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Steering the Self-Assembly of Octadecylamine Monolayers on Mica by Controlled Mechanical Energy Transfer from the AFM Tip

Description: We have studied the effect of mechanical energy transfer from the tip of an Atomic Force Microscope on the dynamics of self-assembly of monolayer films of octadecylamine on mica. The formation of the self-assembled film proceeds in two successive stages, the first being a fast adsorption from solution that follows a Langmuir isotherm. The second is a slower process of island growth by aggregation of the molecules dispersed on the surface. We found that the dynamics of aggregation can be altered substantially by the addition of mechanical energy into the system through controlled tip-surface interactions. This leads to either the creation of pinholes in existing islands as a consequence of vacancy concentration, and to the assembly of residual molecules into more compact islands.
Date: June 24, 2010
Creator: Benitez, J.J.; Heredia-Guerrero, J.A. & Salmeron, M.
Partner: UNT Libraries Government Documents Department

Atomic scale interface structure of In{sub 0.2}Ga{sub 0.8}As/GaAs strained layers studied by cross-sectional scanning tunneling microscopy

Description: A molecular beam epitaxy-grown In{sub 0.2}Ga{sub 0.8}As/GaAs strained layer structure has been studied by scanning tunneling microscopy in cross-section on the (110) cleavage plane perpendicular to [001] the growth direction. Individual indium atoms were differentially imaged in the group III sublattice, allowing, a direct observation of the interface roughness due to the indium compositional fluctuation. In the In{sub 0.2}Ga{sub 0.8}As layers, Indium atoms are found in clusters preferentially along the growth direction with each cluster containing 2--3 indium atoms. Indium segregation induced asymmetrical interface broadening is studied on an atomic scale. The interface of In{sub 0.2}Ga{sub 0.8}As grown on GaAs is sharp within 2--4 atomic layers. The interface of GaAs grown on In{sub 0.2}Ga{sub 0.8}As is found to be broadened to about 5--10 atomic layers. The atomic scale fluctuation due to indium distribution is about 20 {angstrom} alone the interface in this case. The authors conclude that clustering and segregation are the main reason for the In{sub 0.2}Ga{sub 0.8}As/GaAs interface roughness.
Date: November 1, 1993
Creator: Zheng, J.F.; Weber, E.R. & Salmeron, M.B.
Partner: UNT Libraries Government Documents Department

Atomic force microscopy images of T4 bacteriophages on silicon substrates

Description: A new atomic force microscope incorporating microfabricated cantilevers and employing laser beam deflection for force detection has been constructed and is being applied to studied of biological material. In this study, T4 bacteriophage virus particles were deposited from solution onto electronic grade flat silicon wafers and imaged in air with the microscope. Microliter droplets of the solution were deposited and either allowed to dry or removed with blotting paper. The images show both isolated viruses and aggregates of various sizes. The external structure as well as strands believed to be DNA streaming out of the virus could be observed. The construction of the microscope and its performance are also described. 19 refs., 4 figs.
Date: August 1, 1991
Creator: Kolbe, W.F.; Ogletree, D.F. & Salmeron, M.B.
Partner: UNT Libraries Government Documents Department

Molecular conformation changes in alkylthiols ligands as afunction of size in gold nanoparticles

Description: The bonding of hexanethiols to gold nanoparticles of 1.5, 2.0 and 3 nm was studied using X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS). The XANES spectra revealed that a substantial fraction of hexanethiol molecules were weakly bound, in addition to the molecules forming covalent bonds with Au surface atoms. The weakly bound molecules could be easily removed by washing in dichloromethane. After removal of the weakly bound molecules the S K-edge XANES spectrum reveals peaks due to S-Au and S-C bonds with intensities that change as a function of particle size. We explain this as the result of distortions occurring in the molecules adsorbed on the smaller particles. The distortions arise from the poor packing due to the high curvature of the particles. In addition, EXAFS results show that the smaller particles bind more molecules per Au atom than the larger ones, which is again interpreted as a result of the curved nature of the surface.
Date: January 9, 2006
Creator: Ramallo-Lopez, J.M.; Giovanetti, L.J.; Requejo, F.G.; Isaacs,S.R.; Shon, Y.S. & Salmeron, M.
Partner: UNT Libraries Government Documents Department

Growth and structure of water on SiO2 films on Si investigated byKelvin probe microscopy and in situ X-ray Spectroscopies

Description: The growth of water on thin SiO{sub 2} films on Si wafers at vapor pressures between 1.5 and 4 torr and temperatures between -10 and 21 C has been studied in situ using Kelvin Probe Microscopy and X-ray photoemission and absorption spectroscopies. From 0 to 75% relative humidity (RH) water adsorbs forming a uniform film 4-5 layers thick. The surface potential increases in that RH range by about 400 mV and remains constant upon further increase of the RH. Above 75% RH the water film grows rapidly, reaching 6-7 monolayers at around 90% RH and forming a macroscopic drop near 100%. The O K-edge near-edge X-ray absorption spectrum around 75% RH is similar to that of liquid water (imperfect H-bonding coordination) at temperatures above 0 C and ice-like below 0 C.
Date: June 14, 2007
Creator: Verdaguer, A.; Weis, C.; Oncins, G.; Ketteler, G.; Bluhm, H. & Salmeron, M.
Partner: UNT Libraries Government Documents Department

Kinetic Effects in the Self-Assembly of Pure and Mixed Tetradecyland Octadecylamine Molecules on Mica

Description: The self-assembly of tetradecylamine (C14) and of mixtures of tetradecyl and octadecylamine (C18) molecules from chloroform solutions on mica has been studied using atomic force microscopy(AFM). For pure components self-assembly proceeds more slowly for C14 than for C18. In both cases after equilibrium is reached islands of tilted molecules cover a similar fraction of the surface. Images of films formed by mixtures of molecules acquired before equilibrium is reached (short ripening time at room temperature) show only islands with the height corresponding to C18 with many pores. After a long ripening time, when equilibrium is reached, islands of segregated pure components are formed.
Date: September 21, 2005
Creator: Benitez, J. J. & Salmeron, M.
Partner: UNT Libraries Government Documents Department

Scanning tuneling microscopy tip-dependent image contrast of S/Pt(111) by controlled atom transfer

Description: Sudden changes in the detailed appearance and corrugation of atomic-resolution scanning tunneling microscopy (STM) images are commonly observed. In particular, these effects for the Pt(111)({radical}3 {times} {radical} 3)R30{degrees}-sulfur structure have been studied. Corrugation changes have been investigated by the controlled transfer of atoms between tip and sample. Theoretical images simulations using electron scattering quantum chemical methods show that the image contrast is strongly dependent on the chemical identity of the STM tip atom. Observed changes in the sulfur images can be explained by changes from platinum-terminated to sulfur-terminated STM tips.
Date: October 1, 1993
Creator: McIntyre, B.J.; Dunphy, J.C.; Salmeron, M.; Somorjai, G.A. & Sautet, P.
Partner: UNT Libraries Government Documents Department

Effect of oxygen surfactant on the magnetic and structural properties of Co films grown on Cu(110)

Description: It was found that atomically flat Co(110) film could be grown on Cu(110) using O as a surfactant. To obtain detailed knowledge on the effect of O on the growth, as well as on the magnetic properties of Co overlayer, we carried out an investigation on this system using Auger Electron Spectroscopy (AES), Low Energy Electron Diffraction (LEED), Surface Magneto-Optic Kerr Effect (SMOKE), and Scanning Tunneling Microscopy (STM). With O as a surfactant, the initial growth of Co (< 1 ML) results in a flat monolayer structure. When the Co is thicker than 1 ML, three-dimensional clusters begin to form. These clusters become ordered islands at 3 ML Co and coalesce at about 5 ML Co. Above 5 ML Co, layer-by-layer growth resumes. No Cu segregation is observed. SMOKE studies at room temperature show that the Co film is magnetic above about 5 ML Co, with the magnetization easy axis along the [001] direction. On the other hand, without using oxygen as a surfactant, Co grows three-dimensionally on Cu(110). The Co overlayer has its easy magnetization axis along the [001] direction, but the onset of the magnetization was observed at 11 ML Co at room temperature.
Date: April 13, 2000
Creator: Ling, W.L.; Qiu, Z.Q.; Takeuchi, O.; Ogletree, D.F. & Salmeron, M.
Partner: UNT Libraries Government Documents Department

Scanning tunneling microscopy of Si donors in GaAs

Description: Using scanning tunneling microscopy, we have identified and characterized Si donors (Si{sub Ga}) in GaAs located on the (110) surface and in subsurface layers. Si{sub Ga} on the surface shows localized features with characteristic structures in good agreement with a recent theoretical calculation. Si{sub Ga} in subsurface layers appears as delocalized protrusions superimposed on the background lattice, which are interpreted in terms of the modification of the tunneling due to the tip-induced band bending perturbed by the Si{sub Ga} Coulomb potential.
Date: July 1, 1993
Creator: Zheng, J.F.; Weber, E.R.; Liu, X.; Newman, N.; Ogletree, D.F. & Salmeron, M.B.
Partner: UNT Libraries Government Documents Department

Induced water condensation and bridge formation by electric fieldsin Atomic Force Microscopy

Description: We present an analytical model that explains how in humidenvironments the electric field near a sharp tip enhances the formationof water meniscii and bridges between tip and sample. The predictions ofthe model are compared with experimental measurements of the criticaldistance where the field strength causes bridge formation.
Date: February 22, 2006
Creator: Sacha, G.M.; Verdaguer, A. & Salmeron, M.
Partner: UNT Libraries Government Documents Department

Interactions of Oxygen and Hydrogen on Pd(111) surface

Description: The coadsorption and interactions of oxygen and hydrogen on Pd(1 1 1) was studied by scanning tunneling microscopy and density functional theory calculations. In the absence of hydrogen oxygen forms a (2 x 2) ordered structure. Coadsorption of hydrogen leads to a structural transformation from (2 x 2) to a ({radical}3 x {radical}3)R30 degree structure. In addition to this transformation, hydrogen enhances the mobility of oxygen. To explain these observations, the interaction of oxygen and hydrogen on Pd(1 1 1) was studied within the density functional theory. In agreement with the experiment the calculations find a total energy minimum for the oxygen (2 x 2) structure. The interaction between H and O atoms was found to be repulsive and short ranged, leading to a compression of the O islands from (2 x 2) to ({radical}3 x {radical}3)R30 degree ordered structure at high H coverage. The computed energy barriers for the oxygen diffusion were found to be reduced due to the coadsorption of hydrogen, in agreement with the experimentally observed enhancement of oxygen mobility. The calculations also support the finding that at low temperatures the water formation reaction does not occur on Pd(1 1 1).
Date: June 25, 2008
Creator: Demchenko, D.O.; Sacha, G.M.; Salmeron, M. & Wang, L.-W.
Partner: UNT Libraries Government Documents Department

An atomic force microcopy study of the mechanical and electricalproperties of monolayer films of molecules with aromatic end groups

Description: The effect of intermolecular {pi}-{pi} stacking on the electrical and mechanical properties of monolayer films molecules containing aromatic groups was studied using atomic force microscopy. Two types of aromatic molecules, (4-mercaptophenyl) anthrylacetylene (MPAA) and (4-mercaptophenyl)-phenylacetylene (MPPA) were used as model systems with different {pi}-{pi} stacking strength. Monolayer films of these molecules on Au(111) surfaces exhibited conductivities differing by more than one order of magnitude, MPAA being the most conductive and MPPA the least conductive. The response to compressive loads by the AFM tip was also found to be very different for both molecules. In MPAA films distinct molecular conductivity changes are observed upon mechanical perturbation. This effect however was not observed on the MPPA film, where intermolecular {pi}-{pi} interactions are likely weaker.
Date: September 6, 2007
Creator: Fang, Liang; Park, J.Y.; Ma, H.; Jen, A.K.-Y. & Salmeron, M.
Partner: UNT Libraries Government Documents Department

Water adsorption on O(2x2)/Ru(0001) from STM experiments andfirst-principles calculations

Description: We present a combined theoretical and experimental study of water adsorption on Ru(0001) pre-covered with 0.25 monolayers (ML) of oxygen forming a (2 x 2) structure. Several structures were analyzed by means of Density Functional Theory calculations for which STM simulations were performed and compared with experimental data. Up to 0.25 monolayers the molecules bind to the exposed Ru atoms of the 2 x 2 unit cell via the lone pair orbitals. The molecular plane is almost parallel to the surface with its H atoms pointing towards the chemisorbed O atoms of the 2 x 2 unit cell forming hydrogen bonds. The existence of these additional hydrogen bonds increases the adsorption energy of the water molecule to approximately 616 meV, which is {approx}220 meV more stable than on the clean Ru(0001) surface with a similar configuration. The binding energy shows only a weak dependence on water coverage, with a shallow minimum for a row structure at 0.125 ML. This is consistent with the STM experiments that show a tendency of the molecules to form linear rows at intermediate coverage. Our calculations also suggest the possible formation of water dimers near 0.25 ML.
Date: October 15, 2007
Creator: Cabrera-Sanfelix, P.; Sanchez-Portal, D.; Mugarza, A.; Shimizu,T.K.; Salmeron, M. & Arnau, A.
Partner: UNT Libraries Government Documents Department

Manipulation and patterning of the surface hydrogen concentrationon Pd(111) electric fields

Description: Modification of the structure of materials at the nanoscale level is one goal of current nanoscience research. For example, by purposefully modifying the spatial distribution of adsorbates one could control the rate of chemical reactions on a local scale. Here we show how this can be accomplished in the case of H on Pd(111) through the application of local electric fields. Hydrogen adsorption on the platinum group metals is particularly interesting because these metals are used as catalysts in a variety of industrial processes, including hydrogenation and dehydrogenation reactions. Electric fields on surfaces are also of primary interest in electrochemistry where despite the considerable amount of experimental and theoretical work done to date, there still remains more work to be done before a clear understanding of phenomena at the atomic scale can be accomplished.
Date: May 10, 2007
Creator: Mitsui, T.; Fomin, E.; Ogletree, D.F.; Salmeron, M.; Nilekar,A.U. & Mavrikakis, M.
Partner: UNT Libraries Government Documents Department

Water growth on metals and oxides: binding, dissociation and role of hydroxyl groups

Description: The authors discuss the role of the presence of dangling H bonds from water or from surface hydroxyl species on the wetting behavior of surfaces. Using Scanning Tunneling and Atomic Force Microscopies, and Photoelectron Spectroscopy, they have examined a variety of surfaces, including mica, oxides, and pure metals. They find that in all cases, the availability of free, dangling H-bonds at the surface is crucial for the subsequent growth of wetting water films. In the case of mica electrostatic forces and H-bonding to surface O atoms determine the water orientation in the first layer and also in subsequent layers with a strong influence in its wetting characteristics. In the case of oxides like TiO{sub 2}, Cu{sub 2}O, SiO{sub 2} and Al{sub 2}O{sub 3}, surface hydroxyls form readily on defects upon exposure to water vapor and help nucleate the subsequent growth of molecular water films. On pure metals, such as Pt, Pd, and Ru, the structure of the first water layer and whether or not it exhibits dangling H bonds is again crucial. Dangling H-bonds are provided by molecules with their plane oriented vertically, or by OH groups formed by the partial dissociation of water. By tying the two II atoms of the water molecules into strong H-bonds with pre-adsorbed O on Ru can also quench the wettability of the surface.
Date: September 1, 2008
Creator: Salmeron, M.; Bluhm, H.; Tatarkhanov, M.; Ketteler, G.; Shimizu, T.K.; Mugarza, A. et al.
Partner: UNT Libraries Government Documents Department

The structure of mixed H2O-OH monolayer films on Ru(0001)

Description: Scanning tunneling microscopy (STM) and x-ray absorption spectroscopy (XAS) have been used to study the structures produced by water on Ru(0001) at temperatures above 140 K. It was found that while undissociated water layers are metastable below 140 K, heating above this temperature produces drastic transformations whereby a fraction of the water molecules partially dissociate and form mixed H{sub 2}O-OH structures. XPS and XAS revealed the presence of hydroxyl groups with their O-H bond essentially parallel to the surface. STM images show that the mixed H{sub 2}O-OH structures consist of long narrow stripes aligned with the three crystallographic directions perpendicular to the close-packed atomic rows of the Ru(0001) substrate. The internal structure of the stripes is a honeycomb network of H-bonded water and hydroxyl species. We found that the metastable low temperature molecular phase can also be converted to a mixed H{sub 2}O-OH phase through excitation by the tunneling electrons when their energy is 0.5 eV or higher above the Fermi level. Structural models based on the STM images were used for Density Functional Theory optimizations of the stripe geometry. The optimized geometry was then utilized to calculate STM images for comparison with the experiment.
Date: October 20, 2008
Creator: Tatarkhanov, M.; Fomin, E.; Salmeron, M.; Andersson, K.; Ogasawara, H.; Pettersson, L.G.M. et al.
Partner: UNT Libraries Government Documents Department

In-situ X-ray photoelectron spectroscopy studies of water on metals and oxides at ambient conditions

Description: X-ray photoelectron spectroscopy (XPS) is a powerful tool for surface and interface analysis, providing the elemental composition of surfaces and the local chemical environment of adsorbed species. Conventional XPS experiments have been limited to ultrahigh vacuum (UHV) conditions due to a short mean free path of electrons in a gas phase. The recent advances in instrumentation coupled with third-generation synchrotron radiation sources enables in-situ XPS measurements at pressures above 5 Torr. In this review, we describe the basic design of the ambient pressure XPS setup that combines differential pumping with an electrostatic focusing. We present examples of the application of in-situ XPS to studies of water adsorption on the surface of metals and oxides including Cu(110), Cu(111), TiO2(110) under environmental conditions of water vapor pressure. On all these surfaces we observe a general trend where hydroxyl groups form first, followed by molecular water adsorption. The importance of surface OH groups and their hydrogen bonding to water molecules in water adsorption on surfaces is discussed in detail.
Date: October 29, 2007
Creator: Salmeron, Miquel; Yamamoto, S.; Bluhm, H.; Andersson, K.; Ketteler, G.; Ogasawara, H. et al.
Partner: UNT Libraries Government Documents Department

Low-Dimensional Water on Ru(0001); Model System for X-ray Absorption Spectroscopy Studies of Liquid Water

Description: We present an x-ray absorption spectroscopy results for fully broken to a complete H-bond network of water molecules on Ru(0001) by varying the morphology from isolated water molecules via two-dimensional clusters to a fully covered monolayer as probed by scanning tunneling microscopy. The sensitivity of x-ray absorption to the symmetry of H-bonding is further elucidated for the amino (-NH{sub 2}) group in glycine adsorbed on Cu(110) where the E-vector is parallel either to the NH donating an H-bond or to the non-H-bonded NH. The results give further evidence for the interpretation of the various spectral features of liquid water and for the general applicability of x-ray absorption spectroscopy to analyze H-bonded systems.
Date: May 11, 2009
Creator: Nordlund, D.; Ogasawara, H.; Andersson, K.J.; Tatarkhanov, M.; Salmeron, M.; Pettersson, L. G. M. et al.
Partner: UNT Libraries Government Documents Department

Thin film synthesis using miniature pulsed metal vapor vacuum arc plasma guns

Description: Metallic coatings can be fabricated using the intense plasma generated by the metal vapor vacuum arc. We have made and tested an embodiment of vacuum arc plasma source that operates in a pulsed mode, thereby acquiring precise control over the plasma flux and so also over the deposition rate, and that is in the form of a miniature plasma gun, thereby allowing deposition of metallic thin films to be carried out in confined spaces and also allowing a number of such guns to be clustered together. The plasma is created at the cathode spots on the metallic cathode surface, and is highly ionized and of directed energy a few tens of electron volts. Adhesion of the film to the substrate is thus good. Virtually all of the solid metals of the Periodic Table can be used, including highly refractory metals like tantalum and tungsten. Films, including multilayer thin films, can be fabricated of thickness from Angstroms to microns. We have carried out preliminary experiments using several different versions of miniature, pulsed, metal vapor vacuum arc plasma guns to fabricate metallic thin films and multilayers. Here we describe the plasma guns and their operation in this application, and present examples of some of the thin film structures we have fabricated, including yttrium and platinum films of thicknesses from a few hundred Angstroms up to 1 micron and an yttrium-cobalt multilayer structure of layer thickness about 100 Angstroms. 33 refs., 5 figs.
Date: April 1, 1990
Creator: Godechot, X.; Salmeron, M.B.; Ogletree, D.F.; Galvin, J.E.; MacGill, R.A.; Dickinson, M.R. et al.
Partner: UNT Libraries Government Documents Department