Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A42: Focus Session: Quantum Size Effects in Metallic Thin Films |
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Sponsoring Units: DMP Chair: Hanno Weitering, University of Tennessee, Oak Ridge National Laboratory Room: Colorado Convention Center 505 |
Monday, March 5, 2007 8:00AM - 8:36AM |
A42.00001: Quantum size effects in metallic thin films: from thermodynamic stability to superconductivities Invited Speaker: In ultra-thin epitaxial metallic film, confinement of electronic states along the vertical direction leads to the formation of quantum well states (QWS). Over the past few years it has been shown that such QWS have profound effects on the thermodynamic stability of thin metal films. It has also been shown that such QWS can modulate local free energy landscape and influence the kinetic processes of mass transport. More recently, evidences that such QWS can also impact collective electronic properties such as superconductivities have also been reported. This talk will focus on direct correlations of all these aspects. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A42.00002: Non-classical second layer nucleation in Pb/Si(111) and the kinetics of the wetting layer. M. Hupalo, Z. Kuntova , Z. Chvoj, C.Z Wang, K.M Ho, M. C. Tringides By studying the island growth in stepwise deposition experiments with STM we showed two non-classical features i.e. the unusual second layer ring morphology and the crucial role of the wetting layer in the kinetics. The filling of the vacancy island inside the ring is much slower process than the ring formation due to higher radial diffusion barrier towards the island center. In addition Pb is transferred to unstable islands from the continuous spreading of the wetting layer to the island top uncovering the underlying 7x7 reconstruction. Combined Monte Carlo simulations on a novel Potential Energy Surface (PES) constructed with input from first principles calculation can account for most of these unusual non-classical observations. [Preview Abstract] |
Monday, March 5, 2007 8:48AM - 9:00AM |
A42.00003: Quantum Modulation of Island Nucleation on Top of a Metal Nanomesa Yong Han, M. Hupalo, M.C. Tringides, Feng Liu We present a theoretical analysis of selectivity of nucleation location for the two-dimensional island on top of a metal nanomesa. It has been observed experimentally that the nucleation can start either along the periphery of the mesa top or in the middle, depending on the mesa thickness. Such an intriguing nucleation behavior is shown to be originated from the thickness-dependent mesa edge barrier for an adatom to jump off the mesa, induced by the quantum size effect. Based on the experimentally observed nucleation locations, we estimate that the mesa edge barriers for the 5- and 6-layer Pb(111) mesas can differ by $\sim 23\pm 7$ meV. [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A42.00004: In situ x-ray scattering investigation of the Pb/Si(111)7x7 interface Michael Gramlich, Rui Feng, Shawn Hayden, Myron Hupalo, Michael Tringides, Chinkyo Kim, Craig Jeffrey, Philip Ryan, Paul Miceli, Edward Conrad In situ x-ray scattering was used to investigate the structure of Pb deposited on the Si(111)7x7 surface, which exhibits a one- monolayer-thick wetting layer followed by quantum-size-effect nanocrystals at higher coverages. The structure of the wetting layer and its relationship to the nanocrystals is important to understand in order to explain the novel growth kinetics [PRL 96, 106105 (2006)] in this system as well as the charge transfer at the interface. The nanocrystals consume the wetting layer and exhibit a smooth buried interface while displacing the nanocrystal vertically by 0.4 angs. This study examines how the Pb modifies the Si, both in the wetting layer, which exhibits a modified 8x8 structure, and beneath the nanocrystals. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A42.00005: Strongly-Driven Coarsening of Height-Selected Pb Islands on Si(111) Maozhi Li, J. W. Evans, C. Z. Wang, M. Hupalo, M. C. Tringides, T.-L. Chan, K. M. Ho A rapid coarsening behavior was observed experimentally for Pb islands grown on Si(111) surface. It was found that quantum size effects lead to the breakdown of the classical Gibbs-Thomson analysis for this novel behavior. Here we propose a rate equation model where quantum size effects are incorporated by introducing an appropriate dependence of the chemical potential of Pb islands on their heights as well as on their curvatures. The evolution of the chemical potential of the wetting layer between islands is also incorporated. It is shown that rate equations predict evolution of the island density and height distribution in good agreement with experiments. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A42.00006: Stability of magic planar Ag clusters Y.P. Chiu, Y.S. Ou, Y.R. Chang, C.M. Wei, C.S. Chang, Tien T. Tsong 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 [1]. Existence of the magic atom numbers in these clusters is mainly attributed to the electronic confinement effect. Here, we further explore the strength of these magic clusters subject to the temperature rise and oxygen exposure. Detailed calculations based on \textit{ab initio} density functional theory have also been performed. The results help establish the relation between the physical and chemical stability of a magic Ag cluster and its size and shape. Ref:[1] Ya-Ping Chiu, Li-Wei Huang, Ching-Ming Wei, Chia-Seng Chang, and Tien-Tzou Tsong, Phys. Rev. Lett. \textbf{97}, 165504 (2006). [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A42.00007: Self Organization of Pb Islands on Si(111) Caused by Quantum Size Effects Hawoong Hong, Leo Basile, Peter Czoschke, Aaron Gray, Tai-Chang Chiang Growth of metallic Pb islands on Si(111) by vacuum deposition was studied in real time using synchrotron x-ray diffraction. The islands coarsen and order, maintaining a nearly uniform inter-island distance but without angular correlation. The resulting inter-island structure is akin to a two-dimensional liquid. Over a wide temperature range, the inter-island ordering is well correlated with the development of ``magic'' island heights caused by energy minimization of the Pb electrons. The results demonstrate quantum confinement effects as a driving force for self organization, as opposed to strain effects that generally govern the formation of semiconductor quantum dot arrays. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A42.00008: First-principles Study of Pb(111) Nanofilms in the Quantum Regime Yong Han, Raj Ganesh S. Pala, Guang-Hong Lu, Li Huang, Feng Liu We report first-principles calculations to investigate surface free energy, interlayer spacing, surface stress, and surface self-diffusion barrier for Pb(111) films in the thickness range of 1 to 31 monolayers, where the quantum size effect (QSE) dominates. We show that similar to surface free energy, all these properties exhibit an oscillation behavior and a beating pattern as a function of film thickness. We will discuss correlations between these properties in terms of QSE. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A42.00009: Strength modulation of quantum-well states in Pb islands with periodic distortions S.M. Lu, M.C. Yang, W.B. Su, C.L. Jiang, C.S. Chang, Tien T. Tsong We use scanning tunneling microscopy and spectroscopy to revisit the system of three-atomic- layer Pb islands with two types of patterns grown on Si(111) surface. Our results demonstrate that the pattern on the island surface appears as the superposition of geometric corrugation and local variation of the electronic structure. The former originates from two kinds of interface relaxations, resulting in two types of periodic distortions in the Pb island. The latter is due to the periodic strength modulation of quantum-well states in Pb islands, causing inhomogeneity in the integration of the density of states, and the pattern is bias-dependent. This strength modulation of the quantum-well states can be correlated to the electronic screening effect induced by the lattice distortion in Pb islands. [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A42.00010: Coherent Electronic Fringe Structure in Incommensurate Silver-Silicon Quantum Wells Nathan Speer, Shu Tang, Tom Miller, Tai Chiang Atomically uniform Ag films grown on highly doped n-type Si(111) substrates show fine-structured electronic fringes near the Si valence band edge as observed by angle-resolved photoemission. No such fringes are observed for Ag films grown on lightly doped n-type substrates or p-type substrates, although all cases exhibited the usual quantum well states corresponding to electron confinement in the film. The fringes correspond to electronic states extending over the Ag film as a quantum well and reaching into the Si substrate as a quantum slope, with the two parts coherently coupled through an incommensurate interface structure. [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A42.00011: First-principles studies of quantum growth of PbBi alloy films Yu Jia, M. M. Ozer, James R Thompson, H. H. Weitering, Zhenyu Zhang Quantum growth of Pb$_{0.89}$Bi$_{0.11}$ alloy films freestanding or grown on Si(111) and Ge(111) substrates has been studied using first-principles calculations within density functional theory. Our studies show that the surface energy, work function, and lattice relaxation of the quantum alloy films all oscillate strongly with the film thickness. Similar to the case of pure Pb(111) films, the oscillation pattern is bilayer, interrupted with even-odd crossovers. However, the positions of the crossovers and the beating periodicity can be tuned by the contents of Bi in the alloys, with the beating periodicity given by 13ML, 15ML and 17ML for Pb$_{0.89}$Bi$_{0.11}$, Pb$_{0.86}$Bi$_{0.14}$ and Pb$_{0.75}$Bi$_{0.25, }$respectively. These results are in quantitative agreement with experiment. [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A42.00012: Umklapp-Mediated Quantization of Electronic States in Ag Films on Ge(111) Shu-Jung Tang, Yen-Ru Lee, Shih-Lin Chang, Thomas Miller, Tai-Chang Chiang We employ angle-resolved photoemission to study the electronic structure of atomically uniform films of Ag grown on Ge(111). A new kind of quantum well state is observed near a specific emission direction away from the surface normal. In contrast with the usual quantum well state arising from electron confinement by specular reflections at the surface and interface of the film, the new kind involves retroreflections, or umklapp reflections, at the interface. It requires four reflections, instead of the usual two reflections, to complete a coherent interference path. [Preview Abstract] |
Monday, March 5, 2007 10:48AM - 11:00AM |
A42.00013: The Phases of Ag on Ge(111): A Low Energy Electron Microscopy Investigation. Jason Giacomo, Shirley Chiang The phases of Ag on Ge(111) have been investigated with Low Energy Electron Microscopy (LEEM). We have studied the growth of the well known (4x4) and ($\surd $3x$\surd $3)R30$^{o}$ phases of Ag. LEEM images show the (4x4) phase grows on the surface with a high dependency on surface steps. The ($\surd $3x$\surd $3)R30$^{o}$ phase then grows as the Ag concentration increases with little dependence on the steps. These features are explained by the diffusivity of Ag on the surface. LEEM has also been used to study the phase transitions at the Ag desorption temperature. Video rate data shows an interesting phase transition as small domains of Ag abruptly change from the ($\surd $3x$\surd $3)R30$^{o}$ to the (4x4) phase and then from the (4x4) to a disordered 2D gas phase. Although the disordered phase shows no contrast in the LEEM images we know it exists because as the sample is cooled down the remaining Ag on the surface condenses back into the (4x4) and ($\surd $3x$\surd $3)R30$^{o}$ phases depending on how much Ag has desorbed. [Preview Abstract] |
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