Session V47: Dynamics of Surfaces, Adlayers, and Films

Area Determination of Electrodeposited Ni, Co, and NiCo Thin Films

The surface area of electrodeposited thin films of Ni, Co, and NiCo was evaluated using electrochemical double-layer capacitance, electrochemical area measurements using the [Ru(NH$_3$)$_6$]$^{3+}$/[Ru(NH$_3$)$_6$]$^{2+}$ redox couple, and topographic atomic force microscopy (AFM) imaging. The methods were compared to each other for each composition separately and for all the samples regardless of composition. Double-layer capacitance measurements were found to be positively correlated to the roughness factors determined by AFM topography. Electrochemical area measurements were less correlated with measured roughness factors and applicable only to two of the three compositions studied. The results indicate that \textit{in situ} double-layer capacitance measurements are a practical, versatile technique for estimating the accessible surface area of a metal sample.   

Structure and Dynamics of the Au(111) Surface in an Electrochemical Enviroment

The Au(111) surface possesses a well-known herringbone surface reconstruction pattern. However, the character of the reconstruction's response to external variables is not completely understood due to the limited amount of kinetics and dynamics studies of the surface in different environments. Here, we present \textit{in-situ} x-ray scattering measurements from the Advanced Photon Source at Argonne National Laboratory of the Au(111) surface in a controllable electrochemical environment of weak NaF solution. Crystal truncation rod (CTR) measurements were taken to examine how the average surface properties and overall structure change with cell voltage. X-ray photon correlation spectroscopy (XPCS) measurements were also taken to examine how the dynamics of the surface change with voltage. The relation between applied potential, average kinetics, and microstate dynamics will be discussed.   

Atomic diffusion processes in heteroepitaxial metallic systems using SLKMC-II

We have examined the diffusion of small islands of Cu on Ag(111) surface using a self-learning kinetic Monte Carlo (SLKMC-II) [1] method with an improved pattern recognition scheme. Due to strain generated at the interface between metals with different bulk lattice constants, interesting single atom, multi-atom and concerted diffusion processes are automatically revealed in the simulations. Here we will report various processes for small islands in the case of Cu/Ag(111) system. Key processes responsible for island diffusion and their energetics together with trends in effective energy barriers as well as diffusion constants for small islands will also be provided. In addition to 2-D diffusion processes, as an application of SLKMC-II to the 3-dimensional heteroepitaxial systems, we will also report energy barriers of some of the 3-dimensional processes including down the A- and B-steps and exchange processes. [1] Syed Islamuddin Shah, et al., J. Phys.: Condens. Matter 24, 354004 (2012)   

Nanotribological properties of water films adsorbing atop, and absorbing below, graphene layers supported by metal substrates

The tribological properties of graphite, a common lubricant with known sensitivity to the presence of water, have been studied extensively at the macroscopic and microscopic scales. Although far less attention has been devoted to the tribological properties of graphene, it has been established that the tribological response to the presence of water is dissimilar from that of graphite.[1] We report here a quartz crystal microbalance study of the nanotribological properties of water films adsorbed/absorbed on graphene layers prepared by either chemical decomposition on nickel(111) substrates or transfer of freestanding graphene layers to aluminum substrates. Sliding friction levels of the water films were also measured for metal surfaces in the absence of a graphene layer. We observe very high friction levels for water adsorbed atop graphene on Ni(111) and very low levels for water on aluminum. For the case of graphene/aluminum, the data indicate that the water is absorbing between the graphene layer and the aluminum. Dissipation levels moreover indicate the presence of an interstitial water increases sliding friction between the graphene and the aluminum substrate. [1] D. Berman et al., Materials Today 17, 31 (2014)   

Interfacial Assembly of Graphene Oxide Films.

Controlled assembly of monolayer graphene-oxide (GO) films at the air/water interface is of interest for the development of transparent conductive thin films of chemically-derived graphene. We present experimental results from investigations of the assembly of polydisperse GO sheets at the air-water interface. GO nanosheets with lateral dimensions of greater than 10 microns were created using a modified Tour synthesis (Dimiev and Tour, 2014). GO films were generated with conventional Langmuir trough techniques to control lateral packing density. Film morphology was characterized \textit{in situ} with Brewster angle microscopy. Films were transferred unto a substrate via the Langmuir-Blodgett deposition technique and imaged with fluorescence quenching microscopy. Through pH modulation of the aqueous subphase, it was found that GO's intrinsic surface activity to the interface increased with increasing subphase acidity. Finally, we found a dominant elastic contribution during uniaxial film deformation as measured by anisotropic pressure measurements. A. M. Dimiev, and J. M. Tour, ``Mechanism of GO Formation,'' ACS Nano, \textbf{8}, (2014)   

Tuning friction with an external magnetic field: A Quartz Crystal Microbalance study of physisorbed oxygen monolayers and multilayers sliding on nickel substrates.

The sliding friction levels of oxygen monolayer and multilayer films adsorbed on nickel close to the oxygen monolayer solid-liquid melting transition temperature have been monitored by means of a Quartz Crystal Microbalance (QCM) technique in the presence and absence of a weak external magnetic field. Friction levels for the monolayers in the presence of the field were observed to be half of those observed in the absence of a field. For thick films, the reduction was proportionately less, indicating an interfacial effect as the source of the magnetic sensitivity. While the presence of the field is expected to increase the normal force between the paramagnetic oxygen overlayer and the ferromagnetic substrate, the impact of this mechanism on friction appears to be minimal, or possibly masked by more dominant mechanisms. These include magnetically induced structural reorientation (magnetostriction), and/or realignment of adlayer spins in response to the applied field, both of which would reduce the physical or magnetic interfacial commensurability, thus lowering friction levels.   

Effect of charges on the interaction of water with hematite

Hematite is one of the many types of iron oxide that is easily found in nature. It is most commonly used in catalysis and it is rarely present in its pristine form. The influence of charged defects in its properties is very important for the correct geometrical/electronic characterization in more realistic operative conditions, but very few studies focus explicitly on these defects in this system. In this work we perform first principles DFT+U calculations to determine the properties of a hematite slab when both dopant and electrons/holes are added. We focus on the differences between the geometrical/electronic properties between the neutral/charged surfaces and also study their interaction with water (molecule and liquid) by performing molecular dynamics simulations at room temperature. Our results indicate that electric charges strongly influence the properties of these surfaces, changing the binding energies and the molecular arrangement of the water molecules adsorbed on hematite. Negative charges induce a larger binding and favor the partial water dissociation, whereas positive charges weaken the binding energy. We will provide comparative results for different configurations of this system.   

Novel \textbf{Transrotational }Solid State Order Discovered by TEM in Crystallizing Amorphous Films

Exotic thin crystals with unexpected \textbf{transrotational }microstructures [1] have been discovered by transmission electron microscopy (TEM) for crystal growth in thin (10-100 nm) amorphous films of different chemical nature (oxides, chalcogenides, metals and alloys) prepared by various methods. Primarily we use our TEM bend contour technique. The unusual phenomenon can be traced \textbf{\textit{in situ}} in TEM column: dislocation independent regular internal bending of crystal lattice planes in a growing crystal. Such \textbf{transrotation} (unit cell \textbf{trans}lation is complicated by small \textbf{rotation }realized round an axis lying in the film plane) can result in strong regular lattice orientation gradients (up to 300 degrees per micrometer) of different geometries: cylindrical, ellipsoidal, toroidal, saddle, etc. Transrotation is increasing as the film gets thinner. Transrotational crystal resembles ideal single crystal enclosed in a curved space. Transrotational micro crystals have been eventually recognized by other authors in some vital thin film materials, i.e. PCMs for memory, silicides, SrTiO3. Atomic model and possible mechanisms of the phenomenon are discussed. New transrotational nanocrystalline model of amorphous state is also proposed. [1] V.Yu. Kolosov and A.R.Tholen, Acta Mater., 48 (2000) 1829.   

Successively Thresholded Domain Boundary Roughening Driven by Pinning Centers and Missing Bonds: Hard-Spin Mean-Field Theory Applied to d$=$3 Ising Magnets

Hard-spin mean-field theory has recently been applied to Ising magnets, correctly yielding the absence and presence of an interface roughening transition respectively in $d=2$ and $d=3$ dimensions and producing the ordering-roughening phase diagram for isotropic and anisotropic systems. The approach has now been extended to the effects of quenched random pinning centers and missing bonds on the interface of isotropic and anisotropic Ising models in $d=3$ [1]. We find that these frozen impurities cause domain boundary roughening that exhibits consecutive thresholding transitions as a function of interaction anisotropy. For both missing-bond and pinning-center impurities, for moderately large values of the anisotropy, the systems saturate to the "solid-on-solid" limit, exhibiting a single universal curve for the domain boundary width as a function of impurity concentration.\\[4pt] [1] T. Caglar, A.N. Berker, arXiv:1509.01910 [cond-mat.stat-mech]   

Atomic-Scale Mechanism for Hydrogenation of $o$-Cresol on Pt Catalysis

Biofuels derived from lignocellulosic biomass have received significant attention lately due to increasing environmental concerns. With first-principles density-functional theory and \textit{ab initio} molecular dynamic simulations, we investigated the atomic-scale mechanism of $o$-cresol hydrogenation on the Pt(111) surface. The formation of 2-methyl-cyclohexanone (the intermediate product) was found to involve two steps. The first step is the dehydrogenation, that is, the H atom in the hydroxyl group moves to the Pt surface. The second step is the hydrogenation, that is, the H atoms on Pt react with the carbon atoms in the aromatic ring. The first step involves a smaller barrier, suggesting that dehydrogenation occurs first, followed by hydrogenation of the ring. In particular, tautomerization is found to occur via a two-step process over the catalyst. On the other hand, 2-methyl-cyclohexanol (the final product) is produced through two paths. One is direct hydrogenation of the aromatic ring. Another pathway includes partial hydrogenation of the ring, dehydrogenation of --OH group, finally hydrogenation of remaining C atoms and the O atom. Our theoretical results agree well with the experimental observations.   

Multidisciplinary Investigation of the Structural and Electronic Properties of the Pt/$\gamma $-Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ interface

Pt/$\gamma $-Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ is arguably the most important heterogeneous catalyst system. Despite the numerous studies on this system, the detailed structural and electronic properties of this interface remain uncertain. Through controlled oxidation of NiAl (110), we were able to obtain single crystalline Pt/$\gamma $-Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ thin films. We also prepared Pt/$\gamma $-Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ samples. STEM observations show that use of cryo-electron microscopy techniques prevented electron-beam damage, including direct sample damage, and changes in the EELS oxygen K pre-peak (\textasciitilde 532 eV). The oxygen K pre-peak was consistently present in spectra taken at the Pt/$\gamma $-Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ interface at cryo-temperatures, but not in those acquired at room temperature. The theoretically calculated EELS oxygen K signals for the Pt on (110) Pt/$\gamma $-Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ exhibited a similar pre-peak at 532 eV correlating to the experimental EELS oxygen K data, and we attribute to this feature to the formation of Pt-O complex. This points out an important factor in understanding the reactivity of this catalysis.   

First Principles Study of a-U2N3 Surfaces and its Anti-oxidant Mechanism

With the advantages over oxides as fuel materials for fast nuclear reactors, actinide nitrides have been extensively studied in experiments. In particular, a-U2N3 is also the main composition of surface layer obtained by surface nitriding for the enhancement of oxidation resistance of uranium in ambient conditions.[1-4] However, the anti-oxidant mechanism behind is still unclear, which hinders the further development of surface treatment technologies for uranium. Here we perform extensive ab initio studies on the geometric, magnetic and electronic properties of a-U2N3 bulk and (001) surfaces. Then the adsorption and diffusion of O2 near the stable a-U2N3(001) surface will be discussed, focusing on the local atomic arrangements of U-N and U-N-O that cannot be observed easily in experiments. Our theoretical results may give some insights in understanding the anti-oxidant mechanism of surface nitriding.   

Laser driven solid-state diffusional mixing in a Ni-Pt multilayer film probed by hard x-ray diffraction

Intense optical excitation has been utilized for decades to modify atomic scale structure in the condensed phase. When the optically excited systems are probed by hard x-ray radiation, one can reconstruct the modified atomic structure on a sub-angstrom spatial scale. In this work we utilize sub-picosecond optical radiation to rapidly drive atomic diffusion in a Ni-Pt multilayer film. Transient atomic diffusion was measured using hard x-ray diffraction, thereby directly observing the formation of a new metallic alloy as a function of laser excitation. Our observations demonstrate that the diffusional mixing in the multilayer can be completed in only a few individual laser shots, allowing us to directly probe the dynamics of the atomic scale motion.   

NMR Studies of the Dynamics of HD Adsorbed on MCM-41

We report the results of measurements of the nuclear spin-spin and spin-lattice relaxation times of a monolayer of HD molecules adsorbed on MCM-41 for temperatures $1.5 < T < 20$ K. Two distinct characteristic relaxation times are observed. A slow diffusion process for $5 < T < 8.8$ K and a faster rate with a distinctly different activation energy for $8.9 < T < 12$ K. The behavior is fluid-like above 12 K. We discuss the results in terms of an expected cluster formation at low temperatures followed by the diffusion of single molecules at high temperatures.   

Noble gas adsorption in two-dimensional zeolites: a combined experimental and density functional theory study

Zeolites are important industrial catalysts with porous three-dimensional structures. The catalytically active sites are located inside the pores, thus rendering them inaccessible for surface science measurements. We synthesized a two-dimensional (2D) zeolite model system, consisting of an (alumino)silicate bilayer weakly bound to a Ru (0001) surface. The 2D zeolite is suitable for surface science studies; it allows a detailed characterization of the atomic structure of the active site and interrogation of the model system during the catalytic reaction. As an initial step, we use Ar adsorption to obtain a better understanding of the atomic structure of the 2D zeolite. In addition, atomic level studies of rare gas adsorption and separation by zeolite are important for its potential application in nuclear waste sequestration. Experimental studies found that Ar atoms can be trapped inside the 2D-zeolite, raising an interesting question on whether Ar atoms are trapped inside the hexagonal prism nano-cages or at the interface between the (alumino)silicate bilayer and Ru(0001), or both. DFT calculations using van der Waals density functionals were carried out to determine the preferred Ar adsorption sites and the corresponding adsorption energies.