Film Percolation Research Articles

Evidence of local superconductivity in granular Bi nanowires fabricated by electrodeposition

An unusual enhancement of resistance (i.e., a ``superresistivity'') below a certain characteristic temperature ${T}_{\text{sr}}$ was observed in granular Bi nanowires. This ``superresistive'' state was found to be dependent on the applied magnetic field as well as the excitation current. The suppression of ${T}_{\text{sr}}$ by magnetic field resembles that of a superconductor. The observed superresistivity appears to be related to the nucleation of local superconductivity inside the granular nanowire without long-range phase coherence. The phenomenon is reminiscent of the ``Cooper-pair insulator'' observed previously in ultrathin two-dimensional superconducting films and three-dimensional percolative superconducting films.

Sol–gel-derived percolative copper film

Cu–SiO2 films were prepared by the sol–gel method. Two-dimensional fractal copper films were formed after the films were thermally treated in reducing atmosphere. dc resistances of the films decrease 12 orders of magnitude as the content of copper increases from 70 to 80mol%. During the resistance measurement under argon atmosphere, samples showed a sharp increase or decrease of resistance at a transition temperature which is ascribed to the oxidation of Cu into CuO. The oxidation was also observed in the in situ high temperature X-ray diffraction under vacuum condition. The evolution of the morphology of the films was studied by scanning electron microscopy. As the content of copper increases, the forms of copper particles change from discrete to aggregate then to interconnecting. The coverage coefficients of the copper range from 23 to 55% and the fractal dimensions range from 1.65 to 1.77. The percolation thresholds for the coverage coefficient and the fractal dimension are about 33% and 1.71, respectively, which corresponds to the sample containing 72.5mol% of Cu.

Percolative conductor/polymer composite films with significant dielectric properties

A percolative film with conductive acetylene black introduced into β-polyvinylidene fluoride was prepared by dip-coating method. Percolation theory was employed to explain the dielectric behavior of polymer/conductor composite films. Experimental results showed that the dielectric constant of polymer/conductor composite films can reach 56 and the dielectric loss is below 0.15 when acetylene black concentration is in the neighborhood of percolation threshold (fc). The experimental results are in good agreement with classic percolation power law, with fc≈1.3% and exponent q≈0.53. Such composite films have a potential to become capacitors and can be easily fabricated due to its flexibility.

Depth-profiled positronium annihilation lifetime spectroscopy on porous films

Positronium annihilation lifetime spectroscopy (PALS) is a unique porosimetry technique with broad applicability in characterizing nanoporous materials, especially insulators. In beam-based PALS a focused beam of several keV positrons forms positronium (Ps, the electron–positron bound state) with a depth distribution (typically 5 nm–5 μm) that depends on the selected positron beam energy. Ps localizes in the pores where its natural annihilation lifetime of 142 ns is strongly reduced by collisions with pore surfaces. The collisionally reduced Ps lifetime is directly correlated with pore size, the key feature in transforming a Ps lifetime distribution into a pore size distribution over the 0.3–30 nm diameter range. Depth-profiling with PALS has proven to be an ideal way to non-destructively search for depth-dependent changes in the pore structure and to explore porosity hidden beneath dense layers or diffusion barriers whereby the positrons are injected through the barrier into the porous film. Profiling also determines the pore interconnection length, a unique measure of the degree of pore interconnection prior to film percolation. The capability of PALS is rapidly maturing as new intense positron beams around the globe spawn more accessible PALS facilities.

Dielectric behavior of three phase polyimide percolative nanocomposites

AbstractThree phase ceramic–metal–plastic (cermetplas) percolative nanocomposite films were prepared using polyimide, as the matrix to prepare the composite films. Silver nanoparticles with a mean particle size of about 41 nm were prepared and used as the metal phase. Nanocomposites of barium titanate (BaTiO3) particles in polyimide matrix, with silver as metallic inclusion, were formulated using effective medium theory and percolation theory. The concentration and frequency dependence of dielectric permittivity (ε) and loss tangent (tan δ) measurements were reported and discussed for cermetplas below the percolation threshold. Experimental results for cermetplas show that a ε of above 500 can be achieved below the percolation. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci, 2006

A study on percolation threshold of Ag-MgF2 nanoparticle cermet films

Several methods are introduced to calculate the percolation threshold of nanoparticle cermet films made of ceramic matrix with embedded metal nanoparticles. For Ag-MgF2 nanoparticle cermet films, the percolation threshold of 0.14, which reasonably agrees with the experimental results, is calculated by using Priou's percolation threshold theory. The factors that affect cermet film percolation threshold are also discussed.

Deducing nanopore structure and growth mechanisms in porogen-templated silsesquioxane thin films

Adjusting the functional group of a porogen is found to have a tremendous effect on the pore structre of porous low dielectric constant films with silsesquioxane as the matrix precursor. The pore size and interconnection length measured by positronium annihilation lifetime spectroscopy can be used to deduce the pore shape and its evolution with porosity from templates of isolated porogen molecules through film percolation. Inert, self-linkable, and amphiphilic porogens are demonstrated to randomly aggregate three-dimensionally, linearly polymerize, and form micelles, respectively.

Anomalous hopping and tunneling effects on the conductivity ofa wedge-shaped Au film system

A gold thin film percolation system, deposited on glass surfaces by the vapor deposition method, has been fabricated. By using the expansive and movable properties of silicone oil drop, a characteristic wedge - shaped film system with a slope of ～ 10-5 naturally forms during deposition. The electrical conductivity of the band-like gold film, i.e., the uniform part with a fixed thickness, is measured with the four-probe method. It is found that hopping and tunneling effects of the films are stronger than those of the other films.

Growth, morphology, and optical and electrical properties of semicontinuous metallic films

The growth and optical properties of semicontinuous silver films on insulator substrates were studied experimentally and theoretically. In the experimental studies, films were synthesized by the pulsed-laser deposition technique, characterized by electron microscopy and in situ optical and dc electrical resistance measurements and studied using near-field optical microscopy. The percolation threshold of the films was found to be at \ensuremath{\sim}65% metal filling fraction, higher than the 50%--60% range of values predicted for two-dimensional (2D) bond or site percolation films and suggesting the importance of grain coalescence and 3D growth in our system. Local optical properties measured by near-field optical microscopy were compared with theoretical results obtained using the block-elimination method, with good agreement. Local-field distributions were found to depend strongly on the metal concentration and wavelength of illumination. The degree of localization was found to increase at metal concentrations both above and below the percolation threshold. At a very high metal coverage, very strong local fields were observed in submicron voids of a metal dielectric film. These fields are likely due to localized surface plasmon polaritons.

Anomalous electrical conductivity of a gold thin film percolation system

A gold thin film percolation system, deposited on a glass surface by the vapor deposition method, has been fabricated. By using the expansive and mobile properties of the silicone oil drop, a characteristic wedge-shaped film system with a slope of $\ensuremath{\sim}{10}^{\ensuremath{-}5}$ naturally forms during deposition. The electrical conductivity of the bandlike film, i.e., the uniform part of the wedge-shaped film with a fixed thickness, is measured with the four-probe method. It is found that the hopping and tunneling effects of the films are stronger than those of the other films. The dependence between the dc sheet resistance ${R}_{0}$ and temperature T shows that the samples exhibit a negative coefficient ${\mathrm{dR}}_{0}/dT$ below the temperature ${T}^{*}.$ According to our experiment, it is suggested that all the anomalous behaviors of the system should be related to the characteristic microstructure of the samples, which results from the immediate quench processes by the oil drop during deposition. The experiment indicates that the relaxation period of the microstructure of the samples may be longer than 30 min.

Morphological Evolution of Ag/Mica Films Grown by Pulsed Laser Deposition

ABSTRACTMany vapor-deposited metal-on-insulator films exhibit a morphological progression with increasing thickness consisting of several distinct stages: (1) nucleation of 3-dimensional na-nocrystalline islands; (2) elongation of the islands; (3) film percolation. Here we report a study of this progression during Pulsed Laser Deposition (PLD), a technique for film deposition that differs from thermal deposition in that the depositing species arrive in short energetic bursts, leading to instantaneous deposition fluxes orders of magnitude higher than can be achieved in thermal growth. Atomic Force Microscopy reveals that advancement through this same morphological progression occurs at lower thickness in PLD films relative to films grown under comparable conditions by thermal deposition, with PLD films having lower RMS roughness at a given thickness. We also observe that for a constant amount deposited per pulse, films deposited at higher laser pulse frequency are further advanced in morphological state. Kinetic Monte Carlo simulations reveal that PLD nucleation behavior differs from that of thermally deposited films, and this can account for the observed differences. Simulations also reveal a scaling of the percolation thickness with pulse frequency that is consistent with experiment.

Anomalous hopping and tunneling effects in a new aluminum film percolation system

Anomalous hopping and tunneling effects in a new aluminum film percolation system

Near-field optical studies of semicontinuous metal films

Local field distributions in random metal-dielectric films near a percolation threshold are experimentally studied using scanning near-field optical microscopy (SNOM). The surface-plasmon oscillations in such percolation films are localized in small nanometer-scale areas, ``hot spots,'' where the local fields are much larger than the field of an incident electromagnetic wave. The spatial positions of the hot spots vary with the wavelength and polarization of the incident beam. Local near-field spectroscopy of the hot spots is performed using our SNOM. It is shown that the resonance quality-factor of hot spots increases from the visible to the infrared. Giant local optical activity associated with chiral plasmon modes has been obtained. The hot spot's large local fields may result in local, frequency and spatially selective photomodification of percolation films.

Void and grain resonances in discontinuous silver thinfilms close to the percolation threshold

An optical study of discontinuous metallic thin films haspermitted us to observe simultaneously two resonant phenomenain media close to the percolation threshold. In addition to thewell known metal grain resonance, a weaker resonance appears inthe UV spectrum. This phenomenon, the presence of bothresonances in a single sample, has seldom been pointed out. Wedemonstrate experimentally, on a silver percolating film, thatthis resonance is due to the presence of voids in the thin metallayer. We introduce a new model for the calculation of the opticalproperties of heterogeneous media, based on an entropic analysisof the actual medium, which accounts fairly well for bothphenomena.

PERCOLATION AND FRACTAL COMPOSITES: OPTICAL STUDIES

Local field distributions are studied in random metal–dielectric films near percolation (percolation films) and fractal aggregates of colloidal particles. For both systems, it is shown that optical excitations are localized in small nanometer-scale areas, "hot spots," where the local fields are much larger than the field of an incident electromagnetic wave. The large local fields result in giant enhancement of various optical phenomena. The surface-enhanced white-light generation and second-harmonic generation have been obtained in percolation films. For fractal aggregates of silver particles, a giant effect of local optical activity has been observed. The effect is due to surface-plasmon excitations localized on chiral-active particle configurations in fractals.

Superconducting quantum interference in fractal percolation films. Problem of 1/f; noise

An oscillatory magnetic field dependence of the DC voltage is observed when a low-frequency current flows through superconducting SnGe thin-film composites near the percolation threshold. The paper also studies the experimental realisations of temporal voltage fluctuations in these films. Both the structure of the voltage oscillations against the magnetic field and the time series of the electric “noise” possess a fractal pattern. With the help of the fractal analysis procedure, the fluctuations observed have been shown to be neither a noise with a large number of degrees of freedom, nor the realisations of a well-defined dynamic system. On the contrary the model of voltage oscillations induced by the weak fluctuations of a magnetic field of arbitrary nature gives the most appropriate description of the phenomenon observed. The imaging function of such a transformation possesses a fractal nature, thus leading to power-law spectra of voltage fluctuations even for the simplest types of magnetic fluctuations including the monochromatic ones. Thus, the paper suggests a new universal mechanism of a “1/f noise” origin. It consists in a passive transformation of any natural fluctuations with a fractal-type transformation function.

Experimental and theoretical study of metal-dielectric percolating films at microwaves

The detailed experimental study of the microwave properties of macroscopical semicontinuous metal films is presented. Significant absorption has been observed in spite of the fact that losses are almost absent in dielectric as well in metal components of the films. Electric-field distribution is measured over the film surface. The local field appears to be highly localized at some regions of the films. We also present the theory of EM wave interaction with the films. The theory is based on a direct solution of Maxwell's equations and reproduces the experimental results well.

Fringe effect on critical current of a gold film percolation system

In this paper, the fringe effect on critical current has been studied systematically by measuring the electric transport characteristics of a Au-film system with diffuse fringe structure. It is found experimentally that, when the fringe effect becomes obvious, the film will exhibit strange I-R characteristics, in which exist a balance current and a critical current. We find that the critical exponents are related to the diffuse fringe structure of the films. The detailed analysis shows that the characteristics result from the thermionic-activating conductive effect in the diffuse fringe film system.

Critical behavior of a metallic diffuse-fringe film percolation system

Preparation of a metallic diffuse-fringe film system and its electrical properties are reported. The anomalousR-I characteristics, third-harmonic coefficient and critical behavior are observed in the film system. Analysis shows that this behavior is caused by the location-dependent tunneling and hopping effects, which result from. the continuous variation of the diffuse fringe structure of the film due to the increase of the current.

Conductivity measurements of a metallic diffuse-fringe film percolation system

We have fabricated a metallic diffuse-fringe film system by dc-magnetron spulleririg method. The diffuse fringe structure of the film system is obviously observed when during the film deposition process the distance between the shutter slit and the substrate is large enough. An anomalous, nonlinear I-V behavior is found in the system and can be well explained by the Joule heating and hopping effects. The strong dependence between the conductivity behavior and the fringe indicates that the fringe effect is very severe in this inhomogeneous percolation system.