Session P5: Surfaces, Interfaces, and Thin Films: Kinetics, Dynamics, and Reactions

Direct versus hydrogen assisted CO dissociation on metal surfaces

We present investigations of the formation of precursor hydrocarbon species relevant to production of liquid hydrocarbons on low index surfaces of various important noble and transition metals. The formation could occur via the so-called carbide mechanism where direct CO dissociation takes place, followed by stepwise hydrogenation of C yielding CH$_{x }$ species. Formation of precursor CH$_{x }$ species could also potentially take place through hydrogenated CO intermediates. First-principles calculations of energetics and barriers of CO conversion to hydrocarbons species were performed using plane-wave periodic density functional theory. Our calculations indicate that the two pathways are generally competitive on transition metals. A microkinetic model, with input thermodynamics and kinetic parameters estimated from electronic structure calculations, has been developed. The two pathways will be further examined using microkinetic approach to determine whether the aforementioned finding holds at realistic conditions.   

Multi-lattice approach to first-principles kinetic Monte Carlo simulations: Application to catalytic CO oxidation at Pd(100)

First-principles kinetic Monte Carlo (1p-kMC) simulations enable a quantitative microkinetic modeling of heterogeneous chemical reactions while accounting for the full spatial distributions at the surface. Application to reaction-induced surface morphological transitions is hitherto prevented by the inability to describe the system within prevalent fixed-lattice 1p-kMC and the excessive cost of off-lattice 1p-kMC variants. To this end we develop a novel multi-lattice 1p-kMC approach and apply it as a case in point to the CO oxidation at Pd(100). In the catalytically active state this system is suspected to undergo transitions from the pristine metal surface to a PdO surface-oxide film. As a first step towards a comprehensive simulation we focus on the initial oxide destruction step induced by clustering of oxygen vacancies. First simulations confirm the stability of the oxide film at stoichiometric feed as predicted by preceding fixed-lattice 1p-kMC simulations [1].\\[4pt] [1] J. Rogal, K. Reuter, and M. Scheffler, Phys. Rev. Lett. 98, 046101 (2007).   

The role of non-conventional supports for single-atom Platinum-based catalysts in fuel cell technology: A theoretical surface science approach

Platinum-based heterogeneous catalysts are known to play a key role in the fuel cell technology, such as their use in the low-temperature proton exchange membrane (PEM) fuel cells. However, the high cost and low lifecycle of these PEM fuel cells are the major hindrances to its large-scale commercial production. To elevate the high-cost and to optimize its catalytic activity, it was recently proposed that catalysts with single-Pt atom dispersions and a more durable, corrosion-resistant TiN support could play an important role in the next generation of Pt-based PEM fuel cells [1]. As a first step towards a microscopic understanding of single-Pt atom-dispersed catalysts on these new supports, we present density-functional theory (DFT) calculations to investigate the adsorption properties of Pt atoms on pristine TiN(001). Optimized atomic geometries, energetics, and analysis of the electronic structure of the Pt/TiN system are reported for various surface coverages of Pt. We find that atomic Pt does not bind preferably to the clean TiN surface, but under operational conditions, TiN surface vacancies play a crucial role in anchoring the Pt atom for its catalytic function. $[1]$ B. Qiao, $et al.$ Nat. Chem. 3 (2011) 634; B. Avasarala and P. Haldar, Int. J. Hydrog. Energy 36 (2011) 3965   

Water-Pd Interface in Catalytic Biomass Conversion: Atomic-Scale Structure and Properties

Biomass pyrolysis and other relevant catalytic reactions often occur at the liquid-solid interface. It is therefore of great importance to investigate the interfacial structure and other properties in order to achieve a deep understanding about the catalytic reactions for biomass conversion. We used \textit{ab initio }molecular dynamics simulations to study the interfaces formed by liquid water and the palladium surfaces. Such interfaces are involved in many catalytic reactions for biomass conversion. We report results about the structural properties of the water/Pd(100) and water/Pd(111) interfaces, the interaction between liquid water and the metal surfaces, and how the interaction affects the structure. We found that while the interaction between water and the metal surface is weak, it could still cause considerable effects. In particular, the interaction promotes the formation of close-packed local clusters of liquid water.   

Multilayer epitaxial graphene oxide: structural and chemical properties from a combined theoretical and experimental XPS study

Multilayer graphene oxide (GO) is a material holding great promise in future energy storage and nano-electronic technologies. This material remains qualitatively known to date. In this work, we present a combined density functional theory (DFT) and experimental X-ray photo-emission spectroscopy (XPS) study of the structural, chemical, and thermal stability of multilayer GO grown epitaxially on silicon carbide. This investigation shows that at room temperature multilayer GO is a metastable material. GO films undergo spontaneous modifications and chemical reduction with a relaxation time of about one month. These processes lead multilayer GO toward a longer-living quasi-equilibrium state, consisting a structure deprived of epoxide groups, rich of hydroxyl groups, and with a O/C ratio of 0.38. Our study suggests that the presence of excess H chemisorbed on the graphitic sheets is the origin of the metastable character of multilayer GO. These H species favor the reduction of epoxide groups and the consequent transformation of hydroxyls into water molecules intercalated between the graphitic layers. Our DFT calculations show that these molecular transformations are controlled by diffusion processes.   

Switching of molecules at metal surfaces

Understanding the switching ability of molecules on surfaces upon excitation with external stimuli is a prerequisite for the development of functional molecular devices with possible applications to information processing, storage, or switching. While the switching mechanisms in many classes of molecular switches are thoroughly studied and understood in solution, their counterparts on surfaces still remain largely unresolved. In particular, many switching processes are suppressed or irreversible when the molecules are anchored to a metallic substrate. The adsorption configuration and steric hindrance are only one factor influencing the switching capability. More important is the electronic coupling strength between adsorbate and substrate and accordingly the lifetime of molecular excited states which is significantly reduced at metal surfaces. I will discuss several examples of optically and thermally induced conformational changes in molecular switches at noble metal surfaces.   

Competing Atomic and Molecular Mechanisms of Thermal Oxidation of Si vesus SiC

Thermal oxidation is a universal process in solids and is of practical importance in semiconductor technology. The oxidation of Si and SiC provide a unique opportunity for studying the oxidation mechanism because the products are the same oxide SiO$_{2}$. The oxidation of Si follows a linear-parabolic law with molecular oxygen identified as the oxidant. The oxidation of SiC obeys the same linear-parabolic law as Si but with different rates and temperature dependences. Using results from first-principle calculations, we first show that an atomic oxygen mechanism can account for the oxidation of Si-face SiC. We then discuss implications of the results and identify the determining factors in the competition between atomic and molecular mechanism. This work is supported by NSF GOALI grant DMR-0907385.   

Scanning tunneling microscopy uncovers the mechanism of silicon oxidation in aqueous solutions

Because of their immense technological importance, silicon oxidation reactions have been studied intensely for decades under a variety of conditions. However, the disordered nature of the reaction product, silicon oxide, makes these reactions notoriously difficult to understand. In this work, silicon oxidation is coupled with a subsequent etching reaction, allowing the oxidation reactions to literally write an atomic-scale record of their reactivity into the etched surface -- a record that can be decoded into site-specific reaction rates, and thus chemical understanding, with the aid of simulations and infrared spectroscopy. This record overturns the long-standing and much-applied mechanism for the (low-temperature) oxidation of the technologically important face of silicon, Si(100), and shows that the unusually high reactivity of a previously unrecognized surface species leads to a self-propagating etching reaction that produces near-atomically-flat surfaces terminated by a single monolayer of hydrogen atoms. This finding shows that, contrary to expectation, the low-temperature oxidation of Si(100) is a highly site-specific reaction and suggests strategies for the uniform functionalization of the technologically relevant face of silicon by low-temperature reactions.   

Metal pulled-off effect: A unique explanation of different oxidation process on Cu and Al surfaces

One interesting oxidation phenomenon is the difference of the oxidation of Cu and Al. Cu forms disordered domains, large surface reconstructions and oxide islands on the surface with some O atoms diffuse into inner layers to further oxidize inner Cu atoms. Al forms a dense oxide layer which protects the inner Al atoms from oxidation. In this talk, we demonstrate a possible electronic origin of this oxidation difference by using the first-principles method to calculate the initial oxidation of different metal surfaces and nanoclusters. On Cu 55 Icosahedron surface, we found that 2 O atoms at neighboring sites form a structure with a Cu atom in the middle pulled off from the surface. We also found the similar pull-offs on Cu, Pd, Zn surfaces, but not on Al surface, which is not a transition metal. This pulled off effect is explained by the strong metal d and O p coupling. We also checked different O concentration on Cu (111) surface and on Cu cluster surface and found that O atoms form chain or ring like structures. Our first principle molecular dynamic calculation confirms that these structures are stable. With this pull-off effect, additional O atoms can further oxidize inner Cu atoms and make Cu relative easy to oxidize. This finding enhances the scientific understanding of the initial oxidation of metallic nano-particles and surfaces, which may have important applications in catalysis, thermal storage and other surface science fields.   

Electrical and Structural Properties of Thin Films Fabricated by E-Beam Lithography from Gold Nanoparticle Resists

Drop- and spin-coated solutions of ligand-coated nanoparticles act as novel ``direct write'' e-beam resist, which can be prepared with metallic, magnetic and semiconducting nanoparticles. We prepared thin films from gold nanoparticles, in which we varied the film thickness. Small angle X-Ray scattering experiments as well as SEM imaging of the samples were performed to determine structural properties of the nanoparticles films at various stages of the fabrication process, after drop coating, ebeam exposure and annealing. We further performed DC charge transport measurements in the 2-350K temperature range and report different conductivity mechanisms based on the film thickness, ranging from insulating to Mott hopping conduction to metallic.   

Self-Diffusion of small Ag and Ni islands on Ag(111) and Ni(111) using the self-learning kinetic Monte Carlo method

We have applied a modified Self-Learning Kinetic Monte Carlo (SLKMC) method [1] to examine the self-diffusion of small Ag and Ni islands, containing up to 10 atom, on the (111) surface of the respective metal. The pattern recognition scheme in this new SLKMC method allows occupancy of the fcc, hcp and top sites on the fcc(111) surface and employs them to identify the local neighborhood around a central atom. Molecular static calculations with semi empirical interatomic potential and reliable techniques for saddle point search revealed several new diffusion mechanisms that contribute to the diffusion of small islands. For comparison we have also evaluated the diffusion characteristics of Cu clusters on Cu(111) and compared results with previous findings [2]. Our results show a linear increase in effective energy barriers scaling almost as 0.043, 0.051 and 0.064 eV/atom for the Cu/Cu(111), Ag/Ag(111), and Ni/Ni(111) systems, respectively. For all three systems, diffusion of small islands proceeds mainly through concerted motion, although several multiple and single atom processes also contribute. [1] Oleg Trushin et al. Phys. Rev. B 72, 115401 (2005) [2] Altaf Karim et al. Phys. Rev. B 73, 165411 (2006)   

Collective Excitations in Ultrathin Magnesium Films on Silicon

We present a systematic study of plasmon excitation in ultrathin Mg overlayer on Si(111) substrate. Our numerical results qualitatively reproduce the experimentally observed plasmon spectra of the Mg/Si systems [1]. The underlying physics of the formation of various absorption peaks can be understood using the simple hybridization concept. Based on this concept, the coexistence of surface and bulk plasmons in the experimental observation turns out to be a clear evidence for the existence of multiple surface plasmons due to the quantum confinement in Mg thin films [2]. In addition, we clearly see the plasmon enhanced substrate absorption, which comes from the screening of the substrate to the oscillatory charges.\\[4pt] [1] Ao Teng et al.(to be published).\\[0pt] [2] Xiaoguang Li et al.(to be published).   

Scaling Properties of Exciton Binding Energies in Two-Dimensional Insulating Films

Using the GW and Bethe-Salpter Equation (BSE) methods with inclusion of many-electron effects, we carry out a systematic study of the quasiparticle energy and optical absorption spectra of various two-dimensional (2D) thin-film insulators, including boron nitride, graphane, fluorographene, fluorosilicene, etc. All the calculated band gaps of these systems are increased by the GW corrections, and their optical responses are dominated by the strongly bound excitons, which can be attributed to the enhanced Coulomb attraction in 2D. Most strikingly, we find a well-defined linear scaling dependence between the exciton binding energy and band gap, and this scaling relationship is in stark contrast with the established ones in 3D and 1D insulating systems. The likely underlying physical mechanism for the linear scaling relationship will also be discussed.   

Spatially and Temperature Resolved Photoluminescence (PL) Of Excitons in Highly Oriented Phthalocyanine Films

Phthalocyanines and their derivatives are interesting alternative to polymer materials for the development of electronic devices such as organic thin field effect transistors, organic Light Emitting Diodes and photovoltaic cells. The present study focuses on spatially resolved, temperature-dependent PL of highly-oriented metal free and Zn -Octa-butoxy phthalocyanine (OBPc) polycrystalline thin films. Samples were fabricated using an in-house solution processing method\footnote{R. L. Headrick et al, APL, 92, 063302 (2008)} that results in mm-sized grains which can be individually probed using a focused laser beam. The experiments indicate the lowest optically active excitonic state which dominates the PL spectrum at 5K is optically-forbidden at room temperature. Linear Dichroism microscopy experiments indicate a reorientation of molecular planes below T$\sim$200K which may favor a mixing of Frenkel and intermolecular excitons, changing the nature of excitonic ground state.   

Grain Boundary Exploration of Excitonic States in Organic Crystalline Thin Films

The electronic states of Metal-Phthalocyanine (MPc) crystalline thin films are investigated. These samples are fabricated by solution processed pen-writing deposition technique.\footnote{R. Headrick et al, APL, 92, 063302 (2008)} Specifically, a linear dichroism mapping is performed, and excitonic emission is studied both close to and far from the large grain boundaries found in these phthalocyanine thin films. Multiple M-Pc samples are examined, including nickel, zinc and cobalt. In phthalocyanine crystalline films, it is believed that a monomer-like emission feature exclusively associated with a grain boundary is observed. The presence of this feature and its intensity are correlated with the relative orientation of neighboring grains at the boundary. The experiments are performed using a combined Linear Dichroism/Photoluminescence Microscopy experiment developed at the University of Vermont.