Heterogeneous Thin Films Research Articles

Magnetic correlation among nanosized Co particles in CuCo heterogeneous thin films

Room temperature measurements of magnetisation and giant magnetoresistance were performed on heterogeneous CuCo thin films produced with different thickness by means of RF sputtering. Average particle size, particle size distribution, volume fraction of particles were obtained from magnetisation curve analysis. A correlation among moments was found in all examined systems, depending on film thickness.

Equilibrium and Dynamics of Evaporating or Condensing Thin Fluid Domains: Thin Film Stability and Heterogeneous Nucleation

A continuum theory is presented for the equilibrium and dynamics of evaporating/condensing thin (<100 nm) fluid domains on solid surfaces. The local rate of evaporation/condensation depends crucially on the local curvature and disjoining pressure. The phenomena of heterogeneous nucleation and thin film stability on partially wettable surfaces are considered. All of the results for these nanoscale phenomena are consistently interpreted in terms of the macroscopic parameters of wetting, e.g., the apolar van der Waals and the polar (e.g., acid−base) components of the spreading coefficient and the macroscopic equilibrium contact angle. Finite size effects (disjoining pressure) and adsorption can reduce the size, contact angle, and nucleation energy of critical nuclei very substantially as compared to predictions of the classical theory of nucleation. The instability and dewetting (hole formation) of thin evaporating water films on partially wettable surfaces are caused by the hydrophobic attraction, whereas t...

Synthesis and Self-aggregation of Zinc 20-Halogenochlorins as a Model for Bacteriochlorophylls c/d

Zinc 20-halogenochlorins 2(20- F ), 3(20- Cl ) and 4(20- Br ) were synthesized by halogenation of a chlorophyll a derivative at the 20-position as a model for bacteriochlorophyll ( BChl )c, which possesses a methyl group at the 20-position and 20-unsubstituted BChl d. Visible spectra in a polar tetrahydrofuran ( THF ) solution showed that 2-4 were monomeric and the planarity of the chlorin ring, was distorted with increasing bulkiness of the 20-substituent. Visible, circular dichroism and IR spectra revealed that 2-4 self-aggregated to form oligomers similarly with 20-unsubstituted 1 and BChls c/d in the heterogeneous thin film as well as in homogeneous non-polar solvents (1% (v/v) THF-hexane). Therefore, the in vitro self-aggregates of 2-4 are good structural models for in vivo BChls c/d self-aggregates, the main antenna components of photosynthetic green bacteria. Fluorescence spectra showed that monomeric 3 and 4 were less emissive than 1 and 2 due to the heavy atom effect which could not be observed in the oligomeric species, indicating that the in vitro aggregates should be promising as functional (light-harvesting) models.

Interparticle Interaction Effects in Nonmiscible CoAg Thin Films With High Co Concentration

AbstractA series of Co-rich CoAg heterogeneous thin films, with Co concentrations greater than 65 % volume fraction and film thickness in the range 5 nm - 50 nm, was prepared by dc magnetron sputtering. Annealing in high vacuum (P &lt; 3x10-7 Torr) above 400°C was necessary to promote phase separation between the two elements. Maximum room temperature coercivities, Hc ∼ 900 Oe, were obtained at a thickness of 20 nm for CoAg 70:30 vol % films. These coercivities are almost two orders of magnitude larger than the values observed in as-deposited films. The concentration of maximum coercivity, that is usually associated to the threshold for magnetic percolation, is much higher than the 40-50 vol % commonly observed in bulk granular materials. The shift in the magnetic percolation limit was related to the reduced dimensionality of the very thin films. Coercivity was found to vary both with film composition and with thickness. Delta M curves and time decay (magnetic viscosity) measurements were done to determine the interparticle interactions and the activation volumes. A maximum in the remanent coercivity (coincident with the maximum in Hc) was found for a film thickness φ ∼ 15-20 nm in all the alloys. At this same thickness a negative minimum in the interparticle interaction parameter α (indicating that the particle interactions favor the demagnetized state) was observed. The absolute value of a increases for larger Co concentrations. The irreversible susceptibility is an increasing function of film thickness and the magnetic viscosity has a minimum for φ ∼ 15-20 nm. The activation volume also increases with thickness, changing from (15 nm)3 to (40 nm)3 for the more diluted alloys, values similar to the average particle size determined by transmission electron microscopy.

Microstructural stability on aging of an α + β titanium alloy: ti- 6ai- 1.6zr-3.3mo- 0.30si

The development of the microstructure on aging of an (α + β) type titanium alloy containing 6A1-1.6Zr-3.3Mo-0.3Si (VT9) (in weight percent) has been studied. The β-transus temperature of this alloy is approximately 1243 K. Solution treatment in the β-phase field of the alloy followed by quenching in water at room temperature resulted in the formation of a single-phase martensite struc-ture. The martensitic structure was confirmed to be orthorhombic (α″) using X-ray diffraction. The water-quenched (WQ) specimens were subjected to aging treatments at temperatures of 823, 873, and 973 K for various lengths of time. Aging at 823 K for times between 24 and 100 hours did not bring about any noticeable change in the microstructure. Aging at 823 K for 200 and 300 hours resulted in the heterogeneous precipitation ofs2 silicide particles and thin films of β sandwiched between the interplatelet boundaries of martensite. Electron diffraction analysis confirms that the crystal structure of silicide particles is hexagonal with lattice parameters α= 0.70(1) nm andc = 0.36(8) nm. Aging at 873 K for 12 and 24 hours resulted only in the precipitation ofs2 silicide particles, while aging at the same temperatures for longer times (48, 100, and 200 hours) and also at 973 K for 6 to 100 hours resulted in the precipitation of silicides and also thin films of β and acicular martensite. The relative sizes of silicide precipitates and width of thin films of β phase increase with increasing aging time. The sites for silicide precipitation are mainly at α′-α′ boundaries, α-β interfaces, and sometimes within regions of transformed β. The kinetics ofs2 silicide precipi-tation in this alloy is faster than in commercial near-α titanium alloys. This is attributed to the presence of Mo, a strong β stabilizer.

Photoconduction in porous TiO2 sensitized by PbS quantum dots

Photoconduction in heterogeneous thin films consisting of a porous nanocrystalline TiO2 matrix and internally adsorbed quantum size PbS clusters is reported. The TiO2 serves to establish electrical contact to the PbS clusters, and the PbS cluster size is chosen so that photogenerated excess electrons are directly injected from the PbS to the TiO2. The films exhibit strong photoconductance in the visible region, indicating photoelectric sensitization of the TiO2 by the PbS clusters. Comparison of the absorption and photocurrent spectra shows that only PbS clusters of sizes smaller than ∼25 Å contribute to the sensitization. For larger clusters the band alignment at the TiO2/PbS interface appears to be unfavorable for carrier transfer. Further improvement in the photoresponse will have to involve optimization of the transport properties in the nanocrystalline TiO2.

A Thin Film of an Ni–NiO Heterogeneous System for an Optical Recording Medium

The authors have found a write once read many (WORM) type new optical recording medium of an Ni–NiO heterogeneous system thin film. The structure of the recording medium is a single layer Ni–NiO heterogeneous thin film on a transparent resin substrate. Irradiation of a converged laser diode beam causes a volume expansion of the film to form a swell. Information reading is done by using its reduction in reflectivity. The recordable composition region of this film is considered to be the transitive region from the metal to the oxide. The volume expansion is assumed to be induced by the oxidation of the Ni–NiO heterogeneous thin film and the height of the swell is estimated. This value agrees well with the measured top height of the swell.

Investigation of the microstructure effect on the magnetoresistance of heterogeneous Au1-xCox melt-spun ribbons

Giant magnetoresistance (GMR) of up to 65% at room temperature has been achieved in Cu/Co multilayers, far exceeding the 2.5% arising from permalloy, which is presently used in magnetoresistive read heads. The GMR effect is not restricted to multilayered structures only and has been also discovered in heterogeneous CulxCox thin films. At present, experimentally determined and statistically significant microstructural parameters, e.g., Co particle size distribution, composition of Co particles and Co matrix concentration, are still not available for Cu1-xCox films or melt-spun ribbons because of two reasons. First, the small difference in atomic scattering factors of Cu and Co and secondly, the Ashby-Brown contrast, which is expected for coherent precipitates, is not observed. The large volume fraction of Co precipitates results in overlap and cancellation of their strain fields. To overcome limitations in imaging Co precipitates, melt-spun AU1-xCox ribbons have been investigated regarding their GMR. AuCo has a similar phase diagram (Fig. 1) compared with CuCo, but it has the advantage of a large difference in atomic scattering factors of Au and Co. As a consequence Co-rich precipitates can be directly identified by their mass contrast.

Effect of magnetic interactions and multiple magnetic phases on the giant magnetoresistance of heterogeneous cobalt-silver thin films

We have observed magnetic and electrical transport properties in giant magnetoresistive inhomogeneous cobalt-silver films. The material consists of two distinct magnetic phases: large clusters which dominate the magnetization and magnetoresistive processes at room temperature, between which cooperative behavior is observed; and small clusters which dominate the magnetization below 10 K, but make only a minor contribution to the magnetoresistance. Both the variation of the magnetoresistance with magnetization in the film and the difference in the magnetoresistance between the zero-field-cooled and field-cycled state are interpreted by invoking interactions between the active magnetic regions in the sample.

Analysis of solid state reactions at film/substrate interfaces by electron microscopy

Heterogeneous thin film solid state reactions are a matter of topical interest, under both fundamental and applied aspects. The forming phases are in the nanometer size range; the structure and morphology of the involved solid/solid interfaces exerts a strong influence on the reaction kinetics and on the phase formation sequence. Thus electron microscope methods are essential, if the characteristics of film/substrate reactions are to be investigated. Results from a number of electron microscope methods, among them high-resolution and in situ TEM of cross sections, bright- and dark-field diffraction contrast TEM, energy-dispersive X-ray microanalysis and selected area electron diffraction have been used during investigations of interfacial reactions in three thin film systems: TiN/MgO, NiSi2/Si and TiO2/MgO. In addition to the indentification and characterization of the new phases formed, valuable information has been obtained on reaction mechanisms, interfacial kinetic reaction barriers and on the influence of stress on the phase formation.

Materials science aspects in designing giant magnetoresistance in heterogeneous Cu1-xCox thin films

The recent discovery of giant magnetoresistance (GMR) in heterogeneous Cu1-xCox thin films has brought new insights in the phenomenon of GMR, which was previously believed to be restricted to multilayered structures only. Subsequent theoretical analyses of GMR in this new materials class have shown that GMR is mainly controlled by the mean radius and the volume fraction of single domain ferromagnetic particles. In addition to these parameters, the mean free path for electron in the non-magnetic matrix as well as coherency between particles and matrix are influencing the amplitude of GMR. Clearly, the key to increase the amplitude of GMR is to determine the decomposition kinetics and from which to optimize the single domain ferromagnetic Co particle size distribution in heterogeneous Cu1-xCox thin films.Cu81Col9 films, 50 nm in thickness, have been deposited by dc magnetron sputtering from separate Cu and Co targets onto 30 nm thick silicon nitride electron transparent grids.

Giant magnetoresistance in heterogeneous Cu-Co alloys.

We have observed giant magnetoresistance in heterogeneous thin film Cu-Co alloys consisting of ultrafine Co-rich precipitate particles in a Cu-rich matrix. The magnetoresistance scales inversely with the average particle diameter. This behavior is modeled by including spin-dependent scattering at the interfaces between the particles and the matrix, as well as the spin-dependent scattering in the Co-rich particles.

Structural change of a heterogeneous thin film patch by surface electro-migration

A new model is proposed for the mechanism of the structural change of a thin film patch by surface electro-migration. The evolution in time of the structure of a thin film patch is investigated under the influence of the uniform driving force. The computer simulation is executed, based on the lattice gas model with the nearest neighbour atomic interactions. The numerical results can reproduce well the diverse observed behaviours for In/Si(111), Ag/Si(111) and Sn/Si(111).

Structural change of a heterogeneous thin film patch by surface electro-migration

Structural change of a heterogeneous thin film patch by surface electro-migration

Monte-Carlo simulation of heterogeneous thin film growth

The growth mechanism and the resulting atomic structure at initial stages of heterogeneous thin film growth by vapour phase deposition are investigated, and the role of the surface diffusion is clarified. The growth of a film in the island mode on the ideal substrate is studied in the lattice gas model, using Monte-Carlo simulation. The initial process is clearly divided into a nucleation stage and a growth stage, based on the time evolution of the size of growth clusters. With the increase of the deposition rate or of the inter-atomic interaction energy relative to the substrate temperature, the deviation from the thermal equilibrium becomes important and the growth form of a film is determined kinematically. As the evaporation rate increases, the growth character shows a transition from the continuous growth with multi-nucleation to the growth with long incubation time. The growth rate of a cluster tends to be governed by the diffusion length up to the re-evaporation.

Time evolution of heterogeneous thin films during annealing

The dynamical behaviour of heterogeneous thin films during annealing is studied, starting from a random atomic configuration. A computer simulation is performed for the lattice gas model using the Metropolis method. The relaxation behaviour of the atomic structure of the film is clarified for the island mode, layer mode and roughening mode. Fluctuation of the cluster edges appears to be much stronger in the case of two-dimensional clusters in the layer mode than in the case of three-dimensional clusters in the island mode, and the roughening temperature becomes lower in the former case. The condensation process during the annealing is divided into two stages: the initial stage in which single atoms on the substrate are captured into clusters and attain to a quasi-thermal equilibrium with clusters, and the subsequent stage of cluster growth. The clusters grow mainly through the evaporation and condensation of single atoms on clusters in the case of the island mode, and through diffusion and coalescence of clusters in the case of the layer mode.

Time evolution of heterogeneous thin films during annealing

Time evolution of heterogeneous thin films during annealing

Scanning electron microscope identification of weak links in superconducting thin films

We have used a pulsed electron beam in a scanning electron microscope to produce highly localized (∼1 μm) changes in the electrical properties of a heterogeneous thin film of YBa2Cu3O7−δ . The predominant effect of the beam is to locally heat the film, producing a measurable shift in the current-voltage characteristic. In particular, we have precisely located the part of the film responsible for a sharp feature in the current-voltage characteristic corresponding to a single Josephson junction. The technique should also be useful more generally in identifying specific weak links in superconducting thin films.

On the influence of the propagation length of plasmon surface polaritons in the visible energy range for the optical characterization of heterogeneous thin films

Attenuated total reflection measurements in the Kretschmann configuration are performed with various laser wavelengths at Ag surfaces with thin heterogeneous coatings. The plasmon surface polariton (PSP) resonances observed for samples that consist of two different coexisting layer structures — Ag/air and Ag/SiO x /air, respectively — each with a well-defined lateral extension (40 μm) are analyzed taking into account the energy-dependent propagation length, L x , of the excited surface modes. The influence of L x on the lateral resolution for the optical characterization of heterogeneous coatings is discussed.

On the influence of the propagation length of plasmon surface polaritons in the visible energy range for the optical characterization of heterogeneous thin films

On the influence of the propagation length of plasmon surface polaritons in the visible energy range for the optical characterization of heterogeneous thin films