AVS2001 Session SS1-FrM: Dynamics of Metal Surfaces

Friday, November 2, 2001 8:20 AM in Room 120

Friday Morning

Start	Invited?	Item
8:20 AM		SS1-FrM-1 Crossover from Surface- to Bulk-Diffusion-Limited Surface Dynamics on Pt(111)¹ B. Poelsema (University of Twente, The Netherlands); J.B. Hannon (IBM T.J. Watson Research Center); N.C. Bartelt, G.L. Kellogg (Sandia National Laboratories) Microscopic measurements of changes in surface morphology due to the motion of single-atom high steps provide detailed information on atomistic processes occurring at a crystal surface. In this study, we use low energy electron microscopy (LEEM) to measure the time-dependent changes in area of two-dimensional Pt islands on Pt(111) as a function of temperature. We find evidence for a crossover from step motion caused by surface diffusion to step motion caused by the flow of material to and from the bulk crystal as the temperature is raised. At temperatures in the range from 1100-1500 K, circular islands residing at the top of three-dimensional stacks exhibit a linear decay, as long as the stack diameter exceeds approximately 1.5 microns. The temperature dependence of the decay rate yields an activation energy of 1.7 eV. On the other hand, for stacks with smaller diameters and temperatures below 1350 K, the decay is non-linear, having a form more closely resembling surface diffusion-limited kinetics. The high-T behavior is similar to that recently observed for NiAl(110), where it was shown that bulk vacancy transport controls step motion on the surface.² In support of the same interpretation for Pt(111), we observe that the islands grow upon increasing the temperature and shrink upon lowering the temperature in non-equilibrium experiments. The dynamics for Pt(111) are more complicated than NiAl(110), involving a competition between the diffusion of surface adatoms and bulk vacancies. However, preliminary modeling studies indicate that processes involving bulk vacancies can simultaneously explain the temperature dependence of island decay at fixed temperature and the non-equilibrium behavior of islands on Pt(111) as the temperature is varied. ¹Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U. S. DOE under Contract DE-AC04-94AL85000. ² K. McCarty, J.A. Nobel, and N.C. Bartelt, preprint.
8:40 AM		SS1-FrM-2 Dimensionality Investigation of an Alloy-dealloy Phase Transition on Ag/Cu(110) O. Kizilkaya, D.A. Hite, J. Choi, P.T. Sprunger (Louisiana State University) An alloy-dealloy phase transition as a result of 0.4-1 ML deposition of Ag on Cu(110) has been studied by use of angle resolved photoemission spectroscopy (ARUPS). The ARUPS measurements were performed at 3m. TGM beamline at Center for Advanced Microstructure and Devices synchrotron facility. Previous scanning tunneling microscopy studies showed that a surface alloy phase seen at submonolayer coverages of Ag (<0.4 ML) evolves into well ordered Ag chains at higher coverages. These chains consist of Ag dimers and trimers directed on [001] of the substrate and located on monomer and dimer [001] directed vacancies.The ARUPS results for 1 ML coverage of Ag revealed that this unique chain geometry has near 1-dimensional electronic structure showing dispersion along the Ag-chains with no dispersion perpendicular to chains and no dispersion as a function of varying photon energy. The ARUPS were measured in two different geometries, specifically, the vector potential, A, was parallel and perpendicular to [001] plane of Cu(110) in which chain structure was directed. The energy distrubition curves (EDC) as a function of photon energy show the lack of dispersion which imply that Ag chains are not incorporated into Cu(110) substrate. Although no perpendicular dispersion is observed, ARUPS data confirms, by changing the emission angle in surface Brillouin zone along the two high symmetry lines, that the Ag bands in-plane (parallel to the surface) anisotropically disperse. In the experimental geometry probing electronic geometry parallel to the Ag structural chains, that is along the [001] direction of the substrate, the Ag bands disperse as a function of parallel momentum. In contrast, with the experimental geometry probing electronic structure perpendicular to the [001] direction, there is little to no dispersion of the Ag d-bands.These results signify reduced dimensional electronic structure of Ag chains. Research was supported by U.S. DOE contract No. DE-FG02-98ER45712.
9:00 AM	Invited	SS1-FrM-3 Vacancies in Solids and the Stability of Surface Morphology K.F. McCarty, J.A. Nobel, N.C. Bartelt (Sandia National Laboratories) We have examined the kinetics of island decay on the NiAl (110) surface using low-energy electron microscopy (LEEM). Remarkably, the decay rates are constant in time and totally independent of the local environment (e.g., the width of the immediately adja cent terraces and degree of island curvature). Given the lack of surface current between islands of different curvature, we conclude that surface diffusion is not important to the smoothing process. Instead, we find unambiguous evidence that bulk vacancie s are responsible -- we visibly observe exchange between bulk vacancies and the surface when the sample temperature is changed. For a temperature increase, the surface steps advance. For a temperature decrease, the surface steps recess. These changes result from the increase (decrease) in bulk vacancy concentration for a temperature increase (decrease). Remarkably, the size change accompanying a temperature change is strictly proportional to the perimeter (step length) of the island, and again is totally independent of the local environment. Thus, we conclude that the atoms at surface steps undergo direct exchange with bulk vacancies. We will present simple mathematical models showing how this mechanism quantitatively describes the constant-rate kinetics as well as the bulk-derived flux associated with a temperature change. The complete independence of the surface dynamics on the local environment results from the fact that the steps are interacting directly with the bulk, and thus, the local step density and curvature are largely irrelevant. We will also present results illustrating how the bulk/surface exchange affects NiAl oxidation. This work was performed under the U.S. Department of Energy contract DE-AC04-94AL85000 and supported by the Office of Basic Energy Sciences-Division of Materials Sciences.
9:40 AM		SS1-FrM-5 Surface Freezing in Binary Metallic Melts A. Turchanin, W. Freyland, D. Nattland (University of Karlsruhe, Germany) We report on a new phenomena of surface freezing, whereby an ordered solid film forms on top of the bulk liquid at temperatures above the bulk crystallisation. Recently this phenomenon has been observed in dilute Ga-Pb¹ and Ga-Bi² liquid alloys. Here we present new results on the investigation of surface freezing in the entire composition range of the Ga-Bi system. Second Harmonic Generation and Plasma Generation methods were applied to distinguish between the solid and liquid states of the alloy surfaces as a function of temperature and composition. The surface freezing was detected for all investigated alloys with composition between the eutectic (x(Bi)=0.0022, T(eut)=29.48°C) and monotectic point (x(Bi)=0.085, T(mono)=222 °C). On cooling of such alloys a Bi-rich solid-like film forms on top of the bulk liquid phase at temperatures well above the liquidus line. This film disappears on heating at higher temperatures, which characterizes this transition as a first-order phase transition. The maximum difference between the liquidus temperature and the temperature of the surface freezing transition was detected for the eutectic alloy (T(tr)-T(eut)=20 K). The lines of surface freezing and respective melting merge with the liquidus line approaching the monotectic point. The thickness of the forming Bi-rich solid-like films estimated from our measurements lies in the ranges from 0.5-1 nm to 100 nm. We have indications that it has a tendency to increase approaching the monotectic point. The relation of the surface freezing and wetting transitions occurring in Ga-Bi will be discussed. The surface freezing in Ga-Bi system will be compared with the two-dimensional freezing in dilute Ga-Pb alloy detected by x-ray reflectivity and grazing incidence diffraction methods.¹ ¹B. Yang, D. Gidalevitz, D. Li, Z. Huang, S.A. Rice, Proc. Nat. Acad. Svi USA, 96 (1999) 13009 ²A. Turchanin, D. Nattland, W. Freyland, Chem. Phys. Lett. 337 (2001) 5.
10:00 AM		SS1-FrM-6 Submonolayer Growth of Ag on Pt(100): Competition between Alloying and Surface Reconstruction M. Batzill, B.E. Koel (University of Southern California) The growth of Ag on Pt(111) and the formation of a nanophasic alloy, has been thoroughly studied. On the Pt(100) surface, the presence of the energy-minimizing, hex-reconstruction gives rise to phenomena different to that on the (111) face. We present STM, AES, and TPD studies of the Ag/Pt(100) interface for submonolayer Ag coverages in a temperature regime between 300 and 900 K. We observe strongly anisotropic growth of Ag islands governed by the anisotropy of the reconstructed surface at 300 K. Adsorption of Ag lifts the reconstruction, resulting in a kinetically trapped Ag80Pt20 alloy formed by using the excess Pt from the reconstruction ejected into the growing Ag-islands. Annealing causes diffusion of Ag from the Ag80Pt20 adislands to the pure Pt(100) reconstructed regions, lifting its reconstruction and forming a nanophasic Ag-Pt alloy. For surfaces with less than 0.3 ML Ag, domains of Pt(100)-hex reconstruction and a Ag30Pt70 alloy coexist in thermodynamic equilibrium. This phase segregation is a result of energy minimization, and can be explained by the presence of two minima in the Ag-Pt surface phase diagram associated with the pure Pt reconstruction and a strain stabilized alloy phase. Deposition of more than 0.3 ML increases the Ag content in the alloyphase until no Pt remains in the monolayer and an abrupt Pt/Ag interface forms.
10:20 AM		SS1-FrM-7 Direct Observation of Surface Alloying and Interface Roughening: Growth of Au on Fe(001) M.M.J. Bischoff, T. Yamada, A.J. Quinn, R.G.P. van der Kraan, H. van Kempen (University of Nijmegen, The Netherlands) Iron and gold belong to the family of elements which do not form alloys because of the large size difference of the Fe and Au atoms. Nevertheless, it has been predicted by Tersoff¹ that for those elements surface confined mixing may generally occur. In the low coverage limit, this might lead to surface alloying. Because of the immiscibility of these elements, an interesting question is what will happen when this surface alloy becomes saturated, i.e. more Au is deposited than allowed for a surface confined alloy. The Au/Fe combination is especially suitable for the detailed study of possible interface processes like segregation and interface diffusion. The reason is that, while the in-plane lattice match is nearly perfect, there is a large difference in the out-of plane distance between the bcc Fe and the fcc Au (0.14 nm compared to 0.20 nm). This allows to discriminate between interface and surface features. In this paper, we report a scanning tunneling microscopy study on the growth of Au on Fe(001) at 500K. Deposition of 0.5 monolayer (ML) leads to complete coverage by a homogeneous alloyed monolayer and simultaneous formation of islands covering 50% of the terraces. Consecutive deposition of more Au leads to demixing: in this process Au islands embedded in the original Fe(001) terrace are created while Fe islands stick out in the Au covering layers. By using suitable chosen growth and anneal temperatures, the processes at the interface can be followed even for thicker coverages of 5 ML. Scanning tunneling spectroscopy studies on this system will be presented: a new surface state around 0.6-0.7 eV is detected on the homogeneous alloyed surface layer. ¹ ¹J. Tersoff, Phys. Rev. Lett. 74, 434 (1995).
10:40 AM		SS1-FrM-8 Surface Reconstruction STM Studies of Clean and Oxidized Low Index FeAl Surfaces O. Kizilkaya, D.A. Hite (Louisiana State University); D.M. Zehner (Oak Ridge National Laboratory); P.T. Sprunger (Louisiana State University) Details of the atomic reconstruction behavior of clean and oxidized (110), (210) and (310) surfaces of the ordered bimetallic alloy FeAl has been studied with STM. Prefentially sputtering impedes the aluminum concentration on the surfaces. Aluminum preferentially segregates to the surface layer upon sequential annealing of these low index surfaces. Depending on the annealing temperature, there are a sequence of differing surface reconstructions on FeAl(110). At 400 C, a bulk terminated surface is observed. Between the temperatures range of 400-600 C and 800-1000 C an incommensurate FeAl2 overlayer is obtained. At annealing temperatures of 600-800 C, STM images reveal a long-range missing row reconstruction parallel to the [001] direction. For a wide range of annealing temperatures, FeAl(210) reconstructs to a (1x3) structure. The surface has alternating (100) and (110) facets. A similar type of reconstruction is observed for FeAl(310); however, the surface is not stable but facets. The crystal miscuts and kinetic effects are the main reason for instability. In the case of oxidized FeAl(110), STM reveals the formation of a thin-film gamma-alumina at low coverages. This structure is characterized as a hexagonal moiré pattern with a spot-spot distance of 1.8 nm. Research conducted at Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy, under contract DE-AC05-00OR22725. Research was supported by U.S. DOE contract No. DE-FG02-98ER45712.
11:00 AM		SS1-FrM-9 Self-Assembly of Nanoscale Hexagons in S/Cu/Ru(0001)¹ A.P. Baddorf (Oak Ridge National Laboratory); H. Zajonz, D. Gibbs (Brookhaven National Laboratory) Periodic structures on a nanometer length scale exhibit unique dynamics and forces, and offer a template for ordering of quantum dots. Recently, Hrbek, et al.,² have shown that small quantities of S adsorbed on the two layer Cu/Ru stripe phase react at dislocations, shorten stripes, and produce a quasi-hexagonal structure. We have examined the growth of this structure and its dependence on temperature with synchrotron x-ray diffraction. Deposition rates were determined by monitoring the intensity at the specular anti-Bragg position (similar to RHEED oscillations). After S deposition of up to 0.2 ML at room temperature, no new diffraction was observed, even after annealing. This is consistent with the results of Hrbek, et al., who found poor order in STM images. For deposition between 100 and 200°C, additional reflections were observed corresponding to a quasi-hexagonal lattice with a periodicity of approximately 44 Å. This structure persisted when the sample was cooled to room temperature. Long range order was indicated by narrow peak widths, 0.009 Å^-1 FWHM, corresponding to a correlation length of over 700 Å. Heating above 200°C reduced and broadened the diffraction wavevector, indicating a larger, less ordered lattice. Above 300°C, the quasi-hexagonal diffraction pattern disappeared leaving only diffraction from the linear Cu/Ru stripe phase. This process was reversible, so that cooling below 300°C returned the hexagonal pattern. We interpret this as a melting of the S induced structure. ¹ORNL is managed by UT-Battelle, LLC under US DOE contract DE-AC05-00OR22725. BNL is supported by US DOE DE-AC02-98CH10886. ²J. Hrbek, J. de la Figuera, K. Pohl, T. Jirsak, J. A. Rodriguez, A. K. Schmid, N. C. Bartelt, and R. Q. Hwang, J. Phys. Chem. B 103, 10557 (1999).
11:20 AM		SS1-FrM-10 Application of EELFS Spectroscopy to Local Atomic Structure Study Yu.V. Ruts, A.N. Maratkanova, D.V. Surnin, D.E. Guy (Physical-Technical Institute of Ural Branch of RAS, Russia) Fine structures above ionisation edges studied in the EELFS (Extended Energy Loss Fine Structure) technique are similar to those measured in X-ray absorption spectra (EXAFS). A possibility to measure extended structures using both transmission and reflection modes allows to obtain not only bulk parameters of the local atomic structure (partial interatomic distances, coordination numbers, backscattering amplitudes, phase shifts, etc.) but also the surface ones. Surface sensitivity makes the EELFS technique competitive with diffraction techniques especially for studying systems without long-range order. Moreover the EELFS spectroscopy is available in most surface and material physics laboratories having standard equipment with the electron exitation. In contrast to EXAFS the EELFS technique makes possible to study local domains, for example individual blocks or grains, owing to using an electron beam as an exciting source. The essential peculiarity of this method is its high sensitivity to light elements. This is very important for studying surface and bulk oxides, carbides and so on as well as the processes of their formation including gas adsorption. EELFS technique has been proved to be a powerful tool for local structural investigations of clean surfaces and chemisorbed species. Numerous papers have been published to demonstrate the applicability of the EELFS technique in the determination of the structure of different compounds and different metals deposited on clean surfaces. Application of the EELFS technique provides great progress to materials science in regards to atomic structure study. In this paper we give some examples of studying different materials by the EELFS both in transmission and reflection mode using the results obtained by different authors including our own ones. This work was sponsored in part by the Russian Foundation for Basic Research (grants 00-03-33049a and 00-15-97419).