We applied Scanning Probe Microscopy and Density Functional Theory (DFT) to discover the basics of how adsorbates wet insulating substrates, addressing a key question in geochemistry. To allow experiments on insulating samples we added Atomic Force Microscopy (AFM) capability to our existing UHV Scanning Tunneling Microscope (STM). This was accomplished by integrating and debugging a commercial qPlus AFM upgrade. Examining up-to-40-nm-thick water films grown in vacuum we found that the exact nature of the growth spirals forming around dislocations determines what structure of ice, cubic or hexagonal, is formed at low temperature. DFT revealed that wetting of mica is controlled by how exactly a water layer wraps around (hydrates) the K+ ions that protrude from the mica surface. DFT also sheds light on the experimentally observed extreme sensitivity of the mica surface to preparation conditions: K atoms can easily be rinsed off by water flowing past the mica surface.
Density Functional Theory points to a key role of K+ solvation in the low-energy two-dimensional arrangement of water molecules on the basal surface of muscovite. At a coverage of 9 water molecules per 2 surface potassium ions, there is room to accommodate the ions into wetting layers wherein half of them are hydrated by 3 and the other half by 4 water molecules, with no broken H-bonds, or wherein all are hydrated by 4. Relative to the “fully connected network of H-bonded water molecules” that Odelius et al. found to form “a cage around the potassium ions,” the hydrating arrangements are several tens of meV/H2O better bound. Thus, low-temperature wetting on muscovite is not driven towards “ice-like” hexagonal coordination. Instead, solvation forces dominate.
This report summarizes the work completed under the Laboratory Directed Research and Development (LDRD) project 10-0973 of the same title. Understanding the molecular origin of the no-slip boundary condition remains vitally important for understanding molecular transport in biological, environmental and energy-related processes, with broad technological implications. Moreover, the viscoelastic properties of fluids in nanoconfinement or near surfaces are not well-understood. We have critically reviewed progress in this area, evaluated key experimental and theoretical methods, and made unique and important discoveries addressing these and related scientific questions. Thematically, the discoveries include insight into the orientation of water molecules on metal surfaces, the premelting of ice, the nucleation of water and alcohol vapors between surface asperities and the lubricity of these molecules when confined inside nanopores, the influence of water nucleation on adhesion to salts and silicates, and the growth and superplasticity of NaCl nanowires.
The reported theoretical 'average binding of 0.20 eV per C atom, relative to a free graphene sheet and a clean metal slab' was an artifact of faulty evaluation of the energy of the free graphene sheet. Escaping our notice, the error occurred in the electron-density update algorithm, where two of six nearly degenerate eigenvectors were dropped [1]. With the error corrected, the computed binding energy of the graphene layer to Ir(111), is much smaller, just 0.18 eV per moire unit cell, or 0.9 meV per C atom. With a finer, 3 x 3 sample of the 10 x 10 graphene supercell's surface Brillouin zone, it increases to 2 meV/C atom. The cost of having to stretch the graphene sheet by {approx}0.3 linear percent to make it epitaxial on an underlying 9 x 9 Ir(111) supercell is incorporated in these values.
A molecular-scale interpretation of interfacial processes is often downplayed in the analysis of traditional water treatment methods. However, such an approach is critical for the development of enhanced performance in traditional desalination and water treatments. Water confined between surfaces, within channels, or in pores is ubiquitous in technology and nature. Its physical and chemical properties in such environments are unpredictably different from bulk water. As a result, advances in water desalination and purification methods may be accomplished through an improved analysis of water behavior in these challenging environments using state-of-the-art microscopy, spectroscopy, experimental, and computational methods.
We have used scanning tunneling microscopy and low-energy electron microscopy to measure the thermal decay of two-dimensional Cu, Pb-overlayer, and Pb-Cu alloy islands on Pb-Cu(1 1 1) surface alloys. Decay rates covering 6-7 orders of magnitude are accessible by applying the two techniques to the same system. We find that Cu adatom diffusion across the surface alloy is rate-limiting for the decay of both Pb and Pb-Cu islands on the surface alloy and that this rate decreases monotonically with increasing Pb concentration in the alloy. The decrease is attributed to repulsive interactions between Cu adatoms and embedded Pb atoms in the surface alloy. The measured temperature dependences of island decay rates are consistent with first-principles calculations of the Cu binding and diffusion energies related to this 'site-blocking' effect.
Drainage of water from the region between an advancing probe tip and a flat sample is reconsidered under the assumption that the tip and sample surfaces are both coated by a thin water 'interphase' (of width {approx}a few nm) whose viscosity is much higher than the bulk liquid's. A formula derived by solving the Navier-Stokes equations allows one to extract an interphase viscosity of {approx}59 KPa-sec (or {approx}6.6x10{sup 7} times the viscosity of bulk water at 25C) from Interfacial Force Microscope measurements with both tip and sample functionalized hydrophilic by OH-terminated tri(ethylene glycol) undecylthiol, self-assembled monolayers.
Pb deposition on Cu(111) causes the surface to self-assemble into periodically arranged domains of a Pb-rich phase and a Pb-poor phase. Using low-energy electron microscopy (LEEM) we provide evidence that the observed temperature-dependent periodicity of these self-assembled domain patterns is the result of changing domain-boundary free energy. We determine the free energy of boundaries at different temperatures from a capillary wave analysis of the thermal fluctuations of the boundaries and find that it varies from 22 meV/nm at 600 K to 8 meV/nm at 650 K. Combining this result with previous measurements of the surface stress difference between the two phases we find that the theory of surface-stress-induced domain formation can quantitatively account for the observed periodicities.
Drainage of water from the region between an advancing probe tip and a flat sample is reconsidered under the assumption that the tip and sample surfaces are both coated by a thin water "interphase" (of width approximately a few nanometers) whose viscosity is much higher than that of the bulk liquid. A formula derived by solving the Navier-Stokes equations allows one to extract an interphase viscosity of ∼59 kPa·s (or ∼6.6 × 10 7 times the viscosity of bulk water at 25°C) from interfacial force microscope measurements with both tip and sample functionalized hydrophilic by OH-terminated tri(ethylene glycol) undecylthiol, self-assambled monolayers.
We present a combined experimental and theoretical study of submonolayer growth in the presence of predeposited immobile impurities. Scanning tunneling microscopy measurements of Al/Al(1 1 1) epitaxy in the presence of oxygen adsorbates show that immobile O impurities influence all aspects of the early stages of homoepitaxial growth on Al(1 1 1). Possible scenarios for modified growth are investigated using kinetic Monte Carlo simulations. Dependences of island density on temperature, impurity concentration and strength and type of adatom-impurity interaction are compared. The comparison shows that the morphology of the growing Al film cannot result from only one interaction type: attractive or repulsive. An oscillatory interaction, suggested by ab initio calculations, is proposed to explain the behavior of the system.
Ab initio kink-formation energies are about 0.25 and 0.18 eV on the (100)- and (111)-microfacet steps of Pt(111), while the sum of the step-formation energies is approximately 0.75 eV/atom. These results imply a specific ratio of formation energies for the two step types, namely 1.14, in excellent agreement with experiment. If kink-formation costs the same energy on the two step types, an inference recently drawn from scanning probe observations of step wandering, this ratio ought to be unity.
The intrinsic chirality of metal surfaces with kinked steps (e.g. Pt(643)) endows them with enantiospecific adsorption properties (D. S. Shell, Langmuir, 14, 1998, 862). To understand these properties quantitatively the impact of thermally-driven step wandering must be assessed. The authors derive a lattice-gas model of step motion on Pt(111) surfaces using diffusion barriers from Density Functional Theory. This model is used to examine thermal fluctuations of straight and kinked steps.
Ab-initio formation energies for (100)- and (111)-microfacet steps on Pb(111) are in satisfactory agreement with measured values, given that these values are known only as well as the Pb(111) surface energy; the calculated step-energy ratio, 1.29, is within {approximately}8% of experiment. In contrast, calculated kink-formation energies, 41 and 60 meV for the two step types, are 40--50% below published experimental values derived from STM images. The discrepancy results from interpreting the images with a step-stiffness vs. kink-energy relation appropriate to (100) but not (111) surfaces. Good agreement is found when the step-stiffness data are reinterpreted, taking proper account of the trigonal symmetry of Pb(111).
S-decorated Cu trimers are investigated as likely agents of S-enhanced Cu transport between clusters on Cu(111). It is shown what Cu3S3 clusters form more readily on Cu(111) than a Cu adatom and what diffuse easily to determine how S acts. Using a systematic ab initio search, results show that the smallest of such cluster is ad-Cu3S3. approximately 0.5 ev formation energy, lower than that of a Cu adatom, and ≤0.35 eV diffusion barrier, corresponding to tight internal bonding, are obtained.
Notwithstanding half a dozen theoretical publications, well-converged density-functional calculations, whether based on a local or generalized-gradient exchange-correlation potential, whether all-electron or employing pseudopotentials underestimate CO's preference for low-coordination binding sites on Pt(111) and vicinals to it. For example, they imply that CO should prefer hollow- to atop-site adsorption on Pt(111), in apparent contradiction to a host of low temperature experimental studies.
Because of their strong internal bonding, S-decorated Cu trimers are a likely agent of S-enhanced Cu transport between islands on Cu(111). According to ab-initio calculations, excellent healing of dangling Cu valence results in an ad-Cu{sub 3}S{sub 3} formation energy of only {approximately}0.28 eV, compared to 0.79 eV for a self-adsorbed Cu atom, and a diffusion barrier {le}0.35 eV.
Wanting to convert surface impurities from a nuisance to a systematically applicable nano-fabrication tool, the authors have sought to understand how such impurities affect self-diffusion on transition-metal surfaces. Their field-ion microscope experiments reveal that in the presence of surface hydrogen, self-diffusion on Rh(100) is promoted, while on Pt(100), not only is it inhibited, but its mechanism changes. First-principles calculations aimed at learning how oxygen fosters perfect layerwise growth on a growing Pt(111) crystal contradict the idea in the literature that it does so by directly promoting transport over Pt island boundaries. The discovery that its real effect is to burn off adventitious adsorbed carbon monoxide demonstrates the predictive value of state-of-the-art calculation methods.
Bond-counting arguments, supported by ab-initio calculations, predict a lower barrier for "leapfrog" diffusion of Pt addimers on Pt(llO)-lx2 than for adatom dif- fusion or addimer dissociation. This conflicts with experiment, possibly signaling contaminant influence.
In rough agreement with experimental values derived from Cu island shapes vs. temperature, ab-initio calculations yield formation energies of 0.27 and 0.26 eV/ step-edge-atom for (100)- and (111)-micro facet steps on Cu(lll), and 0.09 and 0.12 eV per kink in those steps. Comparison to ab-initio results for Al and Pt shows that as a rule, the average formation energy of straight steps on a close-packed metal surface equals -7% of the metal's cohesive energy.
First-principles total energies of periodic vicinals are used to estimate barriers for Pt-adatom diffusion along straight and kinked steps on Pt(111), and around a corner where straight steps intersect. In all cases studied, hopping diffusion has a lower barrier than concerted substitution. In conflict with simulations of dendritic Pt island formation on Pt(111), hopping from a corner site to a step whose riser is a (111)-micro facet is predicted to be more facile than to one whose riser is a (100).
First-principles density functional calculations and corresponding experimental results underline the importance of basic chemical concepts, such as coordination, valence saturation and promotion-hybridization energetics, in understanding bonding and diffusion of atoms at and on metal surfaces. Several examples are reviewed, including outer-layer relaxations of clean hcp(0001) surfaces, liquid-metal-embrittlement energetics, separation energies of metal-adatom dimers, concerted substitutional self-diffusion on fcc(001) surfaces, and adsorption and diffusion barrier sites for adatoms near steps.
Be is a ``marginal metal.`` The stable phase, hcp-Be, has a low Fermi-level density of states and very anisotropic structural and elastic properties, similar to a semiconductor`s. At the Be(0001) surface, surface states drastically increase the Fermi-level density of states. The different nature of bonding in bulk-Be and at the Be(0001) surface explains the large outward relaxation. The presence of surface states causes large surface core-level shifts by inducing a higher electrostatic potential in the surface layers and by improving the screening at the surface. The authors experimental and theoretical investigations of atomic vibrations at the Be(0001) surface demonstrate clearly that Be screening of atomic motion by the surface states makes the surface phonon dispersion fundamentally different from that of the bulk. Properties of Be(0001) are so different from those of the bulk that the surface can be considered a new ``phase`` of beryllium with unique electronic and structural characteristics. For comparison they also study Be(11{bar 2}0), a very open surface without important surface states. Be(11{bar 2}0) is the only clean s-p metal surface known to reconstruct (1 {times} 3 missing row reconstruction).
This report summarizes research on the aging of Class I components in environments representative of nuclear power plants. It discusses Class IE equipment used in nuclear power plants, typical environments encountered by Class IE components, and aging techniques used to qualify this equipment. General discussions of radiation chemistry of polymers and accelerated aging techniques are also included. Based on the inadequacies of present aging techniques for Class IE equipment, a proposal for an experimental program on electrical cables is presented. One of the main purposes of the proposed work is to obtain relevant data in two areas of particular concern--the effect of radiation dose rate on polymer degradation, and the importance of synergism for combined thermal and radiation environments. A new model that allows combined environment accelerated aging to be carried out is introduced, and it is shown how the experimental data to be generated can be used to test this model.
