associated with H$_2$-ice scattering events, including collision, sticking, diffusion, and ejection. Variables such as angle of incidence, molecular rotational state, substrate temperature ($T_D$), and H$_2$ temperature ($T_{\rm H_2}$) are monitored and allowed to vary. In this talk, we will present our results for the H$_2$-ice sticking coefficient as a function of $T_D$ and $T_{\rm H_2}$.