Interactions of metal ions with monolayers and with Langmuir-Blodgett films

Summary Oriented systems such as monolayers and Langmuir-Blodgett multilayers constitute supramolecular structures appropriate for ion detection. In fact, they are sensitive to small quantities of ions since they have specifically oriented ligand head groups that can easily combine with the ions to be detected in solution. We investigated monolayers of dioctadecyldithiocarbamate (DODTC) specificially synthesized and we studied the interactions of the monolayers with transition metal ions such as Ni²⁺ dissolved in the subphase. The spreading isotherms, the surface potential-area isotherms and the spectroscopic measurements (UV-Vis absorption and ESCA spectroscopy) show that the metal ions can combine with the DODTC monolayers. The present findings show that it is possible to employ these systems to build chemical sensors for transition metal ions Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Theoretical description of the adsorption of transition metal ions on metal surfaces with the formation of submonolayer ferromagnetic films

The influence of the temperature and ferromagnetic ordering on the adsorption of Fe, Co, and Ni transition metal ions on a copper paramagnetic substrate with the formation of a submonolayer film is described in terms of the spin-density functional theory for the first time. The conditions for the formation of magnetic monoatomic films stable with respect to island adsorption with a change in the coverage parameter Θ are revealed. Four model cases of the electron density distribution as a function of the equilibrium distance between the film and the substrate and the equilibrium thickness of the adsorption layer are considered. It is demonstrated that the inclusion of the ferromagnetic ordering substantially affects the adsorption energy and leads to its considerable increase.     

Structure and stability of clusters on metal surfaces

Small clusters of 3d metals Ni/Ni(001), Cu/Cu(001), 4d-Pd/Pd(001), Ag/Ag(001), 5d-Pt/Pt(001), and Au/Au(001) are investigated by semiempirical methods using multiparticle interatomic interaction potentials. It is shown that the same magic numbers (4, 6, and 9) are characteristic for all metals indicated; these numbers are determined by the symmetry characteristics of the clusters, related to the morphology of the fcc (001) substrate. It is shown for Pt/Pt(111) that small clusters of seven, ten, and more atoms are stable for the fcc (111) surface. This confirms that the magic numbers are associated with the symmetry of the clusters. Fiz. Tverd. Tela (St. Petersburg) 41, 1329–1334 (July 1999)     

Dynamics and stability of nanostructures on metal surfaces

Ultrathin metal films consisting of two-dimensional clusters are typically unstable: the cluster ensemble has the tendency to reduce its total free energy via Ostwald ripening or dynamic coalescence of mobile clusters. In this paper we give an overview of recent model experiments addressing these coarsening mechanisms. The experiments have been performed using STM on ensembles consisting of adatom or vacancy clusters with typical diameters in the nanometer range on fcc(111)-metal surfaces. Agreement with and deviations from conventional theories are discussed. Received: 29 March 1999 / Accepted: 17 August 1999 / Published online: 30 September 1999     

Deposition of latex particles encapsulated in polyelectrolyte shells at heterogeneous metal surfaces modified by multilayer films

We used an oblique impinging jet (OIJ) cell to determine the initial deposition rate for the microcapsules deposited on the heterogeneous metal surfaces bare or modified by polyelectrolyte (PE) films. The dependence of reduced particle flux on the Reynolds number of the flow in the OIJ cell was determined by direct counting of particles deposited on the studied surfaces. We used fluorescently labelled latex particles and microcapsules built on these fluorescent cores and the use of fluorescent microscope allowed us to observe “in situ” the deposition processes of particles on rough, highly reflective surfaces. We demonstrated that modification of metallic surfaces of various materials and heterogeneity by the multilayer PE films result in the formation of uniformly charged film of nanometers thickness. The formation of such a film leads to the increase of deposition efficiency and its initial rate is governed by the charge of the film covered surface and the outermost layer of the capsule shell being in agreement with the prediction of the convective-diffusion theory.     

Accommodation coefficients for low-energy ions on metal surfaces

The interaction of low-energy ions (E = 3 to 100 eV) with the surface of a solid cannot be treated in terms of gas dynamics. The scattering of particles at an energy of E _o > 20 eV may be explained in terms of binary collisions with the atoms of the target. The validity of the single collision model with free surface atoms for medium energies from 10 to 100 eV and even down to 1 eV was confirmed on the basis of both experimental and computational data. This paper describes experimental studies of the secondary ion emission from the (100) and (110) faces of a Mo single crystal and both experimental and theoretical studies of alkaline ion accommodation coefficient on polycrystalline Mo within the energy range E = 3 to 50 eV with a varying direction of the primary ion beam from normal to the glancing angle of incidence (ρ = 0 to 75°). On the basis of the retarding potential method using a spherical condenser whose central electrode was the target, we measured the energy distribution of secondary ions. Calculations have been performed for the energy of scattered ions and the high energy portion of accommodation coefficient on the basis of single and double binary collisions using the Born-Mayer potential and taking into consideration the influence of adsorption forces at the surface of the target.     

Limits on stability of positive molecular ions in a homogeneous magnetic field

The problem of stability of positive diatomic molecular ions with the nuclear chargesZ ₁ andZ ₂ andN electrons in a homogeneous magnetic fieldB is studied forZ ₁,Z ₂,N,B. The conditions of instability are obtained for different relations amongZ ₁,Z ₂,N andB. A new version of the HVZ theorem for systems in a homogeneous magnetic field is proved.Supported by the International Erwin Schrödinger Institute, Austria, International Science Foundation Grant N R 94000 and Grant of Russian Fond Fudament. Issled. 94-01-01376.     

Role of various LB parameters on the optical characteristics of mixed Langmuir-Blodgett films

This communication reports the spectroscopic characterizations of mixed Langmuir-Blodgett (LB) films of non-amphiphilic N,N-bis (2,5-di-tert-butylphenyl)- 3,4,9-perylenedicarboximide (DBPI) molecules, mixed with polymethyl methacrylate (PMMA) and stearic acid (SA). J- aggregates of DBPI molecules in the mixed LB films have been confirmed by UV-Vis absorption spectroscopic study. Formation of organized structure of molecular stacking in the mixed LB films gives rise to the strong excimeric emission, which is manifested by a broad structureless band in the longer wavelength region of the fluorescence spectra and is confirmed by excitation spectroscopic study. A weak hump at around 576 nm due to monomeric emission is observed in the fluorescence spectra of 0.1 M of DBPI-PMMA mixed LB films of lower number of layers. The intensity of the 0-0 band at 530 nm in the fluorescence spectra is observed to be a function of the molefraction, number of layers, surface pressure of lifting and the matrix materials.     

Electron microscopy of growth surfaces on epitaxial films

It has been found that the final stage in the growth of an epitaxial film is decoration of the surface; to obtain objective information on the growth relief, it is necessary to examine at least two successive carbon replicas.     

Synchrotron scanning photoemission microscopy of homogeneous and heterogeneous metal sulfide minerals

Scanning photoemission microscopy (SPEM) has been applied to the investigation of homogeneous and heterogeneous metal sulfide mineral surfaces. Three mineral samples were investigated: homogeneous chalcopyrite, heterogeneous chalcopyrite with bornite, and heterogeneous chalcopyrite with pyrite. Sulfur, copper and iron SPEM images, i.e. surface‐selective elemental maps with high spatial resolution acquired using the signal from the S 2p and Cu and Fe 3p photoemission peaks, were obtained for the surfaces after exposure to different oxidation conditions (either exposed to air or oxidized in pH 9 solution), in addition to high‐resolution photoemission spectra from individual pixel areas of the images. Investigation of the homogeneous chalcopyrite sample allowed for the identification of step edges using the topography SPEM image, and high‐resolution S 2p spectra acquired from the different parts of the sample image revealed a similar rate of surface oxidation from solution exposure for both step edge and a nearby terrace site. SPEM was able to successfully distinguish between chalcopyrite and bornite on the heterogeneous sample containing both minerals, based upon sulfur imaging. The high‐resolution S 2p spectra acquired from the two regions highlighted the faster air oxidation of the bornite relative to the chalcopyrite. Differentiation between chalcopyrite and pyrite based upon contrast in SPEM images was not successful, owing to either the poor photoionization cross section of the Cu and Fe 3p electrons or issues with rough fracture of the composite surface. In spite of this, high‐resolution S 2p spectra from each mineral phase were successfully obtained using a step‐scan approach.     

AFM studies on Langmuir-Blodgett films of cholesterol

The Langmuir monolayer of cholesterol at the air-water interface exhibits a condensed phase in which the cholesterol molecules are aligned normal to the water surface. We have transferred the monolayer from water surface to different substrates by Langmuir-Blodgett (LB) technique and have studied their assembly by atomic force microscope (AFM). Our studies reveal that the aggregation of cholesterol molecules on hydrophobic surfaces leads to interesting structures. The cholesterol molecules assemble into a uniform film, elongated domains and uniformly distributed torus-shaped domains (doughnuts) for one, two and four cycles of deposition, respectively. Beyond four cycles, the molecules adsorb and desorb by an equal amount resulting in no further deposition. The formation of uniformly distributed doughnuts can be attributed to the hydrophobic interaction and reorganization of the molecules due to successive adsorption and desorption during deposition cycles. Our studies on hydrophilic surfaces show that cholesterol cannot form more than one layer of deposition.PACS: 68.47.Pe Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces - 68.37.Ps Atomic force microscopy (AFM) - 68.43.Mn Adsorption/desorption kinetics     

Temperature coefficient of resistance of thin metal films on ceramic surfaces

The temperature coefficients (TCR) of vacuum-deposited thin-metal-film resistors were measured in vacuum, air, argon, and silicone oil, and different characteristics were obtained. The resistors were made by evaporating a nickel-chrome alloy (Nichrome 80 to 20) in vacuum onto ceramic substrates to produce a 500-ohms-persquare film. Measurements were made of films which were uncoated and of films which were coated with a protection layer of silicone monoxide in thicknesses from 5000 to 10000 å. When measured under vacuum, the resistors displayed a TCR which was lower than that obtained in air, argon, or silicone oil. In an argon environment, the resistors had lower TCR characteristics than they had in air or silicone oil. Even when the resistor films were protected by a 10000-å-thick layer of silicone monoxide, their TCR behavior differed in different gas environments. Most of the TCR's measured in the temperature range from ?55Ω to +150Ω C were between 5 and 50 parts per million per degree centigrade.     

Ultra-smooth metal surfaces generated by pressure-induced surface deformation of thin metal films

We present a mechanical pressing technique for generating ultra-smooth surfaces on thin metal films by flattening the bumps, asperities, rough grains and spikes of a freshly vacuum deposited metal film. The method was implemented by varying the applied pressure from 100 MPa to 600 MPa on an e-beam evaporated silver film of thickness 1000 Å deposited on double-polished (100)-oriented silicon surfaces, resulting in a varying degree of film smoothness. The surface morphology of the thin film was studied using atomic force microscopy. Notably, at a pressure of ～600 MPa an initial silver surface with 13-nm RMS roughness was plastically deformed and transformed to an ultra-flat plane with better than 0.1 nm RMS. Our demonstration with the e-beam evaporated silver thin film exhibits the potential for applications in decreasing the scattering-induced losses in optical metamaterials, plasmonic nanodevices and electrical shorts in molecular-scale electronic devices.     

Growth of thin, crystalline oxide, nitride and oxynitride films on metal and metal alloy surfaces

This paper reviews the current knowledge of the properties of ultrathin, well-ordered oxide, nitride and oxynitride films grown on metal and metal alloy surfaces. Different modes of preparation are discussed and the vibrational and structural properties are summarized. The focus will be put onto Al-oxides grown on surfaces of the intermetallic alloys NiAl, Ni₃Al and FeAl. The properties of Ga₂O₃ grown on surfaces of the intermetallic alloy CoGa are also described. In these cases Al- and Ga-atoms, respectively, segregate from the substrate and react with adsorbed oxygen. The grown Al- and Ga-oxide films, respectively, order at elevated temperatures. Systems are also discussed where Al-oxide is grown by oxidation of Al-atoms which are evaporated on surfaces of the transition metals Re and Ru. The growth of transition metal oxides CoO (1 1 1)/Co (0 0 0 1), CoO (1 0 0)/Co (1 1 2 0), NiO (1 0 0)/Ni (1 0 0), NiO/Ni (1 1 1) and Cr₂O₃/Cr (1 1 0) are also presented. Thin films of Al- and Ga-nitride, respectively, can be grown on the base of the intermetallic alloy NiAl and CoGa by low-temperature adsorption of ammonia. These nitride films order at elevated temperatures. Al- and Ga-oxynitride, respectively, can be grown on surfaces of NiAl and CoGa substrates by adsorption on nitric oxide. An ordering of these ultrathin oxynitride layers is observed at elevated temperatures.     

Cobalt growth on two related close-packed noble metal surfaces

We report on scanning tunneling microscopy (STM) studies of submonolayer growth of cobalt on the close-packed (1 1 1) surfaces of Au and Ag. Both substrates belong to the category of noble metals, and they both exhibit a lattice misfit of ∼13% with respect to the (0 0 0 1) plane of Co. However, whereas the Au(1 1 1) surface reconstructs into the rather complex herringbone structure that disperses the cobalt into nanoclusters, the Ag(1 1 1) surface does not reconstruct in its clean state, and the surface dispersion of Co on this surface is therefore different. For Ag(1 1 1) at temperatures ranging from 160 to 200 K and for Au(1 1 1) at room temperature, the Co growth is three-dimensional starting with double layer islands followed by additional single layers. For both the Co/Au(1 1 1) and the Co/Ag(1 1 1) system, a Moiré pattern develops in the first bilayer of the Co islands, indicating an epitaxial but not commensurate growth. For Co islands with more than two layers, the subsequent layers are commensurate with the lower Co layers in the islands, but exhibit a decreasing corrugation of the Moiré pattern as observed in STM images. Despite a difference in the Moiré lattice constant and rotational angle, we show that the cobalt lattice constant is the same on both surfaces. We furthermore relate defect nucleation on the herringbone reconstruction on Au(1 1 1) to defect nucleation on steps on Ag(1 1 1).     

Ice films and coatings on metal surfaces: Existing concepts and unsolved problems

Basic theoretical problems concerning the natural formation of an ice coating on a metal surface are discussed. The main types of ice coatings and continuous models of their formation are presented. The results of computational and theoretical studies of different icing modes are described.