Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N12: Focus Session: Alloy and Interface Composition |
Hide Abstracts |
Sponsoring Units: DMP DCMP Chair: Karsten Pohl, University of New Hampshire Room: Baltimore Convention Center 304 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N12.00001: Boron nitride nanostructures: complete layers and nanomeshes Martina Corso, Martin Moerscher, Simon Berner, Thomas Greber, Juerg Osterwalder, Andrii Goriachko A highly ordered self-assembled nanomesh grows on a hot (1000 K) Rh(111) surface during 40 L (1 Langmuir=10$^{-6}$ $torr \times s$) of borazine (HBNH)$_{3}$ exposure [1]. Hexagonal boron nitride (\textit{h}--BN) units aggregate to form this double-layer network of 3 nm periodicity and 2 nm hole size. The two layers are offset so that nearly the entire underlying metal surface is covered. This system can be used as a template for supramolecular structures, as demonstrated with C$_{60}$ molecules, or for any purpose where a nanopatterned surface that is stable at high temperatures (1000 K) is needed. One of the driving forces for its formation is the large lattice mismatch of –6.9 \% between the \textit{h}--BN film and the Rh substrate. The growth of similar nanomeshes on different substrates is investigated, with the purpose to control hole size and shape. It is found that not only the lattice mismatch and the symmetry of the underlying metal play an important role but also the bonding between the nitrogen atoms and the substrate. In fact nanomeshes can be grown on Ru(0001) and on Ir(111) thin films but it does not form on Pd(111) nor on Pd(110). [1] M. Corso et al. Science, 303 (2004) 217. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N12.00002: Measurements of Molecular Dynamics in Atomically Engineered Molecular Nanostructures Joseph A. Stroscio, Robert J. Celotta The quantum yield for exciting the motion of a single atom within a molecular nanostructure was measured with atomic spatial resolution. The molecular nanostructures consisted of a series of CoCu$_{n}$ and CoCu$_{n}$Co linear molecules fabricated on a Cu(111) surface. The Co atoms at the end of the molecules were induced to switch between two lattice positions using electron excitation in a scanning tunneling microscope (STM). The electron excitation and quantum yield were found to be spatially localized on an atomic scale. Above an electron energy threshold, the Co atom motion resulted from a predominantly single electron process. By systematically varying the molecular structure, atom motion within the molecule was shown to be dependent on molecular length and composition. [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N12.00003: Formation and Vibrational Entropy-Driven Disordering of Mo(100) and W(100) Surface Alloys M.S. Altman, K.L. Man, Y.J. Feng, C.T. Chan Atoms that are deposited on a surface of a dissimilar material may either remain on the surface or they may become incorporated in a surface or bulk alloy. Although the energetic differences between alloy and overlayer structures at T = 0 can now be understood from first principles in many systems, the entropic contribution to the system free energy, which governs the equilibrium structure, is less well understood. The formation and stability of Cu, Ag and Au-induced c(2x2) alloys at the Mo(100) and W(100) surfaces have been investigated with low energy electron microscopy and diffraction. The dependence of the c(2x2) diffraction intensity upon metal deposition flux reveals that alloy formation is governed by atomistic processes that are analogous to those that dictate overlayer island nucleation. An order-disorder transition is also observed that converts the surface from ordered alloy to disordered overlayer structure. Combined with knowledge of energetics that is obtained from first principles calculations, a comparison of disordering temperatures for alloys of the different metal species and substrates provides information on the decisive contribution of vibrational entropy to the system free energy. Effective Debye temperatures for metal adatoms are determined that are substantially lower than bulk values, but exhibit the expected mass and bond strength dependence.$^{ }$Vibrational entropy may also play a role in the stability of alloys at other surfaces. [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 9:12AM |
N12.00004: Measuring 3D Alloy Composition Profiles at Surfaces Invited Speaker: A key challenge in thin-film growth is controlling structure and composition. Of particular importance is understanding how and why atomic-scale heterogeneity develops during growth. We have used low-energy electron microscopy (LEEM) to measure how the three-dimensional composition of an alloy film evolves with time at the nanometer length scale. By quantitatively analyzing the reflected electron intensity in LEEM, we determine the alloy composition and structure, layer by layer near a surface, with 9 nm lateral spatial resolution. As an example, we show that heterogeneity during the growth of Pd on Cu(001) arises naturally from a generic step-overgrowth mechanism that is likely to be relevant in many growth systems. This work was performed in collaboration with Jiebing Sun (UNH), Karsten Pohl (UNH), and Gary Kellogg (Sandia Labs). [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N12.00005: Threading Dislocation Pair Annihilation as a Mechanism for the Growth of Ordered 2D Nanocluster Arrays Bogdan Diaconescu, Karsten Pohl The bottom-up approach of growing nanostructured ordered arrays of clusters on the misfit dislocation networks of strained metallic thin films requires a detailed understanding of the nucleation and film-adsorbate interaction processes. In the case of S adsorption on submonolayer Ag / Ru(0001), the Ag's short herring bone rectangular misfit dislocation unit cell of 54{\AA}x40{\AA} (19x16 Ag atoms) reconstructs into a well ordered triangular array of S filled vacancy islands 50{\AA} apart. Atomically resolved VT-STM measurements show that the S cluster growth mechanism involves a local restructuring of the misfit dislocation network of Ag with the final structure free of threading dislocations. The new symmetry and morphology of the composed S/Ag films is obtained via a threading dislocation annihilation mechanism where adjacent and opposite pairs of threading dislocations are replaced by the S filled Ag vacancy islands. The local character of the annihilation process is shown by the conservation of the unit cell size area of 21.5nm$^{2}$. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N12.00006: Highly-spatial resolved surface structure and composition by LEEM image intensity analysis J. Sun, J. B. Hannon, G.L. Kellogg, K. Pohl Controlling the local structure and composition of a surface alloy is of great importance in thin film technologies. ~However, measuring the alloy's heterogeneity is very difficult, because existing experimental techniques either assume lateral homogeneity or have limited subsurface or chemical sensitivity. ~In this work~we have analyzed the electron diffraction intensity vs. incident energy curves of the (00) beam acquired from low-energy electron microscopy (LEEM) images. ~In contrast to conventional LEED-IV we are able to extract structural and local composition in the surface region with a lateral resolution of 8 nm. Two challenges in applying multiple electron scattering calculations to the analysis of LEEM data are the low and limited electron energy range (10 to 100 eV), which we address by a careful choice of the energy-dependent real and imaginary part of the optical potential. Our analysis of the LEEM IV curves for the clean Cu(001) and Pd/Cu(001) surfaces gives excellent agreement between experimental and best-fit data and good agreement with previous structural investigations. Our new analysis technique is capable of determining surface structure and composition with high accuracy. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N12.00007: Structure of an ultra-thin Ag film on the Al(100) surface M. Kopczyk, A. Kayani, R.J. Smith, D.S. Choi The surface structure for one monolayer of Ag deposited on the Al(100) surface at room temperature has been studied using low energy electron diffraction (LEED), ion scattering spectroscopy (ISS) and Rutherford back-scattering spectroscopy (RBS). The Ag coverage was determined with RBS. We conclude that the Ag atoms form two domains of a buckled, quasi-hexagonal structure that is incommensurate with the Al(100) surface unit cell, having a repeat distance of 5 Al(100) interatomic spacings in the [110] direction. The LEED pattern shows a double-domain (5x1) structure with additional intensity in those spots corresponding to a (111) close-packed hexagonal layer. The analysis of the ISS results suggests that the heights of the adsorbed Ag atoms above the Al surface are not all the same, leading to the proposed buckling model. In addition, some Al atoms apparently move from the substrate up into the Ag adlayer. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N12.00008: MEAM Potentials for Al-Mg Alloy: Application to Defects Bohumir Jelinek, Jeffery Houze, Seong-Gon Kim, Mark F. Horstemeyer, M. I. Baskes The ab-initio calculations based on density functional theory (DFT) are performed for the Al and Mg crystals and their alloy in reference structures, such as NaCl structure. The lattice constant (volume), bulk modulus and shear moduli for each element and the alloy are determined from the total energy calculations. These material parameters are then used to determine the Modified Embedded Atom Method (MEAM) potentials for these elements and their alloys. The transferability of these parameters are tested by obtaining relevant physical quantities on structures different than the reference structures and compare them with the results from ab-initio calculations. MEAM potentials determined for these materials are used to study the structure and morphology of various form of defects of these materials. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N12.00009: Disorder and Roughening at Surfaces of Silver/Gold Alloys Gregory Derry, Rundong Wan Attempts to obtain a clean and well-ordered surface in ultrahigh vacuum for several low index faces of a 50 at\% silver/gold alloy gave rise to an unexpected phenomenon. After several cycles of sputtering and annealing, the surfaces appeared clean using Auger spectroscopy but yielded low energy electron diffraction spots of poor quality (in the case of AgAu(110), no diffraction was observed at all). Many further time/temperature annealing protocols were attempted with no improvement. In addition to the diffraction results, which indicate a lack of long-range microscopic order at the surface, continued processing resulted in macroscopic roughening of the surfaces. Electron microscopy revealed the presence of features with sizes on the order of microns at the roughened surfaces. Analysis of the lineshapes of the diffuse LEED beams for the AgAu(111) surface indicates that the sizes of the ordered patches on the surface are less than a nanometer. These results will be presented in more detail and possible explanations for this extraordinary behavior will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N12.00010: Surface Freezing in Liquid Gold-Silicon Eutectic Alloy investigated with Surface X-ray Diffraction Reinhard Streitel, Oleg Shpyrko, Alexei Grigoriev, Venkat Balagurusamy, Peter Pershan The formation of a 2D AuSi crystalline lattice on top of liquid AuSi eutectic alloy was found in surface X-ray diffraction experiments. Up to 12 degree above bulk melting temperature ($T_ {melt}$ = 361$^{\circ}$C) we observe a powder like, 2D crystalline lattice. Increasing the temperature we find a first order phase transition. GID diffraction data was used to determine the 2D lattice parameters and the domain size was estimated to be larger than 0.9 $\mu m$. Normalized X-ray reflectivity shows an increase of a factor of 20 in comparison to classical systems, indicating that atomic layering normal to the surface is significantly enhanced for AuSi. Synchrotron measurements were performed at ChemMatCARS, Advanced Photon Source, Argonne National Lab and supported by DOE grants DE-FG02-88-ER45379 and DE-AC02-98CH10886. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N12.00011: Determination of interface compositions by X-ray three-beam resonance diffraction H.-H. Wu, Y.-R. Lee, C.-M. Hsueh, H.-H. Chen, S.-L. Chang X-ray three-beam diffraction$(200/\bar {3}\bar {1}1)$under resonant conditions is used to measure the concentrations of the constituent elements of the interface between a (100) CdTe thin film and a (100) InSb substrate. The three-beam diffraction profiles versus the azimuth angle of rotation around [200] reveal a wide variety of change in phase shift due to resonance for photon energies in the vicinity of the Cd $L_{III} $ absorption edge. At different momentum transfers $q_r $ along [200], sensitive to the interfacial structure, the phase shift in the resonant state also provides sufficient information about the distributions of Cd and Te concentrations. With theoretical analysis for the crystallographic phase of the structure-factor triplets and the resonance phase shifts involved in the three-beam diffraction, it allows us to determine the composition of Cd and Te as a function of depth normal to the interface. Via the propagation of the secondary$(\bar {3}\bar {1}1)$reflected beam along the surface, possible interface structures parallel to the surface could also be deduced. [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N12.00012: LEED and Ab-Initio Study of the SmSi(111)-3x2 Reconstruction Christopher Eames, Steve Tear, Matthew Probert The Si(111)3x2-Sm reconstruction that has been observed by STM produces a 3x1 pattern when viewed using LEED [1]. It has been suggested that similar behaviour for Si(111)3x2-Ba is due to the interference of the emergent electron amplitudes between adjacent registry shifted unit cells [2]. We have gathered LEED I(V) curves from this surface and here we present a quantitative comparison of these with a structural model that has been suggested in the literature [3] and with the results of our own ab-initio calculations done using the CASTEP [4] code. \\ \ \\ {[}1{]} {C. Wigren {\it et al},{\it Phys. Rev. B.},{\bf 48} (1993) 11014-11021}\\ {[}2{]} {J. Schafer {\it et al}, {\it Phys. Rev. B.}, {\bf 67} (2003) 85411-85415}\\ {[}3{]} {E. Ehret {\it et al}, {\it Surf. Sci.}, {\bf 569} (2004) 23-32}\\ {[}4{]} {M. D. Segall {\it et al}, {\it J. Phys.: Cond. Matt.}, {\bf 14} (2002) 2717-2743} \\ [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N12.00013: Investigation of the interface structure in sputtered WSi$_{2}$/Si multilayers by in-situ synchrotron X-ray scattering. Yiping Wang, Hua Zhou, Lan Zhou, Randall L. Headrick, Albert. T. Macrander, Karl F. Ludwig Multilayer X-ray optics have many applications such as X-ray microscopy, X-ray lithography, and X-ray microanalysis. The interface imperfections are critical to the optical performance of the multilayer structures. We report on the growth of WSi$_{2}$ and Si amorphous thin films by dc magnetron sputtering in a vacuum chamber with 10$^{-9}$ Torr base pressure. In-situ synchrotron X-ray scattering with high temporal resolution has been employed to probe the surface and interface roughness evolution during film deposition. X-ray reflectivity simulations were performed using the IMD software package. It is found that the structure of WSi$_{2}$/Si multilayers is with an alternately smooth and rough interface. While Si layer roughens, WSi$_{2}$ layer is observed to smooth out an initially rough surface. The ion energy and flux assisting the growth may play a role in inducing this asymmetry in the interface roughness. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700