Session W15: Surfaces, Thin Films, and Nanostructures

Electronic Structures of Tungsten (3, 2, 0) and Tungsten (8, 7, 0) Surfaces

The valence-band electronic structures of clean Tungsten (3, 2, 0) surface and Tunsgten (8,7,0) surface are investigated by using angular-resolved photoemission techniques. The experiment was performed at Beamline 7.0.1, Advanced Light Source, LBNL. The surface states related spectral features are distinguished by using 4D-volume scan method, and are compared to the observed electronic structural features of Hydrogen covered Tungsten stepped surfaces. Using Nearly Free Electron Model, Tungsten bulk bands and stepped surface bands are analyzed in band shapes and band widths. Bands reshapings due to Hydrogen adsorption and surface atomic layers change transferring are designated.   

Surfactant Mediated Growth of Co on MgO(111)

Monolayer films of Co were deposited using an electrostatic electron-beam evaporator on single-crystal MgO(111)- $\sqrt 3 \times \sqrt 3 \mbox{ }R\mbox{ }30^{\circ}$ substrates held at room temperature, with subsequent annealing of temperatures 400\r{ }C to 800\r{ }C. These films were characterized using low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and x-ray diffraction (XRD). After short anneals of 400\r{ }C, AFM, LEED, and XPS suggest that islanding has occurred at the surface. XPS and XRD indicate the presence of elemental Co. 1-2 ML films of Ag were examined as a potential surfactant to aid in the growth of smooth Co films. Despite the fact that Ag itself formed islands, it was found that the presence of Ag did have a surfactant effect upon the thin-film growth of Co on Ag/MgO(111)- $\sqrt 3 \times \sqrt 3 \mbox{ }R\mbox{ }30^{\circ}$. Co islands were still present, but much more smooth than islands formed without Ag. XPS and AFM suggest strongly that Ag floated to the top of these samples at temperatures above 400 $^{\circ}$C.   

Single adatom adsorption on metal surfaces by first principles: effects of symmetry gap and surface states on the adatom electronic properties

The adsorption of a single atom on an extended substrate is a fundamental step in processes occurring at surfaces. But its theoretical description is still extremely difficult. After the ``adatom on jellium'' approach by Lang and Williams, only very few methods have been developed in order to treat a single adatom on a more realistic substrate than jellium. In this paper we present a new theoretical approach, within the density functional theory framework, which is able to take into account i) the semi-infinite nature of the substrate, ii) the presence of gap in the surface projected bulk bands, and iii) the existence of surface states. These ingredients are basic for a suitably realistic description of adsorption. We will show the effects of the symmetry gap on the line width of the adsorbate resonances for alkali (Li-Cs), Ba, and magnetic 3$d$ adatoms (Ti-Co) on noble metals.   

Hydrogen on Nickel Surfaces at Elevated Temperatures

Density functional theory, force fields, and Monte-Carlo simulations are used to address the adsorption, dissociation, and migration processes at elevated temperatures of hydrogen at nickel surfaces, which are key steps controlling the electrokinetics of conventional Ni-based anodes in solid-oxide fuel cells. In agreement with previous studies, this work shows that the adsorption and dissociation steps consist of (almost) barrierless processes, leading to the prompt formation of protons confined energetically to the superficial region. The bulk and surface diffusion of a proton in nickel is then addressed by using a multiscale approach. An effective interatomic potential describing the hydrogen-nickel interaction is first derived from density functional theory calculations. Then, molecular dynamics and Monte-Carlo simulations are combined to study in detail the hydrogen diffusion pathways and rates at increasing temperatures in the bulk and on the surface of Ni particles of various size. This work shows that the structural and thus energetic disorder at the surface increases for increasing temperatures, leading to the appearance of an anomalous surface diffusivity and to an increase of bulk migration pathways.   

A Hybrid Density Functional Study of Atomic Hydrogen and Oxygen Adsorptions on the (0001) Surface of Non-Magnetic DHCP Americium

As our group have recently shown$^{+}$, hybrid density functional theory (HDFT) which replaces a fraction (40{\%}) of approximate DFT exchange with exact Hartree-Fock exchange yield structural, magnetic, and electronic properties for Americium-I that are in excellent agreement with experimental data. As a natural progression, \textit{ab initio} calculations for atomic adsorptions on the (0001) surface of non-magnetic americium have been performed using HDFT. The americium surface is modeled by a seven-layer slab using inversion symmetry consisting of one atom per layer and non-magnetic ABAC stacking arrangement of these layers. Top, bridge, hcp and fcc chemisorption sites have been investigated with energies optimized with respect to the adatom distance from the surface. Details of the chemisorptions processes as well as comparisons of different sites will be presented. $^{+ }$R. Atta-Fynn and A. K. Ray, Chemical Physics Letters, \textbf{482}, 223-227 (2009).   

Stress release mechanisms for Cu on Pd(111) in the submonolayer and monolayer regimes

Metallic surfaces and nanostructures are essential systems for heterogeneous catalysis. Cu on Pd(111) is an interesting system, with a large mismatch ( -7.1 percent). We study strain relaxation mechanisms of Cu on Pd(111) up to the monolayer regime using two different computational methodologies, basin hopping global optimization and energy minimization with a repulsive bias potential. Our results are consistent with experimentally observed layer-by-layer growth mode. However, we find that the structure of the Cu layer is not fully pseudomorphic even at low coverages, but forms fcc and hcp stacking domains, separated by partial misfit dislocations. We estimate the minimum energy path and energy barriers for transitions from the epitaxial state to the fcc-hcp domain pattern. This work encourages further experimental investigations directly probing the overlayer structure.   

Electrically driven magnetism on a Pd thin film

Using first-principles density functional calculations we demonstrate that ferromagnetism can be induced and modulated on an otherwise paramagnetic Pd metal thin-film surface through application of an external electric field [1]. As free charges are either accumulated or depleted at the Pd surface to screen the applied electric field there is a corresponding change in the surface density of states. This change can be made sufficient for the Fermi-level density of states to satisfy the Stoner criterion, driving a transition locally at the surface from a paramagnetic state to an itinerant ferromagnetic state above a critical applied electric field, $E_{c}$. Furthermore, due to the second-order nature of this transition, the surface magnetization of the ferromagnetic state just above the transition exhibits a substantial dependence on electric field, as the result of an enhanced magnetoelectric susceptibility. A linearized Stoner model explains the occurrence of the itinerant ferromagnetism and demonstrates that the magnetic moment on the Pd surface follows a square-root variation with electric field, $m \quad \propto $ ($E$ -- $E_{c})^{1/2}$, consistent with our first-principles calculations. Thus, the predicted magnetoelectric effect manifests itself as a critical phenomenon and reveals the magnetoelectric susceptibility strikingly different from that previously known.\\[4pt] [1] Y. Sun, et al. arXiv:911.2678   

Enhancing THz Absorption using Thin-Film Multilayer Stacks

Terahertz imaging has seen significant proliferation in recent years. This band of electromagnetic spectrum has been underutilized for a long time due to the lack of sufficiently powerful sources and sensitive detectors. Because of virtually harmless effects on living tissue, terahertz (THz) radiation is attractive for various applications, ranging from non-invasive medical diagnostics to detection of concealed weapons. Our work focuses on identifying materials, or more specifically a stack of thin-films with increased absorption in the band of interest. In this work, we demonstrate a method that combines finite element modeling, thin-film deposition and experimental characterization to create highly-absorptive multi-layer stacks. Finite element modeling is used to simulate the absorption of a combination of thin dielectric and metallic films. Metals are deposited using e-beam evaporation and dielectric films using plasma enhanced chemical vapor deposition (PECVD). The simulated and measured THz absorption characteristics of the composite thin-film multilayer stacts will be presented.   

Broadband Asymmetric Optical Reflectance of Chemically Grown, Near-Percolation Granular Silver Films

Nanostructured thin metal films with controllable optical dispersions are highly relevant to areas such as optical coatings and photovoltaic devices. We address here the dispersive properties and high reflectance asymmetries observed in chemically grown nanocrystalline silver films. We show that while the optical transmittance of the films is always symmetric, as required by electromagnetic reciprocity conditions, their reflectance asymmetries may exceed those of vacuum deposited bulk films by as much as an order of magnitude. We present here an experimental study of a large number of composite structures comprised of both chemically and vacuum deposited silver films. We show how the optical reflectance asymmetry and its dispersion correlate with measured sheet resistances of these films, and demarcate a unique spectral crossover point of the reflectance asymmetry as an indication of the onset of charge percolation.   

Drude Circle for Ultra-Thin Gold Films Observed by Picometrology

In picometrology, a Gaussian laser beam at normal incidence on the edge of a thin film deposited on a substrate reflects to form a diffraction pattern on a Fourier plane. The complex index of the film is acquired by analyzing the symmetric and anti-symmetric components of the far field diffracted intensity. We have measured the effective index and dielectric constant of gold film as the thickness varies continuously from 0.1 nm to 10 nm at wavelengths of 488 and 532 nm. Three distinct regimes of the dielectric function trajectory on the complex plane are observed as the gold thickness increases. The regimes are interpreted in terms of gold cluster topology acquired from high-resolution SEM imaging. The substrate coverage and the size and aspect ratio of clusters determines the effective dielectric constant of a gold film. Of particular interest is the evolution of the dielectric constant along a circular trajectory in the thickness range of 2 to10 nm. This trajectory is the so-called ``Drude circle'' that occurs as the gold cluster size becomes smaller than the electron mean free path. We have unambiguously observed the Drude circle in the picometrology experiments. We gratefully acknowledge helpful discussions with V. Shalaev and help with SEM data from Kuo-Ping Chen.   

Charge Transport Properties of Continuous and Nanostructured Ruthenium Thin Films

Ru is a p-type transition metal with a 0$^{o}$C resistivity of 6.8 $\mu \Omega $ cm and Hall coefficient of +22 x 10$^{-5}$ cm$^{3}$/C for large grained polycrystalline bulk samples. However, the resistivity and carrier mobility of Ru thin film samples have not been investigated in detail. We have studied the charge transport behavior of continuous polycrystalline Ru thin films and nanorod structured Ru films grown on amorphous silica. Polycrystalline films (200nm thick) were deposited by normal-incidence RF magnetron sputtering of a Ru target in an Ar plasma. Films consisting of highly oriented $\sim $20 nm wide and $\sim $60 nm high nanorods were grown by 80\r{ } glancing angle deposition using DC magnetron sputtering. Van der Pauw resistivity and Hall coefficient measurements were acquired from 20-300$^{o}$C to contrast the transport properties of the continuous and nanorod structures. It was found that film resistivities decreased by as much as 50 {\%} upon first heating due to defect annealing, and the resistivities of the annealed films were significantly higher than the reported bulk value. The resistivities of nanorod-structured films were an order of magnitude higher than the continuous films. Hall coefficients were positive for both film types, indicating holes as majority carriers, but were lower than the bulk value suggesting that both resistivity and hole mobility are a strong function of nanostructure.   

Optical response function reconstruction of the femtosecond electronic dephasing of an individual plasmonic nanostructure

The femtosecond (fs) temporal evolution of the optical polarization in a metal reflects the microscopic electron dynamics associated with carrier scattering and electron-phonon coupling. Conventional incoherent plasmonic light scattering with its lack of phase information provides an incomplete description of the material response and thus its underlying origin. We study the homogeneous electronic dephasing of a surface plasmon polariton excitation of an individual plasmonic nanostructure using sub-10 fs excitation pulses and symmetry-selective second-harmonic scattering. We reconstruct the full optical response function of the plasmon excitation with both amplitude and phase information without any model assumptions from the spectrogram analysis of a two-pulse interferometric autocorrelation measurement, an approach conceptually analogous to frequency resolved optical gating (FROG). Recovered (10$\pm $5) fs dephasing times are consistent with corresponding calculations within the Drude-Sommerfeld model.   

A Debate on the Accuracy of the Results from Transmission Electron Microscopy

We studied the effect of thermal and electron beam (E-beam) radiation on the core shell iron-iron oxide Nanoparticles (NPs). Based on atomic level imaging, electron diffraction, and computer simulation, we have direct evidence that the protecting oxide layer formed on the NPs at room temperature in air or oxygen continues to grow during an E-beam bombardment in the vacuum system transmission electron microscopy (TEM). The oxide layer increases from 3 to 6 nm following 1 hour E-beam exposure with an electron ?ux of 7$\times $10$^{5}$ nm$^{-2}$s$^{-1}$ and a vacuum of 3$\times $10$^{5}$ Pa. We found that the observed growth is related to E-beam facilitated mass transport across the oxide layer by a defect related process. We show the theoretical proof of defect formation in metal due to E-beam radiation, thereby debating on the accuracy of the result from TEM or any microscope which uses electron for scanning and imaging. This research is supported by DOE-AFCI (DE-FC07-08ID14962) and DOE-BES (DE-FG02-06ER15777).   

Preferential Nucleation and Growth of Au Particles on faceted O/Ru$(11\bar {2}0)$ Nanotemplate

Preferential nucleation and growth of Au nanoparticles on a faceted O/Ru$(11\bar {2}0)$ nanotemplate is investigated by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), and Auger electron spectroscopy (AES). The nanotemplate is fabricated on Ru$(11\bar {2}0)$upon annealing in NO$_{2}$ at sample temperatures $\ge $ 650K. The faceted substrate consists of uniform long ridges running along [0001] direction with four facets $(10\bar {1}1)$, $(01\bar {1}1)$, $(10\bar {1}\bar {1})$, and $(01\bar {1}\bar {1})$ on the ridges, with typical dimensions of $\sim $ 6 nm wide. Deposition of Au onto this nanotemplate at room temperature results in preferential nucleation and growth of Au particles with a narrow size distribution within the valleys of the nanofaceted surface. The size of Au nanoparticles can be changed by varying Au coverage. Our work demonstrates that the nanofaceted metal surface is a promising model for future studies of different substrates and metallic overlayers more relevant to catalytic reactions.