Session U20: Focus Session: Metal Surfaces, Interfaces, and Thin Films

Surface islands nucleated by a beam of energetic self-ions on Pt(111): A low-energy electron microscopy study

Using low energy electron microscopy (LEEM), we observe the adatom and advacancy islands nucleate and evolve when clean Pt(111), in the temperature range 750-1300K, is bombarded by a beam of Pt$^{-}$ ions of various energies. The source of negative ion beam is incident on the sample at normal incidence with impact energies selectable in the range of 0 to 5 keV, and with current densities up to 40 $\mu $A/cm$^{2}$. We describe briefly initial experiments done with LEEM-Ion accelerator tandem namely the investigations of relaxing steps extending the range over which surface mass diffusion coefficient D$_{s}$ is known on Pt(111) and observed neutral energy of 245 eV, at which sputtering balances the self-ion input. The results reveal a previously unobserved symmetry between the chemical potentials $\mu $* required to nucleate the adatom and advacancy islands. Linear response theory is employed to relate $\mu $* to ion beam flux; the observations confirm that its use is valid above 1000K.   

The effect of Fe atoms on the adsorption of a W atom on W surfaces

We report ab-initio calculations on the effect of iron (Fe) atoms on the adsorption of a tungsten (W) atom on W(100), W(110), and W(111) surfaces. The adsorption of a W atom on the clean W surfaces is compared with the adsorption of a W atom on a monolayer of Fe atoms covering the W surfaces. The total energy of the system is computed as the function of the height of the W adatom. For the W(100) surface I will show that the W atom first adsorbs onto the Fe monolayer. Then the W atom can replace one of the Fe atoms through a path with a moderate energy barrier and reduce its energy further. This intermediate site makes the adsorption (and desorption) of W atoms a two-step process in the presence of Fe atoms and lowers the overall adsorption energy by nearly 2.4 eV. Similar processes for adsorption will be presented for the (110) and (111) surfaces. Our result provides a fundamental mechanism that can explain the activated sintering of tungsten by Fe atoms.   

Stability of V and Ti on Al surfaces: Searching for suitable interlayer materials to stabilize the Fe-Al interface

There is considerable interest in fabricating thin film multilayer structures with sharp interfaces for a wide variety of applications. Interface intermixing may degrade the desired physical properties of a structure, but this may be reduced in some cases using stabilizing interlayers at the interface. Model calculations predict that both V and Ti will be effective stabilizing interlayers for the Fe-Al interface, a system well known for considerable intermixing at room temperature. We have used X-ray reflectometry (XRR) and Rutherford backscattering spectrometry (RBS) to characterize bilayers and trilayers of the Fe-V-Al and Fe-Ti-Al systems prepared using dc magnetron sputtering. Our analysis revealed that Fe-Al bilayer systems showed considerable intermixing, especially when the Fe layer was deposited on top of the Al. It was also found that with V or Ti as an interlayer at the interface, the intermixing of Fe and Al was reduced.   

Magic planar Ag clusters

The spontaneous assembly of atoms and molecules in a system has attracted many research interests and created numerous potential applications. Utilizing the periodic pattern found on the Pb quantum islands, which are grown on the Si(111) surface, we have recently discovered that self-organized Ag planar clusters formed on these templates exhibit enhanced stability at some particular sizes. Detailed calculations based on ab initio density functional theory have also been performed. The use of a density-functional optimization in geometrical structures and the corresponding binding energy support an examination of the genesis of these magic Ag nanoclusters and their relative stability. When the Ag nanopuck grows to a certain size, the geometrical effect takes hold from the electronic effect as the major attribute, which drive the Ag nanopucks towards well defined hexagonal crystalline structures. The theoretically related electronic and geometrical structures are also correlated with the experimentally energetically favorable structures of these magic clusters.   

Crucial electronic contributions to measures of surface diffusion by He atom scattering

In a He atom scattering (HAS) experiment, the position and motion of atoms or molecules at a surface is inferred indirectly, through the electron distribution at the sample surface. Nevertheless, surface diffusion measurements are typically analyzed assuming that the electron distribution simply follows the position of the surface atoms. We have examined theoretically recent HAS measurements of Na/Cu(001), identifying a non trivial relation between the dynamics of the electron distribution and that of the Na ions. The magnitude of the calculated variations in the charge density, and their dependence on the local density of adsorbates, account for the correlated 3D motion experimentally observed. The results of this study further highlight the sensitivity of HAS to the electron distribution of the sample and point out the role of electronic contributions in high-resolution measures of surface dynamics.   

Sapphire Surface Polymorphs and The Growth of Pb Overlayers

The surface structure of sapphire ($\alpha $-alumina) is an issue of long-standing interest, both scientifically and technologically. Molecular dynamics simulations showed the $\gamma $-alumina structure to have a lower energy than that of $\alpha $-alumina [1], thus suggesting a possibly modified surface structure when sapphire is treated at high temperatures. We have performed x-ray reflectivity measurements at the Advanced Photon Source to address this issue. Standard sapphire substrates were prepared by furnace annealing at 1600$^{\circ}$ C in air. The resulting surfaces showed large terraces with straight step edges. The substrates were then annealed in a UHV chamber at increasingly higher temperatures. Many new features emerged in the reflectivity curves, which could be attributed to various transition alumina structures, including the $\theta $'-, $\delta $-, and $\theta $- polymorphs [2]. Pb films were grown on these surfaces. The resulting structure and morphology was characterized. This talk will summarize our findings. [1] S. Blonski and S. H. Garofalini, Surf. Sci. \textbf{295}, 263 (1993). [2] I. Levin and D. Brandon, J. Am. Ceram. Soc. \textbf{81}, 1995 (1998).   

Molecular dynamics simulations of low temperature Cu/Cu(100) growth

Recent X-ray scattering studies [1] of Cu/Cu(100) growth indicate the existence of a sharp transition from epitaxial growth at high temperatures to growth with a high vacancy concentration at lower temperatures ($T < 150$ K). Here we present the results of molecular dynamics (MD) simulations carried out in order to understand the detailed mechanisms of incorporation of vacancies and voids (vacancy clusters) into the growing film during low temperature deposition. The dependence of compressive strain, vacancy concentration and vacancy-cluster size distribution in deposited thin film, as well as the surface roughness and morphology on deposition conditions, such as the growth temperature, deposition angle, and incident kinetic energy will be discussed and compared with experiments. [1] C. E. Botez, K. Li, E. D. Lu, W. C. Elliott, P. F. Miceli, E. H. Conrad, and P.W. Stephens, Appl. Phys. Lett. 81, 4718 (2002).   

Parallel temperature accelerated dynamics simulations of vacancy formation in low temperature Cu/Cu(100) growth

While molecular dynamics simulations may be used to study thin-film growth at very low temperatures and at very high deposition rates, in order to study growth over time scales close to experiment, accelerated dynamics simulations are needed. Here we present the results of parallel temperature-accelerated dynamics simulations of low-temperature Cu/Cu(100) growth carried out using our recently developed parallel temperature-accelerated dynamics (parTAD) method, in order to understand recent X-ray diffraction experiments showing a surprisingly large vacancy concentration in Cu(100) growth at low temperature. In general, we find that, due to the existence of increased surface relaxation and activated events, the vacancy concentration obtained in our parTAD simulations is smaller than the corresponding concentration obtained in molecular dynamics (MD) simulations. The dependence of compressive strain and vacancy concentration, as well as the surface roughness and morphology, on growth temperature and deposition angle will also be discussed and compared with experiments.   

Atomic Processes responsible for the diffusion of 2D Cu islands on Ag(111): results from self learning KMC

Diffusion on Ag(111) of small 2D-Cu clusters (4 to 30 atoms) is examined using embedded atom method potentials, as a first step towards understanding hetero epitaxial growth. A combination of an Off-Lattice Self-Learning Kinetic Monte Carlo and spherical repulsion scheme for saddle point searches, has revealed novel diffusion mechanisms. For this size range, the diffusion of islands involve 3 classes of processes: i) collective concerted motion (gliding), found to be dominant for small sized islands (4-9); ii) processes involving shear mechanism in which some of the Cu island atoms are commensurate with the substrate and others are not ($>$ 9 atoms), finally iii) processes involving a ``breathing'' mechanism in which the island ``shrinks'' as a whole before ``relaxing'' to a less compact shape with a net displacement of the whole cluster equivalent to an fcc-hcp hop ($>$ 12 atoms). These processes were revealed during the first 100 KMC steps for each island. Diffusion coefficients as a function of temperatures, effective diffusion barriers and frequencies of the responsible events will be presented.   

Semi-coherent Fe(001)/MX(001) interfaces

Using ab initio calculations we investigate interface energies and structures for the semi-coherent Fe(001)/MX(001) interface systems. We apply a continuum approach using the Peierls-Nabarro model in order to account for the elastic displacements arising from the lattice misfit and the periodic misfit dislocations in the interface. The chemical part of the interface energy is obtained by using density functional theory calculations. We find that the Fe/MN systems show decreasing trends along the 3d, 4d and 5d element rows corresponding to stronger bonding to Fe. In contrast the Fe/MC systems show a maximum for the Ti group. The trends and differences have been explored using projected density of states and charge density analysis. The results show a stong covalent bonding between Fe and C(N) when the two atoms are aligned on top of each other, but the hybridization itself can't account for the differences along the rows. Instead the trend appears to be due to the metallic interaction between the Fe atom and M atom in the interface layer. We also find that the Fe/MX-interfaces have large misfits, which causes that many atoms will be unfavorable positioned and the elastic energy will constitute a significant part of the interface energy.   

Unstable and metastable states of dynamics governed by surface diffusion

Under certain kinetic conditions, the dynamics of solid surfaces is governed by surface diffusion processes. This type of dynamics is relevant, for example, in high-temperature sintering processes, and in the coarsening of nanoporous metals coated by catalytic elements. For compact surfaces, the fixed points of this dynamics are surfaces of constant mean curvature (CMC). It is thus natural to ask whether there exist nontrivial CMC's which are stable under dynamics governed by surface diffusion. This question will be addressed in this talk. We will discuss some subtleties concerning an analytic approach to the problem, and will present some numerical results for simple CMC surfaces.   

Morphology evolution of solid thin films in the presence of long range de-wetting interactions

The thin films of metals, such as cobalt or silver on substrates such as sapphire exhibit a striking formation of multilayered islands that reach heights many times larger than the initial film thickness. Here, we theoretically elucidate these phenomena within an interface dynamics model which incorporates both Mullins type surface diffusion relaxation and long range de-wetting forces acting across the film, such as Van der Waals forces and Fermionic Casimir forces (Quantum size effect). The model is used to explore the scaling laws of multilayered island height growth as well as the coarsening laws governing surface evolution. At early times, the surface evolution is dominated by strong up-hill surface currents caused by long range Casimir-like forces. At late times however, the surface coarsening laws are universal and dominated by surface tension effects.   

Charge Transfer Model for Disscociative Barrier Formation; A First Principles Study

The origin of the formation of the barriers has been explained, for two decades, using Pauli repulsion, i.e., exchange term, by Hammer et al.[1] Excluding the exchange term in the electronic total energy calculation, we have however obtained the monotonic increase of the potential energy surface in the dissociation process of the H$_{2}$ molecule on Au(111) system. So we propose another origin for the formation focusing on the charger transfer induced by electronegativity differences between the hydrogen molecule and Au metal surfaces using density functional calculations. We evaluate the charges that belong to atoms in the system during the dissociation process using Bader analysis. The calculated dissociation energy curve along the reaction path coincides with that of the isolated, separated and positively charged hydrogen molecule using the linear combination of the atomic orbital method in real space. No interaction between the hydrogen molecule and the Au surface has been found in the initial stage of the dissociation. The transfer elongates the inter-atomic distance of the hydrogen molecule that raises the energy of the molecule, leading to the formation of the energy barrier in the present case. This charge transfer model is confirmed to be applicable to not only the present system but also H$_{2}$/Mg, H$_{2}$/Pt, O$_{2}$/Pt systems. [1] B.Hammer and J.K.N{\o }rskov, Nature 376, 238 (1995).   

A Density Functional Study of Atomic Hydrogen and Oxygen Chemisorptions on the (0001) Surface of Double Hexagonal Close Packed Americium

\textit{Ab initio} total energy calculations within the framework of density functional theory have been performed for atomic hydrogen and oxygen chemisorptions on the (0001) surface of double hexagonal packed americium using a full-potential all-electron linearized augmented plane wave plus local orbitals (FLAPW+lo) method. The three-fold hollow hcp site was found to be the most stable site for H adsorption, while the two-fold bridge adsorption site was found to be the most stable site for O adsorption. Chemisorption energies and adsorption geometries for different adsorption sites will be discussed. The change in work functions, magnetic moments, partial charges inside muffin-tins, difference charge density distributions and density of states for the bare Am slab and the Am slab after adsorption of the adatom will be discussed. The implications of chemisorption on Am 5$f$ electron localization-delocalization will also be discussed.   

Simulations of adsorption of hydrocarbons on decagonal AlNiCo quasicrystal surfaces

Classical many-body interatomic potentials for hydrocarbon adsorptions on Al-Ni-Co systems are developed by using the Embedded-Atom Method. The potentials are fit to ab-initio energies of Al-Ni-Co ternary phases and hydrocarbons adsorbed on decagonal surface of Al-Ni-Co (d-AlNiCo) approximants. First principle data show that no dissociation occurs for all systems in the training set, indicating no chemisorptions. We extend the study and use the potentials to simulate adsorption of simple hydrocarbons on d-Al$_{73}$Ni$_{10}$Co$_{13}$ quasicrystal surfaces using Grand Canonical Monte Carlo method. Research sponsored by ACS and NSF.