Adam G. Balogh

Morphological evolution of au nanowires controlled by rayleigh instability

A sound knowledge and understanding of the thermal stability of nanowires is a prerequisite for the reliable implementation of nanowire-based devices. We investigate the morphology of Au nanowires annealed isothermally at different temperatures. During the processes, triggered by heating, the wires undergo various configurational changes to finally break up into chains of nanospheres at much lower than bulk melting temperatures due to capillary or so-called Rayleigh instability. The role of three parameters, namely, wire diameter, temperature, and annealing time, on the final morphology is investigated. Both the average sphere diameter and the mean spacing between adjacent spheres are larger than the values predicted for materials with isotropic surface energy. Possible reasons are discussed in the paper.

Instability of irradiation induced defects in nanostructured materials

Abstract The microstructural defect evolution in nanocrystalline Pd and ZrO2 during energetic particle irradiation is examined with emphasis on the correlation of defect density to the grain size. Nanocrystalline bulk Pd and ZrO2, synthesized by the inert gas condensation technique with different initial grain sizes, ranging from 10–300 nm were irradiated using 4 MeV Kr ions with fluences from 1 × 1015 to 2 × 1016 Kr/cm2. The defect density was studied by cross-sectional transmission electron microscopy (TEM). A correlation between grain size and defect density was found showing drastic reduction of defect clusters in the small grains below 50 nm. In the smallest ZrO2 (

Influence of crystallinity on the Rayleigh instability of gold nanowires

The influence of the crystalline structure of nanowires on their thermal instability has been systematically investigated. Both poly- and single-crystalline (SC) cylindrical nanowires with diameters 87 and 132 nm transform into chains of spheres during annealing at 600–700 °C. SC nanowires oriented along the 1 1 0 direction are found to be more stable, i.e. longer annealing times are needed for their complete transformation into sphere chains. Sphere size and spacing between adjacent spheres formed after decay are controlled by the crystallinity of the wires and both are larger in the case of SC nanowires.

Structural evolution of SnO2TiO2 nanocrystalline films for gas sensors

Thin films (50‐200 nm) of SnO2TiO2 were deposited on SiO2:(001)Si substrates by RF-sputtering and by molecular beam before they were annealed in vacuum at 200‐900°C. In-situ TEM, XRD, SEM, Raman and IR-spectroscopy were used to analyze the structure transformations in the SnO2TiO2 films. In the as-deposited state, the films are amorphous. They crystallize at higher temperatures (starting at about 500°C) forming nanosized grains. The problem of the spinodal decomposition in the SnO2TiO2 system observed earlier at high temperatures is discussed also for low-temperature processing. The stoichiometry of the films of both groups (reactive ion sputtered and high-vacuum e-gun sputtered) is being compared.

Phase stability of nanostructured materials under heavy ion irradiation

Abstract Nanocgstalline Pd and ZrO2 samples of theoretical density were irradiated with 4 MeV Kr+ions with doses up to 6.1017 ion/cm2. The irradiated samples were characterized by XRD, RBS, and TEM. Nanocrystalline ZrO2 shows no amorphisation and defect agglomeration after irradiation with Kr+ ions up to a dose of 2·1017 ions/cm2. A phase transformation from the monoclinic phase to a tetragonal phase could be observed. The comparison of irradiated n-Pd with a conventional polycrystalline Pd sample shows a reduced defect agglomeration in n-Pd. For n-Pd samples grain growth after irradiation experiments could be observed.

Oxygen vacancy kinetics in ferroelectric PbZr0.4Ti0.6O3

Oxygen vacancy kinetics in ferroelectric PbZr0.4Ti0.6O3 were studied by oxygen18 (O18) tracer self-diffusion in epitaxial thin films as well as bulk polycrystalline samples. O18 exchange annealing was carried out at an oxygen partial pressure of 250mbar and temperatures between 250 and 400°C. Isotope depth profiling was performed by secondary ion mass spectrometry as well as neutral secondary mass spectrometry. The observed concentration depth profiles of the polycrystalline samples show two distinct diffusion paths, namely, bulk diffusion and grain boundary (GB) diffusion. It appears to be of type B-kinetics in the investigated temperature range, with DGB∕Dbulk⪢100. Donor doped samples with different levels of Nb5+ (1–4mol.%) were also investigated. The effect on the diffusion depth profiles, however, were negligible and can solely be attributed due to the change in the samples microstructure as induced by the dopants. A diffusion coefficient for the bulk diffusion of the O18 isotope, Dbulk=10±5×10−8cm2∕...

Room-temperature growth of ZrO2 thin films using a novel hyperthermal oxygen-atom source

Thin ZrO2 films have been grown on Si(100) and on glassy carbon substrates using a novel atomic oxygen source in a standard molecular beam epitaxy system. The oxygen source produces a flux of hyperthermal oxygen atoms with an ion/atom-ratio ≪0.001 through electron stimulated desorption from a Ag alloy surface at an operating pressure <10−8 Torr. The films were grown at room temperature and analyzed using Rutherford backscattering spectroscopy, x-ray diffraction (XRD), x-ray photoelectron spectroscopy and transmission electron microscopy (TEM). The results show the successful growth of fully stoichiometric ZrO2 films on nonheated Si(100) and on amorphous glassy carbon substrates at a rate of 0.58 μm/hr. The XRD and TEM investigations indicate the formation of a mixed amorphous/orthorhombic film structure. Based on the film growth rate, the O flux produced by the electron stimulated desorption atom source is estimated to be 8×1014 atoms/cm2 s. This flux value is consistent with other determinations using io...

Ion beam mixing and radiation enhanced diffusion in metal/ceramic interfaces

Abstract Ion beam techniques are frequently used to modify the physical properties of materials. It is the aim of this contribution to obtain information on ion beam effects on irradiated metal/ceramic interfaces with bilayer geometry. Ion beam mixing and radiation enhanced diffusion have been investigated in Cu Al 2 O 3 , Au Al 2 O 3 and Au ZrO 2 samples. Specimen, with thicknesses of the metallic film in the range of 60–70 nm, were prepared by vapor deposition and irradiated with 150 keV Ar+ ions in the range of 0.9 × 1016 to 1.5 × 1017 Ar+/cm2. Sample temperature during irradiation was varied between 77 K and 673 K. The mixing behaviour was analysed using concentration depth profiles measured by Rutherford Backscattering Spectroscopy (RBS). The results show that mixing efficiencies for all elements scale linear by the Ar+ ion dose. Radiation enhanced diffusion is separated from temperature independent mixing processes. High resolution scanning electron microscopy (HREM) showed strong surface deterioration for the Au Al 2 O 3 and Au ZrO 2 samples. X-Ray Photoelectron Spectroscopy (XPS) analysis in the particular case of the Cu Al 2 O 3 interface was performed.

Nanostructured ceramics synthesized by chemical vapor condensation

Abstract A modification of the conventional inert gas condensation apparatus for making nanostructured powders, wherein an evaporative source is replaced by a chemical source, is described. This process has been used to synthesize loosely agglomerated amorphous nanoparticles of n-SiCxNyOz, starting from hexamethyldisilazane (HMDS) as precursor compound. The density, surface area, particle size, and composition of as-synthesized n-SiCxNyOz powders can be modified by changing the synthesis temperature and carrier gas. The phase and morphology of as-synthesized powders can also be modified by heat treatment.

Atomic transport in metal/ceramic interfaces under heavy ion irradiation

Abstract Ion beam mixing is a useful tool to modify the physical properties of interfaces in different materials. Metal/metal systems were extensively studied in the last decade. In the last few years research has been focused on the technologically more important metal/ceramic systems. In these systems, however, there is only limited knowledge on stability and diffusion processes under heavy ion irradiation. In order to collect more information about physical processes, which could be important for applications and in tailoring of interfaces, systematic ion beam mixing experiments on several bi-layer samples have been performed. Different oxide-ceramic substrates (MgO, Al2O3, SiO2, ZrO2) were covered by thin metallic films (Fe, Cu, Ni, Au, Ag) using a MBE set-up. Ion beam mixing experiments were performed at three different energies (150 keV, 4 and 12 MeV) with Ar-, Kr-and Xe-ions at different temperatures (77–673 K). Mixing behaviour was studied mainly by RBS measurements after the irradiation. The structure and topography of the films prior to and after irradiation were investigated by high resolution scanning electron microscopy (HRSEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). It was of special interest to determine the enhancement of the mixing rate compared to the ballistic mixing. This enhancement can be explained by thermal spikes (temperature independent effect) or by mobile defects produced by the heavy ions during irradiation and/or by chemical driving forces (temperature dependent effects).