Jouko Lahtinen

Enhanced Magnetic and Dielectric properties of Eu and Co co-doped BiFeO3 nanoparticles

Bi1−xEuxFe1−yCoyO3 (x = 0, 0.01; y = 0, 0.01) nanoparticles, having an average size of 13 nm, were prepared by a simple sol gel route. Strong electronegativity of Eu3+ and smaller oxidation-reduction potential of Co3+/Co2+ (0.55 eV) than Fe3+/Fe2+ (1.3 eV) increase the concentration of Fe3+ ions with doping. Distinct magnetic hysteresis and complete saturation of magnetisation indicate the presence of ferromagnetic phase. The successful co-doping of Eu and Co into BiFeO3 (BFO) lattice dramatically enhances the saturation magnetization (Ms) and coercivity (Hc) by about 20 times than that of pure BiFeO3. A large value of dielectric constant of about 650, low loss (<0.001), and small leakage current density (1.79 × 10−8 A/cm2) are observed for the co-doped sample.

Towards an accurate description of the capillary force in nanoparticle-surface interactions

We present a method to numerically calculate the exact (non-circular) meniscus profile from the Kelvin equation, and compare the results of the obtained capillary force with different previous approximations and experiments. We show that a circular meniscus profile gives correct results in most cases. We also compare different models of pull-off behaviour and show that the often used approximation of humidity independent capillary force is viable for spherical particles above 1 µm radius, but below that there is a strong humidity dependence, as seen in experiments. At the same length scale the direct surface tension force component becomes important. We also discuss the vanishing of the capillary force at very low humidity, the effect of small initial separation between the particle and the surface and the effects of different particle shapes and contact angles on the capillary force. Finally, calculated results are compared with experimental measurements of the capillary force. (Some figures in this article are in colour only in the electronic version)

Hybrid Colloidal Au-CdSe Pentapod Heterostructures Synthesis and their Photocatalytic Properties

In this report, we present a self-driven chemical process to design exclusive Au/CdSe pentapod heterostructures with Au core and CdSe arms. We have analyzed these heterostructures using high-resolution transmission electron microscope (HRTEM), high angle annular dark field-scanning transmission electron microscopic (HAADF-STEM), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS) studies. Microscopic studies suggest that pentapod arms of CdSe are nucleated on the (111) facets of Au and linearly grown only along the [001] direction. From the XPS study, the shifting of peak positions in the higher binding energy region for Au/CdSe heterostructures compared to Au nanoparticles has been found which indicates the charge transfer from CdSe to Au in heterostructures. The steady state and time resolved spectroscopic studies unambiguously confirm the electron transfer from photoexcited CdSe to Au, and the rate of electron transfer is found to be 3.58×10⁸ s⁻¹. It is interesting to note that 87.2% of R6G dye is degraded by the Au/CdSe heterostructures after 150 min UV irradiation, and the apparent rate constant for Au/CdSe heterostructures is found to be 0.013 min⁻¹. This new class of metal-semiconductor heterostructures opens up new possibilities in photocatalytic, solar energy conversion, photovoltaic, and other new emerging applications.

Pull-off test in the assessment of adhesion at printed wiring board metallisation/epoxy interface

It is shown that the value of adhesion strength at the metallisation/polymer interface can be determined directly from a printed wiring board (PWB) by a pull-off method. However, careful optimisation of the test geometry is required. In particular, the mechanical properties of the substrate are important. The use of flexible substrate in the test results in severe underestimation of the adhesion strength. There was a considerable stress peak at the edge of the test area, as shown by a finite element method (FEM) calculation with the flexible construction. This unevenly distributed stress decreases the force needed to detach the coating. In addition, if the test area is not defined to match the stud of the test equipment, the stress peak appears at the adhesive/coating interface rather than at the coating/metallisation interface. The experimental data and the examination of the fracture surfaces by scanning electron microscope (SEM) were consistent with the FEM results. In addition, the effect of environmental stresses on the adhesion was investigated. The environmental tests consisted of exposures to (1) thermal shock, (2) elevated temperature and relative humidity and (3) corrosive gases NO2, SO2, H2S and Cl2. The employment of embedded capacitors during the exposures revealed the diffusion of water into the epoxy, which weakened the adhesion. The corrosive gases induced irreversible deterioration of the dielectric material. An adhesion promoter enhanced the durability of the coating/Cu interface in the environmental tests. X-ray photoelectron spectroscopy was used to analyse the chemical states of the pre-treated copper surfaces.

Thermal annealing of Si/SiO2 materials: Modification of structural and photoluminescence emission properties

We comparatively study two conventional types of Si/SiO2 materials, Si/SiO2 superlattices (SLs) and Si-rich silica (SiOx) films, prepared with a molecular beam deposition method. Raman scattering, photoluminescence (PL), ultraviolet-visible-infrared absorption, and x-ray photoelectron spectroscopies are employed to characterize the samples. The results show clear parallelism in microstructure and emitting properties of Si/SiO2 SLs and SiOx films. The as-grown material is amorphous, and disordered Si areas are seen in Raman spectra for samples with higher Si contents. Annealing at 1150 °C in nitrogen atmosphere leads to ordering of the Si grains and the typical crystalline size is estimated to be 3–4 nm. For all samples, an annealing-induced increase of PL at ∼1.6 eV is observed, and its resulting position is quite independent of the initial sample architecture. Furthermore, this PL is practically identical for continuous wave and pulsed excitation at 488 nm as well as for pulsed excitation at various wave...

Hydrogenation of CO2, acetone, and CO on a Rh foil promoted by titania overlayers

The effects of submonolayer deposits of titania on the hydrogenation of CO2, acetone, and CO on a Rh foil have been investigated. Titania has been found to promote all three of the hydrogenation reactions, with each reaction exhibiting a maximum rate at a titania coverage of 0.5 ML. The maximum rate for CO2 hydrogenation is 15 times that of the bare Rh surface. Acetone hydrogenation shows a 6-fold rate enhancement, while CO displays a 3-fold increase. Changes in the selectivities for each reaction are also observed upon titania promotion. The effects of titania on these reactions are attributed to an interaction between C-O bonds and Ti3+ ions located at the perimeter of titania islands.

C, CO and CO2 hydrogenation on cobalt foil model catalysts: evidence for the need of CoO reduction

The hydrogenation of C, CO, and CO2 has been studied on polycrystalline cobalt foils using a combination of UHV studies and atmospheric pressure reactions in temperature range from 475 to 575 K at 101 kPa total pressure. The reactions produce mainly methane but with selectivities of 98, 80, and 99 wt% at 525 K for C, CO, and CO2, respectively. In the C and CO2 hydrogenation the rest is ethane, whereas in CO hydrogenation hydrocarbons up to C4 were detected. The activation energies of methane formation are 57, 86, and 158 kJ/mol from C, CO, and CO2, respectively. The partial pressure dependencies of the CO and CO2 hydrogenation indicate roughly first order dependence on hydrogen pressure (1.5 and 0.9), negative first order on CO (−0.75) and zero order on CO2 (−0.05). Post reaction spectroscopy revealed carbon deposition from CO and oxygen deposition from CO2 on the surface above 540 K. The reduction of cobalt oxide formed after dissociation of C-O bonds on the surface is proposed to be the rate limiting step in CO and CO2 hydrogenation.

Effect of growth temperature on the electronic energy band and crystal structure of ZnS thin films grown using atomic layer epitaxy

Electroreflectance (ER) technique has been used to study the effect of growth temperature and source materials on the electronic energy band and crystal structure of zinc sulfide thin films grown using atomic layer epitaxy (ALE). The samples studied were grown by ALE at 300–500 °C from zinc acetate or zinc chloride and hydrogen sulfide. The ER spectra were measured at room temperature and at 77 K. The crystal structure of polycrystalline ZnS thin films grown from zinc acetate at 300–375 °C is mainly cubic, whereas the ZnS layers grown from zinc chloride at 425–500 °C are predominantly hexagonal. The ER spectra of the latter samples show peaks belonging to the cubic structure, too. The crystal structure of zinc sulfide thin films is found to depend both on growth temperature and on source materials.

Topographic and electronic contrast of the graphene moire on Ir(111) probed by scanning tunneling microscopy and noncontact atomic force microscopy

Epitaxial graphene grown on transition-metal surfaces typically exhibits a moir´e pattern due to the lattice mismatch between graphene and the underlying metal surface. We use both scanning tunneling microscopy (STM) and atomic force microscopy (AFM) to probe the electronic and topographic contrast of the graphene moir´e on the Ir(111) surface. STM topography is influenced by the local density of states close to the Fermi energy and the local tunneling barrier height. Based on our AFM experiments, we observe a moir´e corrugation of 35 ± 10 pm, where the graphene-Ir(111) distance is the smallest in the areas where the graphene honeycomb is atop the underlying iridium atoms and larger on the fcc or hcp threefold hollow sites.

Preparation and characterization of Co/SiO2, Co-Mg/SiO2 and Mg-Co/SiO2 catalysts and their activity in CO hydrogenation.

Co/SiO2, Mg-Co/SiO2 and Co-Mg/SiO2 catalysts were prepared from acetate, nitrate or carbonyl precursors. The catalysts were characterized by XRD, XPS, SIMS and TGA. The steady-state activity and product distribution of the catalysts were evaluated in synthesis gas reactions at 0.5 MPa and 235-290°C using 3 : 1 : 3 molar ratio of Ar : CO : H2. The activity in CO hydrogenation decreased in the precursor order Co2(CO)8>Co(NO3)2> Co(CH3COO)2, and the probability of chain growth decreased in the precursor order Co(NO3)2>Co2(CO)8>Co(CH3COO)2. Alcohol yields were highest with Co2(CO)8, and lowest with Co(NO3)2, Magnesium promotion influenced the catalyst activity and decreased the CO2 formation, but the promotion effects were less profound than those of the precursor. Surface studies on partially magnesium covered cobalt foil model catalysts suggested that magnesium promotes CO dissociation and chain growth, neither of which were, however, observed in the supported catalysts.