Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B20: Focus Session: Growth, Kinetics and Quantum Effects in Metal Thin Films |
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Sponsoring Units: DMP Chair: Robert Bartynski, Rutgers University Room: Morial Convention Center 212 |
Monday, March 10, 2008 11:15AM - 11:27AM |
B20.00001: Growth instabilities and adsorbed impurities: Nanostructuring of vicinal surfaces controlled by adsorbates Ajmi BHadj Hamouda, T.L. Einstein, P.E. Hoggan, A. Pimpinelli A kinetic Monte Carlo study of the meandering instability of a vicinal surface growing by step flow is performed. Impurities are co-deposited during growth, and they are shown to be responsible for quantitative and qualitative modifications of the surface morphology. In particular, impurities make adatom diffusion less dependent on the deposition rate, affecting thus the wavelength of the meandering. Impurities also act as nucleation centres, causing small stepped pyramids to appear on the surface. Comparison with step- flow experiments on vicinal Cu(100) make plausible the hypothesis that many previously unexplained features of the meandering instability in this system are due to impurities. The density of nano-pyramids can be tuned by varying the impurity concentration. Our results show also that the step bunching instability is strongly affected by adsorbated impurities having lower diffusion rate than adatoms. Such impurities slow the adatoms diffusion and weakens the instability, even removing it at large impurities concentration. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B20.00002: Effects of deposition angle in metal(100) epitaxial growth Jacques G. Amar, Valery Borovikov, Yunsic Shim The effects of oblique incidence on the surface roughness and morphology in Cu/Cu(100) epitaxial growth are investigated via multiscale kinetic Monte Carlo simulations in which the effects of shadowing and short-range (SR) and long-range (LR) attraction during deposition are taken into account via molecular dynamics simulations. Somewhat surprisingly, while the effects of deposition angle are found to be relatively weak at $200$ K, at a slightly lower temperature ($160$ K) both the surface roughness and the growth exponent $\beta$ depend strongly on deposition angle even for moderate deposition angles. These results resolve a long-standing puzzle regarding the growth behavior of Cu/Cu(100) over this temperature range. We also find that while the effects of shadowing and SR attraction play important roles, for moderate deposition angle the effects of LR attraction are relatively weak. Our results also demonstrate that, in general, the effects of deposition angle must be considered in low-temperature growth even for moderate deposition angles. Results for the surface morphology and roughness at higher temperatures and for very large deposition angles are also presented and compared with experiments. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B20.00003: Shape fluctuations of small Pb(111) islands at low temperatures. M. Hupalo, M.C. Tringides With STM we have measured shape fluctuations of monolayer Pb(111) islands grown on top of Pb mesas of controllable height to determine their step energy. Islands as small as 10nm in diameter were used to test the limit of the thermodynamic analysis. It found that the mean square deviations of the fluctuations obey a linear relation on RT where R is the island radius and T the temperature in agreement with the standard analysis.The extracted step energy 140eV/atom is in good agreement with the one obtained in the literature on islands of much larger sizes[1]. Fluctuation magnitude of bilayer island grown on top of 5-layer isladn(which corresponds to a magic 7-layer height) are much lower and the corresponding step free energy is three times higher than for monatomic steps. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B20.00004: Connector Model for Many-Body Interactions at Surfaces from First Principles Yogesh Tiwary, Kristen Fichthorn First-principles calculations based on DFT indicate that pair, trio, four- and five-body atomic interactions are significant in Al clusters on Al(110). These many-body interactions are a signature of ``elastic screening'' of long-ranged, substrate-mediated, elastic interactions between atoms in dilute adlayers by direct bonding and short-ranged substrate relaxation in dense clusters. As a consequence of this screening, we developed the Connector Model to effectively describe the energies of compact clusters. Adsorbate structures and interactions are described in terms of single-atom, many-body connector units, which link to one another to form the structures that can occur in thin-film and crystal growth. The additive connector energies can be effectively incorporated into a lattice-based Hamiltonian to study thermodynamics and kinetics at surfaces. The Connector Model is considerably more efficient than lattice-gas Hamiltonian approaches, which would require a large number of terms to accurately capture many-body effects. Details of the connector model and its application to predicting the shapes of compact clusters on Al(110) including the chain-to-island transition will be discussed. This model can be extended to other crystalline surfaces. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B20.00005: Double-layer island decay on Ag(111): A molecular dynamic simulation Berk Onat, Sondan Durukanoglu We have performed molecular dynamic simulations to investigate double-layer island decays on Ag(111) using the interaction potentials based on the embedded atom method with a specific aim to observe the effect of varying island size and temperature on interlayer mass transport between two layers of the adatom islands. Our preliminary results indicate that decay rates of adatom islands show different characteristics with varying adatom island size. From an analysis of MD simulations, we further examine how the activation barriers for several diffusion processes taking place during adatom island decay change with respect to varying tempterature and island size. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B20.00006: Kinetics of Facile Bilayer Island Formation for Ag on NiAl(110) J.W. Evans, Yong Han, D.-J. Liu, Baris Unal, F. Qin, D. Jin, C.J. Jenks, P.A. Thiel STM studies reveal that deposition of Ag on NiAl(110) at 127 K and above leads to bilayer-by-bilayer growth of a nearly-strain-free film with Ag(110) structure [1]. This growth mode is attributed to Quantum Size Effects (QSE) associated with electron confinement in the Ag film. Our focus here is on analysis the initial nucleation and growth of bilayer Ag(110) islands on NiAl(110) which is facile even at 127K despite requiring uphill transport of Ag. DFT analysis for supported Ag films determines adatom adsorption energies (which display QSE), interaction energies, and various relevant diffusion barriers. Kinetic Monte Carlo simulation of an atomistic lattice-gas model incorporating these energies highlights the role of strongly anisotropic interactions in facilitating bilayer island formation. \newline [1] B. Unal et al., Phys. Rev. B 76 (2007) 195410. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B20.00007: Self-organization and pattern selection under nanosecond pulsed laser-induced melting of ultrathin metal films Ramki Kalyanaraman, Christopher Favazza, Justin Trice, Radhakrishna Sureshkumar When an ultrathin metal film is rapidly melted by nanosecond (ns) laser pulses, ensuing hydrodynamic instabilities and/or fluid flow due to surface tension gradients lead to self-organizing patterns of ordered nanostructures. The extremely fast heating O(100 K/ns) and resolidification (O(10 K/ns) in such ns melting experiments permits quenching in of the morphology. Thus, multiple pulses of such fast melting/resolidification cycles in the film allow different stages of the patterning process to be identified and studied. We show that pattern formation via a thin film hydrodynamic dewetting instability and thermocapillary flow can compete and the dominating mechanism is one which has shorter time scale. We have explored this behavior for a large variety of metals, including Ti, V, Mn, Fe, Co, Ni, Cu, and Ag. This ns melting approach permits the robust self-organization of a wide variety of nanoscale structures, including nanoholes, nanostars, nanoparticles and nanowires. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B20.00008: Quantum engineering of apparent tunneling height in ultra thin Pb films Jungdae Kim, Shengyong Qin, Chih-Kang Ken Shih The thickness dependence of tunneling decay constant ($\kappa )$ for ultra thin Pb films is studied with various sample biases by using low temperature STM. It is found that quantum well states (QWS) have a strong influence on the tunneling decay constant $\kappa $. While the decay constant versus layer thickness ($\kappa $ vs. L) clearly shows bilayer oscillations, we found that the apparent contrast in $\kappa $ vs. L also show strong bias dependence. Depending on the bias voltage, contrast reversal in the apparent oscillation of $\kappa $ vs. L can be precisely tuned when the tunneling into the sample empty states. This result also shows that $\kappa $-oscillation does not necessarily imply the work function oscillation. We further show that in this case, the parallel component of crystal momentum plays a critical role in tunneling process and is largely responsible for the observed phenomena. On the other hand, at negative sample bias, we show that the measured decay constants well reflect the variation of surface workfunction. In this case, the layer-dependent surface work functions indeed show bi-layer oscillations and the signature of phase slip due to non-perfect phase matching between Fermi wavelength and the vertical lattice constant. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B20.00009: Why Is The Size Dependence Of the Scanning Tunneling Microscopy Workfunction Order Of Magnitude Larger Than That Of Photoemission ? Wei-Xue Li, Xin Liu, S.B. Zhang Quantum size effect (QSE) has been studied extensively, as a primary driving force for nano technology. Recent scanning tunneling microscopy workfunction of ultrathin Pb(111) film found however order of magnitude larger of QSE than that of photoemission. By first-principles calculations, we show that the QSE is not merely a size effect but symmetry driven: being maximal at the $\overline{\Gamma}$ point (i.e., the center of the surface Brillouin zone) derived from interlayer coupling of $p_z$ orbital, but could be vanishingly small at other symmetry points from in-plane $p_{x,y}$ orbital. The $\overline{\Gamma}$ valley states have the slowest decay. Thus they are the ones being picked up by near-field techniques such as the scanning tunneling spectroscopy. While whole surface Brillouin zone contributes equally to photoemission, and only marginal QSE effect occurs. For this reason, symmetry could be essential for all near-field physics and chemistry. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B20.00010: Tuning Surface Energy Landscapes in Metallic Quantum Films using Alkali Adsorbates Alexander Khajetoorians, Shengyong Qin, Wenguang Zhu, Holger Eisele, Zhenyu Zhang, Chih-Kang Shih Quantum confinement shows a strong interplay with growth and kinetics in thin metal systems where the Fermi wavelength has a special relationship to the surface normal lattice constant. In the case of Pb/Si(111) systems, this relationship reveals an interesting thickness-dependent bilayer oscillation in the density of states and surface energy up to a phase. In this paper, we report on a novel effect: tuning of the energy landscape of a flat-top quantum Pb mesa using Cs adsorbates. Using STM/STS, we show that depositing Cs adsorbates on a thin Pb mesa promotes quantum stable Pb nanoislands on preferentially unstable thicknesses. Thickness-dependent nanoisland densities show a strong bilayer oscillation correlating with quantum stability. By modifying the Cs coverage on the mesa surface, we can tune the lateral size distribution of the nanoislands and the overall amplitude of the island density oscillation. Nanoisland formation is linked to a step decoration of Cs adatoms along the step edge of the nanoisland. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B20.00011: Super-oscillations in the Interlayer Lattice Relaxation of Quantum Pb Films. Yu Jia, Biao Wu, T.L. Einstein, H.H. Weitering, Zhenyu Zhang Using first-principles total energy calculations, we study the interlayer relaxation in lead films and observe oscillations of the interlayer lattice relaxation with layer number (distance from vacuum) and with film thickness. By analyzing the charge distribution along the direction perpendicular to the film, we show that the former oscillations of lattice relaxation are induced by Friedel oscillations in charge density that decays slowly, following a 1/r law rather than a 1/r$^{2}$ law found in most metal films. The superoscillations are attributable to interference of the Friedel oscillations from the two boundaries of the film. Our results suggest that Friedel oscillations may be responsible for the quantum size effect observed in lead films. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B20.00012: Modifying the Adsorption of Molecules at Metal Surfaces by Quantum Confinement of Electrons Levan Tskipuri, Robert Bartynski We have studied the bonding of CO on several ultrathin Cu and Co films that exhibit metallic quantum well (MQW) states, whose energies change as a function of overlayer thickness, using inverse photoemission (IPE), reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). For Co system, which has unoccupied MQW states that do not cross the Fermi level, a CO 2$\pi $-induced feature is observed in IPE at 3.8 eV above E$_{F}$. CO desorbs at 375 K ( 30 K lower than for hcp Co surfaces) and a second TPD feature at 230 K appears upon low temperature ($\sim $ 100K) dosing. These TPD peak temperatues change as a function of film thickness and are correlated with two different C-O stretch vibrational frequencies observed in the IR spectra. The intensity of the C-O stretch feature in IRAS spectra, and the peak CO desorption temperature in TPD from CO on Cu MQWs both show modulations that are correlated with MQW states crossing the EF. We have also studied the influence of MQW states on the adsorption properties of the dimethyl disulfide (thiol) molecule (CH$_{3}$S)$_{2}$, which forms a self-assembled monolayer when adsorbed on the Cu(100) surface. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B20.00013: Crystal structure and height selection for Indium growth on Si(111) interfaces J. Chen, M. Hupalo, M. Ji, C.Z. Wang, K.M. Ho, M.C. Tringides The growth of In/Si(111) has been studied with SPA-LEED and STM to identify whether QSE-driven height selection with mono-disperse distribution is possible. The motivation is to discover other metals besides Pb/Si(111) with high degree of self-organization. Indium growth only on one substrate (Si(111)-Pb-$\alpha -\surd $3x$\surd $3) has resulted in uniform height In(111) 4-layer flat top islands (with the fcc(111) orientation different from the bct In bulk structure). This allotropic transition is observed at low temperatures T$<$250K and coverages $\theta <$6ML. Otherwise bct(110) oriented Indium islands are observed with continuously increasing height/size aspect ratios. These results suggest two stabilizing energetic effect for the fcc In(111) height selection i.e. Quantum Size Effects (QSE) stabilize uniform height and orientation dependent surface energy favours the different crystallography. The contribution of each effect was studied with first principles calculations and both the height selection and the allotropic transition can be quantitatively explained. -/a [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B20.00014: Non-classical scaling in the Pb/Si(111) coarsening at low temperature Myron Hupalo, R. Feng, E.H. Conrad, C. A. Jeffrey, P. F. Miceli, S. Hayden, M. Gramlich, P. J. Ryan, C. Kim, M.C. Tringides Recent coarsening experiments monitoring the evolution of a mixture of stable and unstable islands in Pb/Si(111) towards a mono-disperse 7-layer height distribution have revealed novel features that extend the classical curvature driven growth. Two complementary techniques are used, X-ray scattering and STM. In particular the coverage $\theta $ , temperature T and flux F dependence are the opposite of what is expected from the classical analysis. The coarsening time $\tau $ increases with increasing temperature T, coverage $\theta $ and decreases with increasing flux rate F according to the scaling relation $\tau $F=constant. These paradoxical results can be understood from the island stability dependence on lateral size L in addition to the QSE-driven well--analyzed height dependence. The decay constant of an unstable island is an increasing function of its lateral size and for sizes larger than Lc$\sim $50nm the unstable islands do not decay but grow in the next stable height. Since the lateral size increases with T, $\theta $ and decreases with F this can account for the novel coarsening results. [Preview Abstract] |
Monday, March 10, 2008 2:03PM - 2:15PM |
B20.00015: Surface, quantum well, and bulk states in Ag films Nathan Speer, Ching Wei, Tom Miller, Tai Chiang Atomically uniform Ag films grown on Si(111) substrates show, in addition to the usual Shockley Surface state, multiple surface states in pockets within the d-band manifold as observed by angle-resolved photoemission spectroscopy. At low coverages, quantum well states are resolved. As the film thickness increases, quantum well states evolve into the bulk band continuum plus separate surface states. The results are compared to a density functional theory calculation. [Preview Abstract] |
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