O. Geszti

Electronic structure of gold nanoparticles deposited on SiOx/Si(100)

Nanosize gold particles were prepared by Ar+ ion sputtering of island-like 10-nm-thick film deposited onto Si/SiOx substrate. The valence band of the gold particles was measured by means of photoelectron spectroscopy and infrared (IR) absorbance. The size of the particles was determined by transmission electron microscopy (TEM). The valence band of Au nanoparticles is strongly redistributed with decreasing size, involving mostly the lowest and the highest binding energy part of the Au 5d valence states. This effect can be attributed more to the redistribution than to the narrowing of the 5d states.

Silica-supported Au nanoparticles decorated by TiO2 : Formation, morphology, and CO oxidation activity

Au-TiO(2) interface on silica support was aimed to be produced in a controlled way by use of Au hydrosol. In method A, the Au colloids were modified by hydrolysis of the water-soluble Ti(IV) bis(ammoniumlactato)dihydroxide (TALH) precursor and then adsorbed on Aerosil SiO(2) surface. In method B, Au sol was first deposited onto the SiO(2) surface and then TALH was adsorbed on it. Regular and high-resolution transmission electron microscopy (TEM and HRTEM) and energy dispersive spectrometry (EDS) analysis allowed us to conclude that, in method A, gold particles were able to retain the precursor of TiO(2) at 1.5 wt % TiO(2) loading, but at 4 wt % TiO(2) content the promoter oxide appeared over the silica surface as well. With method B, titania was detected on silica at each TiO(2) concentration. In Au-TiO(2)/SiO(2) samples, the stability of Au particles against sintering was much higher than in Au/TiO(2). The formation of an active Au-TiO(2) perimeter was proven by the greatly increased CO oxidation activity compared to that of the reference Au/SiO(2).

TEM study of Ag2Se developed by the reaction of polycrystalline silver films and selenium

The formation of Ag 2 Se phase has been studied in the case of high-temperature successive deposition of polycrystalline silver thin films and selenium. It is the first time that the formation of very large Ag 2 Se single crystals of (001) orientation on polycrystalline Ag films are observed and discussed. Continuous 35 nm thick silver films prepared on amorphous substrates have been reacted with various amounts of selenium at different substrate temperatures above and below the bulk polymorphic phase transition temperature of Ag 2 Se (T c = 133°C). All structural investigations were carried out on films transformed to or formed directly in the low temperature orthorhombic phase. TEM investigations revealed that the continuous Ag 2 Se layers undergoing phase transformation during cooling to room temperature consisted of large (100-300 μm) (001) oriented orthorhombic crystals of various azimuthal orientations. No preferred orientation was found in discontinuous Ag 2 Se films. Without phase transformation the orientation remained random with grain sizes below 1 μm. Strong thickness fluctuations showing up in the form of island-channel-like morphology within the Ag 2 Se crystallites has been observed in all continuous layers. The formation of large (001) oriented single crystals is proposed to be related to the polymorphic phase transition from body-centred cubic to orthorhombic phase taking place during the cooling of the film after sample preparation.

TEM study of the structure and morphology of AgI crystals formed on Ag (001), (011) and (111) thin films

Abstract The formation, structure and morphology of thin AgI crystals have been studied by depositing differently oriented single crystalline silver films and iodine vapour successively. Transmission electron microscope studies revealed the formation of oriented hexagonal (low temperature) β-AgI having the c axis parallel to the [001] Ag , [011] Ag and [111] Ag directions on (001), (011) and (111) Ag layers, respectively. The two possible {112¯0} AgI ∥ {100} Ag mutual orientations face at an angle of 90 °, while single positions of {101¯0} AgI ∥ {100} Ag and found on (011) and (111) oriented silver films, respectively. Orientational relationships between Ag films of different orientations and AgI have been discussed by a model representing the atomic arrangements at the Ag film/AgI interface. Iodide particles formed at 80 °C have irregular shape, while particles grown at 200 °C are mostly regularly shaped with well developed facets. No difference in structure and orientation of AgI was found between samples formed below or above the temperature of phase transition (147 °C). The low-temperature cubic (γ) phase was identified in samples iodized at low temperature while in samples iodized at high temperature the (γ)-AgI could be neither proved nor excluded because of the fast decomposition of the AgI crystals under the electron beam.

Structure and spectroscopic properties of C–Ni and CNx–Ni nanocomposite films

Magnetron-sputtered carbon-nickel and carbon-nitride-nickel thin films were investigated by high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy, electron-energy-loss spectroscopy (EELS), and scanning tunneling microscopy∕spectroscopy (STM∕STS) to clarify the relationship between the structure and the spectroscopic properties. The films were deposited in argon or nitrogen plasma at temperatures from 25to800°C. The microstructures can be described as a nanocomposite, built from Ni or Ni3C nanocrystals in a carbon∕CNx matrix. The nanocomposite shows columnar structure up to 200°C deposition temperature, and above this temperature gradually changes to globular morphology. The C∕CNx matrix also grows thicker as the deposition temperature increases. The matrix is amorphous at high nitrogen content and low deposition temperature and it is ordered into 3–5 layer thick graphitic shells around the Ni3C or Ni crystallites at higher temperatures. The surface roughness of the...

The role of chlorine adsorption in the low temperature epitaxial growth of silver, gold and copper on water-treated NaCl

Abstract Chlorine adsorbed on either air-cleaved or water-treated NaCl surfaces prior to metal deposition proved to be active as a surfactant promoting the epitaxial growth of silver, gold and copper at low temperatures (80–120 °C), while no epitaxial orientation of aluminium layers was found under these conditions. Treatment of surfaces of Kyropoulos-grown NaCl with bidistilled water resulted in no significant improvement in the orientation of vacuum-deposited silver, gold and copper, but after exposing the water-treated surfaces to chlorine gas prior to evacuation, epitaxial orientation of the vacuum-deposited metals was found. Changes in the nucleation density of the metals and in the cleavage step structure on NaCl due to different treatments were studied by surface decoration. A correlation between the presence of active chlorine on the NaCl surface and the epitaxial temperature of deposits has been proposed.

Aluminium induced crystallization of amorphous germanium

Crystallization morphologies and kinetic processes have been studied in amorphous germanium (a-Ge) films in contact with aluminium (Al) layers of various thickness. The well-known spherulitic crystallization has been observed in a-Ge/Al/a-Ge trilayers with Al layer thickness above 10 nm. The kinetics of this growth was followed by electrical conductivity measurement of the a-Ge/Al bilayer. In the case of the thinnest Al layer (below 4 nm) fibre-like Ge crystal growth was found. The measurement results suggest that the crystallization of the amorphous Ge layers was controlled by diffusion processes.

Structure of amorphous carbon-nitride thin films

Abstract CN x thin films were prepared in high vacuum by evaporation of C from a pair of graphite rods in a DC arc ignited in N 2 at pressures of up to 1 mbar onto NaCl, Highly ordered pyrolytic graphite (HOPG) and Si substrates. The substrate temperature varied between room temperature and 800 °C. The effect of deposition temperature and N 2 gas pressure on the morphology, structure, composition and electrical properties of CN x thin films was studied by high-resolution transmission electron microscopy (HRTEM), X-ray microanalysis (EDS), and scanning tunneling microscopy and spectroscopy (STM and STS). Morphology ranging from homogeneous layers through spherical or cylindrical particles embedded into the films to low-density globular deposits of CN x was observed as a function of the applied N 2 gas pressure. The N composition found was between 1 and 20 at.% and the structure varied from amorphous through fullerene-like to nanocrystalline diamond composed of amorphous CN x , depending on the temperature and the plasma parameters. The behavior of the films during heat treatment was recorded by electrical conductivity measurements. The activation energy of electrical conductivity was found to be 0.18 eV for carbon and 0.38 eV for CN x films and was independent of the deposition temperature.

Investigation of Noble Metal Nanoparticles (Ag, Au, Pd, Pt) Produced by Chemical Reduction

This work describes atomic-scale, crystalline structure and size distribution for noble metal nanoparticles produced by water-based, environmental friendly technologies. The process was developed and implemented to produce noble metal nanoparticles to be used in water filters, sensors and cosmetics. The particles were investigated by TEM methods and particle size analysis. Growth process of the crystallites in sols made by chemical reduction in aqueous solutions is discussed. Comparison with growth models for vacuum deposited thin films is also identified.

Transmission electron microscope study of the topotactic reaction of (0 0 1), (0 1 1) and (1 1 1) Ag films and Te

The formation, structure and morphology of silver telluride was investigated in the reaction of ( 001 ), ( 011 ) and ( 111 ) single crystalline Ag films with vacuum deposited Te. Silver films 30-40 nm in thickness were deposited by thermal evaporation onto water- and chlorine-treated NaCl. Onto this silver 1-40 nm of tellurium were deposited at 100 and 200 8C. The Ag-Te reaction occurred during Te deposition. Accordingly, formation of the compound phase was investigated from the nucleation stage through complete tellurization on either side of the polymorphic phase transformation temperature (T s150 8C). Transmission c electron microscope and selected area electron diffraction showed that monoclinic silver telluride (Ag Te) of different morphology 2 and texture was always formed. The orientation of silver and monoclinic phase upon differently oriented monocrystalline Ag films and at deposition temperatures around T is discussed. c 2003 Elsevier Science B.V. All rights reserved.